linux-kernel - Re: [PATCH 0/2] x86/asm/bitops: optimize ff{s,z} functions for constant expressions

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Date:   Tue, 10 May 2022 15:14:43 -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
Subject: Re: [PATCH 0/2] x86/asm/bitops: optimize ff{s,z} functions for
 constant expressions

On Tue, May 10, 2022 at 7:26 AM Vincent Mailhol
<mailhol.vincent@...adoo.fr> wrote:
>
> The compilers provides some builtin expression equivalent to the
> ffs(), __ffs() and ffz() function of the kernel. The kernel uses
> optimized assembly which produces better code than the builtin
> functions. However, such assembly code can not be optimized when used
> on constant expression.
>
> This series relies on __builtin_constant_p to select the optimal solution:
>
>   * use kernel assembly for non constant expressions
>
>   * use compiler's __builtin function for constant expressions.
>
> I also think that the fls() and fls64() can be optimized in a similar
> way, using __builtin_ctz() and __builtin_ctzll() but it is a bit less
> trivial so I want to focus on this series first. If it get accepted, I
> will then work on those two additionnal function.
>
>
> ** Statistics **
>
> On a allyesconfig, before applying this series, I get:
>
> | $ objdump -d vmlinux.o | grep bsf | wc -l
> | 1081
>
> After applying this series:
>
> | $ objdump -d vmlinux.o | grep bsf | wc -l
> | 792
>
> So, roughly 26.7% of the call to either ffs() or __ffs() were using
> constant expression and can be optimized (I did not produce the
> figures for ffz()).

These stats are interesting; consider putting them on patch 1/2 commit
message though (in addition to the cover letter). (Sending thoughts on
1/2 next).

>
> (tests done on linux v5.18-rc5 x86_64 using GCC 11.2.1)

Here's the same measure of x86_64 allyesconfig (./scripts/config -d
CONFIG_HINIC) at 9be9ed2612b5aedb52a2c240edb1630b6b743cb6 with ToT
LLVM (~clang-15):

Before:
$ objdump -d vmlinux.o | grep bsf | wc -l
1454

After:
$ objdump -d vmlinux.o | grep bsf | wc -l
1070

-26.4% :)


>
>
> Vincent Mailhol (2):
>   x86/asm/bitops: ffs: use __builtin_ffs to evaluate constant
>     expressions
>   x86/asm/bitops: __ffs,ffz: use __builtin_ctzl to evaluate constant
>     expressions
>
>  arch/x86/include/asm/bitops.h | 65 +++++++++++++++++++++--------------
>  1 file changed, 40 insertions(+), 25 deletions(-)
>
> --
> 2.35.1
>


--
Thanks,
~Nick Desaulniers