diff mbox series

[PATCHv2] Add a -Wcast-align=strict warning

Message ID AM5PR0701MB2657B2B18AD57CBC087A9B33E46F0@AM5PR0701MB2657.eurprd07.prod.outlook.com
State New
Headers show
Series [PATCHv2] Add a -Wcast-align=strict warning | expand

Commit Message

Bernd Edlinger Sept. 14, 2017, 4:33 p.m. UTC 
On 09/04/17 10:07, Bernd Edlinger wrote:
> Hi,

> 

> as you know we have a -Wcast-align warning which works only for

> STRICT_ALIGNMENT targets.  But occasionally it would be nice to be

> able to switch this warning on even for other targets.

> 

> Therefore I would like to add a strict version of this option

> which can be invoked with -Wcast-align=strict.  With the only

> difference that it does not depend on STRICT_ALIGNMENT.

> 

> I used the code from check_effective_target_non_strict_align

> in target-supports.exp for the first version of the test case,

> where we have this:

> 

> return [check_no_compiler_messages non_strict_align assembly {

>       char *y;

>       typedef char __attribute__ ((__aligned__(__BIGGEST_ALIGNMENT__))) c;

>       c *z;

>       void foo(void) { z = (c *) y; }

> } "-Wcast-align"]

> 

> ... and to my big surprise it did _not_ work for C++ as-is,

> because same_type_p considers differently aligned types identical,

> and therefore cp_build_c_cast tries the conversion first via a

> const_cast which succeeds, but did not emit the cast-align warning

> in this case.

> 

> As a work-around I had to check the alignment in build_const_cast_1

> as well.

> 

> 

> Bootstrapped and reg-tested on x86_64-pc-linux-gnu.

> Is it OK for trunk?

> 


Hi,

as suggested by Joseph, here is an updated patch that
uses min_align_of_type instead of TYPE_ALIGN.

Is it OK?


Thanks,
Bernd.
gcc:
2017-09-03  Bernd Edlinger  <bernd.edlinger@hotmail.de>

	* common.opt (Wcast-align=strict): New warning option.
	* doc/invoke.texi: Document -Wcast-align=strict. 

c:
2017-09-03  Bernd Edlinger  <bernd.edlinger@hotmail.de>

	* c-typeck.c (build_c_cast): Implement -Wcast-align=strict.

cp:
2017-09-03  Bernd Edlinger  <bernd.edlinger@hotmail.de>

	* typeck.c (build_reinterpret_cast_1,
	build_const_cast_1): Implement -Wcast-align=strict.

testsuite:
2017-09-03  Bernd Edlinger  <bernd.edlinger@hotmail.de>

	* c-c++-common/Wcast-align.c: New test.

Comments

Joseph Myers Sept. 15, 2017, 3:51 p.m. UTC | #1 
On Thu, 14 Sep 2017, Bernd Edlinger wrote:

> Hi,
> 
> as suggested by Joseph, here is an updated patch that
> uses min_align_of_type instead of TYPE_ALIGN.
> 
> Is it OK?

OK.
diff mbox series

Patch

Index: gcc/c/c-typeck.c
===================================================================
--- gcc/c/c-typeck.c	(revision 251617)
+++ gcc/c/c-typeck.c	(working copy)
@@ -5578,7 +5578,7 @@  build_c_cast (location_t loc, tree type,
 	}
 
       /* Warn about possible alignment problems.  */
-      if (STRICT_ALIGNMENT
+      if ((STRICT_ALIGNMENT || warn_cast_align == 2)
 	  && TREE_CODE (type) == POINTER_TYPE
 	  && TREE_CODE (otype) == POINTER_TYPE
 	  && TREE_CODE (TREE_TYPE (otype)) != VOID_TYPE
@@ -5587,7 +5587,8 @@  build_c_cast (location_t loc, tree type,
 	     restriction is unknown.  */
 	  && !(RECORD_OR_UNION_TYPE_P (TREE_TYPE (otype))
 	       && TYPE_MODE (TREE_TYPE (otype)) == VOIDmode)
-	  && TYPE_ALIGN (TREE_TYPE (type)) > TYPE_ALIGN (TREE_TYPE (otype)))
+	  && min_align_of_type (TREE_TYPE (type))
+	     > min_align_of_type (TREE_TYPE (otype)))
 	warning_at (loc, OPT_Wcast_align,
 		    "cast increases required alignment of target type");
 
Index: gcc/common.opt
===================================================================
--- gcc/common.opt	(revision 251617)
+++ gcc/common.opt	(working copy)
@@ -564,6 +564,10 @@  Wcast-align
 Common Var(warn_cast_align) Warning
 Warn about pointer casts which increase alignment.
 
