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

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Message-ID: <Z8q6wPWXImCeGI8P@visitorckw-System-Product-Name>
Date: Fri, 7 Mar 2025 17:22:08 +0800
From: Kuan-Wei Chiu <visitorckw@...il.com>
To: Jiri Slaby <jirislaby@...nel.org>
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	Yu-Chun Lin <eleanor15x@...il.com>
Subject: Re: [PATCH v3 00/16] Introduce and use generic parity16/32/64 helper

Hi Jiri,

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?
> 
In the previous thread, David and Yury had different opinions regarding
the implementation details of the parity() macro. I am trying to find a
solution that satisfies most people while keeping it as simple as
possible.

I cannot point to any specific users who are particularly concerned
about efficiency, so personally, I am not really concerned about the
generated code either. However, I am not a fan of the #if gcc #else
approach, and Yury also mentioned that he does not like the >> 16 >> 16
hack. At the same time, David pointed out that GCC might generate
double-register math. Given these concerns, I leaned toward reverting
to the parityXX() approach.

If you still prefer using the parity() macro, we can revisit and
discuss its implementation details further.

> > Additionally, I changed parityXX() << y users to !!parityXX() << y
> > because, unlike C++, C does not guarantee that true casts to int as 1.
> 
> How comes? ANSI C99 exactly states:
> ===
> true
> which expands to the integer constant 1,
> ===
> 
I gave a more detailed response in my reply to Peter. If we can confirm
that casting bool to int will only result in 1 or 0, I will remove the
!! hack in the next version.

Regards,
Kuan-Wei