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* [Bug tree-optimization/46781] New: [4.6 Regression] Missing optimization
@ 2010-12-03 12:36 d.g.gorbachev at gmail dot com
2010-12-03 12:38 ` [Bug tree-optimization/46781] " d.g.gorbachev at gmail dot com
` (4 more replies)
0 siblings, 5 replies; 6+ messages in thread
From: d.g.gorbachev at gmail dot com @ 2010-12-03 12:36 UTC (permalink / raw)
To: gcc-bugs
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=46781
Summary: [4.6 Regression] Missing optimization
Product: gcc
Version: 4.6.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
AssignedTo: unassigned@gcc.gnu.org
ReportedBy: d.g.gorbachev@gmail.com
Created attachment 22612
--> http://gcc.gnu.org/bugzilla/attachment.cgi?id=22612
Code produced by GCC 4.6.0
Compiled with -O2
^ permalink raw reply [flat|nested] 6+ messages in thread
* [Bug tree-optimization/46781] [4.6 Regression] Missing optimization
2010-12-03 12:36 [Bug tree-optimization/46781] New: [4.6 Regression] Missing optimization d.g.gorbachev at gmail dot com
@ 2010-12-03 12:38 ` d.g.gorbachev at gmail dot com
2010-12-03 13:55 ` rguenth at gcc dot gnu.org
` (3 subsequent siblings)
4 siblings, 0 replies; 6+ messages in thread
From: d.g.gorbachev at gmail dot com @ 2010-12-03 12:38 UTC (permalink / raw)
To: gcc-bugs
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=46781
--- Comment #1 from Dmitry Gorbachev <d.g.gorbachev at gmail dot com> 2010-12-03 12:38:03 UTC ---
Created attachment 22613
--> http://gcc.gnu.org/bugzilla/attachment.cgi?id=22613
Code produced by GCC 4.5.2
C source is in attachment 19980.
^ permalink raw reply [flat|nested] 6+ messages in thread
* [Bug tree-optimization/46781] [4.6 Regression] Missing optimization
2010-12-03 12:36 [Bug tree-optimization/46781] New: [4.6 Regression] Missing optimization d.g.gorbachev at gmail dot com
2010-12-03 12:38 ` [Bug tree-optimization/46781] " d.g.gorbachev at gmail dot com
@ 2010-12-03 13:55 ` rguenth at gcc dot gnu.org
2010-12-03 15:39 ` d.g.gorbachev at gmail dot com
` (2 subsequent siblings)
4 siblings, 0 replies; 6+ messages in thread
From: rguenth at gcc dot gnu.org @ 2010-12-03 13:55 UTC (permalink / raw)
To: gcc-bugs
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=46781
Richard Guenther <rguenth at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|UNCONFIRMED |NEW
Last reconfirmed| |2010.12.03 13:55:38
Target Milestone|--- |4.6.0
Ever Confirmed|0 |1
--- Comment #2 from Richard Guenther <rguenth at gcc dot gnu.org> 2010-12-03 13:55:38 UTC ---
Confirmed. GCC 4.5 tree DSE removes the first store to p:
fun (void * * q)
{
void * r;
void * * p.1;
<bb 2>:
p.1_1 = p;
p = 0B;
r_3 = *q_2(D);
p = p.1_1;
return r_3;
and then PRE figures out the redundant store.
4.6 correctly preserves the store to p as the read from *q aliases it
(4.5 type-based aliasing concludes that void * doesn't alias void **,
something that people regularly get wrong thus I dumbed down TBAA).
Where did you get this testcase from?
To illustrate the difference, the following testcase is not optimized
by GCC 4.5 either (and that's required):
extern void **p;
void **fun (void ***q)
{
void **t;
void **r;
t = p;
p = 0;
r = *q;
p = t;
return r;
}
The observed change is caused by:
2010-08-25 Richard Guenther <rguenther@suse.de>
* alias.c (get_alias_set): Assign a single alias-set to all pointers.
* gimple.c (gimple_get_alias_set): Remove special handling
for pointers.
