Compiling Lisp Programs for Parallel Execution

Curare, the program restructurer described in this paper automatically transforms a sequential Lisp program into an equivalent concurrent program that runs on a multiprocessor. Data dependences constrain the program's concurrent execution because, in general, two conflicting statements cannot execute in a different order without affecting the program's result. Not all dependences are essential to produce the program's result.Curare attempts to transform the program so it computes its result with fewer conflicts. An optimized program will execute with less synchronization and more concurrency. Curare then examines loops in a program to find those that are unconstrained or lightly constrained by dependences. By necessity,Curare treats recursive functions as loops and does not limit itself to explicit program loops. Recursive functions offer several advantages over explicit loops since they provide a convenient framework for inserting locks and handling the dynamic behavior of symbolic programs. Loops that are suitable for concurrent execution are changed to execute on a set of concurrent server processes. These servers execute single loop iterations and therefore need to be extremely inexpensive to invoke. Restructured programs execute significantly faster than the original sequential programs. This improvement is large enough to attract programmers to a multiprocessor, particularly since it requires little effort on their part.