From: Ammar Faizi <[email protected]>
> Sent: 30 August 2023 14:57
>
> This is an RFC patchset for nolibc x86-64 string functions. There are 5
> patches in this series.
>
> ## Patch 1-3: Use `rep movsb`, `rep stosb`, and `rep cmpsb` for:
> - memcpy() and memmove()
> - memset()
> - memcmp()
> respectively. They can simplify the generated ASM code.
>
...
> After this series:
> ```
> 000000000000140a <memmove>:
> 140a: 48 89 f8 mov %rdi,%rax
> 140d: 48 89 d1 mov %rdx,%rcx
> 1410: 48 8d 7c 0f ff lea -0x1(%rdi,%rcx,1),%rdi
> 1415: 48 8d 74 0e ff lea -0x1(%rsi,%rcx,1),%rsi
> 141a: fd std
> 141b: f3 a4 rep movsb %ds:(%rsi),%es:(%rdi)
> 141d: fc cld
> 141e: c3 ret
Isn't that completely broken?
You need to select between forwards and backwards moves.
Since forwards moves are preferred it is best to do
if (dst - src < len)
backards_copy()
else
formwards_copy()
David
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