diff --git a/docs/Runtime.md b/docs/Runtime.md
index 7514ee4495590..040fb366521c1 100644
--- a/docs/Runtime.md
+++ b/docs/Runtime.md
@@ -340,8 +340,8 @@ runtime.
 0000000000003f50 T _swift_isClassOrObjCExistentialType
 0000000000004130 T _swift_isOptionalType
 00000000000279f0 T _swift_objc_class_usesNativeSwiftReferenceCounting
-000000000002b340 T _swift_objc_class_unknownGetInstancePositiveExtent
-000000000002b350 T _swift_class_getInstancePositiveExtent
+000000000002b340 T _swift_objc_class_unknownGetInstanceExtents
+000000000002b350 T _swift_class_getInstanceExtents
 0000000000004080 T _swift_class_getSuperclass
 ```
 
diff --git a/stdlib/public/SwiftShims/RuntimeShims.h b/stdlib/public/SwiftShims/RuntimeShims.h
index d2bb3aefb06bb..62fa77e0bc525 100644
--- a/stdlib/public/SwiftShims/RuntimeShims.h
+++ b/stdlib/public/SwiftShims/RuntimeShims.h
@@ -29,8 +29,6 @@ namespace swift { extern "C" {
 
 bool swift_objc_class_usesNativeSwiftReferenceCounting(const void *);
 
-__swift_size_t swift_objc_class_unknownGetInstancePositiveExtent(const void *);
-
 /// Return an NSString to be used as the Mirror summary of the object
 void *_swift_objCMirrorSummary(const void * nsObject);
 
diff --git a/stdlib/public/core/Builtin.swift b/stdlib/public/core/Builtin.swift
index e9bde133867d7..0e77e9348b6db 100644
--- a/stdlib/public/core/Builtin.swift
+++ b/stdlib/public/core/Builtin.swift
@@ -327,18 +327,23 @@ internal func _usesNativeSwiftReferenceCounting(theClass: AnyClass) -> Bool {
 }
 
 @warn_unused_result
-@_silgen_name("swift_class_getInstancePositiveExtent")
-func swift_class_getInstancePositiveExtent(theClass: AnyClass) -> UInt
+@_silgen_name("swift_class_getInstanceExtents")
+func swift_class_getInstanceExtents(theClass: AnyClass)
+  -> (negative: UInt, positive: UInt)
 
-/// - Returns: `class_getInstanceSize(theClass)`.
+@warn_unused_result
+@_silgen_name("swift_objc_class_unknownGetInstanceExtents")
+func swift_objc_class_unknownGetInstanceExtents(theClass: AnyClass)
+  -> (negative: UInt, positive: UInt)
+
+/// - Returns: 
 @inline(__always)
 @warn_unused_result
 internal func _class_getInstancePositiveExtentSize(theClass: AnyClass) -> Int {
 #if _runtime(_ObjC)
-  return Int(swift_objc_class_unknownGetInstancePositiveExtent(
-      unsafeAddressOf(theClass)))
+  return Int(swift_objc_class_unknownGetInstanceExtents(theClass).positive)
 #else
-  return Int(swift_class_getInstancePositiveExtent(theClass))
+  return Int(swift_class_getInstanceExtents(theClass).positive)
 #endif
 }
 
diff --git a/stdlib/public/runtime/SwiftObject.mm b/stdlib/public/runtime/SwiftObject.mm
index 6f0889023f8b4..023ed39120a08 100644
--- a/stdlib/public/runtime/SwiftObject.mm
+++ b/stdlib/public/runtime/SwiftObject.mm
@@ -1345,22 +1345,31 @@ static bool usesNativeSwiftReferenceCounting_nonNull(
   return object->refCount.isUniquelyReferencedOrPinned();
 }
 
-#if SWIFT_OBJC_INTEROP
-/// Returns class_getInstanceSize(c)
-///
-/// That function is otherwise unavailable to the core stdlib.
-size_t swift::swift_objc_class_unknownGetInstancePositiveExtent(const void* c) {
-  return class_getInstanceSize((Class)c);
-}
-#endif
+using ClassExtents = TwoWordPair<size_t, size_t>;
 
-extern "C" size_t swift_class_getInstancePositiveExtent(
-    const Metadata *c) {
+extern "C"
+ClassExtents::Return
+swift_class_getInstanceExtents(const Metadata *c) {
   assert(c && c->isClassObject());
   auto metaData = c->getClassObject();
-  return metaData->getInstanceSize() - metaData->getInstanceAddressPoint();
+  return ClassExtents{
+    metaData->getInstanceAddressPoint(),
+    metaData->getInstanceSize() - metaData->getInstanceAddressPoint()
+  };
 }
 
+#if SWIFT_OBJC_INTEROP
+extern "C"
+ClassExtents::Return
+swift_objc_class_unknownGetInstanceExtents(const ClassMetadata* c) {
+  // Pure ObjC classes never have negative extents.
+  if (c->isPureObjC())
+    return ClassExtents{0, class_getInstanceSize((Class)c)};
+  
+  return swift_class_getInstanceExtents(c);
+}
+#endif
+
 const ClassMetadata *swift::getRootSuperclass() {
 #if SWIFT_OBJC_INTEROP
   static Lazy<const ClassMetadata *> SwiftObjectClass;