linux-kernel - Re: [RFC PATCH v1 2/5] tools/nolibc: x86-64: Use `rep stosb` for `memset()`

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Date:   Wed, 30 Aug 2023 23:08:36 +0700
From:   Ammar Faizi <ammarfaizi2@...weeb.org>
To:     Willy Tarreau <w@....eu>
Cc:     Alviro Iskandar Setiawan <alviro.iskandar@...weeb.org>,
        Thomas Weißschuh <linux@...ssschuh.net>,
        Nicholas Rosenberg <inori@...x.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 2/5] tools/nolibc: x86-64: Use `rep stosb` for
 `memset()`

On Wed, Aug 30, 2023 at 05:51:52PM +0200, Willy Tarreau wrote:
> Normal, that's because historically "xchg ax, regX" was a single-byte 0x9X
> on 8086, then it turned to 32-bit keeping the same encoding, like many
> instructions (note that NOP is encoded as xchg ax,ax). It remains short
> when you can sacrifice the other register, or restore it later using yet
> another xchg. For rcx/rdx a push/pop could do it as they should also be
> a single-byte 0x5X even in long mode unless I'm mistaken. Thus if you
> absolutely want to squeeze that 9th byte to end up with a 8-byte function
> you could probably do:
> 
>     xchg %eax, %esi      1
>     push %rdx            1
>     pop %rcx             1
>     push %rdi            1
>     rep movsb            2 [sic]
>     pop %rax             1
>     ret                  1
>     ------------- Total: 8 bytes :-)

Fun!

We're not doing a code golf game, though. So, I think I will leave the
"mov %rdx, %rcx" as is. Otherwise, I would be tempted to do that all
over the place.

-- 
Ammar Faizi