SPEjs: a symbolic partial evaluator for JavaScript

Partial evaluation is widely performed statically, to perform a source to source transformation on a source program that yields a specialized source program. A key observation is that current partial evaluation schemes perform fast but relatively shallow static analyses. In this paper we propose to deepen the reach of such partial evaluation schemes by selectively adding local symbolic execution. Concretely, we describe the SPEjs symbolic partial evaluator for JavaScript that is built on Babel and the SMT solver Z3. To gauge the promise of this approach we compared SPEjs with Facebook's state-of-the-art partial evaluator Prepack. Our results on a set of micro benchmarks and Prepack's test suite indicate that, within Prepack's runtime budget, SPEjs was able to simplify additional expressions and therefore remove dead code branches that Prepack failed to remove, yielding smaller residual programs.