Expose VarHandle.releaseFence (or the JDK8 equivalent) in ScalaRuntime

Author: retronym

src/library/scala/runtime/ScalaRunTime.scala

@@ -268,4 +268,9 @@ object ScalaRunTime {
   def wrapShortArray(xs: Array[Short]): ArraySeq[Short] = if (xs ne null) new ArraySeq.ofShort(xs) else null
   def wrapBooleanArray(xs: Array[Boolean]): ArraySeq[Boolean] = if (xs ne null) new ArraySeq.ofBoolean(xs) else null
   def wrapUnitArray(xs: Array[Unit]): ArraySeq[Unit] = if (xs ne null) new ArraySeq.ofUnit(xs) else null
+
+  /**
+    * Forwards to `VarHandle.releaseFence` or `sun.misc.Unsafe.storeFence` on JDK9+ or JDK8 respectively.
+    */
+  @inline def releaseFence(): Unit = VM.RELEASE_FENCE.invoke()
 }

src/library/scala/runtime/VM.java

@@ -0,0 +1,46 @@
+package scala.runtime;
+
+import java.lang.invoke.MethodHandle;
+import java.lang.invoke.MethodHandles;
+import java.lang.invoke.MethodType;
+import java.lang.reflect.Field;
+
+public class VM {
+    public static final MethodHandle RELEASE_FENCE;
+
+
+    static {
+        RELEASE_FENCE = mkHandle();
+    }
+
+    private static MethodHandle mkHandle() {
+        MethodHandles.Lookup lookup = MethodHandles.lookup();
+        try {
+            return lookup.findStatic(Class.forName("java.lang.invoke.VarHandle"), "releaseFence", MethodType.methodType(Void.TYPE));
+        } catch (ClassNotFoundException e) {
+            try {
+                Class<?> unsafeClass = Class.forName("sun.misc.Unsafe");
+                return lookup.findVirtual(unsafeClass, "storeFence", MethodType.methodType(void.class)).bindTo(findUnsafe(unsafeClass));
+            } catch (NoSuchMethodException | ClassNotFoundException | IllegalAccessException e1) {
+                ExceptionInInitializerError error = new ExceptionInInitializerError(e1);
+                error.addSuppressed(e);
+                throw error;
+            }
+        } catch (NoSuchMethodException | IllegalAccessException e) {
+            throw new ExceptionInInitializerError(e);
+        }
+    }
+
+    private static Object findUnsafe(Class<?> unsafeClass) throws IllegalAccessException {
+        Object found = null;
+        for (Field field : unsafeClass.getDeclaredFields()) {
+            if (field.getType() == unsafeClass) {
+                field.setAccessible(true);
+                found = field.get(null);
+                break;
+            }
+        }
+        if (found == null) throw new IllegalStateException("No instance of Unsafe found");
+        return found;
+    }
+}