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

Uros Bizjak
2025-02-23T20:25:47Z

On 23. 02. 25 17:42, Kuan-Wei Chiu wrote:
> Several parts of the kernel contain redundant implementations of parity
> calculations for 32-bit and 64-bit values. Introduces generic
> parity32() and parity64() helpers in bitops.h, providing a standardized
> and optimized implementation.
>
> Subsequent patches refactor various kernel components to replace
> open-coded parity calculations with the new helpers, reducing code
> duplication and improving maintainability.

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/

Thanks,
Uros.