Use "!=0" instead of ">0" for the 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.
Solidity 0.8.13 has some improvements too but not well tested.
Code Generator: Skip existence check for external contract if return data is expected. In this case, the ABI decoder will revert if the contract does not exist
All Contracts
Tools Used
None
Recommended Mitigation Steps
Consider to upgrade pragma to at least 0.8.10.
C4-006 : ++i is more gas efficient than i++ in loops forwarding
Impact
++i is more gas efficient than i++ in loops forwarding.
It is recommend to use unchecked{++i} and change i declaration to uint256.
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
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.
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.
Some parameters in examples given above are later hashed. It may be beneficial for those parameters to be in memory rather than calldata.
C4-012 : Delete - ABI Coder V2 For Gas Optimization
Impact
From Pragma 0.8.0, ABI coder v2 is activated by default. With upgrading The pragma, abicoder v2 can be deleted from the repository. That will provide gas optimization.
ISSUE LIST
C4-001 : > 0 can be replaced with != 0 for gas 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 : Free gas savings for using solidity 0.8.10+
C4-006 : ++i is more gas efficient than i++ in loops forwarding
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
C4-012 : Delete - ABI Coder V2 For Gas Optimization
C4-001
> 0 can be replaced with != 0 for gas optimizationImpact
!= 0is a cheaper operation compared to> 0, when dealing with uint.Proof of Concept
Tools Used
Code Review
Recommended Mitigation Steps
Use "!=0" instead of ">0" for the 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 : Free gas savings for using solidity 0.8.10+
Impact
Using newer compiler versions and the optimizer gives gas optimizations and additional safety checks are available for free.
Proof of Concept
Solidity 0.8.10 has a useful change which reduced gas costs of external calls which expect a return value: https://blog.soliditylang.org/2021/11/09/solidity-0.8.10-release-announcement/
Solidity 0.8.13 has some improvements too but not well tested.
Code Generator: Skip existence check for external contract if return data is expected. In this case, the ABI decoder will revert if the contract does not exist
All Contracts
Tools Used
None
Recommended Mitigation Steps
Consider to upgrade pragma to at least 0.8.10.
C4-006 : ++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.
C4-007 : 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
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.
C4-012 : Delete - ABI Coder V2 For Gas Optimization
Impact
From Pragma 0.8.0, ABI coder v2 is activated by default. With upgrading The pragma, abicoder v2 can be deleted from the repository. That will provide gas optimization.
Proof of Concept
""" https://github.com/code-423n4/2022-05-sturdy/blob/main/smart-contracts/ConvexCurveLPVault.sol#L3
"""
Tools Used
None
Recommended Mitigation Steps
ABI coder v2 is activated by default. It is recommended to delete redundant codes.
From Solidity v0.8.0 Breaking Changes https://docs.soliditylang.org/en/v0.8.0/080-breaking-changes.html