Gas optimization of calldata input variables

Question

I have some external functions what receives address lists as calldata to save gas. I know that creating a copy from global variables inside functions can be useful to read less from blockchain variables, however I am not sure if doing the same with read-only calldata types will lead to less gas spending.

Is it make sense to prefer one of the below functions in order to save gas?

function testFunctionA (address[] calldata addressList) external {
    address[] memory path;
    path[0] = addressList[0];
    path[1] = addressList[0];

}

In this case I assume it doesn't make sense until I don't use a1 or a2 more than one time within the same function like in testFunctionC. Is it right? Or is it a better idea to copy list objects from calldata lists into local variables inside the function regardless how many times will I use them?

function testFunctionB (address[] calldata addressList) external {
    address a1 = addressList[0];
    address a2 = addressList[1];
    address[] memory path;
    path[0] = a1;
    path[1] = a2;

}

function testFunctionB (address[] calldata addressList) external {
    address a1 = addressList[0];
    address a2 = addressList[1];
    address[] memory path;
    path[0] = a1;
    path[1] = a2;
    path[2] = a1;
    path[3] = a2;

}

Answer 1

After reading about the topic the consequences are the following for me:

• Array reads by index should be avoided when you use the variable more than 1 time. It is cheaper to make a copy and use that inside the function.

• Create fixed length arrays if it is possible in the given situation. Initializing empty, non-fixed arrays and extend the size by pushing new objects will be more expensive.