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* [Bug c/105875] New: Toggling an atomic_bool is inefficient
@ 2022-06-07 15:10 josephcsible at gmail dot com
2022-06-21 6:35 ` [Bug tree-optimization/105875] " pinskia at gcc dot gnu.org
2023-05-17 23:32 ` [Bug c/105875] " pinskia at gcc dot gnu.org
0 siblings, 2 replies; 3+ messages in thread
From: josephcsible at gmail dot com @ 2022-06-07 15:10 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105875
Bug ID: 105875
Summary: Toggling an atomic_bool is inefficient
Product: gcc
Version: 12.1.0
Status: UNCONFIRMED
Keywords: missed-optimization
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: josephcsible at gmail dot com
Target Milestone: ---
Target: x86_64-pc-linux-gnu
Consider this C code:
#include <stdatomic.h>
atomic_bool b;
atomic_char c;
_Bool b2;
void f1(void) {
b ^= 1;
}
void f2(void) {
c ^= 1;
}
void f3(void) {
b2 ^= 1;
}
At -O3, those functions compile into this:
f1:
movzbl b(%rip), %eax
.L5:
movb %al, -1(%rsp)
xorl $1, %eax
movl %eax, %edx
movzbl -1(%rsp), %eax
lock cmpxchgb %dl, b(%rip)
jne .L5
ret
f2:
lock xorb $1, c(%rip)
ret
f3:
xorb $1, b2(%rip)
ret
The code generated for f1 is inefficient. It should have just done a "lock xorb
$1, b(%rip)".
^ permalink raw reply [flat|nested] 3+ messages in thread
* [Bug tree-optimization/105875] Toggling an atomic_bool is inefficient
2022-06-07 15:10 [Bug c/105875] New: Toggling an atomic_bool is inefficient josephcsible at gmail dot com
@ 2022-06-21 6:35 ` pinskia at gcc dot gnu.org
2023-05-17 23:32 ` [Bug c/105875] " pinskia at gcc dot gnu.org
1 sibling, 0 replies; 3+ messages in thread
From: pinskia at gcc dot gnu.org @ 2022-06-21 6:35 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105875
--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
clang/LLVM produces the same ....
^ permalink raw reply [flat|nested] 3+ messages in thread
* [Bug c/105875] Toggling an atomic_bool is inefficient
2022-06-07 15:10 [Bug c/105875] New: Toggling an atomic_bool is inefficient josephcsible at gmail dot com
2022-06-21 6:35 ` [Bug tree-optimization/105875] " pinskia at gcc dot gnu.org
@ 2023-05-17 23:32 ` pinskia at gcc dot gnu.org
1 sibling, 0 replies; 3+ messages in thread
From: pinskia at gcc dot gnu.org @ 2023-05-17 23:32 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105875
Andrew Pinski <pinskia at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|UNCONFIRMED |NEW
Component|tree-optimization |c
Ever confirmed|0 |1
Last reconfirmed| |2023-05-17
--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
It is the front-end that is producing the worse code:
TARGET_EXPR <D.2818, 1>;
TARGET_EXPR <D.2819, (void) (D.2819 =
VIEW_CONVERT_EXPR<atomic_bool>(__atomic_load_1 ((const volatile void *) &b,
5)))>;
<D.2821>:;
TARGET_EXPR <D.2820, ((int) TARGET_EXPR <D.2819, (void) (D.2819 =
VIEW_CONVERT_EXPR<atomic_bool>(__atomic_load_1 ((const volatile void *) &b,
5)))> ^ D.2818) != 0>;
if (__atomic_compare_exchange_1 ((volatile void *) &b, (void *) &D.2819,
(int) VIEW_CONVERT_EXPR<unsigned char>(D.2820), 0, 5, 5))
{
goto <D.2822>;
}
goto <D.2821>;
<D.2822>:;, D.2820;
vs:
TARGET_EXPR <D.2827, (char) __atomic_xor_fetch_1 ((volatile void *) &c, (int)
(unsigned char) TARGET_EXPR <D.2826, 1>, 5)>;, D.2827;
So confirmed.
Using __atomic_xor_fetch_1 directly works.
That is:
__atomic_xor_fetch_1 (&b, 1, 5);
Produces:
lock xorb $1, b(%rip)
^ permalink raw reply [flat|nested] 3+ messages in thread
end of thread, other threads:[~2023-05-17 23:32 UTC | newest]
Thread overview: 3+ messages (download: mbox.gz / follow: Atom feed)
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2022-06-07 15:10 [Bug c/105875] New: Toggling an atomic_bool is inefficient josephcsible at gmail dot com
2022-06-21 6:35 ` [Bug tree-optimization/105875] " pinskia at gcc dot gnu.org
2023-05-17 23:32 ` [Bug c/105875] " pinskia at gcc dot gnu.org
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