move loop invariant variables out from loop

[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++) {
        }
    }

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[songlh]

@nventuro @frangio @Amxx Can you help take a look at this pull request?

[frangio]

it can save 765 units of gas when the length of arr is 100.

First of all, I can't reproduce this benchmark. I only see 5 gas difference in total.

Moreover, if it was 765 gas, it would mean less than 8 gas per iteration. This is marginal, we are not interested in such a small optimization.