Fixed ivar offset checking to take account for alignment.

init.c

@@ -756,38 +756,51 @@ __objc_compute_ivar_offsets (Class class)
     }
   else
     {
-      if (class->ivars != 0)
+      if (NULL == class->ivars)
         {
-          Class super = class_superclass_of_class(class);
-          int start = class->ivars->ivar_list[0].ivar_offset;
-          /* Quick and dirty test */
-          if (Nil != super && start != super->instance_size)
-            {
-              while (NULL == super->ivars) 
-                {
-                  super = class_superclass_of_class(super);
-                }
-              struct objc_ivar *ivar = 
-                &super->ivars->ivar_list[super->ivars->ivar_count-1];
-              if (start != (ivar->ivar_offset + objc_sizeof_type(ivar->ivar_type)))
-                {
-                  /* Panic if this class has an instance variable that overlaps the
-                   * superclass. */
-                  fprintf(stderr, 
-                     "Error: Instance variables in %s overlap superclass %s",
-                     class->name, super->name);
-                  fprintf(stderr, 
-                     "Offset of first instance variable, %s, is %d",
-                     class->ivars->ivar_list[0].ivar_name, start);
-                  fprintf(stderr, 
-                     "Last instance variable in superclass, %s, ends at offset %d",
-                     ivar->ivar_name, 
-                     ivar->ivar_offset + objc_sizeof_type(ivar->ivar_type));
-                  fprintf(stderr, "This probably means that you are subclassing a class from a library, which has changed in a binary-incompatible way.\n");
-                  abort();
-                }
-            }
+          return;
+        }
+      Class super = class_superclass_of_class(class);
+      int start = class->ivars->ivar_list[0].ivar_offset;
+      /* Quick and dirty test.  If the first ivar comes straight after the last
+       * class, then it's fine. */
+      if (Nil == super || start == super->instance_size)
+        {
+          return;
+        }
+      /* Find the last superclass with at least one ivar. */
+      while (NULL == super->ivars) 
+        {
+          super = class_superclass_of_class(super);
+        }
+      struct objc_ivar *ivar = 
+        &super->ivars->ivar_list[super->ivars->ivar_count-1];
+      if (start == (ivar->ivar_offset + objc_sizeof_type(ivar->ivar_type)))
+        {
+          return;
+        }
+      /* The classes don't line up, but don't panic; check that the
+       * difference is not just padding for alignment */
+      int align = objc_alignof_type(class->ivars->ivar_list[0].ivar_type);
+      if (start > ivar->ivar_offset && 
+          start - ivar->ivar_offset < align)
+        {
+          return;
         }
+      /* Panic if this class has an instance variable that overlaps the
+       * superclass. */
+      fprintf(stderr, 
+         "Error: Instance variables in %s overlap superclass %s",
+         class->name, super->name);
+      fprintf(stderr, 
+         "Offset of first instance variable, %s, is %d",
+         class->ivars->ivar_list[0].ivar_name, start);
+      fprintf(stderr, 
+         "Last instance variable in superclass, %s, ends at offset %d",
+         ivar->ivar_name, 
+         ivar->ivar_offset + objc_sizeof_type(ivar->ivar_type));
+      fprintf(stderr, "This probably means that you are subclassing a class from a library, which has changed in a binary-incompatible way.\n");
+      abort();
     }
 }