alloca: port to compilers that can optimize like GCC 4.6.0

* lib/alloca.c (find_stack_direction): New signature, taken from
Autoconf git.  This works with GCC 4.6.0.  This code should never
be used with GCC 4.6.0 itself, as GCC has alloca, but it might
be used with other compilers that optimize as well as GCC 4.6.0 does.
(alloca): Adjust to new signature.
* m4/alloca.m4 (__AC_LIBOBJ_ALLOCA) [Autoconf version < 2.69]:
New macro, which patches Autoconf in a similar way.

1 files changed, 9 insertions, 22 deletions

diff --git a/lib/alloca.c b/lib/alloca.c
index 2f5e27ec69..c7e91ab933 100644
--- a/lib/alloca.c
+++ b/lib/alloca.c
@@ -93,25 +93,15 @@ long i00afunc ();
 static int stack_dir;           /* 1 or -1 once known.  */
 #   define STACK_DIR    stack_dir
 
-static void
-find_stack_direction (char **ptr)
+static int
+find_stack_direction (int *addr, int depth)
 {
-  auto char dummy;              /* To get stack address.  */
-
-  if (*ptr == NULL)
-    {                           /* Initial entry.  */
-      *ptr = ADDRESS_FUNCTION (dummy);
-
-      find_stack_direction (ptr);  /* Recurse once.  */
-    }
-  else
-    {
-      /* Second entry.  */
-      if (ADDRESS_FUNCTION (dummy) > *ptr)
-        stack_dir = 1;          /* Stack grew upward.  */
-      else
-        stack_dir = -1;         /* Stack grew downward.  */
-    }
+  int dir, dummy = 0;
+  if (! addr)
+    addr = &dummy;
+  *addr = addr < &dummy ? 1 : addr == &dummy ? 0 : -1;
+  dir = depth ? find_stack_direction (addr, depth - 1) : 0;
+  return dir + dummy;
 }
 
 #  endif /* STACK_DIRECTION == 0 */
@@ -154,10 +144,7 @@ alloca (size_t size)
 
 #  if STACK_DIRECTION == 0
   if (STACK_DIR == 0)           /* Unknown growth direction.  */
-    {
-      char *addr = NULL;        /* Address of first `dummy', once known.  */
-      find_stack_direction (&addr);
-    }
+    STACK_DIR = find_stack_direction (NULL, (size & 1) + 20);
 #  endif
 
   /* Reclaim garbage, defined as all alloca'd storage that