Extended SSA Numbering: Introducing SSA properties to languages with multi-level pointers

Static Single Assignment (SSA) intermediate representations have become quite popular in compiler development. One advantage of the SSA form is that each use of a variable corresponds to exactly one de nition, and thus two references of the same SSA variable must denote the same value. To date, most SSA forms concentrate on scalar variables, and it is di cult to extend these intermediate representations to languages with multi-level pointers like C. Translating a program into SSA form requires two steps. The rst is inserting -nodes that join values from di erent ow of control paths, and the other renames variables such that each name has exactly one de nition. When designing SSA forms to handle pointers, the insertion of -nodes is quite di cult. Thus, we propose a method that concentrates on the second step, in that we associate an SSA number to each variable reference, including references via pointers. Even without -nodes, we retain some of the properties of SSA such as \one value per name" when the SSA number is used as part of the name. In addition to a primary SSA number for scalar variables we introduce a secondary SSA number for pointers. Given an indirect reference of the form *p, the primary number refers to the contents of p and the secondary number refers to the contents of *p. Using both primary and secondary SSA numbers we can tell when two indirect references denote the same value, as well as derive other SSA properties for indirect references. In this paper we show how we have implemented Extended SSA Numbering in the McCAT C Compiler, and demonstrate its usefulness by strengthening reaching de nitions analysis and developing a new optimizing transformation. i