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Google 'Crankshaft' inspired by Sun Java HotSpot

Bak to 'adaptive compilation'

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Google's new "Crankshaft" JavaScript engine was inspired at least in part by Sun's Java Hotspot performance engine, the reengineered Java virtual machine that Sun released in 1999.

A number of developers who built Crankshaft, including Danish programmer Lars Bak, also worked on HotSpot. Bak led the HotSpot team at Sun, and he's one of the founders of Google's V8 JavaScript engine.

Crankshaft is similar to Mozilla's TraceMonkey-JagerMonkey JavaScript engine setup in that it seeks to optimize certain pieces of code. "At a simple level, they both do dynamic optimization. There's one version of the code that gets 'hot,' gets important, and they take a deeper pass on that code," Google engineer Eric Kay said this morning at the annual Add-on-Con browser conference in Mountain View, California, when we asked about the Crankshaft design and how it compares to Mozilla's engine.

But whereas TraceMonkey uses a technique called "tracing" – detecting code loops and converting them into assembly language – Google's Crankshaft uses an approach known as "adaptive compilation" whereby it identifies all sorts of hot code and optimizes whole methods of code as needed. Sun's HotSpot was based on adaptive compilation, and according to Kay the idea of adding adaptive compilation to the V8 engine has long been on Google's roadmap.

Crankshaft includes four complementary components. In addition to a base compiler, there's a runtime profile that identifies hot code. Then an optimizing compiler recompiles the hot code to offer such optimizations as loop-invariant code motion, linear-scan register allocation, and inlining. Lastly, Google includes "deoptimization support" that identifies cases where the optimizing compiler has promised too much optimization. When this happens, the engine falls back on the base compiler. ®

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