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 14:53, H. Peter Anvin wrote:
> 
> It's pretty bloaty; I don't know exactly how much the rest of the code bloats.
> 
> I would be fine with having mem[n]eq() though as an explicit API. For the 
> out-of-line version used for compiler-generated memcmp() calls it doesn't 
> matter. 
> 
> Here is a very compact way the out-of-line version can be written (showing 
> the 64-bit version; formatting apologies, on a phone):
> 
> xor %eax,%eax    /* Sets ZF */
> repe cmpsb
> jr z,1f
> movzbl -1(%rdi),%edi
> mov -1(%rsi),%al
> sub %edi,%eax
> 1: ret
>
With a C wrapper to work in any mode:

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(%2),%b0 ;"
                     "movzbl -1(%1),%k1 ;"
                     "sub %k1,%0 ;"
                     "1:"
                     : "=&q" (diff), "+D" (a), "+S" (b), "+c" (size)
                     : : "memory");
        return diff;
}