Missed optimization when a constant is a power of two #41602
Description
| Bugzilla Link | 42257 |
| Resolution | FIXED |
| Resolved on | Oct 15, 2019 07:32 |
| Version | trunk |
| OS | Linux |
| CC | @davidbolvansky,@LebedevRI,@RKSimon,@rotateright,@ZviRackover |
| Fixed by commit(s) | r374886 |
Extended Description
The following function is correctly treated as a no-op by clang(trunk, -Ofast --march=skylake)
bool zero(bool b) {
int x = 5;
return (x * b) + (-x * b);
}
=>
zero(bool):
xor eax, eax
ret
Messing around with this function on compiler explorer (https://godbolt.org/z/grc5ZF), I noticed when the constant is set to a power of two (where the power is not 0 or 1) or the negative of a power of two, the code is no longer optimized away.
bool zero(bool b) {
int x = 4;
return (x * b) + (-x * b);
}
=>
zero(bool):
shl edi, 2
cmp edi, edi ;; Always sets the ZF
setne al ;; ZF is set, so al is always 0
ret
This missed optimization still happens when the return type is an int, and the missed optimization is not obvious from the assembly (https://godbolt.org/z/ZEwONp).
// Completely optimized
int zero(bool b) {
int x = 18;
return (x * b) + (-x * b);
}
=>
zero(bool):
xor eax, eax
ret
vs
// Missing opportunity
int zero(bool b) {
int x = 4;
return (x * b) + (-x * b);
}
=>
zero(bool):
mov eax, edi
shl eax, 2
xor dil, 1
movzx ecx, dil
lea eax, [rax + 4*rcx]
add eax, -4
ret
The problem disappears if the function parameter is widened,
int zero(int b) {
int x = 4;
return (x * b) + (-x * b);
}
or if the intermediate values are close to overflowing (https://godbolt.org/z/yHPjz2),
int zero(bool b) {
int x = 1 << 31;
return (x * b) + (-x * b);
}
or (as a comment on stackoverflow pointed out https://stackoverflow.com/questions/56416747/missed-optimization-in-clang-only-on-powers-of-two#comment99429730_56416747) if the value is not known.
int zero(bool b, int x) {
return (x * b) + (-x * b);
}
All compile down to
zero(int):
xor eax, eax
ret