I would really like to write the handlers in terms of C++11 atomics. This would give us the best performance, portablity, and safety tradeoff. Unfortunately I can not yet do that. So I've put the desired code in comments, and reverted the handler getters to the slower but safer legacy atomic intrinsics.

llvm-svn: 153041

libcxxabi/src/cxa_default_handlers.cpp

@@ -94,6 +94,10 @@ static void default_unexpected_handler()
 std::terminate_handler  __cxa_terminate_handler = default_terminate_handler;
 std::unexpected_handler __cxa_unexpected_handler = default_unexpected_handler;
 
+// In the future these will become:
+// std::atomic<std::terminate_handler>  __cxa_terminate_handler(default_terminate_handler);
+// std::atomic<std::unexpected_handler> __cxa_unexpected_handler(default_unexpected_handler);
+
 namespace std
 {
 
@@ -103,6 +107,8 @@ set_unexpected(unexpected_handler func) _NOEXCEPT
 	if (func == 0)
 		func = default_unexpected_handler;
 	return __sync_swap(&__cxa_unexpected_handler, func);
+//  Using of C++11 atomics this should be rewritten
+//  return __cxa_unexpected_handler.exchange(func, memory_order_acq_rel);
 }
 
 terminate_handler
@@ -111,6 +117,8 @@ set_terminate(terminate_handler func) _NOEXCEPT
 	if (func == 0)
 		func = default_terminate_handler;
 	return __sync_swap(&__cxa_terminate_handler, func);
+//  Using of C++11 atomics this should be rewritten
+//  return __cxa_terminate_handler.exchange(func, memory_order_acq_rel);
 }
 
-};
+}

libcxxabi/src/cxa_handlers.cpp

@@ -25,7 +25,10 @@ namespace std
 unexpected_handler
 get_unexpected() _NOEXCEPT
 {
-    return __cxa_unexpected_handler;
+    return __sync_fetch_and_add(&__cxa_unexpected_handler, (unexpected_handler)0);
+//  The above is safe but overkill on x86
+//  Using of C++11 atomics this should be rewritten
+//  return __cxa_unexpected_handler.load(memory_order_acq);
 }
 
 __attribute__((visibility("hidden"), noreturn))
@@ -47,7 +50,10 @@ unexpected()
 terminate_handler
 get_terminate() _NOEXCEPT
 {
-    return __cxa_terminate_handler;
+    return __sync_fetch_and_add(&__cxa_terminate_handler, (terminate_handler)0);
+//  The above is safe but overkill on x86
+//  Using of C++11 atomics this should be rewritten
+//  return __cxa_terminate_handler.load(memory_order_acq);
 }
 
 __attribute__((visibility("hidden"), noreturn))
@@ -96,17 +102,24 @@ terminate() _NOEXCEPT
 }
 
 new_handler __cxa_new_handler = 0;
+// In the future these will become:
+// std::atomic<std::new_handler>  __cxa_new_handler(0);
 
 new_handler
 set_new_handler(new_handler handler) _NOEXCEPT
 {
     return __sync_swap(&__cxa_new_handler, handler);
+//  Using of C++11 atomics this should be rewritten
+//  return __cxa_new_handler.exchange(handler, memory_order_acq_rel);
 }
 
 new_handler
 get_new_handler() _NOEXCEPT
 {
-    return __cxa_new_handler;
+    return __sync_fetch_and_add(&__cxa_new_handler, (new_handler)0);
+//  The above is safe but overkill on x86
+//  Using of C++11 atomics this should be rewritten
+//  return __cxa_new_handler.load(memory_order_acq);
 }
 
 }  // std

libcxxabi/src/cxa_handlers.hpp

@@ -35,6 +35,20 @@ extern void (*__cxa_terminate_handler)();
 extern void (*__cxa_unexpected_handler)();
 extern void (*__cxa_new_handler)();
 
+/*
+
+    At some point in the future these three symbols will become
+    C++11 atomic variables:
+
+    extern std::atomic<std::terminate_handler>  __cxa_terminate_handler;
+    extern std::atomic<std::unexpected_handler> __cxa_unexpected_handler;
+    extern std::atomic<std::new_handler>        __cxa_new_handler;
+
+    This change will not impact their ABI.  But it will allow for a
+    portible performance optimization.
+
+*/
+
 } // extern "C"
 
 #endif  // _CXA_HANDLERS_H