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

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Message-ID: <602e03fd-ce4b-feef-5053-e95834ab35d7@gmail.com>
Date: Sun, 23 Feb 2025 21:25:42 +0100
From: Uros Bizjak <ubizjak@...il.com>
To: Kuan-Wei Chiu <visitorckw@...il.com>, tglx@...utronix.de,
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 Yu-Chun Lin <eleanor15x@...il.com>
Subject: Re: [PATCH 00/17] Introduce and use generic parity32/64 helper



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.