More precise construction of static single assignment programs using reaching definitions

Abstract

The Static Single Assignment (SSA) form is an intermediate representation used for the analysis and optimization of programs in modern compilers. The ϕ-function placement is the most computationally expensive part of converting any program into its SSA form. The most widely-used ϕ-function placement algorithms are based on computing dominance frontiers (DF). However, this kind of algorithms works under the limiting assumption that all variables are defined at the beginning of the program, which is not the case for local variables. In this paper, we introduce an innovative ϕ-placement algorithm based on computing reaching definitions (RD), which generates a precise number of ϕ-functions. We provided theorems and proofs showing the correctness and the theoretical computational complexity of our algorithms. We implemented our approach and a well-known DF-based algorithm in the Clang/LLVM compiler framework, and performed experiments on a number of benchmarks. The results show that the limiting assumption of the DF-based algorithm when compared with the more accurate results of our RD-based approach leads to generating up to 87% (69% on average) superfluous ϕ-functions on all benchmarks, and thus brings about a significant precision loss. Moreover, even though our approach computes more information to generate precise results, it is able to analyze up to 92.96% procedures (65.63% on average) of all benchmarks with execution time within twice the execution time of the reference DF-based approach.