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

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

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;
}