--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/libphobos/libdruntime/rt/critical_.d	Thu Feb 13 11:34:05 2020 +0900
@@ -0,0 +1,68 @@
+/**
+ * Implementation of support routines for synchronized blocks.
+ *
+ * Copyright: Copyright Digital Mars 2000 - 2011.
+ * License:   $(WEB www.boost.org/LICENSE_1_0.txt, Boost License 1.0).
+ * Authors:   Walter Bright, Sean Kelly
+ */
+
+/*          Copyright Digital Mars 2000 - 2011.
+ * Distributed under the Boost Software License, Version 1.0.
+ *    (See accompanying file LICENSE or copy at
+ *          http://www.boost.org/LICENSE_1_0.txt)
+ */
+module rt.critical_;
+
+nothrow:
+
+import rt.monitor_, core.atomic;
+
+extern (C) void _d_critical_init()
+{
+    initMutex(cast(Mutex*)&gcs.mtx);
+    head = &gcs;
+}
+
+extern (C) void _d_critical_term()
+{
+    for (auto p = head; p; p = p.next)
+        destroyMutex(cast(Mutex*)&p.mtx);
+}
+
+extern (C) void _d_criticalenter(D_CRITICAL_SECTION* cs)
+{
+    ensureMutex(cast(shared(D_CRITICAL_SECTION*)) cs);
+    lockMutex(&cs.mtx);
+}
+
+extern (C) void _d_criticalexit(D_CRITICAL_SECTION* cs)
+{
+    unlockMutex(&cs.mtx);
+}
+
+private:
+
+shared D_CRITICAL_SECTION* head;
+shared D_CRITICAL_SECTION gcs;
+
+struct D_CRITICAL_SECTION
+{
+    D_CRITICAL_SECTION* next;
+    Mutex mtx;
+}
+
+void ensureMutex(shared(D_CRITICAL_SECTION)* cs)
+{
+    if (atomicLoad!(MemoryOrder.acq)(cs.next) is null)
+    {
+        lockMutex(cast(Mutex*)&gcs.mtx);
+        if (atomicLoad!(MemoryOrder.raw)(cs.next) is null)
+        {
+            initMutex(cast(Mutex*)&cs.mtx);
+            auto ohead = head;
+            head = cs;
+            atomicStore!(MemoryOrder.rel)(cs.next, ohead);
+        }
+        unlockMutex(cast(Mutex*)&gcs.mtx);
+    }
+}