Subtraction

// SPDX-License-Identifier: GPL-3.0
pragma solidity ^0.8.0;

contract Yul {
    function subtract(uint256 x, uint256 y) public pure returns (bytes32) {
        assembly {
            mstore(0x80, sub(x, y))
            return(0x80, 0x20)
        }
    }
}

The sub(a, b) Yul function takes in two numbers as arguments and returns the difference between the two numbers, a - b.

🚨 Subtractions in Yul are unchecked, meaning that, Yul will also wrap around numbers if the result of the subtraction goes lower than the 0 when subtracting, that is if b > a.

// SPDX-License-Identifier: GPL-3.0
pragma solidity ^0.8.0;

contract Yul {
    function subtract() public pure returns (bytes32) {
        assembly {
            let small := 0x00
            mstore(0x80, sub(small, 0x02))
            return(0x80, 0x20)
        }
    }
}

The above function will return 0xfffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffe.

To write a safe Yul subtraction code, this is the best approach [6].

// SPDX-License-Identifier: GPL-3.0
pragma solidity ^0.8.0;

contract Yul {
    function subtract(uint256 x, uint256 y) public pure returns (bytes32) {
        assembly {
            if gt(y, x) {
                revert(0x00, 0x00)
            }
            
            mstore(0x80, sub(x, y))
            return(0x80, 0x20)
        }
    }
}

The gt(a, b) Yul function takes in two numbers as arguments and returns 1 meaning true if a > b and 0meaning false if a < b.