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

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Message-ID: <CAOG64qNGdPjXHTvrpYxjizOYvMRaB9_2t2raw6DJ3kK852KUHg@mail.gmail.com>
Date:   Wed, 30 Aug 2023 21:24:45 +0700
From:   Alviro Iskandar Setiawan <alviro.iskandar@...weeb.org>
To:     Ammar Faizi <ammarfaizi2@...weeb.org>
Cc:     Willy Tarreau <w@....eu>,
        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 8:57 PM Ammar Faizi wrote:
>   00000000000010b1 <memset>:
>     10b1: 48 89 f0              mov    %rsi,%rax
>     10b4: 48 89 d1              mov    %rdx,%rcx
>     10b7: 48 89 fa              mov    %rdi,%rdx
>     10ba: f3 aa                 rep stos %al,%es:(%rdi)
>     10bc: 48 89 d0              mov    %rdx,%rax
>     10bf: c3                    ret

Just a small idea to shrink this more, "mov %rdi, %rdx" and "mov %rdx,
%rax" can be replaced with "push %rdi" and "pop %rax" (they are just a
byte). So we can save 4 bytes more.

0000000000001500 <memset>:
    1500: 48 89 f0     mov    %rsi,%rax
    1503: 48 89 d1     mov    %rdx,%rcx
    1506: 57           push   %rdi
    1507: f3 aa        rep stos %al,%es:(%rdi)
    1509: 58           pop    %rax
    150a: c3           ret

But I know you don't like it because it costs extra memory access.

-- Viro