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Firefox 18 Beta Out With IonMonkey JavaScript Engine
An anonymous reader writes with a quick bite from The Next Web about the latest Firefox beta, this time featuring some under-the-hood improvements: "Mozilla on Monday announced the release of Firefox 18 beta for Windows, Mac, and Linux. You can download it now from Mozilla.org/Firefox/Beta. The biggest addition in this update is significant JavaScript improvements, courtesy of Mozilla's new JavaScript JIT compiler called IonMonkey. The company promises the performance bump should be noticeable whenever Firefox is displaying Web apps, games, and other JavaScript-heavy pages."
I haven't kept track with the JIT's that have been in Firefox. I recall the days when TraceMonkey and JagerMonkey were added to boost performance. Could somebody recap or tell why Firefox is abandoning the older versions or redoing them? I'm truly curious as to what they learned, what worked and what didn't work. Are they finding new usage patterns that warrant a new JIT design? Thanks.
Its good to see the focus of this release being an attempt to increase javascript speed by leaps and bounds. Modern webpages often use JS that goes way beyond anything people did 10 years ago (Jquery for example) and the complexities of what people do with javascript noticably slow down most webpages considerably.
http://interserver.net/
Wikipedia goes into a bit of detail about it but in basic summary...
TraceMonkey was the first JIT compiler for SpiderMonkey released in Firefox 3.5.
JagerMonkey is a different design on TraceMonkey which outperforms it in certain circumstances (Some differences between TraceMonkey and JagerMonkey)
IonMonkey is another attempt at better perfecting the idea of JagerMonkey allowing even greater optimisations under particular circumstances.
However TraceMonkey, JagerMonkey and IonMonkey are part of SpiderMonkey as JIT compilers, not a replacement of SpiderMonkey itself.
A short summary:
1) TraceMonkey turned out to have very uneven performance. This was partly because it type-specialized very aggressively, and partly because it didn't deal well with very branchy code due to trace-tree explosion. As a result, when it was good it was really good (for back then), but when it hit a case it didn't handle well it was awful. JaegerMonkey was added as a way to address these shortcomings by having a baseline compiler that handled most cases, reserving tracing for very hot type-specialized codepaths.
2) As work on JaegerMonkey progressed and as Brian Hackett's type inference system was being put in place, it turned out that JaegerMonkey + type inference could give performance similar to TraceMonkey, with somewhat less complexity than supporting both compilers on top of type inference. So when TI was enabled, TraceMonkey was switched off, and later removed from the tree. But keep in mind that JaegerMonkey was designed to be a baseline JIT: run fast, compile everything, no fancy optimizations.
3) IonMonkey exists to handle the cases TraceMonkey used to do well. It has a much slower compilation pass than JaegerMonkey, because it does more involved optimizations. So most code gets compiled with JaegerMonkey, and then particularly hot code is compiled with IonMonkey.
This is a common design for JIT systems, actually: a faster JIT that produces slower code and a slower JIT that produces faster code for the cases where it matters.
https://blog.mozilla.org/dmandelin/2011/04/22/mozilla-javascript-2011/ has a bit of discussion about some of this.