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A general description is here.
The JIT byte code compiler is now enabled by default at level 3. This means functions will be compiled on first use and top level loops will be compiled and then run. Thanks to Tomas Kalibera for extensive work to make this possible. For now, the compiler will not compile code containing explicit calls to ‘browser()’. This is to support single stepping from the ‘browser()’ call. The JIT can be disabled using ‘compiler::enableJIT()’.