RFA: new pass to warn on questionable uses of alloca() and VLAs
diff mbox

Message ID 578E02C7.5050400@redhat.com
State New
Headers show

Commit Message

Aldy Hernandez July 19, 2016, 11:03 a.m. UTC
On 07/18/2016 11:14 PM, Martin Sebor wrote:
>> How does this look?
>
> I think it's 99% there.  You've addressed all of my comments so
> far -- thanks for that and for being so patient.  I realize it
> would be a lot more efficient to get all the feedback (or as much
> of it as possible) up front.  Unfortunately, some things don't get
> noticed until round 2 or 3 (or even 4).  Please take this in lieu
> of an apology for not spotting the issues below until now(*).

No problem, although I think we're getting to the point of diminishing 
returns with regards to functionality.  It may be best to involve Jeff 
or another global reviewer at this point for a review.  We can address 
any more minor things as a follow-up patch.  (Unless you find any show 
stoppers before then :)).

>
> For this code:
>
>    void f (void*);
>
>    void g (int n)
>    {
>      int a [n];
>      f (a);
>    }
>
> -Wvla-larger-than=32 prints:
>
>    warning: argument to variable-length array may be too large
>    note: limit is 32 bytes, but argument may be 18446744073709551612
>
> An int argument cannot be that large.  I suspect the printed value
> is actually the size of the VLA in bytes when N is -1, truncated
> to size_t, rather than the value of the VLA bound.  To avoid
> confusion the note should be corrected to say something like:
>
>    note: limit is 32 bytes, but the variable-length array may be
>    as large as 18446744073709551612

Note adjusted.

>
> Also, the checker prints false positives for code like:
>
>    void f (void*);
>
>    void g (unsigned x, int *y)
>    {
>      if (1000 < x) return;
>
>      while (*y) {
>        char a [x];
>        f (a);
>      }
>    }
>
> With -Wvla-larger-than=1000 and greater it prints:
>
>    warning: unbounded use of variable-length array
>
> (Same thing with alloca).  There should be no warning for VLAs,
> and for alloca, the warning should say "use of variable-length
> array within a loop."  The VRP dump suggests the range information
> is available within the loop.  Is the get_range_info() function
> not returning the corresponding bounds?

This is a false positive, but there's little we can do with the current 
range infrastructure.  The range information becomes less precise the 
further down the optimization pipeline we get.  So, even though as far 
as *.c.126t.crited1, we still see appropriate range information:

    # RANGE [0, 1000] NONZERO 1023
    _10 = (sizetype) x_3(D);
...
    a.1_12 = __builtin_alloca_with_align (_10, 8);

The PRE pass cleans things up in such a way that we end up with:

    <bb 2>:
    if (x_3(D) > 1000)
      goto <bb 8>;
    else
      goto <bb 3>;
...
    <bb 3>:
    # VUSE <.MEM_2(D)>
    _16 = *y_1(D);
    if (_16 != 0)
      goto <bb 9>;
    else
      goto <bb 8>;

    <bb 9>:

    <bb 4>:
    # <-----------------NO RANGE INFO--------->
    _4 = (sizetype) x_3(D);

...

a.1_12 = __builtin_alloca_with_align (_4, 8);

The -Walloca pass comes after PRE, which means we no longer have any 
range information for _4, and chasing the IL to glean this information 
would be fragile at best.  We will just have to live with this until we 
have better pervasive range information.

Updated patch tested on x86-64 Linux.

Aldy
gcc/

	* Makefile.in (OBJS): Add gimple-ssa-warn-walloca.o.
	* passes.def: Add two instances of pass_walloca.
	* tree-pass.h (make_pass_walloca): New.
	* gimple-ssa-warn-walloca.c: New file.
	* opts.c (finish_options): Warn when using -Wvla-larger-than= and
	-Walloca-larger-than= without -O2 or greater.
	* doc/invoke.texi: Document -Walloca, -Walloca-larger-than=, and
	-Wvla-larger-than= options.

gcc/c-family/

	* c.opt (Walloca): New.
	(Walloca-larger-than=): New.
	(Wvla-larger-than=): New.

Comments

Jeff Law July 19, 2016, 5:54 p.m. UTC | #1
On 07/19/2016 05:03 AM, Aldy Hernandez wrote:
>> (Same thing with alloca).  There should be no warning for VLAs,
>> and for alloca, the warning should say "use of variable-length
>> array within a loop."  The VRP dump suggests the range information
>> is available within the loop.  Is the get_range_info() function
>> not returning the corresponding bounds?
>
> This is a false positive, but there's little we can do with the current
> range infrastructure.  The range information becomes less precise the
> further down the optimization pipeline we get.  So, even though as far
> as *.c.126t.crited1, we still see appropriate range information:
[ ... ]
So I think we can live with the false positive as an XFAIL while we wait 
for improved infrastructure.

I will note that you could use a two-stage approach to help with this 
kind of issue.  You note the set of potential large allocations early 
(before PRE or anyone else messes it up).  Then you allow the other 
optimizers to run, then go back and recheck the allocations after the 
last optimizer pass.  You end up with

flagged early  && flagged late --> warn
flagged early && ! flagged late -> optimization eliminated the false 
positive (which you can optionally issue a diagnostic for)
! flagged early -- never warn

I don't think you strictly need it here, but it's a way to approach some 
of these problems where you want to run a warning pass late (to allow 
the optimizers to eliminate false positives).

jeff
Aldy Hernandez July 19, 2016, 7:06 p.m. UTC | #2
On 07/19/2016 01:54 PM, Jeff Law wrote:
> On 07/19/2016 05:03 AM, Aldy Hernandez wrote:
>>> (Same thing with alloca).  There should be no warning for VLAs,
>>> and for alloca, the warning should say "use of variable-length
>>> array within a loop."  The VRP dump suggests the range information
>>> is available within the loop.  Is the get_range_info() function
>>> not returning the corresponding bounds?
>>
>> This is a false positive, but there's little we can do with the current
>> range infrastructure.  The range information becomes less precise the
>> further down the optimization pipeline we get.  So, even though as far
>> as *.c.126t.crited1, we still see appropriate range information:
> [ ... ]
> So I think we can live with the false positive as an XFAIL while we wait
> for improved infrastructure.
>
> I will note that you could use a two-stage approach to help with this
> kind of issue.  You note the set of potential large allocations early
> (before PRE or anyone else messes it up).  Then you allow the other
> optimizers to run, then go back and recheck the allocations after the
> last optimizer pass.  You end up with
>
> flagged early  && flagged late --> warn
> flagged early && ! flagged late -> optimization eliminated the false
> positive (which you can optionally issue a diagnostic for)
> ! flagged early -- never warn
>
> I don't think you strictly need it here, but it's a way to approach some
> of these problems where you want to run a warning pass late (to allow
> the optimizers to eliminate false positives).

