jotting ideas

2015-05-20-expression_is_slot.md

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+---
+layout: site
+author: Torsten
+---
+
+Since i got the ideas of Slots and the associated instruction Set, i have been wondering how that
+fits in with the code generation.
+
+I moved the patched AST compiler methods to a Compiler, ok. But still what do all those compile
+methods return.
+
+## Expression
+
+In ruby, everything is an expression. To recap "Expressions have a value, while statements do not",
+or statements represent actions while expressions represent values.
+
+So in ruby everything represents a value, also statements, or functions. There is no such thing
+as the return void in C. Even loops and ifs result in a value, for a loop the last computed value
+and for an if the value of the branch taken.
+
+Having had a vague grasp of this concept i tried to sort of haphazardly return the kind of value
+that i though appropriate. Sometimes literals, sometimes slots. Sometimes "Return" , a slot
+representing the return value of a function.
+
+## Return slot
+
+Today i realized that the Slot representing the return value is special.
+
+It does not hold the value that is returned, but rather the other way around.
+
+A function returns what is in the Return slot, at the time of return.
+
+From there it is easy to see that it must be the Return that holds the last computed value.
+A function can return at any time after all.
+
+The last computed value is the Expression that is currently evaluated. So the compile, which
+initiates the evaluation, returns the Return slot. Always. Easy, simple, nice!
+
+## Example
+
+Constants: say the expression
+
+    true
+
+would compile to a
+
+    ConstantLoad(ReturnSlot , TrueConstant)
+
+While
+
+    2 + 4
+
+would compile to
+
+    ConstantLoad(ReturnSlot , IntegerConstant(2))
+    Set(ReturnSlot , OtherSlot)
+    ConstantLoad(ReturnSlot , IntegerConstant(4))
+    Set(ReturnSlot , EvenOtherSlot)
+    MethodCall() # unspecified details here
+
+
+## Optimisations
+
+But but but i hear that is so totally inefficient. All the time we move data around, to and from
+that one Return slot, just so that the return is simple. Yes but no.
+
+It is very easy to optimize the trivial extra away. Many times the expression moves a value to Return
+just to move it away in the next Instruction. A sequence like in above example
+
+    ConstantLoad(ReturnSlot , IntegerConstant(2))
+    Set(ReturnSlot , OtherSlot)
+
+can easily be optimized into
+
+    ConstantLoad(OtherSlot , IntegerConstant(2))
+
+tbc

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