Comment by RossBencina

2 days ago

> lexer + parser + treewalk interpreter

If all you do is construct an AST and interpret it it's not really a compiler is it. It doesn't compile from source to target. At most you're describing a compiler front end (arguably), or an interpreter.

I would expect:

lexer + parser + AST definition and construction + semantic analysis/type checking + X + codegen to ASM/WASM/C

where X includes

  - definition of an intermediate representation (IR)
  - lowering AST to IR
  - static analysis
  - improvers/optimisation passes (at least some simple stuff)
  - code gen including register allocation

This can still be a simple "complicated" program, but there's more to a compiler than an AST interpreter.

EDIT: I notice that the author of the original article has also started work on an optimizing compiler: https://ssloy.github.io/tinyoptimizer/

8 comments

RossBencina

sieve 2 days ago

> If all you do is construct an AST and interpret it it's not really a compiler is it.

What if I dumped the AST to disk, called it "bytecode" and updated the "interpreter" to run this "bytecode"? Would you consider that to be a compiler? :-)

> where X includes

I don't consider any of these things to be essential. There are entire languages out there which run on interpreters of some kind. There are others which compile down to C/JS. Many compilers use LLVM as their backend. By your definition, none of them use (or are) compilers.

RossBencina 2 days ago
> Would you consider that to be a compiler? :-)
:) No. Under my definition there needs to be some non-trivial transformation into or out of an intermediate and/or target representation (either syntax-directed directly out of the parser, or from a materialized AST). Personally I would argue that even if you trivially "compiled" to sequential bytecode, if the bytecode is then interpreted it is hard to argue that you have created a compiler (the pre-JIT cpython interpreter is still an interpreter, even though it includes a bytecode representation). But I can see that this point can be argued, and historically a school exercise of writing a Pascal-to-pcode converter would be called a compiler, so sure, you can take that perspective if you like. Just don't get confused about whether you are learning to build a compiler (definition 1) or a compiler (definition 2).
- sieve 2 days ago
  
  I have written all kinds of compilers, interpreters and vms over the last 15 years. A couple of them for work. The rest for fun. Some vms used RC, others did mark-and-sweep GC. Some vms even did JIT. A lot of them were simple treewalk interpreters because the point was to play with the syntax of the language.
  Based on that experience, I would say your definition is more academic than realworldly as javac (or any compiler targeting the JVM) is not a real compiler according to it.
  
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