Context Management in VisualWorks 5i

Smalltalk-80 provides a reification of execution state in the form of context objects which represent procedure activation records. Smalltalk-80 also provides full closures with indefinite extent. These features pose interesting implementation challenges because a naïve implementation entails instantiating context objects on every method activation, but typical Smalltalk-80 programs obey stack discipline for the vast majority of activations. Both software and hardware implementations of Smalltalk-80 have mapped contexts and closure activations to stack frames but not without overhead when compared to traditional stack-based activation and return in "conventional" languages. We present a new design for contexts and closures that significantly reduces the overall overhead of these features and imposes overhead only in code that actually manipulates execution state in the form of contexts.