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* [Bug c/45115] New: attribute((pure)) does not work when returning structs
@ 2010-07-28 19:45 marco at technoboredom dot net
2010-07-28 19:48 ` [Bug tree-optimization/45115] pure functions returning structs are not optimized pinskia at gcc dot gnu dot org
2010-07-28 20:05 ` rguenth at gcc dot gnu dot org
0 siblings, 2 replies; 6+ messages in thread
From: marco at technoboredom dot net @ 2010-07-28 19:45 UTC (permalink / raw)
To: gcc-bugs
compiling this with 'gcc -O3 -S x.c'
-------- x.c -----------
#define PURE __attribute__((pure))
struct res {int u; };
extern struct res calc_1() PURE;
extern int calc_2() PURE;
int fun_1()
{
return calc_1().u+calc_1().u;
}
int fun_2()
{
return calc_2()+calc_2();
}
--------------------
yields code which calls calc_2() *once* in fun_2() but calls calc_1() *twice*
in fun_1(). Obviously, fun_1 misses an optimization.
Tested on gcc 4.4.3 (Ubuntu 10.04), gcc 4.5.0/4.4.4/4.3.5 (MacPorts)
--
Summary: attribute((pure)) does not work when returning structs
Product: gcc
Version: unknown
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: marco at technoboredom dot net
GCC host triplet: x86_64-linux-gnu
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45115
^ permalink raw reply [flat|nested] 6+ messages in thread
* [Bug tree-optimization/45115] pure functions returning structs are not optimized.
2010-07-28 19:45 [Bug c/45115] New: attribute((pure)) does not work when returning structs marco at technoboredom dot net
@ 2010-07-28 19:48 ` pinskia at gcc dot gnu dot org
2010-07-28 20:05 ` rguenth at gcc dot gnu dot org
1 sibling, 0 replies; 6+ messages in thread
From: pinskia at gcc dot gnu dot org @ 2010-07-28 19:48 UTC (permalink / raw)
To: gcc-bugs
------- Comment #1 from pinskia at gcc dot gnu dot org 2010-07-28 19:48 -------
Confirmed.
--
pinskia at gcc dot gnu dot org changed:
What |Removed |Added
----------------------------------------------------------------------------
Severity|normal |enhancement
Status|UNCONFIRMED |NEW
Component|c |tree-optimization
Ever Confirmed|0 |1
GCC host triplet|x86_64-linux-gnu |
Keywords| |missed-optimization
Last reconfirmed|0000-00-00 00:00:00 |2010-07-28 19:48:03
date| |
Summary|attribute((pure)) does not |pure functions returning
|work when returning structs |structs are not optimized.
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45115
^ permalink raw reply [flat|nested] 6+ messages in thread
* [Bug tree-optimization/45115] pure functions returning structs are not optimized.
2010-07-28 19:45 [Bug c/45115] New: attribute((pure)) does not work when returning structs marco at technoboredom dot net
2010-07-28 19:48 ` [Bug tree-optimization/45115] pure functions returning structs are not optimized pinskia at gcc dot gnu dot org
@ 2010-07-28 20:05 ` rguenth at gcc dot gnu dot org
1 sibling, 0 replies; 6+ messages in thread
From: rguenth at gcc dot gnu dot org @ 2010-07-28 20:05 UTC (permalink / raw)
To: gcc-bugs
------- Comment #2 from rguenth at gcc dot gnu dot org 2010-07-28 20:05 -------
We can only value-number things which define an SSA name.
--
rguenth at gcc dot gnu dot org changed:
What |Removed |Added
----------------------------------------------------------------------------
CC| |matz at gcc dot gnu dot org
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45115
^ permalink raw reply [flat|nested] 6+ messages in thread
* [Bug tree-optimization/45115] pure functions returning structs are not optimized.
[not found] <bug-45115-4@http.gcc.gnu.org/bugzilla/>
2023-02-02 9:43 ` pinskia at gcc dot gnu.org
2023-02-02 10:15 ` redi at gcc dot gnu.org
@ 2024-04-09 7:06 ` pinskia at gcc dot gnu.org
2 siblings, 0 replies; 6+ messages in thread
From: pinskia at gcc dot gnu.org @ 2024-04-09 7:06 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=45115
Andrew Pinski <pinskia at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|NEW |RESOLVED
Resolution|--- |DUPLICATE
--- Comment #5 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Dup of bug 59739; even though that is a newer bug it has some more analysis and
even been assigned (will add the C++20 testcase there too).
*** This bug has been marked as a duplicate of bug 59739 ***
^ permalink raw reply [flat|nested] 6+ messages in thread
* [Bug tree-optimization/45115] pure functions returning structs are not optimized.
[not found] <bug-45115-4@http.gcc.gnu.org/bugzilla/>
2023-02-02 9:43 ` pinskia at gcc dot gnu.org
@ 2023-02-02 10:15 ` redi at gcc dot gnu.org
2024-04-09 7:06 ` pinskia at gcc dot gnu.org
2 siblings, 0 replies; 6+ messages in thread
From: redi at gcc dot gnu.org @ 2023-02-02 10:15 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=45115
--- Comment #4 from Jonathan Wakely <redi at gcc dot gnu.org> ---
This affects C++20 three-way comparisons, which return trivial structs wrapping
an integer.
From PR 108635:
#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] 6+ messages in thread
* [Bug tree-optimization/45115] pure functions returning structs are not optimized.
[not found] <bug-45115-4@http.gcc.gnu.org/bugzilla/>
@ 2023-02-02 9:43 ` pinskia at gcc dot gnu.org
2023-02-02 10:15 ` redi at gcc dot gnu.org
2024-04-09 7:06 ` pinskia at gcc dot gnu.org
2 siblings, 0 replies; 6+ 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=45115
Andrew Pinski <pinskia at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
CC| |jzwinck at gmail dot com
--- Comment #3 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
*** Bug 108635 has been marked as a duplicate of this bug. ***
^ permalink raw reply [flat|nested] 6+ messages in thread
end of thread, other threads:[~2024-04-09 7:06 UTC | newest]
Thread overview: 6+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2010-07-28 19:45 [Bug c/45115] New: attribute((pure)) does not work when returning structs marco at technoboredom dot net
2010-07-28 19:48 ` [Bug tree-optimization/45115] pure functions returning structs are not optimized pinskia at gcc dot gnu dot org
2010-07-28 20:05 ` rguenth at gcc dot gnu dot org
[not found] <bug-45115-4@http.gcc.gnu.org/bugzilla/>
2023-02-02 9:43 ` pinskia at gcc dot gnu.org
2023-02-02 10:15 ` redi at gcc dot gnu.org
2024-04-09 7:06 ` pinskia at gcc dot gnu.org
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