[LLVMdev] RFC: Patch

[Bill Wendling]

On Sep 22, 2007, at 10:57 AM, Chris Lattner wrote:
> On Sep 21, 2007, at 5:11 PM, Bill Wendling wrote:
>> My question is, is this liable to break something else down the line?
>> Should it be modified to call the getNamedGlobal method only if it's
>> an Objective-C property? Is this even the correct method for an
>> Objective-C property?
>
> There is a bigger question here.  One invariant that is useful is
> that there is only a single decl that corresponds to a given global
> variable.  It sounds like the objc front-end is making two GCC decl
> nodes for the same global variable?  If so, is there a way to fix the
> objc front-end to merge them?
>
According to Fariborz, this is the correct behavior. This second time  
through, it seems as if it's simply trying to do a lookup of the  
(static) global variable "Foo". In essence, it's equivalent to this  
code:

	static struct S {
		int i;
	} Foo; // (1)

	static struct S *Ref = &Foo; // (2)

When I run this through, we only get the "Foo" at (2) going through  
the make_decl_llvm function and not the first one. I think that this  
is because there isn't any metadata that needs to be attached to the  
Foo object.

The code in objc/objc-act.c is acting like this: in finish_objc(), it  
has:

   if (protocol_chain)
     {
       if (flag_objc_abi == 2)
         {
           generate_v2_protocols ();    /* (1) */
         }
       else
         generate_protocols ();
     }
   /* APPLE LOCAL begin radar 4533974 - ObjC new protocol */
   if (protocol_list_chain)
     build_protocol_list_address_table ();
   if (protocollist_ref_chain)
     build_protocollist_translation_table (); /* (2) */

The call at (1) creates the metadata. It calls finish_var_decl(),  
which creates the first decl. The call at (2) builds the reference  
and also calls finish_var_decl(), which tries to do the lookup, but  
fails because the first decl was marked "internal global". So it  
tries to create a new decl, and kablooey. ;-)

The amount that I don't understand the obj-c code can fill  
volumes. :-) We could ask Fariborz if merging them is possible. My  
hunch is "no", but I could be wrong.

-bw