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diff --git a/xpcom/threads/nsMemoryPressure.h b/xpcom/threads/nsMemoryPressure.h
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+/* -*- 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/. */
+
+#ifndef nsMemoryPressure_h__
+#define nsMemoryPressure_h__
+
+#include "nscore.h"
+
+/*
+ * These pre-defined strings are the topic to pass to the observer
+ * service to declare the memory-pressure or lift the memory-pressure.
+ *
+ * 1. Notify kTopicMemoryPressure with kSubTopicLowMemoryNew
+ * New memory pressure deteced
+ * On a new memory pressure, we stop everything to start cleaning
+ * aggresively the memory used, in order to free as much memory as
+ * possible.
+ *
+ * 2. Notify kTopicMemoryPressure with kSubTopicLowMemoryOngoing
+ * Repeated memory pressure.
+ * A repeated memory pressure implies to clean softly recent allocations.
+ * It is supposed to happen after a new memory pressure which already
+ * cleaned aggressivley.  So there is no need to damage the reactivity of
+ * Gecko by stopping the world again.
+ *
+ * In case of conflict with an new memory pressue, the new memory pressure
+ * takes precedence over an ongoing memory pressure.  The reason being
+ * that if no events are processed between 2 notifications (new followed
+ * by ongoing, or ongoing followed by a new) we want to be as aggresive as
+ * possible on the clean-up of the memory.  After all, we are trying to
+ * keep Gecko alive as long as possible.
+ *
+ * 3. Notify kTopicMemoryPressureStop with nullptr
+ * Memory pressure stopped.
+ * We're no longer under acute memory pressure, so we might want to have a
+ * chance of (cautiously) re-enabling some things we previously turned off.
+ * As above, an already enqueued new memory pressure event takes precedence.
+ * The priority ordering between concurrent attempts to queue both stopped
+ * and ongoing memory pressure is currently not defined.
+ */
+extern const char* const kTopicMemoryPressure;
+extern const char* const kTopicMemoryPressureStop;
+extern const char16_t* const kSubTopicLowMemoryNew;
+extern const char16_t* const kSubTopicLowMemoryOngoing;
+
+enum class MemoryPressureState : uint32_t {
+  None,  // For internal use.  Don't use this value.
+  LowMemory,
+  NoPressure,
+};
+
+/**
+ * This function causes the main thread to fire a memory pressure event
+ * before processing the next event, but if there are no events pending in
+ * the main thread's event queue, the memory pressure event would not be
+ * dispatched until one is enqueued. It is infallible and does not allocate
+ * any memory.
+ *
+ * You may call this function from any thread.
+ */
+void NS_NotifyOfEventualMemoryPressure(MemoryPressureState aState);
+
+/**
+ * This function causes the main thread to fire a memory pressure event
+ * before processing the next event. We wake up the main thread by adding a
+ * dummy event to its event loop, so, unlike with
+ * NS_NotifyOfEventualMemoryPressure, this memory-pressure event is always
+ * fired relatively quickly, even if the event loop is otherwise empty.
+ *
+ * You may call this function from any thread.
+ */
+nsresult NS_NotifyOfMemoryPressure(MemoryPressureState aState);
+
+#endif  // nsMemoryPressure_h__