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* [Bug tree-optimization/108997] New: GCC prediction on bool comparisons seems wrong
@ 2023-03-02 22:51 pinskia at gcc dot gnu.org
2023-03-02 23:26 ` [Bug tree-optimization/108997] " nok.raven at gmail dot com
` (5 more replies)
0 siblings, 6 replies; 7+ messages in thread
From: pinskia at gcc dot gnu.org @ 2023-03-02 22:51 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108997
Bug ID: 108997
Summary: GCC prediction on bool comparisons seems wrong
Product: gcc
Version: 13.0
Status: UNCONFIRMED
Keywords: missed-optimization
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: pinskia at gcc dot gnu.org
Target Milestone: ---
I noticed this while looking into 108992.
I suspect we look into this but I am not 100% sure this is a bug.
Testcase:
void use(int *);
void use2(int *);
void foo(bool cond, int * p)
{
if (cond) {
use(p);
}
use2(p);
}
GCC predicts the branch to be taken 66% of the time, but I would have assume it
should be 50% as it is a bool and there is no other information. I suspect this
is due to prediction when comparing against 0.
^ permalink raw reply [flat|nested] 7+ messages in thread
* [Bug tree-optimization/108997] GCC prediction on bool comparisons seems wrong
2023-03-02 22:51 [Bug tree-optimization/108997] New: GCC prediction on bool comparisons seems wrong pinskia at gcc dot gnu.org
@ 2023-03-02 23:26 ` nok.raven at gmail dot com
2023-03-02 23:35 ` pinskia at gcc dot gnu.org
` (4 subsequent siblings)
5 siblings, 0 replies; 7+ messages in thread
From: nok.raven at gmail dot com @ 2023-03-02 23:26 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108997
Nikita Kniazev <nok.raven at gmail dot com> changed:
What |Removed |Added
----------------------------------------------------------------------------
CC| |nok.raven at gmail dot com
--- Comment #1 from Nikita Kniazev <nok.raven at gmail dot com> ---
Is it about bool?
void use(int *);
void use2(int *);
void foo(int * p, int cond)
{
if (cond == 789) {
use(p);
}
use2(p);
}
^ permalink raw reply [flat|nested] 7+ messages in thread
* [Bug tree-optimization/108997] GCC prediction on bool comparisons seems wrong
2023-03-02 22:51 [Bug tree-optimization/108997] New: GCC prediction on bool comparisons seems wrong pinskia at gcc dot gnu.org
2023-03-02 23:26 ` [Bug tree-optimization/108997] " nok.raven at gmail dot com
@ 2023-03-02 23:35 ` pinskia at gcc dot gnu.org
2023-03-03 0:05 ` nok.raven at gmail dot com
` (3 subsequent siblings)
5 siblings, 0 replies; 7+ messages in thread
From: pinskia at gcc dot gnu.org @ 2023-03-02 23:35 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108997
--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
(In reply to Nikita Kniazev from comment #1)
> Is it about bool?
YES.
^ permalink raw reply [flat|nested] 7+ messages in thread
* [Bug tree-optimization/108997] GCC prediction on bool comparisons seems wrong
2023-03-02 22:51 [Bug tree-optimization/108997] New: GCC prediction on bool comparisons seems wrong pinskia at gcc dot gnu.org
2023-03-02 23:26 ` [Bug tree-optimization/108997] " nok.raven at gmail dot com
2023-03-02 23:35 ` pinskia at gcc dot gnu.org
@ 2023-03-03 0:05 ` nok.raven at gmail dot com
2023-03-03 0:10 ` pinskia at gcc dot gnu.org
` (2 subsequent siblings)
5 siblings, 0 replies; 7+ messages in thread
From: nok.raven at gmail dot com @ 2023-03-03 0:05 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108997
--- Comment #3 from Nikita Kniazev <nok.raven at gmail dot com> ---
For cond == 789
if (cond_2(D) == 789)
goto <bb 3>; [20.24%]
else
goto <bb 4>; [79.76%]
For cond != 789
if (cond_2(D) != 789)
goto <bb 3>; [48.88%]
else
goto <bb 4>; [51.12%]
^ permalink raw reply [flat|nested] 7+ messages in thread
* [Bug tree-optimization/108997] GCC prediction on bool comparisons seems wrong
2023-03-02 22:51 [Bug tree-optimization/108997] New: GCC prediction on bool comparisons seems wrong pinskia at gcc dot gnu.org
` (2 preceding siblings ...)
