Phi node ($\varphi$)

A phi node supplies the value at a join in static single-assignment form by selecting among values associated with incoming edges. In LLVM, it lists a value and predecessor basic block for each incoming edge, and phi nodes occur before ordinary operations in their block.^[ref]

For example, if the left branch defines x1=10 and the right defines x2=20, the join can define x3=phi(x1,left; x2,right). Arriving from the left gives ten; arriving from the right gives twenty. It does not average the arguments or evaluate both branches. At a loop header, another phi node can select an initial value on first entry and the updated value on later iterations. The node has one static definition even though its selected value can change across executions.