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

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Message-ID: <CAMZ6Rq+k-5PKesdW7z9DdDMUNSKQR-c1qmQADvJ6VKZA2CM8nw@mail.gmail.com>
Date:   Fri, 12 Aug 2022 20:55:11 +0900
From:   Vincent MAILHOL <mailhol.vincent@...adoo.fr>
To:     Borislav Petkov <bp@...en8.de>
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

Hi Borislav,

On Thu. 11 Aug 2022 at 23:59, Borislav Petkov <bp@...en8.de> wrote:
> 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.

Thank you for the review!
I addressed all your comments and sent a v5:
https://lore.kernel.org/all/20220812114438.1574-1-mailhol.vincent@wanadoo.fr/


Yours sincerely,
Vincent Mailhol