Some generic writeup about common gas optimizations, etc.

generic.org

Upgrade to at least 0.8.4

Using newer compiler versions and the optimizer gives gas optimizations and additional safety checks for free!

The advantages of versions 0.8.* over <0.8.0 are:

• Safemath by default from 0.8.0 (can be more gas efficient than some library based safemath).
• Low level inliner from 0.8.2, leads to cheaper runtime gas. Especially relevant when the contract has small functions. For example, OpenZeppelin libraries typically have a lot of small helper functions and if they are not inlined, they cost an additional 20 to 40 gas because of 2 extra jump instructions and additional stack operations needed for function calls.
• Optimizer improvements in packed structs: Before 0.8.3, storing packed structs, in some cases used an additional storage read operation. After EIP-2929, if the slot was already cold, this means unnecessary stack operations and extra deploy time costs. However, if the slot was already warm, this means additional cost of 100 gas alongside the same unnecessary stack operations and extra deploy time costs.
• Custom errors from 0.8.4, leads to cheaper deploy time cost and run time cost. Note: the run time cost is only relevant when the revert condition is met. In short, replace revert strings by custom errors.

Caching the length in for loops

Consider a generic example of an array arr and the following loop:

for (uint i = 0; i < arr.length; i++) {
    // do something that doesn't change arr.length
}

In the above case, the solidity compiler will always read the length of the array during each iteration. That is,

1. if it is a storage array, this is an extra sload operation (100 additional extra gas (EIP-2929) for each iteration except for the first),
2. if it is a memory array, this is an extra mload operation (3 additional gas for each iteration except for the first),
3. if it is a calldata array, this is an extra calldataload operation (3 additional gas for each iteration except for the first)

This extra costs can be avoided by caching the array length (in stack):

uint length = arr.length;
for (uint i = 0; i < length; i++) {
    // do something that doesn't change arr.length
}

In the above example, the sload or mload or calldataload operation is only called once and subsequently replaced by a cheap dupN instruction. Even though mload, calldataload and dupN have the same gas cost, mload and calldataload needs an additional dupN to put the offset in the stack, i.e., an extra 3 gas.

This optimization is especially important if it is a storage array or if it is a lengthy for loop.

Note that the Yul based optimizer (not enabled by default; only relevant if you are using --experimental-via-ir or the equivalent in standard JSON) can sometimes do this caching automatically. However, this is likely not the case in your project. Reference. Also see this.

Use calldata instead of memory for function parameters

In some cases, having function arguments in calldata instead of memory is more optimal.

Consider the following generic example:

contract C {
    function add(uint[] memory arr) external returns (uint sum) {
        uint length = arr.length;
        for (uint i = 0; i < arr.length; i++) {
            sum += arr[i];
        }
    }
}

In the above example, the dynamic array arr has the storage location memory. When the function gets called externally, the array values are kept in calldata and copied to memory during ABI decoding (using the opcode calldataload and mstore). And during the for loop, arr[i] accesses the value in memory using a mload. However, for the above example this is inefficient. Consider the following snippet instead:

contract C {
    function add(uint[] calldata arr) external returns (uint sum) {
        uint length = arr.length;
        for (uint i = 0; i < arr.length; i++) {
            sum += arr[i];
        }
    }
}

In the above snippet, instead of going via memory, the value is directly read from calldata using calldataload. That is, there are no intermediate memory operations that carries this value.

Gas savings: In the former example, the ABI decoding begins with copying value from calldata to memory in a for loop. Each iteration would cost at least 60 gas. In the latter example, this can be completely avoided. This will also reduce the number of instructions and therefore reduces the deploy time cost of the contract.

In short, use calldata instead of memory if the function argument is only read.

Note that in older Solidity versions, changing some function arguments from memory to calldata may cause “unimplemented feature error”. This can be avoided by using a newer (0.8.*) Solidity compiler.

State variables that can be set to immutable

Solidity 0.6.5 introduced immutable as a major feature. It allows setting contract-level variables at construction time which gets stored in code rather than storage.

Consider the following generic example:

contract C {
    /// The owner is set during contruction time, and never changed afterwards.
    address public owner = msg.sender;
}

In the above example, each call to the function owner() reads from storage, using a sload. After EIP-2929, this costs 2100 gas cold or 100 gas warm. However, the following snippet is more gas efficient:

contract C {
    /// The owner is set during contruction time, and never changed afterwards.
    address public immutable owner = msg.sender;
}

In the above example, each storage read of the owner state variable is replaced by the instruction push32 value, where value is set during contract construction time. Unlike the last example, this costs only 3 gas.

Consider having short revert strings

Consider the following require statement:

// condition is boolean
// str is a string
require(condition, str)

The string str is split into 32-byte sized chunks and then stored in memory using mstore, then the memory offsets are provided to revert(offset, length). For chunks shorter than 32 bytes, and for low --optimize-runs value (usually even the default value of 200), instead of push32 val, where val is the 32 byte hexadecimal representation of the string with 0 padding on the least significant bits, the solidity compiler replaces it by shl(value, short-value)). Where short-value does not have any 0 padding. This saves the total bytes in the deploy code and therefore saves deploy time cost, at the expense of extra 6 gas during runtime. This means that shorter revert strings saves deploy time costs of the contract. Note that this kind of saving is not relevant for high values of --optimize-runs as push32 value will not be replaced by a shl(..., ...) equivalent by the Solidity compiler.

Going back, each 32 byte chunk of the string requires an extra mstore. That is, additional cost for mstore, memory expansion costs, as well as stack operations. Note that, this runtime cost is only relevant when the revert condition is met.

Overall, shorter revert strings can save deploy time as well as runtime costs.

Note that if your contracts already allow using at least Solidity 0.8.4, then consider using Custom errors. This is more gas efficient, while allowing the developer to describe the errors in detail using NatSpec. A disadvantage to this approach is that, some tooling may not have proper support for this.

The increment in for loop post condition can be made unchecked

(This is only relevant if you are using the default solidity checked arithmetic.)

Consider the following generic for loop:

for (uint i = 0; i < length; i++) {
    // do something that doesn't change the value of i
}

In this example, the for loop post condition, i.e., i++ involves checked arithmetic, which is not required. This is because the value of i is always strictly less than length <= 2**256 - 1. Therefore, the theoretical maximum value of i to enter the for-loop body is 2**256 - 2. This means that the i++ in the for loop can never overflow. Regardless, the overflow checks are performed by the compiler.

Unfortunately, the Solidity optimizer is not smart enough to detect this and remove the checks. One can manually do this by:

for (uint i = 0; i < length; i = unchecked_inc(i)) {
    // do something that doesn't change the value of i
}

function unchecked_inc(uint i) returns (uint) {
    unchecked {
        return i + 1;
    }
}

Note that it’s important that the call to unchecked_inc is inlined. This is only possible for solidity versions starting from 0.8.2.

Gas savings: roughly speaking this can save 30-40 gas per loop iteration. For lengthy loops, this can be significant!

Consider using custom errors instead of revert strings

Solidity 0.8.4 introduced custom errors. They are more gas efficient than revert strings, when it comes to deploy cost as well as runtime cost when the revert condition is met. Use custom errors instead of revert strings for gas savings.