https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91502
Bug ID: 91502
Summary: suboptimal atomic_fetch_sub and atomic_fetch_add
Product: gcc
Version: unknown
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: nruslan_devel at yahoo dot com
Target Milestone: ---
I have not specified the gcc version; it seems like it applies to any version.
I have noticed that if I write code:
#include <stdatomic.h>
int func(_Atomic(int) *a)
{
return (atomic_fetch_sub(a, 1) - 1 == 0);
}
gcc generates optimized code (gcc -O2):
func:
.LFB0:
.cfi_startproc
xorl %eax, %eax
lock subl $1, (%rdi)
sete %al
ret
But when I change the condition to <= 0, it does not work. Correct me if I am
wrong, but, I think, it should still be able to use sub:
#include <stdatomic.h>
int func(_Atomic(int) *a)
{
return (atomic_fetch_sub(a, 1) - 1 <= 0);
}
func:
.LFB0:
.cfi_startproc
movl $-1, %eax
lock xaddl %eax, (%rdi)
cmpl $1, %eax
setle %al
movzbl %al, %eax
ret
Seems like the same problem exists for atomic_fetch_add as well.
