[RFC] diagnose built-in declarations without prototype (PR 83656)

With the exception of built-ins with the ellipsis (like sprintf),
GCC silently accepts declarations of built-in functions without
prototypes as well as calls to such functions with any numbers
or types of arguments, compatible or otherwise.  Calls with
arguments whose number and types match exactly those of
the built-in are considered by the middle-end for optimization.
Other calls (compatible or not, irrespective of whether their
number matches the number expected by the function) are then
made to the library functions.

Attached is a small fix to -Wbuiltin-declaration-mismatch to
have it diagnose built-in declarations without a prototype.
The warning is enabled by default so it causes a fair number
of tests to fail because of declarations like 'void abort();'
The breakdown of the built-ins behind the test failures is
below.

Before I take the time to clean up the test suite let me post
what I have in case going this route is not acceptable.  As
an alternative, I could try to avoid some of these warnings,
e.g., by diagnosing incompatible calls instead but I think
it's even less worthwhile for built-ins than trying to do
it for ordinary functions with -Wstrict-prototypes.  There
is, in my view, no justification today for standard functions
to be declared without a prototype.  (I could also make
the warning depend on language mode and -Wpedantic if that
were more preferable.)

Martin

About 115 tests fail due to incompatible declarations of
the built-in functions below (the number shows the number
of warnings for each functions):

428   abort
  58   exit
  36   memcpy
  17   memmove
  15   realloc
  14   cabs
   5   strncpy
   4   strcmp
   3   alloca
   2   rindex
   1   aligned_alloc

On 06/26/2018 08:57 PM, Martin Sebor wrote:
> With the exception of built-ins with the ellipsis (like sprintf),
> GCC silently accepts declarations of built-in functions without
> prototypes as well as calls to such functions with any numbers
> or types of arguments, compatible or otherwise.  Calls with
> arguments whose number and types match exactly those of
> the built-in are considered by the middle-end for optimization.
> Other calls (compatible or not, irrespective of whether their
> number matches the number expected by the function) are then
> made to the library functions.
> 
> Attached is a small fix to -Wbuiltin-declaration-mismatch to
> have it diagnose built-in declarations without a prototype.
> The warning is enabled by default so it causes a fair number
> of tests to fail because of declarations like 'void abort();'
> The breakdown of the built-ins behind the test failures is
> below.
> 
> Before I take the time to clean up the test suite let me post
> what I have in case going this route is not acceptable.  As
> an alternative, I could try to avoid some of these warnings,
> e.g., by diagnosing incompatible calls instead but I think
> it's even less worthwhile for built-ins than trying to do
> it for ordinary functions with -Wstrict-prototypes.  There
> is, in my view, no justification today for standard functions
> to be declared without a prototype.  (I could also make
> the warning depend on language mode and -Wpedantic if that
> were more preferable.)
> 
> Martin
> 
> About 115 tests fail due to incompatible declarations of
> the built-in functions below (the number shows the number
> of warnings for each functions):
> 
> 428   abort
>  58   exit
>  36   memcpy
>  17   memmove
>  15   realloc
>  14   cabs
>   5   strncpy
>   4   strcmp
>   3   alloca
>   2   rindex
>   1   aligned_alloc
I'm supportive of this change.  Though I'm more worried about the
affects on the distros than I am on the testsuite (which I expected to
be in worse shape WRT this issue than your analysis indicates).

Unfortunately I don't have the tester set up to do distro style testing
yet.  So I can't just throw your patch in and see what is spit out.  But
even so, I still think this is the right thing to do.

Jeff

On Wed, Jun 27, 2018 at 09:17:07AM -0600, Jeff Law wrote:
> > About 115 tests fail due to incompatible declarations of
> > the built-in functions below (the number shows the number
> > of warnings for each functions):
> > 
> > 428   abort
> >  58   exit
> >  36   memcpy
> >  17   memmove
> >  15   realloc
> >  14   cabs
> >   5   strncpy
> >   4   strcmp
> >   3   alloca
> >   2   rindex
> >   1   aligned_alloc
> I'm supportive of this change.  Though I'm more worried about the
> affects on the distros than I am on the testsuite (which I expected to
> be in worse shape WRT this issue than your analysis indicates).

I'm mainly worried about configure tests, those will be hit hardest by
this and might result in silently turning off many features of the compiled
programs.

I think
void abort ();
is very common and if people actually don't pass any arguments to abort, I
think warning by default about this is too much.

	Jakub

On 6/26/18, Martin Sebor <msebor@gmail.com> wrote:
> With the exception of built-ins with the ellipsis (like sprintf),
> GCC silently accepts declarations of built-in functions without
> prototypes as well as calls to such functions with any numbers
> or types of arguments, compatible or otherwise.  Calls with
> arguments whose number and types match exactly those of
> the built-in are considered by the middle-end for optimization.
> Other calls (compatible or not, irrespective of whether their
> number matches the number expected by the function) are then
> made to the library functions.
>
> Attached is a small fix to -Wbuiltin-declaration-mismatch to
> have it diagnose built-in declarations without a prototype.
> The warning is enabled by default so it causes a fair number
> of tests to fail because of declarations like 'void abort();'
> The breakdown of the built-ins behind the test failures is
> below.
>
> Before I take the time to clean up the test suite let me post
> what I have in case going this route is not acceptable.  As
> an alternative, I could try to avoid some of these warnings,
> e.g., by diagnosing incompatible calls instead but I think
> it's even less worthwhile for built-ins than trying to do
> it for ordinary functions with -Wstrict-prototypes.  There
> is, in my view, no justification today for standard functions
> to be declared without a prototype.  (I could also make
> the warning depend on language mode and -Wpedantic if that
> were more preferable.)
>
> Martin
>
> About 115 tests fail due to incompatible declarations of
> the built-in functions below (the number shows the number
> of warnings for each functions):
>
> 428   abort
>   58   exit
>   36   memcpy
>   17   memmove
>   15   realloc
>   14   cabs
>    5   strncpy
>    4   strcmp
>    3   alloca
>    2   rindex
>    1   aligned_alloc
>

Could you do something to differentiate it a bit more from the
-Wshadow warning that is also touched in this patch? It'd be kind of
annoying to get warnings from both flags for the same code...

On 06/27/2018 09:27 AM, Jakub Jelinek wrote:
> On Wed, Jun 27, 2018 at 09:17:07AM -0600, Jeff Law wrote:
>>> About 115 tests fail due to incompatible declarations of
>>> the built-in functions below (the number shows the number
>>> of warnings for each functions):
>>>
>>> 428   abort
>>>  58   exit
>>>  36   memcpy
>>>  17   memmove
>>>  15   realloc
>>>  14   cabs
>>>   5   strncpy
>>>   4   strcmp
>>>   3   alloca
>>>   2   rindex
>>>   1   aligned_alloc
>> I'm supportive of this change.  Though I'm more worried about the
>> affects on the distros than I am on the testsuite (which I expected to
>> be in worse shape WRT this issue than your analysis indicates).
> 
> I'm mainly worried about configure tests, those will be hit hardest by
> this and might result in silently turning off many features of the compiled
> programs.
It's certainly a concern.  Sadly, it's a hard one to deal with because
we can't just throw code at it and see what complains.  Instead we end
up with packages that don't configure in the appropriate features.

I think when Roland and I discussed this in the context of gold the
thing we sketched out was to build the distro, capturing the resultant
config.h, then rebuild with gold, capture the new config.h and compare.

We never actually tried that -- we didn't green light the gold
transition due to other reasons.

Just wandering around our own configury bits does show some of these
kinds of problems.  No idea if they'd manifest themselves in a visible
ways though.

Maybe it's a case where it's an opt-in argument in the immediate term
and we work with the autoconf folks to squash out as many problems as we
can and build out some kind of distro-like testing before flipping the
default.

jeff

I was hoping to avoid the complexity but I appreciate
the desire for a more permissive solution, at least for
some transitional period.

I've reworked the patch to avoid diagnosing 'void abort();'
(the only library built-in that takes no arguments).
Instead, calls that pass arguments to it are diagnosed.

While at it, I generalized the convert_arguments() function
to diagnose argument mismatches in calls to any built-in
declared without a prototype.  I think this is a useful
feature regardless because many such calls are not considered
for optimization even when they are safe.  But with it in
place, declarations of built-ins without a prototype could
continue to be silently accepted by default for a period and
only diagnosed with -Wextra until we are ready to turn on
the warnings by default.  (If that's thought necessary --
the patch as is doesn't do that.)

Since these changes involve factoring quite a bit of code into
a new function (with only very minor tweaks), to make review
easier, I attach two diffs: one with -w, and another full one.
The full diff also contains the cleanup of the failing tests.

Martin

On 06/27/2018 09:27 AM, Jakub Jelinek wrote:
> On Wed, Jun 27, 2018 at 09:17:07AM -0600, Jeff Law wrote:
>>> About 115 tests fail due to incompatible declarations of
>>> the built-in functions below (the number shows the number
>>> of warnings for each functions):
>>>
>>> 428   abort
>>>  58   exit
>>>  36   memcpy
>>>  17   memmove
>>>  15   realloc
>>>  14   cabs
>>>   5   strncpy
>>>   4   strcmp
>>>   3   alloca
>>>   2   rindex
>>>   1   aligned_alloc
>> I'm supportive of this change.  Though I'm more worried about the
>> affects on the distros than I am on the testsuite (which I expected to
>> be in worse shape WRT this issue than your analysis indicates).
>
> I'm mainly worried about configure tests, those will be hit hardest by
> this and might result in silently turning off many features of the compiled
> programs.
>
> I think
> void abort ();
> is very common and if people actually don't pass any arguments to abort, I
> think warning by default about this is too much.
>
> 	Jakub
>
PR c/83656 - missing -Wbuiltin-declaration-mismatch on declaration without prototype

gcc/c/ChangeLog:

	PR c/83656
	* c-decl.c (diagnose_mismatched_decls): Diagnose declarations of
	built-in functions without a prototype.
	* c-typeck.c (convert_argument): New function.
	(convert_arguments): Factor code out into convert_argument.
	Detect mismatches between built-in formal arguments in calls
	to built-in without prototype.
	(convert_for_assignment): Add argument.  Conditionally issue
	warnings instead of errors for mismatches.

diff --git a/gcc/c/c-decl.c b/gcc/c/c-decl.c
index af16cfd..07beaa1 100644
--- a/gcc/c/c-decl.c
+++ b/gcc/c/c-decl.c
@@ -1886,8 +1886,9 @@ diagnose_mismatched_decls (tree newdecl, tree olddecl,
 		 built-in.  No point in calling locate_old_decl here, it
 		 won't print anything.  */
 	      warning (OPT_Wbuiltin_declaration_mismatch,
-		       "conflicting types for built-in function %q+D",
-		       newdecl);
+		       "conflicting types for built-in function %q+D; "
+		       "expected %qT",
+		       newdecl, oldtype);
 	      return false;
 	    }
 	}
@@ -2021,17 +2022,35 @@ diagnose_mismatched_decls (tree newdecl, tree olddecl,
 	 can't validate the argument list) the built-in definition is
 	 overridden, but optionally warn this was a bad choice of name.  */
       if (DECL_BUILT_IN (olddecl)
-	  && !C_DECL_DECLARED_BUILTIN (olddecl)
-	  && (!TREE_PUBLIC (newdecl)
+	  && !C_DECL_DECLARED_BUILTIN (olddecl))
+	{
+	  if (!TREE_PUBLIC (newdecl)
 	      || (DECL_INITIAL (newdecl)
-		  && !prototype_p (TREE_TYPE (newdecl)))))
+		  && !prototype_p (TREE_TYPE (newdecl))))
 	    {
-	  warning (OPT_Wshadow, "declaration of %q+D shadows "
+	      warning_at (DECL_SOURCE_LOCATION (newdecl),
+			  OPT_Wshadow, "declaration of %qD shadows "
 			  "a built-in function", newdecl);
 	      /* Discard the old built-in function.  */
 	      return false;
 	    }
 
+	  if (!prototype_p (TREE_TYPE (newdecl)))
+	    {
+	      /* Set for built-ins that take no arguments.  */
+	      bool func_void_args = false;
+	      if (tree at = TYPE_ARG_TYPES (oldtype))
+		func_void_args = VOID_TYPE_P (TREE_VALUE (at));
+
+	      if (extra_warnings || !func_void_args)
+		warning_at (DECL_SOURCE_LOCATION (newdecl),
+			    OPT_Wbuiltin_declaration_mismatch,
+			    "declaration of built-in function %qD without "
+			    "a prototype; expected %qT",
+			    newdecl, TREE_TYPE (olddecl));
+	    }
+	}
+
       if (DECL_INITIAL (newdecl))
 	{
 	  if (DECL_INITIAL (olddecl))
diff --git a/gcc/c/c-typeck.c b/gcc/c/c-typeck.c
index 90ae306..e0584b9 100644
--- a/gcc/c/c-typeck.c
+++ b/gcc/c/c-typeck.c
@@ -97,7 +97,8 @@ static int convert_arguments (location_t, vec<location_t>, tree,
 			      tree);
 static tree pointer_diff (location_t, tree, tree, tree *);
 static tree convert_for_assignment (location_t, location_t, tree, tree, tree,
-				    enum impl_conv, bool, tree, tree, int);
+				    enum impl_conv, bool, tree, tree, int,
+				    int = 0);
 static tree valid_compound_expr_initializer (tree, tree);
 static void push_string (const char *);
 static void push_member_name (tree);
@@ -3179,164 +3180,20 @@ c_build_function_call_vec (location_t loc, vec<location_t> arg_loc,
   return build_function_call_vec (loc, arg_loc, function, params, origtypes);
 }
 
-/* Convert the argument expressions in the vector VALUES
-   to the types in the list TYPELIST.
-
-   If TYPELIST is exhausted, or when an element has NULL as its type,
-   perform the default conversions.
-
-   ORIGTYPES is the original types of the expressions in VALUES.  This
-   holds the type of enum values which have been converted to integral
-   types.  It may be NULL.
-
-   FUNCTION is a tree for the called function.  It is used only for
-   error messages, where it is formatted with %qE.
-
-   This is also where warnings about wrong number of args are generated.
-
-   ARG_LOC are locations of function arguments (if any).
-
-   Returns the actual number of arguments processed (which may be less
-   than the length of VALUES in some error situations), or -1 on
-   failure.  */
-
-static int
-convert_arguments (location_t loc, vec<location_t> arg_loc, tree typelist,
-		   vec<tree, va_gc> *values, vec<tree, va_gc> *origtypes,
-		   tree function, tree fundecl)
-{
-  tree typetail, val;
-  unsigned int parmnum;
-  bool error_args = false;
-  const bool type_generic = fundecl
-    && lookup_attribute ("type generic", TYPE_ATTRIBUTES (TREE_TYPE (fundecl)));
-  bool type_generic_remove_excess_precision = false;
-  bool type_generic_overflow_p = false;
-  tree selector;
-
-  /* Change pointer to function to the function itself for
-     diagnostics.  */
-  if (TREE_CODE (function) == ADDR_EXPR
-      && TREE_CODE (TREE_OPERAND (function, 0)) == FUNCTION_DECL)
-    function = TREE_OPERAND (function, 0);
-
-  /* Handle an ObjC selector specially for diagnostics.  */
-  selector = objc_message_selector ();
-
-  /* For type-generic built-in functions, determine whether excess
-     precision should be removed (classification) or not
-     (comparison).  */
-  if (type_generic
-      && DECL_BUILT_IN (fundecl)
-      && DECL_BUILT_IN_CLASS (fundecl) == BUILT_IN_NORMAL)
-    {
-      switch (DECL_FUNCTION_CODE (fundecl))
-	{
-	case BUILT_IN_ISFINITE:
-	case BUILT_IN_ISINF:
-	case BUILT_IN_ISINF_SIGN:
-	case BUILT_IN_ISNAN:
-	case BUILT_IN_ISNORMAL:
-	case BUILT_IN_FPCLASSIFY:
-	  type_generic_remove_excess_precision = true;
-	  break;
+/* Helper for convert_arguments called to convert the VALue of argument
+   number ARGNUM from ORIGTYPE to the corresponding parameter number
+   PARMNUL and TYPE.  */
 
-	case BUILT_IN_ADD_OVERFLOW_P:
-	case BUILT_IN_SUB_OVERFLOW_P:
-	case BUILT_IN_MUL_OVERFLOW_P:
-	  /* The last argument of these type-generic builtins
-	     should not be promoted.  */
-	  type_generic_overflow_p = true;
-	  break;
-
-	default:
-	  break;
-	}
-    }
-
-  /* Scan the given expressions and types, producing individual
-     converted arguments.  */
-
-  for (typetail = typelist, parmnum = 0;
-       values && values->iterate (parmnum, &val);
-       ++parmnum)
+static tree
+convert_argument (location_t ploc, tree function, tree fundecl,
+		  tree type, tree origtype, tree val,
+		  tree rname, int parmnum, int argnum,
+		  bool excess_precision, int warnopt)
 {
-      tree type = typetail ? TREE_VALUE (typetail) : 0;
   tree valtype = TREE_TYPE (val);
-      tree rname = function;
-      int argnum = parmnum + 1;
-      const char *invalid_func_diag;
-      bool excess_precision = false;
-      bool npc;
-      tree parmval;
-      /* Some __atomic_* builtins have additional hidden argument at
-	 position 0.  */
-      location_t ploc
-	= !arg_loc.is_empty () && values->length () == arg_loc.length ()
-	  ? expansion_point_location_if_in_system_header (arg_loc[parmnum])
-	  : input_location;
-
-      if (type == void_type_node)
-	{
-	  if (selector)
-	    error_at (loc, "too many arguments to method %qE", selector);
-	  else
-	    error_at (loc, "too many arguments to function %qE", function);
-	  inform_declaration (fundecl);
-	  return error_args ? -1 : (int) parmnum;
-	}
 
-      if (selector && argnum > 2)
-	{
-	  rname = selector;
-	  argnum -= 2;
-	}
-
-      npc = null_pointer_constant_p (val);
-
-      /* If there is excess precision and a prototype, convert once to
-	 the required type rather than converting via the semantic
-	 type.  Likewise without a prototype a float value represented
-	 as long double should be converted once to double.  But for
-	 type-generic classification functions excess precision must
-	 be removed here.  */
-      if (TREE_CODE (val) == EXCESS_PRECISION_EXPR
-	  && (type || !type_generic || !type_generic_remove_excess_precision))
-	{
-	  val = TREE_OPERAND (val, 0);
-	  excess_precision = true;
-	}
-      val = c_fully_fold (val, false, NULL);
-      STRIP_TYPE_NOPS (val);
-
-      val = require_complete_type (ploc, val);
-
-      /* Some floating-point arguments must be promoted to double when
-	 no type is specified by a prototype.  This applies to
-	 arguments of type float, and to architecture-specific types
-	 (ARM __fp16), but not to _FloatN or _FloatNx types.  */
-      bool promote_float_arg = false;
-      if (type == NULL_TREE
-	  && TREE_CODE (valtype) == REAL_TYPE
-	  && (TYPE_PRECISION (valtype)
-	      <= TYPE_PRECISION (double_type_node))
-	  && TYPE_MAIN_VARIANT (valtype) != double_type_node
-	  && TYPE_MAIN_VARIANT (valtype) != long_double_type_node
-	  && !DECIMAL_FLOAT_MODE_P (TYPE_MODE (valtype)))
-	{
-	  /* Promote this argument, unless it has a _FloatN or
-	     _FloatNx type.  */
-	  promote_float_arg = true;
-	  for (int i = 0; i < NUM_FLOATN_NX_TYPES; i++)
-	    if (TYPE_MAIN_VARIANT (valtype) == FLOATN_NX_TYPE_NODE (i))
-	      {
-		promote_float_arg = false;
-		break;
-	      }
-	}
+  tree parmval;
 
-      if (type != NULL_TREE)
-	{
   /* Formal parm type is specified by a function prototype.  */
 
   if (type == error_mark_node || !COMPLETE_TYPE_P (type))
@@ -3347,10 +3204,8 @@ convert_arguments (location_t loc, vec<location_t> arg_loc, tree typelist,
     }
   else
     {
-	      tree origtype;
-
-	      /* Optionally warn about conversions that
-		 differ from the default conversions.  */
+      /* Optionally warn about conversions that differ from the default
+	 conversions.  */
       if (warn_traditional_conversion || warn_traditional)
 	{
 	  unsigned int formal_prec = TYPE_PRECISION (type);
@@ -3492,17 +3347,208 @@ convert_arguments (location_t loc, vec<location_t> arg_loc, tree typelist,
 	 sake of better warnings from convert_and_check.  */
       if (excess_precision)
 	val = build1 (EXCESS_PRECISION_EXPR, valtype, val);
-	      origtype = (!origtypes) ? NULL_TREE : (*origtypes)[parmnum];
-	      parmval = convert_for_assignment (loc, ploc, type,
+
+      parmval = convert_for_assignment (ploc, ploc, type,
 					val, origtype, ic_argpass,
-						npc, fundecl, function,
-						parmnum + 1);
+					null_pointer_constant_p (val),
+					fundecl, function,
+					parmnum + 1, warnopt);
 
       if (targetm.calls.promote_prototypes (fundecl ? TREE_TYPE (fundecl) : 0)
 	  && INTEGRAL_TYPE_P (type)
 	  && (TYPE_PRECISION (type) < TYPE_PRECISION (integer_type_node)))
 	parmval = default_conversion (parmval);
     }
+
+  return parmval;
+}
+
+/* Convert the argument expressions in the vector VALUES
+   to the types in the list TYPELIST.
+
+   If TYPELIST is exhausted, or when an element has NULL as its type,
+   perform the default conversions.
+
+   ORIGTYPES is the original types of the expressions in VALUES.  This
+   holds the type of enum values which have been converted to integral
+   types.  It may be NULL.
+
+   FUNCTION is a tree for the called function.  It is used only for
+   error messages, where it is formatted with %qE.
+
+   This is also where warnings about wrong number of args are generated.
+
+   ARG_LOC are locations of function arguments (if any).
+
+   Returns the actual number of arguments processed (which may be less
+   than the length of VALUES in some error situations), or -1 on
+   failure.  */
+
+static int
+convert_arguments (location_t loc, vec<location_t> arg_loc, tree typelist,
+		   vec<tree, va_gc> *values, vec<tree, va_gc> *origtypes,
+		   tree function, tree fundecl)
+{
+  unsigned int parmnum;
+  bool error_args = false;
+  const bool type_generic = fundecl
+    && lookup_attribute ("type generic", TYPE_ATTRIBUTES (TREE_TYPE (fundecl)));
+  bool type_generic_remove_excess_precision = false;
+  bool type_generic_overflow_p = false;
+  tree selector;
+
+  /* Change pointer to function to the function itself for
+     diagnostics.  */
+  if (TREE_CODE (function) == ADDR_EXPR
+      && TREE_CODE (TREE_OPERAND (function, 0)) == FUNCTION_DECL)
+    function = TREE_OPERAND (function, 0);
+
+  /* Handle an ObjC selector specially for diagnostics.  */
+  selector = objc_message_selector ();
+
+  /* For a call to a built-in function declared without a prototype,
+     set to the built-in function's argument list.  */
+  tree builtin_typelist = NULL_TREE;
+
+  if (fundecl
+      && DECL_BUILT_IN (fundecl)
+      && DECL_BUILT_IN_CLASS (fundecl) == BUILT_IN_NORMAL)
+    {
+      built_in_function code = DECL_FUNCTION_CODE (fundecl);
+      if (C_DECL_BUILTIN_PROTOTYPE (fundecl))
+	{
+	  if (tree bdecl = builtin_decl_implicit (code))
+	    builtin_typelist = TYPE_ARG_TYPES (TREE_TYPE (bdecl));
+	}
+
+      /* For type-generic built-in functions, determine whether excess
+	 precision should be removed (classification) or not
+	 (comparison).  */
+      if (type_generic)
+	switch (code)
+	  {
+	  case BUILT_IN_ISFINITE:
+	  case BUILT_IN_ISINF:
+	  case BUILT_IN_ISINF_SIGN:
+	  case BUILT_IN_ISNAN:
+	  case BUILT_IN_ISNORMAL:
+	  case BUILT_IN_FPCLASSIFY:
+	    type_generic_remove_excess_precision = true;
+	    break;
+
+	  case BUILT_IN_ADD_OVERFLOW_P:
+	  case BUILT_IN_SUB_OVERFLOW_P:
+	  case BUILT_IN_MUL_OVERFLOW_P:
+	    /* The last argument of these type-generic builtins
+	       should not be promoted.  */
+	    type_generic_overflow_p = true;
+	    break;
+
+	  default:
+	    break;
+	  }
+    }
+
+  /* Scan the given expressions and types, producing individual
+     converted arguments.  */
+
+  tree typetail, builtin_typetail, val;
+  for (typetail = typelist,
+	 builtin_typetail = builtin_typelist,
+	 parmnum = 0;
+       values && values->iterate (parmnum, &val);
+       ++parmnum)
+    {
+      tree type = typetail ? TREE_VALUE (typetail) : NULL_TREE;
+      tree builtin_type = (builtin_typetail
+			   ? TREE_VALUE (builtin_typetail) : NULL_TREE);
+      tree valtype = TREE_TYPE (val);
+      tree rname = function;
+      int argnum = parmnum + 1;
+      const char *invalid_func_diag;
+      bool excess_precision = false;
+      tree parmval;
+      /* Some __atomic_* builtins have additional hidden argument at
+	 position 0.  */
+      location_t ploc
+	= !arg_loc.is_empty () && values->length () == arg_loc.length ()
+	  ? expansion_point_location_if_in_system_header (arg_loc[parmnum])
+	  : input_location;
+
+      if (type == void_type_node)
+	{
+	  if (selector)
+	    error_at (loc, "too many arguments to method %qE", selector);
+	  else
+	    error_at (loc, "too many arguments to function %qE", function);
+	  inform_declaration (fundecl);
+	  return error_args ? -1 : (int) parmnum;
+	}
+
+      if (builtin_type == void_type_node)
+	{
+	  warning_at (loc, OPT_Wbuiltin_declaration_mismatch,
+		      "too many arguments to built-in function %qE "
+		      "expecting %d",
+		      function, parmnum);
+
+	  inform_declaration (fundecl);
+	  builtin_typetail = NULL_TREE;
+	}
+
+      if (selector && argnum > 2)
+	{
+	  rname = selector;
+	  argnum -= 2;
+	}
+
+      /* If there is excess precision and a prototype, convert once to
+	 the required type rather than converting via the semantic
+	 type.  Likewise without a prototype a float value represented
+	 as long double should be converted once to double.  But for
+	 type-generic classification functions excess precision must
+	 be removed here.  */
+      if (TREE_CODE (val) == EXCESS_PRECISION_EXPR
+	  && (type || !type_generic || !type_generic_remove_excess_precision))
+	{
+	  val = TREE_OPERAND (val, 0);
+	  excess_precision = true;
+	}
+      val = c_fully_fold (val, false, NULL);
+      STRIP_TYPE_NOPS (val);
+
+      val = require_complete_type (ploc, val);
+
+      /* Some floating-point arguments must be promoted to double when
+	 no type is specified by a prototype.  This applies to
+	 arguments of type float, and to architecture-specific types
+	 (ARM __fp16), but not to _FloatN or _FloatNx types.  */
+      bool promote_float_arg = false;
+      if (type == NULL_TREE
+	  && TREE_CODE (valtype) == REAL_TYPE
+	  && (TYPE_PRECISION (valtype)
+	      <= TYPE_PRECISION (double_type_node))
+	  && TYPE_MAIN_VARIANT (valtype) != double_type_node
+	  && TYPE_MAIN_VARIANT (valtype) != long_double_type_node
+	  && !DECIMAL_FLOAT_MODE_P (TYPE_MODE (valtype)))
+	{
+	  /* Promote this argument, unless it has a _FloatN or
+	     _FloatNx type.  */
+	  promote_float_arg = true;
+	  for (int i = 0; i < NUM_FLOATN_NX_TYPES; i++)
+	    if (TYPE_MAIN_VARIANT (valtype) == FLOATN_NX_TYPE_NODE (i))
+	      {
+		promote_float_arg = false;
+		break;
+	      }
+	}
+
+      if (type != NULL_TREE)
+	{
+	  tree origtype = (!origtypes) ? NULL_TREE : (*origtypes)[parmnum];
+	  parmval = convert_argument (ploc, function, fundecl, type, origtype,
+				      val, rname, parmnum, argnum,
+				      excess_precision, 0);
 	}
       else if (promote_float_arg)
         {
@@ -3544,8 +3590,24 @@ convert_arguments (location_t loc, vec<location_t> arg_loc, tree typelist,
       if (parmval == error_mark_node)
 	error_args = true;
 
+      if (!type && builtin_type && TREE_CODE (builtin_type) != VOID_TYPE)
+	{
+	  /* For a call to a built-in function declared without a prototype,
+	     perform the coversions from the argument to the expected type
+	     but issue warnings rather than errors for any mismatches.
+	     Ignore the converted argument and use the PARMVAL obtained
+	     above by applying default coversions instead.  */
+	  tree origtype = (!origtypes) ? NULL_TREE : (*origtypes)[parmnum];
+	  convert_argument (ploc, function, fundecl, builtin_type, origtype,
+			    val, rname, parmnum, argnum, excess_precision,
+			    OPT_Wbuiltin_declaration_mismatch);
+	}
+
       if (typetail)
 	typetail = TREE_CHAIN (typetail);
+
+      if (builtin_typetail)
+	builtin_typetail = TREE_CHAIN (builtin_typetail);
     }
 
   gcc_assert (parmnum == vec_safe_length (values));
@@ -3557,6 +3619,18 @@ convert_arguments (location_t loc, vec<location_t> arg_loc, tree typelist,
       return -1;
     }
 
+  if (builtin_typetail && TREE_VALUE (builtin_typetail) != void_type_node)
+    {
+      unsigned nargs = parmnum;
+      for (tree t = builtin_typetail; t; t = TREE_CHAIN (t))
+	++nargs;
+
+      warning_at (loc, OPT_Wbuiltin_declaration_mismatch,
+		  "too few arguments to built-in function %qE expecting %u",
+		  function, nargs - 1);
+      inform_declaration (fundecl);
+    }
+
   return error_args ? -1 : (int) parmnum;
 }
 
@@ -6328,13 +6402,16 @@ inform_for_arg (tree fundecl, location_t ploc, int parmnum,
 	    ^
 
    FUNCTION is a tree for the function being called.
-   PARMNUM is the number of the argument, for printing in error messages.  */
+   PARMNUM is the number of the argument, for printing in error messages.
+   WARNOPT may be set to a warning option to issue the corresponding warning
+   rather than an error for invalid conversions.  Used for calls to built-in
+   functions declared without a prototype.  */
 
 static tree
 convert_for_assignment (location_t location, location_t expr_loc, tree type,
 			tree rhs, tree origtype, enum impl_conv errtype,
 			bool null_pointer_constant, tree fundecl,
-			tree function, int parmnum)
+			tree function, int parmnum, int warnopt /* = 0 */)
 {
   enum tree_code codel = TREE_CODE (type);
   tree orig_rhs = rhs;
@@ -6523,7 +6600,11 @@ convert_for_assignment (location_t location, location_t expr_loc, tree type,
 	 an unprototyped function, it is compile-time undefined;
 	 making it a constraint in that case was rejected in
 	 DR#252.  */
-      error_at (location, "void value not ignored as it ought to be");
+      const char msg[] = "void value not ignored as it ought to be";
+      if (warnopt)
+	warning_at (location, warnopt, msg);
+      else
+	error_at (location, msg);
       return error_mark_node;
     }
   rhs = require_complete_type (location, rhs);
@@ -6539,7 +6620,11 @@ convert_for_assignment (location_t location, location_t expr_loc, tree type,
     {
       if (!lvalue_p (rhs))
 	{
-	  error_at (location, "cannot pass rvalue to reference parameter");
+	  const char msg[] = "cannot pass rvalue to reference parameter";
+	  if (warnopt)
+	    warning_at (location, warnopt, msg);
+	  else
+	    error_at (location, msg);
 	  return error_mark_node;
 	}
       if (!c_mark_addressable (rhs))
@@ -6551,7 +6636,7 @@ convert_for_assignment (location_t location, location_t expr_loc, tree type,
 				    build_pointer_type (TREE_TYPE (type)),
 				    rhs, origtype, errtype,
 				    null_pointer_constant, fundecl, function,
-				    parmnum);
+				    parmnum, warnopt);
       if (rhs == error_mark_node)
 	return error_mark_node;
 
