Starting off a simple (the simplest perhaps) C com

Question

I came across this: Writing a compiler using Turbo Pascal

I am curious if there are any tutorials or references explaining how to go about creating a simple C compiler. I mean, it is enough if it gets me to the level of making it understand arithmetic operations. I became really curious after reading this article by Ken Thompson. The idea of writing something that understands itself seems exciting.

Why did I put up this question instead of asking Google? I tried Google and the Pascal one was the first link. The rest did no seem relevant and added to that... I am not a CS major (so I still need to learn what all those tools like yacc do) and I want to learn this by doing and am hoping people with more experience are always better at these things than Google. I want to read some article written in the same spirit as the one I listed above but that which highlights at least the bootstrapping phases of building a simple C compiler.

Also, I don't know the best way to learn. Do I start off building a C compiler in C or some other language? Do I write a C compiler or some other language? I feel questions like this are better answered once I have some direction to explore. Any suggestions?

Any suggestions?

Answer 1

I would not recommend starting with C as the language to implement, nor with any of the compiler-generator or parser-generator tools. C is a very tricky language, and it's probably a better idea to just make up a language of your own. It can be a little C-like (e.g. use curly backets if you want to indicate the function body, use the same type names, so you don't have to remember what you called everything).

The tools for making compilers and parsers are great, but have the problem of really being a shorthand notation. If you don't know how to create a compiler in longhand, the shorthand will seem cryptic, needlessly restrictive etc. So write your own simple compiler first, then continue on from there. I also recommend you don't start generating actual machine code unless you eat and breathe assembler. Create your own bytecode interpreter with a VM.

As to what language you should use to create your first compiler: It doesn't really matter, as long as the language is fairly complete. You will be reading input text, building data structures from them and writing out binary data. So if a language makes those things easier in any way, that's a point in favor of it. Pick a language you know well, so you can focus on creating the compiler, not learning the language. I usually use an OO language, which makes the syntax tree easier to write, a functional language would probably also work if you are familiar with that.

I've blogged a lot about programming languages, so you might find some useful postings here: http://orangejuiceliberationfront.com/category/language-design/

In particular, http://orangejuiceliberationfront.com/how-to-write-a-compiler/ is a starter on the particulars of parsing common constructs and generating something useful from that, as well as http://orangejuiceliberationfront.com/generating-machine-code-at-runtime/ which talks about actually spitting out Intel instructions that do something.

Oh, regarding bootstrapping of a compiler: You probably won't be able to do that right from the start. There is a fair amount of work involved in creating a compiler. So not only would writing a bootstrapping compiler involve writing the compiler (in some other language), once you have it, you would then have to write a second version of the compiler using itself. That's twice the work, plus the debugging needed in the existing and the bootstrapped new compiler until it all works. That said, once you have a working compiler, it is a good way to test its completeness. OK, maybe not twice the work, but more work. I'd go for the easy successes first, then move on from there.

In any event, have fun!

Answer 2

A compiler consists of three pieces:

1. A parser
2. A abstract syntax tree (AST)
3. A code generator

There are lots of nice parser generators that start with language grammars. Maybe ANTLR would be a good place for you to start. If you want to stick to C roots, try lex/yacc or bison.

There are grammars for C, but I think C in its entirety is complex. You'd do well to start off with a subset of the language and work your way up.

Once you have an AST, you use it to generate the machine code that you'll run.

It's doable, but not trivial.

I'd also check Amazon for books about writing compilers. The Dragon Book is the classic, but there are more modern ones available.

UPDATE: There have been similar questions on Stack overflow, like this one. Check out those resources as well.

Answer 3

How do I [start writing] a simple C compiler?

There's nothing simple about compiling C. The best simple C compiler is lcc by Chris Fraser and David Hanson. They spent 10 years working on the design to make it as simple as they possibly could, while still generating reasonably good code. If you have access to a university library, you should be able to get their book.

Do I start off building a C compiler in C or some other language?

Some other language. One time I got to ask Hanson what lessons he and Fraser had learned by spending 10 years on the lcc project. The main thing Hanson said was

C is a lousy language to write a compiler in.

You're better off using Haskell or some dialect of ML. Both languages offer functions over algebraic data types, which is a perfect match to the problems faced by the compiler writer. If you still want to pursue C, you could start with George Necula's CIL, which is a big chunk of a C compiler written in ML.

I want to read some article written in the same spirit as the one I listed above but that which highlights at least the bootstrapping phases...

You won't find another article like Ken's. But Andrew Appel has written a nice article called Axiomatic Bootstrapping: A Guide for Compiler Hackers I couldn't find a free version but many people have access to the ACM Digital Library.

Any suggestions?

If you want to write a compiler,

• Use Haskell or ML as your implementation language.

• For your first compiler, pick a very simple language like Oberon or like P0 from Niklaus Wirth's book Algorithms + Data Structures = Programs. Wirth is famous for designing languages that are easy to compile.

You can write a C compiler for your second compiler.

Answer 4

You might be interested in the book/course The Elements of Computing Systems:Building a Modern Computer from First Principles.

Note that this isn't about building a "pc" from stuff you bought off newegg. It begins with a description of Boolean logic fundamentals, and builds a virtual computer from the lowest levels of abstraction to progressively higher levels of abstraction. The course materials are all online, and the book itself is fairly inexpensive from Amazon.

In the course, in addition to "building the hardware", you'll also implement an assembler, virtual machine, compiler, and rudimentary OS, in a step-wise fashion. I think this would give you enough of a background to delve deeper into the subject area with some of the more commonly recommended resources listed in the other answers.

Answer 5

It might be worthwhile to learn about functional programming, too. Functional languages are well-suited to writing a compiler both in and for. My school's intro compilers class contained an intro to functional languages and the assignments were all in OCaml.

Funny you should ask this today, since just a couple days ago I wrote a lambda calculus interpreter. Lambda calculus is the granddaddy of all functional languages. It's just 200 lines long (in C++, incl. error reporting, some pretty printing, some unicode) and has a two-phase structure, with an intermediate format that could be used to generate code.

Not only is starting small and building up the most practical approach to compilers, it also encourages good, modular, organizational practice.

Answer 6

A compiler is a very large project, although I suppose it wouldn't hurt to try.

I know of at least one C compiler written in Pascal, so it's not the most insane thing you could do. I personally would pick a more modern language in which to implement my C compiler project, both for the simplicity (it's easy to d/l packages for Python, Ruby, C, C++ or Java) and because it will look better on your resume.

In order to do a compiler as a beginner project, though, you will need to drink all of the Agile kool-aid.

Always have something running, even if it doesn't do much of anything. Add things to your compiler only in small steps. ("Frequent releases".) Pick a viciously tiny subset of the language and implement that first. (Support only i = 0; at first and expand things from there.)