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

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Message-ID: <YxhbXE0lWtyNtRHh@nazgul.tnic>
Date:   Wed, 7 Sep 2022 10:50:47 +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>,
        Yury Norov <yury.norov@...il.com>
Subject: Re: [PATCH v5 2/2] x86/asm/bitops: __ffs,ffz: use __builtin_ctzl to
 evaluate constant expressions

On Wed, Sep 07, 2022 at 02:35:41PM +0900, Vincent MAILHOL wrote:
> I rephrased the beginning of the commit message as below:
> 
> 
> If x is not 0, __ffs(x) is equivalent to:
>   (unsigned long)__builtin_ctzl(x)
> And if x is not ~0UL, ffz(x) is equivalent to:
>   (unsigned long)__builtin_ctzl(~x)
> Because __builting_ctzl() returns an int, a cast to (unsigned long) is
> necessary to avoid potential warnings on implicit casts.
> 
> Concerning the edge cases, __builtin_ctzl(0) is always undefined,
> whereas __ffs(0) and ffz(~0UL) may or may not be defined, depending on
> the processor. Regardless, for both functions, developers are asked to
> check against 0 or ~0UL so replacing __ffs() or ffz() by
> __builting_ctzl() is safe.
> 
> 
> 
> Does this solve the issue?

Yes, that sounds better.

Thx.

-- 
Regards/Gruss,
    Boris.

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