If you feel strongly about this I can certainly do so, but until we get 
better range info, I'd prefer to work on other stuff ;-).

Let me know.

Aldy

Patch
diff mbox

diff --git a/gcc/Makefile.in b/gcc/Makefile.in
index 776f6d7..2a13b8f 100644
--- a/gcc/Makefile.in
+++ b/gcc/Makefile.in
@@ -1284,6 +1284,7 @@  OBJS = \
 	gimple-ssa-nonnull-compare.o \
 	gimple-ssa-split-paths.o \
 	gimple-ssa-strength-reduction.o \
+	gimple-ssa-warn-alloca.o \
 	gimple-streamer-in.o \
 	gimple-streamer-out.o \
 	gimple-walk.o \
diff --git a/gcc/c-family/c-opts.c b/gcc/c-family/c-opts.c
index ff6339c..dc2be2d 100644
--- a/gcc/c-family/c-opts.c
+++ b/gcc/c-family/c-opts.c
@@ -376,6 +376,16 @@  c_common_handle_option (size_t scode, const char *arg, int value,
       cpp_opts->warn_num_sign_change = value;
       break;
 
+    case OPT_Walloca_larger_than_:
+      if (!value)
+	inform (loc, "-Walloca-larger-than=0 is meaningless");
+      break;
+
+    case OPT_Wvla_larger_than_:
+      if (!value)
+	inform (loc, "-Wvla-larger-than=0 is meaningless");
+      break;
+
     case OPT_Wunknown_pragmas:
       /* Set to greater than 1, so that even unknown pragmas in
 	 system headers will be warned about.  */
diff --git a/gcc/c-family/c.opt b/gcc/c-family/c.opt
index 83fd84c..1d4ebf0 100644
--- a/gcc/c-family/c.opt
+++ b/gcc/c-family/c.opt
@@ -275,6 +275,16 @@  Wall
 C ObjC C++ ObjC++ Warning
 Enable most warning messages.
 
+Walloca
+C ObjC C++ ObjC++ Var(warn_alloca) Warning
+Warn on any use of alloca.
+
+Walloca-larger-than=
+C ObjC C++ ObjC++ Var(warn_alloca_limit) Warning Joined RejectNegative UInteger
+-Walloca-larger-than=<number> Warn on unbounded uses of
+alloca, and on bounded uses of alloca whose bound can be larger than
+<number> bytes.
+
 Warray-bounds
 LangEnabledBy(C ObjC C++ ObjC++,Wall)
 ; in common.opt
@@ -980,6 +990,12 @@  Wvla
 C ObjC C++ ObjC++ Var(warn_vla) Init(-1) Warning
 Warn if a variable length array is used.
 
+Wvla-larger-than=
+C ObjC C++ ObjC++ Var(warn_vla_limit) Warning Joined RejectNegative UInteger
+-Wvla-larger-than=<number> Warn on unbounded uses of variable-length arrays, and
+on bounded uses of variable-length arrays whose bound can be
+larger than <number> bytes.
+
 Wvolatile-register-var
 C ObjC C++ ObjC++ Var(warn_volatile_register_var) Warning LangEnabledBy(C ObjC C++ ObjC++,Wall)
 Warn when a register variable is declared volatile.
diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
index 2105351..3847b24 100644
--- a/gcc/doc/invoke.texi
+++ b/gcc/doc/invoke.texi
@@ -253,6 +253,7 @@  Objective-C and Objective-C++ Dialects}.
 @gccoptlist{-fsyntax-only  -fmax-errors=@var{n}  -Wpedantic @gol
 -pedantic-errors @gol
 -w  -Wextra  -Wall  -Waddress  -Waggregate-return  @gol
+-Walloca -Walloca-larger-than=@var{n} @gol
 -Wno-aggressive-loop-optimizations -Warray-bounds -Warray-bounds=@var{n} @gol
 -Wno-attributes -Wbool-compare -Wno-builtin-macro-redefined @gol
 -Wc90-c99-compat -Wc99-c11-compat @gol
@@ -309,7 +310,7 @@  Objective-C and Objective-C++ Dialects}.
 -Wunused-const-variable -Wunused-const-variable=@var{n} @gol
 -Wunused-but-set-parameter -Wunused-but-set-variable @gol
 -Wuseless-cast -Wvariadic-macros -Wvector-operation-performance @gol
--Wvla -Wvolatile-register-var  -Wwrite-strings @gol
+-Wvla -Wvla-larger-than=@var{n} -Wvolatile-register-var  -Wwrite-strings @gol
 -Wzero-as-null-pointer-constant -Whsa}
 
 @item C and Objective-C-only Warning Options
@@ -4618,6 +4619,61 @@  annotations.
 Warn about overriding virtual functions that are not marked with the override
 keyword.
 
