Optimize zero-value memory variable declarations using CALLDATACOPY

[iamdefinitelyahuman]

Description

A common pattern I use is to declare multiple memory variables with zero-values at the beginning of a function. For example:

@external
def foo():
    a: int128 = 0
    b: uint256[3] = empty(uint256[3])
    c: address = ZERO_ADDRESS

These intializations produce the following LLL:

[mstore, 320, 0],
[calldatacopy, 352, calldatasize, 96],
[mstore, 448, 0],

Which could be optimized to:

[calldatacopy, 320, calldatasize, 160]

How to Implement

• After generating the LLL, look for sequential zero-value memory assignments or calldatacopy actions and merge them into a single calldatacopy.
• It might also be possible to mutate the AST by moving all the AnnAssign nodes to the top of the function body (if the right-hand-side is a zero value).