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* [Bug target/98977] New: [x86] Failure to optimize consecutive sub flags usage
@ 2021-02-05 14:20 gabravier at gmail dot com
2021-02-06 6:30 ` [Bug target/98977] " pinskia at gcc dot gnu.org
` (3 more replies)
0 siblings, 4 replies; 5+ messages in thread
From: gabravier at gmail dot com @ 2021-02-05 14:20 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98977
Bug ID: 98977
Summary: [x86] Failure to optimize consecutive sub flags usage
Product: gcc
Version: 11.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: target
Assignee: unassigned at gcc dot gnu.org
Reporter: gabravier at gmail dot com
Target Milestone: ---
extern bool z, c;
uint8_t f(uint8_t dest, uint8_t src)
{
u8 res = dest - src;
z = !res;
c = src > dest;
return res;
}
With -O3, LLVM outputs this:
f(unsigned char, unsigned char):
mov eax, edi
sub al, sil
sete byte ptr [rip + z]
setb byte ptr [rip + c]
ret
GCC outputs this:
f(unsigned char, unsigned char):
mov eax, edi
sub al, sil
sete BYTE PTR z[rip]
cmp dil, sil
setb BYTE PTR c[rip]
ret
It seems desirable to eliminate the `cmp`, unless there's some weird flag stall
thing I'm not aware of.
^ permalink raw reply [flat|nested] 5+ messages in thread
* [Bug target/98977] [x86] Failure to optimize consecutive sub flags usage
2021-02-05 14:20 [Bug target/98977] New: [x86] Failure to optimize consecutive sub flags usage gabravier at gmail dot com
@ 2021-02-06 6:30 ` pinskia at gcc dot gnu.org
2021-12-23 21:13 ` pinskia at gcc dot gnu.org
` (2 subsequent siblings)
3 siblings, 0 replies; 5+ messages in thread
From: pinskia at gcc dot gnu.org @ 2021-02-06 6:30 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98977
Andrew Pinski <pinskia at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Severity|normal |enhancement
^ permalink raw reply [flat|nested] 5+ messages in thread
* [Bug target/98977] [x86] Failure to optimize consecutive sub flags usage
2021-02-05 14:20 [Bug target/98977] New: [x86] Failure to optimize consecutive sub flags usage gabravier at gmail dot com
2021-02-06 6:30 ` [Bug target/98977] " pinskia at gcc dot gnu.org
@ 2021-12-23 21:13 ` pinskia at gcc dot gnu.org
2021-12-23 21:16 ` [Bug rtl-optimization/98977] " pinskia at gcc dot gnu.org
2021-12-30 9:34 ` [Bug rtl-optimization/98977] " crazylht at gmail dot com
3 siblings, 0 replies; 5+ messages in thread
From: pinskia at gcc dot gnu.org @ 2021-12-23 21:13 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98977
Andrew Pinski <pinskia at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Last reconfirmed| |2021-12-23
Status|UNCONFIRMED |NEW
Ever confirmed|0 |1
Depends on| |3507
--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Confirmed, PR 3507 is part of it (maybe all of it) as shown by:
#include <stdbool.h>
#include <stdint.h>
extern bool z, c;
uint8_t f(uint8_t dest, uint8_t src)
{
uint8_t res = dest - src;
//z = !res;
c = src > dest;
return res;
}
Referenced Bugs:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3507
[Bug 3507] appalling optimisation with sub/cmp on multiple targets
^ permalink raw reply [flat|nested] 5+ messages in thread
* [Bug rtl-optimization/98977] [x86] Failure to optimize consecutive sub flags usage
2021-02-05 14:20 [Bug target/98977] New: [x86] Failure to optimize consecutive sub flags usage gabravier at gmail dot com
2021-02-06 6:30 ` [Bug target/98977] " pinskia at gcc dot gnu.org
2021-12-23 21:13 ` pinskia at gcc dot gnu.org
@ 2021-12-23 21:16 ` pinskia at gcc dot gnu.org
2021-12-30 9:34 ` [Bug rtl-optimization/98977] " crazylht at gmail dot com
3 siblings, 0 replies; 5+ messages in thread
From: pinskia at gcc dot gnu.org @ 2021-12-23 21:16 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98977
Andrew Pinski <pinskia at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Component|target |rtl-optimization
Target|x86_64-*-* i?86-*-* |x86_64-*-* i?86-*-*
| |aarch64*-*-*
--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Here is a testcase which shows the issue on other targets (aarch64) too:
#include <stdbool.h>
#include <stdint.h>
extern bool z, c;
uint32_t f(uint32_t dest, uint32_t src)
{
uint32_t res = dest - src;
z = !res;
c = src > dest;
return res;
}
^ permalink raw reply [flat|nested] 5+ messages in thread
* [Bug rtl-optimization/98977] Failure to optimize consecutive sub flags usage
2021-02-05 14:20 [Bug target/98977] New: [x86] Failure to optimize consecutive sub flags usage gabravier at gmail dot com
` (2 preceding siblings ...)
2021-12-23 21:16 ` [Bug rtl-optimization/98977] " pinskia at gcc dot gnu.org
@ 2021-12-30 9:34 ` crazylht at gmail dot com
3 siblings, 0 replies; 5+ messages in thread
From: crazylht at gmail dot com @ 2021-12-30 9:34 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98977
--- Comment #3 from Hongtao.liu <crazylht at gmail dot com> ---
LLVM has a separate module to merge sub and cmp, GCC can do similar thing.
Alternative choice is canonicalizing cmp patterns to be same as subs' with a
unused dest(result of sub), then CSE/PRE would be able to do the elimination
work?
^ permalink raw reply [flat|nested] 5+ messages in thread
end of thread, other threads:[~2021-12-30 9:34 UTC | newest]
Thread overview: 5+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2021-02-05 14:20 [Bug target/98977] New: [x86] Failure to optimize consecutive sub flags usage gabravier at gmail dot com
2021-02-06 6:30 ` [Bug target/98977] " pinskia at gcc dot gnu.org
2021-12-23 21:13 ` pinskia at gcc dot gnu.org
2021-12-23 21:16 ` [Bug rtl-optimization/98977] " pinskia at gcc dot gnu.org
2021-12-30 9:34 ` [Bug rtl-optimization/98977] " crazylht at gmail dot com
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