Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Date:   Tue, 23 Aug 2022 19:43:59 +0200
From:   Borislav Petkov <bp@alien8.de>
To:     Nick Desaulniers <ndesaulniers@google.com>
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>
Subject: Re: [PATCH v5 2/2] x86/asm/bitops: __ffs,ffz: use __builtin_ctzl to
 evaluate constant expressions
Message-ID: <YwUR35I7+5JbLvMM@zn.tnic>
References: <20220511160319.1045812-1-mailhol.vincent@wanadoo.fr>
 <20220812114438.1574-1-mailhol.vincent@wanadoo.fr>
 <20220812114438.1574-3-mailhol.vincent@wanadoo.fr>
 <YwT+5GGCOKoTjfQZ@zn.tnic>
 <CAKwvOdnc-Js8x4sv0j23crtYP73sRkNexom5ydm=r=8rYgc_5Q@mail.gmail.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=utf-8
Content-Disposition: inline
In-Reply-To: <CAKwvOdnc-Js8x4sv0j23crtYP73sRkNexom5ydm=r=8rYgc_5Q@mail.gmail.com>
Precedence: bulk

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