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

[Kevin Bowling]

FreeBSD does not require pre-commit approval unless called out
specifically.  Are you volunteering to review the changes, and if so
where is your guidance?  These messages are otherwise unhelpful.

On Mon, Feb 8, 2021 at 12:37 PM Jessica Clarke <jrtc27 at freebsd.org> wrote:
>
> 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
>
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