+@item -Walloca
+@opindex Wno-alloca
+@opindex Walloca
+This option warns on all uses of @code{alloca} in the source.
+
+@item -Walloca-larger-than=@var{n}
+This option warns on calls to @code{alloca} that are not bounded by a
+controlling predicate limiting its size to @var{n} bytes, or calls to
+@code{alloca} where the bound is unknown.
+
+For example, a bounded case of @code{alloca} could be:
+
+@smallexample
+unsigned int n;
+...
+if (n <= 1000)
+  alloca (n);
+@end smallexample
+
+In the above example, passing @code{-Walloca=1000} would not issue a
+warning because the call to @code{alloca} is known to be at most 1000
+bytes.  However, if @code{-Walloca=500} was passed, the compiler would
+have emitted a warning.
+
+Unbounded uses, on the other hand, are uses of @code{alloca} with no
+controlling predicate verifying its size.  For example:
+
+@smallexample
+stuff ();
+alloca (n);
+@end smallexample
+
+If @code{-Walloca=500} was passed, the above would trigger a warning,
+but this time because of the lack of bounds checking.
+
+Note, that even seemingly correct code involving signed integers could
+cause a warning:
+
+@smallexample
+signed int n;
+...
+if (n < 500)
+  alloca (n);
+@end smallexample
+
+In the above example, @var{n} could be negative, causing a larger than
+expected argument to be implicitly casted into the @code{alloca} call.
+
+This option also warns when @code{alloca} is used in a loop.
+
+This warning is not enabled by @option{-Wall}, and is only active when
+@option{-ftree-vrp} is active (default for @option{-O2} and above).
+
+See also @option{-Wvla-larger-than=@var{n}}.
+
 @item -Warray-bounds
 @itemx -Warray-bounds=@var{n}
 @opindex Wno-array-bounds
@@ -5782,9 +5838,25 @@  moving from a moved-from object, this warning can be disabled.
 @item -Wvla
 @opindex Wvla
 @opindex Wno-vla
-Warn if variable length array is used in the code.
+Warn if a variable-length array is used in the code.
 @option{-Wno-vla} prevents the @option{-Wpedantic} warning of
-the variable length array.
+the variable-length array.
+
+@item -Wvla-larger-than=@var{n}
+If this option is used, the compiler will warn on uses of
+variable-length arrays where the size is either unbounded, or bounded
+by an argument that can be larger than @var{n} bytes.  This is similar
+to how @option{-Walloca-larger-than=@var{n}} works, but with
+variable-length arrays.
+
+Note that GCC may optimize small variable-length arrays of a known
+value into plain arrays, so this warning may not get triggered for
+such arrays.
+
+This warning is not enabled by @option{-Wall}, and is only active when
+@option{-ftree-vrp} is active (default for @option{-O2} and above).
+
+See also @option{-Walloca-larger-than=@var{n}}.
 
