Re: [PATCH v9 2/5] x86/asm, x86/boot: expose inline memcmp()

[David Laight]

On Wed, 9 Sep 2026 06:43:01 -0700
"H. Peter Anvin" <hpa@xxxxxxxxx> wrote:

> On 2026-09-09 01:38, David Laight wrote:
> >>
> >> Here is an out-of-line compact memcmp() which works for both 16/32 and 64 bits:
> >>  
> <broken code removed>
> 
> >>
> >> On 64 bits it compiles to:
> >>
> >> 0000000000000000 <memcmp>:
> >>    0:   48 89 d1                mov    %rdx,%rcx
> >>    3:   31 d2                   xor    %edx,%edx
> >>    5:   31 c0                   xor    %eax,%eax
> >>    7:   f3 a6                   repz cmpsb (%rdi),(%rsi)
> >>    9:   0f 97 c2                seta   %dl
> >>    c:   0f 92 c0                setb   %al
> >>    f:   29 d0                   sub    %edx,%eax  
> > 
> > That isn't the object code from the source ...
> >   
> And that's the ultimate hint that a cut and paste error had happened.
> 
> This was the actual source code.
> 
> int memcmp(const void *s1, const void *s2, size_t len)
> {
>     int lt, gt;
> 
>     /*
>      * Note: for the benefit of 64-bit code, xDI and xSI are reversed
>      * compared with what CMPSB uses; hence SETA and SETB are also reversed.
>      *
>      * The XOR statements set ZF = 1, CF = 0, which is required to handle
>      * the case len == 0 correctly.
>      */
>     asm volatile("xor %[lt],%[lt] ; "
>                  "xor %[gt],%[gt] ; "
>                  "repe cmpsb ; "
>                  "seta %b[lt] ; "
>                  "setb %b[gt]"
>                  : "+D" (s1), "+S" (s2), "+c" (len),
>                    [lt] "=&q" (lt), [gt] "=&q" (gt)
>                  : : "cc", "memory");
>     return gt - lt;
> }
> 

Try:

int memcmp_2(const void *s1, const void *s2, unsigned long len)
{
    signed char lt, gt;

    asm volatile("repe cmpsb ; "
                 "seta %[lt] ; "
                 "setb %[gt]"
                 : "+D" (s1), "+S" (s2), "+c" (len),
                   [lt] "=&q" (lt), [gt] "=&q" (gt)
                 : : "cc", "memory");
    return (signed char)(gt - lt);
}

https://www.godbolt.org/z/6hrxGxb18

Saves the XORs - go away completely in the usual case of 'if (memcpy(....))'.
The 'mess' on the return statement moves the sign extend after the
subtract.

David