Fix for PR libobjc/49883 ("clang + gcc 4.6 runtime = broken") and a small related clang fix

["Nicola Pero" <nicola dot pero at meta-innovation dot com>]

This patch fixes PR libobjc/49883.  To fix it, I installed clang and tried out what 
happens if you compile Objective-C code using clang and targetting the GCC runtime.

Unfortunately, the report was correct in that clang is producing incorrect code and
abusing the higher bits of the class->info field to store some other information.  On
the good side, the fix I proposed in the discussion of PR libobjc/49883 actually works. 
:-)

So, I applied that fix.

I also found that clang still emits calls to the objc_lookup_class() function, so this
patch also adds that function back into the runtime to get code compiled with clang
work.

Committed to trunk.

Thanks

PS: In case anyone wonders, I do want the GNU Objective-C Runtime to be usable with
free, non-GCC Objective-C compilers.  It should obviously work perfectly with GCC, the 
GNU compiler, which is its natural partner, but some people would like to use it with
other free compilers and that seems a reasonable request.  Refusing that request just 
provides an incentive to write and support other Objective-C runtimes, which is a 
waste of time and resources. ;-)

Index: init.c
===================================================================
--- init.c      (revision 179711)
+++ init.c      (working copy)
@@ -643,6 +643,15 @@
       assert (CLS_ISMETA (class->class_pointer));
       DEBUG_PRINTF (" installing class '%s'\n", class->name);
 
+      /* Workaround for a bug in clang: Clang may set flags other than
+        _CLS_CLASS and _CLS_META even when compiling for the
+        traditional ABI (version 8), confusing our runtime.  Try to
+        wipe these flags out.  */
+      if (CLS_ISCLASS (class))
+       __CLS_INFO (class) = _CLS_CLASS;
+      else
+       __CLS_INFO (class) = _CLS_META;
+
       /* Initialize the subclass list to be NULL.  In some cases it
         isn't and this crashes the program.  */
       class->subclass_list = NULL;
Index: class.c
===================================================================
--- class.c     (revision 179711)
+++ class.c     (working copy)
@@ -764,6 +764,15 @@
   return objc_get_class (name)->class_pointer;
 }
 
+/* This is not used by GCC, but the clang compiler seems to use it
+   when targetting the GNU runtime.  That's wrong, but we have it to
+   be compatible.  */
+Class
+objc_lookup_class (const char *name)
+{
+  return objc_getClass (name);
+}
+
 /* This is used when the implementation of a method changes.  It goes
    through all classes, looking for the ones that have these methods
    (either method_a or method_b; method_b can be NULL), and reloads
Index: ChangeLog
===================================================================
--- ChangeLog   (revision 179711)
+++ ChangeLog   (working copy)
@@ -1,3 +1,18 @@
+2011-10-09  Nicola Pero  <nicola.pero@meta-innovation.com>
+
+       PR libobjc/49883
+       * init.c (__objc_exec_class): Work around a bug in clang's code
+       generation.  Clang sets the class->info field to values different
+       from 0x1 or 0x2 (the only allowed values in the traditional GNU
+       Objective-C runtime ABI) to store some additional information, but
+       this breaks backwards compatibility.  Wipe out all the bits in the
+       fields other than the first two upon loading a class.
+
+2011-10-09  Nicola Pero  <nicola.pero@meta-innovation.com>
+
+       * class.c (objc_lookup_class): Added back for compatibility with
+       clang which seems to emit calls to it.
+
 2011-10-08  Richard Frith-Macdonald <rfm@gnu.org>
             Nicola Pero  <nicola.pero@meta-innovation.com>