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18 Copy propagation

File: copyprop

This phase is optional, but should be done whenever speed or space is more important than compile speed. We use global flow analysis to find the reaching definitions for each TN. This information is used here to eliminate unnecessary TNs, and is also used later on by loop invariant optimization.

In some cases, VMR conversion will unnecessarily copy the value of a TN into another TN, since it may not be able to tell that the initial TN has the same value at the time the second TN is referenced. This can happen when ICR optimize is unable to eliminate a trivial variable binding, or when the user does a setq, or may also result from creation of expression evaluation temporaries during VMR conversion. Whatever the cause, we would like to avoid the unnecessary creation and assignment of these TNs.

What we do is replace TN references whose only reaching definition is a Move VOP with a reference to the TN moved from, and then delete the Move VOP if the copy TN has no remaining references. There are several restrictions on copy propagation:

• The TNs must be “ordinary” TNs, not restricted or otherwise unusual. Extending the life of restricted (or wired) TNs can make register allocation impossible. Some other TN kinds have hidden references.
• We don’t want to defeat source-level debugging by replacing named variables with anonymous temporaries.
• We can’t delete moves that representation selected might want to change into a representation conversion, since we need the primitive types of both TNs to select a conversion.

Some cleverness reduces the cost of flow analysis. As for lifetime analysis, we only need to do flow analysis on global packed TNs. We can’t do the real local TN assignment pass before this, since we allocate TNs afterward, so we do a pre-pass that marks the TNs that are local for our purposes. We don’t care if block splitting eventually causes some of them to be considered global.

Note also that we are really only interested in knowing if there is a unique reaching definition, which we can mash into our flow analysis rules by doing an intersection. Then a definition only appears in the set when it is unique. We then propagate only definitions of TNs with only one write, which allows the TN to stand for the definition.