linux-kernel - Re: [PATCH v3 00/16] Introduce and use generic parity16/32/64 helper

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Message-ID: <Z8sW4c5LxV-ITdCi@thinkpad>
Date: Fri, 7 Mar 2025 10:55:13 -0500
From: Yury Norov <yury.norov@...il.com>
To: Jiri Slaby <jirislaby@...nel.org>
Cc: Kuan-Wei Chiu <visitorckw@...il.com>, tglx@...utronix.de,
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	Yu-Chun Lin <eleanor15x@...il.com>
Subject: Re: [PATCH v3 00/16] Introduce and use generic parity16/32/64 helper

On Fri, Mar 07, 2025 at 07:57:48AM +0100, Jiri Slaby wrote:
> On 06. 03. 25, 17:25, Kuan-Wei Chiu wrote:
> > Several parts of the kernel contain redundant implementations of parity
> > calculations for 16/32/64-bit values. Introduces generic
> > parity16/32/64() 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.
> > 
> > Co-developed-by: Yu-Chun Lin <eleanor15x@...il.com>
> > Signed-off-by: Yu-Chun Lin <eleanor15x@...il.com>
> > Signed-off-by: Kuan-Wei Chiu <visitorckw@...il.com>
> > ---
> > In v3, I use parityXX() instead of the parity() macro since the
> > parity() macro may generate suboptimal code and requires special hacks
> > to make GCC happy. If anyone still prefers a single parity() macro,
> > please let me know.
> 
> What is suboptimal and where exactly it matters? Have you actually measured
> it?

I asked exactly this question at least 3 times, and have never
received perf tests or asm listings - nothing. I've never received
any comments from driver maintainers about how performance of the
parity() is important for them, as well.

With the absence of _any_ feedback, I'm not going to take this series,
of course, for the reason: overengineering.

With that said, the simplest way would be replacing parity8(u8) with
parity(u64) 'one size fits all' thing. I even made a one extra step,
suggesting a macro that would generate a better code for smaller types
with almost no extra maintenance burden. This is another acceptable
option to me.

Thanks,
Yury