Overview of A Scheme Compiler

Posted on June 7, 2014

For the last few months I’ve been spending a fair amount of time on a fun little Scheme to C compiler, c_of_scheme.

In this post I’ll outline the high level overview of c_of_scheme and in future posts detail the specifics of each component.


c_of_scheme is divided into 11 modules: 2 utility modules, 6 modules which each handle one step of compilation, a module with definitions of ASTs, a driver, and of course Main.

First, let’s discuss the utility modules, Utils.Gen and Utils.Error. Gen defines a monad, Gen. This is used to generate unique integers to be used as identifiers. For example:

    data Var = Name String | Gen Integer

    genVar :: Gen Var
    genVar = Gen <$> gen

Other stages of the compiler (continuation passing style, closure conversion, and lambda lifting) need lots of temporaries so this is used throughout the compiler.

Gen also comes with a monad transformer that implements a handful of useful MTL type classes. Overall, nothing too stunning.

The other uninteresting utility module is Error, this is just a wrapper around Either with a few functions for throwing errors and good pretty printing of errors. This is used internally to signal a major internal error.

The precise interface is given by a set of functions failRW, failCPS, failClos, etc., which correspond to each stage of compilation. These generate lovely pretty printed error messages for each stage. This will become clearer as we go over each phase individually and it’s clear what needs to signal failure.

A module that’s worth mentioning that’s not a compilation stage but not quite a utility module is AST. This defines the various abstract syntax trees and primops for our representation of Scheme. This also defines the compiler monad, which combines our error monad with Gen and some other bits and bobs useful for our compiler. More on AST in future posts.

Stages of Compilation

Now let’s actually go over the individual phases of compilation.

Parsing (Parser.hs)

This is the least interesting phase of compilation.. I personally just dislike parsing so I don’t have much to say about this.

A legal Scheme program is a list of definitions, we don’t currently allow top level expressions. We also don’t currently support the usual define sugar for functions.

The parser uses Parsec because I just happen to know the Parsec API, ironically because of this. If anyone cares enough to write a proper lexer and/or parser or something, I’m more than happy to help!

Rewrite Top Levels (RewriteTopLevel.hs)

This phase is a little peculiar. It exists because we’re targeting C and C has a fairly annoying restriction on what it allows top levels to initialized to.

In C, we can’t write something like

    int c = 1 + 1 + 1;

but in our dialect of Scheme, this is the only way to write interesting computations! This phase of compilation rewrites top levels (Shocking!) to match the C definition of top levels.

This is done by changing each definition to an Init, this will later turn into a C declaration without initialization. Next we create a new function, our main function, that is a series of assignments which pair each top level definition to its initializer.

For example

    (define foo 1)
    (define bar 2)
    (define quux (+ foo bar))

    (define _ (display quux))

will become

    (init foo)
    (init bar)
    (init quux)
    (init _)

    (define magical-main
       (lambda ()
          (set! foo 1)
          (set! bar 1)
          (set! quux (+ foo bar))
          (set! _ (display quux))))

where magical-main will be the first thing called in the generated code.

A caveat, we turn (define foo (lambda (..) ...)) into something different since it’s more efficient to directly convert these to functions.

Continuations Passing Style Conversion (CPS.hs)

This is the first interesting bit of compilation, CPS is a style where each function call is a tail call. Here’s an example non-CPS code converted to CPS.

    (define foo
       (lambda (y)
          (+ 1 y)))

     (define foo-cps
       (lambda (cont x y)
         ((lambda (+')
            ((lambda (one)
               ((lambda (x')
                  ((lambda (result)
                     (cont result))
                   (+' one x')))

Notice how with the CPS’ed version we’ve actually made evaluation order explicit and have removed non-primitive expressions.

CPS.hs converts the AST to use CPS. We’ll detail this process later but for now I’ll mention one more interesting tidbit.

CPS.hs is also where we implement call/cc! In fact it’s trivial to do. All we do as add the declaration for

    (define call/cc
       (lambda (c f)
          (f c
             (lambda (ignored x) (c x)))))

Optimizations (OptimizeCPS.hs)

This module implements the simple optimizations we perform. For now this is limited to simple inlining and constant folding, but this should improve in the future.

These optimizations are implemented quite pleasantly with recursion schemes.

Closure Conversion + Lambda Lifting (ClosureConvert.hs)

This is the most difficult phase of compilation, for me anyways. In concept it’s quite simple though.

The idea is that we take the implicit closure “argument” that all scheme procedures take and make it explicit. To this end we add three new primops, NewClos, ReadClos, and WriteClos. These do much what you would expect and let us treat closures opaquely as first class values.

Next we change each procedure to take an extra argument, its closure, and change closed over variables to be selected from this closure. Finally we change each lambda to be paired with its closure when constructed.

This sounded pretty feasible to me on paper, but in practice it seems to be the greatest source of bugs in c_of_scheme. It finally seems to work nicely now so I’ll be sure to blog about it soon.

Code Generation (CodeGen.hs)

This is the final stop in our compilation pipeline - we generate C code.

To do this we use one of my libraries. This is actually quite a simple step in the compiler since closure-converted, CPS-ed code is quite close to C.

Some of the details that code generation handles:

While this might sound daunting, this isn’t actually so bad.

Driver (Driver.hs)

While I might not write a post on it, Driver is my personal favorite module. It glues together all of the previous compilation phases and provides a bunch of nice high level functions like compileScheme.

The reason I like it so much is that all the code in it is a very nice, clean example of composing components as good old functions.

If you’re looking to understand c_of_scheme’s particular implementation, I’d urge you to start with Driver. It’ll provide a bit of an intuition from what goes to where.

The Runtime System

Currently c_of_scheme has an incredibly naive runtime system. Mostly because it’s being written by an incredibly naive C programmer (hi!).

I already wrote about the most interesting bit of the RTS: tail calls.

I plan on talking a bit about the RTS in the context of code generation (since it’d be impossible not to), and perhaps a post on c_of_scheme’s simple little mark and sweep GC.

Wrap Up

So that’s the high level overview of c_of_scheme, I think the compiler is best exemplified by one particular function in Driver.hs:

    compileScheme :: [SDec UserPrim] -> Compiler [CExtDecl]
    compileScheme = addPrimops >=> makeMain >=> cpsify >=> optimizeCPS >=> closConvert >=> codegen
      where addPrimops = return . (++prims)

This chains together all the phases of compilation into one big old function from the Scheme AST to the C one.

Now, if you’re really interested in c_of_scheme, go ahead and grab the source with

hg clone ssh://hg@bitbucket.org/jozefg/c_of_scheme

I do use mercurial so you can also grab a zip from bitbucket if you’re unwilling to use mercurial for one command :)

I should have posts about each specific phase of compilation up in Real Soon Now. I’ll edit with a list of links to posts below as they are written.

Thanks to @tylerholien for proofreading

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