 @item -Wvolatile-register-var
 @opindex Wvolatile-register-var
diff --git a/gcc/gimple-ssa-warn-alloca.c b/gcc/gimple-ssa-warn-alloca.c
new file mode 100644
index 0000000..065a72b
--- /dev/null
+++ b/gcc/gimple-ssa-warn-alloca.c
@@ -0,0 +1,517 @@ 
+/* Warn on problematic uses of alloca and variable length arrays.
+   Copyright (C) 2016 Free Software Foundation, Inc.
+   Contributed by Aldy Hernandez <aldyh@redhat.com>.
+
+This file is part of GCC.
+
+GCC is free software; you can redistribute it and/or modify it under
+the terms of the GNU General Public License as published by the Free
+Software Foundation; either version 3, or (at your option) any later
+version.
+
+GCC is distributed in the hope that it will be useful, but WITHOUT ANY
+WARRANTY; without even the implied warranty of MERCHANTABILITY or
+FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+for more details.
+
+You should have received a copy of the GNU General Public License
+along with GCC; see the file COPYING3.  If not see
+<http://www.gnu.org/licenses/>.  */
+
+#include "config.h"
+#include "system.h"
+#include "coretypes.h"
+#include "backend.h"
+#include "tree.h"
+#include "gimple.h"
+#include "tree-pass.h"
+#include "ssa.h"
+#include "gimple-pretty-print.h"
+#include "diagnostic-core.h"
+#include "fold-const.h"
+#include "gimple-iterator.h"
+#include "tree-ssa.h"
+#include "params.h"
+#include "tree-cfg.h"
+#include "calls.h"
+#include "cfgloop.h"
+
+const pass_data pass_data_walloca = {
+  GIMPLE_PASS,
+  "walloca",
+  OPTGROUP_NONE,
+  TV_NONE,
+  PROP_cfg, // properties_required
+  0,	    // properties_provided
+  0,	    // properties_destroyed
+  0,	    // properties_start
+  0,	    // properties_finish
+};
+
+class pass_walloca : public gimple_opt_pass
+{
+public:
+  pass_walloca (gcc::context *ctxt)
+    : gimple_opt_pass(pass_data_walloca, ctxt), first_time_p (false)
+  {}
+  opt_pass *clone () { return new pass_walloca (m_ctxt); }
+  void set_pass_param (unsigned int n, bool param)
+    {
+      gcc_assert (n == 0);
+      first_time_p = param;
+    }
+  virtual bool gate (function *);
+  virtual unsigned int execute (function *);
+
+ private:
+  // Set to TRUE the first time we run this pass on a function.
+  bool first_time_p;
+};
+
+bool
+pass_walloca::gate (function *fun ATTRIBUTE_UNUSED)
+{
+  // The first time this pass is called, it is called before
+  // optimizations have been run and range information is unavailable,
+  // so we can only perform strict alloca checking.
+  if (first_time_p)
+    return warn_alloca != 0;
+
+  return warn_alloca_limit > 0 || warn_vla_limit > 0;
+}
+
+// Possible problematic uses of alloca.
+enum alloca_type {
+  // Alloca argument is within known bounds that are appropriate.
+  ALLOCA_OK,
+
+  // Alloca argument is KNOWN to have a value that is too large.
+  ALLOCA_BOUND_DEFINITELY_LARGE,
+
+  // Alloca argument may be too large.
+  ALLOCA_BOUND_MAYBE_LARGE,
+
+  // Alloca argument is bounded but of an indeterminate size.
+  ALLOCA_BOUND_UNKNOWN,
+
+  // Alloca argument was casted from a signed integer.
+  ALLOCA_CAST_FROM_SIGNED,
+
+  // Alloca appears in a loop.
+  ALLOCA_IN_LOOP,
+
+  // Alloca argument is 0.
+  ALLOCA_ARG_IS_ZERO,
+
+  // Alloca call is unbounded.  That is, there is no controlling
+  // predicate for its argument.
+  ALLOCA_UNBOUNDED
+};
+
+// We have a few heuristics up our sleeve to determine if a call to
+// alloca() is within bounds.  Try them out and return the type of
+// alloca call this is based on its argument.
+//
+// Given a known argument (ARG) to alloca() and an EDGE (E)
+// calculating said argument, verify that the last statement in the BB
+// in E->SRC is a gate comparing ARG to an acceptable bound for
+// alloca().  See examples below.
+//
+// MAX_SIZE is WARN_ALLOCA= adjusted for VLAs.  It is the maximum size
+// in bytes we allow for arg.
+//
+// If the alloca bound is determined to be too large, ASSUMED_LIMIT is
+// set to the bound used to determine this.  ASSUMED_LIMIT is only set
+// for ALLOCA_BOUND_MAYBE_LARGE and ALLOCA_BOUND_DEFINITELY_LARGE.
+//
+// Returns the alloca type.
+
+static enum alloca_type
+alloca_call_type_by_arg (tree arg, edge e, unsigned max_size,
+			 wide_int *assumed_limit)
+{
+  // All the tests bellow depend on the jump being on the TRUE path.
+  if (!(e->flags & EDGE_TRUE_VALUE))
+    return ALLOCA_UNBOUNDED;
+
+  basic_block bb = e->src;
+  gimple_stmt_iterator gsi = gsi_last_bb (bb);
+  gimple *last = gsi_stmt (gsi);
+  if (!last || gimple_code (last) != GIMPLE_COND)
+    return ALLOCA_UNBOUNDED;
+
+  /* Check for:
+     if (ARG <= N)
+       goto <bb 3>;
+      else
+        goto <bb 4>;
+      <bb 3>:
+      alloca(ARG);
+  */
+  if (gimple_cond_code (last) == LE_EXPR
+      && gimple_cond_lhs (last) == arg)
+    {
+      if (TREE_CODE (gimple_cond_rhs (last)) == INTEGER_CST)
+	{
+	  tree rhs = gimple_cond_rhs (last);
+	  if (tree_to_uhwi (rhs) > max_size)
+	    {
+	      *assumed_limit = rhs;
+	      return ALLOCA_BOUND_MAYBE_LARGE;
+	    }
+	  return ALLOCA_OK;
+	}
+      else
+	return ALLOCA_BOUND_UNKNOWN;
+    }
+
+  /* Check for:
+     if (arg .cond. LIMIT) -or- if (LIMIT .cond. arg)
+       alloca(arg);
+
+     Where LIMIT has a bound of unknown range.  */
+  tree limit = NULL;
+  if (gimple_cond_lhs (last) == arg)
+    limit = gimple_cond_rhs (last);
+  else if (gimple_cond_rhs (last) == arg)
+    limit = gimple_cond_lhs (last);
+  if (limit && TREE_CODE (limit) == SSA_NAME)
+    {
+      wide_int min, max;
+      value_range_type range_type = get_range_info (limit, &min, &max);
