move loop invariant variables out from loop to save gas

[molly-ting]

Moving loop invariant codes out from loop can save gas. In the following demo, it can save 765 units of gas when the length of arr is 100.

    function test1(uint256[] calldata arr) public {     
        for(uint256 i = 0; i < arr.length; i++) {
        }
    }

    function test2(uint256[] calldata arr) public {
        uint256 len = arr.length;
        for(uint256 i = 0; i < len; i++) {
        }
    }

PR Checklist

• [ ] Tests
• [ ] Documentation
• [ ] Changeset entry (run npx changeset add)

[songlh]

@nventuro can you take a look at this pull request? it looks good to me. The original functionality is preserved and some gases can be saved.

[songlh]

Can anyone take a look at this pull request?

[frangio]

https://github.com/OpenZeppelin/openzeppelin-contracts/pull/4182#issuecomment-1586310971