Nicolas Petton

Nicolas Petton

Web developer, Lisper, Smalltalker & Emacs maniac.

Amber's new compiler

Amber's new compiler

October 18, 2013

Amber's compiler has been improved some months ago and a post explaining its new architecture was a long overdue.

Following Opal (the new Pharo compiler) architecture, the compiler has been entirely rewritten from a single, complex and stateful AST visitor into several smaller, single-purpose classes, making it a more flexible and extensible compiler.

The new architecture

The AST produced by the parser is first annotated and validated by the SemanticAnalyzer. The annotation step is important as it adds semantic knowledge to the AST (scope, variable semantics, etc.). This way we first make sure that the produced AST is both valid syntactically and semantically, and keep semantic information directly in nodes.

   | ast ir output |

   ast := Smalltalk current parse: 'foo true ifTrue: [ ^ self asString ]'.
   (SemanticAnalyzer on: Object) visit: ast.

The second step is to produce an intermediate representation (IR) through an IRASTTranslator. The IR tree is similar to the AST but is simpler and flatten. It doesn't includes cascades for instance, and cut down most of the tree into a flat list of instructions.

   ir := IRASTTranslator new visit: ast.

From the IR it's a lot easier to perform efficient inlinings. This optional step is performed by the IRInliner visitor. The IRInlner replaces parts of the tree, with the inlined equivalent. You can find the currently inlined send nodes with IRSendInliner class >> inlinedSelectors.

The final step is then to produce the JavaScript output. This is the role of the IRJSTranslator. The compiler can then install the method in the appropriate class.

   output := IRJSTranslator new
        currentClass: Object;
        visit: ir;

The following diagram shows the current architecture.


The public API

The Compiler facade hides all the internals away and provides the public API to compile and install methods in Amber. It internally delegates compiling to an instance of CodeGenerator, or its subclass InliningCodeGenerator (the default), that will perform the optional inlining step.

   "Compiling without installing"
   Compiler new
        codeGeneratorClass: InliningCodeGenerator; "Default, can be omitted"
        compile: 'foo true ifTrue: [ ^ self asString ]'
        forClass: Object.

   "Compiling and installing"
   Compiler new
        codeGeneratorClass: InliningCodeGenerator; "Default, can be omitted"
        install: 'foo true ifTrue: [ ^ self asString ]'
        forClass: Object
        category: 'foo'

The end

That's all for now. The new compiler is a very important brick for Amber. There is still room for improvement, as the generated code could sometimes be simpler, but it is overall a great improvement over the old compiler. Together with the new ASTInterpreter, it will allow fancy new features in Amber, like the new stepping debugger.

I plan to write another post with more details about the inlining steps. Stay tuned!

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