GNUstep Objective-C v2 ABI update

[David Chisnall]

Hello the list,

It seems that Christmas is about the only time I get to work on 
Objective-C things these days.  I started the new ABI project two 
Christmases ago and this Christmas I've managed to update all of the 
FreeBSD GNUstep ports to use it.  As far as I can tell, everything is 
working (I've tried running a few GUI apps and everything seemed happy).

A few highlights of the new ABI:

- A new constant string representation, including space to store a hash 
and better unicode support.  This should improve performance for 
anything that uses constant strings as dictionary keys.  The compiler 
will now emit tiny strings (up to 8 ASCII characters) embedded in the 
pointer.

- Better introspection metadata.  We now have full introspection 
metadata for properties (including class properties and properties 
declared in categories) and the extended type encodings that 
JavaScriptCore (WebKit) needs for the JavaScript bridge.

- Significantly reduced redundancy for selectors and protocols.  The 
linker can now deduplicate these within a library, so we get (for 
example) one copy of the +alloc selector per library, not one per 
compilation unit.  This means that we have significantly smaller (5-10%) 
binary sizes, even with the richer reflection metadata.

- Protocols and classes are referenced via an indirection layer, so a 
future version of the ABI can be backwards compatible.

- @private and @package are now enforced by the linker.  You cannot 
refer to a @package or @private ivar from a different library.  This 
means that, aside from reflection, @package and @private ivars are not 
part of the public ABI and can be removed without breaking any code that 
does not use reflection.

The Windows version still has a few rough corners to iron out.  On 
Windows, a DLL cannot contain a global initialised to be a pointer to a 
global in another library.  This is normally not a bad thing, because it 
enforces modularity, but it does mean that Objective-C classes, for 
example, can't be statically initialised to point to their superclasses 
(though we can emit code that does this).  Dustin has some patches for 
clang that should fix these issues.

I've pushed patches to -base to support the new string representation 
and to Gorm to make it use reflection instead of directly accessing 
ivars in -gui classes.  I also have a patch to Pantomime that prevents 
it from doing the same.

David