yeyo8711/gassavingtips

How to save gas in Solidity

Gas Saving Tips

How to save gas in Solidity

1. Function Names

• Function Selector = The first 4 bytes of the hashed keccak256 function name
• The compiler will sort them in hexidecimal order
• Solidity will check to see which function was included in transaction.data until it reaches the right one
• If you have a gas sensitive function, try to have its function selector at the top
• Example:

function setArray() external {}//gas: 144

function aSetArray() external {} //gas: 122

2. Less than VS. Less than or equal to

• There is no bytecode to compare if one integer is greater or equal to (or <=)
• Solidity will use an extra opcode "ISZERO" to invert the equation hence more gas
• Always use > or < instead of >= and <=
• Strict inequalities only require one opcode, LT or GT
• Example:

function compare() external pure returns(bool) { return 3 > 2; // 327 gas }

function compare() external pure returns(bool) { return 3 >= 2; // 330 gas }

3. Bit Shifting

• Shifting bits using the Bitwise Operators >> and << can help reduce gas but is very dangerous due to overflow

• Opcode for multiplication (MUL) cost 5 gas, whereas the opcode for shifting(SHL or SHR) cost 3 gas

• Example:

  function shiftLeft() external pure returns(uint256){ return 10 << 1; // gas used 21445 }

  function multiply() external pure returns(uint256){ return 10 * 2; // gas used 21379 }

4. Reverting Early

• When a transaction reverts, the gas is paid up until it runs out of gas or until it hits a revert (such as a require)
• Its best to revert as early as possible before accessing state variables or doing any computation
• Example:

function claim() external { require(msg.sender == owner); // rest of logic }

5. Short Circuiting

• Soldity also has short circuiting just like Javascript ( || or &&)
• Having the less gas costly statement on the left will be a good way to save gas because there is a chance the other statement gets ignored
• Example: require(block.timeStamp > 126548623 || whiteListed[msg.sender]) the left one cost less gas and if it returns true, the state variable "whiteListed" will not be read

6. Payable

• Although it is not recommended to make ALL your functions payable, it can save gas.
• In a non payable function, Solidity checks if ether was sent(opcode CALLVALUE) and reverts if that is true, this causes more gas being used

7. Unchecked

• Overflow happens when the sum of two uints returns a smaller value
• Underflow is just the opposite, subtracting a number from another number returns a higher value
• Solidity 8.0 now has an overflow/underflow checker implemented
• You can put your logic inside unchecked{} to avoid the extra opcodes and save gas but you must be very careful
• One ideal place to use unchecked is if you are using a counter and that counter increases by a small amount like 1
• Example: unchecked{ counter++ }