Xcode 3.2.1 GCC CLANG and LLVM demystification

Question

The readme included with the new Xcode 3.2.1 this week says the following:

• Static code analysis is fully integrated within the Xcode IDE via the Build and Analyze option under the Build menu or via custom build settings
• GCC 4.2 is the default system compiler for the 10.6 SDK
• The optional LLVM compiler is included using two different front ends - the Clang compiler is a leading-edge parser that offers dramatically improved compile times. For maximum compatibility, the GCC LLVM compiler utilizes the LLVM back-end with the GCC 4.2 parser.
• New optional Clang-LLVM 1.0 compiler uses the much faster Clang front-end parser coupled with the LLVM back-end compiler for fast compiles and fast executable code. Many projects will benefit from this compiler combination, although GCC 4.2 is still the system default. The Clang-LLVM 1.0 compiler will fall back to using LLVM-GCC 4.2 when it encounters C++ code.

Our company has existing projects that are pure C, Objective-C, and Objective-C++ for desktop and iphone. Can someone summarize at a high-level the differences between LLVM, GCC, CLANG, CLANG-LLVM, WordFoo et. al. and explain what they are and when we should be using each and for what? It would be nice to have links to more a detailed explanation, but I'm really just looking for a high-level overview.

Answer 1

In a nutshell:

Compilers are basically split into two parts. One being the front-end that contains the parser and semantic analysis for the programming language. The front-end produces some kind of intermediate representation of your code. Then there's the backend which takes the stuff the front-end produced, optimizes it, and eventually generates assembly code.

• GCC: well known compiler, contains both front-ends for various languages and back-ends for many processor architectures
• LLVM: a set of back-ends for various architectures (and other low-level stuff)
• clang: a new front-end for C, Objective-C, and C++; uses the LLVM back-ends. You'll get more readable errors and warnings from your compiler and shorter compile times. You might also encounter incompatibilities or bugs; clang is a very young project.
• LLVM-GCC: GCC's front-end with LLVM's back-end. LLVM's back-end is faster than GCC's.

clang's (Objective-)C++ support is far from being complete so it calls llvm-gcc when it encounters a C++ source file. It also contains the static analyzer that is now integrated into Xcode. Some people say LLVM's back-end generates better code than GCC's but your mileage may vary. LLVM also supports link-time optimizations (which you can enable in Xcode's project settings). They may produce faster code.

Apple wants to replace GCC with clang in the future because they have a policy against GPLv3 licensed code (GCC 4.2 is the last version that's licensed under GPLv2).