@@ -6778,21 +6863,45 @@ convert_for_assignment (location_t location, location_t expr_loc, tree type,
 	  switch (errtype)
 	    {
 	    case ic_argpass:
-	      error_at (expr_loc, "passing argument %d of %qE from pointer to "
-			"non-enclosed address space", parmnum, rname);
+	      {
+		const char msg[] = G_("passing argument %d of %qE from "
+				      "pointer to non-enclosed address space");
+		if (warnopt)
+		  warning_at (expr_loc, warnopt, msg, parmnum, rname);
+		else
+		  error_at (expr_loc, msg, parmnum, rname);
 	      break;
+	      }
 	    case ic_assign:
-	      error_at (location, "assignment from pointer to "
+	      {
+		const char msg[] = G_("assignment from pointer to "
 				      "non-enclosed address space");
+		if (warnopt)
+		  warning_at (location, warnopt, msg);
+		else
+		  error_at (location, msg);
 		break;
+	      }
 	    case ic_init:
-	      error_at (location, "initialization from pointer to "
+	      {
+		const char msg[] = G_("initialization from pointer to "
 				      "non-enclosed address space");
+		if (warnopt)
+		  warning_at (location, warnopt, msg);
+		else
+		  error_at (location, msg);
 		break;
+	      }
 	    case ic_return:
-	      error_at (location, "return from pointer to "
+	      {
+		const char msg[] = G_("return from pointer to "
 				      "non-enclosed address space");
+		if (warnopt)
+		  warning_at (location, warnopt, msg);
+		else
+		  error_at (location, msg);
 		break;
+	      }
 	    default:
 	      gcc_unreachable ();
 	    }
@@ -7007,7 +7116,11 @@ convert_for_assignment (location_t location, location_t expr_loc, tree type,
     {
       /* ??? This should not be an error when inlining calls to
 	 unprototyped functions.  */
-      error_at (location, "invalid use of non-lvalue array");
+      const char msg[] = "invalid use of non-lvalue array";
+      if (warnopt)
+	warning_at (location, warnopt, msg);
+      else
+	error_at (location, msg);
       return error_mark_node;
     }
   else if (codel == POINTER_TYPE && coder == INTEGER_TYPE)
@@ -7089,24 +7202,45 @@ convert_for_assignment (location_t location, location_t expr_loc, tree type,
   switch (errtype)
     {
     case ic_argpass:
-      error_at (expr_loc, "incompatible type for argument %d of %qE", parmnum,
-		rname);
+      {
+	const char msg[] = G_("incompatible type for argument %d of %qE");
+	if (warnopt)
+	  warning_at (expr_loc, 0, msg, parmnum, rname);
+	else
+	  error_at (expr_loc, msg, parmnum, rname);
 	inform_for_arg (fundecl, expr_loc, parmnum, type, rhstype);
 	break;
+      }
     case ic_assign:
-      error_at (location, "incompatible types when assigning to type %qT from "
-		"type %qT", type, rhstype);
+      {
+	const char msg[]
+	  = G_("incompatible types when assigning to type %qT from type %qT");
+	if (warnopt)
+	  warning_at (expr_loc, 0, msg, type, rhstype);
+	else
+	  error_at (expr_loc, msg, type, rhstype);
 	break;
+      }
     case ic_init:
-      error_at (location,
-		"incompatible types when initializing type %qT using type %qT",
-		type, rhstype);
+      {
+	const char msg[]
+	  = G_("incompatible types when initializing type %qT using type %qT");
+	if (warnopt)
+	  warning_at (location, 0, msg, type, rhstype);
+	else
+	  error_at (location, msg, type, rhstype);
 	break;
+      }
     case ic_return:
-      error_at (location,
-		"incompatible types when returning type %qT but %qT was "
-		"expected", rhstype, type);
+      {
+	const char msg[]
+	  = G_("incompatible types when returning type %qT but %qT was expected");
+	if (warnopt)
+	  warning_at (location, 0, msg, rhstype, type);
+	else
+	  error_at (location, msg, rhstype, type);
 	break;
+      }
     default:
       gcc_unreachable ();
     }
diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
index df4c55f..ad7c067 100644
--- a/gcc/doc/invoke.texi
+++ b/gcc/doc/invoke.texi
@@ -6569,7 +6569,12 @@ attributes.
 @opindex Wno-builtin-declaration-mismatch
 @opindex Wbuiltin-declaration-mismatch
 Warn if a built-in function is declared with the wrong signature or
-as non-function.
+as non-function.  Also warn when a built-in function that takes arguments
+is declared without a prototype, and for calls to built-in functions
+declared ithout a prototype made with the wrong number of arguments or
+with arguments with incompatible types.  When @option{-Wextra} is used
+declarations without a prototype of built-in functions that take no
+arguments are diagnosed as well.
 This warning is enabled by default.
 
 @item -Wno-builtin-macro-redefined
PR c/83656 - missing -Wbuiltin-declaration-mismatch on declaration without prototype

gcc/c/ChangeLog:

	PR c/83656
	* c-decl.c (diagnose_mismatched_decls): Diagnose declarations of
	built-in functions without a prototype.
	* c-typeck.c (convert_argument): New function.
	(convert_arguments): Factor code out into convert_argument.
	Detect mismatches between built-in formal arguments in calls
	to built-in without prototype.
	(convert_for_assignment): Add argument.  Conditionally issue
	warnings instead of errors for mismatches.

gcc/testsuite/ChangeLog:

	PR c/83656
	* gcc.dg/Wbuiltin-declaration-mismatch-5.c: New test.
	* gcc.dg/Wbuiltin-declaration-mismatch-6.c: New test.
	* gcc.dg/Walloca-16.c: Adjust.
	* gcc.dg/Wrestrict-4.c: Adjust.
	* gcc.dg/Wrestrict-5.c: Adjust.
	* gcc.dg/pr15698-1.c: Adjust.
	* gcc.dg/pr83463.c: Adjust.
	* gcc.dg/redecl-4.c: Adjust.
	* gcc.dg/torture/pr55890-2.c: Adjust.
	* gcc.dg/torture/pr55890-3.c: Adjust.
	* gcc.dg/torture/pr67741.c: Adjust.
	* gcc.dg/torture/stackalign/sibcall-1.c: Adjust.

diff --git a/gcc/c/c-decl.c b/gcc/c/c-decl.c
index af16cfd..07beaa1 100644
--- a/gcc/c/c-decl.c
+++ b/gcc/c/c-decl.c
@@ -1886,8 +1886,9 @@ diagnose_mismatched_decls (tree newdecl, tree olddecl,
 		 built-in.  No point in calling locate_old_decl here, it
 		 won't print anything.  */
 	      warning (OPT_Wbuiltin_declaration_mismatch,
-		       "conflicting types for built-in function %q+D",
-		       newdecl);
+		       "conflicting types for built-in function %q+D; "
+		       "expected %qT",
+		       newdecl, oldtype);
 	      return false;
 	    }
 	}
@@ -2021,15 +2022,33 @@ diagnose_mismatched_decls (tree newdecl, tree olddecl,
 	 can't validate the argument list) the built-in definition is
 	 overridden, but optionally warn this was a bad choice of name.  */
       if (DECL_BUILT_IN (olddecl)
-	  && !C_DECL_DECLARED_BUILTIN (olddecl)
-	  && (!TREE_PUBLIC (newdecl)
-	      || (DECL_INITIAL (newdecl)
-		  && !prototype_p (TREE_TYPE (newdecl)))))
+	  && !C_DECL_DECLARED_BUILTIN (olddecl))
 	{
-	  warning (OPT_Wshadow, "declaration of %q+D shadows "
-		   "a built-in function", newdecl);
-	  /* Discard the old built-in function.  */
-	  return false;
+	  if (!TREE_PUBLIC (newdecl)
+	      || (DECL_INITIAL (newdecl)
+		  && !prototype_p (TREE_TYPE (newdecl))))
+	    {
+	      warning_at (DECL_SOURCE_LOCATION (newdecl),
+			  OPT_Wshadow, "declaration of %qD shadows "
+			  "a built-in function", newdecl);
+	      /* Discard the old built-in function.  */
+	      return false;
+	    }
+
+	  if (!prototype_p (TREE_TYPE (newdecl)))
+	    {
+	      /* Set for built-ins that take no arguments.  */
+	      bool func_void_args = false;
+	      if (tree at = TYPE_ARG_TYPES (oldtype))
+		func_void_args = VOID_TYPE_P (TREE_VALUE (at));
+
+	      if (extra_warnings || !func_void_args)
+		warning_at (DECL_SOURCE_LOCATION (newdecl),
+			    OPT_Wbuiltin_declaration_mismatch,
+			    "declaration of built-in function %qD without "
+			    "a prototype; expected %qT",
+			    newdecl, TREE_TYPE (olddecl));
+	    }
 	}
 
       if (DECL_INITIAL (newdecl))
diff --git a/gcc/c/c-typeck.c b/gcc/c/c-typeck.c
index 90ae306..e0584b9 100644
--- a/gcc/c/c-typeck.c
+++ b/gcc/c/c-typeck.c
@@ -97,7 +97,8 @@ static int convert_arguments (location_t, vec<location_t>, tree,
 			      tree);
 static tree pointer_diff (location_t, tree, tree, tree *);
 static tree convert_for_assignment (location_t, location_t, tree, tree, tree,
-				    enum impl_conv, bool, tree, tree, int);
+				    enum impl_conv, bool, tree, tree, int,
+				    int = 0);
 static tree valid_compound_expr_initializer (tree, tree);
 static void push_string (const char *);
 static void push_member_name (tree);
@@ -3179,6 +3180,189 @@ c_build_function_call_vec (location_t loc, vec<location_t> arg_loc,
   return build_function_call_vec (loc, arg_loc, function, params, origtypes);
 }
 
+/* Helper for convert_arguments called to convert the VALue of argument
+   number ARGNUM from ORIGTYPE to the corresponding parameter number
+   PARMNUL and TYPE.  */
+
+static tree
+convert_argument (location_t ploc, tree function, tree fundecl,
+		  tree type, tree origtype, tree val,
+		  tree rname, int parmnum, int argnum,
+		  bool excess_precision, int warnopt)
+{
+  tree valtype = TREE_TYPE (val);
+
+  tree parmval;
+
+  /* Formal parm type is specified by a function prototype.  */
+
+  if (type == error_mark_node || !COMPLETE_TYPE_P (type))
+    {
+      error_at (ploc, "type of formal parameter %d is incomplete",
+		parmnum + 1);
+      parmval = val;
+    }
+  else
+    {
+      /* Optionally warn about conversions that differ from the default
+	 conversions.  */
+      if (warn_traditional_conversion || warn_traditional)
+	{
+	  unsigned int formal_prec = TYPE_PRECISION (type);
+
+	  if (INTEGRAL_TYPE_P (type)
+	      && TREE_CODE (valtype) == REAL_TYPE)
+	    warning_at (ploc, OPT_Wtraditional_conversion,
+			"passing argument %d of %qE as integer rather "
+			"than floating due to prototype",
+			argnum, rname);
+	  if (INTEGRAL_TYPE_P (type)
+	      && TREE_CODE (valtype) == COMPLEX_TYPE)
+	    warning_at (ploc, OPT_Wtraditional_conversion,
+			"passing argument %d of %qE as integer rather "
+			"than complex due to prototype",
+			argnum, rname);
+	  else if (TREE_CODE (type) == COMPLEX_TYPE
+		   && TREE_CODE (valtype) == REAL_TYPE)
+	    warning_at (ploc, OPT_Wtraditional_conversion,
+			"passing argument %d of %qE as complex rather "
+			"than floating due to prototype",
+			argnum, rname);
+	  else if (TREE_CODE (type) == REAL_TYPE
+		   && INTEGRAL_TYPE_P (valtype))
+	    warning_at (ploc, OPT_Wtraditional_conversion,
+			"passing argument %d of %qE as floating rather "
+			"than integer due to prototype",
+			argnum, rname);
+	  else if (TREE_CODE (type) == COMPLEX_TYPE
+		   && INTEGRAL_TYPE_P (valtype))
+	    warning_at (ploc, OPT_Wtraditional_conversion,
+			"passing argument %d of %qE as complex rather "
+			"than integer due to prototype",
+			argnum, rname);
+	  else if (TREE_CODE (type) == REAL_TYPE
+		   && TREE_CODE (valtype) == COMPLEX_TYPE)
+	    warning_at (ploc, OPT_Wtraditional_conversion,
+			"passing argument %d of %qE as floating rather "
+			"than complex due to prototype",
+			argnum, rname);
+	  /* ??? At some point, messages should be written about
+	     conversions between complex types, but that's too messy
+	     to do now.  */
+	  else if (TREE_CODE (type) == REAL_TYPE
+		   && TREE_CODE (valtype) == REAL_TYPE)
+	    {
+	      /* Warn if any argument is passed as `float',
+		 since without a prototype it would be `double'.  */
+	      if (formal_prec == TYPE_PRECISION (float_type_node)
+		  && type != dfloat32_type_node)
+		warning_at (ploc, 0,
+			    "passing argument %d of %qE as %<float%> "
+			    "rather than %<double%> due to prototype",
+			    argnum, rname);
+
+	      /* Warn if mismatch between argument and prototype
+		 for decimal float types.  Warn of conversions with
+		 binary float types and of precision narrowing due to
+		 prototype. */
+	      else if (type != valtype
+		       && (type == dfloat32_type_node
+			   || type == dfloat64_type_node
+			   || type == dfloat128_type_node
+			   || valtype == dfloat32_type_node
+			   || valtype == dfloat64_type_node
+			   || valtype == dfloat128_type_node)
+		       && (formal_prec
+			   <= TYPE_PRECISION (valtype)
+			   || (type == dfloat128_type_node
+			       && (valtype
+				   != dfloat64_type_node
+				   && (valtype
+				       != dfloat32_type_node)))
+			   || (type == dfloat64_type_node
+			       && (valtype
+				   != dfloat32_type_node))))
+		warning_at (ploc, 0,
+			    "passing argument %d of %qE as %qT "
+			    "rather than %qT due to prototype",
+			    argnum, rname, type, valtype);
+
+	    }
+	  /* Detect integer changing in width or signedness.
+	     These warnings are only activated with
+	     -Wtraditional-conversion, not with -Wtraditional.  */
+	  else if (warn_traditional_conversion
+		   && INTEGRAL_TYPE_P (type)
+		   && INTEGRAL_TYPE_P (valtype))
+	    {
+	      tree would_have_been = default_conversion (val);
+	      tree type1 = TREE_TYPE (would_have_been);
+
+	      if (val == error_mark_node)
+		/* VAL could have been of incomplete type.  */;
+	      else if (TREE_CODE (type) == ENUMERAL_TYPE
+		       && (TYPE_MAIN_VARIANT (type)
+			   == TYPE_MAIN_VARIANT (valtype)))
+		/* No warning if function asks for enum
+		   and the actual arg is that enum type.  */
+		;
+	      else if (formal_prec != TYPE_PRECISION (type1))
+		warning_at (ploc, OPT_Wtraditional_conversion,
+			    "passing argument %d of %qE "
+			    "with different width due to prototype",
+			    argnum, rname);
+	      else if (TYPE_UNSIGNED (type) == TYPE_UNSIGNED (type1))
+		;
+	      /* Don't complain if the formal parameter type
+		 is an enum, because we can't tell now whether
+		 the value was an enum--even the same enum.  */
+	      else if (TREE_CODE (type) == ENUMERAL_TYPE)
+		;
+	      else if (TREE_CODE (val) == INTEGER_CST
+		       && int_fits_type_p (val, type))
+		/* Change in signedness doesn't matter
+		   if a constant value is unaffected.  */
+		;
+	      /* If the value is extended from a narrower
+		 unsigned type, it doesn't matter whether we
+		 pass it as signed or unsigned; the value
+		 certainly is the same either way.  */
+	      else if (TYPE_PRECISION (valtype) < TYPE_PRECISION (type)
+		       && TYPE_UNSIGNED (valtype))
+		;
+	      else if (TYPE_UNSIGNED (type))
+		warning_at (ploc, OPT_Wtraditional_conversion,
+			    "passing argument %d of %qE "
+			    "as unsigned due to prototype",
+			    argnum, rname);
+	      else
+		warning_at (ploc, OPT_Wtraditional_conversion,
+			    "passing argument %d of %qE "
+			    "as signed due to prototype",
+			    argnum, rname);
+	    }
+	}
+
+      /* Possibly restore an EXCESS_PRECISION_EXPR for the
+	 sake of better warnings from convert_and_check.  */
+      if (excess_precision)
+	val = build1 (EXCESS_PRECISION_EXPR, valtype, val);
+
+      parmval = convert_for_assignment (ploc, ploc, type,
+					val, origtype, ic_argpass,
+					null_pointer_constant_p (val),
+					fundecl, function,
+					parmnum + 1, warnopt);
+
+      if (targetm.calls.promote_prototypes (fundecl ? TREE_TYPE (fundecl) : 0)
+	  && INTEGRAL_TYPE_P (type)
+	  && (TYPE_PRECISION (type) < TYPE_PRECISION (integer_type_node)))
+	parmval = default_conversion (parmval);
+    }
+
+  return parmval;
+}
+
 /* Convert the argument expressions in the vector VALUES
    to the types in the list TYPELIST.
 
@@ -3205,7 +3389,6 @@ convert_arguments (location_t loc, vec<location_t> arg_loc, tree typelist,
 		   vec<tree, va_gc> *values, vec<tree, va_gc> *origtypes,
 		   tree function, tree fundecl)
 {
-  tree typetail, val;
   unsigned int parmnum;
   bool error_args = false;
   const bool type_generic = fundecl
@@ -3223,51 +3406,67 @@ convert_arguments (location_t loc, vec<location_t> arg_loc, tree typelist,
   /* Handle an ObjC selector specially for diagnostics.  */
   selector = objc_message_selector ();
 
-  /* For type-generic built-in functions, determine whether excess
-     precision should be removed (classification) or not
-     (comparison).  */
-  if (type_generic
+  /* For a call to a built-in function declared without a prototype,
+     set to the built-in function's argument list.  */
+  tree builtin_typelist = NULL_TREE;
+
+  if (fundecl
       && DECL_BUILT_IN (fundecl)
       && DECL_BUILT_IN_CLASS (fundecl) == BUILT_IN_NORMAL)
     {
-      switch (DECL_FUNCTION_CODE (fundecl))
+      built_in_function code = DECL_FUNCTION_CODE (fundecl);
+      if (C_DECL_BUILTIN_PROTOTYPE (fundecl))
 	{
-	case BUILT_IN_ISFINITE:
-	case BUILT_IN_ISINF:
-	case BUILT_IN_ISINF_SIGN:
-	case BUILT_IN_ISNAN:
-	case BUILT_IN_ISNORMAL:
-	case BUILT_IN_FPCLASSIFY:
-	  type_generic_remove_excess_precision = true;
-	  break;
+	  if (tree bdecl = builtin_decl_implicit (code))
+	    builtin_typelist = TYPE_ARG_TYPES (TREE_TYPE (bdecl));
+	}
 
-	case BUILT_IN_ADD_OVERFLOW_P:
-	case BUILT_IN_SUB_OVERFLOW_P:
-	case BUILT_IN_MUL_OVERFLOW_P:
-	  /* The last argument of these type-generic builtins
-	     should not be promoted.  */
-	  type_generic_overflow_p = true;
-	  break;
+      /* For type-generic built-in functions, determine whether excess
+	 precision should be removed (classification) or not
+	 (comparison).  */
+      if (type_generic)
+	switch (code)
+	  {
+	  case BUILT_IN_ISFINITE:
+	  case BUILT_IN_ISINF:
+	  case BUILT_IN_ISINF_SIGN:
+	  case BUILT_IN_ISNAN:
+	  case BUILT_IN_ISNORMAL:
+	  case BUILT_IN_FPCLASSIFY:
+	    type_generic_remove_excess_precision = true;
+	    break;
 
-	default:
-	  break;
-	}
+	  case BUILT_IN_ADD_OVERFLOW_P:
+	  case BUILT_IN_SUB_OVERFLOW_P:
+	  case BUILT_IN_MUL_OVERFLOW_P:
+	    /* The last argument of these type-generic builtins
+	       should not be promoted.  */
+	    type_generic_overflow_p = true;
+	    break;
+
+	  default:
+	    break;
+	  }
     }
 
   /* Scan the given expressions and types, producing individual
      converted arguments.  */
 
-  for (typetail = typelist, parmnum = 0;
+  tree typetail, builtin_typetail, val;
+  for (typetail = typelist,
+	 builtin_typetail = builtin_typelist,
+	 parmnum = 0;
        values && values->iterate (parmnum, &val);
        ++parmnum)
     {
-      tree type = typetail ? TREE_VALUE (typetail) : 0;
+      tree type = typetail ? TREE_VALUE (typetail) : NULL_TREE;
+      tree builtin_type = (builtin_typetail
+			   ? TREE_VALUE (builtin_typetail) : NULL_TREE);
       tree valtype = TREE_TYPE (val);
       tree rname = function;
       int argnum = parmnum + 1;
       const char *invalid_func_diag;
       bool excess_precision = false;
-      bool npc;
       tree parmval;
       /* Some __atomic_* builtins have additional hidden argument at
 	 position 0.  */
@@ -3286,14 +3485,23 @@ convert_arguments (location_t loc, vec<location_t> arg_loc, tree typelist,
 	  return error_args ? -1 : (int) parmnum;
 	}
 
+      if (builtin_type == void_type_node)
+	{
+	  warning_at (loc, OPT_Wbuiltin_declaration_mismatch,
+		      "too many arguments to built-in function %qE "
+		      "expecting %d",
+		      function, parmnum);
+
+	  inform_declaration (fundecl);
+	  builtin_typetail = NULL_TREE;
+	}
+
       if (selector && argnum > 2)
 	{
 	  rname = selector;
 	  argnum -= 2;
 	}
 
-      npc = null_pointer_constant_p (val);
-
       /* If there is excess precision and a prototype, convert once to
 	 the required type rather than converting via the semantic
 	 type.  Likewise without a prototype a float value represented
@@ -3337,172 +3545,10 @@ convert_arguments (location_t loc, vec<location_t> arg_loc, tree typelist,
 
       if (type != NULL_TREE)
 	{
-	  /* Formal parm type is specified by a function prototype.  */
-
-	  if (type == error_mark_node || !COMPLETE_TYPE_P (type))
-	    {
-	      error_at (ploc, "type of formal parameter %d is incomplete",
-			parmnum + 1);
-	      parmval = val;
-	    }
-	  else
-	    {
-	      tree origtype;
-
-	      /* Optionally warn about conversions that
-		 differ from the default conversions.  */
-	      if (warn_traditional_conversion || warn_traditional)
-		{
-		  unsigned int formal_prec = TYPE_PRECISION (type);
-
-		  if (INTEGRAL_TYPE_P (type)
-		      && TREE_CODE (valtype) == REAL_TYPE)
-		    warning_at (ploc, OPT_Wtraditional_conversion,
-				"passing argument %d of %qE as integer rather "
-				"than floating due to prototype",
-				argnum, rname);
-		  if (INTEGRAL_TYPE_P (type)
-		      && TREE_CODE (valtype) == COMPLEX_TYPE)
-		    warning_at (ploc, OPT_Wtraditional_conversion,
-				"passing argument %d of %qE as integer rather "
-				"than complex due to prototype",
-				argnum, rname);
-		  else if (TREE_CODE (type) == COMPLEX_TYPE
-			   && TREE_CODE (valtype) == REAL_TYPE)
-		    warning_at (ploc, OPT_Wtraditional_conversion,
-				"passing argument %d of %qE as complex rather "
-				"than floating due to prototype",
-				argnum, rname);
-		  else if (TREE_CODE (type) == REAL_TYPE
-			   && INTEGRAL_TYPE_P (valtype))
-		    warning_at (ploc, OPT_Wtraditional_conversion,
-				"passing argument %d of %qE as floating rather "
-				"than integer due to prototype",
-				argnum, rname);
-		  else if (TREE_CODE (type) == COMPLEX_TYPE
-			   && INTEGRAL_TYPE_P (valtype))
-		    warning_at (ploc, OPT_Wtraditional_conversion,
-				"passing argument %d of %qE as complex rather "
-				"than integer due to prototype",
-				argnum, rname);
-		  else if (TREE_CODE (type) == REAL_TYPE
-			   && TREE_CODE (valtype) == COMPLEX_TYPE)
-		    warning_at (ploc, OPT_Wtraditional_conversion,
-				"passing argument %d of %qE as floating rather "
-				"than complex due to prototype",
-				argnum, rname);
-		  /* ??? At some point, messages should be written about
-		     conversions between complex types, but that's too messy
-		     to do now.  */
-		  else if (TREE_CODE (type) == REAL_TYPE
-			   && TREE_CODE (valtype) == REAL_TYPE)
-		    {
-		      /* Warn if any argument is passed as `float',
-			 since without a prototype it would be `double'.  */
-		      if (formal_prec == TYPE_PRECISION (float_type_node)
-			  && type != dfloat32_type_node)
-			warning_at (ploc, 0,
-				    "passing argument %d of %qE as %<float%> "
-				    "rather than %<double%> due to prototype",
-				    argnum, rname);
-
-		      /* Warn if mismatch between argument and prototype
-			 for decimal float types.  Warn of conversions with
-			 binary float types and of precision narrowing due to
-			 prototype. */
- 		      else if (type != valtype
-			       && (type == dfloat32_type_node
-				   || type == dfloat64_type_node
-				   || type == dfloat128_type_node
-				   || valtype == dfloat32_type_node
-				   || valtype == dfloat64_type_node
-				   || valtype == dfloat128_type_node)
-			       && (formal_prec
-				   <= TYPE_PRECISION (valtype)
-				   || (type == dfloat128_type_node
-				       && (valtype
-					   != dfloat64_type_node
-					   && (valtype
-					       != dfloat32_type_node)))
-				   || (type == dfloat64_type_node
-				       && (valtype
-					   != dfloat32_type_node))))
-			warning_at (ploc, 0,
-				    "passing argument %d of %qE as %qT "
-				    "rather than %qT due to prototype",
-				    argnum, rname, type, valtype);
-
-		    }
-		  /* Detect integer changing in width or signedness.
-		     These warnings are only activated with
-		     -Wtraditional-conversion, not with -Wtraditional.  */
-		  else if (warn_traditional_conversion
-			   && INTEGRAL_TYPE_P (type)
-			   && INTEGRAL_TYPE_P (valtype))
-		    {
-		      tree would_have_been = default_conversion (val);
-		      tree type1 = TREE_TYPE (would_have_been);
-
-		      if (val == error_mark_node)
-			/* VAL could have been of incomplete type.  */;
-		      else if (TREE_CODE (type) == ENUMERAL_TYPE
-			       && (TYPE_MAIN_VARIANT (type)
-				   == TYPE_MAIN_VARIANT (valtype)))
-			/* No warning if function asks for enum
-			   and the actual arg is that enum type.  */
-			;
-		      else if (formal_prec != TYPE_PRECISION (type1))
-			warning_at (ploc, OPT_Wtraditional_conversion,
-				    "passing argument %d of %qE "
-				    "with different width due to prototype",
-				    argnum, rname);
-		      else if (TYPE_UNSIGNED (type) == TYPE_UNSIGNED (type1))
-			;
-		      /* Don't complain if the formal parameter type
-			 is an enum, because we can't tell now whether
-			 the value was an enum--even the same enum.  */
-		      else if (TREE_CODE (type) == ENUMERAL_TYPE)
-			;
-		      else if (TREE_CODE (val) == INTEGER_CST
-			       && int_fits_type_p (val, type))
-			/* Change in signedness doesn't matter
-			   if a constant value is unaffected.  */
-			;
-		      /* If the value is extended from a narrower
-			 unsigned type, it doesn't matter whether we
-			 pass it as signed or unsigned; the value
-			 certainly is the same either way.  */
-		      else if (TYPE_PRECISION (valtype) < TYPE_PRECISION (type)
-			       && TYPE_UNSIGNED (valtype))
-			;
-		      else if (TYPE_UNSIGNED (type))
-			warning_at (ploc, OPT_Wtraditional_conversion,
-				    "passing argument %d of %qE "
-				    "as unsigned due to prototype",
-				    argnum, rname);
-		      else
-			warning_at (ploc, OPT_Wtraditional_conversion,
-				    "passing argument %d of %qE "
-				    "as signed due to prototype",
-				    argnum, rname);
-		    }
-		}
-
-	      /* Possibly restore an EXCESS_PRECISION_EXPR for the
-		 sake of better warnings from convert_and_check.  */
-	      if (excess_precision)
-		val = build1 (EXCESS_PRECISION_EXPR, valtype, val);
-	      origtype = (!origtypes) ? NULL_TREE : (*origtypes)[parmnum];
-	      parmval = convert_for_assignment (loc, ploc, type,
-						val, origtype, ic_argpass,
-						npc, fundecl, function,
-						parmnum + 1);
-
-	      if (targetm.calls.promote_prototypes (fundecl ? TREE_TYPE (fundecl) : 0)
-		  && INTEGRAL_TYPE_P (type)
-		  && (TYPE_PRECISION (type) < TYPE_PRECISION (integer_type_node)))
-		parmval = default_conversion (parmval);
-	    }
+	  tree origtype = (!origtypes) ? NULL_TREE : (*origtypes)[parmnum];
+	  parmval = convert_argument (ploc, function, fundecl, type, origtype,
+				      val, rname, parmnum, argnum,
+				      excess_precision, 0);
 	}
       else if (promote_float_arg)
         {
@@ -3544,8 +3590,24 @@ convert_arguments (location_t loc, vec<location_t> arg_loc, tree typelist,
       if (parmval == error_mark_node)
 	error_args = true;
 
+      if (!type && builtin_type && TREE_CODE (builtin_type) != VOID_TYPE)
+	{
+	  /* For a call to a built-in function declared without a prototype,
+	     perform the coversions from the argument to the expected type
+	     but issue warnings rather than errors for any mismatches.
+	     Ignore the converted argument and use the PARMVAL obtained
+	     above by applying default coversions instead.  */
+	  tree origtype = (!origtypes) ? NULL_TREE : (*origtypes)[parmnum];
+	  convert_argument (ploc, function, fundecl, builtin_type, origtype,
+			    val, rname, parmnum, argnum, excess_precision,
+			    OPT_Wbuiltin_declaration_mismatch);
+	}
+
       if (typetail)
 	typetail = TREE_CHAIN (typetail);
+
+      if (builtin_typetail)
+	builtin_typetail = TREE_CHAIN (builtin_typetail);
     }
 
   gcc_assert (parmnum == vec_safe_length (values));
@@ -3557,6 +3619,18 @@ convert_arguments (location_t loc, vec<location_t> arg_loc, tree typelist,
       return -1;
     }
 
