move loop invariant codes 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 265 units of gas when the length of arr is 100.

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

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

Initializing variables before the loop instead of inside the loop can also save gas. In the example below, function test4 consumes 553 units less gas than function test3 when the length of arr is 100.

    function test3(uint256[] memory arr) public {
        for(uint256 i = 0; i < arr.length; i++) {
            uint256 a = arr[i];
        }
    }

    function test4(uint256[] memory arr) public {
        uint256 a;
        for(uint256 i = 0; i < arr.length; i++) {
            a = arr[i];
        }
    }

[stale[bot]]

Is this still relevant? If so, what is blocking it? Is there anything you can do to help move it forward?

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