Optimize gas cost in for loop.

[dbadoy]

AS-IS

function getWhitelistAllowanceLeft(address wallet) external view returns (uint256 allowance) {
  uint256 length = _whitelist.length; // gas saving
  for (uint256 i; i != length;) {
      if (_whitelist[i] == wallet) {
          allowance++;
      }
      unchecked {
          ++i;
      }
  }

  return allowance;
}

function getWhitelistIndex(address wallet) external view returns (uint256) {
  uint256 length = _whitelist.length; // gas saving
  for (uint256 i; i != length;) {
      if (_whitelist[i] == wallet) {
          return i;
      }
      unchecked {
          ++i;
      }
  }

  revert DixelClubV2__NotWhitelisted();
}

That clone _whitelist from storage to use in loop is expected the effect of reducing SLOAD calls.

First, this is the test result.

address[20] addrs = [ ... ]

function origin() external {
    uint256 dummy = 0;
    uint256 len = addrs.length;
    for (uint256 i; i < len;) {
        if (addrs[i] == address(0)) {
            dummy += i;
        }
        unchecked {
            i++;
        }
    }
}

function suggest1() external {
    uint256 dummy = 0;
    address[] memory temp = addrs;
    uint256 len = temp.length;
    for (uint256 i; i < len;) {
        if (temp[i] == address(0)) {
            dummy += i;
        }
        unchecked {
            i++;
        }
    }
}

function suggest2() external {
    uint256 dummy = 0;
    address[] storage temp = addrs;
    uint256 len = temp.length;
    for (uint256 i; i < len;) {
        if (temp[i] == address(0)) {
            dummy += i;
        }
        unchecked {
            i++;
        }
    }
}

origin() gasUsed : 57873
suggest1() gasUsed : 43636
suggest2() gasUsed : 57925

suggest1() what clone storage data to memory variable seems to be the best case.

---

So, i suggest this.

TO-BE

function getWhitelistAllowanceLeft(address wallet) external view returns (uint256 allowance) {
    // gas saving
    address[] memory clone = _whitelist;
    uint256 length = clone.length;
    for (uint256 i; i != length;) {
        if (clone[i] == wallet) {
            allowance++;
        }
        unchecked {
            ++i;
        }
    }

    return allowance;
}

function getWhitelistIndex(address wallet) external view returns (uint256) {
    // gas saving
    address[] memory clone = _whitelist;
    uint256 length = clone.length;
    for (uint256 i; i != length;) {
        if (clone[i] == wallet) {
            return i;
        }
        unchecked {
            ++i;
        }
    }

    revert DixelClubV2__NotWhitelisted();
}