# dietC: a C backend for chibicc

What: desugar C code

Why: easier program analysis and transformation

How: fork of Rui Ueyama's great chibicc compiler, replacing the x86 assembly
backend with a C backend

Quick Example:

  $ cat tests/super_simple.c
  int main(void) {
    int x = 1;
    int y = 2;
    return y;
  }
  $ make
  ...
  $ ./dietc tests/super_simple.c
  #include "/home/matthew/repos/dietc/scripts/dietc_helpers.h"
  typedef int Type_1 ;
  ...
  extern Type_5 main ;
  ...
  Type_1 main ( ) {
          Type_1 t1 ;
          ...
          t3 = 1 ;
          * t5 = t3 ;
          ...
          return t1 ;
  }

Selling points:

  - Guaranteed space-separated tokens
  - Extremely restricted grammar, super easy to parse; see docs/LANGUAGE.txt
  - Maintains all type information (except qualifiers like const)
  - Comes with a wrapper to replace GCC in existing build scripts; see
    docs/DIETCC.txt

Known to be unsupported:

- dietC is ... optimistic about identifier conflicts with its generated
  labels. It likely will not work if you run it on its own output. This is very
  fixable, will do soon.
- chibicc does not have good support for long double; specifically, long
  doubles cannot be used in constant expressions. This is fixable, but requires
  non-trivial modifications to the chibicc side of the code.
- chibicc does not parse const, volatile, etc. type qualifiers. it would not be
  too hard to support this if desired.

Why not cilly?

  - cilly performs both not enough and too much lowering.
  - it often compiles a struct field acces a->foo into a literal offset like
    *(a + 16).
  - even after simplification it has multiple control flow constructs: loop,
    etc.
  - it's written in OCAML, and hard to parse the result outside of OCAML

Why not C--, assembly, QBE?

  - all these are strictly lower level than cilly. They preserve almost no type
    information at all.

Why not LLVM?

  - LLVM has breaking changes approximately every 5ns and preservation of
    high-level type information is not guaranteed.
  - mostly forced to use LLVM's C++ interfaces and then compile with LLVM

License:
  - the underlying chibicc code is licensed under the MIT license
  - my modifications are licensed under the AGPLv3