Re: [PATCH v4] arch/x86: Improve 'rep movs{b|q}' usage in memmove_64.S

[Noah Goldstein <goldstein.w.n@gmail.com>]

On Fri, Nov 19, 2021 at 6:05 PM Noah Goldstein <goldstein.w.n@gmail.com> wrote:
>
> On Fri, Nov 19, 2021 at 4:31 PM David Laight <David.Laight@aculab.com> wrote:
> >
> > From: Noah Goldstein
> > > Sent: 17 November 2021 22:45
> > >
> > > On Wed, Nov 17, 2021 at 4:31 PM David Laight <David.Laight@aculab.com> wrote:
> > > >
> > > > From: Noah Goldstein
> > > > > Sent: 17 November 2021 21:03
> > > > >
> > > > > Add check for "short distance movsb" for forwards FSRM usage and
> > > > > entirely remove backwards 'rep movsq'. Both of these usages hit "slow
> > > > > modes" that are an order of magnitude slower than usual.
> > > > >
> > > > > 'rep movsb' has some noticeable VERY slow modes that the current
> > > > > implementation is either 1) not checking for or 2) intentionally
> > > > > using.
> > > >
> > > > How does this relate to the decision that glibc made a few years
> > > > ago to use backwards 'rep movs' for non-overlapping copies?
> > >
> > > GLIBC doesn't use backwards `rep movs`.  Since the regions are
> > > non-overlapping it just uses forward copy. Backwards `rep movs` is
> > > from setting the direction flag (`std`) and is a very slow byte
> > > copy. For overlapping regions where backwards copy is necessary GLIBC
> > > uses 4x vec copy loop.
> >
> > Try to find this commit 6fb8cbcb58a29fff73eb2101b34caa19a7f88eba
> >
> > Or follow links from https://www.win.tue.nl/~aeb/linux/misc/gcc-semibug.html
> > But I can't find the actual patch.
> >
> > The claims were a massive performance increase for the reverse copy.
> >
>
> I don't think that's referring to optimizations around `rep movs`. It
> appears to be referring to fallout from this patch:
> https://sourceware.org/git/?p=glibc.git;a=commit;h=6fb8cbcb58a29fff73eb2101b34caa19a7f88eba
>
> which broken programs misusing `memcpy` with overlapping regions
> resulting in this fix:
> https://sourceware.org/git/?p=glibc.git;a=commit;h=0354e355014b7bfda32622e0255399d859862fcd
>
> AFAICT support for ERMS was only added around:
> https://sourceware.org/git/?p=glibc.git;a=commit;h=13efa86ece61bf84daca50cab30db1b0902fe2db
>
> Either way GLIBC memcpy/memmove moment most certainly does not
> use backwards `rep movs`:
> https://sourceware.org/git/?p=glibc.git;a=blob;f=sysdeps/x86_64/multiarch/memmove-vec-unaligned-erms.S;hb=HEAD#l655
>
> as it is very slow.
>
> > The pdf from www.agner.org/optimize may well indicate why some
> > copies are unexpectedly slow due to cache access aliasing.
>
> Even in the `4k` aliasing case `rep movsb` seems to stay within a
> factor of 2 of optimal whereas the `std` backwards `rep movs` loses
> by a factor of 10.
>
> Either way, `4k` aliasing detection is mostly a concern of `memcpy` as
> the direction of copy for `memmove` is a correctness question, not
> an optimization.
>
>
> >
> > I'm pretty sure that Intel cpu (possibly from Ivy bridge onwards)
> > can be persuaded to copy 8 bytes/clock for in-cache data with
> > a fairly simple loop that contains 2 reads (maybe misaligned)
> > and two writes (so 16 bytes per iteration).
> > Extra unrolling just adds extra code top and bottom.
> >
> > You might want a loop like:
> >         1:      mov     0(%rsi, %rcx),%rax
> >                 mov     8(%rsi, %rcx),%rdx
> >                 mov     %rax, 0(%rdi, %rcx)
> >                 mov     %rdx, 8(%rdi, %rcx)
> >                 add     $16, %rcx
> >                 jnz     1b
> >
> >         David
>
> The backwards loop already has 4x unrolled `movq` loop.
ping.
>
> >
> > -
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