[llvm-bugs] [Bug 46707] New: Failure to use al for sete after cmpxchg
via llvm-bugs
llvm-bugs at lists.llvm.org
Mon Jul 13 09:26:31 PDT 2020
https://bugs.llvm.org/show_bug.cgi?id=46707
Bug ID: 46707
Summary: Failure to use al for sete after cmpxchg
Product: libraries
Version: trunk
Hardware: PC
OS: Linux
Status: NEW
Severity: enhancement
Priority: P
Component: Backend: X86
Assignee: unassignedbugs at nondot.org
Reporter: gabravier at gmail.com
CC: craig.topper at gmail.com, llvm-bugs at lists.llvm.org,
llvm-dev at redking.me.uk, spatel+llvm at rotateright.com
int f(unsigned char* addr, unsigned char old_val, unsigned char new_val)
{
auto old_val_cpy = old_val;
__atomic_compare_exchange_n(addr, &old_val, new_val, 0, 0, 0);
return old_val == old_val_cpy;
}
With -O3, LLVM outputs this :
f(unsigned char*, unsigned char, unsigned char):
mov eax, esi
xor ecx, ecx
lock cmpxchg byte ptr [rdi], dl
sete cl
mov eax, ecx
ret
Is there some weird hardware problem that makes the sete slower if used on al
or something ? If not, I see no reason why the sete could not have al as the
operand, which would avoid the mov afterwards.
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