From: Ammar Faizi <[email protected]>
To: Alviro Iskandar Setiawan <[email protected]>
Cc: "Willy Tarreau" <[email protected]>,
"Thomas Weißschuh" <[email protected]>,
"Nicholas Rosenberg" <[email protected]>,
"Michael William Jonathan" <[email protected]>,
"GNU/Weeb Mailing List" <[email protected]>,
"Linux Kernel Mailing List" <[email protected]>
Subject: Re: [RFC PATCH v1 2/5] tools/nolibc: x86-64: Use `rep stosb` for `memset()`
Date: Wed, 30 Aug 2023 22:09:51 +0700 [thread overview]
Message-ID: <[email protected]> (raw)
In-Reply-To: <CAOG64qNGdPjXHTvrpYxjizOYvMRaB9_2t2raw6DJ3kK852KUHg@mail.gmail.com>
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
next prev parent reply other threads:[~2023-08-30 15:09 UTC|newest]
Thread overview: 29+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-08-30 13:57 [RFC PATCH v1 0/5] nolibc x86-64 string functions Ammar Faizi
2023-08-30 13:57 ` [RFC PATCH v1 1/5] tools/nolibc: x86-64: Use `rep movsb` for `memcpy()` and `memmove()` Ammar Faizi
2023-08-30 13:57 ` [RFC PATCH v1 2/5] tools/nolibc: x86-64: Use `rep stosb` for `memset()` Ammar Faizi
2023-08-30 14:08 ` Alviro Iskandar Setiawan
2023-08-30 14:13 ` Ammar Faizi
2023-08-30 14:24 ` Alviro Iskandar Setiawan
2023-08-30 15:09 ` Ammar Faizi [this message]
2023-08-30 15:23 ` Willy Tarreau
2023-08-30 15:44 ` Ammar Faizi
2023-08-30 15:51 ` Willy Tarreau
2023-08-30 16:08 ` Ammar Faizi
2023-08-30 16:11 ` Alviro Iskandar Setiawan
2023-08-30 13:57 ` [RFC PATCH v1 3/5] tools/nolibc: x86-64: Use `rep cmpsb` for `memcmp()` Ammar Faizi
2023-08-30 21:26 ` Willy Tarreau
2023-09-01 3:24 ` Ammar Faizi
2023-09-01 3:35 ` Willy Tarreau
2023-09-01 7:27 ` Ammar Faizi
2023-09-04 8:26 ` David Laight
2023-08-30 13:57 ` [RFC PATCH v1 4/5] tools/nolibc: string: Remove the `_nolibc_memcpy_down()` function Ammar Faizi
2023-08-30 21:27 ` Willy Tarreau
2023-08-30 13:57 ` [RFC PATCH v1 5/5] tools/nolibc: string: Remove the `_nolibc_memcpy_up()` function Ammar Faizi
2023-08-30 21:32 ` [RFC PATCH v1 0/5] nolibc x86-64 string functions Willy Tarreau
2023-09-01 11:34 ` David Laight
2023-09-01 11:46 ` Willy Tarreau
2023-09-01 13:06 ` Ammar Faizi
2023-09-01 14:23 ` David Laight
2023-09-01 14:41 ` Ammar Faizi
2023-09-01 14:54 ` David Laight
2023-09-01 15:20 ` Ammar Faizi
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