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. ®