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

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Message-ID: <YwT+5GGCOKoTjfQZ@zn.tnic>
Date:   Tue, 23 Aug 2022 18:23:00 +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 Fri, Aug 12, 2022 at 08:44:38PM +0900, Vincent Mailhol wrote:
> __ffs(x) is equivalent to (unsigned long)__builtin_ctzl(x)

Are you sure about this?

My gcc documentation says:

"Built-in Function: int __builtin_ctz (unsigned int x)

    Returns the number of trailing 0-bits in x, starting at the least significant bit position. If x is 0, the result is undefined."

Note the undefined part.

Also,

__builtin_ctzl(0): 0x40
ffs(0): 0x0

I'm using the kernel ffs() version in a small program which is basically
a wrapper around BSF.

-- 
Regards/Gruss,
    Boris.

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