An Aggressive Approach to Loop Unrolling

Authors:Davidson, Jack, Department of Computer ScienceUniversity of Virginia Jinturkar, Sanjay, Department of Computer ScienceUniversity of Virginia

A well-known code transformation for improving the execution performance of a program is loop unrolling. The unrolled loop usually requires fewer instruction executions than the original loop. The reduction in instruction executions comes from two sources: the number of branch instructions executed is reduced, and the index variable is modified fewer times. In addition, for architectures with features designed to exploit instruction-level parallelism, loop unrolling can expose greater levels of instruction-level parallelism. Loop unrolling is an effective code transformation often improving the execution performance of programs that spend much of their execution time in loops by 10 to 30 percent. Unfortunately, it has not been studied as extensively as other code improvements such as register allocation or common subexpression elimination. The result is that many compilers employ simplistic loop unrolling algorithms that miss many opportunities for improving run-time performance. This paper describes how aggressive loop unrolling is done in a retargetable optimizing compiler. Using a set of 32 benchmark programs, the effectiveness of this more aggressive approach to loop unrolling is evaluated. Interactions between loop unrolling and other common optimizations are also discussed.

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Source Citation:

Davidson, Jack, and Sanjay Jinturkar. "An Aggressive Approach to Loop Unrolling." University of Virginia Dept. of Computer Science Tech Report (1995).

University of Virginia, Department of Computer Science
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