linux-kernel - Re: [PATCH v2 1/2] x86/asm/bitops: ffs: use __builtin

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Message-ID: <CAMZ6Rq+d_eKXQQZ1p-yUpqC-SOjpa83o_QpZci-GN9qd9UKa5g@mail.gmail.com>
Date:   Thu, 12 May 2022 08:30:45 +0900
From:   Vincent MAILHOL <mailhol.vincent@...adoo.fr>
To:     Christophe JAILLET <christophe.jaillet@...adoo.fr>
Cc:     Nick Desaulniers <ndesaulniers@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        "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>
Subject: Re: [PATCH v2 1/2] x86/asm/bitops: ffs: use __builtin_ffs to evaluate
 constant expressions

On Thu. 12 May 2022 at 05:56, Christophe JAILLET
<christophe.jaillet@...adoo.fr> wrote:
> Le 11/05/2022 à 18:03, Vincent Mailhol a écrit :
> > For x86_64, the current ffs() implementation does not produce
> > optimized code when called with a constant expression. On the
> > contrary, the __builtin_ffs() function of both GCC and clang is able
> > to simplify the expression into a single instruction.
> >
>
> [...]
>
> >
> > ** Statistics **
> >
> > On a allyesconfig, before applying this patch...:
> >
> > | $ objdump -d vmlinux.o | grep bsf | wc -l
> > | 3607
> >
> > ...and after:
> >
> > | $ objdump -d vmlinux.o | grep bsf | wc -l
> > | 792
> >
> > So, roughly 26.7% of the call to ffs() were using constant expression
> > and were optimized out.
> >
> >
> nitpicking: numbers look odd.

And they are. Thanks for spotting the issue.

>     3607 is the exact same number as in patch 2/2. (ok, could be)
>     26.7% is surprising with these numbers. (I guess it is (total_before
> - remaining) / total_before x 100 = (3607-792)/36.07 = 78.0%)

The 3607 is incorrect (copy/paste issue, sorry). The correct figure is
1081. And:

(1081-792)/1081 = 26.7%

Will amend the comment and send v3 right away.

> (but patch looks great to me :)

Thanks! :)

> CJ