Re: [PATCH 00/17] Introduce and use generic parity32/64 helper

Yu-Chun Lin
2025-02-24T15:27:16Z

On Sun, Feb 23, 2025 at 09:25:42PM +0100, Uros Bizjak wrote:
>
> Please note that GCC (and clang) provide __builtin_parity{,l,ll}() family of
> builtin functions. Recently, I have tried to use this builtin in a couple of
> places [1], [2], but I had to retract the patches, because __builtin
> functions aren't strictly required to be inlined and can generate a library
> call [3].
>
> As explained in [2], the compilers are able to emit optimized
> target-dependent code (also automatically using popcnt insn when avaialble),
> so ideally the generic parity64() and parity32() would be implemented using
> __builtin_parity(), where the generic library would provide a fallback
> __paritydi2() and __paritysi2() functions, otherwise provided by the
> compiler support library.
>
> For x86, we would like to exercise the hardware parity calculation or
> optimized code sequences involving HW parity calculation, as shown in [1]
> and [2].
>
> [1] https://lore.kernel.org/lkml/20250129205746.10963-1-ubizjak@gmail.com/
>
> [2] https://lore.kernel.org/lkml/20250129154920.6773-2-ubizjak@gmail.com/
>
> [3] https://lore.kernel.org/linux-mm/CAKbZUD0N7bkuw_Le3Pr9o1V2BjjcY_YiLm8a8DPceubTdZ00GQ@mail.gmail.com/

Hi Uros,
Thanks for your information.

We originally planned to implement hardware optimizations after this
patch series. However, for V2, We will incorporate __builtin_parity(),
while keeping our current implementation as the fallback function.

Best regards,
Yu-Chun Lin