Signed integer extension with number of bytes

signext.c

int64_t signext(uint64_t val, uint64_t bytes) {
    uint64_t bits = 8 * bytes;
    uint64_t bitmask = (1 << bits) - 1;
    uint64_t value = bitmask & val;
    uint64_t mask = 1 << (bits-1);
    if (mask & val) {
        value |= ((uint64_t)-1) ^ bitmask;
    }
    return value;
}