diff --git a/dtool/src/dtoolbase/atomicAdjust.h b/dtool/src/dtoolbase/atomicAdjust.h
index 6ce85c3589..9dc7832e46 100644
--- a/dtool/src/dtoolbase/atomicAdjust.h
+++ b/dtool/src/dtoolbase/atomicAdjust.h
@@ -39,14 +39,19 @@ typedef AtomicAdjustI386Impl AtomicAdjust;
 #define HAVE_ATOMIC_COMPARE_AND_EXCHANGE 1
 #define HAVE_ATOMIC_COMPARE_AND_EXCHANGE_PTR 1
 
-#elif defined(__GNUC__) && (__GNUC__ > 4 || (__GNUC__ == 4 && __GNUC_MINOR__ >= 7))
+#elif (defined(__GNUC__) && (__GNUC__ > 4 || (__GNUC__ == 4 && __GNUC_MINOR__ >= 7))) || (defined(__clang__) && (__clang_major__ >= 3))
 // GCC 4.7 and above has built-in __atomic functions for atomic operations.
+// Clang 3.0 and above also supports them.
 
 #include "atomicAdjustGccImpl.h"
 typedef AtomicAdjustGccImpl AtomicAdjust;
 
+#if (__GCC_ATOMIC_INT_LOCK_FREE + __GCC_ATOMIC_INT_LOCK_FREE) > 0
 #define HAVE_ATOMIC_COMPARE_AND_EXCHANGE 1
+#endif
+#if __GCC_ATOMIC_POINTER_LOCK_FREE > 0
 #define HAVE_ATOMIC_COMPARE_AND_EXCHANGE_PTR 1
+#endif
 
 #elif defined(THREAD_WIN32_IMPL)
 
diff --git a/dtool/src/dtoolbase/atomicAdjustGccImpl.I b/dtool/src/dtoolbase/atomicAdjustGccImpl.I
index 6831a16a1e..3e462f01cf 100644
--- a/dtool/src/dtoolbase/atomicAdjustGccImpl.I
+++ b/dtool/src/dtoolbase/atomicAdjustGccImpl.I
@@ -125,7 +125,7 @@ compare_and_exchange(TVOLATILE AtomicAdjustGccImpl::Integer &mem,
                      AtomicAdjustGccImpl::Integer old_value,
                      AtomicAdjustGccImpl::Integer new_value) {
 
-  __atomic_compare_exchange_n(&mem, &old_value, new_value, false,
+  __atomic_compare_exchange_n(&mem, &old_value, new_value, true,
                               __ATOMIC_SEQ_CST, __ATOMIC_SEQ_CST);
   return old_value;
 }
@@ -142,7 +142,7 @@ compare_and_exchange_ptr(TVOLATILE AtomicAdjustGccImpl::Pointer &mem,
                          AtomicAdjustGccImpl::Pointer old_value,
                          AtomicAdjustGccImpl::Pointer new_value) {
 
-  __atomic_compare_exchange_n(&mem, &old_value, new_value, false,
+  __atomic_compare_exchange_n(&mem, &old_value, new_value, true,
                               __ATOMIC_SEQ_CST, __ATOMIC_SEQ_CST);
   return old_value;
 }
diff --git a/dtool/src/dtoolbase/atomicAdjustGccImpl.h b/dtool/src/dtoolbase/atomicAdjustGccImpl.h
index 2c8003948d..2059addbbe 100644
--- a/dtool/src/dtoolbase/atomicAdjustGccImpl.h
+++ b/dtool/src/dtoolbase/atomicAdjustGccImpl.h
@@ -18,7 +18,7 @@
 #include "dtoolbase.h"
 #include "selectThreadImpl.h"
 
-#if defined(__GNUC__) && (__GNUC__ > 4 || (__GNUC__ == 4 && __GNUC_MINOR__ >= 7))
+#if (defined(__GNUC__) && (__GNUC__ > 4 || (__GNUC__ == 4 && __GNUC_MINOR__ >= 7))) || (defined(__clang__) && (__clang_major__ >= 3))
 
 ////////////////////////////////////////////////////////////////////
 //       Class : AtomicAdjustGccImpl