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* [Bug tree-optimization/100080] New: missed optimization for dead code elimination at -O3 (vs. -O2)
@ 2021-04-14 14:14 zhendong.su at inf dot ethz.ch
2021-04-15 6:43 ` [Bug rtl-optimization/100080] " 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 @ 2021-04-14 14:14 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100080
Bug ID: 100080
Summary: missed optimization for dead code elimination at -O3
(vs. -O2)
Product: gcc
Version: unknown
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: zhendong.su at inf dot ethz.ch
Target Milestone: ---
[604] % 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 20210414 (experimental) [master revision
006783f4b16:29da9c11552:0589be0c59767cf4cbb0ef0e7d918cf6aa3d606c] (GCC)
[605] %
[605] % gcctk -O2 -S -o O2.s small.c
[606] % gcctk -O3 -S -o O3.s small.c
[607] %
[607] % wc O2.s O3.s
68 147 999 O2.s
95 202 1373 O3.s
163 349 2372 total
[608] %
[608] % grep foo O2.s
[609] % grep foo O3.s
call foo
[610] %
[610] % cat small.c
extern void foo(void);
int a, b, d, f;
static unsigned c;
void e(int g, int *k) {
int h, *i = &b, *j = &b;
*i = g;
c = &b == i;
d = c >= (*j = b | *k) && b & (*i == 0);
h = a ? 0 : d;
if (h)
foo();
}
int main() {
while (f) {
int l;
e(1, &l);
}
return 0;
}
^ permalink raw reply [flat|nested] 4+ messages in thread
* [Bug rtl-optimization/100080] missed optimization for dead code elimination at -O3 (vs. -O2)
2021-04-14 14:14 [Bug tree-optimization/100080] New: missed optimization for dead code elimination at -O3 (vs. -O2) zhendong.su at inf dot ethz.ch
@ 2021-04-15 6:43 ` rguenth at gcc dot gnu.org
2023-06-02 5:03 ` pinskia at gcc dot gnu.org
2023-08-18 6:52 ` pinskia at gcc dot gnu.org
2 siblings, 0 replies; 4+ messages in thread
From: rguenth at gcc dot gnu.org @ 2021-04-15 6:43 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100080
Richard Biener <rguenth at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Keywords| |missed-optimization
Ever confirmed|0 |1
Last reconfirmed| |2021-04-15
Status|UNCONFIRMED |NEW
Component|tree-optimization |rtl-optimization
Version|unknown |11.0
--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
Confirmed. At -O2 combine manages to drop the call to foo () (indirectly), at
-O3 it does not. There's not much difference on the GIMPLE level
^ permalink raw reply [flat|nested] 4+ messages in thread
* [Bug rtl-optimization/100080] missed optimization for dead code elimination at -O3 (vs. -O2)
2021-04-14 14:14 [Bug tree-optimization/100080] New: missed optimization for dead code elimination at -O3 (vs. -O2) zhendong.su at inf dot ethz.ch
2021-04-15 6:43 ` [Bug rtl-optimization/100080] " rguenth at gcc dot gnu.org
@ 2023-06-02 5:03 ` pinskia at gcc dot gnu.org
2023-08-18 6:52 ` pinskia at gcc dot gnu.org
2 siblings, 0 replies; 4+ messages in thread
From: pinskia at gcc dot gnu.org @ 2023-06-02 5:03 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100080
--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
(In reply to Richard Biener from comment #1)
> Confirmed. At -O2 combine manages to drop the call to foo () (indirectly),
> at -O3 it does not. There's not much difference on the GIMPLE level
For the gimple level it comes down to:
_5 = (unsigned int) l_20(D);
if (_5 > 1)
goto <bb 5>; [50.00%]
else
goto <bb 6>; [50.00%]
What should we do there, l_20(D) is definitely uninitialized?
We even get a warning about it:
In function 'e',
inlined from 'main' at <source>:17:5:
<source>:9:20: warning: 'l' may be used uninitialized [-Wmaybe-uninitialized]
9 | d = c >= (*j = b | *k) && b & (*i == 0);
| ~~^~~~
<source>: In function 'main':
<source>:16:9: note: 'l' was declared here
16 | int l;
| ^
^ permalink raw reply [flat|nested] 4+ messages in thread
* [Bug rtl-optimization/100080] missed optimization for dead code elimination at -O3 (vs. -O2)
2021-04-14 14:14 [Bug tree-optimization/100080] New: missed optimization for dead code elimination at -O3 (vs. -O2) zhendong.su at inf dot ethz.ch
2021-04-15 6:43 ` [Bug rtl-optimization/100080] " rguenth at gcc dot gnu.org
2023-06-02 5:03 ` pinskia at gcc dot gnu.org
@ 2023-08-18 6:52 ` pinskia at gcc dot gnu.org
2 siblings, 0 replies; 4+ messages in thread
From: pinskia at gcc dot gnu.org @ 2023-08-18 6:52 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100080
--- Comment #3 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
I am thinking we should mark this as won't fix.
In VRP2 we have:
<bb 4> [local count: 105119324]:
_7 = l_20(D) | 1;
_8 = (unsigned int) _7;
if (_8 <= 1)
goto <bb 6>; [41.00%]
else
goto <bb 13>; [59.00%]
...
4->6 (T) _7 : [irange] int [0, 1] MASK 0x1 VALUE 0x0
4->6 (T) _8 : [irange] unsigned int [0, 1]
4->6 (T) l_20(D) : [irange] UNDEFINED
...
=========== BB 6 ============
Imports: l_20(D)
Exports: _5 l_20(D)
_5 : l_20(D)(I)
_7 [irange] int [0, 1] MASK 0x1 VALUE 0x0
l_20(D) [irange] UNDEFINED
<bb 6> [local count: 43098922]:
_5 = (unsigned int) l_20(D);
b = _7;
if (_5 > 1)
goto <bb 14>; [37.49%]
else
goto <bb 15>; [62.51%]
We techincally could have [0,1] as the range for l_20(D) on the edge 4->6
And then that if goes away.
^ permalink raw reply [flat|nested] 4+ messages in thread
end of thread, other threads:[~2023-08-18 6:52 UTC | newest]
Thread overview: 4+ messages (download: mbox.gz / follow: Atom feed)
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2021-04-14 14:14 [Bug tree-optimization/100080] New: missed optimization for dead code elimination at -O3 (vs. -O2) zhendong.su at inf dot ethz.ch
2021-04-15 6:43 ` [Bug rtl-optimization/100080] " rguenth at gcc dot gnu.org
2023-06-02 5:03 ` pinskia at gcc dot gnu.org
2023-08-18 6:52 ` pinskia at gcc dot gnu.org
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