ipow.c

int64_t ipow(int64_t base, uint8_t exp) {
    static const uint8_t highest_bit_set[] = {
        0, 1, 2, 2, 3, 3, 3, 3,
        4, 4, 4, 4, 4, 4, 4, 4,
        5, 5, 5, 5, 5, 5, 5, 5,
        5, 5, 5, 5, 5, 5, 5, 5,
        6, 6, 6, 6, 6, 6, 6, 6,
        6, 6, 6, 6, 6, 6, 6, 6,
        6, 6, 6, 6, 6, 6, 6, 6,
        6, 6, 6, 6, 6, 6, 6, 255, // anything past 63 is a guaranteed overflow with base > 1
        255, 255, 255, 255, 255, 255, 255, 255,
        255, 255, 255, 255, 255, 255, 255, 255,
        255, 255, 255, 255, 255, 255, 255, 255,
        255, 255, 255, 255, 255, 255, 255, 255,
        255, 255, 255, 255, 255, 255, 255, 255,
        255, 255, 255, 255, 255, 255, 255, 255,
        255, 255, 255, 255, 255, 255, 255, 255,
        255, 255, 255, 255, 255, 255, 255, 255,
        255, 255, 255, 255, 255, 255, 255, 255,
        255, 255, 255, 255, 255, 255, 255, 255,
        255, 255, 255, 255, 255, 255, 255, 255,
        255, 255, 255, 255, 255, 255, 255, 255,
        255, 255, 255, 255, 255, 255, 255, 255,
        255, 255, 255, 255, 255, 255, 255, 255,
        255, 255, 255, 255, 255, 255, 255, 255,
        255, 255, 255, 255, 255, 255, 255, 255,
        255, 255, 255, 255, 255, 255, 255, 255,
        255, 255, 255, 255, 255, 255, 255, 255,
        255, 255, 255, 255, 255, 255, 255, 255,
        255, 255, 255, 255, 255, 255, 255, 255,
        255, 255, 255, 255, 255, 255, 255, 255,
        255, 255, 255, 255, 255, 255, 255, 255,
        255, 255, 255, 255, 255, 255, 255, 255,
        255, 255, 255, 255, 255, 255, 255, 255,
    };

    int64_t result = 1;

    switch (highest_bit_set[exp]) {
    case 255: // we use 255 as an overflow marker and return 0 on overflow/underflow
        if (base == 1) {
            return 1;
        }
        
        if (base == -1) {
            return 1 - 2 * (exp & 1);
        }
        
        return 0;
    case 6:
        if (exp & 1) result *= base;
        exp >>= 1;
        base *= base;
    case 5:
        if (exp & 1) result *= base;
        exp >>= 1;
        base *= base;
    case 4:
        if (exp & 1) result *= base;
        exp >>= 1;
        base *= base;
    case 3:
        if (exp & 1) result *= base;
        exp >>= 1;
        base *= base;
    case 2:
        if (exp & 1) result *= base;
        exp >>= 1;
        base *= base;
    case 1:
        if (exp & 1) result *= base;
    default:
        return result;
    }
}

the lookup table for highest bit set can be replaced with __builtin_clz if it is available (gcc, clang). It will make it faster (especially if the lookup table is not in cache, which is likely in a real-world application doing other things than that) and smaller.

Simply replace highest_bit_set[exp] with (exp>0) ? 32 - __builtin_clz(exp) : 0