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Optimizes a Program's variable allocation strategy.

The variable allocation strategies determine how much memory the Program consumes, and how costly its memory access operations are (see instructions.VariableAllocation). In general, a variable holding data with a longer or more complex lifetime will need a more expensive storage strategy. This analysis examines variables' liveness and opportunistically selects inexpensive sound allocation strategies.

Specifically, the algorithm is to:

  • Run liveness analysis to determine the lifespan of each variable.
  • Assume optimistically that no variable needs to be stored at all (instructions.VariableAllocation.NULL).
  • Traverse the instructions and pattern-match conditions that require some storage:
    • If a variable is read by an instruction, it must be at least instructions.VariableAllocation.TEMPORARY.
    • If a variable is live out of some block (i.e., crosses a block boundary), it must be at least instructions.VariableAllocation.REGISTER. This is because temporaries do not appear in the loop state in execute.
    • If a variable is alive across a call to an autobatched Function, it must be instructions.VariableAllocation.FULL, because that Function may push values to it that must not overwrite the value present at the call point. (This can be improved by examining the call graph to see whether the callee really does push values to this variable, but that's future work.)

program Program to optimize.

program A newly allocated Program with the same semantics but possibly different allocation strategies for some (or all) variables. Each new strategy may be more efficient than the input Program's allocation strategy for that variable (if the analysis can prove it safe), but will not be less efficient.