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

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Date:   Wed, 30 Aug 2023 17:23:22 +0200
From:   Willy Tarreau <w@....eu>
To:     Ammar Faizi <ammarfaizi2@...weeb.org>
Cc:     Alviro Iskandar Setiawan <alviro.iskandar@...weeb.org>,
        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 10:09:51PM +0700, Ammar Faizi wrote:
> 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".

Then "xchg %esi, %eax" is just one byte with no memory access ;-)

Willy