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* [Bug rtl-optimization/94838] New: Failure to optimize out useless zero-ing after register was already zero-ed
@ 2020-04-29 1:32 gabravier at gmail dot com
2020-04-29 2:06 ` [Bug target/94838] " pinskia at gcc dot gnu.org
` (5 more replies)
0 siblings, 6 replies; 7+ messages in thread
From: gabravier at gmail dot com @ 2020-04-29 1:32 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94838
Bug ID: 94838
Summary: Failure to optimize out useless zero-ing after
register was already zero-ed
Product: gcc
Version: 10.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: rtl-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: gabravier at gmail dot com
Target Milestone: ---
int f(bool b, int *p)
{
return b && *p;
}
GCC generates this with -O3:
f(bool, int*):
xor eax, eax
test dil, dil
je .L1
mov edx, DWORD PTR [rsi]
xor eax, eax ; This can be removed, since eax is already 0 here
test edx, edx
setne al
.L1:
ret
^ permalink raw reply [flat|nested] 7+ messages in thread
* [Bug target/94838] Failure to optimize out useless zero-ing after register was already zero-ed
2020-04-29 1:32 [Bug rtl-optimization/94838] New: Failure to optimize out useless zero-ing after register was already zero-ed gabravier at gmail dot com
@ 2020-04-29 2:06 ` pinskia at gcc dot gnu.org
2020-04-29 6:50 ` gabravier at gmail dot com
` (4 subsequent siblings)
5 siblings, 0 replies; 7+ messages in thread
From: pinskia at gcc dot gnu.org @ 2020-04-29 2:06 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94838
Andrew Pinski <pinskia at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Severity|normal |enhancement
--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Most likely it is due to splitting of the setting of eax from the cmp happens
after reload. THIS IS a target issue because of that.
^ permalink raw reply [flat|nested] 7+ messages in thread
* [Bug target/94838] Failure to optimize out useless zero-ing after register was already zero-ed
2020-04-29 1:32 [Bug rtl-optimization/94838] New: Failure to optimize out useless zero-ing after register was already zero-ed gabravier at gmail dot com
2020-04-29 2:06 ` [Bug target/94838] " pinskia at gcc dot gnu.org
@ 2020-04-29 6:50 ` gabravier at gmail dot com
2020-04-29 6:51 ` gabravier at gmail dot com
` (3 subsequent siblings)
5 siblings, 0 replies; 7+ messages in thread
From: gabravier at gmail dot com @ 2020-04-29 6:50 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94838
Gabriel Ravier <gabravier at gmail dot com> changed:
What |Removed |Added
----------------------------------------------------------------------------
Target|x86_64-linux-gnu |x86_64-* i?86-*-*
--- Comment #2 from Gabriel Ravier <gabravier at gmail dot com> ---
This also occurs on i68* :
f(bool, int*):
xor eax, eax ; Already 0
cmp BYTE PTR [esp+4], 0
je .L1
mov eax, DWORD PTR [esp+8] ; Could use different caller-saved register such
as ecx or edx
mov eax, DWORD PTR [eax]
test eax, eax
setne al
movzx eax, al
.L1:
ret
^ permalink raw reply [flat|nested] 7+ messages in thread
* [Bug target/94838] Failure to optimize out useless zero-ing after register was already zero-ed
2020-04-29 1:32 [Bug rtl-optimization/94838] New: Failure to optimize out useless zero-ing after register was already zero-ed gabravier at gmail dot com
2020-04-29 2:06 ` [Bug target/94838] " pinskia at gcc dot gnu.org
2020-04-29 6:50 ` gabravier at gmail dot com
@ 2020-04-29 6:51 ` gabravier at gmail dot com
2020-04-29 6:53 ` gabravier at gmail dot com
` (2 subsequent siblings)
5 siblings, 0 replies; 7+ messages in thread
From: gabravier at gmail dot com @ 2020-04-29 6:51 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94838
--- Comment #3 from Gabriel Ravier <gabravier at gmail dot com> ---
This also occurs on i68* :
f(bool, int*):
xor eax, eax ; Already 0
cmp BYTE PTR [esp+4], 0
je .L1
mov eax, DWORD PTR [esp+8] ; Could use different caller-saved register such
as ecx or edx
mov eax, DWORD PTR [eax]
test eax, eax
setne al
movzx eax, al ; This could then be avoided since eax would already be 0
.L1:
ret
^ permalink raw reply [flat|nested] 7+ messages in thread
* [Bug target/94838] Failure to optimize out useless zero-ing after register was already zero-ed
2020-04-29 1:32 [Bug rtl-optimization/94838] New: Failure to optimize out useless zero-ing after register was already zero-ed gabravier at gmail dot com
` (2 preceding siblings ...)