+      if (range_type == VR_UNDEFINED || range_type == VR_VARYING)
+	return ALLOCA_BOUND_UNKNOWN;
+      // FIXME: We could try harder here and handle a possible range
+      // or anti-range.  Hopefully the upcoming changes to range info
+      // will give us finer grained info, and we can avoid somersaults
+      // here.
+    }
+
+  return ALLOCA_UNBOUNDED;
+}
+
+// Return TRUE if SSA's definition is a cast from a signed type.
+// If so, set *INVALID_CASTED_TYPE to the signed type.
+
+static bool
+cast_from_signed_p (tree ssa, tree *invalid_casted_type)
+{
+  gimple *def = SSA_NAME_DEF_STMT (ssa);
+  if (def
+      && !gimple_nop_p (def)
+      && gimple_assign_cast_p (def)
+      && !TYPE_UNSIGNED (TREE_TYPE (gimple_assign_rhs1 (def))))
+    {
+      *invalid_casted_type = TREE_TYPE (gimple_assign_rhs1 (def));
+      return true;
+    }
+  return false;
+}
+
+// Return TURE if X has a maximum range of MAX, basically covering the
+// entire domain, in which case it's no range at all.
+
+static bool
+is_max (tree x, wide_int max)
+{
+  return wi::max_value (TREE_TYPE (x)) == max;
+}
+
+// Analyze the alloca call in STMT and return an `enum alloca_type'
+// explaining what type of alloca it is.  IS_VLA is set if the alloca
+// call is really a BUILT_IN_ALLOCA_WITH_ALIGN, signifying a VLA.
+//
+// If the alloca bound is determined to be too large, ASSUMED_LIMIT is
+// set to the bound used to determine this.  ASSUMED_LIMIT is only set
+// for ALLOCA_BOUND_MAYBE_LARGE and ALLOCA_BOUND_DEFINITELY_LARGE.
+//
+// If the alloca call may be too large because of a cast from a signed
+// type to an unsigned type, set *INVALID_CASTED_TYPE to the
+// problematic signed type.
+
+static enum alloca_type
+alloca_call_type (gimple *stmt, bool is_vla, wide_int *assumed_limit,
+		  tree *invalid_casted_type)
+{
+  gcc_assert (gimple_alloca_call_p (stmt));
+  tree len = gimple_call_arg (stmt, 0);
+  enum alloca_type w = ALLOCA_UNBOUNDED;
+  wide_int min, max;
+
+  gcc_assert (!is_vla || warn_vla_limit > 0);
+  gcc_assert (is_vla || warn_alloca_limit > 0);
+
+  // Adjust warn_alloca_max_size for VLAs, by taking the underlying
+  // type into account.
+  unsigned HOST_WIDE_INT max_size;
+  if (is_vla)
+    max_size = (unsigned HOST_WIDE_INT) warn_vla_limit;
+  else
+    max_size = (unsigned HOST_WIDE_INT) warn_alloca_limit;
+
+  // Check for the obviously bounded case.
+  if (TREE_CODE (len) == INTEGER_CST)
+    {
+      if (tree_to_uhwi (len) > max_size)
+	{
+	  *assumed_limit = len;
+	  return ALLOCA_BOUND_DEFINITELY_LARGE;
+	}
+      if (integer_zerop (len))
+	return ALLOCA_ARG_IS_ZERO;
+      w = ALLOCA_OK;
+    }
+  else if (TREE_CODE (len) != SSA_NAME)
+    return ALLOCA_UNBOUNDED;
+  // Check the range info if available.
+  else
+    {
+      if (value_range_type range_type = get_range_info (len, &min, &max))
+	{
+	  if (range_type == VR_RANGE)
+	    {
+	      if (wi::leu_p (max, max_size))
+		w = ALLOCA_OK;
+	      else if (is_max (len, max))
+		{
+		  // A cast may have created a range we don't care
+		  // about.  For instance, a cast from 16-bit to
+		  // 32-bit creates a range of 0..65535, even if there
+		  // is not really a determinable range in the
+		  // underlying code.  In this case, look through the
+		  // cast at the original argument, and fall through
+		  // to look at other alternatives.
+		  gimple *def = SSA_NAME_DEF_STMT (len);
+		  if (gimple_assign_cast_p (def))
+		    len = gimple_assign_rhs1 (def);
+		}
+	      else
+		{
+		  /* If `len' is merely a cast that is being
+		     calculated right before the call to alloca, look
+		     at the range for the original value.
+
+		     This avoids the cast creating a range where the
+		     original expression did not have a range:
+
+		     # RANGE [0, 18446744073709551615] NONZERO 4294967295
+		     _2 = (long unsigned int) n_7(D);
+		     p_9 = __builtin_alloca (_2);
+
+		     The correct thing would've been for the user to
+		     use size_t, which in the case above would've been
+		     'long unsigned int', and everything would've
+		     worked.  But we have to catch cases where the
+		     user is using some other compatible type for the
+		     call argument to alloca (say unsigned short).  */
+		  gimple *def = SSA_NAME_DEF_STMT (len);
+		  if (gimple_assign_cast_p (def))
+		    {
+		      len = gimple_assign_rhs1 (def);
+		      range_type = get_range_info (len, &min, &max);
+		    }
+
+		  if (range_type != VR_VARYING && is_max (len, max))
+		    {
+		      // Treat a max of the entire domain as if it had no
+		      // range info, and fall through the try other
+		      // alternatives.
+		    }
+		  else
+		    {
+		      *assumed_limit = max;
+		      return ALLOCA_BOUND_MAYBE_LARGE;
+		    }
+		}
+	    }
+	  else if (range_type == VR_ANTI_RANGE)
+	    {
+	      // There may be some wrapping around going on.  Catch it
+	      // with this heuristic.  Hopefully, this VR_ANTI_RANGE
+	      // nonsense will go away, and we won't have to catch the
+	      // sign conversion problems with this crap.
+	      if (cast_from_signed_p (len, invalid_casted_type))
+		return ALLOCA_CAST_FROM_SIGNED;
+
+	      // Fall thru and try other things.
+	    }
+	  else if (range_type == VR_VARYING)
+	    {
+	      // No easily determined range.  Try other things.
+	    }
+	}
+    }
+
+  // If we couldn't find anything, try a few heuristics for things we
+  // can easily determine.  Check these misc cases but only accept
+  // them if all predecessors have a known bound.
+  basic_block bb = gimple_bb (stmt);