+  if (builtin_typetail && TREE_VALUE (builtin_typetail) != void_type_node)
+    {
+      unsigned nargs = parmnum;
+      for (tree t = builtin_typetail; t; t = TREE_CHAIN (t))
+	++nargs;
+
+      warning_at (loc, OPT_Wbuiltin_declaration_mismatch,
+		  "too few arguments to built-in function %qE expecting %u",
+		  function, nargs - 1);
+      inform_declaration (fundecl);
+    }
+
   return error_args ? -1 : (int) parmnum;
 }
 
@@ -6328,13 +6402,16 @@ inform_for_arg (tree fundecl, location_t ploc, int parmnum,
 	    ^
 
    FUNCTION is a tree for the function being called.
-   PARMNUM is the number of the argument, for printing in error messages.  */
+   PARMNUM is the number of the argument, for printing in error messages.
+   WARNOPT may be set to a warning option to issue the corresponding warning
+   rather than an error for invalid conversions.  Used for calls to built-in
+   functions declared without a prototype.  */
 
 static tree
 convert_for_assignment (location_t location, location_t expr_loc, tree type,
 			tree rhs, tree origtype, enum impl_conv errtype,
 			bool null_pointer_constant, tree fundecl,
-			tree function, int parmnum)
+			tree function, int parmnum, int warnopt /* = 0 */)
 {
   enum tree_code codel = TREE_CODE (type);
   tree orig_rhs = rhs;
@@ -6523,7 +6600,11 @@ convert_for_assignment (location_t location, location_t expr_loc, tree type,
 	 an unprototyped function, it is compile-time undefined;
 	 making it a constraint in that case was rejected in
 	 DR#252.  */
-      error_at (location, "void value not ignored as it ought to be");
+      const char msg[] = "void value not ignored as it ought to be";
+      if (warnopt)
+	warning_at (location, warnopt, msg);
+      else
+	error_at (location, msg);
       return error_mark_node;
     }
   rhs = require_complete_type (location, rhs);
@@ -6539,7 +6620,11 @@ convert_for_assignment (location_t location, location_t expr_loc, tree type,
     {
       if (!lvalue_p (rhs))
 	{
-	  error_at (location, "cannot pass rvalue to reference parameter");
+	  const char msg[] = "cannot pass rvalue to reference parameter";
+	  if (warnopt)
+	    warning_at (location, warnopt, msg);
+	  else
+	    error_at (location, msg);
 	  return error_mark_node;
 	}
       if (!c_mark_addressable (rhs))
@@ -6551,7 +6636,7 @@ convert_for_assignment (location_t location, location_t expr_loc, tree type,
 				    build_pointer_type (TREE_TYPE (type)),
 				    rhs, origtype, errtype,
 				    null_pointer_constant, fundecl, function,
-				    parmnum);
+				    parmnum, warnopt);
       if (rhs == error_mark_node)
 	return error_mark_node;
 
@@ -6778,21 +6863,45 @@ convert_for_assignment (location_t location, location_t expr_loc, tree type,
 	  switch (errtype)
 	    {
 	    case ic_argpass:
-	      error_at (expr_loc, "passing argument %d of %qE from pointer to "
-			"non-enclosed address space", parmnum, rname);
+	      {
+		const char msg[] = G_("passing argument %d of %qE from "
+				      "pointer to non-enclosed address space");
+		if (warnopt)
+		  warning_at (expr_loc, warnopt, msg, parmnum, rname);
+		else
+		  error_at (expr_loc, msg, parmnum, rname);
 	      break;
+	      }
 	    case ic_assign:
-	      error_at (location, "assignment from pointer to "
-			"non-enclosed address space");
-	      break;
+	      {
+		const char msg[] = G_("assignment from pointer to "
+				      "non-enclosed address space");
+		if (warnopt)
+		  warning_at (location, warnopt, msg);
+		else
+		  error_at (location, msg);
+		break;
+	      }
 	    case ic_init:
-	      error_at (location, "initialization from pointer to "
-			"non-enclosed address space");
-	      break;
+	      {
+		const char msg[] = G_("initialization from pointer to "
+				      "non-enclosed address space");
+		if (warnopt)
+		  warning_at (location, warnopt, msg);
+		else
+		  error_at (location, msg);
+		break;
+	      }
 	    case ic_return:
-	      error_at (location, "return from pointer to "
-			"non-enclosed address space");
-	      break;
+	      {
+		const char msg[] = G_("return from pointer to "
+				      "non-enclosed address space");
+		if (warnopt)
+		  warning_at (location, warnopt, msg);
+		else
+		  error_at (location, msg);
+		break;
+	      }
 	    default:
 	      gcc_unreachable ();
 	    }
@@ -7007,7 +7116,11 @@ convert_for_assignment (location_t location, location_t expr_loc, tree type,
     {
       /* ??? This should not be an error when inlining calls to
 	 unprototyped functions.  */
-      error_at (location, "invalid use of non-lvalue array");
+      const char msg[] = "invalid use of non-lvalue array";
+      if (warnopt)
+	warning_at (location, warnopt, msg);
+      else
+	error_at (location, msg);
       return error_mark_node;
     }
   else if (codel == POINTER_TYPE && coder == INTEGER_TYPE)
@@ -7089,24 +7202,45 @@ convert_for_assignment (location_t location, location_t expr_loc, tree type,
   switch (errtype)
     {
     case ic_argpass:
-      error_at (expr_loc, "incompatible type for argument %d of %qE", parmnum,
-		rname);
-      inform_for_arg (fundecl, expr_loc, parmnum, type, rhstype);
-      break;
+      {
+	const char msg[] = G_("incompatible type for argument %d of %qE");
+	if (warnopt)
+	  warning_at (expr_loc, 0, msg, parmnum, rname);
+	else
+	  error_at (expr_loc, msg, parmnum, rname);
+	inform_for_arg (fundecl, expr_loc, parmnum, type, rhstype);
+	break;
+      }
     case ic_assign:
-      error_at (location, "incompatible types when assigning to type %qT from "
-		"type %qT", type, rhstype);
-      break;
+      {
+	const char msg[]
+	  = G_("incompatible types when assigning to type %qT from type %qT");
+	if (warnopt)
+	  warning_at (expr_loc, 0, msg, type, rhstype);
+	else
+	  error_at (expr_loc, msg, type, rhstype);
+	break;
+      }
     case ic_init:
-      error_at (location,
-		"incompatible types when initializing type %qT using type %qT",
-		type, rhstype);
-      break;
+      {
+	const char msg[]
+	  = G_("incompatible types when initializing type %qT using type %qT");
+	if (warnopt)
+	  warning_at (location, 0, msg, type, rhstype);
+	else
+	  error_at (location, msg, type, rhstype);
+	break;
+      }
     case ic_return:
-      error_at (location,
-		"incompatible types when returning type %qT but %qT was "
-		"expected", rhstype, type);
-      break;
+      {
+	const char msg[]
+	  = G_("incompatible types when returning type %qT but %qT was expected");
+	if (warnopt)
+	  warning_at (location, 0, msg, rhstype, type);
+	else
+	  error_at (location, msg, rhstype, type);
+	break;
+      }
     default:
       gcc_unreachable ();
     }
diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
index df4c55f..ad7c067 100644
--- a/gcc/doc/invoke.texi
+++ b/gcc/doc/invoke.texi
@@ -6568,8 +6568,13 @@ attributes.
 @item -Wno-builtin-declaration-mismatch
 @opindex Wno-builtin-declaration-mismatch
 @opindex Wbuiltin-declaration-mismatch
-Warn if a built-in function is declared with the wrong signature or 
-as non-function.
+Warn if a built-in function is declared with the wrong signature or
+as non-function.  Also warn when a built-in function that takes arguments
+is declared without a prototype, and for calls to built-in functions
+declared ithout a prototype made with the wrong number of arguments or
+with arguments with incompatible types.  When @option{-Wextra} is used
+declarations without a prototype of built-in functions that take no
+arguments are diagnosed as well.
 This warning is enabled by default.
 
 @item -Wno-builtin-macro-redefined
diff --git a/gcc/testsuite/gcc.dg/Walloca-16.c b/gcc/testsuite/gcc.dg/Walloca-16.c
index 3ee96a9..866594c 100644
--- a/gcc/testsuite/gcc.dg/Walloca-16.c
+++ b/gcc/testsuite/gcc.dg/Walloca-16.c
@@ -4,3 +4,5 @@
 
 void *alloca ();
 __typeof__(alloca ()) a () { return alloca (); }
+
+/* { dg-prune-output "\\\[-Wbuiltin-declaration-mismatch]" } */
diff --git a/gcc/testsuite/gcc.dg/Wbuiltin-declaration-mismatch-5.c b/gcc/testsuite/gcc.dg/Wbuiltin-declaration-mismatch-5.c
new file mode 100644
index 0000000..d268215
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/Wbuiltin-declaration-mismatch-5.c
@@ -0,0 +1,63 @@
+/* PR c/83656 - missing -Wbuiltin-declaration-mismatch on declaration
+   without prototype
+   { dg-do compile }
+   { dg-options "-Wbuiltin-declaration-mismatch" } */
+
+/* Verify that built-ins that take no arguments (i.e., abort) are
+   not diagnosed by default.  */
+void abort ();
+
+void exit ();       /* { dg-warning "declaration of built-in function .exit. without a prototype; expected .void\\\(int\\\)." } */
+
+void* memcpy ();    /* { dg-warning "declaration of built-in function .memcpy. without a prototype; expected .void \\\*\\\(void \\\*, const void \\\*, \(long \)?unsigned int\\\)." } */
+
+int strcmp ();      /* { dg-warning "declaration of built-in function .strcmp. without a prototype; expected .int\\\(const char* \\\*, const char \\\*\\\)." } */
+
+int strcpy ();      /* { dg-warning "conflicting types for built-in function .strcpy.; expected .char \\\*\\\(char \\\*, const char \\\*\\\)." } */
+
+
+void test_call_abort (void)
+{
+  /* Verify that a valid call to abort() is not diagnosed.  */
+  abort ();
+
+  abort (1);        /* { dg-warning "too many arguments to built-in function 'abort' expecting 0" } */
+
+  abort (1, 2);     /* { dg-warning "too many arguments" } */
+}
+
+
+void test_call_exit (void)
+{
+  /* Verify that a valid call to exit is not diagnosed.  */
+  exit (0);
+
+  exit ();          /* { dg-warning "too few arguments to built-in function 'exit' expecting 1" } */
+
+  exit (1, 2);      /* { dg-warning "too many arguments" } */
+
+  /* Verify that passing incompatible arguments is diagnosed by
+     a warning.  */
+  exit ("");        /* { dg-warning "\\\[-Wint-conversion]" } */
+
+  struct S { int i; } s = { 0 };
+  exit (s);         /* { dg-warning "incompatible type for argument 1" } */
+}
+
+
+void test_call_memcpy (void *p, const void *q, unsigned n)
+{
+  memcpy (p, q, n);
+
+  memcpy ();        /* { dg-warning "too few arguments to built-in function 'memcpy' expecting 3" } */
+
+  memcpy (p);       /* { dg-warning "too few arguments to built-in function 'memcpy' expecting 3" } */
+
+  memcpy (p, q);     /* { dg-warning "too few arguments to built-in function 'memcpy' expecting 3" } */
+
+  memcpy (q, p, n); /* { dg-warning "\\\[-Wdiscarded-qualifiers]" } */
+
+  memcpy (p, n, q); /* { dg-warning "\\\[-Wint-conversion]" } */
+
+  memcpy (p, q, n, 0); /* { dg-warning "too many arguments to built-in function 'memcpy' expecting 3" } */
+}
diff --git a/gcc/testsuite/gcc.dg/Wbuiltin-declaration-mismatch-6.c b/gcc/testsuite/gcc.dg/Wbuiltin-declaration-mismatch-6.c
new file mode 100644
index 0000000..b66d6f4
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/Wbuiltin-declaration-mismatch-6.c
@@ -0,0 +1,8 @@
+/* PR c/83656 - missing -Wbuiltin-declaration-mismatch on declaration
+   without prototype
+   { dg-do compile }
+   { dg-options "-Wbuiltin-declaration-mismatch -Wextra" } */
+
+/* Verify that with -Wextra, even declarations of built-ins that take
+   no arguments (i.e., abort) are diagnosed.  */
+void abort ();      /* { dg-warning "declaration of built-in function .abort. without a prototype; expected .void\\\(void\\\)." } */
diff --git a/gcc/testsuite/gcc.dg/Wrestrict-4.c b/gcc/testsuite/gcc.dg/Wrestrict-4.c
index f2398ef..55ea6c0 100644
--- a/gcc/testsuite/gcc.dg/Wrestrict-4.c
+++ b/gcc/testsuite/gcc.dg/Wrestrict-4.c
@@ -108,3 +108,5 @@ void* test_strncpy_2 (char *d, const char *s)
 {
   return strncpy (d, s);
 }
+
+/* { dg-prune-output "\\\[-Wbuiltin-declaration-mismatch]" } */
diff --git a/gcc/testsuite/gcc.dg/Wrestrict-5.c b/gcc/testsuite/gcc.dg/Wrestrict-5.c
index 4912ee5..7064e1e 100644
--- a/gcc/testsuite/gcc.dg/Wrestrict-5.c
+++ b/gcc/testsuite/gcc.dg/Wrestrict-5.c
@@ -41,3 +41,6 @@ void test_strncpy_nowarn (char *d)
 {
   strncpy (d + 1, d + 3, "");
 }
+
+/* { dg-prune-output "\\\[-Wbuiltin-declaration-mismatch]" }
+   { dg-prune-output "\\\[-Wint-conversion]" } */
diff --git a/gcc/testsuite/gcc.dg/pr15698-1.c b/gcc/testsuite/gcc.dg/pr15698-1.c
index 5a75a10..cbe613c 100644
--- a/gcc/testsuite/gcc.dg/pr15698-1.c
+++ b/gcc/testsuite/gcc.dg/pr15698-1.c
@@ -21,3 +21,5 @@ char *rindex(a, b)
 {
   return 0;
 }
+
+/* { dg-prune-output "\\\[-Wbuiltin-declaration-mismatch]" } */
diff --git a/gcc/testsuite/gcc.dg/pr83463.c b/gcc/testsuite/gcc.dg/pr83463.c
index ddf662f..cd11f64 100644
--- a/gcc/testsuite/gcc.dg/pr83463.c
+++ b/gcc/testsuite/gcc.dg/pr83463.c
@@ -15,3 +15,5 @@ p ()
 {
   m (p + (long) a);
 }
+
+/* { dg-prune-output "\\\[-Wbuiltin-declaration-mismatch]" } */
diff --git a/gcc/testsuite/gcc.dg/redecl-4.c b/gcc/testsuite/gcc.dg/redecl-4.c
index 2ba74a3..8f12488 100644
--- a/gcc/testsuite/gcc.dg/redecl-4.c
+++ b/gcc/testsuite/gcc.dg/redecl-4.c
@@ -27,3 +27,6 @@ f (void)
 
 /* Should still diagnose incompatible prototype for strcmp.  */
 int strcmp (void); /* { dg-error "conflict" } */
+
+/* { dg-prune-output "\\\[-Wbuiltin-declaration-mismatch]" }
+   { dg-prune-output "\\\[-Wint-conversion]" } */
diff --git a/gcc/testsuite/gcc.dg/torture/pr55890-2.c b/gcc/testsuite/gcc.dg/torture/pr55890-2.c
index 1cf71d7..31779e0 100644
--- a/gcc/testsuite/gcc.dg/torture/pr55890-2.c
+++ b/gcc/testsuite/gcc.dg/torture/pr55890-2.c
@@ -2,3 +2,5 @@
 
 extern void *memcpy();
 int main() { memcpy(); }
+
+/* { dg-prune-output "\\\[-Wbuiltin-declaration-mismatch]" } */
diff --git a/gcc/testsuite/gcc.dg/torture/pr55890-3.c b/gcc/testsuite/gcc.dg/torture/pr55890-3.c
index c7f77be..21a3d98 100644
--- a/gcc/testsuite/gcc.dg/torture/pr55890-3.c
+++ b/gcc/testsuite/gcc.dg/torture/pr55890-3.c
@@ -7,3 +7,5 @@ bar ()
 {
   return memmove ();
 }
+
+/* { dg-prune-output "\\\[-Wbuiltin-declaration-mismatch]" } */
diff --git a/gcc/testsuite/gcc.dg/torture/pr67741.c b/gcc/testsuite/gcc.dg/torture/pr67741.c
index 1ffc707..833e53e 100644
--- a/gcc/testsuite/gcc.dg/torture/pr67741.c
+++ b/gcc/testsuite/gcc.dg/torture/pr67741.c
@@ -3,11 +3,12 @@
 struct singlecomplex { float real, imag ; } ;
 struct doublecomplex { double real, imag ; } ;
 struct extendedcomplex { long double real, imag ; } ;
-extern double cabs();
+extern double cabs(); /* { dg-warning "\\\[-Wbuiltin-declaration-mismatch]" } */
 float cabsf(fc)
      struct singlecomplex fc;  /* { dg-warning "doesn't match" } */
 {
   struct doublecomplex dc ;
   dc.real=fc.real; dc.imag=fc.imag;
-  return (float) cabs(dc);
+  return (float) cabs(dc);   /* { dg-warning "incompatible type for argument 1 of .cabs." } */
 }
+
diff --git a/gcc/testsuite/gcc.dg/torture/stackalign/sibcall-1.c b/gcc/testsuite/gcc.dg/torture/stackalign/sibcall-1.c
index 8c17475..c4992df 100644
--- a/gcc/testsuite/gcc.dg/torture/stackalign/sibcall-1.c
+++ b/gcc/testsuite/gcc.dg/torture/stackalign/sibcall-1.c
@@ -1,7 +1,7 @@
 /* { dg-do run } */
 
 extern int ok (int);
-extern void exit ();
+extern void exit (int);
 static int gen_x86_64_shrd (int);
 static int
 gen_x86_64_shrd(int a __attribute__ ((__unused__)))

On 06/27/2018 12:44 PM, Eric Gallager wrote:
> On 6/26/18, Martin Sebor <msebor@gmail.com> wrote:
>> With the exception of built-ins with the ellipsis (like sprintf),
>> GCC silently accepts declarations of built-in functions without
>> prototypes as well as calls to such functions with any numbers
>> or types of arguments, compatible or otherwise.  Calls with
>> arguments whose number and types match exactly those of
>> the built-in are considered by the middle-end for optimization.
>> Other calls (compatible or not, irrespective of whether their
>> number matches the number expected by the function) are then
>> made to the library functions.
>>
>> Attached is a small fix to -Wbuiltin-declaration-mismatch to
>> have it diagnose built-in declarations without a prototype.
>> The warning is enabled by default so it causes a fair number
>> of tests to fail because of declarations like 'void abort();'
>> The breakdown of the built-ins behind the test failures is
>> below.
>>
>> Before I take the time to clean up the test suite let me post
>> what I have in case going this route is not acceptable.  As
>> an alternative, I could try to avoid some of these warnings,
>> e.g., by diagnosing incompatible calls instead but I think
>> it's even less worthwhile for built-ins than trying to do
>> it for ordinary functions with -Wstrict-prototypes.  There
>> is, in my view, no justification today for standard functions
>> to be declared without a prototype.  (I could also make
>> the warning depend on language mode and -Wpedantic if that
>> were more preferable.)
>>
>> Martin
>>
>> About 115 tests fail due to incompatible declarations of
>> the built-in functions below (the number shows the number
>> of warnings for each functions):
>>
>> 428   abort
>>   58   exit
>>   36   memcpy
>>   17   memmove
>>   15   realloc
>>   14   cabs
>>    5   strncpy
>>    4   strcmp
>>    3   alloca
>>    2   rindex
>>    1   aligned_alloc
>>
>
> Could you do something to differentiate it a bit more from the
> -Wshadow warning that is also touched in this patch? It'd be kind of
> annoying to get warnings from both flags for the same code...

I don't think it's possible to get both warnings.  The block
with the -Wshadow warning returns before the one
-Wbuiltin-declaration-mismatch is entered.

Martin

On 06/27/2018 03:53 PM, Jeff Law wrote:
> On 06/27/2018 09:27 AM, Jakub Jelinek wrote:
>> On Wed, Jun 27, 2018 at 09:17:07AM -0600, Jeff Law wrote:
>>>> About 115 tests fail due to incompatible declarations of
>>>> the built-in functions below (the number shows the number
>>>> of warnings for each functions):
>>>>
>>>> 428   abort
>>>>  58   exit
>>>>  36   memcpy
>>>>  17   memmove
>>>>  15   realloc
>>>>  14   cabs
>>>>   5   strncpy
>>>>   4   strcmp
>>>>   3   alloca
>>>>   2   rindex
>>>>   1   aligned_alloc
>>> I'm supportive of this change.  Though I'm more worried about the
>>> affects on the distros than I am on the testsuite (which I expected to
>>> be in worse shape WRT this issue than your analysis indicates).
>>
>> I'm mainly worried about configure tests, those will be hit hardest by
>> this and might result in silently turning off many features of the compiled
>> programs.
> It's certainly a concern.  Sadly, it's a hard one to deal with because
> we can't just throw code at it and see what complains.  Instead we end
> up with packages that don't configure in the appropriate features.

I checked all GCC's config logs and although there are 543
instances of the -Wbuiltin-declaration-mismatch warning in
an x86_64-linux build, none of them is an error and
the number is the same as before the patch.

Martin

>
> I think when Roland and I discussed this in the context of gold the
> thing we sketched out was to build the distro, capturing the resultant
> config.h, then rebuild with gold, capture the new config.h and compare.
>
> We never actually tried that -- we didn't green light the gold
> transition due to other reasons.
>
> Just wandering around our own configury bits does show some of these
> kinds of problems.  No idea if they'd manifest themselves in a visible
> ways though.
>
> Maybe it's a case where it's an opt-in argument in the immediate term
> and we work with the autoconf folks to squash out as many problems as we
> can and build out some kind of distro-like testing before flipping the
> default.
>
> jeff
>

On 06/27/2018 08:40 PM, Martin Sebor wrote:
> On 06/27/2018 03:53 PM, Jeff Law wrote:
>> On 06/27/2018 09:27 AM, Jakub Jelinek wrote:
>>> On Wed, Jun 27, 2018 at 09:17:07AM -0600, Jeff Law wrote:
>>>>> About 115 tests fail due to incompatible declarations of
>>>>> the built-in functions below (the number shows the number
>>>>> of warnings for each functions):
>>>>>
>>>>> 428   abort
>>>>>  58   exit
>>>>>  36   memcpy
>>>>>  17   memmove
>>>>>  15   realloc
>>>>>  14   cabs
>>>>>   5   strncpy
>>>>>   4   strcmp
>>>>>   3   alloca
>>>>>   2   rindex
>>>>>   1   aligned_alloc
>>>> I'm supportive of this change.  Though I'm more worried about the
>>>> affects on the distros than I am on the testsuite (which I expected to
>>>> be in worse shape WRT this issue than your analysis indicates).
>>>
>>> I'm mainly worried about configure tests, those will be hit hardest by
>>> this and might result in silently turning off many features of the
>>> compiled
>>> programs.
>> It's certainly a concern.  Sadly, it's a hard one to deal with because
>> we can't just throw code at it and see what complains.  Instead we end
>> up with packages that don't configure in the appropriate features.
> 
> I checked all GCC's config logs and although there are 543
> instances of the -Wbuiltin-declaration-mismatch warning in
> an x86_64-linux build, none of them is an error and
> the number is the same as before the patch.
That's both depressing and promising at the same time.  Depressing
there's so many, promising that none trigger an error.

I wonder if stepping forward to a more modern version of autoconf is
going to help here and if we should be feeding them updates to make this
kind of stuff less pervasive, at least in standard autoconf tests.
jeff

On Fri, Jun 29, 2018 at 09:11:39AM -0600, Jeff Law wrote:
> > I checked all GCC's config logs and although there are 543
> > instances of the -Wbuiltin-declaration-mismatch warning in
> > an x86_64-linux build, none of them is an error and
> > the number is the same as before the patch.
> That's both depressing and promising at the same time.  Depressing
> there's so many, promising that none trigger an error.
> 
> I wonder if stepping forward to a more modern version of autoconf is
> going to help here and if we should be feeding them updates to make this
> kind of stuff less pervasive, at least in standard autoconf tests.

For autoconf, the problem is that the tests usually scan for any compiler
messages, so even a warning results in test failure.
Even current autoconf for AC_CHECK_FUNCS declares
#ifdef __cplusplus
extern "C"
#endif
char whatever();

and in main body does return whatever();, so if we start warning on this if
we haven't warned before, then it might be a problem.

	Jakub

On 06/29/2018 09:11 AM, Jeff Law wrote:
> On 06/27/2018 08:40 PM, Martin Sebor wrote:
>> On 06/27/2018 03:53 PM, Jeff Law wrote:
>>> On 06/27/2018 09:27 AM, Jakub Jelinek wrote:
>>>> On Wed, Jun 27, 2018 at 09:17:07AM -0600, Jeff Law wrote:
>>>>>> About 115 tests fail due to incompatible declarations of
>>>>>> the built-in functions below (the number shows the number
>>>>>> of warnings for each functions):
>>>>>>
>>>>>> 428   abort
>>>>>>  58   exit
>>>>>>  36   memcpy
>>>>>>  17   memmove
>>>>>>  15   realloc
>>>>>>  14   cabs
>>>>>>   5   strncpy
>>>>>>   4   strcmp
>>>>>>   3   alloca
>>>>>>   2   rindex
>>>>>>   1   aligned_alloc
>>>>> I'm supportive of this change.  Though I'm more worried about the
>>>>> affects on the distros than I am on the testsuite (which I expected to
>>>>> be in worse shape WRT this issue than your analysis indicates).
>>>>
>>>> I'm mainly worried about configure tests, those will be hit hardest by
>>>> this and might result in silently turning off many features of the
>>>> compiled
>>>> programs.
>>> It's certainly a concern.  Sadly, it's a hard one to deal with because
>>> we can't just throw code at it and see what complains.  Instead we end
>>> up with packages that don't configure in the appropriate features.
>>
>> I checked all GCC's config logs and although there are 543
>> instances of the -Wbuiltin-declaration-mismatch warning in
>> an x86_64-linux build, none of them is an error and
>> the number is the same as before the patch.
> That's both depressing and promising at the same time.  Depressing
> there's so many, promising that none trigger an error.

All the warnings I have seen are because of declarations like
the one in the test below that checks for the presence of symbol
sin in the library:

   char sin ();
   int main () { return sin (); }

GCC has warned for this code by default since at least 4.1 so if
it is, in fact, a problem it has been with us for over a decade.

There's a comment in the test that explains that the char return
type is deliberate to prevent GCC from treating the function as
a built-in.  (That, of course, relies on undefined behavior
because names of extern library functions are reserved and
conforming compilers could simply avoid emitting the call or
replace it with a trap.)

> I wonder if stepping forward to a more modern version of autoconf is
> going to help here and if we should be feeding them updates to make this
> kind of stuff less pervasive, at least in standard autoconf tests.

That would make sense to me.  The tests should not rely on
undefined behavior.  They should declare standard functions with
the right prototypes.  IMO, for GCC and compatible compilers they
should disable built-in expansion instead via -fno-builtin.  For
all other compilers, they could store the address of each function
in a (perhaps volatile) pointer and use it to make the call instead.

But since the number of warnings here hasn't changed, the ones
in GCC logs predate my changes.  So updating the tests seems
like an improvement to consider independently of the patch.

Martin

On 6/29/18, Jeff Law <law@redhat.com> wrote:
> On 06/27/2018 08:40 PM, Martin Sebor wrote:
>> On 06/27/2018 03:53 PM, Jeff Law wrote:
>>> On 06/27/2018 09:27 AM, Jakub Jelinek wrote:
>>>> On Wed, Jun 27, 2018 at 09:17:07AM -0600, Jeff Law wrote:
>>>>>> About 115 tests fail due to incompatible declarations of
>>>>>> the built-in functions below (the number shows the number
>>>>>> of warnings for each functions):
>>>>>>
>>>>>> 428   abort
>>>>>>  58   exit
>>>>>>  36   memcpy
>>>>>>  17   memmove
>>>>>>  15   realloc
>>>>>>  14   cabs
>>>>>>   5   strncpy
>>>>>>   4   strcmp
>>>>>>   3   alloca
>>>>>>   2   rindex
>>>>>>   1   aligned_alloc
>>>>> I'm supportive of this change.  Though I'm more worried about the
>>>>> affects on the distros than I am on the testsuite (which I expected to
>>>>> be in worse shape WRT this issue than your analysis indicates).
>>>>
>>>> I'm mainly worried about configure tests, those will be hit hardest by
>>>> this and might result in silently turning off many features of the
>>>> compiled
>>>> programs.
>>> It's certainly a concern.  Sadly, it's a hard one to deal with because
>>> we can't just throw code at it and see what complains.  Instead we end
>>> up with packages that don't configure in the appropriate features.
>>
>> I checked all GCC's config logs and although there are 543
>> instances of the -Wbuiltin-declaration-mismatch warning in
>> an x86_64-linux build, none of them is an error and
>> the number is the same as before the patch.
> That's both depressing and promising at the same time.  Depressing
> there's so many, promising that none trigger an error.
>
> I wonder if stepping forward to a more modern version of autoconf is
> going to help here and if we should be feeding them updates to make this
> kind of stuff less pervasive, at least in standard autoconf tests.
> jeff
>

If you're going to be feeding the autoconf people updates, please help
them do a release, too; 2.70 has been stuck in limbo for too long now
due to nobody being able to do a release.
Thanks!

On 06/29/2018 09:54 AM, Martin Sebor wrote:
> 
> All the warnings I have seen are because of declarations like
> the one in the test below that checks for the presence of symbol
> sin in the library:
> 
>   char sin ();
>   int main () { return sin (); }
> 
> GCC has warned for this code by default since at least 4.1 so if
> it is, in fact, a problem it has been with us for over a decade.
> 
> There's a comment in the test that explains that the char return
> type is deliberate to prevent GCC from treating the function as
> a built-in.  (That, of course, relies on undefined behavior
> because names of extern library functions are reserved and
> conforming compilers could simply avoid emitting the call or
> replace it with a trap.)
As you noted, by doing this the test can verify if there's a sin()
function in the library or not, regardless of whether or not the
compiler has a builtin for sin().


> 
>> I wonder if stepping forward to a more modern version of autoconf is
>> going to help here and if we should be feeding them updates to make this
>> kind of stuff less pervasive, at least in standard autoconf tests.
> 
> That would make sense to me.  The tests should not rely on
> undefined behavior.  They should declare standard functions with
> the right prototypes.  IMO, for GCC and compatible compilers they
> should disable built-in expansion instead via -fno-builtin.  For
> all other compilers, they could store the address of each function
> in a (perhaps volatile) pointer and use it to make the call instead.
I think the problem is they can't rely on the compiler having the
-fno-builtin flag.  Of course they're relying on other compilers having
the same kind of behavior as GCC WRT which is possibly worse.