+Wcast-align=strict
+Common Var(warn_cast_align,2) Warning
+Warn about pointer casts which increase alignment.
+
 Wcpp
 Common Var(warn_cpp) Init(1) Warning
 Warn when a #warning directive is encountered.
Index: gcc/cp/typeck.c
===================================================================
--- gcc/cp/typeck.c	(revision 251617)
+++ gcc/cp/typeck.c	(working copy)
@@ -7265,15 +7265,16 @@  build_reinterpret_cast_1 (tree type, tre
 					       complain))
 	return error_mark_node;
       /* Warn about possible alignment problems.  */
-      if (STRICT_ALIGNMENT && warn_cast_align
-          && (complain & tf_warning)
+      if ((STRICT_ALIGNMENT || warn_cast_align == 2)
+	  && (complain & tf_warning)
 	  && !VOID_TYPE_P (type)
 	  && TREE_CODE (TREE_TYPE (intype)) != FUNCTION_TYPE
 	  && COMPLETE_TYPE_P (TREE_TYPE (type))
 	  && COMPLETE_TYPE_P (TREE_TYPE (intype))
-	  && TYPE_ALIGN (TREE_TYPE (type)) > TYPE_ALIGN (TREE_TYPE (intype)))
+	  && min_align_of_type (TREE_TYPE (type))
+	     > min_align_of_type (TREE_TYPE (intype)))
 	warning (OPT_Wcast_align, "cast from %qH to %qI "
-                 "increases required alignment of target type", intype, type);
+		 "increases required alignment of target type", intype, type);
 
       /* We need to strip nops here, because the front end likes to
 	 create (int *)&a for array-to-pointer decay, instead of &a[0].  */
@@ -7447,6 +7448,14 @@  build_const_cast_1 (tree dst_type, tree
 		 the user is making a potentially unsafe cast.  */
 	      check_for_casting_away_constness (src_type, dst_type,
 						CAST_EXPR, complain);
+	      /* ??? comp_ptr_ttypes_const ignores TYPE_ALIGN.  */
+	      if ((STRICT_ALIGNMENT || warn_cast_align == 2)
+		  && (complain & tf_warning)
+		  && min_align_of_type (TREE_TYPE (dst_type))
+		     > min_align_of_type (TREE_TYPE (src_type)))
+		warning (OPT_Wcast_align, "cast from %qH to %qI "
+			 "increases required alignment of target type",
+			 src_type, dst_type);
 	    }
 	  if (reference_type)
 	    {
Index: gcc/doc/invoke.texi
===================================================================
--- gcc/doc/invoke.texi	(revision 251617)
+++ gcc/doc/invoke.texi	(working copy)
@@ -266,7 +266,8 @@  Objective-C and Objective-C++ Dialects}.
 -Wno-attributes  -Wbool-compare  -Wbool-operation @gol
 -Wno-builtin-declaration-mismatch @gol
 -Wno-builtin-macro-redefined  -Wc90-c99-compat  -Wc99-c11-compat @gol
--Wc++-compat  -Wc++11-compat  -Wc++14-compat  -Wcast-align  -Wcast-qual  @gol
+-Wc++-compat  -Wc++11-compat  -Wc++14-compat  @gol
+-Wcast-align  -Wcast-align=strict  -Wcast-qual  @gol
 -Wchar-subscripts  -Wchkp  -Wcatch-value  -Wcatch-value=@var{n} @gol
 -Wclobbered  -Wcomment  -Wconditionally-supported @gol
 -Wconversion  -Wcoverage-mismatch  -Wno-cpp  -Wdangling-else  -Wdate-time @gol
@@ -5923,6 +5924,12 @@  target is increased.  For example, warn if a @code
 an @code{int *} on machines where integers can only be accessed at
 two- or four-byte boundaries.
 
+@item -Wcast-align=strict
+@opindex Wcast-align=strict
+Warn whenever a pointer is cast such that the required alignment of the
+target is increased.  For example, warn if a @code{char *} is cast to
+an @code{int *} regardless of the target machine.
+
 @item -Wwrite-strings
 @opindex Wwrite-strings
 @opindex Wno-write-strings
Index: gcc/testsuite/c-c++-common/Wcast-align.c
===================================================================
--- gcc/testsuite/c-c++-common/Wcast-align.c	(revision 0)
+++ gcc/testsuite/c-c++-common/Wcast-align.c	(working copy)
@@ -0,0 +1,23 @@ 
+/* { dg-do compile } */
+/* { dg-options "-Wcast-align=strict" } */
+
+typedef char __attribute__ ((__aligned__(__BIGGEST_ALIGNMENT__))) c;
+typedef struct __attribute__ ((__aligned__(__BIGGEST_ALIGNMENT__)))
+{
+  char x;
+} d;
+
+char *x;
+c *y;
+d *z;
+struct s { long long x; } *p;
+struct t { double x; } *q;
+
+void
+foo (void)
+{
+  y = (c *) x;  /* { dg-warning "alignment" } */
+  z = (d *) x;  /* { dg-warning "alignment" } */
+  (long long *) p;  /* { dg-bogus "alignment" } */
+  (double *) q;     /* { dg-bogus "alignment" } */
+}