| Issue |
130549
|
| Summary |
[X86] `acc -= val < 0` converted to less efficient `acc += (val >> 7) & 1`
|
| Labels |
|
| Assignees |
|
| Reporter |
dzaima
|
The code:
```c
#include <immintrin.h>
#include <stdint.h>
__m256i count_negative_values(__m256i* src, size_t n) {
__m256i acc = _mm256_setzero_si256();
for (size_t i = 0; i < n; i++) {
__m256i c = _mm256_loadu_si256(src + i);
__m256i m = _mm256_cmpgt_epi8(_mm256_setzero_si256(), c);
acc = _mm256_sub_epi8(acc, m);
}
return acc;
}
```
with `-O3 -march=haswell` generates a sequence of `vpand` + `vpaddb` for adding to the accumulator, whereas it could produce just `vpsubb`. (if desired, in the unrolled case, it could, to add e.g. `a`,`b`,`c`,`d` to the accumulator without a long `vpsubb` chain, do `acc -= (a+b)+(c+d)`)
https://godbolt.org/z/Wo7qE8MjG
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