diff --git a/src/hb-atomic-private.hh b/src/hb-atomic-private.hh
index 149600268..61d333654 100644
--- a/src/hb-atomic-private.hh
+++ b/src/hb-atomic-private.hh
@@ -46,21 +46,37 @@
 /* Defined externally, i.e. in config.h; must have typedef'ed hb_atomic_int_impl_t as well. */
 
 
+#elif !defined(HB_NO_MT) && defined(__ATOMIC_ACQUIRE)
+
+/* C++11-style GCC primitives. */
+
+typedef int hb_atomic_int_impl_t;
+#define hb_atomic_int_impl_add(AI, V)		__atomic_fetch_add (&(AI), (V), __ATOMIC_ACQ_REL)
+
+#define hb_atomic_ptr_impl_get(P)		__atomic_load_n ((P), __ATOMIC_ACQUIRE)
+static inline bool
+_hb_atomic_ptr_impl_cmplexch (const void **P, const void *O_, const void *N)
+{
+  const void *O = O_; // Need lvalue
+  return __atomic_compare_exchange_n ((void **) P, (void **) O, (void *) N, true, __ATOMIC_ACQ_REL, __ATOMIC_RELAXED);
+}
+#define hb_atomic_ptr_impl_cmpexch(P,O,N)	(_hb_atomic_ptr_impl_cmplexch ((const void **) (P), (O), (N)))
+
 #elif !defined(HB_NO_MT) && __cplusplus >= 201103L
 
-/* Prefer C++11 atomics. */
+/* C++11 atomics. */
 
 #include <atomic>
 
 typedef int hb_atomic_int_impl_t;
-#define hb_atomic_int_impl_add(AI, V)		(reinterpret_cast<std::atomic<int> *> (&AI)->fetch_add (V, std::memory_order_acq_rel))
+#define hb_atomic_int_impl_add(AI, V)		(reinterpret_cast<std::atomic<int> *> (&AI)->fetch_add ((V), std::memory_order_acq_rel))
 
 #define hb_atomic_ptr_impl_get(P)		(reinterpret_cast<std::atomic<void*> *> (P)->load (std::memory_order_acquire))
 static inline bool
 _hb_atomic_ptr_impl_cmplexch (const void **P, const void *O_, const void *N)
 {
   const void *O = O_; // Need lvalue
-  return reinterpret_cast<std::atomic<const void*> *> (P)->compare_exchange_weak ((O), (N), std::memory_order_acq_rel);
+  return reinterpret_cast<std::atomic<const void*> *> (P)->compare_exchange_weak (O, N, std::memory_order_acq_rel, std::memory_order_relaxed);
 }
 #define hb_atomic_ptr_impl_cmpexch(P,O,N)	(_hb_atomic_ptr_impl_cmplexch ((const void **) (P), (O), (N)))
 
diff --git a/src/hb-object-private.hh b/src/hb-object-private.hh
index fcdc9256d..8199c4b84 100644
--- a/src/hb-object-private.hh
+++ b/src/hb-object-private.hh
@@ -94,7 +94,7 @@ struct hb_user_data_array_t
 struct hb_object_header_t
 {
   hb_reference_count_t ref_count;
-  hb_user_data_array_t *user_data;
+  mutable hb_user_data_array_t *user_data;
 
 #define HB_OBJECT_HEADER_STATIC {HB_REFERENCE_COUNT_INIT, nullptr}