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IonMonkey is the next generation JavaScript JIT compiler for SpiderMonkey. It is a whole-method JIT with the ability to perform type specialization. It has two goals: a cleanly engineered design that makes future optimization work possible, and excellent performance.


See Platform/Features/IonMonkey for IonMonkey's planning page.


See IonMonkey/Overview for an overview of the IonMonkey architecture and its files.


To get started, see the Build Documentation for SpiderMonkey. Use the '-h' shell option to see all of IonMonkey's options.

Currently, IonMonkey is enabled by default for those processor architectures it supports. Use '--disable-ion' to disable it.


A few tools exist to help debug IonMonkey.

  • Debug spew, controlled by the environment variable IONFLAGS. Set it to help to see available options.
  • Instruction spew, via setting the environment variable JMFLAGS to insns.
  • c1visualizer IonMonkey writes a spew file to /tmp/ion.cfg, which can be imported into the c1visualizer. It can display a clickable control-flow graph, MIR, LIR, and liveness intervals for LSRA.
  • iongraph by Sean Stangl, using IonMonkey's JSON spew.
  • Some gdb tricks related to IonMonkey are listed on the Hacking Tips page of spider monkey.