Which of My Transient Type Checks Are Not (Almost) Free?

TL;DR

This paper investigates the performance impact of transient type checks in gradually typed languages, refining a protocol to identify which annotations cause significant overhead and how to optimize or eliminate them.

Contribution

It refines the Takikawa protocol to pinpoint specific type annotations that significantly affect performance in virtual machines.

Findings

Certain type annotations cause notable performance overhead.

Removing specific annotations can make transient checks nearly free.

Graphing performance variations reveals patterns in overhead impact.

Abstract

One form of type checking used in gradually typed language is transient type checking: whenever an object 'flows' through code with a type annotation, the object is dynamically checked to ensure it has the methods required by the annotation. Just-in-time compilation and optimisation in virtual machines can eliminate much of the overhead of run-time transient type checks. Unfortunately this optimisation is not uniform: some type checks will significantly decrease, or even increase, a program's performance. In this paper, we refine the so called "Takikawa" protocol, and use it to identify which type annotations have the greatest effects on performance. In particular, we show how graphing the performance of such benchmarks when varying which type annotations are present in the source code can be used to discern potential patterns in performance. We demonstrate our approach by testing the Moth virtual machine: for many of the benchmarks where Moth's transient type checking impacts performance, we have been able to identify one or two specific type annotations that are the likely cause. Without these type annotations, the performance impact of transient type checking becomes negligible. Using our technique programmers can optimise programs by removing expensive type checks, and VM engineers can identify new opportunities for compiler optimisation.