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* [Bug tree-optimization/108635] New: Redundant calls to C++ spaceship operator<=> with attribute pure or const
@ 2023-02-02 8:00 jzwinck at gmail dot com
2023-02-02 9:36 ` [Bug tree-optimization/108635] " pinskia at gcc dot gnu.org
` (3 more replies)
0 siblings, 4 replies; 5+ messages in thread
From: jzwinck at gmail dot com @ 2023-02-02 8:00 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108635
Bug ID: 108635
Summary: Redundant calls to C++ spaceship operator<=> with
attribute pure or const
Product: gcc
Version: 12.2.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: jzwinck at gmail dot com
Target Milestone: ---
This code seems to have a missed optimization in GCC 10/11/12:
#include <compare>
struct S
{
std::weak_ordering operator<=>(const S&) const __attribute__((const));
};
int compare3way(S& a, S& b)
{
return (a < b) ? -1 : (a > b) ? 1 : 0;
}
I expect operator<=> to be called once, but it is called twice. This can be a
major missed optimization if operator<=> is expensive. It happens regardless
of:
1. Using attribute((const)) or attribute((pure)).
2. Making operator<=> a free function or a member.
3. Comparing (a > b) or (a < b) in the second ternary expression. This is
especially strange, because it's really calling the same pure function twice,
and that's optimized correctly when the function being called is operator<
instead of operator<=>.
Clang optimizes it as expected.
Demo: https://godbolt.org/z/jP51E6xaz
^ permalink raw reply [flat|nested] 5+ messages in thread
* [Bug tree-optimization/108635] Redundant calls to C++ spaceship operator<=> with attribute pure or const
2023-02-02 8:00 [Bug tree-optimization/108635] New: Redundant calls to C++ spaceship operator<=> with attribute pure or const jzwinck at gmail dot com
@ 2023-02-02 9:36 ` pinskia at gcc dot gnu.org
2023-02-02 9:38 ` redi at gcc dot gnu.org
` (2 subsequent siblings)
3 siblings, 0 replies; 5+ messages in thread
From: pinskia at gcc dot gnu.org @ 2023-02-02 9:36 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108635
--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
I suspect you need noexcept too. Otherwise you could in theory have an
exception.
^ permalink raw reply [flat|nested] 5+ messages in thread
* [Bug tree-optimization/108635] Redundant calls to C++ spaceship operator<=> with attribute pure or const
2023-02-02 8:00 [Bug tree-optimization/108635] New: Redundant calls to C++ spaceship operator<=> with attribute pure or const jzwinck at gmail dot com
2023-02-02 9:36 ` [Bug tree-optimization/108635] " pinskia at gcc dot gnu.org
@ 2023-02-02 9:38 ` redi at gcc dot gnu.org
2023-02-02 9:42 ` pinskia at gcc dot gnu.org
2023-02-02 9:43 ` pinskia at gcc dot gnu.org
3 siblings, 0 replies; 5+ messages in thread
From: redi at gcc dot gnu.org @ 2023-02-02 9:38 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108635
Jonathan Wakely <redi at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Last reconfirmed| |2023-02-02
Status|UNCONFIRMED |NEW
Ever confirmed|0 |1
--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
GCC still makes two calls even if it's noexcept.
^ permalink raw reply [flat|nested] 5+ messages in thread
* [Bug tree-optimization/108635] Redundant calls to C++ spaceship operator<=> with attribute pure or const
2023-02-02 8:00 [Bug tree-optimization/108635] New: Redundant calls to C++ spaceship operator<=> with attribute pure or const jzwinck at gmail dot com
2023-02-02 9:36 ` [Bug tree-optimization/108635] " pinskia at gcc dot gnu.org
2023-02-02 9:38 ` redi at gcc dot gnu.org
@ 2023-02-02 9:42 ` pinskia at gcc dot gnu.org
2023-02-02 9:43 ` pinskia at gcc dot gnu.org
3 siblings, 0 replies; 5+ messages in thread
From: pinskia at gcc dot gnu.org @ 2023-02-02 9:42 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108635
Andrew Pinski <pinskia at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
See Also| |https://gcc.gnu.org/bugzill
| |a/show_bug.cgi?id=45115
--- Comment #3 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Then it might be a dup of bug 45115
^ permalink raw reply [flat|nested] 5+ messages in thread
* [Bug tree-optimization/108635] Redundant calls to C++ spaceship operator<=> with attribute pure or const
2023-02-02 8:00 [Bug tree-optimization/108635] New: Redundant calls to C++ spaceship operator<=> with attribute pure or const jzwinck at gmail dot com
` (2 preceding siblings ...)
2023-02-02 9:42 ` pinskia at gcc dot gnu.org
@ 2023-02-02 9:43 ` pinskia at gcc dot gnu.org
3 siblings, 0 replies; 5+ messages in thread
From: pinskia at gcc dot gnu.org @ 2023-02-02 9:43 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108635
Andrew Pinski <pinskia at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|NEW |RESOLVED
Resolution|--- |DUPLICATE
--- Comment #4 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Yes it is a dup.
*** This bug has been marked as a duplicate of bug 45115 ***
^ permalink raw reply [flat|nested] 5+ messages in thread
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2023-02-02 8:00 [Bug tree-optimization/108635] New: Redundant calls to C++ spaceship operator<=> with attribute pure or const jzwinck at gmail dot com
2023-02-02 9:36 ` [Bug tree-optimization/108635] " pinskia at gcc dot gnu.org
2023-02-02 9:38 ` redi at gcc dot gnu.org
2023-02-02 9:42 ` pinskia at gcc dot gnu.org
2023-02-02 9:43 ` pinskia at gcc dot gnu.org
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