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From:   Nick Desaulniers <ndesaulniers@google.com>
Date:   Tue, 23 Aug 2022 10:12:17 -0700
Message-ID: <CAKwvOdnc-Js8x4sv0j23crtYP73sRkNexom5ydm=r=8rYgc_5Q@mail.gmail.com>
Subject: Re: [PATCH v5 2/2] x86/asm/bitops: __ffs,ffz: use __builtin_ctzl to
 evaluate constant expressions
To:     Borislav Petkov <bp@alien8.de>
Cc:     Vincent Mailhol <mailhol.vincent@wanadoo.fr>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, x86@kernel.org,
        Peter Zijlstra <peterz@infradead.org>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        "H . Peter Anvin" <hpa@zytor.com>,
        Nathan Chancellor <nathan@kernel.org>,
        Tom Rix <trix@redhat.com>, linux-kernel@vger.kernel.org,
        llvm@lists.linux.dev, David Howells <dhowells@redhat.com>,
        Jan Beulich <JBeulich@suse.com>,
        Christophe Jaillet <christophe.jaillet@wanadoo.fr>,
        Joe Perches <joe@perches.com>,
        Josh Poimboeuf <jpoimboe@kernel.org>
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On Tue, Aug 23, 2022 at 9:23 AM Borislav Petkov <bp@alien8.de> wrote:
>
> 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.

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.
-- 
Thanks,
~Nick Desaulniers