git: af366d353b84 - main - amd64: implement strlen in assembly

[Jessica Clarke]

On 8 Feb 2021, at 19:15, Mateusz Guzik <mjg at FreeBSD.org> wrote:
> 
> The branch main has been updated by mjg:
> 
> URL: https://cgit.FreeBSD.org/src/commit/?id=af366d353b84bdc4e730f0fc563853abc338271c
> 
> commit af366d353b84bdc4e730f0fc563853abc338271c
> Author:     Mateusz Guzik <mjg at FreeBSD.org>
> AuthorDate: 2021-02-08 17:01:48 +0000
> Commit:     Mateusz Guzik <mjg at FreeBSD.org>
> CommitDate: 2021-02-08 19:15:21 +0000
> 
>    amd64: implement strlen in assembly
> 
>    The C variant in libkern performs excessive branching to find the
>    non-zero byte instead of using the bsfq instruction. The same code
>    patched to use it is still slower than the routine implemented here
>    as the compiler keeps neglecting to perform certain optimizations
>    (like using leaq).
> 
>    On top of that the routine can is a starting point for copyinstr
>    which operates on words instead of bytes.
> 
>    Tested with glibc test suite.
> 
>    Sample results (calls/s):
> 
>    Haswell:
>    $(perl -e "print 'A' x 3"):
>    stock:  211198039
>    patched:338626619
>    asm:    465609618
> 
>    $(perl -e "print 'A' x 100"):
>    stock:   83151997
>    patched: 98285919
>    asm:    120719888
> 
>    AMD EPYC 7R32:
>    $(perl -e "print 'A' x 3"):
>    stock:  282523617
>    asm:    491498172
> 
>    $(perl -e "print 'A' x 100"):
>    stock:  114857172
>    asm:    112082057

No Reviewed by? More than one pair of eyes on non-trivial assembly is
almost always a good idea.

Jess