Lazy Stack Copying and Stack Copy Sharing for the Efficient Implementation of Continuations

Abstract

In order to capture first-class continuations, most stack-based implementations copy contents of the stack to the heap. While various implementation strategies for copying have been proposed, many implementations employ the stack strategy. With this strategy, the entire stack contents is copied to the heap whenever a continuation is captured. This simple strategy is easy to implement and can be used for implementations with foreign language interface. However, this strategy requires a lot of time and memory for creation and invocation of continuations. We propose a lazy stack copying technique. The contents of the stack to copy are preserved on the stack until the function returns that has captured the continuation. So we delay stack copying for the continuation until the function returns. We can avoid stack copying if it is detected that the continuation has become garbage before the function returns. In addition, we propose stack copy sharing, which is realized by using lazy stack copying. We present three models for stack copy sharing. We applied these techniques to Scheme systems and found that the proposed techniques improve runtime and memory efficiency of programs that use first-class continuations.