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Related Work
For the Smalltalk devotee, nothing is more natural than the desire to attack all programming problems with Smalltalk. Thus, there has long been a tradition of using Smalltalk to describe and debug a low-level system before its final implementation. As mentioned earlier, the Blue Book used Smalltalk as a high-level description of a Smalltalk virtual machine, and this description was actually checked for accuracy by running it. In LOOM [Kaeh86], the kernel of a virtual object memory was written and executed in a separate, simplified Smalltalk virtual machine whenever an "object fault" occurred. For better performance, this kernel was later translated into BCPL semi-automatically, then fixed up by hand. This experience planted the seed for the approach taken in Squeak two decades later.
A number of recent systems translate complete Smalltalk programs into lower-level languages to gain speed, portability, or application packaging advantages. Smalltalk/X [Gitt95] and SPiCE [YaDo95] generate C code from programs using the full range of Smalltalk semantics, including blocks. Babel [MWH94] translates Smalltalk applications into CLOS, and includes a facility for automatically winnowing out unused classes and methods.
Producer [Cox87] translated Smalltalk programs into Objective C, but required the programmer to supply type declarations and rules for mapping dynamically allocated objects such as Points into Objective C record structures. Producer only supported a subset of Smalltalk semantics because it depended on the Objective C runtime and thus did not support blocks as first-class objects. Squeak's Smalltalk-to-C translator restricts the programmer to an even more limited subset of Smalltalk, but that subset closely mirrors the underlying processor architecture, allowing the translated code to run nearly as efficiently as if it were written in C directly. The difference arises from a difference in goals: The goal of Squeak's translation is merely to support the construction of its own virtual machine, a much simpler task than translating full-blown Smalltalk programs into C. Squeak's translator is more in the spirit of QUICKTALK [Ball86], a system that used Smalltalk to define new primitive methods for the virtual machine. Another Smalltalk-to-primitive compiler, Hurricane [Atki86], used a combination of user-supplied declarations and simple type inference to eliminate class checks and to inline integer arithmetic. Unlike Squeak's translator, Hurricane allowed the programmer to also use polymorphic arithmetic in the Smalltalk code to be translated. Neither QUICKTALK nor Hurricane attempted to produce an entire virtual machine via translation.
Type information can help a translator produce more efficient code by eliminating run-time type tests and enabling inlining. Typed Smalltalk [JGZ88] added optional type declarations to Smalltalk and used that type information to generate faster code. The quality of its code was comparable to that of QUICKTALK but, to the best of the authors' knowledge, the project's ultimate goal of producing a complete, stand-alone Smalltalk virtual machine was never realized. A different approach is to use type information gathered during program execution to guide on-the-fly optimization, as done in the Self [ChUn91] [Hölz94] and Animorphic [Anim96] virtual machines. Note that using types for optimization is independent of whether the programming language has type declarations. The Self and Animorphic virtual machines use type information to optimize declaration-free languages whereas Strongtalk [BrGr93], which augments Smalltalk with an optional type system to support the specification and verification of interfaces, ran on a virtual machine that knew nothing about those types. The subset of Smalltalk used for the Squeak virtual machine maps so directly to the fundamental data types of the hardware that the translator would not benefit from additional type information. However, we have contemplated building a separate primitive compiler that supports polymorphic arithmetic, in which case the declaration-driven optimization techniques of Hurricane and Typed Smalltalk would be beneficial.
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