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* [Bug tree-optimization/102929] New: [missed optimization] two ways to rounddown-to-next-multiple
@ 2021-10-25 12:53 jengelh at inai dot de
2021-10-25 22:11 ` [Bug tree-optimization/102929] " pinskia at gcc dot gnu.org
` (2 more replies)
0 siblings, 3 replies; 4+ messages in thread
From: jengelh at inai dot de @ 2021-10-25 12:53 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102929
Bug ID: 102929
Summary: [missed optimization] two ways to
rounddown-to-next-multiple
Product: gcc
Version: 11.2.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: jengelh at inai dot de
Target Milestone: ---
Input
=====
unsigned long calc(unsigned long x, unsigned long y)
{
return x/y*y;
}
unsigned long calc2(unsigned long x, unsigned long y)
{
return x - x % y;
}
Observed
========
» g++ -O3 -c x.c; objdump -Mintel -d x.o
gcc version 11.2.1 20210816 [revision 056e324ce46a7924b5cf10f61010cf9dd2ca10e9]
(SUSE Linux) x86_64
0000000000000000 <_Z4calcmm>:
0: 48 89 f8 mov rax,rdi
3: 31 d2 xor edx,edx
5: 48 f7 f6 div rsi
8: 48 0f af c6 imul rax,rsi
c: c3 ret
d: 0f 1f 00 nop DWORD PTR [rax]
0000000000000010 <_Z5calc2mm>:
10: 48 89 f8 mov rax,rdi
13: 31 d2 xor edx,edx
15: 48 f7 f6 div rsi
18: 48 89 f8 mov rax,rdi
1b: 48 29 d0 sub rax,rdx
1e: c3 ret
Expected
========
I do not see any obvious differences in the outcome of the two C functions, so
I would expect that, ideally, both should lead to the same asm. (Either by
making calc use div-mov-sub, or by making calc2 using div-imul; whichever
happens to be determined more beneficial as per the machine descriptions).
^ permalink raw reply [flat|nested] 4+ messages in thread
* [Bug tree-optimization/102929] [missed optimization] two ways to rounddown-to-next-multiple
2021-10-25 12:53 [Bug tree-optimization/102929] New: [missed optimization] two ways to rounddown-to-next-multiple jengelh at inai dot de
@ 2021-10-25 22:11 ` pinskia at gcc dot gnu.org
2021-10-26 7:00 ` rguenth at gcc dot gnu.org
2021-11-17 0:55 ` navidrahimi at microsoft dot com
2 siblings, 0 replies; 4+ messages in thread
From: pinskia at gcc dot gnu.org @ 2021-10-25 22:11 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102929
Andrew Pinski <pinskia at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Keywords| |missed-optimization
Severity|normal |enhancement
^ permalink raw reply [flat|nested] 4+ messages in thread
* [Bug tree-optimization/102929] [missed optimization] two ways to rounddown-to-next-multiple
2021-10-25 12:53 [Bug tree-optimization/102929] New: [missed optimization] two ways to rounddown-to-next-multiple jengelh at inai dot de
2021-10-25 22:11 ` [Bug tree-optimization/102929] " pinskia at gcc dot gnu.org
@ 2021-10-26 7:00 ` rguenth at gcc dot gnu.org
2021-11-17 0:55 ` navidrahimi at microsoft dot com
2 siblings, 0 replies; 4+ messages in thread
From: rguenth at gcc dot gnu.org @ 2021-10-26 7:00 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102929
Richard Biener <rguenth at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Ever confirmed|0 |1
Status|UNCONFIRMED |NEW
Last reconfirmed| |2021-10-26
--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
Confirmed. I do remember patches flowing by for x/y*y -> x - x%y.
^ permalink raw reply [flat|nested] 4+ messages in thread
* [Bug tree-optimization/102929] [missed optimization] two ways to rounddown-to-next-multiple
2021-10-25 12:53 [Bug tree-optimization/102929] New: [missed optimization] two ways to rounddown-to-next-multiple jengelh at inai dot de
2021-10-25 22:11 ` [Bug tree-optimization/102929] " pinskia at gcc dot gnu.org
2021-10-26 7:00 ` rguenth at gcc dot gnu.org
@ 2021-11-17 0:55 ` navidrahimi at microsoft dot com
2 siblings, 0 replies; 4+ messages in thread
From: navidrahimi at microsoft dot com @ 2021-11-17 0:55 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102929
Navid Rahimi <navidrahimi at microsoft dot com> changed:
What |Removed |Added
----------------------------------------------------------------------------
CC| |navidrahimi at microsoft dot com
--- Comment #2 from Navid Rahimi <navidrahimi at microsoft dot com> ---
https://compiler-explorer.com/z/rMKnddh3G
Seems the patch has been landed. We can close this bug.
^ permalink raw reply [flat|nested] 4+ messages in thread
end of thread, other threads:[~2021-11-17 0:55 UTC | newest]
Thread overview: 4+ messages (download: mbox.gz / follow: Atom feed)
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2021-10-25 12:53 [Bug tree-optimization/102929] New: [missed optimization] two ways to rounddown-to-next-multiple jengelh at inai dot de
2021-10-25 22:11 ` [Bug tree-optimization/102929] " pinskia at gcc dot gnu.org
2021-10-26 7:00 ` rguenth at gcc dot gnu.org
2021-11-17 0:55 ` navidrahimi at microsoft dot com
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