Partial Evaluation, Whole-Program Compilation

There is a tension in dynamic language runtime design between speed and correctness. State-of-the-art JIT compilation, the result of enormous industrial investment and significant research, achieves heroic speedups at the cost of complexity. This complexity leads to subtle and sometimes catastrophic correctness bugs. Much of this complexity comes from the existence of multiple tiers and the need to maintain correspondence between these separate definitions of the language’s semantics; it also comes from the indirect nature of the semantics implicitly encoded in a compiler backend. One way to address this complexity is to automatically derive, as much as possible, the compiled code from a single source-of-truth, such as the interpreter tier. In this work, we introduce a partial evaluator that can compile a whole guest-language function ahead-of-time, without tracing or profiling, “for free.” This transform unrolls an interpreter function expressed in a standard compiler intermediate representation (static single assignment or SSA) and uses partial evaluation of the interpreter function and its regular control flow to drive the guest-language compilation. The effect of this is that the transform is applicable to almost unmodified existing interpreters in systems languages such as C or C++, producing ahead-of-time guest-language compilers. We show the effectiveness of this new tool by applying it to the interpreter tier of an existing industrial JavaScript engine, SpiderMonkey, yielding 2.17× speedups, and the PUC-Rio Lua interpreter, yielding 1.84× speedups. Finally, we outline an approach to carry this work further, deriving more of the capabilities of a JIT backend from first principles while retaining correctness.