+  if (w == ALLOCA_UNBOUNDED)
+    {
+      w = ALLOCA_OK;
+      for (unsigned ix = 0; ix < EDGE_COUNT (bb->preds); ix++)
+	{
+	  enum alloca_type w
+	    = alloca_call_type_by_arg (len, EDGE_PRED (bb, ix), max_size,
+				       assumed_limit);
+	  if (w != ALLOCA_OK)
+	    return w;
+	}
+    }
+
+  return w;
+}
+
+// Return TRUE if the alloca call in STMT is in a loop, otherwise
+// return FALSE. As an exception, ignore alloca calls for VLAs that
+// occur in a loop since those will be cleaned up when they go out of
+// scope.
+
+static bool
+in_loop_p (bool is_vla, gimple *stmt)
+{
+  basic_block bb = gimple_bb (stmt);
+  if (bb->loop_father
+      // ?? Functions with no loops get a loop_father?  I
+      // don't get it.  The following conditional seems to do
+      // the trick to exclude such nonsense.
+      && bb->loop_father->header != ENTRY_BLOCK_PTR_FOR_FN (cfun))
+    {
+      // Do not warn on VLAs occurring in a loop, since VLAs are
+      // guaranteed to be cleaned up when they go out of scope.
+      // That is, there is a corresponding __builtin_stack_restore
+      // at the end of the scope in which the VLA occurs.
+      tree fndecl = gimple_call_fn (stmt);
+      while (TREE_CODE (fndecl) == ADDR_EXPR)
+	fndecl = TREE_OPERAND (fndecl, 0);
+      if (DECL_BUILT_IN_CLASS (fndecl) == BUILT_IN_NORMAL
+	  && is_vla
+	  && DECL_FUNCTION_CODE (fndecl) == BUILT_IN_ALLOCA_WITH_ALIGN)
+	return false;
+
+      return true;
+    }
+  return false;
+}
+
+unsigned int
+pass_walloca::execute (function *fun)
+{
+  basic_block bb;
+  FOR_EACH_BB_FN (bb, fun)
+    {
+      for (gimple_stmt_iterator si = gsi_start_bb (bb); !gsi_end_p (si);
+	   gsi_next (&si))
+	{
+	  gimple *stmt = gsi_stmt (si);
+	  location_t loc = gimple_location (stmt);
+
+	  if (!gimple_alloca_call_p (stmt))
+	    continue;
+	  gcc_assert (gimple_call_num_args (stmt) >= 1);
+
+	  bool is_vla = gimple_alloca_call_p (stmt)
+	    && gimple_call_alloca_for_var_p (as_a <gcall *> (stmt));
+
+	  // Strict mode whining for VLAs is handled by the front-end,
+	  // so we can safely ignore this case.  Also, ignore VLAs if
+	  // the user doesn't care about them.
+	  if (is_vla
+	      && (warn_vla > 0 || !warn_vla_limit))
+	    continue;
+
+	  if (!is_vla && (warn_alloca || !warn_alloca_limit))
+	    {
+	      if (warn_alloca)
+		warning_at (loc, OPT_Walloca, "use of %<alloca%>");
+	      continue;
+	    }
+
+	  wide_int assumed_limit
+	    = wi::to_wide (integer_zero_node,
+			   TYPE_PRECISION (size_type_node));
+	  tree invalid_casted_type = NULL;
+	  enum alloca_type w = alloca_call_type (stmt, is_vla, &assumed_limit,
+						 &invalid_casted_type);
+
+	  // Even if we think the alloca call is OK, make sure it's
+	  // not in a loop.
+	  if (w == ALLOCA_OK && in_loop_p (is_vla, stmt))
+	    w = ALLOCA_IN_LOOP;
+
+	  enum opt_code wcode
+	    = is_vla ? OPT_Wvla_larger_than_ : OPT_Walloca_larger_than_;
+	  char buff[WIDE_INT_MAX_PRECISION / 4 + 4];
+	  switch (w)
+	    {
+	    case ALLOCA_OK:
+	      break;
+	    case ALLOCA_BOUND_MAYBE_LARGE:
+	      gcc_assert (assumed_limit != 0);
+	      if (warning_at (loc, wcode,
+			      is_vla ? "argument to variable-length array "
+			      "may be too large"
+			      : "argument to %<alloca%> may be too large"))
+		{
+		  print_decu (assumed_limit, buff);
+		  inform (loc, "limit is %u bytes, but argument "
+			  "may be as large as %s",
+			  is_vla ? warn_vla_limit : warn_alloca_limit, buff);
+		}
+	      break;
+	    case ALLOCA_BOUND_DEFINITELY_LARGE:
+	      gcc_assert (assumed_limit != 0);
+	      if (warning_at (loc, wcode,
+			      is_vla ? "argument to variable-length array "
+			      "is too large"
+			      : "argument to %<alloca%> is too large"))
+		{
+		  print_decu (assumed_limit, buff);
+		  inform (loc, "limit is %u bytes, but argument is %s",
+			  is_vla ? warn_vla_limit : warn_alloca_limit, buff);
+		}
+	      break;
+	    case ALLOCA_BOUND_UNKNOWN:
+	      warning_at (loc, wcode,
+			  is_vla ? "variable-length array bound is unknown"
+			  : "%<alloca%> bound is unknown");
+	      break;
+	    case ALLOCA_UNBOUNDED:
+	      warning_at (loc, wcode,
+			  is_vla ? "unbounded use of variable-length array"
+			  : "unbounded use of %<alloca%>");
+	      break;
+	    case ALLOCA_IN_LOOP:
+	      warning_at (loc, wcode,
+			  is_vla ? "use of variable-length array "
+			  "within a loop"
+			  : "use of %<alloca%> within a loop");
+	      break;
+	    case ALLOCA_CAST_FROM_SIGNED:
+	      gcc_assert (invalid_casted_type != NULL_TREE);
+	      warning_at (loc, wcode,
+			  is_vla ? "argument to variable-length array "
+			  "may be too large due to "
+			  "conversion from %qT to %qT"
+			  : "argument to %<alloca%> may be too large due to "
+			  "conversion from %qT to %qT",
+			  invalid_casted_type, size_type_node);
+	      break;
+	    case ALLOCA_ARG_IS_ZERO:
+	      warning_at (loc, wcode,
+			  is_vla ? "argument to variable-length array is zero"
+			  : "argument to %<alloca%> is zero");
+	      break;
+	    default:
+	      gcc_unreachable ();
+	    }
+	}
+    }
+  return 0;
+}
+
+gimple_opt_pass *
+make_pass_walloca (gcc::context *ctxt)
+{
+  return new pass_walloca (ctxt);
+}
diff --git a/gcc/opts.c b/gcc/opts.c
index e80331f..7cdb4bf 100644
--- a/gcc/opts.c
+++ b/gcc/opts.c
@@ -743,6 +743,20 @@  finish_options (struct gcc_options *opts, struct gcc_options *opts_set,
   if (opts->x_flag_tm && opts->x_flag_non_call_exceptions)
     sorry ("transactional memory is not supported with non-call exceptions");
 
