Spidermonkey JIT improvements in FF52

Last week we signed off our hard work on FF52 and we will start working on FF53. The expected release date of this version is the 6th of March. In the meantime we still have time to stabilize the code and fix issues that we encounter.

In the JavaScript JIT engine a lot of important work was done.

WebAssembly has made great advances. We have fully implemented the draft specification and are requesting final feedback as part of a cross-browser Browser Preview in the W3C WebAssembly Community Group. Assuming the Browser Preview concludes without major changes before 52 is released, we’ll enable WebAssembly by default in 52.

Step by step our CacheIR infrastructure is improving. In this release primitive value getprop was ported to the CacheIR infrastructure.

GC sometimes needs to discard the compilations happening in the helper threads. It seemed like we waited for the compilation to stop one after another. As a result it could take a while till all compilations were discarded. Now we signal all threads at the same time to stop compiling and afterwards wait for all of them to finish. This was a major win in our investment to make sure GC doesn’t interrupt the execution for too long.

The register allocator also received a nice improvement. Sometimes we were adding spills (stack to/from registers moves) while they were not needed. A fix was added to combat this.

Like in every release a long list of differential bugs and crashes have been fixed as well.

This release also include code from contributors:

• Emanuel Hoogeveen improved our crash annotations. He noticed that we didn’t save the crash reason in our crash reports when using “MOZ_CRASH”.
• Tooru Fujisawa has been doing incredible work throughout the codebase.
• Johannes Schulte has landed a patch that improves code for “testArg ? testArg : 0.0” and also eliminates needless control flow.
• Sander Mathijs van Veen made sure we could use unsigned integer modulo instead of a double modulo operation and also added code to fold additions with multiple constants.
• André Bargull added code to inline String.fromCodePoint in IonMonkey. As a result the performance should now be equal to using String.fromCharCode
• Robin Templeton noticed that we were spewing incorrect information in our debug logs and fixed that.
• Heiher has been updating MIPS to account for all changes we did to platform dependent code for WebAssembly.

I want to thank every one of them for their help! They did a tremendous job! If you are interested in helping out, we have a list of mentored bugs at bugsahoy or you can contact me (h4writer) online at irc.mozilla.org #jsapi.