This section describes how the C2 compiler (c2c) is structured.
Each component will be described in detail below. The colors mean:
- green: used unmodified
- light blue: not used
- blue: new components
- orange: modified from the original
The LLVM Project provides great building blocks for compilers. Just what we need!
IR (Intermediate Representation) is roughly an assembly-like language and is used as input for LLVM. On the IR code, various optimizations can be applied and code can be generated for various targets (like ARM, PowerPC, X86, etc.). Additionally the LLVM code holds much utility functionality.
For C2 we use the LLVM unmodified!
Clang is a C/C++/Objective-C compiler that uses LLVM as its back-end. Very briefly, it works like this:
Clang has a Lexer that provides symbols to the Parser. The Parser then calls the Sema (Semantic Analyser) to do analysis and build up the AST (abstract syntax tree). The CodeGen components take the AST and convert it into IR code for LLVM. For this, there are various Util libraries and a Diagostics component to display beautiful error messages.
Both LLVM and Clang are compiled into roughly 50 static libraries that can be linked into other applications.
C2C does not use Clang's AST and Parser/Sema. This would have provided a lot of functionality out of the box. However, since C2 does not have forward declarations and does not require any ordering on file level, a multi-pass analysis is required. This does not fit into Clang's infrastructure all, since it is based on C-like languages, where can be just a single pass compilation.
C2 has it's own Parser that uses Clang's Lexer. The Parser builds up the AST via a very small Sema (Semantic Analyser). Not much analysis can be done after the parsing phase, since all .c2 files need to be parsed before proper analysis can start.
After parsing all files, the Analyser will start its multi-pass analysis over all files. Since C2C has the full build-scope, it can provide very detailed diagnostics, like which fields on a struct are unused, etc
C2C has two back-ends; one that generates Ansi-C code and a second one that generates LLVM's IR code.