^ permalink raw reply [flat|nested] 6+ messages in thread
* [Bug tree-optimization/46781] [4.6 Regression] Missing optimization
2010-12-03 12:36 [Bug tree-optimization/46781] New: [4.6 Regression] Missing optimization d.g.gorbachev at gmail dot com
2010-12-03 12:38 ` [Bug tree-optimization/46781] " d.g.gorbachev at gmail dot com
2010-12-03 13:55 ` rguenth at gcc dot gnu.org
@ 2010-12-03 15:39 ` d.g.gorbachev at gmail dot com
2011-01-03 20:32 ` rguenth at gcc dot gnu.org
2011-02-03 8:55 ` jakub at gcc dot gnu.org
4 siblings, 0 replies; 6+ messages in thread
From: d.g.gorbachev at gmail dot com @ 2010-12-03 15:39 UTC (permalink / raw)
To: gcc-bugs
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=46781
Dmitry Gorbachev <d.g.gorbachev at gmail dot com> changed:
What |Removed |Added
----------------------------------------------------------------------------
Attachment #22612|application/octet-stream |text/plain
mime type| |
--- Comment #3 from Dmitry Gorbachev <d.g.gorbachev at gmail dot com> 2010-12-03 15:39:16 UTC ---
Comment on attachment 22612
--> http://gcc.gnu.org/bugzilla/attachment.cgi?id=22612
Code produced by GCC 4.6.0
(In reply to comment #2)
> Where did you get this testcase from?
May I know why do you ask?
IIRC, it was distilled from some large program. There was a difference when
compiling it with and without LTO, and I reported it as a bug 43201.
^ permalink raw reply [flat|nested] 6+ messages in thread
* [Bug tree-optimization/46781] [4.6 Regression] Missing optimization
2010-12-03 12:36 [Bug tree-optimization/46781] New: [4.6 Regression] Missing optimization d.g.gorbachev at gmail dot com
` (2 preceding siblings ...)
2010-12-03 15:39 ` d.g.gorbachev at gmail dot com
@ 2011-01-03 20:32 ` rguenth at gcc dot gnu.org
2011-02-03 8:55 ` jakub at gcc dot gnu.org
4 siblings, 0 replies; 6+ messages in thread
From: rguenth at gcc dot gnu.org @ 2011-01-03 20:32 UTC (permalink / raw)
To: gcc-bugs
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=46781
--- Comment #4 from Richard Guenther <rguenth at gcc dot gnu.org> 2011-01-03 20:31:55 UTC ---
(In reply to comment #3)
> Comment on attachment 22612 [details]
> Code produced by GCC 4.6.0
>
> (In reply to comment #2)
> > Where did you get this testcase from?
>
> May I know why do you ask?
To see if the change indeed causes important regressions in real-world
code. The change was done to be less surprising when doing TBAA based
disambiguations as people regularly expect void * to be similar to char *
and also do not really handle multiple-indirect pointers the correct
way. Thus, we now miscompile less not strictly conforming programs.
> IIRC, it was distilled from some large program. There was a difference when
> compiling it with and without LTO, and I reported it as a bug 43201.
Ah, that explains it (LTO did the pointer TBAA thing for a long time).
^ permalink raw reply [flat|nested] 6+ messages in thread
* [Bug tree-optimization/46781] [4.6 Regression] Missing optimization
2010-12-03 12:36 [Bug tree-optimization/46781] New: [4.6 Regression] Missing optimization d.g.gorbachev at gmail dot com
` (3 preceding siblings ...)
2011-01-03 20:32 ` rguenth at gcc dot gnu.org
@ 2011-02-03 8:55 ` jakub at gcc dot gnu.org
4 siblings, 0 replies; 6+ messages in thread
From: jakub at gcc dot gnu.org @ 2011-02-03 8:55 UTC (permalink / raw)
To: gcc-bugs
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=46781
Jakub Jelinek <jakub at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|NEW |RESOLVED
CC| |jakub at gcc dot gnu.org
Resolution| |WONTFIX
--- Comment #5 from Jakub Jelinek <jakub at gcc dot gnu.org> 2011-02-03 08:55:23 UTC ---
The change was intentional.
^ permalink raw reply [flat|nested] 6+ messages in thread
end of thread, other threads:[~2011-02-03 8:55 UTC | newest]
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2010-12-03 12:36 [Bug tree-optimization/46781] New: [4.6 Regression] Missing optimization d.g.gorbachev at gmail dot com
2010-12-03 12:38 ` [Bug tree-optimization/46781] " d.g.gorbachev at gmail dot com
2010-12-03 13:55 ` rguenth at gcc dot gnu.org
2010-12-03 15:39 ` d.g.gorbachev at gmail dot com
2011-01-03 20:32 ` rguenth at gcc dot gnu.org
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