Function sz_rfind_byte_serial

include/stringzilla/find.h:417–448

Find the last occurrence of a @b single-character needle in an arbitrary length haystack. * This implementation uses hardware-agnostic SWAR technique, to process 8 characters at a time. * Identical to `memrchr(haystack, needle[0], haystack_length)`. */

 *  Identical to `memrchr(haystack, needle[0], haystack_length)`.
 */
sz_cptr_t sz_rfind_byte_serial(sz_cptr_t h_chars, sz_size_t h_length, sz_cptr_t n_chars) {

    if (!h_length) return SZ_NULL_CHAR;
    // Reinterpret as unsigned bytes so the SWAR broadcast below cannot sign-extend
    // on platforms where `char` is signed (e.g. `-fsigned-char`). See issue #306.
    sz_u8_t const *const h_start = (sz_u8_t const *)h_chars;
    sz_u8_t const *const n = (sz_u8_t const *)n_chars;

    // Reposition the `h` pointer to the end, as we will be walking backwards.
    sz_u8_t const *h = h_start + h_length - 1;

#if !SZ_IS_BIG_ENDIAN_       // Use SWAR only on little-endian platforms for brevity.
#if !SZ_USE_MISALIGNED_LOADS // Process the misaligned head, to void UB on unaligned 64-bit loads.
    for (; ((sz_size_t)(h + 1) & 7ull) && h >= h_start; --h)
        if (*h == *n) return (sz_cptr_t)h;
#endif

    // Broadcast the n into every byte of a 64-bit integer to use SWAR
    // techniques and process eight characters at a time.
    sz_u64_vec_t h_vec, n_vec, match_vec;
    n_vec.u64 = (sz_u64_t)*n * 0x0101010101010101ull;
    for (; h >= h_start + 7; h -= 8) {
        h_vec.u64 = *(sz_u64_t const *)(h - 7);
        match_vec = sz_u64_each_byte_equal_(h_vec, n_vec);
        if (match_vec.u64) return (sz_cptr_t)(h - sz_u64_clz(match_vec.u64) / 8);
    }
#endif

    for (; h >= h_start; --h)
        if (*h == *n) return (sz_cptr_t)h;
    return SZ_NULL_CHAR;
}

/**
 *  @brief  2Byte-level equality comparison between two 64-bit integers.