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

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Message-ID: <YwUR35I7+5JbLvMM@zn.tnic>
Date:   Tue, 23 Aug 2022 19:43:59 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Vincent Mailhol <mailhol.vincent@...adoo.fr>,
        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 Tue, Aug 23, 2022 at 10:12:17AM -0700, Nick Desaulniers wrote:
> Callers of these need to guard against zero input, as the pre-existing
> comment notes:
> 
> >> Undefined if no bit exists, so code should check against 0 first.

This is just silly.

And then there's

 * ffs(value) returns 0 if value is 0 or the position of the first
 * set bit if value is nonzero. The first (least significant) bit
 * is at position 1.
 */
static __always_inline int ffs(int x)

Can we unify the two and move the guard against 0 inside the function
or, like ffs() does, have it return 0 if value is 0?

-- 
Regards/Gruss,
    Boris.

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