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

[Ammar Faizi <ammarfaizi2@gnuweeb.org>]

On Wed, Aug 30, 2023 at 09:24:45PM +0700, Alviro Iskandar Setiawan wrote:
> 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.

Yes, that's an extra memory access. But I believe it doesn't hurt
someone targetting -Os. In many cases, the compilers use push/pop to
align the stack before a 'call' instruction. If they want to avoid extra
memory access, they could have used "subq $8, %rsp" and "addq $8, %rsp".

For example: https://godbolt.org/z/Tzc1xWGEn

C code:
```
int fx(int b);
int fy(int a)
{
    return 1 + fx(a);
}
```

Targetting -Os, both clang and gcc compile it to:
```
fy:
    pushq   %rax
    call    fx
    popq    %rdx
    incl    %eax
    ret
```

Targetting -O2:
```
fy:
    subq    $8, %rsp
    call    fx
    addq    $8, %rsp
    addl    $1, %eax
    ret
```

That pushq/popq pair doesn't actually preserve anything; it's just to
align the %rsp at 16 bytes on 'call'. IOW, sometimes having extra memory
access to get a smaller code size is acceptable.

-- 
Ammar Faizi