I guess there's a reason why they didn't extract the set of symbols from
the library. :-)

> 
> But since the number of warnings here hasn't changed, the ones
> in GCC logs predate my changes.  So updating the tests seems
> like an improvement to consider independently of the patch.
Agreed.  I'm still wary of proceeding given the general concerns about
configure tests.  It's good that GCC's configury bits aren't affected,
but I'm not sure we can generalize a whole lot from that.

jeff

On 07/03/2018 08:33 PM, Jeff Law wrote:
> On 06/29/2018 09:54 AM, Martin Sebor wrote:
>>
>> All the warnings I have seen are because of declarations like
>> the one in the test below that checks for the presence of symbol
>> sin in the library:
>>
>>   char sin ();
>>   int main () { return sin (); }
>>
>> GCC has warned for this code by default since at least 4.1 so if
>> it is, in fact, a problem it has been with us for over a decade.
>>
>> There's a comment in the test that explains that the char return
>> type is deliberate to prevent GCC from treating the function as
>> a built-in.  (That, of course, relies on undefined behavior
>> because names of extern library functions are reserved and
>> conforming compilers could simply avoid emitting the call or
>> replace it with a trap.)
> As you noted, by doing this the test can verify if there's a sin()
> function in the library or not, regardless of whether or not the
> compiler has a builtin for sin().
>>> I wonder if stepping forward to a more modern version of autoconf is
>>> going to help here and if we should be feeding them updates to make this
>>> kind of stuff less pervasive, at least in standard autoconf tests.
>>
>> That would make sense to me.  The tests should not rely on
>> undefined behavior.  They should declare standard functions with
>> the right prototypes.  IMO, for GCC and compatible compilers they
>> should disable built-in expansion instead via -fno-builtin.  For
>> all other compilers, they could store the address of each function
>> in a (perhaps volatile) pointer and use it to make the call instead.
> I think the problem is they can't rely on the compiler having the
> -fno-builtin flag.  Of course they're relying on other compilers having
> the same kind of behavior as GCC WRT which is possibly worse.
>
> I guess there's a reason why they didn't extract the set of symbols from
> the library. :-)
>
>>
>> But since the number of warnings here hasn't changed, the ones
>> in GCC logs predate my changes.  So updating the tests seems
>> like an improvement to consider independently of the patch.
> Agreed.  I'm still wary of proceeding given the general concerns about
> configure tests.  It's good that GCC's configury bits aren't affected,
> but I'm not sure we can generalize a whole lot from that.

So what's the next step?  I'm open to relaxing the warning
so it only triggers with -Wall or -Wextra and not by default
if that's considered necessary.

At the same time, the instances of the warning we have seen
have all been issued for the configure tests for years and
we have not seen any new instances of it as a result of
this change, so the concern that the patch might lead to some
more while at the same time accepting the ones we know about
doesn't make sense to me.

Any new warning that's enabled by default has the potential
to trigger for invalid configure tests or any other such code.
I would expect tests deliberately written to rely on undefined
behavior to be prepared for compiler warnings, so I'm not sure
I understand the basis for the concern in this case.  I also
don't recall the concern being raised for newly added warnings
in the past that were enabled by default and I'm not sure I see
what makes this one different.

Martin

On 07/04/2018 11:32 AM, Martin Sebor wrote:
> On 07/03/2018 08:33 PM, Jeff Law wrote:
>>
>>>
>>> But since the number of warnings here hasn't changed, the ones
>>> in GCC logs predate my changes.  So updating the tests seems
>>> like an improvement to consider independently of the patch.
>> Agreed.  I'm still wary of proceeding given the general concerns about
>> configure tests.  It's good that GCC's configury bits aren't affected,
>> but I'm not sure we can generalize a whole lot from that.
> 
> So what's the next step?  I'm open to relaxing the warning
> so it only triggers with -Wall or -Wextra and not by default
> if that's considered necessary.
I'm not sure :-)  The problem is we have notable potential to break
things and do so in ways that are going to be painful to find.

Having them only turn on for -Wextra might be an compromise position.
But even if we do that I don't really see how we take the next step (ie,
adding it to Wall).


> 
> At the same time, the instances of the warning we have seen
> have all been issued for the configure tests for years and
> we have not seen any new instances of it as a result of
> this change, so the concern that the patch might lead to some
> more while at the same time accepting the ones we know about
> doesn't make sense to me.
Again, I don't think we can generalize much from the GCC autoconf
scripts and the failure modes are going to be extremely painful to track
down to this change.

While we have this concern with every new warning or enhancements to
existing warnings, this specific instance is worse because of how it
interacts with relatively common configury code.

Jeff

I have reworked the patch to resolve any lingering concerns about
warnings in configure tests.  The attached revision only warns
with -Wextra and only for incompatible declarations of built-ins
that take arguments.  For void built-ins like abort() it only
warns with -Wpedantic (this required adjustments to several
tests that are being compiled with -pedantic-errors).

The revised patch also detects incompatibilities om uses of
built-ins declared with no prototype and warns for those.  This
includes argument passing and function pointer conversions.

The effect of these changes is that for example the following
snippet is only diagnosed with -Wpedantic because it isn't
incorrect (so only on the basis that the declaration style is
deprecated in C):

   void abort ();   // warning only with -Wpedantic here

   void f (void)
   {
     abort ();      // safe, not diagnosed
   }

while the following is diagnosed by default because the call is
definitely undefined:

   char* strncpy ();          // warning only with -Wextra

   void f (char *d)
   {
     strncpy (d, 4, "123");   // undefined, warning by default
   }

Tested on x86_64-linux.

Martin

On 07/05/2018 01:44 PM, Jeff Law wrote:
> On 07/04/2018 11:32 AM, Martin Sebor wrote:
>> On 07/03/2018 08:33 PM, Jeff Law wrote:
>>>
>>>>
>>>> But since the number of warnings here hasn't changed, the ones
>>>> in GCC logs predate my changes.  So updating the tests seems
>>>> like an improvement to consider independently of the patch.
>>> Agreed.  I'm still wary of proceeding given the general concerns about
>>> configure tests.  It's good that GCC's configury bits aren't affected,
>>> but I'm not sure we can generalize a whole lot from that.
>>
>> So what's the next step?  I'm open to relaxing the warning
>> so it only triggers with -Wall or -Wextra and not by default
>> if that's considered necessary.
> I'm not sure :-)  The problem is we have notable potential to break
> things and do so in ways that are going to be painful to find.
>
> Having them only turn on for -Wextra might be an compromise position.
> But even if we do that I don't really see how we take the next step (ie,
> adding it to Wall).
>
>
>>
>> At the same time, the instances of the warning we have seen
>> have all been issued for the configure tests for years and
>> we have not seen any new instances of it as a result of
>> this change, so the concern that the patch might lead to some
>> more while at the same time accepting the ones we know about
>> doesn't make sense to me.
> Again, I don't think we can generalize much from the GCC autoconf
> scripts and the failure modes are going to be extremely painful to track
> down to this change.
>
> While we have this concern with every new warning or enhancements to
> existing warnings, this specific instance is worse because of how it
> interacts with relatively common configury code.
>
> Jeff
>
>
PR c/83656 - missing -Wbuiltin-declaration-mismatch on declaration without prototype

gcc/c/ChangeLog:

	PR c/83656
	* c-decl.c (header_for_builtin_fn): Declare.
	(diagnose_mismatched_decls): Diagnose declarations of built-in
	functions without a prototype.
	* c-typeck.c (convert_argument): New function.
	(convert_arguments): Factor code out into convert_argument.
	Detect mismatches between built-in formal arguments in calls
	to built-in without prototype.
	(type_or_builtin_type): New function.
	(convert_for_assignment): Add argument.  Conditionally issue
	warnings instead of errors for mismatches.

gcc/testsuite/ChangeLog:

	PR c/83656
	* gcc.dg/20021006-1.c
	* gcc.dg/Wbuiltin-declaration-mismatch.c: New test.
	* gcc.dg/Wbuiltin-declaration-mismatch-2.c: New test.
	* gcc.dg/Wbuiltin-declaration-mismatch-3.c: New test.
	* gcc.dg/Walloca-16.c: Adjust.
	* gcc.dg/Wrestrict-4.c: Adjust.
	* gcc.dg/Wrestrict-5.c: Adjust.
	* gcc.dg/atomic/stdatomic-generic.c: Adjust.
	* gcc.dg/atomic/stdatomic-lockfree.c: Adjust.
	* gcc.dg/initpri1.c: Adjust.
	* gcc.dg/pr15698-1.c: Adjust.
	* gcc.dg/pr69156.c: Adjust.
	* gcc.dg/pr83463.c: Adjust.
	* gcc.dg/redecl-4.c: Adjust.
	* gcc.dg/tls/thr-init-2.c: Adjust.
	* gcc.dg/torture/pr55890-2.c: Adjust.
	* gcc.dg/torture/pr55890-3.c: Adjust.
	* gcc.dg/torture/pr67741.c: Adjust.
	* gcc.dg/torture/stackalign/sibcall-1.c: Adjust.
	* gcc.dg/torture/tls/thr-init-1.c: Adjust.

diff --git a/gcc/c/c-decl.c b/gcc/c/c-decl.c
index cbbf7eb..1572f45 100644
--- a/gcc/c/c-decl.c
+++ b/gcc/c/c-decl.c
@@ -604,6 +604,7 @@ static tree grokparms (struct c_arg_info *, bool);
 static void layout_array_type (tree);
 static void warn_defaults_to (location_t, int, const char *, ...)
     ATTRIBUTE_GCC_DIAG(3,4);
+static const char *header_for_builtin_fn (enum built_in_function);
 
 /* T is a statement.  Add it to the statement-tree.  This is the
    C/ObjC version--C++ has a slightly different version of this
@@ -1887,12 +1888,25 @@ diagnose_mismatched_decls (tree newdecl, tree olddecl,
 	    *oldtypep = oldtype = trytype;
 	  else
 	    {
+	      const char *header
+		= header_for_builtin_fn (DECL_FUNCTION_CODE (olddecl));
+	      location_t loc = DECL_SOURCE_LOCATION (newdecl);
+	      if (warning_at (loc, OPT_Wbuiltin_declaration_mismatch,
+			      "conflicting types for built-in function %q+D; "
+			      "expected %qT",
+			      newdecl, oldtype)
+		  && header)
+		{
+		  /* Suggest the right header to include as the preferred
+		     solution rather than the spelling of the declaration.  */
+		  rich_location richloc (line_table, loc);
+		  maybe_add_include_fixit (&richloc, header, true);
+		  inform (&richloc,
+			  "%qD is declared in header %qs", olddecl, header);
+		}
 	      /* If types don't match for a built-in, throw away the
 		 built-in.  No point in calling locate_old_decl here, it
 		 won't print anything.  */
-	      warning (OPT_Wbuiltin_declaration_mismatch,
-		       "conflicting types for built-in function %q+D",
-		       newdecl);
 	      return false;
 	    }
 	}
@@ -2026,15 +2040,33 @@ diagnose_mismatched_decls (tree newdecl, tree olddecl,
 	 can't validate the argument list) the built-in definition is
 	 overridden, but optionally warn this was a bad choice of name.  */
       if (fndecl_built_in_p (olddecl)
-	  && !C_DECL_DECLARED_BUILTIN (olddecl)
-	  && (!TREE_PUBLIC (newdecl)
-	      || (DECL_INITIAL (newdecl)
-		  && !prototype_p (TREE_TYPE (newdecl)))))
+	  && !C_DECL_DECLARED_BUILTIN (olddecl))
 	{
-	  warning (OPT_Wshadow, "declaration of %q+D shadows "
-		   "a built-in function", newdecl);
-	  /* Discard the old built-in function.  */
-	  return false;
+	  if (!TREE_PUBLIC (newdecl)
+	      || (DECL_INITIAL (newdecl)
+		  && !prototype_p (TREE_TYPE (newdecl))))
+	    {
+	      warning_at (DECL_SOURCE_LOCATION (newdecl),
+			  OPT_Wshadow, "declaration of %qD shadows "
+			  "a built-in function", newdecl);
+	      /* Discard the old built-in function.  */
+	      return false;
+	    }
+
+	  if (!prototype_p (TREE_TYPE (newdecl)))
+	    {
+	      /* Set for built-ins that take no arguments.  */
+	      bool func_void_args = false;
+	      if (tree at = TYPE_ARG_TYPES (oldtype))
+		func_void_args = VOID_TYPE_P (TREE_VALUE (at));
+
+	      if (pedantic || (extra_warnings && !func_void_args))
+		warning_at (DECL_SOURCE_LOCATION (newdecl),
+			    OPT_Wbuiltin_declaration_mismatch,
+			    "declaration of built-in function %qD without "
+			    "a prototype; expected %qT",
+			    newdecl, TREE_TYPE (olddecl));
+	    }
 	}
 
       if (DECL_INITIAL (newdecl))
diff --git a/gcc/c/c-typeck.c b/gcc/c/c-typeck.c
index 9d09b8d..7c0047a 100644
--- a/gcc/c/c-typeck.c
+++ b/gcc/c/c-typeck.c
@@ -97,7 +97,8 @@ static int convert_arguments (location_t, vec<location_t>, tree,
 			      tree);
 static tree pointer_diff (location_t, tree, tree, tree *);
 static tree convert_for_assignment (location_t, location_t, tree, tree, tree,
-				    enum impl_conv, bool, tree, tree, int);
+				    enum impl_conv, bool, tree, tree, int,
+				    int = 0);
 static tree valid_compound_expr_initializer (tree, tree);
 static void push_string (const char *);
 static void push_member_name (tree);
@@ -3183,6 +3184,188 @@ c_build_function_call_vec (location_t loc, vec<location_t> arg_loc,
   return build_function_call_vec (loc, arg_loc, function, params, origtypes);
 }
 
+/* Helper for convert_arguments called to convert the VALue of argument
+   number ARGNUM from ORIGTYPE to the corresponding parameter number
+   PARMNUL and TYPE.  */
+
+static tree
+convert_argument (location_t ploc, tree function, tree fundecl,
+		  tree type, tree origtype, tree val, bool npc,
+		  tree rname, int parmnum, int argnum,
+		  bool excess_precision, int warnopt)
+{
+  tree valtype = TREE_TYPE (val);
+
+  tree parmval;
+
+  /* Formal parm type is specified by a function prototype.  */
+
+  if (type == error_mark_node || !COMPLETE_TYPE_P (type))
+    {
+      error_at (ploc, "type of formal parameter %d is incomplete",
+		parmnum + 1);
+      parmval = val;
+    }
+  else
+    {
+      /* Optionally warn about conversions that differ from the default
+	 conversions.  */
+      if (warn_traditional_conversion || warn_traditional)
+	{
+	  unsigned int formal_prec = TYPE_PRECISION (type);
+
+	  if (INTEGRAL_TYPE_P (type)
+	      && TREE_CODE (valtype) == REAL_TYPE)
+	    warning_at (ploc, OPT_Wtraditional_conversion,
+			"passing argument %d of %qE as integer rather "
+			"than floating due to prototype",
+			argnum, rname);
+	  if (INTEGRAL_TYPE_P (type)
+	      && TREE_CODE (valtype) == COMPLEX_TYPE)
+	    warning_at (ploc, OPT_Wtraditional_conversion,
+			"passing argument %d of %qE as integer rather "
+			"than complex due to prototype",
+			argnum, rname);
+	  else if (TREE_CODE (type) == COMPLEX_TYPE
+		   && TREE_CODE (valtype) == REAL_TYPE)
+	    warning_at (ploc, OPT_Wtraditional_conversion,
+			"passing argument %d of %qE as complex rather "
+			"than floating due to prototype",
+			argnum, rname);
+	  else if (TREE_CODE (type) == REAL_TYPE
+		   && INTEGRAL_TYPE_P (valtype))
+	    warning_at (ploc, OPT_Wtraditional_conversion,
+			"passing argument %d of %qE as floating rather "
+			"than integer due to prototype",
+			argnum, rname);
+	  else if (TREE_CODE (type) == COMPLEX_TYPE
+		   && INTEGRAL_TYPE_P (valtype))
+	    warning_at (ploc, OPT_Wtraditional_conversion,
+			"passing argument %d of %qE as complex rather "
+			"than integer due to prototype",
+			argnum, rname);
+	  else if (TREE_CODE (type) == REAL_TYPE
+		   && TREE_CODE (valtype) == COMPLEX_TYPE)
+	    warning_at (ploc, OPT_Wtraditional_conversion,
+			"passing argument %d of %qE as floating rather "
+			"than complex due to prototype",
+			argnum, rname);
+	  /* ??? At some point, messages should be written about
+	     conversions between complex types, but that's too messy
+	     to do now.  */
+	  else if (TREE_CODE (type) == REAL_TYPE
+		   && TREE_CODE (valtype) == REAL_TYPE)
+	    {
+	      /* Warn if any argument is passed as `float',
+		 since without a prototype it would be `double'.  */
+	      if (formal_prec == TYPE_PRECISION (float_type_node)
+		  && type != dfloat32_type_node)
+		warning_at (ploc, 0,
+			    "passing argument %d of %qE as %<float%> "
+			    "rather than %<double%> due to prototype",
+			    argnum, rname);
+
+	      /* Warn if mismatch between argument and prototype
+		 for decimal float types.  Warn of conversions with
+		 binary float types and of precision narrowing due to
+		 prototype. */
+	      else if (type != valtype
+		       && (type == dfloat32_type_node
+			   || type == dfloat64_type_node
+			   || type == dfloat128_type_node
+			   || valtype == dfloat32_type_node
+			   || valtype == dfloat64_type_node
+			   || valtype == dfloat128_type_node)
+		       && (formal_prec
+			   <= TYPE_PRECISION (valtype)
+			   || (type == dfloat128_type_node
+			       && (valtype
+				   != dfloat64_type_node
+				   && (valtype
+				       != dfloat32_type_node)))
+			   || (type == dfloat64_type_node
+			       && (valtype
+				   != dfloat32_type_node))))
+		warning_at (ploc, 0,
+			    "passing argument %d of %qE as %qT "
+			    "rather than %qT due to prototype",
+			    argnum, rname, type, valtype);
+
+	    }
+	  /* Detect integer changing in width or signedness.
+	     These warnings are only activated with
+	     -Wtraditional-conversion, not with -Wtraditional.  */
+	  else if (warn_traditional_conversion
+		   && INTEGRAL_TYPE_P (type)
+		   && INTEGRAL_TYPE_P (valtype))
+	    {
+	      tree would_have_been = default_conversion (val);
+	      tree type1 = TREE_TYPE (would_have_been);
+
+	      if (val == error_mark_node)
+		/* VAL could have been of incomplete type.  */;
+	      else if (TREE_CODE (type) == ENUMERAL_TYPE
+		       && (TYPE_MAIN_VARIANT (type)
+			   == TYPE_MAIN_VARIANT (valtype)))
+		/* No warning if function asks for enum
+		   and the actual arg is that enum type.  */
+		;
+	      else if (formal_prec != TYPE_PRECISION (type1))
+		warning_at (ploc, OPT_Wtraditional_conversion,
+			    "passing argument %d of %qE "
+			    "with different width due to prototype",
+			    argnum, rname);
+	      else if (TYPE_UNSIGNED (type) == TYPE_UNSIGNED (type1))
+		;
+	      /* Don't complain if the formal parameter type
+		 is an enum, because we can't tell now whether
+		 the value was an enum--even the same enum.  */
+	      else if (TREE_CODE (type) == ENUMERAL_TYPE)
+		;
+	      else if (TREE_CODE (val) == INTEGER_CST
+		       && int_fits_type_p (val, type))
+		/* Change in signedness doesn't matter
+		   if a constant value is unaffected.  */
+		;
+	      /* If the value is extended from a narrower
+		 unsigned type, it doesn't matter whether we
+		 pass it as signed or unsigned; the value
+		 certainly is the same either way.  */
+	      else if (TYPE_PRECISION (valtype) < TYPE_PRECISION (type)
+		       && TYPE_UNSIGNED (valtype))
+		;
+	      else if (TYPE_UNSIGNED (type))
+		warning_at (ploc, OPT_Wtraditional_conversion,
+			    "passing argument %d of %qE "
+			    "as unsigned due to prototype",
+			    argnum, rname);
+	      else
+		warning_at (ploc, OPT_Wtraditional_conversion,
+			    "passing argument %d of %qE "
+			    "as signed due to prototype",
+			    argnum, rname);
+	    }
+	}
+
+      /* Possibly restore an EXCESS_PRECISION_EXPR for the
+	 sake of better warnings from convert_and_check.  */
+      if (excess_precision)
+	val = build1 (EXCESS_PRECISION_EXPR, valtype, val);
+
+      parmval = convert_for_assignment (ploc, ploc, type,
+					val, origtype, ic_argpass,
+					npc, fundecl, function,
+					parmnum + 1, warnopt);
+
+      if (targetm.calls.promote_prototypes (fundecl ? TREE_TYPE (fundecl) : 0)
+	  && INTEGRAL_TYPE_P (type)
+	  && (TYPE_PRECISION (type) < TYPE_PRECISION (integer_type_node)))
+	parmval = default_conversion (parmval);
+    }
+
+  return parmval;
+}
+
 /* Convert the argument expressions in the vector VALUES
    to the types in the list TYPELIST.
 
@@ -3209,7 +3392,6 @@ convert_arguments (location_t loc, vec<location_t> arg_loc, tree typelist,
 		   vec<tree, va_gc> *values, vec<tree, va_gc> *origtypes,
 		   tree function, tree fundecl)
 {
-  tree typetail, val;
   unsigned int parmnum;
   bool error_args = false;
   const bool type_generic = fundecl
@@ -3227,50 +3409,69 @@ convert_arguments (location_t loc, vec<location_t> arg_loc, tree typelist,
   /* Handle an ObjC selector specially for diagnostics.  */
   selector = objc_message_selector ();
 
+  /* For a call to a built-in function declared without a prototype,
+     set to the built-in function's argument list.  */
+  tree builtin_typelist = NULL_TREE;
+
   /* For type-generic built-in functions, determine whether excess
      precision should be removed (classification) or not
      (comparison).  */
-  if (type_generic
+  if (fundecl
       && fndecl_built_in_p (fundecl, BUILT_IN_NORMAL))
     {
-      switch (DECL_FUNCTION_CODE (fundecl))
+      built_in_function code = DECL_FUNCTION_CODE (fundecl);
+      if (C_DECL_BUILTIN_PROTOTYPE (fundecl))
 	{
-	case BUILT_IN_ISFINITE:
-	case BUILT_IN_ISINF:
-	case BUILT_IN_ISINF_SIGN:
-	case BUILT_IN_ISNAN:
-	case BUILT_IN_ISNORMAL:
-	case BUILT_IN_FPCLASSIFY:
-	  type_generic_remove_excess_precision = true;
-	  break;
+	  if (tree bdecl = builtin_decl_implicit (code))
+	    builtin_typelist = TYPE_ARG_TYPES (TREE_TYPE (bdecl));
+	}
 
-	case BUILT_IN_ADD_OVERFLOW_P:
-	case BUILT_IN_SUB_OVERFLOW_P:
-	case BUILT_IN_MUL_OVERFLOW_P:
-	  /* The last argument of these type-generic builtins
-	     should not be promoted.  */
-	  type_generic_overflow_p = true;
-	  break;
+      /* For type-generic built-in functions, determine whether excess
+	 precision should be removed (classification) or not
+	 (comparison).  */
+      if (type_generic)
+	switch (code)
+	  {
+	  case BUILT_IN_ISFINITE:
+	  case BUILT_IN_ISINF:
+	  case BUILT_IN_ISINF_SIGN:
+	  case BUILT_IN_ISNAN:
+	  case BUILT_IN_ISNORMAL:
+	  case BUILT_IN_FPCLASSIFY:
+	    type_generic_remove_excess_precision = true;
+	    break;
 
-	default:
-	  break;
-	}
+	  case BUILT_IN_ADD_OVERFLOW_P:
+	  case BUILT_IN_SUB_OVERFLOW_P:
+	  case BUILT_IN_MUL_OVERFLOW_P:
+	    /* The last argument of these type-generic builtins
+	       should not be promoted.  */
+	    type_generic_overflow_p = true;
+	    break;
+
+	  default:
+	    break;
+	  }
     }
 
   /* Scan the given expressions and types, producing individual
      converted arguments.  */
 
-  for (typetail = typelist, parmnum = 0;
+  tree typetail, builtin_typetail, val;
+  for (typetail = typelist,
+	 builtin_typetail = builtin_typelist,
+	 parmnum = 0;
        values && values->iterate (parmnum, &val);
        ++parmnum)
     {
-      tree type = typetail ? TREE_VALUE (typetail) : 0;
+      tree type = typetail ? TREE_VALUE (typetail) : NULL_TREE;
+      tree builtin_type = (builtin_typetail
+			   ? TREE_VALUE (builtin_typetail) : NULL_TREE);
       tree valtype = TREE_TYPE (val);
       tree rname = function;
       int argnum = parmnum + 1;
       const char *invalid_func_diag;
       bool excess_precision = false;
-      bool npc;
       tree parmval;
       /* Some __atomic_* builtins have additional hidden argument at
 	 position 0.  */
@@ -3289,13 +3490,25 @@ convert_arguments (location_t loc, vec<location_t> arg_loc, tree typelist,
 	  return error_args ? -1 : (int) parmnum;
 	}
 
+      if (builtin_type == void_type_node)
+	{
+	  warning_at (loc, OPT_Wbuiltin_declaration_mismatch,
+		      "too many arguments to built-in function %qE "
+		      "expecting %d",
+		      function, parmnum);
+
+	  inform_declaration (fundecl);
+	  builtin_typetail = NULL_TREE;
+	}
+
       if (selector && argnum > 2)
 	{
 	  rname = selector;
 	  argnum -= 2;
 	}
 
-      npc = null_pointer_constant_p (val);
+      /* Determine if VAL is a null pointer constant before folding it.  */
+      bool npc = null_pointer_constant_p (val);
 
       /* If there is excess precision and a prototype, convert once to
 	 the required type rather than converting via the semantic
@@ -3340,172 +3553,10 @@ convert_arguments (location_t loc, vec<location_t> arg_loc, tree typelist,
 
       if (type != NULL_TREE)
 	{
-	  /* Formal parm type is specified by a function prototype.  */
-
-	  if (type == error_mark_node || !COMPLETE_TYPE_P (type))
-	    {
-	      error_at (ploc, "type of formal parameter %d is incomplete",
-			parmnum + 1);
-	      parmval = val;
-	    }
-	  else
-	    {
-	      tree origtype;
-
-	      /* Optionally warn about conversions that
-		 differ from the default conversions.  */
-	      if (warn_traditional_conversion || warn_traditional)
-		{
-		  unsigned int formal_prec = TYPE_PRECISION (type);
-
-		  if (INTEGRAL_TYPE_P (type)
-		      && TREE_CODE (valtype) == REAL_TYPE)
-		    warning_at (ploc, OPT_Wtraditional_conversion,
-				"passing argument %d of %qE as integer rather "
-				"than floating due to prototype",
-				argnum, rname);
-		  if (INTEGRAL_TYPE_P (type)
-		      && TREE_CODE (valtype) == COMPLEX_TYPE)
-		    warning_at (ploc, OPT_Wtraditional_conversion,
-				"passing argument %d of %qE as integer rather "
-				"than complex due to prototype",
-				argnum, rname);
-		  else if (TREE_CODE (type) == COMPLEX_TYPE
-			   && TREE_CODE (valtype) == REAL_TYPE)
-		    warning_at (ploc, OPT_Wtraditional_conversion,
-				"passing argument %d of %qE as complex rather "
-				"than floating due to prototype",
-				argnum, rname);
-		  else if (TREE_CODE (type) == REAL_TYPE
-			   && INTEGRAL_TYPE_P (valtype))
-		    warning_at (ploc, OPT_Wtraditional_conversion,
-				"passing argument %d of %qE as floating rather "
-				"than integer due to prototype",
-				argnum, rname);
-		  else if (TREE_CODE (type) == COMPLEX_TYPE
-			   && INTEGRAL_TYPE_P (valtype))
-		    warning_at (ploc, OPT_Wtraditional_conversion,
-				"passing argument %d of %qE as complex rather "
-				"than integer due to prototype",
-				argnum, rname);
-		  else if (TREE_CODE (type) == REAL_TYPE
-			   && TREE_CODE (valtype) == COMPLEX_TYPE)
-		    warning_at (ploc, OPT_Wtraditional_conversion,
-				"passing argument %d of %qE as floating rather "
-				"than complex due to prototype",
-				argnum, rname);
-		  /* ??? At some point, messages should be written about
-		     conversions between complex types, but that's too messy
-		     to do now.  */
-		  else if (TREE_CODE (type) == REAL_TYPE
-			   && TREE_CODE (valtype) == REAL_TYPE)
-		    {
-		      /* Warn if any argument is passed as `float',
-			 since without a prototype it would be `double'.  */
-		      if (formal_prec == TYPE_PRECISION (float_type_node)
-			  && type != dfloat32_type_node)
-			warning_at (ploc, 0,
-				    "passing argument %d of %qE as %<float%> "
-				    "rather than %<double%> due to prototype",
-				    argnum, rname);
-
-		      /* Warn if mismatch between argument and prototype
-			 for decimal float types.  Warn of conversions with
-			 binary float types and of precision narrowing due to
-			 prototype. */
- 		      else if (type != valtype
-			       && (type == dfloat32_type_node
-				   || type == dfloat64_type_node
-				   || type == dfloat128_type_node
-				   || valtype == dfloat32_type_node
-				   || valtype == dfloat64_type_node
-				   || valtype == dfloat128_type_node)
-			       && (formal_prec
-				   <= TYPE_PRECISION (valtype)
-				   || (type == dfloat128_type_node
-				       && (valtype
-					   != dfloat64_type_node
-					   && (valtype
-					       != dfloat32_type_node)))
-				   || (type == dfloat64_type_node
-				       && (valtype
-					   != dfloat32_type_node))))
-			warning_at (ploc, 0,
-				    "passing argument %d of %qE as %qT "
-				    "rather than %qT due to prototype",
-				    argnum, rname, type, valtype);
-
-		    }
-		  /* Detect integer changing in width or signedness.
-		     These warnings are only activated with
-		     -Wtraditional-conversion, not with -Wtraditional.  */
-		  else if (warn_traditional_conversion
-			   && INTEGRAL_TYPE_P (type)
-			   && INTEGRAL_TYPE_P (valtype))
-		    {
-		      tree would_have_been = default_conversion (val);
-		      tree type1 = TREE_TYPE (would_have_been);
-
-		      if (val == error_mark_node)
-			/* VAL could have been of incomplete type.  */;
-		      else if (TREE_CODE (type) == ENUMERAL_TYPE
-			       && (TYPE_MAIN_VARIANT (type)
-				   == TYPE_MAIN_VARIANT (valtype)))
-			/* No warning if function asks for enum
-			   and the actual arg is that enum type.  */
-			;
-		      else if (formal_prec != TYPE_PRECISION (type1))
-			warning_at (ploc, OPT_Wtraditional_conversion,
-				    "passing argument %d of %qE "
-				    "with different width due to prototype",
-				    argnum, rname);
-		      else if (TYPE_UNSIGNED (type) == TYPE_UNSIGNED (type1))
-			;
-		      /* Don't complain if the formal parameter type
-			 is an enum, because we can't tell now whether
-			 the value was an enum--even the same enum.  */
-		      else if (TREE_CODE (type) == ENUMERAL_TYPE)
-			;
-		      else if (TREE_CODE (val) == INTEGER_CST
-			       && int_fits_type_p (val, type))
-			/* Change in signedness doesn't matter
-			   if a constant value is unaffected.  */
-			;
-		      /* If the value is extended from a narrower
-			 unsigned type, it doesn't matter whether we
-			 pass it as signed or unsigned; the value
-			 certainly is the same either way.  */
-		      else if (TYPE_PRECISION (valtype) < TYPE_PRECISION (type)
-			       && TYPE_UNSIGNED (valtype))
-			;
-		      else if (TYPE_UNSIGNED (type))
-			warning_at (ploc, OPT_Wtraditional_conversion,
-				    "passing argument %d of %qE "
-				    "as unsigned due to prototype",
-				    argnum, rname);
-		      else
-			warning_at (ploc, OPT_Wtraditional_conversion,
-				    "passing argument %d of %qE "
-				    "as signed due to prototype",
-				    argnum, rname);
-		    }
-		}
-
-	      /* Possibly restore an EXCESS_PRECISION_EXPR for the
-		 sake of better warnings from convert_and_check.  */
-	      if (excess_precision)
-		val = build1 (EXCESS_PRECISION_EXPR, valtype, val);
-	      origtype = (!origtypes) ? NULL_TREE : (*origtypes)[parmnum];
-	      parmval = convert_for_assignment (loc, ploc, type,
-						val, origtype, ic_argpass,
-						npc, fundecl, function,
-						parmnum + 1);
-
-	      if (targetm.calls.promote_prototypes (fundecl ? TREE_TYPE (fundecl) : 0)
-		  && INTEGRAL_TYPE_P (type)
-		  && (TYPE_PRECISION (type) < TYPE_PRECISION (integer_type_node)))
-		parmval = default_conversion (parmval);
-	    }
+	  tree origtype = (!origtypes) ? NULL_TREE : (*origtypes)[parmnum];
+	  parmval = convert_argument (ploc, function, fundecl, type, origtype,
+				      val, npc, rname, parmnum, argnum,
+				      excess_precision, 0);
 	}
       else if (promote_float_arg)
         {
@@ -3547,8 +3598,24 @@ convert_arguments (location_t loc, vec<location_t> arg_loc, tree typelist,
       if (parmval == error_mark_node)
 	error_args = true;
 
