Introduction to Compiler Construction in a Java World

220 An Introduction to Compiler Construction in a Java World

Of course, a redundant Phi function will not be changed. If the w is never modified in the

loop body, the Phi function instruction takes the form

w

2

= [w

1

w

2

]

and can be removed as it is apparent that w has not been changed.

Phi functions are tightly bound to state vectors. When a block is processed:

• If the block has just a single predecessor, then it may inherit the state vector of that

predecessor; the states are simply copied.

• If the block has more than one predecessor, then those states in the vectors that differ

must be merged using Phi functions.

• For loop headers we conservatively