c69's Browser Pit

Unlike in other browsers, Chakra’s just-in-time (JIT) compiler runs on a separate dedicated thread and thus does not interfere with script execution.

To further reduce the time to first executed instruction, Chakra processes and emits bytecode only for functions that are about to be executed using a mechanism called deferred parsing.

We enhanced Chakra’s interpreter to collect it at runtime, a technique we call dynamic profiling. When a function is scheduled for JIT compilation, the compiler examines the runtime profile gathered by the interpreter and emits code tailored to the expected inputs.

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The interpreter gathers information for the runs it observes, but it’s possible that the execution of the program will lead to runtime values which violate assumptions made in the generated optimized code. For every assumption it makes, the compiler emits a runtime check. If a later execution results in an unexpected value, the check fails, execution bails out of the specialized machine code, and is continued in the interpreter. The reason for bailout (the failed check) is recorded, additional profile information is collected by the interpreter, and the function is recompiled with different assumptions. Bailout and re-compilation are two fundamentally new capabilities in IE10.