Irregular Data-parallel Objects in C++ 1 Irregularity and Data-parallelism

Most data-parallel languages use arrays to support paral-lelism. This regular data structure allows a natural development of regular parallel algorithms. The implementation of irregular algorithms requires a programming eeort to project the irregular data structures onto regular structures. We rst propose in this paper a classiication of existing data-parallel languages. We brieey describe their irregular and dynamic aspects, and derive diierent levels where irregularity and dy-namicity may be introduced. We propose then a new irregular and dynamic data-parallel programming model, called Idole. Finally we discuss its integration in the C++ language, and present an overview of the Idole extension of C++. The evolution of data-parallel languages mimics closely the evolution of sequential languages. Keeping in mind eeciency and simplicity, compilers have supported, in a rst step, only regular data structures: arrays in sequential languages , vectors and matrices in parallel languages. Handling irregular data structures imposes then to the programmer an explicit management of the memory. A consequence is that irregularity will take place at the level of the algorithm (gather/scatter operations). The integration of irregular data structures in data-parallel languages can be compared to the emergence of dynamic memory management and pointers in sequential languages. While preserving the semantics of the data-parallel model, irregular data structures essentially allow the speciica-tion of interdependencies between parallel object elements, which are similar to the interdependencies expressed by pointers in sequential data structures. Let's consider the following example : in a binary tree, each node wants to compute the minimum of a value held by its two child nodes. (Parallel happy birthday: in a family and on several generations, each parent wants to know the next birthday to wish among its children.) This algorithm is eeectively a data-parallel algorithm. The same algorithm is applied to a set of data of similar type. For a data-parallel language supporting irregular data structures, the translation is straightforward: if (rchild and lchild exist) then min_child = min (rchild.val, lchild.val)