| Issue |
204535
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| Summary |
[Analysis] `@llvm.pdep` can be transformed into squaring with `@llvm.clmul`, and vice versa
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| Labels |
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| Assignees |
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| Reporter |
eisenwave
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`pdep` with an alternating bit pattern like `0b01010101...` is used to perform bitwise interleaving. See https://stackoverflow.com/a/79962549/5740428
On platforms where there exists `clmul` but no `pdep` (e.g. RISC-V) or x86_64 CPUs with the PCLMUL extensions but no BMI2, it is beneficial to transform `pdep` into `clmul` if such a mask is detected.
That is, `pdep(x, 0b10101010...) -> clmul(x, x)`.
The inverse is also possible and potentially beneficial because on x86_64, CLMUL is done through vector registers, so there is a bit of fluff just to get things set up and extract the result in the case of scalar operations. That is, `clmul(x, x) -> pdep(x, 0b1010101010...)`.
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