https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98737
Bug ID: 98737
Summary: Atomic operation on x86 no optimized to use flags
Product: gcc
Version: 11.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: target
Assignee: unassigned at gcc dot gnu.org
Reporter: drepper.fsp+rhbz at gmail dot com
Target Milestone: ---
Consider the following code:
long a;
_Bool f(long b)
{
return __atomic_sub_fetch(&a, b, __ATOMIC_RELEASE) == 0;
}
_Bool g(long b)
{
return (a -= b) == 0;
}
When compiling for x86-64 with the current HEAD as of 20210118 the resulting
code is:
0000000000000000 <f>:
0: 48 f7 df neg %rdi
3: 48 89 f8 mov %rdi,%rax
6: f0 48 0f c1 05 00 00 lock xadd %rax,0x0(%rip) # f <f+0xf>
d: 00 00
f: 48 01 f8 add %rdi,%rax
12: 0f 94 c0 sete %al
15: c3 retq
16: 66 2e 0f 1f 84 00 00 nopw %cs:0x0(%rax,%rax,1)
1d: 00 00 00
0000000000000020 <g>:
20: 48 29 3d 00 00 00 00 sub %rdi,0x0(%rip) # 27 <g+0x7>
27: 0f 94 c0 sete %al
2a: c3 retq
The code for f is far too complicated. All that needs to be different from the
code in g is that the lock prefix must be used for sub.
Probably all __atomic_* builtins have problems with using flags when possible.
This is not an esoteric problem. I was specifically looking at optimizing the
std::latch implementation for C++20 and this is what would be needed. Without
a fix a special version would be needed or the current, much worse code is
used.
