Search

The aliasing that results in a variable being known by more than one name has greatly complicated efforts to derive data flow information. The approach we take involves the use of a series of claims that, after we compute the data flow for some of the aliasing possibilities, allows us to produce good approximations for the remaining cases. The method can thus limit the potential combinatorial explosion of aliasing computations while providing results that are frequently exact and almost always very good. The method is illustrated in the context of data flow analysis involving multiple procedures and their calling interactions. It is applicable also in the treatment of recursive procedures.

A data definition facility is presented that provides a consistent description of both primitive and user data. It is based on a value-oriented storage model which carefully distinguishes between values and objects. It is values that are typed in this model, and operations of the type work explicitly on the values. Objects are accessible only via reference values. Objects are described via descriptors called templates, which ultimately yield reference type values. Operations, both primitive and user-defined, are part of a "machine interface," and all executable language constructs can ultimately be defined as explicit operations of the interface. Importantly, these operations must respect the typing constraints imposed by both the primitive types and the user extensions. The interactions of definition facility, storage model, and execution model are illustrated via a series of examples in which commonly used data constructs are defined.

A storage management scheme is described that supports the invalidation of addresses to freed storage and thus, in that sense, provides a secure system. Unlike previous virtual memory techniques, the allocated areas of our scheme can vary from the very large, requiring multiple pages of storage, to the very small, in which several can be contained on a single page. Special treatment is accorded procedure activation storage so as to provide increased effectiveness for this important case. The interaction of this deletion scheme with garbage collection techniques is also examined. Finally, the relative advantages of retention and deletion strategies of storage management are considered.