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Message-ID: <ZO9bv+GvgpphtGEi@biznet-home.integral.gnuweeb.org>
Date: Wed, 30 Aug 2023 22:09:51 +0700
From: Ammar Faizi <ammarfaizi2@...weeb.org>
To: Alviro Iskandar Setiawan <alviro.iskandar@...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 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
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