jotting ideas

2015-06-20-closing-the-chasm.md

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+---
+layout: site
+author: Torsten
+---
+
+As noted in previous posts, differentiating between compile- and run-time is one of the more
+difficult things in doing the vm. That is because the computing that needs to happen is so similar,
+in other words almost all of the vm - level is available at run-time too.
+
+But off course we try to do as much as posible at compile-time.
+
+One hears or reads that exactly this is a topic causing (also) other vms problems.
+Specifically how one assures that what is compiled at compile-time and and run-time are
+identical or at least compatible.
+
+The obvious answer seems to me to **use the same code**.In a way that "just" moves the question
+around a bit, becuase then one would have to know how to do that. I'll go into that below,
+but find that the concept is worth exploring first.
+
+Let's take a simple example of accessing an instance variable. This is off course available at
+run-time through the function *instance_variable_get* , which could go something like:
+
+    def instance_variable_get name
+      index = @layout.index name
+      return nil unless index
+      at_index(index)
+    end
+
+Let's assume the *builtin* at_index function and take the layout to be an array like structure.
+As noted in previous posts, when this is compiled we get a Method with Blocks, and exactly one
+Block will initiate the return. The previous post detailed how at that time the return value will
+be in the ReturnSlot.
+
+So then we get to the idea of how: We "just" need to take the blocks from the method and paste
+them where the instance variable is accessed. Following code will pick the value from the ReturnSlot
+as it would any other value and continue.
+
+The only glitch in this plan is that the code will assume a new message and frame. But if we just
+paste it it will use message/frame/self from the enclosing method. So that is where the work is:
+translating slots from the inner, inlined fuction to the outer one. Possibly creating new frame
+entries.
+
+tbc