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Date: Sun, 3 Sep 2023 23:19:42 +0200
From: Willy Tarreau <w@1wt.eu>
To: David Laight <David.Laight@ACULAB.COM>
Cc: "'Ammar Faizi'" <ammarfaizi2@gnuweeb.org>,
        Thomas =?iso-8859-1?Q?Wei=DFschuh?= <linux@weissschuh.net>,
        Nicholas Rosenberg <inori@vnlx.org>,
        Alviro Iskandar Setiawan <alviro.iskandar@gnuweeb.org>,
        Michael William Jonathan <moe@gnuweeb.org>,
        GNU/Weeb Mailing List <gwml@vger.gnuweeb.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [RFC PATCH v3 0/4] nolibc x86-64 string functions
Message-ID: <20230903211942.GA31739@1wt.eu>
References: <20230902133505.2176434-1-ammarfaizi2@gnuweeb.org>
 <e4862b01c0e4440f949e240360e438a3@AcuMS.aculab.com>
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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