The hierarchical task graph as a universal intermediate representation

Abstract

This paper presents an intermediate program representation called the Hierarchical Task Graph (HTG), and argues that it is not only suitable as the basis for program optimization and code generation, but it fully encapsulates program parallelism at all levels of granularity. As such, the HTG can be used as the basis for a variety of restructuring and optimization techniques, and hence as the target for front-end compilers as well as the input to source and code generators. Our implementation and testing of the HTG in the Parafrase-2 compiler has demonstrated its suitability and versatility as a potentially universal intermediate representation. In addition to encapsulating semantic information, data and control dependences, the HTG provides more information vital to efficient code generation and optimizations related to parallel code generation. In particular, we introduce the notion of precedence between nodes of the structure whose grain size can range from atomic operations to entire subprograms.