It's not clear to me why calling into C++ is expensive to begin with. My best guess is it has to do with switching to a native stack (switching "folders"). So m

It's not clear to me why calling into C++ is expensive to begin with. My best guess is it has to do with switching to a native stack (switching "folders"). So making the stub directly callable from the JIT means not having to change activations. Also presumably it uses the parameter passing conventions and semantics of all other JIT functions (whereas with a C++ function it presumably has to marshal the parameters to the function).