phis with identical operands don't get eliminated

[charles-cooper]

Version Information

• vyper Version (output of vyper --version): d395dd0

What's your issue about?

phi instructions where both branches produce the same value don't get reduced.

the following vyper source code demonstrates the issue:

@external
def foo(x: uint256) -> uint256:
    y: uint256 = x + 1
    z: uint256 = 0
    if msg.sender == self:
        z = y
    else:
        z = y
    return z

the resulting output looks like this:

%alloca_2_18_1 could get reduced to %13, but it doesn't, leading to suboptimally branching and joining.