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

[David Laight]

From: Noah Goldstein
> Sent: 17 November 2021 22:45
> 
> On Wed, Nov 17, 2021 at 4:31 PM David Laight <David.Laight@xxxxxxxxxx> 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.

The pdf from www.agner.org/optimize may well indicate why some
copies are unexpectedly slow due to cache access aliasing.

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

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