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

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/ (local)

[2] https://lore.kernel.org/lkml/20250129154920.6773-2-ubizjak@gmail.com/ (local)

[3] https://lore.kernel.org/linux-mm/CAKbZUD0N7bkuw_Le3Pr9o1V2BjjcY_YiLm8a8DPceubTdZ00GQ@mail.gmail.com/ (local)

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