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diff --git a/mfbt/Likely.h b/mfbt/Likely.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/. */
+
+/*
+ * MOZ_LIKELY and MOZ_UNLIKELY macros to hint to the compiler how a
+ * boolean predicate should be branch-predicted.
+ */
+
+#ifndef mozilla_Likely_h
+#define mozilla_Likely_h
+
+#if defined(__clang__) || defined(__GNUC__)
+# define MOZ_LIKELY(x) (__builtin_expect(!!(x), 1))
+# define MOZ_UNLIKELY(x) (__builtin_expect(!!(x), 0))
+#else
+# define MOZ_LIKELY(x) (!!(x))
+# define MOZ_UNLIKELY(x) (!!(x))
+#endif
+
+#endif /* mozilla_Likely_h */