Cache the length of actions when looping over them

[cleanunicorn]

Description

When the total reported amount of assets is estimated by the actions, the storage variable actions.length is used. This value does not change over time (in this loop) and can be cached in a local variable, instead of retrieving the value from the storage.

https://github.com/monoceros-alpha/review-opyn-perp-vault-templates-2021-07/blob/518e4f6d174cae6ee75e316ad56789aaeb695069/code/contracts/core/OpynPerpVault.sol#L393-L395

Similarly for _closeAndWithdraw():

https://github.com/monoceros-alpha/review-opyn-perp-vault-templates-2021-07/blob/3d44603300dd9abffe5a1c1e1c2647e9f6b80c7b/code/contracts/core/OpynPerpVault.sol#L420

Recommendation

Cache the length locally and use the local variable in the loop.

Reference

An example was created to illustrate the gas difference with or without length cache.

Not using a cache, for a set of 10 iterations uses 49157 gas.

contract SumNumbers {
    uint[] public numbers;
    
    constructor(uint size) {
        // Add some data to work with
        for (uint i = 0; i < size; i++) {
            numbers.push(i);
        }
    }
    
    function sumNumbers() public view returns (uint) {
        uint sum;
        for(uint i = 0; i < numbers.length; i++) {
            sum += numbers[i];
        }
        
        return sum;
    }
}

Using a cache, for the same set of 10 iterations uses 48168 gas.

contract SumNumbersWithCache {
    uint[] public numbers;
    
    constructor(uint size) {
        // Add some data to work with
        for (uint i = 0; i < size; i++) {
            numbers.push(i);
        }
    }
    
    function sumNumbers() public view returns (uint) {
        uint sum;
        uint size = numbers.length;
        
        for(uint i = 0; i < size; i++) {
            sum += numbers[i];
        }
        
        return sum;
    }
}

Both contracts were compiled with 200 optimization rounds.