Partial evaluation allows specializing programs by substituting some of their inputs with static values and evaluating the resulting static expressions. When applied to a language interpreter, it specializes the interpreter to a fixed source program, resulting in a specialized interpreter that existing compilers can optimize and generate efficient machine code for. Although this approach achieves good peak performance in just-in-time compilers, partial evaluation is costly with respect to compilation time. In the case of bytecode interpreters, partial evaluation must process and transform each bytecode instruction. Since instructions with the same opcodes share common functionality, partial evaluation applies the same transformations repeatedly. Consequently, partial evaluation significantly increases the compilation time of just-in-time compilation.

We propose partial-evaluation templates, i.e., reusable collections of ahead-of-time-compiled graphs that are generated by a hand-written cogen approach. The templates are parametric in their static inputs, and cover common functionality in a bytecode interpreter implementation. At compile time, templates specialize themselves by substituting their static inputs, reducing themselves to a single pre-compiled graph that the compiler inlines and optimizes. This reduces the size of the intermediate representation and speeds up partial evaluation, as well as optimizations.

We integrated our approach into GraalVM, a state-of-the-art high-performance polyglot virtual machine, and GraalWasm, a bytecode-interpreter-based WebAssembly runtime. We extended the existing GraalVM compiler with a binding-time analysis that generates the templates and introduced a specializer for the templates into the existing partial evaluator. We enabled template generation for nearly all opcodes in GraalWasm, which reduced partial-evaluation time by up to 36% and warmup time by up to 17% without impacting peak performance.