问C/C++中两种缓冲区的差异

EN

inline size_t ComputeDifferenceSmall(
    _In_reads_bytes_(size) char* buf1, 
    _In_reads_bytes_(size) char* buf2, 
    size_t size) {

    /* size should be <= 0x1000 bytes */
    /* In my case, I expect frequent differences if any at all are present. */
    size_t res = 0;

    for (size_t i = 0; i < (size & ~0xF); i += 0x10) {
        uint64_t diff1 = *reinterpret_cast<uint64_t*>(buf1) ^ 
            *reinterpret_cast<uint64_t*>(buf2);
        if (!diff1) continue;

        /* Bit fiddle to make each byte 1 if they're different and 0 if the same */
        diff1 = ((diff1 & 0xF0F0F0F0F0F0F0F0ULL) >> 4) | (diff1 & 0x0F0F0F0F0F0F0F0FULL);
        diff1 = ((diff1 & 0x0C0C0C0C0C0C0C0CULL) >> 2) | (diff1 & 0x0303030303030303ULL);
        diff1 = ((diff1 & 0x0202020202020202ULL) >> 1) | (diff1 & 0x0101010101010101ULL);

        /* Sum the bytes */
        diff1 = (diff1 >> 32) + (diff1 & 0xFFFFFFFFULL);
        diff1 = (diff1 >> 16) + (diff1 & 0xFFFFULL);
        diff1 = (diff1 >> 8) + (diff1 & 0xFFULL);
        diff1 = (diff1 >> 4) + (diff1 & 0xFULL);

        res += diff1;
    }

    for (size_t i = (size & ~0xF); i < size; i++) {
        res += (buf1[i] != buf2[i]);
    }

    return res;
}

size_t ComputeDifference(
    _In_reads_bytes_(size) char* buf1, 
    _In_reads_bytes_(size) char* buf2, 
    size_t size) {

    size_t res = 0;

    /* I expect most pages to be identical, and both buffers should be page aligned if 
     * larger than a page. memcmp has more optimizations than I'll ever come up with, 
     * so I can just use that to determine if I need to check for differences 
     * in the page. */
    for (size_t pn = 0; pn < (size & ~0xFFF); pn += 0x1000) {
        if (memcmp(&buf1[pn], &buf2[pn], 0x1000)) {
            res += ComputeDifferenceSmall(&buf1[pn], &buf2[pn], 0x1000);
        }
    }

    return res + ComputeDifferenceSmall(
        &buf1[size & ~0xFFF], &buf2[size & ~0xFFF], size & 0xFFF);
}