Gas cost research on retrieving constants

constantGasChecker.sol

pragma solidity ^0.6.8;

/// Trying to answer the question: Is it more expensive to check variable from a constant or from a memory variable?
/// The answer is that they both have near 0 gas costs (191 vs 213 gas) but checking from constant gives a lower value


contract externalContract {
    
    uint256 storedConstant;
    
    function getConstant() public returns (uint256) {
        return 10**27;
    }
    
    // 20k gas
    function storeConstant(uint256 _storedConstant) public {
        storedConstant = _storedConstant;
    }
}


contract constantGasChecker {
    
    uint256 immutable private UINT_IMMUTABLE; 
    
    uint192 immutable private UINT_PRECISE; 
    
    uint256 private numVariable;
    
    constructor(uint256 _numPrecision, uint256 _numVariable, uint192 _uintPrecise) public {
        UINT_IMMUTABLE = _numPrecision;
        numVariable = _numVariable;
        UINT_PRECISE = _uintPrecise;
    }
    
    // Returns 257 gas 
    function checkWithConstant() public returns (uint256) {
        return UINT_IMMUTABLE;
    }
    
    // Returns 197 gas 
    function checkWithConstantPrecise() public returns (uint192) {
        return UINT_PRECISE;
    }
    
    // Returns 235 gas 
    function checkWithRunningVariable() public returns (uint256) {
        return 10**27;
    }
    
    // Returns 1013 gas that comes mostly from an SLOAD (800gas)
    function checkWithStoredVariable() public returns (uint256) {
        return numVariable;
    }
    
    // 20k gas
    function storeConstant(uint256 _storedConstant) public {
        storedConstant = _storedConstant;
    }
}
    
    // 2194 gas
    function getConstantFromExternalContract(address _externalContractAddress) public returns (uint256) {
        return externalContract(_externalContractAddress).getConstant();
    }

}