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

[Alviro Iskandar Setiawan]

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