A Hierarchical Region-Based Static Single Assignment Form

Abstract

Modern compilation systems face the challenge of incrementally reanalyzing a program’s intermediate representation each time a code transformation is performed. Current approaches typically either re-analyze the entire program after an individual transformation or limit the analysis information that is available after a transformation. To address both efficiency and precision goals in an optimizing compiler, we introduce a hierarchical static single-assignment form called Region Static Single-Assignment (Region-SSA) form. Static single assignment (SSA) form is an efficient intermediate representation that is well suited for solving many data flow analysis and optimization problems. By partitioning the program into hierarchical regions, Region-SSA form maintains a local SSA form for each region. Region-SSA supports a demand-driven re-computation of SSA form after a transformation is performed, since only the updated region’s SSA form needs to be reconstructed along with a potential propagation of exposed defs and uses. In this paper, we introduce the Region-SSA data structure, and present algorithms for construction and incremental reconstruction of Region-SSA form. The Region-SSA data structure includes a tree based region hierarchy, a region based control flow graph, and region-based SSA forms. We have implemented in Region SSA form in the Habanero-Java (HJ) research compiler. Our experimental results show significant improvements in compile-time compared to traditional approaches that recomputed the entire procedure’s SSA form exhaustively after transformation. For loop unrolling transformations, compile-time speedups up to 35.8× were observed using Region-SSA form relative to standard SSA form. For loop interchange transformations, compile-time speedups up to 205.6× were observed. We believe that Region-SSA form is an attractive foundation for future compiler frameworks that need to incorporate sophisticated incremental program analyses and transformations.