Compares the cost of mulmod versus the cost of modexp

compare-mulmod-modexp.sol

pragma solidity ^0.5.0;

contract CompareMulExp
{
    // altBN curve order
    uint256 constant CURVE_ORDER = 0x30644e72e131a029b85045b68181585d2833e84879b9709143e1f593f0000001;

    /**
    * @dev Computes (base ^ exponent) % modulus over big numbers.
    */
    function modexp(uint256 base, uint256 exponent, uint256 modulus)
        internal view returns (uint256)
    {
        bool success;
        uint256[6] memory input;
        uint256[1] memory output;

        input[0] = 32;
        input[1] = 32;
        input[2] = 32;
        input[3] = base;
        input[4] = exponent;
        input[5] = modulus;

        assembly {
            success := staticcall(sub(gas, 2000), 5, input, 0xc0, output, 0x20)
        }
        
        require( success );
        
        return output[0];
    }

    function TestMulmod( uint256 round_count )
        public pure returns(uint256)
    {
        uint256 t = uint256(keccak256(abi.encodePacked(round_count)));
        uint256 j = t;

        for( uint256 i = 0; i < round_count; i++ )
        {
            t = mulmod(t, j, CURVE_ORDER);  // t^i
        }

        return t;
    }
    
    function TestModexp( uint256 round_count )
        public view returns(uint256)
    {
        uint256  t = uint256(keccak256(abi.encodePacked(round_count)));

        for( uint256 i = 0; i < round_count; i++ )
        {
            t = modexp(t, CURVE_ORDER - 2, CURVE_ORDER);  // 1/t
        }

        return t;
    }
}