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

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Message-ID: <CAMZ6Rq+Uo4r9DXsOTgEhNEv7wWkHjBnU0498+1++qaD+4WCPKw@mail.gmail.com>
Date:   Wed, 7 Sep 2022 14:35:41 +0900
From:   Vincent MAILHOL <mailhol.vincent@...adoo.fr>
To:     Borislav Petkov <bp@...en8.de>
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. 7 Sep 2022 at 13:06, Borislav Petkov <bp@...en8.de> wrote:
> On Sat, Aug 27, 2022 at 06:32:05AM +0900, Vincent MAILHOL wrote:
> > Agree that this is only the surface. But, my patch series is about
> > constant folding, not about the text of *ffs(). Here, I just *move*
> > the existing text, I did not modify anything.
> > Can we agree that this is a separate topic?
>
> Sure we can.
>
> But then you can't start your commit message with:
>
> "__ffs(x) is equivalent to (unsigned long)__builtin_ctzl(x) and ffz(x)
> is equivalent to (unsigned long)__builtin_ctzl(~x)."
>
> which will bring unenlightened readers like me into the very same mess.
>
> So at least mention that there's a difference between the kernel
> implementation using hw insns which are well defined on some machines
> and what the glibc API does. So that at least people are aware that
> there's something dangerous to be cautious about.
>
> Ok?

OK.

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? If yes, I will prepare the v8 right away.

Yours sincerely,
Vincent Mailhol