+  // -Walloca-larger-than=N needs range info which is only available at -O2.
+  if (opts->x_warn_alloca_limit > 0 && opts->x_optimize < 2)
+    {
+      warning_at (loc, 0, "-Walloca-larger-than=N ignored without -O2");
+      opts->x_warn_alloca_limit = 0;
+    }
+
+  // -Wvla-larger-than= needs range info which is only available at -O2.
+  if (opts->x_warn_vla_limit > 0 && opts->x_optimize < 2)
+    {
+      warning_at (loc, 0, "-Wvla-larger-than= ignored without -O2");
+      opts->x_warn_vla_limit = 0;
+    }
+
   /* Unless the user has asked for section anchors, we disable toplevel
      reordering at -O0 to disable transformations that might be surprising
      to end users and to get -fno-toplevel-reorder tested.  */
diff --git a/gcc/passes.def b/gcc/passes.def
index 3647e90..591add2 100644
--- a/gcc/passes.def
+++ b/gcc/passes.def
@@ -43,6 +43,7 @@  along with GCC; see the file COPYING3.  If not see
   NEXT_PASS (pass_warn_function_return);
   NEXT_PASS (pass_expand_omp);
   NEXT_PASS (pass_build_cgraph_edges);
+  NEXT_PASS (pass_walloca, /*strict_mode_p=*/true);
   TERMINATE_PASS_LIST (all_lowering_passes)
 
