Shared Source CLI Essentials

Verifying and Compiling CIL

After the EEClass and its related MethodTable have been laid out, all of the type information necessary for compilation and most of the runtime structures necessary for execution are finished. At this point, the execution engine is ready to compile and execute the code for the type. But what is it that triggers JIT compilation?

In traditional toolchains (such as that of C++), compilation often occurs as far forward as the language can make it—the C++ compiler wants to eliminate as much information as possible from being needed at runtime, so as to minimize the amount of processing required. Frequently, this approach results in situations in which the assumptions used while compiling no longer apply—methods are compiled that are never called in a normal run of the program, for example, or precomputed layouts cannot be used against newer libraries.

The CLI adheres to a principle of maximal deferral: compilation (along with many other activities) does not occur until the last possible moment. In the case of method compilation, the “last possible moment” is the moment that a method is required to run. We need some kind of tripwire to inform us of this event, something that will fire just before method execution, giving the CLI a chance to invoke the JIT compiler on the CIL for that method. It would be possible to track all method invocations and force JIT compilation when necessary, but this would be a naïve implementation and would perform poorly, since ...