+      if (!type && builtin_type && TREE_CODE (builtin_type) != VOID_TYPE)
+	{
+	  /* For a call to a built-in function declared without a prototype,
+	     perform the coversions from the argument to the expected type
+	     but issue warnings rather than errors for any mismatches.
+	     Ignore the converted argument and use the PARMVAL obtained
+	     above by applying default coversions instead.  */
+	  tree origtype = (!origtypes) ? NULL_TREE : (*origtypes)[parmnum];
+	  convert_argument (ploc, function, fundecl, builtin_type, origtype,
+			    val, npc, rname, parmnum, argnum, excess_precision,
+			    OPT_Wbuiltin_declaration_mismatch);
+	}
+
       if (typetail)
 	typetail = TREE_CHAIN (typetail);
+
+      if (builtin_typetail)
+	builtin_typetail = TREE_CHAIN (builtin_typetail);
     }
 
   gcc_assert (parmnum == vec_safe_length (values));
@@ -3560,6 +3627,18 @@ convert_arguments (location_t loc, vec<location_t> arg_loc, tree typelist,
       return -1;
     }
 
+  if (builtin_typetail && TREE_VALUE (builtin_typetail) != void_type_node)
+    {
+      unsigned nargs = parmnum;
+      for (tree t = builtin_typetail; t; t = TREE_CHAIN (t))
+	++nargs;
+
+      warning_at (loc, OPT_Wbuiltin_declaration_mismatch,
+		  "too few arguments to built-in function %qE expecting %u",
+		  function, nargs - 1);
+      inform_declaration (fundecl);
+    }
+
   return error_args ? -1 : (int) parmnum;
 }
 
@@ -4922,6 +5001,40 @@ ep_convert_and_check (location_t loc, tree type, tree expr,
   return convert (type, expr);
 }
 
+/* If EXPR refers to a built-in declared without a prototype returns
+   the actual type of the built-in and, if non-null, set *BLTIN to
+   a pointer to the built-in.  Otherwise return the type of EXPR
+   and clear *BLTIN if non-null. */
+
+static tree
+type_or_builtin_type (tree expr, tree *bltin = NULL)
+{
+  tree dummy;
+  if (!bltin)
+    bltin = &dummy;
+
+  *bltin = NULL_TREE;
+
+  tree type = TREE_TYPE (expr);
+  if (TREE_CODE (expr) != ADDR_EXPR)
+    return type;
+
+  tree oper = TREE_OPERAND (expr, 0);
+  if (!DECL_P (oper)
+      || TREE_CODE (oper) != FUNCTION_DECL
+      || !fndecl_built_in_p (oper, BUILT_IN_NORMAL))
+    return type;
+
+  built_in_function code = DECL_FUNCTION_CODE (oper);
+  if (!C_DECL_BUILTIN_PROTOTYPE (oper))
+    return type;
+
+  if ((*bltin = builtin_decl_implicit (code)))
+    type = build_pointer_type (TREE_TYPE (*bltin));
+
+  return type;
+}
+
 /* Build and return a conditional expression IFEXP ? OP1 : OP2.  If
    IFEXP_BCP then the condition is a call to __builtin_constant_p, and
    if folded to an integer constant then the unselected half may
@@ -4966,9 +5079,11 @@ build_conditional_expr (location_t colon_loc, tree ifexp, bool ifexp_bcp,
       || TREE_CODE (TREE_TYPE (op2)) == ERROR_MARK)
     return error_mark_node;
 
-  type1 = TREE_TYPE (op1);
+  tree bltin1 = NULL_TREE;
+  tree bltin2 = NULL_TREE;
+  type1 = type_or_builtin_type (op1, &bltin1);
   code1 = TREE_CODE (type1);
-  type2 = TREE_TYPE (op2);
+  type2 = type_or_builtin_type (op2, &bltin2);
   code2 = TREE_CODE (type2);
 
   if (code1 == POINTER_TYPE && reject_gcc_builtin (op1))
@@ -5206,9 +5321,14 @@ build_conditional_expr (location_t colon_loc, tree ifexp, bool ifexp_bcp,
       else
 	{
 	  int qual = ENCODE_QUAL_ADDR_SPACE (as_common);
-
-	  pedwarn (colon_loc, 0,
-		   "pointer type mismatch in conditional expression");
+	  if (bltin1 && bltin2)	
+	    warning_at (colon_loc, OPT_Wincompatible_pointer_types,
+			"pointer type mismatch between %qT and %qT "
+			"of %qD and %qD in conditional expression",
+			type1, type2, bltin1, bltin2);
+	  else
+	    pedwarn (colon_loc, 0,
+		     "pointer type mismatch in conditional expression");
 	  result_type = build_pointer_type
 			  (build_qualified_type (void_type_node, qual));
 	}
@@ -6346,13 +6466,16 @@ inform_for_arg (tree fundecl, location_t ploc, int parmnum,
 	    ^
 
    FUNCTION is a tree for the function being called.
-   PARMNUM is the number of the argument, for printing in error messages.  */
+   PARMNUM is the number of the argument, for printing in error messages.
+   WARNOPT may be set to a warning option to issue the corresponding warning
+   rather than an error for invalid conversions.  Used for calls to built-in
+   functions declared without a prototype.  */
 
 static tree
 convert_for_assignment (location_t location, location_t expr_loc, tree type,
 			tree rhs, tree origtype, enum impl_conv errtype,
 			bool null_pointer_constant, tree fundecl,
-			tree function, int parmnum)
+			tree function, int parmnum, int warnopt /* = 0 */)
 {
   enum tree_code codel = TREE_CODE (type);
   tree orig_rhs = rhs;
@@ -6550,7 +6673,11 @@ convert_for_assignment (location_t location, location_t expr_loc, tree type,
 	 an unprototyped function, it is compile-time undefined;
 	 making it a constraint in that case was rejected in
 	 DR#252.  */
-      error_at (location, "void value not ignored as it ought to be");
+      const char msg[] = "void value not ignored as it ought to be";
+      if (warnopt)
+	warning_at (location, warnopt, msg);
+      else
+	error_at (location, msg);
       return error_mark_node;
     }
   rhs = require_complete_type (location, rhs);
@@ -6566,7 +6693,11 @@ convert_for_assignment (location_t location, location_t expr_loc, tree type,
     {
       if (!lvalue_p (rhs))
 	{
-	  error_at (location, "cannot pass rvalue to reference parameter");
+	  const char msg[] = "cannot pass rvalue to reference parameter";
+	  if (warnopt)
+	    warning_at (location, warnopt, msg);
+	  else
+	    error_at (location, msg);
 	  return error_mark_node;
 	}
       if (!c_mark_addressable (rhs))
@@ -6578,7 +6709,7 @@ convert_for_assignment (location_t location, location_t expr_loc, tree type,
 				    build_pointer_type (TREE_TYPE (type)),
 				    rhs, origtype, errtype,
 				    null_pointer_constant, fundecl, function,
-				    parmnum);
+				    parmnum, warnopt);
       if (rhs == error_mark_node)
 	return error_mark_node;
 
@@ -6742,6 +6873,12 @@ convert_for_assignment (location_t location, location_t expr_loc, tree type,
   else if ((codel == POINTER_TYPE || codel == REFERENCE_TYPE)
 	   && (coder == codel))
     {
+      /* If RHS refers to a built-in declared without a prototype
+	 BLTIN is the declaration of the built-in with a prototype
+	 and RHSTYPE is set to the actual type of the built-in.  */
+      tree bltin;
+      rhstype = type_or_builtin_type (rhs, &bltin);
+
       tree ttl = TREE_TYPE (type);
       tree ttr = TREE_TYPE (rhstype);
       tree mvl = ttl;
@@ -6805,21 +6942,45 @@ convert_for_assignment (location_t location, location_t expr_loc, tree type,
 	  switch (errtype)
 	    {
 	    case ic_argpass:
-	      error_at (expr_loc, "passing argument %d of %qE from pointer to "
-			"non-enclosed address space", parmnum, rname);
+	      {
+		const char msg[] = G_("passing argument %d of %qE from "
+				      "pointer to non-enclosed address space");
+		if (warnopt)
+		  warning_at (expr_loc, warnopt, msg, parmnum, rname);
+		else
+		  error_at (expr_loc, msg, parmnum, rname);
 	      break;
+	      }
 	    case ic_assign:
-	      error_at (location, "assignment from pointer to "
-			"non-enclosed address space");
-	      break;
+	      {
+		const char msg[] = G_("assignment from pointer to "
+				      "non-enclosed address space");
+		if (warnopt)
+		  warning_at (location, warnopt, msg);
+		else
+		  error_at (location, msg);
+		break;
+	      }
 	    case ic_init:
-	      error_at (location, "initialization from pointer to "
-			"non-enclosed address space");
-	      break;
+	      {
+		const char msg[] = G_("initialization from pointer to "
+				      "non-enclosed address space");
+		if (warnopt)
+		  warning_at (location, warnopt, msg);
+		else
+		  error_at (location, msg);
+		break;
+	      }
 	    case ic_return:
-	      error_at (location, "return from pointer to "
-			"non-enclosed address space");
-	      break;
+	      {
+		const char msg[] = G_("return from pointer to "
+				      "non-enclosed address space");
+		if (warnopt)
+		  warning_at (location, warnopt, msg);
+		else
+		  error_at (location, msg);
+		break;
+	      }
 	    default:
 	      gcc_unreachable ();
 	    }
@@ -7017,19 +7178,38 @@ convert_for_assignment (location_t location, location_t expr_loc, tree type,
 	      }
 	      break;
 	    case ic_assign:
-	      pedwarn (location, OPT_Wincompatible_pointer_types,
-		       "assignment to %qT from incompatible pointer type %qT",
-		       type, rhstype);
+	      if (bltin)
+		pedwarn (location, OPT_Wincompatible_pointer_types,
+			 "assignment to %qT from pointer to "
+			 "%qD with incompatible type %qT",
+			 type, bltin, rhstype);
+	      else
+		pedwarn (location, OPT_Wincompatible_pointer_types,
+			 "assignment to %qT from incompatible pointer type %qT",
+			 type, rhstype);
 	      break;
 	    case ic_init:
-	      pedwarn_init (location, OPT_Wincompatible_pointer_types,
-			    "initialization of %qT from incompatible pointer "
-			    "type %qT", type, rhstype);
+	      if (bltin)
+		pedwarn_init (location, OPT_Wincompatible_pointer_types,
+			      "initialization of %qT from pointer to "
+			      "%qD with incompatible type %qT",
+			      type, bltin, rhstype);
+	      else
+		pedwarn_init (location, OPT_Wincompatible_pointer_types,
+			      "initialization of %qT from incompatible "
+			      "pointer type %qT",
+			      type, rhstype);
 	      break;
 	    case ic_return:
-	      pedwarn (location, OPT_Wincompatible_pointer_types,
-		       "returning %qT from a function with incompatible "
-		       "return type %qT", rhstype, type);
+	      if (bltin)
+		pedwarn (location, OPT_Wincompatible_pointer_types,
+			 "returning pointer to %qD of type %qT from "
+			 "a function with incompatible type %qT",
+			 bltin, rhstype, type);
+	      else
+		pedwarn (location, OPT_Wincompatible_pointer_types,
+			 "returning %qT from a function with incompatible "
+			 "return type %qT", rhstype, type);
 	      break;
 	    default:
 	      gcc_unreachable ();
@@ -7042,7 +7222,11 @@ convert_for_assignment (location_t location, location_t expr_loc, tree type,
     {
       /* ??? This should not be an error when inlining calls to
 	 unprototyped functions.  */
-      error_at (location, "invalid use of non-lvalue array");
+      const char msg[] = "invalid use of non-lvalue array";
+      if (warnopt)
+	warning_at (location, warnopt, msg);
+      else
+	error_at (location, msg);
       return error_mark_node;
     }
   else if (codel == POINTER_TYPE && coder == INTEGER_TYPE)
@@ -7138,25 +7322,44 @@ convert_for_assignment (location_t location, location_t expr_loc, tree type,
 	auto_diagnostic_group d;
 	range_label_for_type_mismatch rhs_label (rhstype, type);
 	gcc_rich_location richloc (expr_loc, &rhs_label);
-	error_at (&richloc, "incompatible type for argument %d of %qE", parmnum,
-		  rname);
+	const char msg[] = G_("incompatible type for argument %d of %qE");
+	if (warnopt)
+	  warning_at (expr_loc, warnopt, msg, parmnum, rname);
+	else
+	  error_at (&richloc, msg, parmnum, rname);
 	inform_for_arg (fundecl, expr_loc, parmnum, type, rhstype);
       }
       break;
     case ic_assign:
-      error_at (location, "incompatible types when assigning to type %qT from "
-		"type %qT", type, rhstype);
-      break;
+      {
+	const char msg[]
+	  = G_("incompatible types when assigning to type %qT from type %qT");
+	if (warnopt)
+	  warning_at (expr_loc, 0, msg, type, rhstype);
+	else
+	  error_at (expr_loc, msg, type, rhstype);
+	break;
+      }
     case ic_init:
-      error_at (location,
-		"incompatible types when initializing type %qT using type %qT",
-		type, rhstype);
-      break;
+      {
+	const char msg[]
+	  = G_("incompatible types when initializing type %qT using type %qT");
+	if (warnopt)
+	  warning_at (location, 0, msg, type, rhstype);
+	else
+	  error_at (location, msg, type, rhstype);
+	break;
+      }
     case ic_return:
-      error_at (location,
-		"incompatible types when returning type %qT but %qT was "
-		"expected", rhstype, type);
-      break;
+      {
+	const char msg[]
+	  = G_("incompatible types when returning type %qT but %qT was expected");
+	if (warnopt)
+	  warning_at (location, 0, msg, rhstype, type);
+	else
+	  error_at (location, msg, rhstype, type);
+	break;
+      }
     default:
       gcc_unreachable ();
     }
diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
index 284594d..b879a74 100644
--- a/gcc/doc/invoke.texi
+++ b/gcc/doc/invoke.texi
@@ -6809,9 +6809,16 @@ attributes.
 @item -Wno-builtin-declaration-mismatch
 @opindex Wno-builtin-declaration-mismatch
 @opindex Wbuiltin-declaration-mismatch
-Warn if a built-in function is declared with the wrong signature or 
-as non-function.
-This warning is enabled by default.
+Warn if a built-in function is declared with an incompatible signature
+or as a non-function.  When @option{-Wextra} is specified, also warn when
+a built-in function that takes arguments is declared without a prototype,
+and for calls to built-in functions declared without a prototype made with
+the wrong number of arguments or with arguments with incompatible types.
+When @option{-Wpedantic} is specified, declarations without a prototype
+of built-in functions that take no arguments are diagnosed as well.  The
+@option{-Wno-builtin-declaration-mismatch} warning is enabled by default.
+To avoid the warning include the appropriate header to bring the declarations
+of built-in functions into scope.
 
 @item -Wno-builtin-macro-redefined
 @opindex Wno-builtin-macro-redefined
diff --git a/gcc/testsuite/gcc.dg/20021006-1.c b/gcc/testsuite/gcc.dg/20021006-1.c
index 63cc8a5..92df2c5 100644
--- a/gcc/testsuite/gcc.dg/20021006-1.c
+++ b/gcc/testsuite/gcc.dg/20021006-1.c
@@ -2,7 +2,7 @@
    This testcase was miscompiled on x86-64 due to wrong access to the struct
    members.  */
 
-extern void abort();
+extern void abort(void);
 
 struct A {
   long x;
diff --git a/gcc/testsuite/gcc.dg/Walloca-16.c b/gcc/testsuite/gcc.dg/Walloca-16.c
index 3ee96a9..866594c 100644
--- a/gcc/testsuite/gcc.dg/Walloca-16.c
+++ b/gcc/testsuite/gcc.dg/Walloca-16.c
@@ -4,3 +4,5 @@
 
 void *alloca ();
 __typeof__(alloca ()) a () { return alloca (); }
+
+/* { dg-prune-output "\\\[-Wbuiltin-declaration-mismatch]" } */
diff --git a/gcc/testsuite/gcc.dg/Wbuiltin-declaration-mismatch-2.c b/gcc/testsuite/gcc.dg/Wbuiltin-declaration-mismatch-2.c
new file mode 100644
index 0000000..9d75cd8
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/Wbuiltin-declaration-mismatch-2.c
@@ -0,0 +1,25 @@
+/* PR c/83656 - missing -Wbuiltin-declaration-mismatch on declaration
+   without prototype
+   { dg-do compile }
+   { dg-options "-Wall -Wextra" } */
+
+typedef __SIZE_TYPE__ size_t;
+
+/* Verify that ordinary library built-ins are not diagnosed with -Wextra
+   when they take no arguments (except in cases of return type mismatches).
+   This is in anticipation that C may some day adopt the same syntax as
+   C++ for declaring functions that take no arguments.  */
+
+void abort ();
+
+/* Verify that ordinary library built-ins are diagnosed with -Wextra
+   when they take arguments.  */
+
+void* memcpy ();    /* { dg-warning "declaration of built-in function .memcpy. without a prototype; expected .void \\\*\\\(void \\\*, const void \\\*, \(long \)*unsigned int\\\)." } */
+void* memset ();    /* { dg-warning "declaration of built-in function .memset. without a prototype; expected .void \\\*\\\(void \\\*, int, *\(long \)*unsigned int\\\)." } */
+size_t strlen ();   /* { dg-warning "declaration of built-in function .strlen. without a prototype; expected .\(long \)*unsigned int\\\(const char \\\*\\\)." } */
+
+/* Variadic built-ins are diagnosed even without -Wextra (they are,
+   in fact, diagnosed by default).  */
+int printf ();      /* { dg-warning "\\\[-Wbuiltin-declaration-mismatch]" } */
+int sprintf ();     /* { dg-warning "\\\[-Wbuiltin-declaration-mismatch]" } */
diff --git a/gcc/testsuite/gcc.dg/Wbuiltin-declaration-mismatch-3.c b/gcc/testsuite/gcc.dg/Wbuiltin-declaration-mismatch-3.c
new file mode 100644
index 0000000..110267f
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/Wbuiltin-declaration-mismatch-3.c
@@ -0,0 +1,110 @@
+/* PR c/83656 - missing -Wbuiltin-declaration-mismatch on declaration
+   without prototype
+   { dg-do compile }
+   { dg-options "-Wbuiltin-declaration-mismatch" } */
+
+typedef __SIZE_TYPE__ size_t;
+
+/* Built-ins declared without a prototype are not diagnosed by default
+   (without -Wextra) except when their return type doesn't match.  */
+int abort ();       /* { dg-warning "\\\[-Wbuiltin-declaration-mismatch]" } */
+
+/* Built-ins declared without a prototype are not diagnosed without -Wextra.  */
+void exit ();
+void* memcpy ();
+void* memset ();
+
+
+void test_call_abort (void)
+{
+  /* Verify that a valid call to abort() is not diagnosed.  */
+  abort ();
+
+  /* Unfortunately, the incompatible declaration above makes GCC "forget"
+     that abort() is a built-in and so the invalid calls below aren't
+     diagnosed.  The only saving grace is that the invalid declaration
+     that differs in the return type is diagnosed by default. */
+  abort (1);        /* { dg-warning "too many arguments to built-in function .abort. expecting 0" "pr?????" { xfail *-*-* } } */
+
+  abort (1, 2);     /* { dg-warning "too many arguments" "pr?????" { xfail *-*-* } } */
+}
+
+
+void test_call_exit (void)
+{
+  /* Verify that a valid call to exit is not diagnosed.  */
+  exit (0);
+
+  exit ();          /* { dg-warning "too few arguments to built-in function 'exit' expecting 1" } */
+
+  exit (1, 2);      /* { dg-warning "too many arguments" } */
+
+  /* Verify that passing incompatible arguments triggers a warning.  */
+  exit ("");        /* { dg-warning "\\\[-Wint-conversion]" } */
+
+  struct S { int i; } s = { 0 };
+  exit (s);         /* { dg-warning "incompatible type for argument 1" } */
+}
+
+
+void test_call_memcpy (void *p, const void *q, unsigned n)
+{
+  memcpy (p, q, n);
+
+  memcpy ();        /* { dg-warning "too few arguments to built-in function 'memcpy' expecting 3" } */
+
+  memcpy (p);       /* { dg-warning "too few arguments to built-in function 'memcpy' expecting 3" } */
+
+  memcpy (p, q);     /* { dg-warning "too few arguments to built-in function 'memcpy' expecting 3" } */
+
+  memcpy (q, p, n); /* { dg-warning "\\\[-Wdiscarded-qualifiers]" } */
+
+  memcpy (p, n, q); /* { dg-warning "\\\[-Wint-conversion]" } */
+
+  memcpy (p, q, n, 0); /* { dg-warning "too many arguments to built-in function 'memcpy' expecting 3" } */
+}
+
+
+typedef void* (memcpy_t)(void*, const void*, size_t);
+typedef void* (memset_t)(void*, int, size_t);
+
+void test_init (void)
+{
+  /* Verify that initialization of a pointer by the address of a built-in
+     function of a matching type declared without a prototype doesn't
+     trigger a warning...  */
+  memset_t *pmemset = memset;
+
+  /* ...but initialization by the address of an incompatible built-in
+     does even without -Wextra.  */
+  memcpy_t *pmemcpy = memset;           /* { dg-warning "\\\[-Wincompatible-pointer-types]" } */
+}
+
+
+void test_assign (void)
+{
+  /* Same as above but for assignment.  */
+  memset_t *pmemset;
+  pmemset = memset;
+
+  memcpy_t *pmemcpy;
+  pmemcpy = memset;                     /* { dg-warning "\\\[-Wincompatible-pointer-types]" } */
+}
+
+
+/* Verify that passing built-ins declared without a prototype to
+   functions that expect a pointer to a function of a specific type
+   is diagnosed.  Ditto for return statements.  */
+
+void take_memcpy (memcpy_t*);
+void take_any (int, ...);
+
+memset_t* pass_args (int i)
+{
+  take_memcpy (memcpy);
+  take_memcpy (memset);                 /* { dg-warning "\\\[-Wincompatible-pointer-types]" } */
+
+  take_any (0, i ? memcpy : memset);    /* { dg-warning "\\\[-Wincompatible-pointer-types]" } */
+
+  return memcpy;                        /* { dg-warning "\\\[-Wincompatible-pointer-types]" } */
+}
diff --git a/gcc/testsuite/gcc.dg/Wbuiltin-declaration-mismatch.c b/gcc/testsuite/gcc.dg/Wbuiltin-declaration-mismatch.c
new file mode 100644
index 0000000..ed66fd3
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/Wbuiltin-declaration-mismatch.c
@@ -0,0 +1,23 @@
+/* PR c/83656 - missing -Wbuiltin-declaration-mismatch on declaration
+   without prototype
+   { dg-do compile }
+   { dg-options "-Wall" } */
+
+typedef __SIZE_TYPE__ size_t;
+
+/* Verify that ordinary library built-ins are not diagnosed with -Wall
+   (or by default) whether or not they take arguments (even though they
+   should be).  */
+
+void abort ();
+void* memcpy ();
+void* memset ();
+size_t strlen ();
+
+/* Verify mismatches in return types are diagnosed.  */
+int exit ();        /* { dg-warning "\\\[-Wbuiltin-declaration-mismatch]" } */
+
+/* Variadic built-ins are diagnosed with -Wall (they are, in fact,
+   diagnosed by default).  */
+int printf ();      /* { dg-warning "\\\[-Wbuiltin-declaration-mismatch]" } */
+int sprintf ();     /* { dg-warning "\\\[-Wbuiltin-declaration-mismatch]" } */
diff --git a/gcc/testsuite/gcc.dg/Wrestrict-4.c b/gcc/testsuite/gcc.dg/Wrestrict-4.c
index f2398ef..55ea6c0 100644
--- a/gcc/testsuite/gcc.dg/Wrestrict-4.c
+++ b/gcc/testsuite/gcc.dg/Wrestrict-4.c
@@ -108,3 +108,5 @@ void* test_strncpy_2 (char *d, const char *s)
 {
   return strncpy (d, s);
 }
+
+/* { dg-prune-output "\\\[-Wbuiltin-declaration-mismatch]" } */
diff --git a/gcc/testsuite/gcc.dg/Wrestrict-5.c b/gcc/testsuite/gcc.dg/Wrestrict-5.c
index 4912ee5..7064e1e 100644
--- a/gcc/testsuite/gcc.dg/Wrestrict-5.c
+++ b/gcc/testsuite/gcc.dg/Wrestrict-5.c
@@ -41,3 +41,6 @@ void test_strncpy_nowarn (char *d)
 {
   strncpy (d + 1, d + 3, "");
 }
+
+/* { dg-prune-output "\\\[-Wbuiltin-declaration-mismatch]" }
+   { dg-prune-output "\\\[-Wint-conversion]" } */
diff --git a/gcc/testsuite/gcc.dg/atomic/stdatomic-generic.c b/gcc/testsuite/gcc.dg/atomic/stdatomic-generic.c
index 8033c53..96582b4 100644
--- a/gcc/testsuite/gcc.dg/atomic/stdatomic-generic.c
+++ b/gcc/testsuite/gcc.dg/atomic/stdatomic-generic.c
@@ -4,7 +4,7 @@
 
 #include <stdatomic.h>
 
-extern void abort ();
+extern void abort (void);
 extern int memcmp (const void *, const void *, __SIZE_TYPE__);
 
 typedef struct test {
diff --git a/gcc/testsuite/gcc.dg/atomic/stdatomic-lockfree.c b/gcc/testsuite/gcc.dg/atomic/stdatomic-lockfree.c
index 29310e9..c6d06a5 100644
--- a/gcc/testsuite/gcc.dg/atomic/stdatomic-lockfree.c
+++ b/gcc/testsuite/gcc.dg/atomic/stdatomic-lockfree.c
@@ -5,7 +5,7 @@
 #include <stdatomic.h>
 #include <stdint.h>
 
-extern void abort ();
+extern void abort (void);
 
 _Atomic _Bool aba;
 atomic_bool abt;
diff --git a/gcc/testsuite/gcc.dg/initpri1.c b/gcc/testsuite/gcc.dg/initpri1.c
index 794ea2b..b6afd76 100644
--- a/gcc/testsuite/gcc.dg/initpri1.c
+++ b/gcc/testsuite/gcc.dg/initpri1.c
@@ -1,6 +1,6 @@
 /* { dg-do run { target init_priority } } */
 
-extern void abort ();
+extern void abort (void);
 
 int i;
 int j;
diff --git a/gcc/testsuite/gcc.dg/pr15698-1.c b/gcc/testsuite/gcc.dg/pr15698-1.c
index 5a75a10..cbe613c 100644
--- a/gcc/testsuite/gcc.dg/pr15698-1.c
+++ b/gcc/testsuite/gcc.dg/pr15698-1.c
@@ -21,3 +21,5 @@ char *rindex(a, b)
 {
   return 0;
 }
+
+/* { dg-prune-output "\\\[-Wbuiltin-declaration-mismatch]" } */
diff --git a/gcc/testsuite/gcc.dg/pr69156.c b/gcc/testsuite/gcc.dg/pr69156.c
index 1addfa3..522ac00 100644
--- a/gcc/testsuite/gcc.dg/pr69156.c
+++ b/gcc/testsuite/gcc.dg/pr69156.c
@@ -5,6 +5,6 @@
 _Bool
 foo ()
 {
-  _Bool (*f) () = __builtin_abs;	/* { dg-warning "initialization of '_Bool \\(\\*\\)\\(\\)' from incompatible pointer type" } */
+  _Bool (*f) () = __builtin_abs;	/* { dg-warning "initialization of '_Bool \\(\\*\\)\\(\\)' from pointer to .__builtin_abs. with incompatible type .int \\\(\\\*\\\)." } */
   return f (0);
 }
diff --git a/gcc/testsuite/gcc.dg/pr83463.c b/gcc/testsuite/gcc.dg/pr83463.c
index ddf662f..cd11f64 100644
--- a/gcc/testsuite/gcc.dg/pr83463.c
+++ b/gcc/testsuite/gcc.dg/pr83463.c
@@ -15,3 +15,5 @@ p ()
 {
   m (p + (long) a);
 }
+
+/* { dg-prune-output "\\\[-Wbuiltin-declaration-mismatch]" } */
diff --git a/gcc/testsuite/gcc.dg/redecl-4.c b/gcc/testsuite/gcc.dg/redecl-4.c
index 2ba74a3..8f12488 100644
--- a/gcc/testsuite/gcc.dg/redecl-4.c
+++ b/gcc/testsuite/gcc.dg/redecl-4.c
@@ -27,3 +27,6 @@ f (void)
 
 /* Should still diagnose incompatible prototype for strcmp.  */
 int strcmp (void); /* { dg-error "conflict" } */
+
+/* { dg-prune-output "\\\[-Wbuiltin-declaration-mismatch]" }
+   { dg-prune-output "\\\[-Wint-conversion]" } */
diff --git a/gcc/testsuite/gcc.dg/tls/thr-init-2.c b/gcc/testsuite/gcc.dg/tls/thr-init-2.c
index 22c96ea..a540389 100644
--- a/gcc/testsuite/gcc.dg/tls/thr-init-2.c
+++ b/gcc/testsuite/gcc.dg/tls/thr-init-2.c
@@ -2,7 +2,7 @@
 /* { dg-require-effective-target tls_runtime } */
 /* { dg-add-options tls } */
 
-extern void abort() ;
+extern void abort (void);
 
 static __thread int fstat ;
 static __thread int fstat = 1;
diff --git a/gcc/testsuite/gcc.dg/torture/pr55890-2.c b/gcc/testsuite/gcc.dg/torture/pr55890-2.c
index 1cf71d7..31779e0 100644
--- a/gcc/testsuite/gcc.dg/torture/pr55890-2.c
+++ b/gcc/testsuite/gcc.dg/torture/pr55890-2.c
@@ -2,3 +2,5 @@
 
 extern void *memcpy();
 int main() { memcpy(); }
+
+/* { dg-prune-output "\\\[-Wbuiltin-declaration-mismatch]" } */
diff --git a/gcc/testsuite/gcc.dg/torture/pr55890-3.c b/gcc/testsuite/gcc.dg/torture/pr55890-3.c
index c7f77be..21a3d98 100644
--- a/gcc/testsuite/gcc.dg/torture/pr55890-3.c
+++ b/gcc/testsuite/gcc.dg/torture/pr55890-3.c
@@ -7,3 +7,5 @@ bar ()
 {
   return memmove ();
 }
+
+/* { dg-prune-output "\\\[-Wbuiltin-declaration-mismatch]" } */
diff --git a/gcc/testsuite/gcc.dg/torture/pr67741.c b/gcc/testsuite/gcc.dg/torture/pr67741.c
index 1ffc707..833e53e 100644
--- a/gcc/testsuite/gcc.dg/torture/pr67741.c
+++ b/gcc/testsuite/gcc.dg/torture/pr67741.c
@@ -3,11 +3,12 @@
 struct singlecomplex { float real, imag ; } ;
 struct doublecomplex { double real, imag ; } ;
 struct extendedcomplex { long double real, imag ; } ;
-extern double cabs();
+extern double cabs(); /* { dg-warning "\\\[-Wbuiltin-declaration-mismatch]" } */
 float cabsf(fc)
      struct singlecomplex fc;  /* { dg-warning "doesn't match" } */
 {
   struct doublecomplex dc ;
   dc.real=fc.real; dc.imag=fc.imag;
-  return (float) cabs(dc);
+  return (float) cabs(dc);   /* { dg-warning "incompatible type for argument 1 of .cabs." } */
 }
+
diff --git a/gcc/testsuite/gcc.dg/torture/stackalign/sibcall-1.c b/gcc/testsuite/gcc.dg/torture/stackalign/sibcall-1.c
index 8c17475..c4992df 100644
--- a/gcc/testsuite/gcc.dg/torture/stackalign/sibcall-1.c
+++ b/gcc/testsuite/gcc.dg/torture/stackalign/sibcall-1.c
@@ -1,7 +1,7 @@
 /* { dg-do run } */
 
 extern int ok (int);
-extern void exit ();
+extern void exit (int);
 static int gen_x86_64_shrd (int);
 static int
 gen_x86_64_shrd(int a __attribute__ ((__unused__)))
diff --git a/gcc/testsuite/gcc.dg/torture/tls/thr-init-1.c b/gcc/testsuite/gcc.dg/torture/tls/thr-init-1.c
index ff3338f..3ae2e29 100644
--- a/gcc/testsuite/gcc.dg/torture/tls/thr-init-1.c
+++ b/gcc/testsuite/gcc.dg/torture/tls/thr-init-1.c
@@ -3,7 +3,7 @@
 /* { dg-add-options tls } */
 
 extern int printf (char *,...);
-extern void abort() ;
+extern void abort(void) ;
 
 int test_code(int b)
 {

On Fri, 2 Nov 2018, Martin Sebor wrote:

> I have reworked the patch to resolve any lingering concerns about
> warnings in configure tests.  The attached revision only warns
> with -Wextra and only for incompatible declarations of built-ins
> that take arguments.  For void built-ins like abort() it only
> warns with -Wpedantic (this required adjustments to several
> tests that are being compiled with -pedantic-errors).

