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diff --git a/xpcom/base/nsMemory.h b/xpcom/base/nsMemory.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 nsMemory_h__
+#define nsMemory_h__
+
+#include "nsError.h"
+
+/**
+ *
+ * A client that wishes to be notified of low memory situations (for
+ * example, because the client maintains a large memory cache that
+ * could be released when memory is tight) should register with the
+ * observer service (see nsIObserverService) using the topic
+ * "memory-pressure".  There are specific types of notifications
+ * that can occur.  These types will be passed as the |aData|
+ * parameter of the of the "memory-pressure" notification:
+ *
+ * "low-memory"
+ * This will be passed as the extra data when the pressure
+ * observer is being asked to flush for low-memory conditions.
+ *
+ * "low-memory-ongoing"
+ * This will be passed when we continue to be in a low-memory
+ * condition and we want to flush caches and do other cheap
+ * forms of memory minimization, but heavy handed approaches like
+ * a GC are unlikely to succeed.
+ *
+ * "heap-minimize"
+ * This will be passed as the extra data when the pressure
+ * observer is being asked to flush because of a heap minimize
+ * call.
+ */
+
+// This is implemented in nsMemoryImpl.cpp.
+
+namespace nsMemory {
+
+/**
+ * Attempts to shrink the heap.
+ * @param immediate - if true, heap minimization will occur
+ *   immediately if the call was made on the main thread. If
+ *   false, the flush will be scheduled to happen when the app is
+ *   idle.
+ * @throws NS_ERROR_FAILURE if 'immediate' is set and the call
+ *   was not on the application's main thread.
+ */
+nsresult HeapMinimize(bool aImmediate);
+
+/**
+ * This predicate can be used to determine if the platform is a "low-memory"
+ * platform. Callers may use this to dynamically tune their behaviour
+ * to favour reduced memory usage at the expense of performance. The value
+ * returned by this function will not change over the lifetime of the process.
+ */
+bool IsLowMemoryPlatform();
+}  // namespace nsMemory
+
+#endif  // nsMemory_h__