| Issue |
124993
|
| Summary |
Suboptimal code generation for vectorized version of llvm.ctlz() for int64 on x86-64
|
| Labels |
new issue
|
| Assignees |
|
| Reporter |
aneshlya
|
LLVM generates suboptimal code for `llvm.ctlz()` on the int64 type across various x86-64 instruction sets (SSE4–AVX2) before AVX512. Performance measurements indicate that extracting individual 64-bit values from the `ymm` register and applying `lzcnt` separately to each yields a 25% improvement on AVX2 and a 124% improvement on SSE4, compared to `llvm.ctlz` vectorized implementation.
Please see the example here: https://ispc.godbolt.org/z/EEErrednx
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