I don't think this use of -Wpedantic is appropriate.  -Wpedantic is not a 
catch-all for warnings we don't want to enable with some other option; 
it's specifically for programs doing something that is disallowed by ISO C 
(such warnings may or may not also be enabled by other relevant options).

Since this declaration is not disallowed by ISO C, -Wpedantic should not 
result in a warning for it.

(I do consider declarations with () for built-in functions without 
arguments to be more dubious than for user-defined functions without 
arguments, simply because good practice would be to include the standard 
header to get declarations of those functions, whereas for user-defined 
functions the code might simply be using C++ style for declaring functions 
without arguments.)

On 11/02/2018 04:52 PM, Joseph Myers wrote:
> On Fri, 2 Nov 2018, Martin Sebor wrote:
>
>> I have reworked the patch to resolve any lingering concerns about
>> warnings in configure tests.  The attached revision only warns
>> with -Wextra and only for incompatible declarations of built-ins
>> that take arguments.  For void built-ins like abort() it only
>> warns with -Wpedantic (this required adjustments to several
>> tests that are being compiled with -pedantic-errors).
>
> I don't think this use of -Wpedantic is appropriate.  -Wpedantic is not a
> catch-all for warnings we don't want to enable with some other option;
> it's specifically for programs doing something that is disallowed by ISO C
> (such warnings may or may not also be enabled by other relevant options).
>
> Since this declaration is not disallowed by ISO C, -Wpedantic should not
> result in a warning for it.
>
> (I do consider declarations with () for built-in functions without
> arguments to be more dubious than for user-defined functions without
> arguments, simply because good practice would be to include the standard
> header to get declarations of those functions, whereas for user-defined
> functions the code might simply be using C++ style for declaring functions
> without arguments.)

-Wpedantic alone doesn't cause a warning, only in conjunction
with -Wno-builtin-declaration-mismatch.

But I have no preference for what option to put it under, or
necessarily think that using -Wpedantic (or any other "group"
option) like this is a great idea (it doesn't work with #pragma
GCC diagnostic that way I think it should).  In fact, with
the latest approach of diagnosing unsafe calls to these functions
regardless of the declaration form it doesn't seem that important
that declarations of built-ins with no arguments be diagnosed at
all.  Either way, there aren't enough of them for it to matter
much.  I think there's just one: abort.  I'm fine with removing
this part of the patch.

Is there anything else?

Martin

In response to Joseph's comment I've removed the interaction
with -Wpedantic from the updated patch.

In addition, to help detect bugs like the one in the test case
for pr87886, I have also enhanced the detection of incompatible
calls to include integer/real type mismatches so that calls like
the one below are diagnosed:

   extern double sqrt ();
   int f (int x)
   {
     return sqrt (x);   // passing int where double is expected
   }

With the removal of the -Wpedantic interaction declaring abort()
without a prototype is no longer diagnosed and so the test suite
changes to add the prototype are not necessary.  I decided not
to back them out because Jeff indicated a preference for making
these kinds of improvements in general in an unrelated
discussion.

On 11/02/2018 05:40 PM, Martin Sebor wrote:
> On 11/02/2018 04:52 PM, Joseph Myers wrote:
>> On Fri, 2 Nov 2018, Martin Sebor wrote:
>>
>>> I have reworked the patch to resolve any lingering concerns about
>>> warnings in configure tests.  The attached revision only warns
>>> with -Wextra and only for incompatible declarations of built-ins
>>> that take arguments.  For void built-ins like abort() it only
>>> warns with -Wpedantic (this required adjustments to several
>>> tests that are being compiled with -pedantic-errors).
>>
>> I don't think this use of -Wpedantic is appropriate.  -Wpedantic is not a
>> catch-all for warnings we don't want to enable with some other option;
>> it's specifically for programs doing something that is disallowed by
>> ISO C
>> (such warnings may or may not also be enabled by other relevant options).
>>
>> Since this declaration is not disallowed by ISO C, -Wpedantic should not
>> result in a warning for it.
>>
>> (I do consider declarations with () for built-in functions without
>> arguments to be more dubious than for user-defined functions without
>> arguments, simply because good practice would be to include the standard
>> header to get declarations of those functions, whereas for user-defined
>> functions the code might simply be using C++ style for declaring
>> functions
>> without arguments.)
>
> -Wpedantic alone doesn't cause a warning, only in conjunction
> with -Wno-builtin-declaration-mismatch.
>
> But I have no preference for what option to put it under, or
> necessarily think that using -Wpedantic (or any other "group"
> option) like this is a great idea (it doesn't work with #pragma
> GCC diagnostic that way I think it should).  In fact, with
> the latest approach of diagnosing unsafe calls to these functions
> regardless of the declaration form it doesn't seem that important
> that declarations of built-ins with no arguments be diagnosed at
> all.  Either way, there aren't enough of them for it to matter
> much.  I think there's just one: abort.  I'm fine with removing
> this part of the patch.
>
> Is there anything else?
>
> Martin
PR c/83656 - missing -Wbuiltin-declaration-mismatch on declaration without prototype

gcc/c/ChangeLog:

	PR c/83656
	* c-decl.c (header_for_builtin_fn): Declare.
	(diagnose_mismatched_decls): Diagnose declarations of built-in
	functions without a prototype.
	* c-typeck.c (maybe_warn_builtin_no_proto_arg): New function.
	(convert_argument): Same.
	(convert_arguments): Factor code out into convert_argument.
	Detect mismatches between built-in formal arguments in calls
	to built-in without prototype.
	(build_conditional_expr): Same.
	(type_or_builtin_type): New function.
	(convert_for_assignment): Add argument.  Conditionally issue
	warnings instead of errors for mismatches.

gcc/testsuite/ChangeLog:

	PR c/83656
	* gcc.dg/20021006-1.c
	* gcc.dg/Wbuiltin-declaration-mismatch.c: New test.
	* gcc.dg/Wbuiltin-declaration-mismatch-2.c: New test.
	* gcc.dg/Wbuiltin-declaration-mismatch-3.c: New test.
	* gcc.dg/Wbuiltin-declaration-mismatch-4.c: New test.
	* gcc.dg/Walloca-16.c: Adjust.
	* gcc.dg/Wrestrict-4.c: Adjust.
	* gcc.dg/Wrestrict-5.c: Adjust.
	* gcc.dg/atomic/stdatomic-generic.c: Adjust.
	* gcc.dg/atomic/stdatomic-lockfree.c: Adjust.
	* gcc.dg/initpri1.c: Adjust.
	* gcc.dg/pr15698-1.c: Adjust.
	* gcc.dg/pr69156.c: Adjust.
	* gcc.dg/pr83463.c: Adjust.
	* gcc.dg/redecl-4.c: Adjust.
	* gcc.dg/tls/thr-init-2.c: Adjust.
	* gcc.dg/torture/pr55890-2.c: Adjust.
	* gcc.dg/torture/pr55890-3.c: Adjust.
	* gcc.dg/torture/pr67741.c: Adjust.
	* gcc.dg/torture/stackalign/sibcall-1.c: Adjust.
	* gcc.dg/torture/tls/thr-init-1.c: Adjust.
	* gcc.dg/tree-ssa/builtins-folding-gimple-ub.c: Adjust.

diff --git a/gcc/c/c-decl.c b/gcc/c/c-decl.c
index cbbf7eb..524ac83 100644
--- a/gcc/c/c-decl.c
+++ b/gcc/c/c-decl.c
@@ -604,6 +604,7 @@ static tree grokparms (struct c_arg_info *, bool);
 static void layout_array_type (tree);
 static void warn_defaults_to (location_t, int, const char *, ...)
     ATTRIBUTE_GCC_DIAG(3,4);
+static const char *header_for_builtin_fn (enum built_in_function);
 
 /* T is a statement.  Add it to the statement-tree.  This is the
    C/ObjC version--C++ has a slightly different version of this
@@ -1887,12 +1888,25 @@ diagnose_mismatched_decls (tree newdecl, tree olddecl,
 	    *oldtypep = oldtype = trytype;
 	  else
 	    {
+	      const char *header
+		= header_for_builtin_fn (DECL_FUNCTION_CODE (olddecl));
+	      location_t loc = DECL_SOURCE_LOCATION (newdecl);
+	      if (warning_at (loc, OPT_Wbuiltin_declaration_mismatch,
+			      "conflicting types for built-in function %q+D; "
+			      "expected %qT",
+			      newdecl, oldtype)
+		  && header)
+		{
+		  /* Suggest the right header to include as the preferred
+		     solution rather than the spelling of the declaration.  */
+		  rich_location richloc (line_table, loc);
+		  maybe_add_include_fixit (&richloc, header, true);
+		  inform (&richloc,
+			  "%qD is declared in header %qs", olddecl, header);
+		}
 	      /* If types don't match for a built-in, throw away the
 		 built-in.  No point in calling locate_old_decl here, it
 		 won't print anything.  */
-	      warning (OPT_Wbuiltin_declaration_mismatch,
-		       "conflicting types for built-in function %q+D",
-		       newdecl);
 	      return false;
 	    }
 	}
@@ -2026,15 +2040,33 @@ diagnose_mismatched_decls (tree newdecl, tree olddecl,
 	 can't validate the argument list) the built-in definition is
 	 overridden, but optionally warn this was a bad choice of name.  */
       if (fndecl_built_in_p (olddecl)
-	  && !C_DECL_DECLARED_BUILTIN (olddecl)
-	  && (!TREE_PUBLIC (newdecl)
-	      || (DECL_INITIAL (newdecl)
-		  && !prototype_p (TREE_TYPE (newdecl)))))
+	  && !C_DECL_DECLARED_BUILTIN (olddecl))
 	{
-	  warning (OPT_Wshadow, "declaration of %q+D shadows "
-		   "a built-in function", newdecl);
-	  /* Discard the old built-in function.  */
-	  return false;
+	  if (!TREE_PUBLIC (newdecl)
+	      || (DECL_INITIAL (newdecl)
+		  && !prototype_p (TREE_TYPE (newdecl))))
+	    {
+	      warning_at (DECL_SOURCE_LOCATION (newdecl),
+			  OPT_Wshadow, "declaration of %qD shadows "
+			  "a built-in function", newdecl);
+	      /* Discard the old built-in function.  */
+	      return false;
+	    }
+
+	  if (!prototype_p (TREE_TYPE (newdecl)))
+	    {
+	      /* Set for built-ins that take no arguments.  */
+	      bool func_void_args = false;
+	      if (tree at = TYPE_ARG_TYPES (oldtype))
+		func_void_args = VOID_TYPE_P (TREE_VALUE (at));
+
+	      if (extra_warnings && !func_void_args)
+		warning_at (DECL_SOURCE_LOCATION (newdecl),
+			    OPT_Wbuiltin_declaration_mismatch,
+			    "declaration of built-in function %qD without "
+			    "a prototype; expected %qT",
+			    newdecl, TREE_TYPE (olddecl));
+	    }
 	}
 
       if (DECL_INITIAL (newdecl))
diff --git a/gcc/c/c-typeck.c b/gcc/c/c-typeck.c
index 9d09b8d..45331be 100644
--- a/gcc/c/c-typeck.c
+++ b/gcc/c/c-typeck.c
@@ -97,7 +97,8 @@ static int convert_arguments (location_t, vec<location_t>, tree,
 			      tree);
 static tree pointer_diff (location_t, tree, tree, tree *);
 static tree convert_for_assignment (location_t, location_t, tree, tree, tree,
-				    enum impl_conv, bool, tree, tree, int);
+				    enum impl_conv, bool, tree, tree, int,
+				    int = 0);
 static tree valid_compound_expr_initializer (tree, tree);
 static void push_string (const char *);
 static void push_member_name (tree);
@@ -3183,6 +3184,188 @@ c_build_function_call_vec (location_t loc, vec<location_t> arg_loc,
   return build_function_call_vec (loc, arg_loc, function, params, origtypes);
 }
 
+/* Helper for convert_arguments called to convert the VALue of argument
+   number ARGNUM from ORIGTYPE to the corresponding parameter number
+   PARMNUL and TYPE.  */
+
+static tree
+convert_argument (location_t ploc, tree function, tree fundecl,
+		  tree type, tree origtype, tree val, bool npc,
+		  tree rname, int parmnum, int argnum,
+		  bool excess_precision, int warnopt)
+{
+  tree valtype = TREE_TYPE (val);
+
+  tree parmval;
+
+  /* Formal parm type is specified by a function prototype.  */
+
+  if (type == error_mark_node || !COMPLETE_TYPE_P (type))
+    {
+      error_at (ploc, "type of formal parameter %d is incomplete",
+		parmnum + 1);
+      parmval = val;
+    }
+  else
+    {
+      /* Optionally warn about conversions that differ from the default
+	 conversions.  */
+      if (warn_traditional_conversion || warn_traditional)
+	{
+	  unsigned int formal_prec = TYPE_PRECISION (type);
+
+	  if (INTEGRAL_TYPE_P (type)
+	      && TREE_CODE (valtype) == REAL_TYPE)
+	    warning_at (ploc, OPT_Wtraditional_conversion,
+			"passing argument %d of %qE as integer rather "
+			"than floating due to prototype",
+			argnum, rname);
+	  if (INTEGRAL_TYPE_P (type)
+	      && TREE_CODE (valtype) == COMPLEX_TYPE)
+	    warning_at (ploc, OPT_Wtraditional_conversion,
+			"passing argument %d of %qE as integer rather "
+			"than complex due to prototype",
+			argnum, rname);
+	  else if (TREE_CODE (type) == COMPLEX_TYPE
+		   && TREE_CODE (valtype) == REAL_TYPE)
+	    warning_at (ploc, OPT_Wtraditional_conversion,
+			"passing argument %d of %qE as complex rather "
+			"than floating due to prototype",
+			argnum, rname);
+	  else if (TREE_CODE (type) == REAL_TYPE
+		   && INTEGRAL_TYPE_P (valtype))
+	    warning_at (ploc, OPT_Wtraditional_conversion,
+			"passing argument %d of %qE as floating rather "
+			"than integer due to prototype",
+			argnum, rname);
+	  else if (TREE_CODE (type) == COMPLEX_TYPE
+		   && INTEGRAL_TYPE_P (valtype))
+	    warning_at (ploc, OPT_Wtraditional_conversion,
+			"passing argument %d of %qE as complex rather "
+			"than integer due to prototype",
+			argnum, rname);
+	  else if (TREE_CODE (type) == REAL_TYPE
+		   && TREE_CODE (valtype) == COMPLEX_TYPE)
+	    warning_at (ploc, OPT_Wtraditional_conversion,
+			"passing argument %d of %qE as floating rather "
+			"than complex due to prototype",
+			argnum, rname);
+	  /* ??? At some point, messages should be written about
+	     conversions between complex types, but that's too messy
+	     to do now.  */
+	  else if (TREE_CODE (type) == REAL_TYPE
+		   && TREE_CODE (valtype) == REAL_TYPE)
+	    {
+	      /* Warn if any argument is passed as `float',
+		 since without a prototype it would be `double'.  */
+	      if (formal_prec == TYPE_PRECISION (float_type_node)
+		  && type != dfloat32_type_node)
+		warning_at (ploc, 0,
+			    "passing argument %d of %qE as %<float%> "
+			    "rather than %<double%> due to prototype",
+			    argnum, rname);
+
+	      /* Warn if mismatch between argument and prototype
+		 for decimal float types.  Warn of conversions with
+		 binary float types and of precision narrowing due to
+		 prototype. */
+	      else if (type != valtype
+		       && (type == dfloat32_type_node
+			   || type == dfloat64_type_node
+			   || type == dfloat128_type_node
+			   || valtype == dfloat32_type_node
+			   || valtype == dfloat64_type_node
+			   || valtype == dfloat128_type_node)
+		       && (formal_prec
+			   <= TYPE_PRECISION (valtype)
+			   || (type == dfloat128_type_node
+			       && (valtype
+				   != dfloat64_type_node
+				   && (valtype
+				       != dfloat32_type_node)))
+			   || (type == dfloat64_type_node
+			       && (valtype
+				   != dfloat32_type_node))))
+		warning_at (ploc, 0,
+			    "passing argument %d of %qE as %qT "
+			    "rather than %qT due to prototype",
+			    argnum, rname, type, valtype);
+
+	    }
+	  /* Detect integer changing in width or signedness.
+	     These warnings are only activated with
+	     -Wtraditional-conversion, not with -Wtraditional.  */
+	  else if (warn_traditional_conversion
+		   && INTEGRAL_TYPE_P (type)
+		   && INTEGRAL_TYPE_P (valtype))
+	    {
+	      tree would_have_been = default_conversion (val);
+	      tree type1 = TREE_TYPE (would_have_been);
+
+	      if (val == error_mark_node)
+		/* VAL could have been of incomplete type.  */;
+	      else if (TREE_CODE (type) == ENUMERAL_TYPE
+		       && (TYPE_MAIN_VARIANT (type)
+			   == TYPE_MAIN_VARIANT (valtype)))
+		/* No warning if function asks for enum
+		   and the actual arg is that enum type.  */
+		;
+	      else if (formal_prec != TYPE_PRECISION (type1))
+		warning_at (ploc, OPT_Wtraditional_conversion,
+			    "passing argument %d of %qE "
+			    "with different width due to prototype",
+			    argnum, rname);
+	      else if (TYPE_UNSIGNED (type) == TYPE_UNSIGNED (type1))
+		;
+	      /* Don't complain if the formal parameter type
+		 is an enum, because we can't tell now whether
+		 the value was an enum--even the same enum.  */
+	      else if (TREE_CODE (type) == ENUMERAL_TYPE)
+		;
+	      else if (TREE_CODE (val) == INTEGER_CST
+		       && int_fits_type_p (val, type))
+		/* Change in signedness doesn't matter
+		   if a constant value is unaffected.  */
+		;
+	      /* If the value is extended from a narrower
+		 unsigned type, it doesn't matter whether we
+		 pass it as signed or unsigned; the value
+		 certainly is the same either way.  */
+	      else if (TYPE_PRECISION (valtype) < TYPE_PRECISION (type)
+		       && TYPE_UNSIGNED (valtype))
+		;
+	      else if (TYPE_UNSIGNED (type))
+		warning_at (ploc, OPT_Wtraditional_conversion,
+			    "passing argument %d of %qE "
+			    "as unsigned due to prototype",
+			    argnum, rname);
+	      else
+		warning_at (ploc, OPT_Wtraditional_conversion,
+			    "passing argument %d of %qE "
+			    "as signed due to prototype",
+			    argnum, rname);
+	    }
+	}
+
+      /* Possibly restore an EXCESS_PRECISION_EXPR for the
+	 sake of better warnings from convert_and_check.  */
+      if (excess_precision)
+	val = build1 (EXCESS_PRECISION_EXPR, valtype, val);
+
+      parmval = convert_for_assignment (ploc, ploc, type,
+					val, origtype, ic_argpass,
+					npc, fundecl, function,
+					parmnum + 1, warnopt);
+
+      if (targetm.calls.promote_prototypes (fundecl ? TREE_TYPE (fundecl) : 0)
+	  && INTEGRAL_TYPE_P (type)
+	  && (TYPE_PRECISION (type) < TYPE_PRECISION (integer_type_node)))
+	parmval = default_conversion (parmval);
+    }
+
+  return parmval;
+}
+
 /* Convert the argument expressions in the vector VALUES
    to the types in the list TYPELIST.
 
@@ -3209,7 +3392,6 @@ convert_arguments (location_t loc, vec<location_t> arg_loc, tree typelist,
 		   vec<tree, va_gc> *values, vec<tree, va_gc> *origtypes,
 		   tree function, tree fundecl)
 {
-  tree typetail, val;
   unsigned int parmnum;
   bool error_args = false;
   const bool type_generic = fundecl
@@ -3227,50 +3409,69 @@ convert_arguments (location_t loc, vec<location_t> arg_loc, tree typelist,
   /* Handle an ObjC selector specially for diagnostics.  */
   selector = objc_message_selector ();
 
+  /* For a call to a built-in function declared without a prototype,
+     set to the built-in function's argument list.  */
+  tree builtin_typelist = NULL_TREE;
+
   /* For type-generic built-in functions, determine whether excess
      precision should be removed (classification) or not
      (comparison).  */
-  if (type_generic
+  if (fundecl
       && fndecl_built_in_p (fundecl, BUILT_IN_NORMAL))
     {
-      switch (DECL_FUNCTION_CODE (fundecl))
+      built_in_function code = DECL_FUNCTION_CODE (fundecl);
+      if (C_DECL_BUILTIN_PROTOTYPE (fundecl))
 	{
-	case BUILT_IN_ISFINITE:
-	case BUILT_IN_ISINF:
-	case BUILT_IN_ISINF_SIGN:
-	case BUILT_IN_ISNAN:
-	case BUILT_IN_ISNORMAL:
-	case BUILT_IN_FPCLASSIFY:
-	  type_generic_remove_excess_precision = true;
-	  break;
+	  if (tree bdecl = builtin_decl_implicit (code))
+	    builtin_typelist = TYPE_ARG_TYPES (TREE_TYPE (bdecl));
+	}
 
-	case BUILT_IN_ADD_OVERFLOW_P:
-	case BUILT_IN_SUB_OVERFLOW_P:
-	case BUILT_IN_MUL_OVERFLOW_P:
-	  /* The last argument of these type-generic builtins
-	     should not be promoted.  */
-	  type_generic_overflow_p = true;
-	  break;
+      /* For type-generic built-in functions, determine whether excess
+	 precision should be removed (classification) or not
+	 (comparison).  */
+      if (type_generic)
+	switch (code)
+	  {
+	  case BUILT_IN_ISFINITE:
+	  case BUILT_IN_ISINF:
+	  case BUILT_IN_ISINF_SIGN:
+	  case BUILT_IN_ISNAN:
+	  case BUILT_IN_ISNORMAL:
+	  case BUILT_IN_FPCLASSIFY:
+	    type_generic_remove_excess_precision = true;
+	    break;
 
-	default:
-	  break;
-	}
+	  case BUILT_IN_ADD_OVERFLOW_P:
+	  case BUILT_IN_SUB_OVERFLOW_P:
+	  case BUILT_IN_MUL_OVERFLOW_P:
+	    /* The last argument of these type-generic builtins
+	       should not be promoted.  */
+	    type_generic_overflow_p = true;
+	    break;
+
+	  default:
+	    break;
+	  }
     }
 
   /* Scan the given expressions and types, producing individual
      converted arguments.  */
 
-  for (typetail = typelist, parmnum = 0;
+  tree typetail, builtin_typetail, val;
+  for (typetail = typelist,
+	 builtin_typetail = builtin_typelist,
+	 parmnum = 0;
        values && values->iterate (parmnum, &val);
        ++parmnum)
     {
-      tree type = typetail ? TREE_VALUE (typetail) : 0;
+      tree type = typetail ? TREE_VALUE (typetail) : NULL_TREE;
+      tree builtin_type = (builtin_typetail
+			   ? TREE_VALUE (builtin_typetail) : NULL_TREE);
       tree valtype = TREE_TYPE (val);
       tree rname = function;
       int argnum = parmnum + 1;
       const char *invalid_func_diag;
       bool excess_precision = false;
-      bool npc;
       tree parmval;
       /* Some __atomic_* builtins have additional hidden argument at
 	 position 0.  */
@@ -3289,13 +3490,25 @@ convert_arguments (location_t loc, vec<location_t> arg_loc, tree typelist,
 	  return error_args ? -1 : (int) parmnum;
 	}
 
+      if (builtin_type == void_type_node)
+	{
+	  warning_at (loc, OPT_Wbuiltin_declaration_mismatch,
+		      "too many arguments to built-in function %qE "
+		      "expecting %d",
+		      function, parmnum);
+
+	  inform_declaration (fundecl);
+	  builtin_typetail = NULL_TREE;
+	}
+
       if (selector && argnum > 2)
 	{
 	  rname = selector;
 	  argnum -= 2;
 	}
 
-      npc = null_pointer_constant_p (val);
+      /* Determine if VAL is a null pointer constant before folding it.  */
+      bool npc = null_pointer_constant_p (val);
 
       /* If there is excess precision and a prototype, convert once to
 	 the required type rather than converting via the semantic
@@ -3340,172 +3553,10 @@ convert_arguments (location_t loc, vec<location_t> arg_loc, tree typelist,
 
       if (type != NULL_TREE)
 	{
-	  /* Formal parm type is specified by a function prototype.  */
-
-	  if (type == error_mark_node || !COMPLETE_TYPE_P (type))
-	    {
-	      error_at (ploc, "type of formal parameter %d is incomplete",
-			parmnum + 1);
-	      parmval = val;
-	    }
-	  else
-	    {
-	      tree origtype;
-
-	      /* Optionally warn about conversions that
-		 differ from the default conversions.  */
-	      if (warn_traditional_conversion || warn_traditional)
-		{
-		  unsigned int formal_prec = TYPE_PRECISION (type);
-
-		  if (INTEGRAL_TYPE_P (type)
-		      && TREE_CODE (valtype) == REAL_TYPE)
-		    warning_at (ploc, OPT_Wtraditional_conversion,
-				"passing argument %d of %qE as integer rather "
-				"than floating due to prototype",
-				argnum, rname);
-		  if (INTEGRAL_TYPE_P (type)
-		      && TREE_CODE (valtype) == COMPLEX_TYPE)
-		    warning_at (ploc, OPT_Wtraditional_conversion,
-				"passing argument %d of %qE as integer rather "
-				"than complex due to prototype",
-				argnum, rname);
-		  else if (TREE_CODE (type) == COMPLEX_TYPE
-			   && TREE_CODE (valtype) == REAL_TYPE)
-		    warning_at (ploc, OPT_Wtraditional_conversion,
-				"passing argument %d of %qE as complex rather "
-				"than floating due to prototype",
-				argnum, rname);
-		  else if (TREE_CODE (type) == REAL_TYPE
-			   && INTEGRAL_TYPE_P (valtype))
-		    warning_at (ploc, OPT_Wtraditional_conversion,
-				"passing argument %d of %qE as floating rather "
-				"than integer due to prototype",
-				argnum, rname);
-		  else if (TREE_CODE (type) == COMPLEX_TYPE
-			   && INTEGRAL_TYPE_P (valtype))
-		    warning_at (ploc, OPT_Wtraditional_conversion,
-				"passing argument %d of %qE as complex rather "
-				"than integer due to prototype",
-				argnum, rname);
-		  else if (TREE_CODE (type) == REAL_TYPE
-			   && TREE_CODE (valtype) == COMPLEX_TYPE)
-		    warning_at (ploc, OPT_Wtraditional_conversion,
-				"passing argument %d of %qE as floating rather "
-				"than complex due to prototype",
-				argnum, rname);
-		  /* ??? At some point, messages should be written about
-		     conversions between complex types, but that's too messy
-		     to do now.  */
-		  else if (TREE_CODE (type) == REAL_TYPE
-			   && TREE_CODE (valtype) == REAL_TYPE)
-		    {
-		      /* Warn if any argument is passed as `float',
-			 since without a prototype it would be `double'.  */
-		      if (formal_prec == TYPE_PRECISION (float_type_node)
-			  && type != dfloat32_type_node)
-			warning_at (ploc, 0,
-				    "passing argument %d of %qE as %<float%> "
-				    "rather than %<double%> due to prototype",
-				    argnum, rname);
-
-		      /* Warn if mismatch between argument and prototype
-			 for decimal float types.  Warn of conversions with
-			 binary float types and of precision narrowing due to
-			 prototype. */
- 		      else if (type != valtype
-			       && (type == dfloat32_type_node
-				   || type == dfloat64_type_node
-				   || type == dfloat128_type_node
-				   || valtype == dfloat32_type_node
-				   || valtype == dfloat64_type_node
-				   || valtype == dfloat128_type_node)
-			       && (formal_prec
-				   <= TYPE_PRECISION (valtype)
-				   || (type == dfloat128_type_node
-				       && (valtype
-					   != dfloat64_type_node
-					   && (valtype
-					       != dfloat32_type_node)))
-				   || (type == dfloat64_type_node
-				       && (valtype
-					   != dfloat32_type_node))))
-			warning_at (ploc, 0,
-				    "passing argument %d of %qE as %qT "
-				    "rather than %qT due to prototype",
-				    argnum, rname, type, valtype);
-
-		    }
-		  /* Detect integer changing in width or signedness.
-		     These warnings are only activated with
-		     -Wtraditional-conversion, not with -Wtraditional.  */
-		  else if (warn_traditional_conversion
-			   && INTEGRAL_TYPE_P (type)
-			   && INTEGRAL_TYPE_P (valtype))
-		    {
-		      tree would_have_been = default_conversion (val);
-		      tree type1 = TREE_TYPE (would_have_been);
-
-		      if (val == error_mark_node)
-			/* VAL could have been of incomplete type.  */;
-		      else if (TREE_CODE (type) == ENUMERAL_TYPE
-			       && (TYPE_MAIN_VARIANT (type)
-				   == TYPE_MAIN_VARIANT (valtype)))
-			/* No warning if function asks for enum
-			   and the actual arg is that enum type.  */
-			;
-		      else if (formal_prec != TYPE_PRECISION (type1))
-			warning_at (ploc, OPT_Wtraditional_conversion,
-				    "passing argument %d of %qE "
-				    "with different width due to prototype",
-				    argnum, rname);
-		      else if (TYPE_UNSIGNED (type) == TYPE_UNSIGNED (type1))
-			;
-		      /* Don't complain if the formal parameter type
-			 is an enum, because we can't tell now whether
-			 the value was an enum--even the same enum.  */
-		      else if (TREE_CODE (type) == ENUMERAL_TYPE)
-			;
-		      else if (TREE_CODE (val) == INTEGER_CST
-			       && int_fits_type_p (val, type))
-			/* Change in signedness doesn't matter
-			   if a constant value is unaffected.  */
-			;
-		      /* If the value is extended from a narrower
-			 unsigned type, it doesn't matter whether we
-			 pass it as signed or unsigned; the value
-			 certainly is the same either way.  */
-		      else if (TYPE_PRECISION (valtype) < TYPE_PRECISION (type)
-			       && TYPE_UNSIGNED (valtype))
-			;
-		      else if (TYPE_UNSIGNED (type))
-			warning_at (ploc, OPT_Wtraditional_conversion,
-				    "passing argument %d of %qE "
-				    "as unsigned due to prototype",
-				    argnum, rname);
-		      else
-			warning_at (ploc, OPT_Wtraditional_conversion,
-				    "passing argument %d of %qE "
-				    "as signed due to prototype",
-				    argnum, rname);
-		    }
-		}
-
-	      /* Possibly restore an EXCESS_PRECISION_EXPR for the
-		 sake of better warnings from convert_and_check.  */
-	      if (excess_precision)
-		val = build1 (EXCESS_PRECISION_EXPR, valtype, val);
-	      origtype = (!origtypes) ? NULL_TREE : (*origtypes)[parmnum];
-	      parmval = convert_for_assignment (loc, ploc, type,
-						val, origtype, ic_argpass,
-						npc, fundecl, function,
-						parmnum + 1);
-
-	      if (targetm.calls.promote_prototypes (fundecl ? TREE_TYPE (fundecl) : 0)
-		  && INTEGRAL_TYPE_P (type)
-		  && (TYPE_PRECISION (type) < TYPE_PRECISION (integer_type_node)))
-		parmval = default_conversion (parmval);
-	    }
+	  tree origtype = (!origtypes) ? NULL_TREE : (*origtypes)[parmnum];
+	  parmval = convert_argument (ploc, function, fundecl, type, origtype,
+				      val, npc, rname, parmnum, argnum,
+				      excess_precision, 0);
 	}
       else if (promote_float_arg)
         {
@@ -3547,8 +3598,24 @@ convert_arguments (location_t loc, vec<location_t> arg_loc, tree typelist,
       if (parmval == error_mark_node)
 	error_args = true;
 
