Removing Impediments to Loop Fusion Through Code Transformations

Loop fusion is a common optimization technique that takes several loops and combines them into a single large loop. Most of the existing work on loop fusion concentrates on the heuristics required to optimize an objective function, such. as data reuse or creation of instruction level parallelism opportunities. Often, however, the code provided to a compiler has only small sets of loops that are control flow equivalent, normalized, have the same iteration count, are adjacent, and have no fusion-preventing dependences. This paper focuses on code transformations that create more opportunities for loop fusion in the IBM®XL compiler suite that generates code for the IBM family of PowerPC®processors. In this compiler an objective function is used at the loop distributor to decide which portions of a loop should remain in the same loop nest and which portions should be redistributed. Our algorithm focuses on eliminating conditions that prevent loop fusion. By generating maximal fusion our algorithm increases the scope of later transformations. We tested our improved code generator in an IBM pSeries 690 machine equipped with a POWER4 processor using the SPEC CPU2000 benchmark suite. Our improvements to loop fusion resulted in three times as many loops fused in a subset of CFP2000 benchmarks, and four times as many for a subset of CINT2000 benchmarks.