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Date: Thu, 29 Feb 2024 01:57:48 +0100
From: Robert Clausecker <fuz@....su>
To: musl@...ts.openwall.com
Subject: Re: [PATCH] add memcmpeq: memcmp that returns length of first
mismatch
Greetings,
Am Thu, Feb 29, 2024 at 12:10:05AM +0000 schrieb Thorsten Glaser:
> Pedro Falcato dixit:
>
> >Small note: This isn't quite true for remotely modern x86, unaligned
>
> It’s very much true, e.g. it breaks atomicity (ok, not relevant
> *here*, but in general).
>
> AIUI, even modern amd64 chips of all vendors are reverting to
> optimising rep movsb/lodsb instead again, for stringops.
That is not the case. REP MOVSB and friends have a high startup latency,
so you only want to use them for large-ish blocks. Too large and all of
the sudden AVX-512 is faster again. For small blocks however, you do not
want to use this instruction. It's indeed much better to do a pair of
overlapping stores. They do not perform crazy well, but it's still better
than all alternatives.
Also note that REP LODSB is pretty useless; did you perhaps mean REP STOSB?
Source: have spent a good part of last year implementing <string.h> in
x86 assembly for FreeBSD's libc.
> Of course the status on other architectures should be sufficient to
> not use unaligned accesses.
Yours,
Robert Clausecker
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