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_posts/2015-05-20-expression-is-slot.md

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+---
+layout: news
+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