CONS should not CONS its arguments, or, a lazy alloc is a smart alloc

• Authors: Henry G. Baker
• Venue: ACM SIGPLAN Notices
• Year: 1992

Lazy allocation is a model for allocating objects on the execution stack of a high-level language which does not create dangling references. Our model provides safe transportation into the heap for objects that may survive the deallocation of the surrounding stack frame. Space for objects that do not survive the deallocation of the surrounding stack frame is reclaimed without additional effort when the stack is popped. Lazy allocation thus performs a first-level garbage collection, and if the language supports garbage collection of the heap, then our model can reduce the amortized cost of allocation in such a heap by filtering out the short-lived objects that can be more efficiently managed in LIFO order. A run-time mechanism called result expectation further filters out unneeded results from functions called only for their effects. In a shared-memory multi-processor environment, this filtering reduces contention for the allocation and management of global memory.Our model performs simple local operations, and is therefore suitable for an interpreter or a hardware implementation. Its overheads for functional data are associated only with assignments, making lazy allocation attractive for mostly functional programming styles. Many existing stack allocation optimizations can be seen as instances of this generic model, in which some portion of these local operations have been optimized away through static analysis techniques.Important applications of our model include the efficient allocation of temporary data structures that are passed as arguments to anonymous procedures which may or may not use these data structures in a stack-like fashion. The most important of these objects are functional arguments (funargs), which require some run-time allocation to preserve the local environment. Since a funarg is sometimes returned as a first-class value, its lifetime can survive the stack frame in which it was created. Arguments which are evaluated in a lazy fashion (Scheme delays or "suspensions") are similarly handled. Variable-length argument "lists" themselves can be allocated in this fashion, allowing these objects to become "first-class". Finally, lazy allocation correctly handles the allocation of a Scheme control stack, allowing Scheme continuations to become first-class values.