EVALUATING COMPILER OPTIMIZATIONS FOR FORTRAN-D

The Fortran D compiler uses data decomposition specifications to autom atically translate Fortran programs for execution on MIMD distributed- memory machines. This paper introduces and classifies a number of adva nced optimizations needed to achieve acceptable performance; they are analyzed and empirically evaluated for stencil computations. Communica tion optimizations reduce communication overhead by decreasing the num ber of messages and hide communication overhead by overlapping the cos t of remaining messages with local computation. Parallelism optimizati ons exploit parallel and pipelined computations and may need to restru cture the computation to increase parallelism. Profitability formulas are derived for each optimization. Empirical results show that exploit ing parallelism for pipelined computations, reductions, and scans is v ital. Message vectorization, collective communication, and efficient c oarse-grain pipelining also significantly affect performance. Scalabil ity of communication and parallelism optimizations are analyzed. The e ffectiveness of communication optimizations is dictated by the ratio o f communication to computation in the program. An optimization strateg y is developed based on these analyses. (C) 1994 Academic Press, Inc.