Demo.sol

pragma solidity ^0.4.23;

contract Calldata {
    
    // Alters the values on the stack for _a and _b, and returns values not located in calldata
    function ignoreCalldata(address _a, bytes memory _b) public view returns (address, bytes memory) {
        _a = address(this);
        _b = new bytes(5);
        _b[0] = 0xaa;
        _b[4] = 0xdd;
        return (_a, _b);
    }
    
    // Alters the values on the stack for _a and _b, but returns the calldata anyway
    function returnCalldata(address _a, bytes memory _b) public view returns (address, bytes memory) {
        _a = address(this);
        _b = new bytes(5);
        _b[0] = 0xaa;
        _b[4] = 0xdd;
        assembly {
            calldatacopy(0, 0, calldatasize) // directly copy from calldata
            return(0x04, sub(calldatasize, 0x04)) // return copied values
        }
    }
}

contract Returndata {
    
    Calldata internal cd_contract;
    
    constructor () public {
        cd_contract = new Calldata();
    }
    
    address internal constant addr = address(0xa);
    bytes internal constant bts = "Hello, world!";
    
    // Gets returned data from the Calldata contract and returns values not located in returndata
    function ignoreReturndata() public view returns (address a, bytes memory b) {
        // Get values from Calldata.ignoreCalldata -
        (a, b) = cd_contract.ignoreCalldata(msg.sender, 'test');
        // Alter values on stack -
        a = addr;
        b = bts;
        // Return altered values -
        return (a, b);
    }
    
    // Gets returned data from the Calldata contract and returns values not located in returndata
    function returnReturndata() public view returns (address a, bytes memory b) {
        // Get values from Calldata.ignoreCalldata -
        (a, b) = cd_contract.ignoreCalldata(msg.sender, 'test');
        // Alter values on stack -
        a = addr;
        b = bts;
        // Return values returned from Calldata, instead of the altered a and b -
        assembly {
            returndatacopy(0, 0, returndatasize)
            return(0, returndatasize)
        }
    }
}