   /* Interprocedural optimization passes.  */
@@ -303,6 +304,7 @@  along with GCC; see the file COPYING3.  If not see
       NEXT_PASS (pass_simduid_cleanup);
       NEXT_PASS (pass_lower_vector_ssa);
       NEXT_PASS (pass_cse_reciprocals);
+      NEXT_PASS (pass_walloca, /*strict_mode_p=*/false);
       NEXT_PASS (pass_reassoc, false /* insert_powi_p */);
       NEXT_PASS (pass_strength_reduction);
       NEXT_PASS (pass_split_paths);
diff --git a/gcc/testsuite/gcc.dg/Walloca-1.c b/gcc/testsuite/gcc.dg/Walloca-1.c
new file mode 100644
index 0000000..34a20c3
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/Walloca-1.c
@@ -0,0 +1,63 @@ 
+/* { dg-do compile } */
+/* { dg-options "-Walloca-larger-than=2000 -O2" } */
+
+#define alloca __builtin_alloca
+
+typedef __SIZE_TYPE__ size_t;
+extern size_t strlen(const char *);
+
+extern void useit (char *);
+
+int num;
+
+void foo1 (size_t len, size_t len2, size_t len3)
+{
+  int i;
+
+  for (i=0; i < 123; ++i)
+    {
+      char *s = alloca (566);	/* { dg-warning "'alloca' within a loop" } */
+      useit (s);
+    }
+
+  char *s = alloca (123);
+  useit (s);			// OK, constant argument to alloca
+
+  s = alloca (num);		// { dg-warning "large due to conversion" }
+  useit (s);
+
+  s = alloca(90000);		/* { dg-warning "is too large" } */
+  useit (s);
+
+  if (len < 2000)
+    {
+      s = alloca(len);		// OK, bounded
+      useit (s);
+    }
+
+  if (len + len2 < 2000)	// OK, bounded
+    {
+      s = alloca(len + len2);
+      useit (s);
+    }
+
+  if (len3 <= 2001)
+    {
+      s = alloca(len3);		/* { dg-warning "may be too large" } */
+      useit(s);
+    }
+}
+
+void foo2 (__SIZE_TYPE__ len)
+{
+  // Test that a direct call to __builtin_alloca_with_align is not confused
+  // with a VLA.
+  void *p = __builtin_alloca_with_align (len, 8); // { dg-warning "unbounded use of 'alloca'" }
+  useit (p);
+}
+
+void foo3 (unsigned char a)
+{
+  if (a == 0)
+    useit (__builtin_alloca (a)); // { dg-warning "argument to 'alloca' is zero" }
+}
diff --git a/gcc/testsuite/gcc.dg/Walloca-2.c b/gcc/testsuite/gcc.dg/Walloca-2.c
new file mode 100644
index 0000000..284b34e
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/Walloca-2.c
@@ -0,0 +1,40 @@ 
+/* { dg-do compile } */
+/* { dg-options "-Walloca-larger-than=2000 -O2" } */
+
+void f (void *);
+
+void
+g1 (int n)
+{
+  void *p;
+  if (n > 0 && n < 2000)
+    p = __builtin_alloca (n);
+  else
+    p = __builtin_malloc (n);
+  f (p);
+}
+
+void
+g2 (int n)
+{
+  void *p;
+  if (n < 2000)
+    p = __builtin_alloca (n); // { dg-warning "large due to conversion" }
+  else
+    p = __builtin_malloc (n);
+  f (p);
+}
+
+void
+g3 (int n)
+{
+  void *p;
+  if (n > 0 && n < 3000)
+    {
+      p = __builtin_alloca (n); // { dg-warning "'alloca' may be too large" }
+      // { dg-message "note:.*argument may be as large as 2999" "note" { target *-*-* } 34 }
+    }
+  else
+    p = __builtin_malloc (n);
+  f (p);
+}
diff --git a/gcc/testsuite/gcc.dg/Walloca-3.c b/gcc/testsuite/gcc.dg/Walloca-3.c
new file mode 100644
index 0000000..5345197
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/Walloca-3.c
@@ -0,0 +1,33 @@ 
+/* { dg-do compile } */
+/* { dg-options "-Walloca-larger-than=2000 -O2" } */
+
+void f (void *);
+
+__SIZE_TYPE__ LIMIT;
+
+// Warn when there is an alloca bound, but it is an unknown bound.
+
+void
+g1 (__SIZE_TYPE__ n)
+{
+  void *p;
+  if (n < LIMIT)
+    p = __builtin_alloca (n); // { dg-warning "'alloca' bound is unknown" }
+  else
+    p = __builtin_malloc (n);
+  f (p);
+}
+
+// Similar to the above, but do not get confused by the upcast.
+
+unsigned short SHORT_LIMIT;
+void
+g2 (unsigned short n)
+{
+  void *p;
+  if (n < SHORT_LIMIT)
+    p = __builtin_alloca (n); // { dg-warning "'alloca' bound is unknown" }
+  else
+    p = __builtin_malloc (n);
+  f (p);
+}
diff --git a/gcc/testsuite/gcc.dg/Walloca-4.c b/gcc/testsuite/gcc.dg/Walloca-4.c
new file mode 100644
index 0000000..d96cc4e
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/Walloca-4.c
@@ -0,0 +1,20 @@ 
+/* { dg-do compile } */
+/* { dg-options "-Walloca-larger-than=5000 -O2" } */
+/* { dg-xfail-if "Currently broken but Andrew's work should fix this" { *-*-* } } */
+
+// Should be another variant of Walloca-7.c.
+// This should not warn, as we have a known bound within limits.
+
+ char *
+ _i18n_number_rewrite (char *w, char *rear_ptr)
+{
+
+  char *src;
+ _Bool 
+      use_alloca = (((rear_ptr - w) * sizeof (char)) < 4096U);
+ if (use_alloca)
+    src = (char *) __builtin_alloca ((rear_ptr - w) * sizeof (char));
+  else
+    src = (char *) __builtin_malloc ((rear_ptr - w) * sizeof (char));
+  return src;
+}
diff --git a/gcc/testsuite/gcc.dg/Walloca-5.c b/gcc/testsuite/gcc.dg/Walloca-5.c
new file mode 100644
index 0000000..5ed1171
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/Walloca-5.c
@@ -0,0 +1,32 @@ 
+/* { dg-do compile } */
+/* { dg-options "-Walloca-larger-than=123 -O2" } */
+/* { dg-xfail-if "Currently broken but Andrew's work should fix this" { *-*-* } } */
+
+/* The argument to alloca ends up having a range of 0..MAXINT(32bits),
+   so we think we have a range because of the upcast.  Consequently,
+   we warn with "alloca may be too large", but we should technically
+   warn with "unbounded use of alloca".
+
+   We currently drill through casts to figure this stuff out, but we
+   get confused because it's not just a cast.  It's a cast, plus a
+   multiply.
+
+   <bb 2>:
+  # RANGE [0, 4294967295] NONZERO 4294967295
+  _1 = (long unsigned int) something_4(D);
+  # RANGE [0, 34359738360] NONZERO 34359738360
+  _2 = _1 * 8;
+  _3 = __builtin_alloca (_2);
+
+  I don't know whether it's even worth such fine-grained warnings.
+  Perhaps we should generically warn everywhere with "alloca may be
+  too large".
+
+  I'm hoping that this particular case will be easier to diagnose with
+  Andrew's work.  */
+
+void useit(void *);
+void foobar(unsigned int something)
+{
+  useit(__builtin_alloca (something * sizeof (const char *))); // { dg-warning "unbounded use of alloca" "" { xfail *-*-* } }
+}
diff --git a/gcc/testsuite/gcc.dg/Walloca-6.c b/gcc/testsuite/gcc.dg/Walloca-6.c
new file mode 100644
index 0000000..b4d8d41
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/Walloca-6.c
@@ -0,0 +1,11 @@ 
+/* { dg-do compile } */
+/* { dg-options "-Walloca-larger-than=256 -O2" } */
+/* { dg-xfail-if "Currently broken but Andrew's work should fix this" { *-*-* } } */
+
+void f (void*);
+void g (__SIZE_TYPE__ n)
+{
+  // No warning on this case.  Range is easily determinable.
+  if (n > 0 && n < 256)
+    f (__builtin_alloca (n));
+}
diff --git a/gcc/testsuite/gcc.dg/Walloca-7.c b/gcc/testsuite/gcc.dg/Walloca-7.c
new file mode 100644
index 0000000..d6581a5
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/Walloca-7.c
@@ -0,0 +1,17 @@ 
+/* { dg-do compile } */
+/* { dg-options "-Walloca -O0" } */
+
+extern void f(void *);
+
+void foo(void)
+{
+  // Test that strict -Walloca works even without optimization.
+  f (__builtin_alloca(500)); // { dg-warning "use of 'alloca'" }
+}
+
+void bar(void)
+{
+  // Test that we warn on alloca() calls, not just __builtin_alloca calls.
+  extern void *alloca(__SIZE_TYPE__);
+  f (alloca (123)); // { dg-warning "use of 'alloca'" }
+}
diff --git a/gcc/testsuite/gcc.dg/Wvla-1.c b/gcc/testsuite/gcc.dg/Wvla-1.c
new file mode 100644
index 0000000..384c930
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/Wvla-1.c
@@ -0,0 +1,24 @@ 
+/* { dg-do compile } */
+/* { dg-options "-Wvla-larger-than=100 -O2" } */
+
+typedef __SIZE_TYPE__ size_t;
+
+extern void useit (char *);
+
+int num;
+
+void test_vlas (size_t num)
+{
+  char str2[num];		/* { dg-warning "unbounded use" } */
+  useit(str2);
+
+  num = 98;
+  for (int i=0; i < 1234; ++i) {
+    char str[num];	        // OK, VLA in a loop, but it is a
+				// known size *AND* the compiler takes
+				// care of cleaning up between
+				// iterations with
+				// __builtin_stack_restore.
+    useit(str);
+  }
+}
diff --git a/gcc/testsuite/gcc.dg/Wvla-2.c b/gcc/testsuite/gcc.dg/Wvla-2.c
new file mode 100644
index 0000000..96814dc
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/Wvla-2.c
@@ -0,0 +1,70 @@ 
+/* { dg-do compile } */
+/* { dg-require-effective-target stdint_types } */
+/* { dg-options "-O2 -Wvla-larger-than=40" } */
+
+#include <stdint.h>
+
+void f0 (void *);
+void
+f1 (__SIZE_TYPE__ a)
+{
+  if (a <= 10)
+    {
+      // 10 * 4 bytes = 40: OK!
+      uint32_t x[a];
+      f0 (x);
+    }
+}
+
+void
+f2 (__SIZE_TYPE__ a)
+{
+  if (a <= 11)
+    {
+      // 11 * 4 bytes = 44: Not OK.
+      uint32_t x[a]; // { dg-warning "array may be too large" }
+      // { dg-message "note:.*argument may be as large as 44" "note" { target *-*-* } 25 }
+      f0 (x);
+    }
+}
+
+void
+f3 (__SIZE_TYPE__ a, __SIZE_TYPE__ b)
+{
+  if (a <= 5 && b <= 3)
+    {
+      // 5 * 3 * 4 bytes = 60: Not OK.
+      uint32_t x[a][b]; // { dg-warning "array may be too large" }
+      f0 (x);
+    }
+}
+
+void
+f4 (__SIZE_TYPE__ a, __SIZE_TYPE__ b)
+{
+  if (a <= 5 && b <= 2)
+    {
+      // 5 * 2 * 4 bytes = 40 bytes: OK!
+      uint32_t x[a][b];
+      f0 (x);
+    }
+}
+
+void
+f5 (__SIZE_TYPE__ len)
+{
+  // Test that a direct call to __builtin_alloca_with_align is not
+  // confused with a VLA.
+  void *p = __builtin_alloca_with_align (len, 8);
+  f0 (p);
+}
+
+void
+f6 (unsigned stuff)
+{
+  int n = 7000;
+  do {
+    char a[n]; // { dg-warning "variable-length array is too large" }
+    f0 (a);
+  } while (stuff--);
+}
diff --git a/gcc/testsuite/gcc.dg/Wvla-3.c b/gcc/testsuite/gcc.dg/Wvla-3.c
new file mode 100644
index 0000000..5124476
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/Wvla-3.c
@@ -0,0 +1,12 @@ 
+/* { dg-do compile } */
+/* { dg-options "-Walloca -O2" } */
+
+// Make sure we don't warn on VLA with -Walloca.
+
+void f (void*);
+
+void h1 (unsigned n)
+{
+  int a [n];
+  f (a);
+}
diff --git a/gcc/tree-pass.h b/gcc/tree-pass.h
index 36299a6..57b61f4 100644
--- a/gcc/tree-pass.h
+++ b/gcc/tree-pass.h
@@ -469,6 +469,7 @@  extern simple_ipa_opt_pass *make_pass_ipa_oacc (gcc::context *ctxt);
 extern simple_ipa_opt_pass *make_pass_ipa_oacc_kernels (gcc::context *ctxt);
 extern gimple_opt_pass *make_pass_gen_hsail (gcc::context *ctxt);
 extern gimple_opt_pass *make_pass_warn_nonnull_compare (gcc::context *ctxt);
+extern gimple_opt_pass *make_pass_walloca (gcc::context *ctxt);
 
 /* IPA Passes */
 extern simple_ipa_opt_pass *make_pass_ipa_lower_emutls (gcc::context *ctxt);