RFC: Non-fragile ivar implementation

[David Chisnall]

Hi Everyone,

I now have clang correctly implementing all of Objective-C 2 on the  
GNU runtime except for:

- Property introspection (properties work, but you can not introspect  
them)

- Garbage collection (simple to do, but requires something similar to  
Autozone to be available)

- Non-fragile instance variables.

The last of these is the topic of this email.  I've attached patches  
to both clang and libobjc for your inspection.  The goals of this  
implementation are:

- The ability to link classes that use static ivar layouts and non- 
fragile layouts, with non-fragile classes subclassing fragile classes  
and not having problems if the fragile classes are modified (i.e. not  
breaking the ABI, not requiring existing frameworks to be recompiled).

- The ability to add and rearrange ivars in a class without breaking  
linked modules (i.e. non-fragile ivars must actually work).

- Only a small run time penalty (i.e. not making it horribly slow).

The proposed approach makes a small change to the data that the  
compiler fills in for the objc_class structure.  If the instance_size  
is negative, then this class is compiled with non-fragile ivars.  The  
ivar_offset fields are then taken as being relative to the start of  
the ivars declared in this class, rather than relative to the start of  
all objects.

Every access to an ivar will emit code that adds  
__objc_ivar_offset_{class name}.{ivar_name} to the object address.

When loading a module, the Objective-C runtime will update the  
ivar_list structure to the correct values and set the  
__objc_ivar_offset variables correctly.

To test this, you will need to add -fpic -fobjc-nonfragile-abi to the  
flags you pass to clang (-fpic is irritating; you don't need position- 
independent code, you just need the dlsym to have symbol information  
which, for me, only happens with PIC).

Pending comments from here, I will push these patches up to libobjc  
and clang, respectively.

David

P.S. If anyone knows of a way of building GNU libobjc without building  
all of GCC, please let me know...
P.P.S. The version in GNUstep svn is far too old to be useful in this  
case.

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