linux-kernel - RE: [PATCH v4] x86, mem: move memmove to out of line assembler

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Date:   Fri, 30 Sep 2022 09:55:27 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Nick Desaulniers' <ndesaulniers@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>
CC:     "x86@...nel.org" <x86@...nel.org>,
        "H . Peter Anvin" <hpa@...or.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Kees Cook <keescook@...omium.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        "llvm@...ts.linux.dev" <llvm@...ts.linux.dev>,
        Andy Lutomirski <luto@...nel.org>,
        Rasmus Villemoes <linux@...musvillemoes.dk>
Subject: RE: [PATCH v4] x86, mem: move memmove to out of line assembler

From: Nick Desaulniers
> Sent: 28 September 2022 22:05
...
> memmove is quite large and probably shouldn't be inlined due to size
> alone
..
> +	/* Decide forward/backward copy mode */
> +	cmpl	dest, src
> +	jb	.Lbackwards_header

It has to be better to do the slightly more complicated
test 'dest - src < size' (as unsigned) so that reverse
copies are only done when absolutely necessary.

Ignoring pathological cases on some cpu families the
forwards copy will benefit from hardware cache prefetch.

I also have a vague recollection of std and cld being slow.

Oh - and why do all the labels have 'byteswap' in them?

	David

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