Re: [PATCH v7 2/5] x86/asm: add volatile, clobbers and zero-length check in inline memcmp

["H. Peter Anvin" <hpa@xxxxxxxxx>]

On 2026-07-24 16:57, H. Peter Anvin wrote:
> 
> And of course I got the comparison backwards (negative means a < b, thus we
> want a - b not b - a). Here is a fixed version.
> 
> int memcmp(const void *a, const void *b, size_t size)
> {
>         int diff;
> 
>         asm volatile("xor %0,%0 ;"      /* Sets ZF for the size = 0 case */
>                      "repe cmpsb ;"
>                      "jz 1f ;"          /* When size = 0 loading is unsafe */
>                      "movb -1(%1),%b0 ;"
>                      "movzbl -1(%2),%k2 ;"
>                      "sub %k2,%0 ;"
>                      "1:"
>                      : "=&q" (diff), "+D" (a), "+S" (b), "+c" (size)
>                      : : "memory");
>         return diff;
> }
> 
For extra credit, this version is even smaller in 16- and 32-bit mode, but
larger in 64-bit mode (because it depends on the order of the CMPSB operands,
which is the inverse of what the x86-64 ABI expects; swapping the order of "a"
and "b" and adding a cmc instruction improves the x86-64 size, but 64 bits is
not where the really tight code is...)

int memcmp(const void *a, const void *b, size_t size)
{
        int diff;

        asm volatile("xor %0,%0 ;"
                     "repe cmpsb ;"
                     "jz 1f ;"
                     "sbb %0,%0 ;"
                     "or $1,%0 ;"
                     "1:"
                     : "=&r" (diff), "+S" (a), "+D" (b), "+c" (size)
                     : : "memory");
        return diff;
}