[PATCH] Use ECF_MAY_BE_ALLOCA for __builtin_alloca_with_align (PR tree-optimization/68680)

[Jakub Jelinek <jakub@redhat.com>]

Hi!

As mentioned in the PR, GCC 4.7+ seems to have regressed for
-fstack-protector*, functions containing VLAs and no other arrays are not
protected anymore.  Before 4.7, VLAs were gimplified as __builtin_alloca
call, which sets ECF_MAY_BE_ALLOCA and in turn cfun->calls_alloca.
These two are used in various places:
1) for stack protector purposes (this issue), early during expansion
2) in the inliner
3) for tail call optimization
4) for some non-NULL optimizations
and tons of places in RTL.  As 4.7+ emits __builtin_alloca_with_align
instead and special_function_p has not been adjusted, this does not happen
any longer, though cfun->calls_alloca gets set during the expansion of
__builtin_alloca_with_align, so for RTL optimizers it is already set.

The following patch restores the previous behavior, making VLAs be
ECF_MAY_BE_ALLOCA and cfun->calls_alloca already during GIMPLE passes.
It could be also done by testing the name, but I thought that it would be
too ugly (would need another case anyway, as the current tests are for
names with length <= 16).

1) and 4) surely want to treat the VLAs like the patch does, I'm not 100%
sure about 2) and 3), as VLAs are slightly different, they release
the stack afterwards at the end of scope of the VLA var.  If we wanted to
treat the two differently, maybe we'd need another ECF* flag and another
cfun bitfield for VLAs.

The following patch has been bootstrapped/regtested on x86_64-linux and
i686-linux.

2015-12-03  Jakub Jelinek  <jakub@redhat.com>

	PR tree-optimization/68680
	* calls.c (special_function_p): Return ECF_MAY_BE_ALLOCA for
	BUILT_IN_ALLOCA{,_WITH_ALIGN}.

	* gcc.target/i386/pr68680.c: New test.

--- gcc/calls.c.jj	2015-11-26 11:17:25.000000000 +0100
+++ gcc/calls.c	2015-12-03 19:03:59.342306457 +0100
@@ -553,6 +553,17 @@ special_function_p (const_tree fndecl, i
 	flags |= ECF_NORETURN;
     }
 
+  if (DECL_BUILT_IN_CLASS (fndecl) == BUILT_IN_NORMAL)
+    switch (DECL_FUNCTION_CODE (fndecl))
+      {
+      case BUILT_IN_ALLOCA:
+      case BUILT_IN_ALLOCA_WITH_ALIGN:
+	flags |= ECF_MAY_BE_ALLOCA;
+	break;
+      default:
+	break;
+      }
+
   return flags;
 }
 
--- gcc/testsuite/gcc.target/i386/pr68680.c.jj	2015-12-03 19:10:14.836037923 +0100
+++ gcc/testsuite/gcc.target/i386/pr68680.c	2015-12-03 19:09:57.000000000 +0100
@@ -0,0 +1,15 @@
+/* PR tree-optimization/68680 */
+/* { dg-do compile } */
+/* { dg-options "-O2 -fstack-protector-strong" } */
+
+int foo (char *);
+
+int
+bar (unsigned long x)
+{
+  char a[x];
+  return foo (a);
+}
+
+/* Verify that this function is stack protected.  */
+/* { dg-final { scan-assembler "stack_chk_fail" } } */

	Jakub