2015-12-04  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	(revision 231278)
+++ gcc/calls.c	(revision 231279)
@@ -564,6 +564,17 @@ 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	(revision 0)
+++ gcc/testsuite/gcc.target/i386/pr68680.c	(revision 231279)
@@ -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" } } */