Section: New Results

Array Contraction in Parallel Programs

Participants : Alain Darte, Alexandre Isoard.

Array contraction is a technique to reuse array elements when they are dead, in a form of array folding. A standard technique for array contraction is to use affine remappings with modulos. When the modulo is equal to 1, this corresponds to the removal of the corresponding array dimension. Array contraction is well-known for sequential programs, after element-wise array liveness analysis. It has also been customized for parallel codes obtained through affine schedules by Lefebvre and Feautrier, and Quilleré-Rajopadhye, both frameworks being generalized by the lattice-based memory allocation framework of Darte, Schreiber, and Villard  [17] and the construction of the set of conflicting array indices. We showed how the same framework can be used for a larger range of parallel programs, including programs with outer parallel loops, programs exhibiting pipelining, a subset of X10, etc. The optimality of the construction can be shown, despite a related (but actually non-contradictory here) NP-completeness result for worst-case of register pressure in the context of register allocation. A research report on this topic is in preparation.