Move folly/Assume.h

[folly.git] / folly / lang / Assume.h

diff --git a/folly/lang/Assume.h b/folly/lang/Assume.h
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+/*
+ * 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