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