linux-kernel - Re: [RFC PATCH v2 1/4] tools/nolibc: x86-64: Use `rep movsb` for `memcpy()` and `memmove()`

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Message-ID: <CAOG64qNw+ZovcC4+5Sqsvi-m=wyoe47eb5K4KAdbO5hcp8ipAA@mail.gmail.com>
Date:   Sat, 2 Sep 2023 13:07:50 +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>,
        David Laight <David.Laight@...lab.com>,
        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 v2 1/4] tools/nolibc: x86-64: Use `rep movsb` for
 `memcpy()` and `memmove()`

On Sat, Sep 2, 2023 at 12:51 PM Ammar Faizi wrote:
> +__asm__ (
> +".section .text.nolibc_memmove\n"
> +".weak memmove\n"
> +"memmove:\n"
> +       "movq %rdx, %rcx\n"
> +       "movq %rdi, %rdx\n"
> +       "movq %rdi, %rax\n"
> +       "subq %rsi, %rdx\n"
> +       "cmpq %rcx, %rdx\n"
> +       "jnb  .Lforward_copy\n"
> +       "leaq -1(%rdi, %rcx, 1), %rdi\n"
> +       "leaq -1(%rsi, %rcx, 1), %rsi\n"
> +       "std\n"
> +       "rep movsb\n"
> +       "cld\n"
> +       "retq\n"
> +".Lforward_copy:\n"
> +       "rep movsb\n"
> +       "retq\n"
> +
> +".section .text.nolibc_memcpy\n"
> +".weak memcpy\n"
> +"memcpy:\n"
> +       "movq %rdi, %rax\n"
> +       "movq %rdx, %rcx\n"
> +       "rep movsb\n"
> +       "retq\n"
> +);

Btw, sir, this can be simplified more by merging the forward copy
path, only using two "rep movsb" for both memmove() and memcpy()
should be enough?
```
__asm__ (
".section .text.nolibc_memmove_memcpy\n"
".weak memmove\n"
".weak memcpy\n"
"memmove:\n"
        "movq %rdx, %rcx\n"
        "movq %rdi, %rdx\n"
        "movq %rdi, %rax\n"
        "subq %rsi, %rdx\n"
        "cmpq %rcx, %rdx\n"
        "jnb __nolibc_forward_copy\n"
        "leaq -1(%rdi, %rcx, 1), %rdi\n"
        "leaq -1(%rsi, %rcx, 1), %rsi\n"
        "std\n"
        "rep movsb\n"
        "cld\n"
        "retq\n"

"memcpy:\n"
        "movq %rdi, %rax\n"
        "movq %rdx, %rcx\n"
"__nolibc_forward_copy:\n"
        "rep movsb\n"
        "retq\n"
);
```
Thought?

-- Viro