[LLVMdev] Running Objective-C in the JIT

[Jean-Daniel Dupas]

Le 7 mai 2012 à 17:07, Jules a écrit :

> Hello, I've been trying to get some OSX code to execute within the JIT,
> and it's been causing me some major headaches!
> 
> I'm attempting to JIT-compile some code which uses external OSX obj-C 
> classes (Cocoa, etc), and also contains its own embedded obj-C classes.
> 
> My first hurdle in doing this was that when the code tried to call Cocoa
> classes, the obj-C selectors weren't being recognised by the host 
> process's objc_msgSend function.. After quite a bit of hair-pulling, I 
> copied a trick from the lldb::IRForTarget class, which scans the compiled
> module and replaces any selector constants with explicit function calls 
> to sel_registerName().. By applying a similar transformation to my own 
> modules, I've now managed to successfully invoke selectors on Cocoa classes.
> 
> But now I've hit a brick wall trying to use internal obj-C classes (i.e. 
> classes that are defined within the JIT-ed code itself). With these, even
> trying to alloc them causes a crash. Does anyone have enough of a grasp of
> what's going on in the JIT and obj-C ABI to be able to give me some 
> pointers here!?
> 
> Thanks!    


Obj-C classes are registered by the runtime at module load time.
When a module is loaded, the Obj-C runtime create classes and register them by parsing the module Obj-C data segments.

My guess is that as you are using JIT, the runtime hook used to detect loading of new module is never called, and your classes are not registered.
You can try to load them manually by using the runtime functions (see https://developer.apple.com/library/mac/#documentation/Cocoa/Reference/ObjCRuntimeRef/Reference/reference.html ) and if it's not enough, you may have to download the runtime source code (http://opensource.apple.com/source/objc4/objc4-493.11/) to see what it does when a new module is loaded.


-- Jean-Daniel