linux-kernel - Re: [RESEND PATCH v4 1/2] x86/asm/bitops: ffs: use __builtin

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Message-ID: <YvUZVYxbOMcZtR5G@zn.tnic>
Date:   Thu, 11 Aug 2022 16:59:33 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Vincent Mailhol <mailhol.vincent@...adoo.fr>
Cc:     Nick Desaulniers <ndesaulniers@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, x86@...nel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H . Peter Anvin" <hpa@...or.com>,
        Nathan Chancellor <nathan@...nel.org>,
        Tom Rix <trix@...hat.com>, linux-kernel@...r.kernel.org,
        llvm@...ts.linux.dev, David Howells <dhowells@...hat.com>,
        Jan Beulich <JBeulich@...e.com>,
        Christophe Jaillet <christophe.jaillet@...adoo.fr>,
        Joe Perches <joe@...ches.com>,
        Josh Poimboeuf <jpoimboe@...nel.org>
Subject: Re: [RESEND PATCH v4 1/2] x86/asm/bitops: ffs: use __builtin_ffs to
 evaluate constant expressions

On Sun, Jul 24, 2022 at 12:15:20AM +0900, Vincent Mailhol wrote:
> For x86_64, the current ffs() implementation does not produce
> optimized code when called with a constant expression. On the
> contrary, the __builtin_ffs() function of both GCC and clang is able
> to simplify the expression into a single instruction.
> 
> * Example *
> 
> Let's consider two dummy functions foo() and bar() as below:
> 
> | #include <linux/bitops.h>
> | #define CONST 0x01000000

Those code examples you can simply indent with two spaces.

> In both examples, we clearly see the benefit of using __builtin_ffs()

Who's "we"?

Please use passive voice in your commit message: no "we" or "I", etc,
and describe your changes in imperative mood.

> instead of the kernel's asm implementation for constant expressions.
> 
> However, for non constant expressions, the ffs() asm version of the
> kernel remains better for x86_64 because, contrary to GCC, it doesn't
> emit the CMOV assembly instruction, c.f. [1] (noticeably, clang is
> able optimize out the CMOV call).
> 
> This patch uses the __builtin_constant_p() to select between the

Avoid having "This patch" or "This commit" in the commit message. It is
tautologically useless.

Also, do

$ git grep 'This patch' Documentation/process

for more details.

> kernel's ffs() and the __builtin_ffs() depending on whether the
> argument is constant or not.

In general, you don't have to say what the patch does - that should be
visible from the diff. The more important part is the *why*. And that
you do.

Rest looks ok.

Thx.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette