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* [Bug tree-optimization/94757] New: GCC does not optimise unsigned multiplication known not to overflow
@ 2020-04-25 8:55 felix.von.s at posteo dot de
2020-04-27 6:35 ` [Bug tree-optimization/94757] " rguenth at gcc dot gnu.org
` (2 more replies)
0 siblings, 3 replies; 4+ messages in thread
From: felix.von.s at posteo dot de @ 2020-04-25 8:55 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94757
Bug ID: 94757
Summary: GCC does not optimise unsigned multiplication known
not to overflow
Product: gcc
Version: 10.0
Status: UNCONFIRMED
Keywords: missed-optimization
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: felix.von.s at posteo dot de
Target Milestone: ---
This code:
#include <limits.h>
unsigned f(unsigned x) {
if (x > UINT_MAX / 3)
__builtin_unreachable();
return (x * 3) / 3;
}
should be possible to optimise into the identity function, but (on x86_64) gcc
-O3 generates a multiplication (via LEA) then a division (via multiplication).
GCC knows that the multiplication cannot overflow, because replacing the
returned expression with __builtin_mul_overflow_p(x, 3, x) is makes it optimise
to returning constant 0.
^ permalink raw reply [flat|nested] 4+ messages in thread
* [Bug tree-optimization/94757] GCC does not optimise unsigned multiplication known not to overflow
2020-04-25 8:55 [Bug tree-optimization/94757] New: GCC does not optimise unsigned multiplication known not to overflow felix.von.s at posteo dot de
@ 2020-04-27 6:35 ` rguenth at gcc dot gnu.org
2020-06-19 8:09 ` qianjh at cn dot fujitsu.com
2021-12-15 23:56 ` pinskia at gcc dot gnu.org
2 siblings, 0 replies; 4+ messages in thread
From: rguenth at gcc dot gnu.org @ 2020-04-27 6:35 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94757
Richard Biener <rguenth at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Last reconfirmed| |2020-04-27
Status|UNCONFIRMED |NEW
Ever confirmed|0 |1
--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
Confirmed.
^ permalink raw reply [flat|nested] 4+ messages in thread
* [Bug tree-optimization/94757] GCC does not optimise unsigned multiplication known not to overflow
2020-04-25 8:55 [Bug tree-optimization/94757] New: GCC does not optimise unsigned multiplication known not to overflow felix.von.s at posteo dot de
2020-04-27 6:35 ` [Bug tree-optimization/94757] " rguenth at gcc dot gnu.org
@ 2020-06-19 8:09 ` qianjh at cn dot fujitsu.com
2021-12-15 23:56 ` pinskia at gcc dot gnu.org
2 siblings, 0 replies; 4+ messages in thread
From: qianjh at cn dot fujitsu.com @ 2020-06-19 8:09 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94757
Qian Jianhua <qianjh at cn dot fujitsu.com> changed:
What |Removed |Added
----------------------------------------------------------------------------
CC| |qianjh at cn dot fujitsu.com
--- Comment #2 from Qian Jianhua <qianjh at cn dot fujitsu.com> ---
>GCC knows that the multiplication cannot overflow, because replacing the
>returned expression with __builtin_mul_overflow_p(x, 3, x) is makes it optimise
>to returning constant 0.
__builtin_mul_overflow_p function only check the value range. So it could be
optimised by VRP.
The process like this
x range [0, UINT_MAX/3]
x*3 range [0, UINT_MAX]
x*3/3 range [0, UINT_MAX/3], but not calculate the result(=x).
I tested some other cases.
It seems that the optimiser in gcc only check the value range of the
expression, but not track/calculate the result of the expression, if the
variable is not constant int the expression.
So i think such optimisation of expression is not supported in gcc now.
In clang, the expression "(x * 3) / 3" is optimised to "x".
^ permalink raw reply [flat|nested] 4+ messages in thread
* [Bug tree-optimization/94757] GCC does not optimise unsigned multiplication known not to overflow
2020-04-25 8:55 [Bug tree-optimization/94757] New: GCC does not optimise unsigned multiplication known not to overflow felix.von.s at posteo dot de
2020-04-27 6:35 ` [Bug tree-optimization/94757] " rguenth at gcc dot gnu.org
2020-06-19 8:09 ` qianjh at cn dot fujitsu.com
@ 2021-12-15 23:56 ` pinskia at gcc dot gnu.org
2 siblings, 0 replies; 4+ messages in thread
From: pinskia at gcc dot gnu.org @ 2021-12-15 23:56 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94757
Andrew Pinski <pinskia at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Target Milestone|--- |11.0
Severity|normal |enhancement
See Also| |https://gcc.gnu.org/bugzill
| |a/show_bug.cgi?id=97997
Status|NEW |RESOLVED
Resolution|--- |FIXED
--- Comment #3 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Fixed for GCC 11 by r11-5444-ga3ebc13492ff23 (PR 97997), this is almost a dup
of bug 97997 but not exactly.
^ permalink raw reply [flat|nested] 4+ messages in thread
end of thread, other threads:[~2021-12-15 23:56 UTC | newest]
Thread overview: 4+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2020-04-25 8:55 [Bug tree-optimization/94757] New: GCC does not optimise unsigned multiplication known not to overflow felix.von.s at posteo dot de
2020-04-27 6:35 ` [Bug tree-optimization/94757] " rguenth at gcc dot gnu.org
2020-06-19 8:09 ` qianjh at cn dot fujitsu.com
2021-12-15 23:56 ` pinskia at gcc dot gnu.org
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