OpenSea Seaport contest - sirhashalot's results

1. Unnecessary explicit initialization of variables with default values

If a variable is not set/initialized, it has the default value (0 for uint, false for bool, etc.). Explicitly initializing a variable with its default value is an anti-pattern and wastes gas.

The OrderCombiner.sol contract has more than ten explicit initializations of default values. Other contract have more examples. Examples include:

• line 181
• line 247
• line 291
• line 373
• line 470
• line 473

To fix this, remove explicit initializations for default values.

2. Use unchecked for increment

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;
    }
}

Above code and explanation from hrkrshnn.

The OrdrCombiner.sol contract has more than 5 loops that can be improved this way. Examples include:

• line 181
• line 247
• line 291
• line 373
• line 473
• line 498
• line 577

To fix, use unchecked increment in loops.

3. iszero assembly should replace == 0

The iszero opcode is useful when comparing variables to zero. t11s suggested overusing this improvement on twitter.

The Assertions.sol and OrderCombiner.sol contracts have more than 5 zero comparisons that can be improved this way. Examples include:

• line 93
• line 113
• line 114
• line 115
• line 186
• line 217
• line 537
• line 582

To fix, replace == 0 with iszero.

4. Use vyper instead of solidity

Vyper contracts normally use less gas than their solidity counterparts. The possible savings was demonstrated for Seaport by doggo and a more general example was shown here. Seaport is written in solidity and yul, not vyper. Switching some or all of the contracts to vyper can result in gas savings.

To fix, replace solidity contracts with vyper code.