linux-kernel - Re: [PATCH v3 2/2] x86/asm/bitops: __ffs,ffz: use __builtin

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Message-ID: <CAKwvOdntx-ZBqVYhdeOO4C1tF4Mx=OpFNYBQUfB=iXjfQO9bvw@mail.gmail.com>
Date:   Wed, 11 May 2022 17:19:54 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Vincent Mailhol <mailhol.vincent@...adoo.fr>
Cc:     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 v3 2/2] x86/asm/bitops: __ffs,ffz: use __builtin_ctzl to
 evaluate constant expressions

On Wed, May 11, 2022 at 5:04 PM Vincent Mailhol
<mailhol.vincent@...adoo.fr> wrote:
>
> __ffs(x) is equivalent to (unsigned long)__builtin_ctzl(x) and 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.
>
> For x86_64, the current __ffs() and ffz() implementations do not
> produce optimized code when called with a constant expression. On the
> contrary, the __builtin_ctzl() gets simplified into a single
> instruction.
>
> However, for non constant expressions, the __ffs() and ffz() asm
> versions of the kernel remains slightly better than the code produced
> by GCC (it produces a useless instruction to clear eax).
>
> This patch uses the __builtin_constant_p() to select between the
> kernel's __ffs()/ffz() and the __builtin_ctzl() depending on whether
> the argument is constant or not.
>
> ** Statistics **
>
> On a allyesconfig, before applying this patch...:
>
> | $ objdump -d vmlinux.o | grep tzcnt | wc -l
> | 3607
>
> ...and after:
>
> | $ objdump -d vmlinux.o | grep tzcnt | wc -l
> | 2600
>
> So, roughly 27.9% of the calls to either __ffs() or ffz() were using
> constant expressions and could be optimized out.
>
> (tests done on linux v5.18-rc5 x86_64 using GCC 11.2.1)
>
> Note: on x86_64, the asm bsf instruction produces tzcnt when used with
> the ret prefix (which is why we grep tzcnt instead of bsf in above
> benchmark). c.f. [1]
>
> [1] commit e26a44a2d618 ("x86: Use REP BSF unconditionally")
> http://lkml.kernel.org/r/5058741E020000780009C014@nat28.tlf.novell.com
>
> CC: Nick Desaulniers <ndesaulniers@...gle.com>
> Signed-off-by: Vincent Mailhol <mailhol.vincent@...adoo.fr>

Thanks for the patches!
Reviewed-by: Nick Desaulniers <ndesaulniers@...gle.com>


-- 
Thanks,
~Nick Desaulniers