RECONSIDER: Working around a WinObjC regression that hit after switch…

…ing to the libobjc2 runtime

Clang occasionally emits a negative offest for the first ivar in a subclass. When that occurs, we need to increase the padding between the superclass and the subclass by that amount to prevent ivars from stomping each other.

This is a temporary workaround until we can fix this in clang; not intended for upstream consumption.

ivar.c

@@ -32,7 +32,7 @@ PRIVATE void objc_compute_ivar_offsets(Class class)
 			ivar_start = super->instance_size;
 		}
 		long class_size = 0 - class->instance_size;
-		class->instance_size = ivar_start - class->instance_size;
+
 		/* For each instance variable, we add the offset if required (it will be zero
 		* if this class is compiled with a static ivar layout).  We then set the
 		* value of a global variable to the offset value.  
@@ -56,6 +56,29 @@ PRIVATE void objc_compute_ivar_offsets(Class class)
 				long ivar_size = (i+1 == class->ivars->count)
 					? (class_size - ivar->offset)
 					: ivar->offset - class->ivars->ivar_list[i+1].offset;
+
+				// TODO: Working around a WinObjC regression that hit after switching to the libobjc2 runtime.
+				// Clang occasionally emits a negative offest for the first ivar in a subclass. 
+				// When that occurs, we need to increase the padding between the superclass and the subclass by that amount to prevent ivars from stomping each other.
+				// This is a temporary workaround until we can fix this in clang; not intended for upstream consumption.
+				if (ivar->offset < 0) {
+					if (i != 0) {
+						// We only expect to see negative offsets for the *first* ivar in a class.
+						fprintf(stderr,
+							"Error: Class %hs has an unexpected ivar %hs with a negative offset of %d!\n",
+							class->name,
+							ivar->name,
+							ivar->offset);
+						abort();
+					}
+
+					printf("Warning: Class %hs has first ivar %hs with a negative offset of %d!\n",
+						class->name,
+						ivar->name,
+						ivar->offset);
+					ivar_start += -(ivar->offset);
+				}
+
 #if 0
 				// We only need to do the realignment for things that are
 				// bigger than a pointer, and we don't need to do it in GC mode
@@ -90,6 +113,10 @@ PRIVATE void objc_compute_ivar_offsets(Class class)
 				}
 			}
 		}
+
+		// TODO: Move this back under line 34 once the TODO above is fixed.
+		// Update the instance_size to account for any adjustment to the ivar layout
+		class->instance_size = ivar_start - class->instance_size;
 	}
 	else
 	{