53 lines
2.7 KiB
Markdown
53 lines
2.7 KiB
Markdown
---
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layout: news
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author: Torsten
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---
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I am not stuck. I know i'm not. Just because there is little visible progress doesn't mean i'm stuck. It may just feel like it though.
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But like little cogweels in the clock, i can hear the background process ticking away and sometime there is a gong.
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What i wasn't stuck with is where to draw the layer for the vm.
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### Layers
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Software engineers like layers. Like the onion boy. You can draw boxes, make presentation and convince your boss.
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They help us to reason about the software.
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In this case the model was to go from ast layer to a vm layer. Via a compile method, that could just as well have been a
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visitor.
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That didn't work, too big astep and so it was from ast, to vm, to neumann. But i couldn't decide on the abstraction of the
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virtual machine layer. Specifically, when you have a send (and you have soo many sends in ruby), do you:
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- model it as a vm instruction (a bit like java)
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- implement it in a couple instructions like resolve, a loop and call
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- go to a version that is clearly translatable to neumann, say without the value type implementation
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Obviously the third is where we need to get to, as the next step is the neumann layer and somewhow we need to get there.
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In effect one could take those three and present them as layers, not as alternatives like i have.
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### Passes
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And then the little cob went click, and the idea of passes resurfaced. LLvm has these passes on the code tree, is probably
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where it surfaced from.
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So we can have as high of a degree of abstraction as possible when going from ast to code. And then have as many passes
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over that as we want / need.
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Passes can be order dependend, and create more an more datail. To solve the above layer conundrum, we just do a pass for each
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of those options.
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The two main benefits that come from this are:
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1 - At each point, ie after and during each pass we can analyse the data. Imagine for example that we would have picked the
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second layer option, that means there would never have been a representation where the sends would have been explicit. Thus
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any analasis of them would be impossible or need reverse engineering (eg call graph analysis)
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2 - Passes can be gems or come from other sources. The mechanism can be relatively oblivious to explicit passes. And they
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make the transformation explicit, ie easier to understand. In the example of having picked the second layer level, one
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would have to patch the implementation of that transformation to achieve a different result. With pases it would be a matter
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of replacing a pass, thus explicitly stating "i want a non-standard send implementation"
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Actually a third benefit is that it makes testing simpler. More modular. Just test the initial ast->code and then mostly
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the results of passes. |