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Message-ID: <ZO9phB2Ehukb1kab@biznet-home.integral.gnuweeb.org>
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
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