Exploiting the parallelism available in loops

Because a loop's body often executes many times, loops provide a rich opportunity for exploiting parallelism. This article explains scheduling techniques and compares results on different architectures. Since parallel architectures differ in synchronization overhead, instruction scheduling constraints, memory latencies, and implementation details, determining the best approach for exploiting parallelism can be difficult. To indicate their performance potential, this article surveys several architectures and compilation techniques using a common notation and consistent terminology. First we develop the critical dependence ratio to determine a loop's maximum possible parallelism, given infinite hardware. Then we look at specific architectures and techniques. Loops can provide a large portion of the parallelism available in an application program, since the iterations of a loop may be executed many times. To exploit this parallelism, however, one must look beyond a single basic block or a single iteration for independent operations. The choice of technique depends on the underlying architecture of the parallel machine and the characteristics of each individual loop.<
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