+      if (!type && builtin_type && TREE_CODE (builtin_type) != VOID_TYPE)
+	{
+	  /* For a call to a built-in function declared without a prototype,
+	     perform the coversions from the argument to the expected type
+	     but issue warnings rather than errors for any mismatches.
+	     Ignore the converted argument and use the PARMVAL obtained
+	     above by applying default coversions instead.  */
+	  tree origtype = (!origtypes) ? NULL_TREE : (*origtypes)[parmnum];
+	  convert_argument (ploc, function, fundecl, builtin_type, origtype,
+			    val, npc, rname, parmnum, argnum, excess_precision,
+			    OPT_Wbuiltin_declaration_mismatch);
+	}
+
       if (typetail)
 	typetail = TREE_CHAIN (typetail);
+
+      if (builtin_typetail)
+	builtin_typetail = TREE_CHAIN (builtin_typetail);
     }
 
   gcc_assert (parmnum == vec_safe_length (values));
@@ -3560,6 +3627,18 @@ convert_arguments (location_t loc, vec<location_t> arg_loc, tree typelist,
       return -1;
     }
 
+  if (builtin_typetail && TREE_VALUE (builtin_typetail) != void_type_node)
+    {
+      unsigned nargs = parmnum;
+      for (tree t = builtin_typetail; t; t = TREE_CHAIN (t))
+	++nargs;
+
+      warning_at (loc, OPT_Wbuiltin_declaration_mismatch,
+		  "too few arguments to built-in function %qE expecting %u",
+		  function, nargs - 1);
+      inform_declaration (fundecl);
+    }
+
   return error_args ? -1 : (int) parmnum;
 }
 
@@ -4922,6 +5001,40 @@ ep_convert_and_check (location_t loc, tree type, tree expr,
   return convert (type, expr);
 }
 
+/* If EXPR refers to a built-in declared without a prototype returns
+   the actual type of the built-in and, if non-null, set *BLTIN to
+   a pointer to the built-in.  Otherwise return the type of EXPR
+   and clear *BLTIN if non-null. */
+
+static tree
+type_or_builtin_type (tree expr, tree *bltin = NULL)
+{
+  tree dummy;
+  if (!bltin)
+    bltin = &dummy;
+
+  *bltin = NULL_TREE;
+
+  tree type = TREE_TYPE (expr);
+  if (TREE_CODE (expr) != ADDR_EXPR)
+    return type;
+
+  tree oper = TREE_OPERAND (expr, 0);
+  if (!DECL_P (oper)
+      || TREE_CODE (oper) != FUNCTION_DECL
+      || !fndecl_built_in_p (oper, BUILT_IN_NORMAL))
+    return type;
+
+  built_in_function code = DECL_FUNCTION_CODE (oper);
+  if (!C_DECL_BUILTIN_PROTOTYPE (oper))
+    return type;
+
+  if ((*bltin = builtin_decl_implicit (code)))
+    type = build_pointer_type (TREE_TYPE (*bltin));
+
+  return type;
+}
+
 /* Build and return a conditional expression IFEXP ? OP1 : OP2.  If
    IFEXP_BCP then the condition is a call to __builtin_constant_p, and
    if folded to an integer constant then the unselected half may
@@ -4966,9 +5079,11 @@ build_conditional_expr (location_t colon_loc, tree ifexp, bool ifexp_bcp,
       || TREE_CODE (TREE_TYPE (op2)) == ERROR_MARK)
     return error_mark_node;
 
-  type1 = TREE_TYPE (op1);
+  tree bltin1 = NULL_TREE;
+  tree bltin2 = NULL_TREE;
+  type1 = type_or_builtin_type (op1, &bltin1);
   code1 = TREE_CODE (type1);
-  type2 = TREE_TYPE (op2);
+  type2 = type_or_builtin_type (op2, &bltin2);
   code2 = TREE_CODE (type2);
 
   if (code1 == POINTER_TYPE && reject_gcc_builtin (op1))
@@ -5206,9 +5321,14 @@ build_conditional_expr (location_t colon_loc, tree ifexp, bool ifexp_bcp,
       else
 	{
 	  int qual = ENCODE_QUAL_ADDR_SPACE (as_common);
-
-	  pedwarn (colon_loc, 0,
-		   "pointer type mismatch in conditional expression");
+	  if (bltin1 && bltin2)	
+	    warning_at (colon_loc, OPT_Wincompatible_pointer_types,
+			"pointer type mismatch between %qT and %qT "
+			"of %qD and %qD in conditional expression",
+			type1, type2, bltin1, bltin2);
+	  else
+	    pedwarn (colon_loc, 0,
+		     "pointer type mismatch in conditional expression");
 	  result_type = build_pointer_type
 			  (build_qualified_type (void_type_node, qual));
 	}
@@ -6322,6 +6442,46 @@ inform_for_arg (tree fundecl, location_t ploc, int parmnum,
 	  expected_type, actual_type);
 }
 
+/* Issue a warning when an argument of ARGTYPE is passed to a built-in
+   function FUNDECL declared without prototype to parameter PARMNUM of
+   PARMTYPE when ARGTYPE does not promote to PARMTYPE.  */
+
+static void
+maybe_warn_builtin_no_proto_arg (location_t loc, tree fundecl, int parmnum,
+				 tree parmtype, tree argtype)
+{
+  tree_code parmcode = TREE_CODE (parmtype);
+  tree_code argcode = TREE_CODE (argtype);
+  tree promoted = c_type_promotes_to (argtype);
+
+  /* Avoid warning for enum arguments that promote to an integer type
+     of the same size/mode.  */
+  if (parmcode == INTEGER_TYPE
+      && argcode == ENUMERAL_TYPE
+      && TYPE_MODE (parmtype) == TYPE_MODE (argtype))
+    return;
+
+  if (parmcode == argcode
+      && TYPE_MAIN_VARIANT (parmtype) == TYPE_MAIN_VARIANT (promoted))
+    return;
+
+  /* This diagnoses even signed/unsigned mismatches.  Those might be
+     safe in many cases but GCC may emit suboptimal code for them so
+     warning on those cases drives efficiency improvements.  */
+  if (warning_at (loc, OPT_Wbuiltin_declaration_mismatch,
+		  TYPE_MAIN_VARIANT (promoted) == argtype
+		  ? G_("%qD argument %d type is %qT where %qT is expected "
+		       "in a call to built-in function declared without "
+		       "prototype")
+		  : G_("%qD argument %d promotes to %qT where %qT is expected "
+		       "in a call to built-in function declared without "
+		       "prototype"),
+		  fundecl, parmnum, promoted, parmtype))
+    inform (DECL_SOURCE_LOCATION (fundecl),
+	    "built-in %qD declared here",
+	    fundecl);
+}
+
 /* Convert value RHS to type TYPE as preparation for an assignment to
    an lvalue of type TYPE.  If ORIGTYPE is not NULL_TREE, it is the
    original type of RHS; this differs from TREE_TYPE (RHS) for enum
@@ -6346,13 +6506,16 @@ inform_for_arg (tree fundecl, location_t ploc, int parmnum,
 	    ^
 
    FUNCTION is a tree for the function being called.
-   PARMNUM is the number of the argument, for printing in error messages.  */
+   PARMNUM is the number of the argument, for printing in error messages.
+   WARNOPT may be set to a warning option to issue the corresponding warning
+   rather than an error for invalid conversions.  Used for calls to built-in
+   functions declared without a prototype.  */
 
 static tree
 convert_for_assignment (location_t location, location_t expr_loc, tree type,
 			tree rhs, tree origtype, enum impl_conv errtype,
 			bool null_pointer_constant, tree fundecl,
-			tree function, int parmnum)
+			tree function, int parmnum, int warnopt /* = 0 */)
 {
   enum tree_code codel = TREE_CODE (type);
   tree orig_rhs = rhs;
@@ -6550,7 +6713,11 @@ convert_for_assignment (location_t location, location_t expr_loc, tree type,
 	 an unprototyped function, it is compile-time undefined;
 	 making it a constraint in that case was rejected in
 	 DR#252.  */
-      error_at (location, "void value not ignored as it ought to be");
+      const char msg[] = "void value not ignored as it ought to be";
+      if (warnopt)
+	warning_at (location, warnopt, msg);
+      else
+	error_at (location, msg);
       return error_mark_node;
     }
   rhs = require_complete_type (location, rhs);
@@ -6566,7 +6733,11 @@ convert_for_assignment (location_t location, location_t expr_loc, tree type,
     {
       if (!lvalue_p (rhs))
 	{
-	  error_at (location, "cannot pass rvalue to reference parameter");
+	  const char msg[] = "cannot pass rvalue to reference parameter";
+	  if (warnopt)
+	    warning_at (location, warnopt, msg);
+	  else
+	    error_at (location, msg);
 	  return error_mark_node;
 	}
       if (!c_mark_addressable (rhs))
@@ -6578,7 +6749,7 @@ convert_for_assignment (location_t location, location_t expr_loc, tree type,
 				    build_pointer_type (TREE_TYPE (type)),
 				    rhs, origtype, errtype,
 				    null_pointer_constant, fundecl, function,
-				    parmnum);
+				    parmnum, warnopt);
       if (rhs == error_mark_node)
 	return error_mark_node;
 
@@ -6600,15 +6771,18 @@ convert_for_assignment (location_t location, location_t expr_loc, tree type,
 	       || coder == ENUMERAL_TYPE || coder == COMPLEX_TYPE
 	       || coder == BOOLEAN_TYPE))
     {
-      tree ret;
+      if (warnopt && errtype == ic_argpass)
+	maybe_warn_builtin_no_proto_arg (expr_loc, fundecl, parmnum, type,
+					 rhstype);
+
       bool save = in_late_binary_op;
       if (codel == BOOLEAN_TYPE || codel == COMPLEX_TYPE
 	  || (coder == REAL_TYPE
 	      && (codel == INTEGER_TYPE || codel == ENUMERAL_TYPE)
 	      && sanitize_flags_p (SANITIZE_FLOAT_CAST)))
 	in_late_binary_op = true;
-      ret = convert_and_check (expr_loc != UNKNOWN_LOCATION
-			       ? expr_loc : location, type, orig_rhs);
+      tree ret = convert_and_check (expr_loc != UNKNOWN_LOCATION
+				    ? expr_loc : location, type, orig_rhs);
       in_late_binary_op = save;
       return ret;
     }
@@ -6742,6 +6916,12 @@ convert_for_assignment (location_t location, location_t expr_loc, tree type,
   else if ((codel == POINTER_TYPE || codel == REFERENCE_TYPE)
 	   && (coder == codel))
     {
+      /* If RHS refers to a built-in declared without a prototype
+	 BLTIN is the declaration of the built-in with a prototype
+	 and RHSTYPE is set to the actual type of the built-in.  */
+      tree bltin;
+      rhstype = type_or_builtin_type (rhs, &bltin);
+
       tree ttl = TREE_TYPE (type);
       tree ttr = TREE_TYPE (rhstype);
       tree mvl = ttl;
@@ -6805,21 +6985,45 @@ convert_for_assignment (location_t location, location_t expr_loc, tree type,
 	  switch (errtype)
 	    {
 	    case ic_argpass:
-	      error_at (expr_loc, "passing argument %d of %qE from pointer to "
-			"non-enclosed address space", parmnum, rname);
+	      {
+		const char msg[] = G_("passing argument %d of %qE from "
+				      "pointer to non-enclosed address space");
+		if (warnopt)
+		  warning_at (expr_loc, warnopt, msg, parmnum, rname);
+		else
+		  error_at (expr_loc, msg, parmnum, rname);
 	      break;
+	      }
 	    case ic_assign:
-	      error_at (location, "assignment from pointer to "
-			"non-enclosed address space");
-	      break;
+	      {
+		const char msg[] = G_("assignment from pointer to "
+				      "non-enclosed address space");
+		if (warnopt)
+		  warning_at (location, warnopt, msg);
+		else
+		  error_at (location, msg);
+		break;
+	      }
 	    case ic_init:
-	      error_at (location, "initialization from pointer to "
-			"non-enclosed address space");
-	      break;
+	      {
+		const char msg[] = G_("initialization from pointer to "
+				      "non-enclosed address space");
+		if (warnopt)
+		  warning_at (location, warnopt, msg);
+		else
+		  error_at (location, msg);
+		break;
+	      }
 	    case ic_return:
-	      error_at (location, "return from pointer to "
-			"non-enclosed address space");
-	      break;
+	      {
+		const char msg[] = G_("return from pointer to "
+				      "non-enclosed address space");
+		if (warnopt)
+		  warning_at (location, warnopt, msg);
+		else
+		  error_at (location, msg);
+		break;
+	      }
 	    default:
 	      gcc_unreachable ();
 	    }
@@ -7017,19 +7221,38 @@ convert_for_assignment (location_t location, location_t expr_loc, tree type,
 	      }
 	      break;
 	    case ic_assign:
-	      pedwarn (location, OPT_Wincompatible_pointer_types,
-		       "assignment to %qT from incompatible pointer type %qT",
-		       type, rhstype);
+	      if (bltin)
+		pedwarn (location, OPT_Wincompatible_pointer_types,
+			 "assignment to %qT from pointer to "
+			 "%qD with incompatible type %qT",
+			 type, bltin, rhstype);
+	      else
+		pedwarn (location, OPT_Wincompatible_pointer_types,
+			 "assignment to %qT from incompatible pointer type %qT",
+			 type, rhstype);
 	      break;
 	    case ic_init:
-	      pedwarn_init (location, OPT_Wincompatible_pointer_types,
-			    "initialization of %qT from incompatible pointer "
-			    "type %qT", type, rhstype);
+	      if (bltin)
+		pedwarn_init (location, OPT_Wincompatible_pointer_types,
+			      "initialization of %qT from pointer to "
+			      "%qD with incompatible type %qT",
+			      type, bltin, rhstype);
+	      else
+		pedwarn_init (location, OPT_Wincompatible_pointer_types,
+			      "initialization of %qT from incompatible "
+			      "pointer type %qT",
+			      type, rhstype);
 	      break;
 	    case ic_return:
-	      pedwarn (location, OPT_Wincompatible_pointer_types,
-		       "returning %qT from a function with incompatible "
-		       "return type %qT", rhstype, type);
+	      if (bltin)
+		pedwarn (location, OPT_Wincompatible_pointer_types,
+			 "returning pointer to %qD of type %qT from "
+			 "a function with incompatible type %qT",
+			 bltin, rhstype, type);
+	      else
+		pedwarn (location, OPT_Wincompatible_pointer_types,
+			 "returning %qT from a function with incompatible "
+			 "return type %qT", rhstype, type);
 	      break;
 	    default:
 	      gcc_unreachable ();
@@ -7042,7 +7265,11 @@ convert_for_assignment (location_t location, location_t expr_loc, tree type,
     {
       /* ??? This should not be an error when inlining calls to
 	 unprototyped functions.  */
-      error_at (location, "invalid use of non-lvalue array");
+      const char msg[] = "invalid use of non-lvalue array";
+      if (warnopt)
+	warning_at (location, warnopt, msg);
+      else
+	error_at (location, msg);
       return error_mark_node;
     }
   else if (codel == POINTER_TYPE && coder == INTEGER_TYPE)
@@ -7138,25 +7365,44 @@ convert_for_assignment (location_t location, location_t expr_loc, tree type,
 	auto_diagnostic_group d;
 	range_label_for_type_mismatch rhs_label (rhstype, type);
 	gcc_rich_location richloc (expr_loc, &rhs_label);
-	error_at (&richloc, "incompatible type for argument %d of %qE", parmnum,
-		  rname);
+	const char msg[] = G_("incompatible type for argument %d of %qE");
+	if (warnopt)
+	  warning_at (expr_loc, warnopt, msg, parmnum, rname);
+	else
+	  error_at (&richloc, msg, parmnum, rname);
 	inform_for_arg (fundecl, expr_loc, parmnum, type, rhstype);
       }
       break;
     case ic_assign:
-      error_at (location, "incompatible types when assigning to type %qT from "
-		"type %qT", type, rhstype);
-      break;
+      {
+	const char msg[]
+	  = G_("incompatible types when assigning to type %qT from type %qT");
+	if (warnopt)
+	  warning_at (expr_loc, 0, msg, type, rhstype);
+	else
+	  error_at (expr_loc, msg, type, rhstype);
+	break;
+      }
     case ic_init:
-      error_at (location,
-		"incompatible types when initializing type %qT using type %qT",
-		type, rhstype);
-      break;
+      {
+	const char msg[]
+	  = G_("incompatible types when initializing type %qT using type %qT");
+	if (warnopt)
+	  warning_at (location, 0, msg, type, rhstype);
+	else
+	  error_at (location, msg, type, rhstype);
+	break;
+      }
     case ic_return:
-      error_at (location,
-		"incompatible types when returning type %qT but %qT was "
-		"expected", rhstype, type);
-      break;
+      {
+	const char msg[]
+	  = G_("incompatible types when returning type %qT but %qT was expected");
+	if (warnopt)
+	  warning_at (location, 0, msg, rhstype, type);
+	else
+	  error_at (location, msg, rhstype, type);
+	break;
+      }
     default:
       gcc_unreachable ();
     }
diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
index 284594d..45cb222 100644
--- a/gcc/doc/invoke.texi
+++ b/gcc/doc/invoke.texi
@@ -6809,9 +6809,26 @@ attributes.
 @item -Wno-builtin-declaration-mismatch
 @opindex Wno-builtin-declaration-mismatch
 @opindex Wbuiltin-declaration-mismatch
-Warn if a built-in function is declared with the wrong signature or 
-as non-function.
-This warning is enabled by default.
+Warn if a built-in function is declared with an incompatible signature
+or as a non-function, or when a built-in function declared with a type
+that does not include a prototype is called with arguments whose promoted
+types do not match those expected by the function.  When @option{-Wextra}
+is specified, also warn when a built-in function that takes arguments is
+declared without a prototype.  The @option{-Wno-builtin-declaration-mismatch}
+warning is enabled by default.  To avoid the warning include the appropriate
+header to bring the prototypes of built-in functions into scope.
+
+For example, the call to @code{memset} below is diagnosed by the warning
+because the function expects a value of type @code{size_t} as its argument
+but the type of @code{32} is @code{int}.  With @option{-Wextra},
+the declaration of the function is diagnosed as well.
+@smallexample
+extern void* memset ();
+void f (void *d)
+@{
+  memset (d, '\0', 32);
+@}
+@end smallexample
 
 @item -Wno-builtin-macro-redefined
 @opindex Wno-builtin-macro-redefined
diff --git a/gcc/testsuite/gcc.dg/20021006-1.c b/gcc/testsuite/gcc.dg/20021006-1.c
index 63cc8a5..92df2c5 100644
--- a/gcc/testsuite/gcc.dg/20021006-1.c
+++ b/gcc/testsuite/gcc.dg/20021006-1.c
@@ -2,7 +2,7 @@
    This testcase was miscompiled on x86-64 due to wrong access to the struct
    members.  */
 
-extern void abort();
+extern void abort(void);
 
 struct A {
   long x;
diff --git a/gcc/testsuite/gcc.dg/Walloca-16.c b/gcc/testsuite/gcc.dg/Walloca-16.c
index 3ee96a9..866594c 100644
--- a/gcc/testsuite/gcc.dg/Walloca-16.c
+++ b/gcc/testsuite/gcc.dg/Walloca-16.c
@@ -4,3 +4,5 @@
 
 void *alloca ();
 __typeof__(alloca ()) a () { return alloca (); }
+
+/* { dg-prune-output "\\\[-Wbuiltin-declaration-mismatch]" } */
diff --git a/gcc/testsuite/gcc.dg/Wbuiltin-declaration-mismatch-2.c b/gcc/testsuite/gcc.dg/Wbuiltin-declaration-mismatch-2.c
new file mode 100644
index 0000000..9d75cd8
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/Wbuiltin-declaration-mismatch-2.c
@@ -0,0 +1,25 @@
+/* PR c/83656 - missing -Wbuiltin-declaration-mismatch on declaration
+   without prototype
+   { dg-do compile }
+   { dg-options "-Wall -Wextra" } */
+
+typedef __SIZE_TYPE__ size_t;
+
+/* Verify that ordinary library built-ins are not diagnosed with -Wextra
+   when they take no arguments (except in cases of return type mismatches).
+   This is in anticipation that C may some day adopt the same syntax as
+   C++ for declaring functions that take no arguments.  */
+
+void abort ();
+
+/* Verify that ordinary library built-ins are diagnosed with -Wextra
+   when they take arguments.  */
+
+void* memcpy ();    /* { dg-warning "declaration of built-in function .memcpy. without a prototype; expected .void \\\*\\\(void \\\*, const void \\\*, \(long \)*unsigned int\\\)." } */
+void* memset ();    /* { dg-warning "declaration of built-in function .memset. without a prototype; expected .void \\\*\\\(void \\\*, int, *\(long \)*unsigned int\\\)." } */
+size_t strlen ();   /* { dg-warning "declaration of built-in function .strlen. without a prototype; expected .\(long \)*unsigned int\\\(const char \\\*\\\)." } */
+
+/* Variadic built-ins are diagnosed even without -Wextra (they are,
+   in fact, diagnosed by default).  */
+int printf ();      /* { dg-warning "\\\[-Wbuiltin-declaration-mismatch]" } */
+int sprintf ();     /* { dg-warning "\\\[-Wbuiltin-declaration-mismatch]" } */
diff --git a/gcc/testsuite/gcc.dg/Wbuiltin-declaration-mismatch-3.c b/gcc/testsuite/gcc.dg/Wbuiltin-declaration-mismatch-3.c
new file mode 100644
index 0000000..f2ec325
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/Wbuiltin-declaration-mismatch-3.c
@@ -0,0 +1,116 @@
+/* PR c/83656 - missing -Wbuiltin-declaration-mismatch on declaration
+   without prototype
+   { dg-do compile }
+   { dg-options "-Wbuiltin-declaration-mismatch" } */
+
+typedef __SIZE_TYPE__ size_t;
+
+/* Built-ins declared without a prototype are not diagnosed by default
+   (without -Wextra) except when their return type doesn't match.  */
+int abort ();       /* { dg-warning "\\\[-Wbuiltin-declaration-mismatch]" } */
+
+/* Built-ins declared without a prototype are not diagnosed without -Wextra.  */
+void exit ();
+void* memcpy ();
+void* memset ();
+
+
+void test_call_abort (void)
+{
+  /* Verify that a valid call to abort() is not diagnosed.  */
+  abort ();
+
+  /* Unfortunately, the incompatible declaration above makes GCC "forget"
+     that abort() is a built-in and so the invalid calls below aren't
+     diagnosed.  The only saving grace is that the invalid declaration
+     that differs in the return type is diagnosed by default. */
+  abort (1);        /* { dg-warning "too many arguments to built-in function .abort. expecting 0" "pr?????" { xfail *-*-* } } */
+
+  abort (1, 2);     /* { dg-warning "too many arguments" "pr?????" { xfail *-*-* } } */
+}
+
+
+void test_call_exit (void)
+{
+  /* Verify that valid calls to exit are not diagnosed.  */
+  exit ('\0');
+  exit (0);
+
+  /* Also verify calls to the built-in.  */
+  __builtin_exit ('\0');
+  __builtin_exit (0);
+  __builtin_exit (0.0);
+
+  exit ();          /* { dg-warning "too few arguments to built-in function 'exit' expecting 1" } */
+
+  exit (1, 2);      /* { dg-warning "too many arguments" } */
+
+  /* Verify that passing incompatible arguments triggers a warning.  */
+  exit ("");        /* { dg-warning "\\\[-Wint-conversion]" } */
+
+  struct S { int i; } s = { 0 };
+  exit (s);         /* { dg-warning "incompatible type for argument 1" } */
+}
+
+
+void test_call_memcpy (void *p, const void *q, size_t n)
+{
+  memcpy (p, q, n);
+
+  memcpy ();        /* { dg-warning "too few arguments to built-in function 'memcpy' expecting 3" } */
+
+  memcpy (p);       /* { dg-warning "too few arguments to built-in function 'memcpy' expecting 3" } */
+
+  memcpy (p, q);     /* { dg-warning "too few arguments to built-in function 'memcpy' expecting 3" } */
+
+  memcpy (q, p, n); /* { dg-warning "\\\[-Wdiscarded-qualifiers]" } */
+
+  memcpy (p, n, q); /* { dg-warning "\\\[-Wint-conversion]" } */
+
+  memcpy (p, q, n, 0); /* { dg-warning "too many arguments to built-in function 'memcpy' expecting 3" } */
+}
+
+
+typedef void* (memcpy_t)(void*, const void*, size_t);
+typedef void* (memset_t)(void*, int, size_t);
+
+void test_init (void)
+{
+  /* Verify that initialization of a pointer by the address of a built-in
+     function of a matching type declared without a prototype doesn't
+     trigger a warning...  */
+  memset_t *pmemset = memset;
+
+  /* ...but initialization by the address of an incompatible built-in
+     does even without -Wextra.  */
+  memcpy_t *pmemcpy = memset;           /* { dg-warning "\\\[-Wincompatible-pointer-types]" } */
+}
+
+
+void test_assign (void)
+{
+  /* Same as above but for assignment.  */
+  memset_t *pmemset;
+  pmemset = memset;
+
+  memcpy_t *pmemcpy;
+  pmemcpy = memset;                     /* { dg-warning "\\\[-Wincompatible-pointer-types]" } */
+}
+
+
+/* Verify that passing built-ins declared without a prototype to
+   functions that expect a pointer to a function of a specific type
+   is diagnosed.  Ditto for return statements.  */
+
+void take_memcpy (memcpy_t*);
+void take_any (int, ...);
+
+memset_t* pass_args (int i)
+{
+  take_memcpy (memcpy);
+  take_memcpy (memset);                 /* { dg-warning "\\\[-Wincompatible-pointer-types]" } */
+
+  take_any (0, i ? memcpy : memset);    /* { dg-warning "\\\[-Wincompatible-pointer-types]" } */
+
+  return memcpy;                        /* { dg-warning "\\\[-Wincompatible-pointer-types]" } */
+}
diff --git a/gcc/testsuite/gcc.dg/Wbuiltin-declaration-mismatch-4.c b/gcc/testsuite/gcc.dg/Wbuiltin-declaration-mismatch-4.c
new file mode 100644
index 0000000..3c82e9d
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/Wbuiltin-declaration-mismatch-4.c
@@ -0,0 +1,152 @@
+/* PR c/83656 - missing -Wbuiltin-declaration-mismatch on declaration
+   without prototype
+   { dg-do compile }
+   { dg-options "-Wbuiltin-declaration-mismatch" } */
+
+typedef __PTRDIFF_TYPE__ ptrdiff_t;
+typedef __SIZE_TYPE__    size_t;
+
+char c;
+signed char sc;
+unsigned char uc;
+short si;
+unsigned short usi;
+int i;
+unsigned ui;
+long li;
+unsigned long uli;
+
+size_t szi;
+typedef size_t SizeType;
+SizeType szti;
+
+ptrdiff_t diffi;
+
+enum E { e0 } e;
+
+float f;
+double d;
+long double ld;
+
+
+/* Verify warnings for undefined calls to built-ins expecting integer
+   arguments.  */
+
+int abs ();         /* { dg-message "built-in .abs. declared here" } */
+
+void test_integer_conversion_abs (void)
+{
+  i = abs (c);
+  i = abs (sc);
+  i = abs (uc);
+
+  i = abs (si);
+  i = abs (usi);
+
+  i = abs (i);
+  i = abs (ui);     /* { dg-warning ".abs. argument 1 type is .unsigned int. where .int. is expected in a call to built-in function declared without prototype" } */
+
+  /* Verify that the same call as above but to the built-in doesn't
+     trigger a warning.  */
+  i = __builtin_abs (ui);
+
+  i = abs (li);     /* { dg-warning ".abs. argument 1 type is .long int. where .int. is expected in a call to built-in function declared without prototype" } */
+  i = abs (uli);    /* { dg-warning ".abs. argument 1 type is .long unsigned int. where .int. is expected in a call to built-in function declared without prototype" } */
+
+  i = abs (e0);
+  i = abs (e);
+
+  i = abs (-1.0);   /* { dg-warning ".abs. argument 1 type is .double. where .int. is expected in a call to built-in function declared without prototype" } */
+  i = abs (f);      /* { dg-warning ".abs. argument 1 promotes to .double. where .int. is expected in a call to built-in function declared without prototype" } */
+  i = abs (ld);     /* { dg-warning ".abs. argument 1 type is .long double. where .int. is expected in a call to built-in function declared without prototype" } */
+
+  /* Verify that the same call as above but to the built-in doesn't
+     trigger a warning.  */
+  i = __builtin_abs (ld);
+}
+
+
+extern void* memset ();
+
+void test_integer_conversion_memset (void *d)
+{
+  memset (d, 0, sizeof (int));
+  memset (d, '\0', szi);
+  memset (d, i, szti);
+
+  /* Passing a ptrdiff_t where size_t is expected may not be unsafe
+     but because GCC may emits suboptimal code for such calls warning
+     for them helps improve efficiency.  */
+  memset (d, 0, diffi);       /* { dg-warning ".memset. argument 3 promotes to .ptrdiff_t. {aka .long int.} where .long unsigned int. is expected" } */
+
+  memset (d, 0, 2.0);         /* { dg-warning ".memset. argument 3 type is .double. where 'long unsigned int' is expected" } */
+
+  /* Verify that the same call as above but to the built-in doesn't
+     trigger a warning.  */
+  __builtin_memset (d, 0.0, 4.0);
+}
+
+
+/* Verify warnings for undefined calls to built-ins expecting floating
+   arguments.  */
+
+double fabs ();           /* { dg-message "built-in .fabs. declared here" } */
+
+/* Expect a warning for fabsf below because even a float argument promotes
+   to double.  Unfortunately, invalid calls to fabsf() are not diagnosed.  */
+float fabsf ();           /* { dg-warning "conflicting types for built-in function .fabsf.; expected .float\\\(float\\\)." } */
+long double fabsl ();     /* { dg-message "built-in .fabsl. declared here" } */
+
+void test_real_conversion_fabs (void)
+{
+  d = fabs (c);     /* { dg-warning ".fabs. argument 1 promotes to .int. where .double. is expected in a call to built-in function declared without prototype" } */
+
+  d = fabs (i);     /* { dg-warning ".fabs. argument 1 type is .int. where .double. is expected in a call to built-in function declared without prototype" } */
+
+  d = fabs (li);    /* { dg-warning ".fabs. argument 1 type is .long int. where .double. is expected in a call to built-in function declared without prototype" } */
+
+  /* In C, the type of an enumeration constant is int.  */
+  d = fabs (e0);    /* { dg-warning ".fabs. argument 1 type is .int. where .double. is expected in a call to built-in function declared without prototype" } */
+
+  d = fabs (e);     /* { dg-warning ".fabs. argument 1 type is .enum E. where .double. is expected in a call to built-in function declared without prototype" } */
+
+  /* No warning here since float is promoted to double.  */
+  d = fabs (f);
+
+  d = fabs (ld);    /* { dg-warning ".fabs. argument 1 type is .long double. where .double. is expected in a call to built-in function declared without prototype" } */
+
+  d = fabsf (c);    /* { dg-warning ".fabsf. argument 1 promotes to .int. where .float. is expected in a call to built-in function declared without prototype" "pr87890" { xfail *-*-* } } */
+
+  d = fabsl (c);    /* { dg-warning ".fabsl. argument 1 promotes to .int. where .long double. is expected in a call to built-in function declared without prototype" } */
+
+  d = fabsl (f);    /* { dg-warning ".fabsl. argument 1 promotes to .double. where .long double. is expected in a call to built-in function declared without prototype" } */
+
+  /* Verify that the same call as above but to the built-in doesn't
+     trigger a warning.  */
+  d = __builtin_fabsl (f);
+}
+
+/* Verify warnings for calls to a two-argument real function.  */
+
+double pow ();      /* { dg-message "built-in .pow. declared here" } */
+
+void test_real_conversion_pow (void)
+{
+  d = pow (2.0, 2.0);
+  d = pow (d, 3.0);
+  d = pow (d, d);
+
+  d = pow (2, 3.0); /* { dg-warning ".pow. argument 1 type is .int. where .double. is expected in a call to built-in function declared without prototype" } */
+  d = pow (3.0, 2); /* { dg-warning ".pow. argument 2 type is .int. where .double. is expected in a call to built-in function declared without prototype" } */
+}
+
+
+/* Verify warnings for calls that discard qualifiers.  */
+
+extern void* memcpy ();
+
+void test_qual_conversion_memcpy (void *d, const void *s)
+{
+  memcpy (d, s, sizeof (int));
+  memcpy (s, d, sizeof (int));    /* { dg-warning "passing argument 1 of .memcpy. discards 'const' qualifier from pointer target type" } */
+}
diff --git a/gcc/testsuite/gcc.dg/Wbuiltin-declaration-mismatch.c b/gcc/testsuite/gcc.dg/Wbuiltin-declaration-mismatch.c
new file mode 100644
index 0000000..ed66fd3
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/Wbuiltin-declaration-mismatch.c
@@ -0,0 +1,23 @@
+/* PR c/83656 - missing -Wbuiltin-declaration-mismatch on declaration
+   without prototype
+   { dg-do compile }
+   { dg-options "-Wall" } */
+
+typedef __SIZE_TYPE__ size_t;
+
+/* Verify that ordinary library built-ins are not diagnosed with -Wall
+   (or by default) whether or not they take arguments (even though they
+   should be).  */
+
+void abort ();
+void* memcpy ();
+void* memset ();
+size_t strlen ();
+
+/* Verify mismatches in return types are diagnosed.  */
+int exit ();        /* { dg-warning "\\\[-Wbuiltin-declaration-mismatch]" } */
+
+/* Variadic built-ins are diagnosed with -Wall (they are, in fact,
+   diagnosed by default).  */
+int printf ();      /* { dg-warning "\\\[-Wbuiltin-declaration-mismatch]" } */
+int sprintf ();     /* { dg-warning "\\\[-Wbuiltin-declaration-mismatch]" } */
diff --git a/gcc/testsuite/gcc.dg/Wrestrict-4.c b/gcc/testsuite/gcc.dg/Wrestrict-4.c
index f2398ef..55ea6c0 100644
--- a/gcc/testsuite/gcc.dg/Wrestrict-4.c
+++ b/gcc/testsuite/gcc.dg/Wrestrict-4.c
@@ -108,3 +108,5 @@ void* test_strncpy_2 (char *d, const char *s)
 {
   return strncpy (d, s);
 }
+
+/* { dg-prune-output "\\\[-Wbuiltin-declaration-mismatch]" } */
diff --git a/gcc/testsuite/gcc.dg/Wrestrict-5.c b/gcc/testsuite/gcc.dg/Wrestrict-5.c
index 4912ee5..7064e1e 100644
--- a/gcc/testsuite/gcc.dg/Wrestrict-5.c
+++ b/gcc/testsuite/gcc.dg/Wrestrict-5.c
@@ -41,3 +41,6 @@ void test_strncpy_nowarn (char *d)
 {
   strncpy (d + 1, d + 3, "");
 }
+
+/* { dg-prune-output "\\\[-Wbuiltin-declaration-mismatch]" }
+   { dg-prune-output "\\\[-Wint-conversion]" } */
diff --git a/gcc/testsuite/gcc.dg/atomic/stdatomic-generic.c b/gcc/testsuite/gcc.dg/atomic/stdatomic-generic.c
index 8033c53..96582b4 100644
--- a/gcc/testsuite/gcc.dg/atomic/stdatomic-generic.c
+++ b/gcc/testsuite/gcc.dg/atomic/stdatomic-generic.c
@@ -4,7 +4,7 @@
 
