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* [Bug middle-end/99087] New: suboptimal codegen for division by constant 3
@ 2021-02-13 15:33 vanyacpp at gmail dot com
2021-02-15 8:26 ` [Bug target/99087] " rguenth at gcc dot gnu.org
2023-10-19 16:35 ` vanyacpp at gmail dot com
0 siblings, 2 replies; 3+ messages in thread
From: vanyacpp at gmail dot com @ 2021-02-13 15:33 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99087
Bug ID: 99087
Summary: suboptimal codegen for division by constant 3
Product: gcc
Version: unknown
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: middle-end
Assignee: unassigned at gcc dot gnu.org
Reporter: vanyacpp at gmail dot com
Target Milestone: ---
These two are functionally the same, but generate different code with g++ -O2:
unsigned long long foo(unsigned long long a)
{
return a / 3;
}
unsigned long long bar(unsigned long long a)
{
return (unsigned __int128)a * 0xAAAA'AAAA'AAAA'AAAB >> 65;
}
foo(unsigned long long):
movabs rdx, -6148914691236517205
mov rax, rdi
mul rdx
mov rax, rdx
shr rax
ret
bar(unsigned long long):
movabs rax, -6148914691236517205
mul rdi
mov rax, rdx
shr rax
ret
For some reason for division GCC chooses different argument order which causes
generation of one extra mov.
^ permalink raw reply [flat|nested] 3+ messages in thread
* [Bug target/99087] suboptimal codegen for division by constant 3
2021-02-13 15:33 [Bug middle-end/99087] New: suboptimal codegen for division by constant 3 vanyacpp at gmail dot com
@ 2021-02-15 8:26 ` rguenth at gcc dot gnu.org
2023-10-19 16:35 ` vanyacpp at gmail dot com
1 sibling, 0 replies; 3+ messages in thread
From: rguenth at gcc dot gnu.org @ 2021-02-15 8:26 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99087
Richard Biener <rguenth at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Ever confirmed|0 |1
Status|UNCONFIRMED |NEW
Last reconfirmed| |2021-02-15
--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
Confirmed.
^ permalink raw reply [flat|nested] 3+ messages in thread
* [Bug target/99087] suboptimal codegen for division by constant 3
2021-02-13 15:33 [Bug middle-end/99087] New: suboptimal codegen for division by constant 3 vanyacpp at gmail dot com
2021-02-15 8:26 ` [Bug target/99087] " rguenth at gcc dot gnu.org
@ 2023-10-19 16:35 ` vanyacpp at gmail dot com
1 sibling, 0 replies; 3+ messages in thread
From: vanyacpp at gmail dot com @ 2023-10-19 16:35 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99087
Ivan Sorokin <vanyacpp at gmail dot com> changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|NEW |RESOLVED
Resolution|--- |FIXED
--- Comment #2 from Ivan Sorokin <vanyacpp at gmail dot com> ---
Since GCC 12 the issue no longer reproduces. Closing as fixed.
https://godbolt.org/z/ss7Y84a9f
^ permalink raw reply [flat|nested] 3+ messages in thread
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2021-02-13 15:33 [Bug middle-end/99087] New: suboptimal codegen for division by constant 3 vanyacpp at gmail dot com
2021-02-15 8:26 ` [Bug target/99087] " rguenth at gcc dot gnu.org
2023-10-19 16:35 ` vanyacpp at gmail dot com
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