[Pharo-dev] The evil twin in 40516

Marcus Denker marcus.denker at inria.fr
Thu Feb 26 18:07:43 EST 2015


In 40516 there is now a strange, but rather powerful mechanism for CompiledMethod: the Twin.

What does that do?

you can call on a method #createTwin

	(ReflectivityExamples>>#exampleMethod) createTwin. 

after this, the CompiledMethod has a high-level representation (the AST) attached that itself references the CompiledMethod
(they form a twin).

	(ReflectivityExamples>>#exampleMethod) reflectiveMethod 

The fun thing is now that one can install either in the class. Call #invalidate to make sure the reflective method is installed.

ReflectiveMethod implements #run:with:in: which calls a compilation hook (too re-create from the AST) and then installs that in the 
method dict and then executes the method:

	(ReflectivityExamples>>#exampleMethod) createTwin. 
	(ReflectivityExamples>>#exampleMethod) invalidate. 
	self assert: (ReflectivityExamples>>#exampleMethod) class = ReflectiveMethod.
	self assert: ReflectivityExamples new exampleMethod = 5.
	self assert: (ReflectivityExamples>>#exampleMethod) class = CompiledMethod. 

Which means that this gives us an in-image, on-demand JIT compiler AST->Bytecode. In 50 lines of code.

e.g. try on Morph:

Morph methods do: #createTwin.
Morph methods do: #invalidate.

Counting which twin is installed shows us the working set of Morph:

(Morph methods select: [ :each | each class = CompiledMethod ]) size

some 330 method out of nearly 900….

So what can one do with this? In essence this turns the AST into a reflective representation for Methods
(if you care to set up twin creation + invalidation correctly).

What will this allow us to do? stay tuned...


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