https://bugs.llvm.org/show_bug.cgi?id=46203
Bug ID: 46203
Summary: Failure to optimize __builtin_ffs minus 1 to
tzcnt+cmovne
Product: libraries
Version: trunk
Hardware: PC
OS: Linux
Status: NEW
Severity: enhancement
Priority: P
Component: Backend: X86
Assignee: [email protected]
Reporter: [email protected]
CC: [email protected], [email protected],
[email protected], [email protected]
int f(int mask)
{
return __builtin_ffs(mask) - 1;
}
With -O3, GCC outputs this :
f(int):
bsf edi, edi
mov eax, -1
cmovne eax, edi
ret
LLVM outputs this :
f(int): # @f(int)
bsf ecx, edi
xor eax, eax
cmp edi, 1
sbb eax, eax
or eax, ecx
ret
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