Re: GNUstep base almost builds with clang

[David Chisnall]

On 1 Apr 2009, at 01:51, Pete French wrote:

> > Apple has moved away from GCC so you can no longer depend on them.
>
> This I did not know. Interesting. I assumed Xcode was still using
> gcc.

XCode currently ships with gcc and llvm-gcc.  Only llvm-gcc is  
supported for iPhone developement (if you take a look at the LLVM  
commit logs, you'll notice a massive amount of ARM-backend code was  
committed the day the first iPhone developer kits were released).

llvm-gcc is a hybrid compiler, which uses GCC to parse, then  
translates GIMPLE (the GCC intermediate representation) into LLVM IR  
and then uses LLVM for optimisation and code generation.  Because it  
includes parts of GCC, it is GPL'd.

Unfortunately for us, llvm-gcc uses Apple's branch of GCC, which is  
GPLv2, and (more importantly) only supports the Apple runtime.  You  
can use it with to compile C and C++ code on non-Apple platforms, but  
you can't use it with Objective-C, so GNUstep can't use it.

Clang is a new front-end for LLVM, written completely from scratch,  
which is more-or-less feature complete for C99 and Objective-C 2  
(parsing anyway - code generation is only finished for the Apple  
runtimes), and now working towards C++ support.

My understanding is that Apple intends to drop llvm-gcc once clang is  
a drop-in replacement (it almost is now for C and Objective-C on the  
Apple runtimes, but it's still probably about a year away from being  
ready for C++ / ObjC++).

There are a few other reasons why clang might interest the GNUstep  
community:

- It contains a static analyser, which can find a lot of different  
types of Objective-C bug.

- It is built from several libraries, which means that you can easily  
reuse the parser in other programs, for example to implement code  
completion and syntax highlighting, or to write a version of autogsdoc  
that is aware of macros.

- One of the demos is a rewriter, which translates ObjC code into pure  
C code (for the Apple runtime).  Modifying this to use the GNU runtime  
calls would give a mechanism for compiling GNUstep on any platform  
with a working C compiler.  Alternatively, LLVM has a C back end which  
can generate (completely unreadable, but semantically valid) C code  
from the IR.

If you want an overview of LLVM, you can read this article:

http://www.informit.com/articles/article.aspx?p=1215438

David