Running Objective-C in the JIT

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!?


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 ) and if it's not enough, you may have to download the runtime source code ( to see what it does when a new module is loaded.

-- Jean-Daniel

Thanks for the info!

Yes, the problem is certainly that the JITed code isn't registering
its classes, but even after digging through all the runtime code I
can't find anything that seems suitable for doing this.. The nearest
function I could find was _objc_init_image, but that seems to be for

I assume that the way it works must be that clang creates some static
data structures defining all the class's methods and ivars, and then
these data structures are picked up automatically (somehow) by the
runtime, which parses them and dynamically registers the classes

So the worst-case here seems to be that I might need to write my own
utility to do this parsing/registering.. But it'd be nice to at least
be able to see some code from the runtime that does this job, so I can
understand what's going on, but I can't find anything like that!

The runtime sources contains all the code to load a Mach-O binary.

I think the main function is
PRIVATE_EXTERN void _read_images(header_info **hList, uint32_t hCount)

And also, the hook to load/unload image is installed in objc-os.m using dyld_register_image_state_change_handler() function.

Excellent, thanks very much for your help, I'll keep digging away. I
think I'm starting to understand how it all fits together...