[v3] 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.

[v3: Re-base to current trunk.]
[v2: Drop inclusion of hard register variables, as requested by
     Jakub and Richard.]

gcc/
2015-12-10  Jan Beulich  <jbeulich@suse.com>

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

gcc/testsuite/
2015-12-10  Jan Beulich  <jbeulich@suse.com>

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

--- 2015-12-09/gcc/cfgexpand.c
+++ 2015-12-09/gcc/cfgexpand.c
@@ -1544,12 +1544,15 @@ 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);
+
       /* 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
@@ -1578,7 +1581,8 @@ expand_one_var (tree var, bool toplevel,
 	align = POINTER_SIZE;
     }
 
-  record_alignment_for_reg_var (align);
+  if (stack)
+    record_alignment_for_reg_var (align);
 
   if (TREE_CODE (origvar) == SSA_NAME)
     {
--- 2015-12-09/gcc/testsuite/gcc.c-torture/execute/stkalign.c
+++ 2015-12-09/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
Description: Text document