signed_promote.c

static void signed_promote(void const * in, size_t in_size, void * out, size_t out_size) {
    assert(in_size <= out_size);
    assert(in_size == 1 || in_size == 2 || in_size == 4 || in_size == 8);
    assert(out_size == 1 || out_size == 2 || out_size == 4 || out_size == 8);

    uint64_t word_in = 0;

    memcpy(&word_in, in, in_size);

    if (word_in & (1 << ((in_size * 8) - 1))) {
        memset(out, 0xFF, out_size);
    }

    memcpy(out, in, in_size);
}