Shorten the revert strings to fit in 32 bytes. That will affect gas optimization.
C4-002 : Adding unchecked directive can save gas
Impact
For the arithmetic operations that will never over/underflow, using the unchecked directive (Solidity v0.8 has default overflow/underflow checks) can save some gas from the unnecessary internal over/underflow checks.
Consider applying unchecked arithmetic where overflow/underflow is not possible. Example can be seen from below.
Unchecked{i++};
C4-003 : Check if amount > 0 before token transfer can save gas
Impact
Since _amount can be 0. Checking if (_amount != 0) before the transfer can potentially save an external call and the unnecessary gas cost of a 0 token transfer.
C4-007 : Use Custom Errors instead of Revert Strings to save Gas
Custom errors from Solidity 0.8.4 are cheaper than revert strings (cheaper deployment cost and runtime cost when the revert condition is met)
Source Custom Errors in Solidity:
Starting from Solidity v0.8.4, there is a convenient and gas-efficient way to explain to users why an operation failed through the use of custom errors. Until now, you could already use strings to give more information about failures (e.g., revert("Insufficient funds.");), but they are rather expensive, especially when it comes to deploy cost, and it is difficult to use dynamic information in them.
Custom errors are defined using the error statement, which can be used inside and outside of contracts (including interfaces and libraries).
Instances include:
All require Statements
Tools Used
Code Review
Recommended Mitigation Steps
Recommended to replace revert strings with custom errors.
C4-008 : Use Shift Right/Left instead of Division/Multiplication if possible
Impact
A division/multiplication by any number x being a power of 2 can be calculated by shifting log2(x) to the right/left.
While the DIV opcode uses 5 gas, the SHR opcode only uses 3 gas. Furthermore, Solidity's division operation also includes a division-by-0 prevention which is bypassed using shifting.
C4-010 : State Variables that can be changed to immutable
Impact
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.
C4-011 : Use calldata instead of memory for function parameters
Impact
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.
C4-001: Revert String Size Optimization
C4-002 : Adding unchecked directive can save gas
C4-003 : Check if amount > 0 before token transfer can save gas
C4-004 : There is no need to assign default values to variables
C4-005 : ++i is more gas efficient than i++ in loops forwarding
C4-006 : Non-strict inequalities are cheaper than strict ones
C4-007 : Use Custom Errors instead of Revert Strings to save Gas
C4-008 : Use Shift Right/Left instead of Division/Multiplication if possible
C4-009 : Cache array length in for loops can save gas
C4-010 : State Variables that can be changed to immutable
C4-011 : Use calldata instead of memory for function parameters
C4-001: Revert String Size Optimization
Impact
Shortening revert strings to fit in 32 bytes will decrease deploy time gas and will decrease runtime gas when the revert condition has been met.
Revert strings that are longer than 32 bytes require at least one additional mstore, along with additional overhead for computing memory offset, etc.
Proof of Concept
Revert strings > 32 bytes are here:
Tools Used
Manual Review
Recommended Mitigation Steps
Shorten the revert strings to fit in 32 bytes. That will affect gas optimization.
C4-002 : Adding unchecked directive can save gas
Impact
For the arithmetic operations that will never over/underflow, using the unchecked directive (Solidity v0.8 has default overflow/underflow checks) can save some gas from the unnecessary internal over/underflow checks.
Proof of Concept
Tools Used
None
Recommended Mitigation Steps
Consider applying unchecked arithmetic where overflow/underflow is not possible. Example can be seen from below.
C4-003 : Check if amount > 0 before token transfer can save gas
Impact
Since _amount can be 0. Checking if (_amount != 0) before the transfer can potentially save an external call and the unnecessary gas cost of a 0 token transfer.
Proof of Concept
All Contracts
Tools Used
None
Recommended Mitigation Steps
Consider checking amount != 0.
C4-004 : There is no need to assign default values to variables
Impact - Gas Optimization
When a variable is declared solidity assigns the default value. In case the contract assigns the value again, it costs extra gas.
Example: uint x = 0 costs more gas than uint x without having any different functionality.
Proof of Concept
Tools Used
Code Review
Recommended Mitigation Steps
uint x = 0 costs more gas than uint x without having any different functionality.
C4-005 : ++i is more gas efficient than i++ in loops forwarding
Impact
++i is more gas efficient than i++ in loops forwarding.
Proof of Concept
Tools Used
Code Review
Recommended Mitigation Steps
It is recommend to use unchecked{++i} and change i declaration to uint256.
C4-006 : Non-strict inequalities are cheaper than strict ones
Impact
Strict inequalities add a check of non equality which costs around 3 gas.
Proof of Concept
Tools Used
Code Review
Recommended Mitigation Steps
Use >= or <= instead of > and < when possible.
C4-007 : Use Custom Errors instead of Revert Strings to save Gas
Custom errors from Solidity 0.8.4 are cheaper than revert strings (cheaper deployment cost and runtime cost when the revert condition is met)
Source Custom Errors in Solidity:
Starting from Solidity v0.8.4, there is a convenient and gas-efficient way to explain to users why an operation failed through the use of custom errors. Until now, you could already use strings to give more information about failures (e.g., revert("Insufficient funds.");), but they are rather expensive, especially when it comes to deploy cost, and it is difficult to use dynamic information in them.
Custom errors are defined using the error statement, which can be used inside and outside of contracts (including interfaces and libraries).
Instances include:
All require Statements
Tools Used
Code Review
Recommended Mitigation Steps
Recommended to replace revert strings with custom errors.
C4-008 : Use Shift Right/Left instead of Division/Multiplication if possible
Impact
A division/multiplication by any number x being a power of 2 can be calculated by shifting log2(x) to the right/left.
While the DIV opcode uses 5 gas, the SHR opcode only uses 3 gas. Furthermore, Solidity's division operation also includes a division-by-0 prevention which is bypassed using shifting.
Proof of Concept
Contracts - https://github.com/code-423n4/2022-05-cally/blob/main/contracts/src/CallyNft.sol#L245
Tools Used
None
Recommended Mitigation Steps
A division/multiplication by any number x being a power of 2 can be calculated by shifting log2(x) to the right/left.
C4-009 : Cache array length in for loops can save gas
Impact
Reading array length at each iteration of the loop takes 6 gas (3 for mload and 3 to place memory_offset) in the stack.
Caching the array length in the stack saves around 3 gas per iteration.
Proof of Concept
Tools Used
None
Recommended Mitigation Steps
Consider to cache array length.
C4-010 : State Variables that can be changed to immutable
Impact
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:
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:
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.
Proof of Concept
Tools Used
None
Recommended Mitigation Steps
Consider using immutable variable.
C4-011 : Use calldata instead of memory for function parameters
Impact
In some cases, having function arguments in calldata instead of memory is more optimal.
Consider the following generic example:
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:
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.
Proof of Concept
Tools Used
None
Recommended Mitigation Steps
Some parameters in examples given above are later hashed. It may be beneficial for those parameters to be in memory rather than calldata.