[cfe-dev] Objective-C Code Generation

Chris Lattner clattner at apple.com
Sat Feb 23 11:06:40 PST 2008

On Feb 23, 2008, at 9:55 AM, David Chisnall wrote:
> I'm interested in working on Objective-C code generation for the GNU
> and √Čtoil√© Objective-C runtimes (I've worked on the former and wrote
> the latter).


> I imagine the easiest way of getting this working is to
> transform the Objective-C AST into a pure-C AST with calls to the
> relevant runtime libraries.

This is the way that GCC works, but I don't think it is really the  
best way.  I'd much rather have the clang codegen module directly  
produce the LLVM IR for the constructs it needs.  I don't anticipate  
that this will be a problem for metadata or other objc constructs.

This does require an understanding of the LLVM IR, but it is well  
documented, and there are lots of examples. If you get stuck, please  
ask on this list, we'd be happy to help.

> Presumably someone at Apple will want to add support for their runtime
> libraries as well at some point, so having a clean interface to allow
> easily switching between the three is going to be important (rather
> than the 10,000 lines of runtime-specific, unreadable, code in GCC).

Yes, absolutely :)

> Are there any existing hooks for inserting this abstraction layer?  If
> so, can someone point me in the right direction, and if not can
> someone suggest a good place to put them?

At this point, I'd suggest starting with the simple constructs (e.g.  
the stand alone objc expressions like @"foo", add pointers to  
interfaces, etc) and then move on the metadata.  If you do each patch  
cleanly and incrementally I don't expect a big problem.  When we start  
on codegen support for the next runtime, we can generalize the code  
and figure out what abstractions are best on a case-by-case basis.


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