WebKit Unifies JavaScript Compilation With LLVM Optimizer

An anonymous reader tips this post at Webkit.org: "Just a decade ago, JavaScript – the programming language used to drive web page interactions – was thought to be too slow for serious application development. But thanks to continuous optimization efforts, it's now possible to write sophisticated, high-performance applications – even graphics-intensive games – using portable standards-compliant JavaScript and HTML5. This post describes a new advancement in JavaScript optimization: the WebKit project has unified its existing JavaScript compilation infrastructure with the state-of-the-art LLVM optimizer. This will allow JavaScript programs to leverage sophisticated optimizations that were previously only available to native applications written in languages like C++ or Objective-C. ... I'm happy to report that our LLVM-based just-in-time (JIT) compiler, dubbed the FTL – short for Fourth Tier LLVM – has been enabled by default on the Mac and iOS ports. This post summarizes the FTL engineering that was undertaken over the past year. It first reviews how WebKit's JIT compilers worked prior to the FTL. Then it describes the FTL architecture along with how we solved some of the fundamental challenges of using LLVM as a dynamic language JIT. Finally, this post shows how the FTL enables a couple of JavaScript-specific optimizations."

Cool but not finished yet

[StripedCow]

Cool stuff, but until one can write an optimized javascript virtual machine in it, I don't consider this project finished.

Speed space trade-off

[Required Snark]

I read the article, and it's clear that they are trading space for speed. At every step they create multiple versions of JavaScript code, each with a specific optimization goal. As far as I can tell, they are not garbage collected as long as the code is in use, because at any point they can switch from a more optimized version to a less optimized version.

Not only do they have many copies of the code around, they also keep a lot of information about how each version behaves as well as mapping structures so they can switch between the versions while they are running.

I infer that this means a lot of code bloat. I have no sense of how this memory usage compares to the memory used for DOM storage and the like. Does anyone know if code memory is a significant contributor to the overall memory footprint of a WebKit based browser?

Using FIrefox on the Mac, I experience an ever increasing memory footprint if I keep the browser running for a long period of time, i.e. over the course of a few days. This is partly laziness, but it also reflects how I use online references. Often I have multiple pages open at the same time for references, and I don't want to close them until I finish what I am working on (typically coding). After I have found a lot of relevant information online, I really don't want to end the browser session and then restore everything when I return two work.

So how will WebKit perform in this environment? How does it compare to Chrome and Firefox for memory usage? If something besides FF didn't suffer from memory bloat I might consider changing. Any experience or benchmarks would be interesting to hear about.