Summit2008/Sessions/Proposals/Tracing and JavaScript performance

Andreas Gal, Jim Blandy, Brendan Eich, Mike Shaver

Tracing and JavaScript performance

Tracing is a technique for implementation of just-in-time compilation well-suited to the needs of JavaScript on the web: it exhibits extremely good compilation performance (in space and speed), even when performing aggressive optimizations, and can efficiently cope with dynamic typing. In addition, it can be implemented in a minimally invasive and incremental manner. Tracing is employed in the experimental Hotpath Java virtual machine, the upcoming luajit2 implementation of the Lua language, the "Tamarin Tracing" ActionScript/JavaScript runtime, and now in the "Tracemonkey" project to bring native code performance to Mozilla's Spidermonkey JavaScript engine.

This session will provide a background in trace-based JIT compilation, an overview of the current state of Mozilla's tracing-related work, and future plans. Spirited discussion of relative implementation priorities and schedules will no doubt ensue.

Fundamentals of tracing: an introduction to the basic principles of tracing (link to papers)

• trace specialization and guarding
  • shape induction
• interaction between interpreter and JIT compiler
• applying optimizations to traces

nanojit: a lightweight native code generator specialized for tracing

• implementation of TSSA
• back-end implementation status

Tamarin Tracing status:

• relationship to "Tamarin Central"
• status and near-term goals
• performance characteristics
• current challenges

Tracemonkey status: an overview of current Tracemonkey capabilities, and discussion

• differences from Tamarin Tracing
• work so far (timeline, design choices)
• near-term goals
• preliminary performance results
• current challenges

Future work: making more of our code available to tracing, additional optimizations

• self-hosting the DOM
• escape analysis and allocation removal

Tracing beyond JavaScript: can we apply tracing techniques to other areas of code?

Further reading:

• HotpathVM: an effective JIT compiler for resource-constrained devices.
• One Method At A Time Is Quite A Waste Of Time
• Efficient Just-In-Time Execution of Dynamically Typed Languages Via Code Specialization Using Precise Runtime Type Inference
• Incremental Dynamic Code Generation with Trace Trees