 #include <stdatomic.h>
 
-extern void abort ();
+extern void abort (void);
 extern int memcmp (const void *, const void *, __SIZE_TYPE__);
 
 typedef struct test {
diff --git a/gcc/testsuite/gcc.dg/atomic/stdatomic-lockfree.c b/gcc/testsuite/gcc.dg/atomic/stdatomic-lockfree.c
index 29310e9..c6d06a5 100644
--- a/gcc/testsuite/gcc.dg/atomic/stdatomic-lockfree.c
+++ b/gcc/testsuite/gcc.dg/atomic/stdatomic-lockfree.c
@@ -5,7 +5,7 @@
 #include <stdatomic.h>
 #include <stdint.h>
 
-extern void abort ();
+extern void abort (void);
 
 _Atomic _Bool aba;
 atomic_bool abt;
diff --git a/gcc/testsuite/gcc.dg/initpri1.c b/gcc/testsuite/gcc.dg/initpri1.c
index 794ea2b..b6afd76 100644
--- a/gcc/testsuite/gcc.dg/initpri1.c
+++ b/gcc/testsuite/gcc.dg/initpri1.c
@@ -1,6 +1,6 @@
 /* { dg-do run { target init_priority } } */
 
-extern void abort ();
+extern void abort (void);
 
 int i;
 int j;
diff --git a/gcc/testsuite/gcc.dg/pr15698-1.c b/gcc/testsuite/gcc.dg/pr15698-1.c
index 5a75a10..cbe613c 100644
--- a/gcc/testsuite/gcc.dg/pr15698-1.c
+++ b/gcc/testsuite/gcc.dg/pr15698-1.c
@@ -21,3 +21,5 @@ char *rindex(a, b)
 {
   return 0;
 }
+
+/* { dg-prune-output "\\\[-Wbuiltin-declaration-mismatch]" } */
diff --git a/gcc/testsuite/gcc.dg/pr69156.c b/gcc/testsuite/gcc.dg/pr69156.c
index 1addfa3..522ac00 100644
--- a/gcc/testsuite/gcc.dg/pr69156.c
+++ b/gcc/testsuite/gcc.dg/pr69156.c
@@ -5,6 +5,6 @@
 _Bool
 foo ()
 {
-  _Bool (*f) () = __builtin_abs;	/* { dg-warning "initialization of '_Bool \\(\\*\\)\\(\\)' from incompatible pointer type" } */
+  _Bool (*f) () = __builtin_abs;	/* { dg-warning "initialization of '_Bool \\(\\*\\)\\(\\)' from pointer to .__builtin_abs. with incompatible type .int \\\(\\\*\\\)." } */
   return f (0);
 }
diff --git a/gcc/testsuite/gcc.dg/pr83463.c b/gcc/testsuite/gcc.dg/pr83463.c
index ddf662f..cd11f64 100644
--- a/gcc/testsuite/gcc.dg/pr83463.c
+++ b/gcc/testsuite/gcc.dg/pr83463.c
@@ -15,3 +15,5 @@ p ()
 {
   m (p + (long) a);
 }
+
+/* { dg-prune-output "\\\[-Wbuiltin-declaration-mismatch]" } */
diff --git a/gcc/testsuite/gcc.dg/redecl-4.c b/gcc/testsuite/gcc.dg/redecl-4.c
index 2ba74a3..8f12488 100644
--- a/gcc/testsuite/gcc.dg/redecl-4.c
+++ b/gcc/testsuite/gcc.dg/redecl-4.c
@@ -27,3 +27,6 @@ f (void)
 
 /* Should still diagnose incompatible prototype for strcmp.  */
 int strcmp (void); /* { dg-error "conflict" } */
+
+/* { dg-prune-output "\\\[-Wbuiltin-declaration-mismatch]" }
+   { dg-prune-output "\\\[-Wint-conversion]" } */
diff --git a/gcc/testsuite/gcc.dg/tls/thr-init-2.c b/gcc/testsuite/gcc.dg/tls/thr-init-2.c
index 22c96ea..a540389 100644
--- a/gcc/testsuite/gcc.dg/tls/thr-init-2.c
+++ b/gcc/testsuite/gcc.dg/tls/thr-init-2.c
@@ -2,7 +2,7 @@
 /* { dg-require-effective-target tls_runtime } */
 /* { dg-add-options tls } */
 
-extern void abort() ;
+extern void abort (void);
 
 static __thread int fstat ;
 static __thread int fstat = 1;
diff --git a/gcc/testsuite/gcc.dg/torture/pr55890-2.c b/gcc/testsuite/gcc.dg/torture/pr55890-2.c
index 1cf71d7..31779e0 100644
--- a/gcc/testsuite/gcc.dg/torture/pr55890-2.c
+++ b/gcc/testsuite/gcc.dg/torture/pr55890-2.c
@@ -2,3 +2,5 @@
 
 extern void *memcpy();
 int main() { memcpy(); }
+
+/* { dg-prune-output "\\\[-Wbuiltin-declaration-mismatch]" } */
diff --git a/gcc/testsuite/gcc.dg/torture/pr55890-3.c b/gcc/testsuite/gcc.dg/torture/pr55890-3.c
index c7f77be..21a3d98 100644
--- a/gcc/testsuite/gcc.dg/torture/pr55890-3.c
+++ b/gcc/testsuite/gcc.dg/torture/pr55890-3.c
@@ -7,3 +7,5 @@ bar ()
 {
   return memmove ();
 }
+
+/* { dg-prune-output "\\\[-Wbuiltin-declaration-mismatch]" } */
diff --git a/gcc/testsuite/gcc.dg/torture/pr67741.c b/gcc/testsuite/gcc.dg/torture/pr67741.c
index 1ffc707..833e53e 100644
--- a/gcc/testsuite/gcc.dg/torture/pr67741.c
+++ b/gcc/testsuite/gcc.dg/torture/pr67741.c
@@ -3,11 +3,12 @@
 struct singlecomplex { float real, imag ; } ;
 struct doublecomplex { double real, imag ; } ;
 struct extendedcomplex { long double real, imag ; } ;
-extern double cabs();
+extern double cabs(); /* { dg-warning "\\\[-Wbuiltin-declaration-mismatch]" } */
 float cabsf(fc)
      struct singlecomplex fc;  /* { dg-warning "doesn't match" } */
 {
   struct doublecomplex dc ;
   dc.real=fc.real; dc.imag=fc.imag;
-  return (float) cabs(dc);
+  return (float) cabs(dc);   /* { dg-warning "incompatible type for argument 1 of .cabs." } */
 }
+
diff --git a/gcc/testsuite/gcc.dg/torture/stackalign/sibcall-1.c b/gcc/testsuite/gcc.dg/torture/stackalign/sibcall-1.c
index 8c17475..c4992df 100644
--- a/gcc/testsuite/gcc.dg/torture/stackalign/sibcall-1.c
+++ b/gcc/testsuite/gcc.dg/torture/stackalign/sibcall-1.c
@@ -1,7 +1,7 @@
 /* { dg-do run } */
 
 extern int ok (int);
-extern void exit ();
+extern void exit (int);
 static int gen_x86_64_shrd (int);
 static int
 gen_x86_64_shrd(int a __attribute__ ((__unused__)))
diff --git a/gcc/testsuite/gcc.dg/torture/tls/thr-init-1.c b/gcc/testsuite/gcc.dg/torture/tls/thr-init-1.c
index ff3338f..3ae2e29 100644
--- a/gcc/testsuite/gcc.dg/torture/tls/thr-init-1.c
+++ b/gcc/testsuite/gcc.dg/torture/tls/thr-init-1.c
@@ -3,7 +3,7 @@
 /* { dg-add-options tls } */
 
 extern int printf (char *,...);
-extern void abort() ;
+extern void abort(void) ;
 
 int test_code(int b)
 {
diff --git a/gcc/testsuite/gcc.dg/tree-ssa/builtins-folding-gimple-ub.c b/gcc/testsuite/gcc.dg/tree-ssa/builtins-folding-gimple-ub.c
index a313998..0912b68 100644
--- a/gcc/testsuite/gcc.dg/tree-ssa/builtins-folding-gimple-ub.c
+++ b/gcc/testsuite/gcc.dg/tree-ssa/builtins-folding-gimple-ub.c
@@ -31,4 +31,5 @@ main (void)
   return 0;
 }
 
-/* { dg-final { scan-tree-dump-times "__builtin_memchr" 2 "optimized" } } */
+/* { dg-prune-output "-Wbuiltin-declaration-mismatch" }
+   { dg-final { scan-tree-dump-times "__builtin_memchr" 2 "optimized" } } */

On 11/6/18 4:56 PM, Martin Sebor wrote:
> In response to Joseph's comment I've removed the interaction
> with -Wpedantic from the updated patch.
> 
> In addition, to help detect bugs like the one in the test case
> for pr87886, I have also enhanced the detection of incompatible
> calls to include integer/real type mismatches so that calls like
> the one below are diagnosed:
> 
>   extern double sqrt ();
>   int f (int x)
>   {
>     return sqrt (x);   // passing int where double is expected
>   }
> 
> With the removal of the -Wpedantic interaction declaring abort()
> without a prototype is no longer diagnosed and so the test suite
> changes to add the prototype are not necessary.  I decided not
> to back them out because Jeff indicated a preference for making
> these kinds of improvements in general in an unrelated
> discussion.
> 
> 
> 
> gcc-83656.diff
> 
> PR c/83656 - missing -Wbuiltin-declaration-mismatch on declaration without prototype
> 
> gcc/c/ChangeLog:
> 
> 	PR c/83656
> 	* c-decl.c (header_for_builtin_fn): Declare.
> 	(diagnose_mismatched_decls): Diagnose declarations of built-in
> 	functions without a prototype.
> 	* c-typeck.c (maybe_warn_builtin_no_proto_arg): New function.
> 	(convert_argument): Same.
> 	(convert_arguments): Factor code out into convert_argument.
> 	Detect mismatches between built-in formal arguments in calls
> 	to built-in without prototype.
> 	(build_conditional_expr): Same.
> 	(type_or_builtin_type): New function.
> 	(convert_for_assignment): Add argument.  Conditionally issue
> 	warnings instead of errors for mismatches.
> 
> gcc/testsuite/ChangeLog:
> 
> 	PR c/83656
> 	* gcc.dg/20021006-1.c
> 	* gcc.dg/Wbuiltin-declaration-mismatch.c: New test.
> 	* gcc.dg/Wbuiltin-declaration-mismatch-2.c: New test.
> 	* gcc.dg/Wbuiltin-declaration-mismatch-3.c: New test.
> 	* gcc.dg/Wbuiltin-declaration-mismatch-4.c: New test.
> 	* gcc.dg/Walloca-16.c: Adjust.
> 	* gcc.dg/Wrestrict-4.c: Adjust.
> 	* gcc.dg/Wrestrict-5.c: Adjust.
> 	* gcc.dg/atomic/stdatomic-generic.c: Adjust.
> 	* gcc.dg/atomic/stdatomic-lockfree.c: Adjust.
> 	* gcc.dg/initpri1.c: Adjust.
> 	* gcc.dg/pr15698-1.c: Adjust.
> 	* gcc.dg/pr69156.c: Adjust.
> 	* gcc.dg/pr83463.c: Adjust.
> 	* gcc.dg/redecl-4.c: Adjust.
> 	* gcc.dg/tls/thr-init-2.c: Adjust.
> 	* gcc.dg/torture/pr55890-2.c: Adjust.
> 	* gcc.dg/torture/pr55890-3.c: Adjust.
> 	* gcc.dg/torture/pr67741.c: Adjust.
> 	* gcc.dg/torture/stackalign/sibcall-1.c: Adjust.
> 	* gcc.dg/torture/tls/thr-init-1.c: Adjust.
> 	* gcc.dg/tree-ssa/builtins-folding-gimple-ub.c: Adjust.
> 


> @@ -3547,8 +3598,24 @@ convert_arguments (location_t loc, vec<location_t> arg_loc, tree typelist,
>        if (parmval == error_mark_node)
>  	error_args = true;
>  
> +      if (!type && builtin_type && TREE_CODE (builtin_type) != VOID_TYPE)
> +	{
> +	  /* For a call to a built-in function declared without a prototype,
> +	     perform the coversions from the argument to the expected type
> +	     but issue warnings rather than errors for any mismatches.
> +	     Ignore the converted argument and use the PARMVAL obtained
> +	     above by applying default coversions instead.  */
s/coversions/conversions/

Two of 'em in that comment.  OK with that nit fixed.

jeff

On Wed, Nov 14, 2018 at 7:08 PM Jeff Law <law@redhat.com> wrote:
>
> On 11/6/18 4:56 PM, Martin Sebor wrote:
> > In response to Joseph's comment I've removed the interaction
> > with -Wpedantic from the updated patch.
> >
> > In addition, to help detect bugs like the one in the test case
> > for pr87886, I have also enhanced the detection of incompatible
> > calls to include integer/real type mismatches so that calls like
> > the one below are diagnosed:
> >
> >   extern double sqrt ();
> >   int f (int x)
> >   {
> >     return sqrt (x);   // passing int where double is expected
> >   }
> >
> > With the removal of the -Wpedantic interaction declaring abort()
> > without a prototype is no longer diagnosed and so the test suite
> > changes to add the prototype are not necessary.  I decided not
> > to back them out because Jeff indicated a preference for making
> > these kinds of improvements in general in an unrelated
> > discussion.
> >
> >
> >
> > gcc-83656.diff
> >
> > PR c/83656 - missing -Wbuiltin-declaration-mismatch on declaration without prototype
> >
> > gcc/c/ChangeLog:
> >
> >       PR c/83656
> >       * c-decl.c (header_for_builtin_fn): Declare.
> >       (diagnose_mismatched_decls): Diagnose declarations of built-in
> >       functions without a prototype.
> >       * c-typeck.c (maybe_warn_builtin_no_proto_arg): New function.
> >       (convert_argument): Same.
> >       (convert_arguments): Factor code out into convert_argument.
> >       Detect mismatches between built-in formal arguments in calls
> >       to built-in without prototype.
> >       (build_conditional_expr): Same.
> >       (type_or_builtin_type): New function.
> >       (convert_for_assignment): Add argument.  Conditionally issue
> >       warnings instead of errors for mismatches.
> >
> > gcc/testsuite/ChangeLog:
> >
> >       PR c/83656
> >       * gcc.dg/20021006-1.c
> >       * gcc.dg/Wbuiltin-declaration-mismatch.c: New test.
> >       * gcc.dg/Wbuiltin-declaration-mismatch-2.c: New test.
> >       * gcc.dg/Wbuiltin-declaration-mismatch-3.c: New test.
> >       * gcc.dg/Wbuiltin-declaration-mismatch-4.c: New test.
> >       * gcc.dg/Walloca-16.c: Adjust.
> >       * gcc.dg/Wrestrict-4.c: Adjust.
> >       * gcc.dg/Wrestrict-5.c: Adjust.
> >       * gcc.dg/atomic/stdatomic-generic.c: Adjust.
> >       * gcc.dg/atomic/stdatomic-lockfree.c: Adjust.
> >       * gcc.dg/initpri1.c: Adjust.
> >       * gcc.dg/pr15698-1.c: Adjust.
> >       * gcc.dg/pr69156.c: Adjust.
> >       * gcc.dg/pr83463.c: Adjust.
> >       * gcc.dg/redecl-4.c: Adjust.
> >       * gcc.dg/tls/thr-init-2.c: Adjust.
> >       * gcc.dg/torture/pr55890-2.c: Adjust.
> >       * gcc.dg/torture/pr55890-3.c: Adjust.
> >       * gcc.dg/torture/pr67741.c: Adjust.
> >       * gcc.dg/torture/stackalign/sibcall-1.c: Adjust.
> >       * gcc.dg/torture/tls/thr-init-1.c: Adjust.
> >       * gcc.dg/tree-ssa/builtins-folding-gimple-ub.c: Adjust.
> >
>
>
> > @@ -3547,8 +3598,24 @@ convert_arguments (location_t loc, vec<location_t> arg_loc, tree typelist,
> >        if (parmval == error_mark_node)
> >       error_args = true;
> >
> > +      if (!type && builtin_type && TREE_CODE (builtin_type) != VOID_TYPE)
> > +     {
> > +       /* For a call to a built-in function declared without a prototype,
> > +          perform the coversions from the argument to the expected type
> > +          but issue warnings rather than errors for any mismatches.
> > +          Ignore the converted argument and use the PARMVAL obtained
> > +          above by applying default coversions instead.  */
> s/coversions/conversions/
>
> Two of 'em in that comment.  OK with that nit fixed.

I once had -Wunprototyped-calls...  (attached).  Because the issue doesn't
exist only for builtins... (originally inspired by a Xorg bug)

Richard.

> jeff
>
>
Index: gcc/c-family/c.opt
===================================================================
--- gcc/c-family/c.opt.orig	2014-11-11 09:38:31.286867405 +0100
+++ gcc/c-family/c.opt	2014-11-17 16:38:44.438078729 +0100
@@ -868,6 +868,10 @@ Wunused-local-typedefs
 C ObjC C++ ObjC++ Var(warn_unused_local_typedefs) Warning EnabledBy(Wunused)
 Warn when typedefs locally defined in a function are not used
 
+Wunprototyped-calls
+C Var(warn_unprototyped_calls) Warning LangEnabledBy(C,Wall)
+Warn about calls to unprototyped functions with at least one argument
+
 Wunused-macros
 C ObjC C++ ObjC++ CppReason(CPP_W_UNUSED_MACROS) Var(cpp_warn_unused_macros) Warning
 Warn about macros defined in the main file that are not used
Index: gcc/c/c-typeck.c
===================================================================
--- gcc/c/c-typeck.c.orig	2014-11-11 09:38:32.068867371 +0100
+++ gcc/c/c-typeck.c	2014-11-17 16:38:44.442078729 +0100
@@ -2957,6 +2957,19 @@ build_function_call_vec (location_t loc,
       && !check_builtin_function_arguments (fundecl, nargs, argarray))
     return error_mark_node;
 
+  /* If we cannot check function arguments because a prototype is
+     missing for the callee, warn here.  */
+  if (warn_unprototyped_calls
+      && nargs > 0 && !TYPE_ARG_TYPES (fntype)
+      && fundecl && !DECL_BUILT_IN (fundecl) && !C_DECL_IMPLICIT (fundecl)
+      && !DECL_ARGUMENTS (fundecl))
+    {
+      if (warning (OPT_Wunprototyped_calls,
+		   "call to function %qD without a real prototype", fundecl))
+	inform (DECL_SOURCE_LOCATION (fundecl), "%qD was declared here",
+		fundecl);
+    }
+
   /* Check that the arguments to the function are valid.  */
   check_function_arguments (fntype, nargs, argarray);
 
Index: gcc/testsuite/gcc.dg/cleanup-1.c
===================================================================
--- gcc/testsuite/gcc.dg/cleanup-1.c.orig	2010-02-23 12:43:21.000000000 +0100
+++ gcc/testsuite/gcc.dg/cleanup-1.c	2014-11-17 16:38:44.442078729 +0100
@@ -6,7 +6,7 @@
 #define C(x)	__attribute__((cleanup(x)))
 
 static int f1(void *x U) { return 0; }
-static void f2() { }
+static void f2() { } /* { dg-message "declared here" "" } */
 static void f3(void) { } /* { dg-message "note: declared here" } */
 static void f4(void *x U) { }
 static void f5(int *x U) { }
@@ -18,7 +18,7 @@ static void f9(int x U) { } /* { dg-mess
 void test(void)
 {
   int o1 C(f1);
-  int o2 C(f2);
+  int o2 C(f2);		/* { dg-warning "without a real prototype" } */
   int o3 C(f3);		/* { dg-error "too many arguments" } */
   int o4 C(f4);
   int o5 C(f5);

On 11/15/2018 03:10 AM, Richard Biener wrote:
> On Wed, Nov 14, 2018 at 7:08 PM Jeff Law <law@redhat.com> wrote:
>>
>> On 11/6/18 4:56 PM, Martin Sebor wrote:
>>> In response to Joseph's comment I've removed the interaction
>>> with -Wpedantic from the updated patch.
>>>
>>> In addition, to help detect bugs like the one in the test case
>>> for pr87886, I have also enhanced the detection of incompatible
>>> calls to include integer/real type mismatches so that calls like
>>> the one below are diagnosed:
>>>
>>>   extern double sqrt ();
>>>   int f (int x)
>>>   {
>>>     return sqrt (x);   // passing int where double is expected
>>>   }
>>>
>>> With the removal of the -Wpedantic interaction declaring abort()
>>> without a prototype is no longer diagnosed and so the test suite
>>> changes to add the prototype are not necessary.  I decided not
>>> to back them out because Jeff indicated a preference for making
>>> these kinds of improvements in general in an unrelated
>>> discussion.
>>>
>>>
>>>
>>> gcc-83656.diff
>>>
>>> PR c/83656 - missing -Wbuiltin-declaration-mismatch on declaration without prototype
>>>
>>> gcc/c/ChangeLog:
>>>
>>>       PR c/83656
>>>       * c-decl.c (header_for_builtin_fn): Declare.
>>>       (diagnose_mismatched_decls): Diagnose declarations of built-in
>>>       functions without a prototype.
>>>       * c-typeck.c (maybe_warn_builtin_no_proto_arg): New function.
>>>       (convert_argument): Same.
>>>       (convert_arguments): Factor code out into convert_argument.
>>>       Detect mismatches between built-in formal arguments in calls
>>>       to built-in without prototype.
>>>       (build_conditional_expr): Same.
>>>       (type_or_builtin_type): New function.
>>>       (convert_for_assignment): Add argument.  Conditionally issue
>>>       warnings instead of errors for mismatches.
>>>
>>> gcc/testsuite/ChangeLog:
>>>
>>>       PR c/83656
>>>       * gcc.dg/20021006-1.c
>>>       * gcc.dg/Wbuiltin-declaration-mismatch.c: New test.
>>>       * gcc.dg/Wbuiltin-declaration-mismatch-2.c: New test.
>>>       * gcc.dg/Wbuiltin-declaration-mismatch-3.c: New test.
>>>       * gcc.dg/Wbuiltin-declaration-mismatch-4.c: New test.
>>>       * gcc.dg/Walloca-16.c: Adjust.
>>>       * gcc.dg/Wrestrict-4.c: Adjust.
>>>       * gcc.dg/Wrestrict-5.c: Adjust.
>>>       * gcc.dg/atomic/stdatomic-generic.c: Adjust.
>>>       * gcc.dg/atomic/stdatomic-lockfree.c: Adjust.
>>>       * gcc.dg/initpri1.c: Adjust.
>>>       * gcc.dg/pr15698-1.c: Adjust.
>>>       * gcc.dg/pr69156.c: Adjust.
>>>       * gcc.dg/pr83463.c: Adjust.
>>>       * gcc.dg/redecl-4.c: Adjust.
>>>       * gcc.dg/tls/thr-init-2.c: Adjust.
>>>       * gcc.dg/torture/pr55890-2.c: Adjust.
>>>       * gcc.dg/torture/pr55890-3.c: Adjust.
>>>       * gcc.dg/torture/pr67741.c: Adjust.
>>>       * gcc.dg/torture/stackalign/sibcall-1.c: Adjust.
>>>       * gcc.dg/torture/tls/thr-init-1.c: Adjust.
>>>       * gcc.dg/tree-ssa/builtins-folding-gimple-ub.c: Adjust.
>>>
>>
>>
>>> @@ -3547,8 +3598,24 @@ convert_arguments (location_t loc, vec<location_t> arg_loc, tree typelist,
>>>        if (parmval == error_mark_node)
>>>       error_args = true;
>>>
>>> +      if (!type && builtin_type && TREE_CODE (builtin_type) != VOID_TYPE)
>>> +     {
>>> +       /* For a call to a built-in function declared without a prototype,
>>> +          perform the coversions from the argument to the expected type
>>> +          but issue warnings rather than errors for any mismatches.
>>> +          Ignore the converted argument and use the PARMVAL obtained
>>> +          above by applying default coversions instead.  */
>> s/coversions/conversions/
>>
>> Two of 'em in that comment.  OK with that nit fixed.
>
> I once had -Wunprototyped-calls...  (attached).  Because the issue doesn't
> exist only for builtins... (originally inspired by a Xorg bug)

You're right, thanks for the suggestion.  I'm still planning to
resubmit the patch I posted last June to enable -Wstrict-prototypes
(PR 82922).  Detecting this problem there would be a good enhancement.
If it's too late to do this for GCC 9 I will submit the patch for GCC
10.

Martin