1 files changed, 77 insertions, 0 deletions
diff --git a/ipc/mscom/SpinEvent.cpp b/ipc/mscom/SpinEvent.cpp
new file mode 100644
index 0000000000..493a1646b7
--- /dev/null
+++ b/ipc/mscom/SpinEvent.cpp
@@ -0,0 +1,77 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "mozilla/mscom/SpinEvent.h"
+
+#include "mozilla/ArrayUtils.h"
+#include "mozilla/Assertions.h"
+#include "mozilla/TimeStamp.h"
+#include "nsServiceManagerUtils.h"
+#include "nsString.h"
+#include "nsSystemInfo.h"
+
+namespace mozilla {
+namespace mscom {
+
+static const TimeDuration kMaxSpinTime = TimeDuration::FromMilliseconds(30);
+bool SpinEvent::sIsMulticore = false;
+
+/* static */
+bool SpinEvent::InitStatics() {
+  SYSTEM_INFO sysInfo;
+  ::GetSystemInfo(&sysInfo);
+  sIsMulticore = sysInfo.dwNumberOfProcessors > 1;
+  return true;
+}
+
+SpinEvent::SpinEvent() : mDone(false) {
+  static const bool gotStatics = InitStatics();
+  MOZ_ALWAYS_TRUE(gotStatics);
+
+  mDoneEvent.own(::CreateEventW(nullptr, FALSE, FALSE, nullptr));
+  MOZ_ASSERT(mDoneEvent);
+}
+
+bool SpinEvent::Wait(HANDLE aTargetThread) {
+  MOZ_ASSERT(aTargetThread);
+  if (!aTargetThread) {
+    return false;
+  }
+
+  if (sIsMulticore) {
+    // Bug 1311834: Spinning allows for faster response than waiting on an
+    // event, as events are constrained by the system's timer resolution.
+    // Bug 1429665: However, we only want to spin for a very short time. If
+    // we're waiting for a while, we don't want to be burning CPU for the
+    // entire time. At that point, a few extra ms isn't going to make much
+    // difference to perceived responsiveness.
+    TimeStamp start(TimeStamp::Now());
+    while (!mDone) {
+      TimeDuration elapsed(TimeStamp::Now() - start);
+      if (elapsed >= kMaxSpinTime) {
+        break;
+      }
+      YieldProcessor();
+    }
+    if (mDone) {
+      return true;
+    }
+  }
+
+  MOZ_ASSERT(mDoneEvent);
+  HANDLE handles[] = {mDoneEvent, aTargetThread};
+  DWORD waitResult = ::WaitForMultipleObjects(mozilla::ArrayLength(handles),
+                                              handles, FALSE, INFINITE);
+  return waitResult == WAIT_OBJECT_0;
+}
+
+void SpinEvent::Signal() {
+  ::SetEvent(mDoneEvent);
+  mDone = true;
+}
+
+}  // namespace mscom
+}  // namespace mozilla