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by pansa2 192 days ago
> Jack Crenshaw's tutorial takes the syntax-directed translation approach, where code is emitted while parsing, without having to divide the compiler into explicit phases with IRs.

Is "syntax-directed translation" just another term for a single-pass compiler, e.g. as used by Lua (albeit to generate bytecode instead of assembly / machine code)? Or is it something more specific?

> in the latter parts of the tutorial it starts showing its limitations. Especially once we get to types [...] it's easy to generate working code; it's just not easy to generate optimal code

So, using a single-pass compiler for a statically-typed language makes it difficult to apply type-based optimizations. (Of course, Lua sidesteps this problem because the language is dynamically typed.)

Are there any other downsides? Does single-pass compilation also restrict the level of type checking that can be performed?
2 comments
dist1ll 192 days ago
As long as your target language has a strict define-before-use rule and no advanced inference is required you will know the types of expressions, and can perform type-based optimizations. You can also do constant folding and (very rudimentary) inlining. But the best optimizations are done on IRs, which you don't have access to in an old-school single pass design. LICM, CSE, GVN, DCE, and all the countless loop opts are not available to you. You'll also spill to memory a lot, because you can't run a decent regalloc in a single pass.

I'm actually a big fan a function-by-function dual-pass compilation. You generate IR from the parser in one pass, and do codegen right after. Most intermediate state is thrown out (including the AST, for non-polymorphic functions) and you move on to the next function. This give you an extremely fast data-oriented baseline compiler with reasonable codegen (much better than something like tcc).

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pjmlp 192 days ago
It is more specific, it means emiting code as you go along throught the source file.

A sigle pass compiler can still split the various phases, and only do the code generation on the last phase.

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