+/*
+ * Copyright 2017 Facebook, Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#pragma once
+
+#include <cstdlib>
+
+#include <folly/Portability.h>
+
+namespace folly {
+
+namespace detail {
+
+extern void assume_check(bool cond);
+
+} // namespace detail
+
+/**
+ * Inform the compiler that the argument can be assumed true. It is
+ * undefined behavior if the argument is not actually true, so use
+ * with care.
+ *
+ * Implemented as a function instead of a macro because
+ * __builtin_assume does not evaluate its argument at runtime, so it
+ * cannot be used with expressions that have side-effects.
+ */
+
+FOLLY_ALWAYS_INLINE void assume(bool cond) {
+ if (kIsDebug) {
+ detail::assume_check(cond);
+ } else {
+#if defined(__clang__) // Must go first because Clang also defines __GNUC__.
+ __builtin_assume(cond);
+#elif defined(__GNUC__)
+ if (!cond) { __builtin_unreachable(); }
+#elif defined(_MSC_VER)
+ __assume(cond);
+#else
+ // Do nothing.
+#endif
+ }
+}
+
+[[noreturn]] FOLLY_ALWAYS_INLINE void assume_unreachable() {
+ assume(false);
+ // Do a bit more to get the compiler to understand
+ // that this function really will never return.
+#if defined(__GNUC__)
+ __builtin_unreachable();
+#elif defined(_MSC_VER)
+ __assume(0);
+#else
+ // Well, it's better than nothing.
+ std::abort();
+#endif
+}
+
+} // namespace folly