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

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Message-ID: <ZO9j9WhSj7PmsM8b@biznet-home.integral.gnuweeb.org>
Date:   Wed, 30 Aug 2023 22:44:53 +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 05:23:22PM +0200, Willy Tarreau wrote:
> Then "xchg %esi, %eax" is just one byte with no memory access ;-)

Perfect!

Now I got this, shorter than "movl %esi, %eax":
```
0000000000001500 <memset>:
    1500: 96          xchg   %eax,%esi
    1501: 48 89 d1    mov    %rdx,%rcx
    1504: 57          push   %rdi
    1505: f3 aa       rep stos %al,%es:(%rdi)
    1507: 58          pop    %rax
    1508: c3          ret
```

Unfortunately, the xchg trick doesn't yield smaller machine code for
%rdx, %rcx. Lol.

-- 
Ammar Faizi