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* [Bug ipa/99834] New: missed optimization for dead code elimination at -O3 (vs. -O2)
@ 2021-03-30 17:20 zhendong.su at inf dot ethz.ch
2021-03-31 8:04 ` [Bug ipa/99834] " rguenth at gcc dot gnu.org
` (2 more replies)
0 siblings, 3 replies; 4+ messages in thread
From: zhendong.su at inf dot ethz.ch @ 2021-03-30 17:20 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99834
Bug ID: 99834
Summary: missed optimization for dead code elimination at -O3
(vs. -O2)
Product: gcc
Version: unknown
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: ipa
Assignee: unassigned at gcc dot gnu.org
Reporter: zhendong.su at inf dot ethz.ch
CC: marxin at gcc dot gnu.org
Target Milestone: ---
[590] % gcctk -v
Using built-in specs.
COLLECT_GCC=gcctk
COLLECT_LTO_WRAPPER=/local/suz-local/software/local/gcc-trunk/libexec/gcc/x86_64-pc-linux-gnu/11.0.1/lto-wrapper
Target: x86_64-pc-linux-gnu
Configured with: ../gcc-trunk/configure --disable-bootstrap
--prefix=/local/suz-local/software/local/gcc-trunk --enable-languages=c,c++
--disable-werror --enable-multilib --with-system-zlib
Thread model: posix
Supported LTO compression algorithms: zlib
gcc version 11.0.1 20210330 (experimental) [master revision
8aac913adfc:ff4ebc2f17c:65374af219f9c5c594951a07e766fe70c1136a1f] (GCC)
[591] %
[591] % gcctk -O2 -S -o O2.s small.c
[592] % gcctk -O3 -S -o O3.s small.c
[593] %
[593] % wc O2.s O3.s
43 90 704 O2.s
61 126 950 O3.s
104 216 1654 total
[594] %
[594] % grep foo O2.s
[595] % grep foo O3.s
call foo
call foo
[596] %
[596] % cat small.c
extern void foo(void);
static int a, b, c;
static int d() {
for (a = 0; a < 1; a++)
b = 1;
for (; b; b--)
for (; c; c--)
;
return 0;
}
static void e() {
if (d())
foo();
}
int main() {
e();
e();
return 0;
}
^ permalink raw reply [flat|nested] 4+ messages in thread
* [Bug ipa/99834] missed optimization for dead code elimination at -O3 (vs. -O2)
2021-03-30 17:20 [Bug ipa/99834] New: missed optimization for dead code elimination at -O3 (vs. -O2) zhendong.su at inf dot ethz.ch
@ 2021-03-31 8:04 ` rguenth at gcc dot gnu.org
2021-03-31 13:49 ` zhendong.su at inf dot ethz.ch
2023-08-18 1:39 ` pinskia at gcc dot gnu.org
2 siblings, 0 replies; 4+ messages in thread
From: rguenth at gcc dot gnu.org @ 2021-03-31 8:04 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99834
Richard Biener <rguenth at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|UNCONFIRMED |NEW
Last reconfirmed| |2021-03-31
Version|unknown |11.0
CC| |hubicka at gcc dot gnu.org
Keywords| |missed-optimization
Ever confirmed|0 |1
--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
Similar to the other bug - when 'main' is named 'bar' then we optimize it OK at
-O3, the difference being again loop header copying not applied in "cold"
context
but inlining is.
Not sure if we want to call this a bug, but then the inconsistency is odd.
^ permalink raw reply [flat|nested] 4+ messages in thread
* [Bug ipa/99834] missed optimization for dead code elimination at -O3 (vs. -O2)
2021-03-30 17:20 [Bug ipa/99834] New: missed optimization for dead code elimination at -O3 (vs. -O2) zhendong.su at inf dot ethz.ch
2021-03-31 8:04 ` [Bug ipa/99834] " rguenth at gcc dot gnu.org
@ 2021-03-31 13:49 ` zhendong.su at inf dot ethz.ch
2023-08-18 1:39 ` pinskia at gcc dot gnu.org
2 siblings, 0 replies; 4+ messages in thread
From: zhendong.su at inf dot ethz.ch @ 2021-03-31 13:49 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99834
--- Comment #2 from Zhendong Su <zhendong.su at inf dot ethz.ch> ---
Hi Richard and all, thanks for analyzing these reports!
I have some more cases, and wonder whether you folks would prefer that I open a
meta issue report and append these (and others that we find) to that report
(rather than filing these one by one). Thanks.
^ permalink raw reply [flat|nested] 4+ messages in thread
* [Bug ipa/99834] missed optimization for dead code elimination at -O3 (vs. -O2)
2021-03-30 17:20 [Bug ipa/99834] New: missed optimization for dead code elimination at -O3 (vs. -O2) zhendong.su at inf dot ethz.ch
2021-03-31 8:04 ` [Bug ipa/99834] " rguenth at gcc dot gnu.org
2021-03-31 13:49 ` zhendong.su at inf dot ethz.ch
@ 2023-08-18 1:39 ` pinskia at gcc dot gnu.org
2 siblings, 0 replies; 4+ messages in thread
From: pinskia at gcc dot gnu.org @ 2023-08-18 1:39 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99834
--- Comment #3 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
The other way of optimizing this is to see that the return value of d is 0
(even early on) and do a const prop of that ... IN a sense an IPA-CCP for
return values rather than for argument passing.
^ permalink raw reply [flat|nested] 4+ messages in thread
end of thread, other threads:[~2023-08-18 1:39 UTC | newest]
Thread overview: 4+ messages (download: mbox.gz / follow: Atom feed)
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2021-03-30 17:20 [Bug ipa/99834] New: missed optimization for dead code elimination at -O3 (vs. -O2) zhendong.su at inf dot ethz.ch
2021-03-31 8:04 ` [Bug ipa/99834] " rguenth at gcc dot gnu.org
2021-03-31 13:49 ` zhendong.su at inf dot ethz.ch
2023-08-18 1:39 ` pinskia at gcc dot gnu.org
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