linux-kernel - Re: [RFC PATCH v3 0/4] nolibc x86-64 string functions

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Date:   Sun, 3 Sep 2023 23:19:42 +0200
From:   Willy Tarreau <w@....eu>
To:     David Laight <David.Laight@...LAB.COM>
Cc:     "'Ammar Faizi'" <ammarfaizi2@...weeb.org>,
        Thomas Weißschuh <linux@...ssschuh.net>,
        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 v3 0/4] nolibc x86-64 string functions

On Sun, Sep 03, 2023 at 08:38:22PM +0000, David Laight wrote:
> From: Ammar Faizi
> > Sent: 02 September 2023 14:35
> > 
> > This is an RFC patchset v3 for nolibc x86-64 string functions.
> > 
> > There are 4 patches in this series:
> > 
> > ## Patch 1-2: Use `rep movsb`, `rep stosb` for:
> >     - memcpy() and memmove()
> >     - memset()
> > respectively. They can simplify the generated ASM code.
> 
> It is worth pointing out that while the code size for 'rep xxxb'
> is smaller, the performance is terrible.
> The only time it is ever good is for the optimised forwards
> copies on cpu that support it.
> 
> reverse, stos and scas are always horrid.

It's terrible compared to other approaches but not *that* bad. Also we
absolutely don't care about performance here, rather about correctness
and compact size.

Regards,
Willy