Re: [RFC PATCH v1 3/5] tools/nolibc: x86-64: Use `rep cmpsb` for `memcmp()`

[Willy Tarreau]

On Wed, Aug 30, 2023 at 08:57:24PM +0700, Ammar Faizi wrote:
> Simplify memcmp() on the x86-64 arch.
> 
> The x86-64 arch has a 'rep cmpsb' instruction, which can be used to
> implement the memcmp() function.
> 
>     %rdi = source 1
>     %rsi = source 2
>     %rcx = length
> 
> Signed-off-by: Ammar Faizi <ammarfaizi2@xxxxxxxxxxx>
> ---
>  tools/include/nolibc/arch-x86_64.h | 19 +++++++++++++++++++
>  tools/include/nolibc/string.h      |  2 ++
>  2 files changed, 21 insertions(+)
> 
> diff --git a/tools/include/nolibc/arch-x86_64.h b/tools/include/nolibc/arch-x86_64.h
> index 42f2674ad1ecdd64..6c1b54ba9f774e7b 100644
> --- a/tools/include/nolibc/arch-x86_64.h
> +++ b/tools/include/nolibc/arch-x86_64.h
> @@ -214,4 +214,23 @@ __asm__ (
>  	"retq\n"
>  );
>  
> +#define NOLIBC_ARCH_HAS_MEMCMP
> +static int memcmp(const void *s1, const void *s2, size_t n)
> +{
> +	const unsigned char *p1 = s1;
> +	const unsigned char *p2 = s2;
> +
> +	if (!n)
> +		return 0;
> +
> +	__asm__ volatile (
> +		"rep cmpsb"
> +		: "+D"(p2), "+S"(p1), "+c"(n)
> +		: "m"(*(const unsigned char (*)[n])s1),
> +		  "m"(*(const unsigned char (*)[n])s2)
> +	);
> +
> +	return p1[-1] - p2[-1];
> +}

Out of curiosity, given that you implemented the 3 other ones directly
in an asm statement, is there a particular reason this one mixes a bit
of C and asm ? It would probably be something around this, in the same
vein:

  memcmp:
    xchg  %esi,%eax   // source1
    mov   %rdx,%rcx   // count
    rep   cmpsb       // source2 in rdi; sets ZF on equal, CF if src1<src2
    seta  %al         // 0 if src2 <= src1, 1 if src2 > src1
    sbb   $0, %al     // 0 if src2 == src1, -1 if src2 < src1, 1 if src2 > src1
    movsx %al, %eax   // sign extend to %eax
    ret

Note that the output logic could have to be revisited, I'm not certain but
at first glance it looks valid.

Regards,
Willy