2020-04-29 6:51 ` gabravier at gmail dot com
@ 2020-04-29 6:53 ` gabravier at gmail dot com
2020-04-29 7:13 ` rguenth at gcc dot gnu.org
2020-04-29 9:00 ` [Bug rtl-optimization/94838] " ubizjak at gmail dot com
5 siblings, 0 replies; 7+ messages in thread
From: gabravier at gmail dot com @ 2020-04-29 6:53 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94838
--- Comment #4 from Gabriel Ravier <gabravier at gmail dot com> ---
Oops, seems like there was a weird collision. Don't pay attention to the second
to last comment before this one, it's identical to the last comment before this
one except for a single comment being added in the last comment before this
one.
^ permalink raw reply [flat|nested] 7+ messages in thread
* [Bug target/94838] Failure to optimize out useless zero-ing after register was already zero-ed
2020-04-29 1:32 [Bug rtl-optimization/94838] New: Failure to optimize out useless zero-ing after register was already zero-ed gabravier at gmail dot com
` (3 preceding siblings ...)
2020-04-29 6:53 ` gabravier at gmail dot com
@ 2020-04-29 7:13 ` rguenth at gcc dot gnu.org
2020-04-29 9:00 ` [Bug rtl-optimization/94838] " ubizjak at gmail dot com
5 siblings, 0 replies; 7+ messages in thread
From: rguenth at gcc dot gnu.org @ 2020-04-29 7:13 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94838
Richard Biener <rguenth at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Ever confirmed|0 |1
Last reconfirmed| |2020-04-29
Status|UNCONFIRMED |NEW
^ permalink raw reply [flat|nested] 7+ messages in thread
* [Bug rtl-optimization/94838] Failure to optimize out useless zero-ing after register was already zero-ed
2020-04-29 1:32 [Bug rtl-optimization/94838] New: Failure to optimize out useless zero-ing after register was already zero-ed gabravier at gmail dot com
` (4 preceding siblings ...)
2020-04-29 7:13 ` rguenth at gcc dot gnu.org
@ 2020-04-29 9:00 ` ubizjak at gmail dot com
5 siblings, 0 replies; 7+ messages in thread
From: ubizjak at gmail dot com @ 2020-04-29 9:00 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94838
Uroš Bizjak <ubizjak at gmail dot com> changed:
What |Removed |Added
----------------------------------------------------------------------------
Component|target |rtl-optimization
--- Comment #5 from Uroš Bizjak <ubizjak at gmail dot com> ---
(In reply to Gabriel Ravier from comment #0)
> int f(bool b, int *p)
> {
> return b && *p;
> }
>
> GCC generates this with -O3:
>
> f(bool, int*):
> xor eax, eax
> test dil, dil
> je .L1
> mov edx, DWORD PTR [rsi]
> xor eax, eax ; This can be removed, since eax is already 0 here
> test edx, edx
> setne al
> .L1:
> ret
The first xor is return value load and the second is from peephole2 pass that
converts:
11: NOTE_INSN_BASIC_BLOCK 3
12: flags:CCZ=cmp([si:DI],0)
REG_DEAD si:DI
13: NOTE_INSN_DELETED
32: ax:QI=flags:CCZ!=0
REG_DEAD flags:CCZ
33: ax:SI=zero_extend(ax:QI)
to:
11: NOTE_INSN_BASIC_BLOCK 3
40: {ax:SI=0;clobber flags:CC;}
43: dx:SI=[si:DI]
44: flags:CCZ=cmp(dx:SI,0)
42: strict_low_part(ax:QI)=flags:CCZ!=0
The follow-up cprop-hardreg pass does not notice that we already have zero
loaded to a register.
There is nothing that target-dependent part can do here. A follow-up RTL
hardreg propagation pass should fix this.
^ permalink raw reply [flat|nested] 7+ messages in thread
end of thread, other threads:[~2020-04-29 9:00 UTC | newest]
Thread overview: 7+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2020-04-29 1:32 [Bug rtl-optimization/94838] New: Failure to optimize out useless zero-ing after register was already zero-ed gabravier at gmail dot com
2020-04-29 2:06 ` [Bug target/94838] " pinskia at gcc dot gnu.org
2020-04-29 6:50 ` gabravier at gmail dot com
2020-04-29 6:51 ` gabravier at gmail dot com
2020-04-29 6:53 ` gabravier at gmail dot com
2020-04-29 7:13 ` rguenth at gcc dot gnu.org
2020-04-29 9:00 ` [Bug rtl-optimization/94838] " ubizjak at gmail dot com
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