| Issue |
95255
|
| Summary |
[Missed optimization] `has_single_bit` when the value is known to be non-zero
|
| Labels |
new issue
|
| Assignees |
|
| Reporter |
MitalAshok
|
```c++
bool test1(unsigned n) {
__builtin_assume(n);
return __builtin_popcount(n) == 1;
}
bool test2(unsigned n) {
__builtin_assume(n);
return (n != 0) && ((n & (n-1u)) == 0);
}
```
Where `test1` looks like libstdc++'s `std::has_single_bit` and `test2` looks like libc++'s `std::has_single_bit`.
These are optimized differently with the non-zero assumption: https://godbolt.org/z/b8jTae91x
```asm
test1: # @test1
popcnt eax, edi
cmp eax, 1
sete al
ret
test2: # @test2
blsr eax, edi
sete al
ret
```
(without the assumption, they are both optimized to use `popcnt`)
`return __builtin_popcount(n) <= 1;` *is* optimized to `blsr`
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