From: "Jan Beulich" <JBeulich@suse.com>
To: <gcc-patches@gcc.gnu.org>
Subject: avoid alignment of static variables affecting stack's
Date: Thu, 23 Oct 2014 06:50:00 -0000 [thread overview]
Message-ID: <5448BCA30200007800041508@mail.emea.novell.com> (raw)
[-- Attachment #1: Type: text/plain, Size: 3572 bytes --]
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 #2: gcc-trunk-stack-align-ignore-static.patch --]
[-- Type: text/plain, Size: 3623 bytes --]
avoid alignment of static variables affecting stack's
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));
+}
next reply other threads:[~2014-10-23 6:30 UTC|newest]
Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top
2014-10-23 6:50 Jan Beulich [this message]
2014-10-23 6:54 ` Jakub Jelinek
2014-10-23 7:11 ` Jan Beulich
2014-10-23 18:14 ` Jeff Law
2014-10-24 9:11 ` Jan Beulich
2014-10-24 9:12 ` Richard Biener
2014-10-24 9:18 ` Jan Beulich
2014-10-24 9:19 ` Jakub Jelinek
2014-10-24 9:54 ` Richard Biener
2014-10-24 10:16 ` Jan Beulich
2014-10-24 10:42 ` Richard Biener
2014-10-24 16:42 ` Jeff Law
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