Practical improvements to the construction and destruction of static single assignment form

Abstract

Static Single Assignment (SSA) form is a program representation that is becoming increasingly popular for compiler-based code optimization. In this paper, we address three problems that have arisen in our use of SSA form. Two are variations to the SSA construction algorithms presented by Cytron et al.1 The first variation is a version of SSA form that we call ‘semi-pruned’ SSA. It offers an attractive trade-off between the cost of global data-flow analysis required to build ‘pruned’ SSA and the large number of unused ϕ-functions found in minimal SSA. The second variation speeds up the program renaming process by efficiently manipulating the stacks of names used during renaming. Our improvement reduces the number of pushes performed, in addition to more efficiently locating the stacks that should be popped. To convert code in SSA form back into an executable form, the compiler must use an algorithm that replaces ϕ-functions with appropriately-placed copy instructions. The algorithm given by Cytron et al. for inserting copies produces incorrect results in some situations; particularly in cases like instruction scheduling, where the compiler may not be able to split ‘critical edges’, and in the aftermath of optimizations that aggressively rewrite the name space, like some forms of global value numbering.2 We present a new algorithm for inserting copy instructions to replace ϕ-functions. It fixes the problems that we have encountered with the original copy insertion algorithm. We present experimental results that demonstrate the effectiveness of the first two improvements not only during the construction of SSA form, but also in the time saved by subsequent optimization passes that use a smaller representation of the program. © 1998 John Wiley & Sons, Ltd.