linux-kernel - RE: [RFC PATCH v1 0/5] nolibc x86-64 string functions

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Message-ID: <5a821292d96a4dbc84c96ccdc6b5b666@AcuMS.aculab.com>
Date:   Fri, 1 Sep 2023 11:34:18 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Ammar Faizi' <ammarfaizi2@...weeb.org>, Willy Tarreau <w@....eu>,
        Thomas Weißschuh <linux@...ssschuh.net>
CC:     Nicholas Rosenberg <inori@...x.org>,
        Alviro Iskandar Setiawan <alviro.iskandar@...weeb.org>,
        Michael William Jonathan <moe@...weeb.org>,
        GNU/Weeb Mailing List <gwml@...r.gnuweeb.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: RE: [RFC PATCH v1 0/5] nolibc x86-64 string functions

From: Ammar Faizi <ammarfaizi2@...weeb.org>
> Sent: 30 August 2023 14:57
> 
> This is an RFC patchset for nolibc x86-64 string functions. There are 5
> patches in this series.
> 
> ## Patch 1-3: Use `rep movsb`, `rep stosb`, and `rep cmpsb` for:
>     - memcpy() and memmove()
>     - memset()
>     - memcmp()
> respectively. They can simplify the generated ASM code.
> 
...
> After this series:
> ```
>   000000000000140a <memmove>:
>     140a: 48 89 f8              mov    %rdi,%rax
>     140d: 48 89 d1              mov    %rdx,%rcx
>     1410: 48 8d 7c 0f ff        lea    -0x1(%rdi,%rcx,1),%rdi
>     1415: 48 8d 74 0e ff        lea    -0x1(%rsi,%rcx,1),%rsi
>     141a: fd                    std
>     141b: f3 a4                 rep movsb %ds:(%rsi),%es:(%rdi)
>     141d: fc                    cld
>     141e: c3                    ret

Isn't that completely broken?

You need to select between forwards and backwards moves.
Since forwards moves are preferred it is best to do
	if (dst - src < len)
		backards_copy()
	else
		formwards_copy()

	David

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