2023-03-03 0:05 ` nok.raven at gmail dot com
@ 2023-03-03 0:10 ` pinskia at gcc dot gnu.org
2023-03-03 8:43 ` rguenth at gcc dot gnu.org
2023-03-07 14:49 ` marxin at gcc dot gnu.org
5 siblings, 0 replies; 7+ messages in thread
From: pinskia at gcc dot gnu.org @ 2023-03-03 0:10 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108997
--- Comment #4 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
(In reply to Nikita Kniazev from comment #3)
> For cond == 789
> if (cond_2(D) == 789)
> goto <bb 3>; [20.24%]
> else
> goto <bb 4>; [79.76%]
>
> For cond != 789
> if (cond_2(D) != 789)
> goto <bb 3>; [48.88%]
> else
> goto <bb 4>; [51.12%]
an intergal type equaling a specific # is handled this way on purpose. Bool
should be handled differently because it only has two values, 0/1.
Anyways != is not handled specially except for the zero value case ...
^ permalink raw reply [flat|nested] 7+ messages in thread
* [Bug tree-optimization/108997] GCC prediction on bool comparisons seems wrong
2023-03-02 22:51 [Bug tree-optimization/108997] New: GCC prediction on bool comparisons seems wrong pinskia at gcc dot gnu.org
` (3 preceding siblings ...)
2023-03-03 0:10 ` pinskia at gcc dot gnu.org
@ 2023-03-03 8:43 ` rguenth at gcc dot gnu.org
2023-03-07 14:49 ` marxin at gcc dot gnu.org
5 siblings, 0 replies; 7+ messages in thread
From: rguenth at gcc dot gnu.org @ 2023-03-03 8:43 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108997
Richard Biener <rguenth at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
CC| |hubicka at gcc dot gnu.org
--- Comment #5 from Richard Biener <rguenth at gcc dot gnu.org> ---
Predictions for bb 2
DS theory heuristics: 33.00%
combined heuristics: 33.00%
call heuristics of edge 2->3: 33.00%
so it seems it's about "conditional call" which is reasonable I think.
Doing
void use2(int *);
int x = 1;
void foo(bool cond, int * p)
{
if (cond) {
x = 1;
}
use2(p);
}
shows even probabilities, so does
if (cond) use1 (p); else use2 (p);
^ permalink raw reply [flat|nested] 7+ messages in thread
* [Bug tree-optimization/108997] GCC prediction on bool comparisons seems wrong
2023-03-02 22:51 [Bug tree-optimization/108997] New: GCC prediction on bool comparisons seems wrong pinskia at gcc dot gnu.org
` (4 preceding siblings ...)
2023-03-03 8:43 ` rguenth at gcc dot gnu.org
@ 2023-03-07 14:49 ` marxin at gcc dot gnu.org
5 siblings, 0 replies; 7+ messages in thread
From: marxin at gcc dot gnu.org @ 2023-03-07 14:49 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108997
Martin Liška <marxin at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|UNCONFIRMED |RESOLVED
CC| |marxin at gcc dot gnu.org
Resolution|--- |INVALID
--- Comment #6 from Martin Liška <marxin at gcc dot gnu.org> ---
Yes, one can see the heuristics with -fdump-tree-profile_estimate as Richi
showed.
^ permalink raw reply [flat|nested] 7+ messages in thread
end of thread, other threads:[~2023-03-07 14:49 UTC | newest]
Thread overview: 7+ messages (download: mbox.gz / follow: Atom feed)
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2023-03-02 22:51 [Bug tree-optimization/108997] New: GCC prediction on bool comparisons seems wrong pinskia at gcc dot gnu.org
2023-03-02 23:26 ` [Bug tree-optimization/108997] " nok.raven at gmail dot com
2023-03-02 23:35 ` pinskia at gcc dot gnu.org
2023-03-03 0:05 ` nok.raven at gmail dot com
2023-03-03 0:10 ` pinskia at gcc dot gnu.org
2023-03-03 8:43 ` rguenth at gcc dot gnu.org
2023-03-07 14:49 ` marxin at gcc dot gnu.org
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