Loop Parallelization by Utpal Banerjee

By Utpal Banerjee

Automatic transformation of a sequential application right into a parallel shape is a topic that offers an outstanding highbrow problem and provides an outstanding sensible award. there's a super funding in present sequential courses, and scientists and engineers proceed to write down their program courses in sequential languages (primarily in Fortran). The call for for better speedups raises. The task of a restructuring compiler is to find the dependence constitution and the features of the given laptop. a lot consciousness has been excited by the Fortran do loop. this is often the place one expects to discover significant chunks of computation that must be played again and again for various values of the index variable. Many loop changes were designed through the years, and a number of other of them are available in any parallelizing compiler presently in use in or at a college study facility.
The e-book sequence on KappaLoop modifications for Restructuring Compilerskappa presents a rigorous conception of loop differences and dependence research. we wish to enhance the variations in a constant mathematical framework utilizing items like directed graphs, matrices, and linear equations. Then, the algorithms that enforce the ameliorations may be accurately defined by way of yes summary mathematical algorithms. the 1st quantity, LoopTransformations for Restructuring Compilers: The Foundations, supplied the final mathematical heritage wanted for loop changes (including these uncomplicated mathematical algorithms), mentioned info dependence, and brought the foremost variations. the present quantity, Loop Parallelization, builds a close concept of iteration-level loop differences in accordance with the fabric built within the earlier publication.

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Consider next the interchange of two arbitrary loops. 5, if there is no dependence at levels p through q - 1, then the loops Lp and Lq can be interchanged. This is only a sufficient condition, however. 6 and its corollary. 3. PREVENTING PERMUTATIONS 45 It is often useful to know if we can leg~lly move a given loop in L to a certain position in the nest, without making any other changes. Let 1 S p < q S m. Left circulation of loops Lp through Lq moves Lp inward to the qth position, while right circulation of the same loops moves Lq outward to the pth position.

In the previous exercise, prove that L2 is equivalent to Ll iff the following condition holds: An iteration H(j) depends on an iteration H(i) in L2 iff H(j) depends on H(i) in L 1 . 7. Let £ denote a set of nests of do loops such that each nest has the same set of iterations. ) Define a relation rv in £ by requiring that for any two nests Ll and L2 in £, we have Ll rv L2 if L2 is equivalent to L 1 . Prove that rv is an equivalence relation. 5 CHAPTER 1. BACKGROUND Loop Parallelization Let L denote a nest of m do loops of the type considered in this chapter.

Let Lp denote the program consisting of the iterations of L, such that an iteration H(i) is executed before another iteration H(j) in Lp iff iP -< jP. The program Lp is the transformed program defined by P, and the transformation L f--t Lp is the (loop) permutation of L defined by P. In a loop permutation, the index variables retain their identity; only their ordering is changed. We retain the labels for the individual loops, remembering that their limits may change. If P = [11" (1) 11" (2) ...

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