https://bugs.llvm.org/show_bug.cgi?id=37557
Bug ID: 37557
Summary: Suboptimal atomic_fetch_add
Product: libraries
Version: trunk
Hardware: PC
OS: Linux
Status: NEW
Severity: enhancement
Priority: P
Component: Backend: C
Assignee: [email protected]
Reporter: [email protected]
CC: [email protected]
Consider the following example:
#include <stdatomic.h>
int func(_Atomic(unsigned long) * obj, unsigned long a)
{
return atomic_fetch_add(obj, a) == -a;
}
Basically, it suffices to use 'lock add' and check whether ZF flag is set.
However, the following code (that uses xadd) is generated (-O2):
func: # @func
.cfi_startproc
# %bb.0:
movq %rsi, %rcx
lock xaddq %rcx, (%rdi)
xorl %eax, %eax
addq %rsi, %rcx
sete %al
retq
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