Faster, With Guardrails

Back in March, I wrote about JSSE, a JavaScript engine an agent built from scratch in Rust. It was correct, but slow. In May, I wrote about making it faster without touching its architecture, and promised that post was not about bytecode. JSSE v0.9.0 was tagged on September 23, so it is time to take stock. In this post, I will go through where JSSE stands six months after its first release, how it compares with other engines, and the guardrails that allowed an agent to make it faster without breaking it. The short version: v0.9.0 runs 47 of the 48 JetStream workloads, up from 22 in March. On the 22 that both versions run, it is 2.92x faster (geometric mean). And it still passes every one of the 99,911 test262 scenarios we run. I still haven't written a single line of JSSE. What I did build is the guardrails and, as it turns out, I think the guardrails are the actual story. Agent-driven performance work is possible, however it is only trustworthy if the agent cannot quietly trade…