linux-kernel - Re: [PATCH v5 2/2] x86/asm/bitops: __ffs,ffz: use __builtin

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Message-ID: <YxgY2MBmBIkBsdlu@nazgul.tnic>
Date:   Wed, 7 Sep 2022 06:06:58 +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: [PATCH v5 2/2] x86/asm/bitops: __ffs,ffz: use __builtin_ctzl to
 evaluate constant expressions

On Sat, Aug 27, 2022 at 06:32:05AM +0900, Vincent MAILHOL wrote:
> Agree that this is only the surface. But, my patch series is about
> constant folding, not about the text of *ffs(). Here, I just *move*
> the existing text, I did not modify anything.
> Can we agree that this is a separate topic?

Sure we can.

But then you can't start your commit message with:

"__ffs(x) is equivalent to (unsigned long)__builtin_ctzl(x) and ffz(x)
is equivalent to (unsigned long)__builtin_ctzl(~x)."

which will bring unenlightened readers like me into the very same mess.

So at least mention that there's a difference between the kernel
implementation using hw insns which are well defined on some machines
and what the glibc API does. So that at least people are aware that
there's something dangerous to be cautious about.

Ok?

Thx.

-- 
Regards/Gruss,
    Boris.

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