avoid alignment of static variables affecting stack's

["Jan Beulich" <JBeulich at suse dot com>]

Function (or more narrow) scope static variables (as well as others not
placed on the stack) should also not have any effect on the stack
alignment. I noticed the issue first with Linux'es dynamic_pr_debug()
construct using an 8-byte aligned sub-file-scope local variable.

According to my checking bad behavior started with 4.6.x (4.5.3 was
still okay), but generated code got quite a bit worse as of 4.9.0.

gcc/
2014-10-23  Jan Beulich  <jbeulich@suse.com>

	* cfgexpand.c (expand_one_var): Exclude static, external, and
	hard register variables when adjusting stack alignment related
	state.

gcc/testsuite/
2014-10-23  Jan Beulich  <jbeulich@suse.com>

	* gcc.c-torture/execute/stkalign.c: New.

--- a/gcc/cfgexpand.c
+++ b/gcc/cfgexpand.c
@@ -1233,12 +1233,16 @@ static HOST_WIDE_INT
 expand_one_var (tree var, bool toplevel, bool really_expand)
 {
   unsigned int align = BITS_PER_UNIT;
+  bool stack = true;
   tree origvar = var;
 
   var = SSAVAR (var);
 
   if (TREE_TYPE (var) != error_mark_node && TREE_CODE (var) == VAR_DECL)
     {
+      stack = !TREE_STATIC (var) && !DECL_EXTERNAL (var)
+	      && !DECL_HARD_REGISTER (var);
+
       /* Because we don't know if VAR will be in register or on stack,
 	 we conservatively assume it will be on stack even if VAR is
 	 eventually put into register after RA pass.  For non-automatic
@@ -1267,22 +1271,25 @@ expand_one_var (tree var, bool toplevel,
 	align = POINTER_SIZE;
     }
 
-  if (SUPPORTS_STACK_ALIGNMENT
-      && crtl->stack_alignment_estimated < align)
+  if (stack)
     {
-      /* stack_alignment_estimated shouldn't change after stack
-         realign decision made */
-      gcc_assert (!crtl->stack_realign_processed);
-      crtl->stack_alignment_estimated = align;
+      if (SUPPORTS_STACK_ALIGNMENT
+	  && crtl->stack_alignment_estimated < align)
+	{
+	  /* stack_alignment_estimated shouldn't change after stack
+	     realign decision made */
+	  gcc_assert (!crtl->stack_realign_processed);
+	  crtl->stack_alignment_estimated = align;
+	}
+
+      /* stack_alignment_needed > PREFERRED_STACK_BOUNDARY is permitted.
+	 So here we only make sure stack_alignment_needed >= align.  */
+      if (crtl->stack_alignment_needed < align)
+	crtl->stack_alignment_needed = align;
+      if (crtl->max_used_stack_slot_alignment < align)
+	crtl->max_used_stack_slot_alignment = align;
     }
 
-  /* stack_alignment_needed > PREFERRED_STACK_BOUNDARY is permitted.
-     So here we only make sure stack_alignment_needed >= align.  */
-  if (crtl->stack_alignment_needed < align)
-    crtl->stack_alignment_needed = align;
-  if (crtl->max_used_stack_slot_alignment < align)
-    crtl->max_used_stack_slot_alignment = align;
-
   if (TREE_CODE (origvar) == SSA_NAME)
     {
       gcc_assert (TREE_CODE (var) != VAR_DECL
--- a/gcc/testsuite/gcc.c-torture/execute/stkalign.c
+++ b/gcc/testsuite/gcc.c-torture/execute/stkalign.c
@@ -0,0 +1,26 @@
+/* { dg-options "-fno-inline" } */
+
+#include <assert.h>
+
+#define ALIGNMENT 64
+
+unsigned test(unsigned n, unsigned p)
+{
+  static struct { char __attribute__((__aligned__(ALIGNMENT))) c; } s;
+  unsigned x;
+
+  assert(__alignof__(s) == ALIGNMENT);
+  asm ("" : "=g" (x), "+m" (s) : "0" (&x));
+
+  return n ? test(n - 1, x) : (x ^ p);
+}
+
+int main (int argc, char *argv[] __attribute__((unused)))
+{
+  unsigned int x = test(argc, 0);
+
+  x |= test(argc + 1, 0);
+  x |= test(argc + 2, 0);
+
+  return !(x & (ALIGNMENT - 1));
+}

Attachment: gcc-trunk-stack-align-ignore-static.patch
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