Implement assume()

Summary: In some cases hinting the compiler about conditions that are guaranteed
to be true can help it generate better code. For instance, remove
unnecessary bound checks. Clang has `__builtin_assume` and on GCC we
can simulate it with `__builtin_unreachable`, so better just abstract
both into a function.

Reviewed By: @philippv

Differential Revision: D2419937

folly/Assume.h

+/*
+ * Copyright 2015 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.
+ */
+
+#ifndef FOLLY_BASE_ASSUME_H_
+#define FOLLY_BASE_ASSUME_H_
+
+#include <folly/Portability.h>
+#include <glog/logging.h>
+
+namespace folly {
+
+/**
+ * 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) {
+#ifndef NDEBUG
+  DCHECK(cond);
+#elif defined(__clang__)  // Must go first because Clang also defines __GNUC__.
+  __builtin_assume(cond);
+#elif defined(__GNUC__)
+  if (!cond) { __builtin_unreachable(); }
+#else
+  // Do nothing.
+#endif
+}
+
+}  // namespace folly
+
+#endif

folly/Makefile.am

@@ -21,6 +21,7 @@ nobase_follyinclude_HEADERS = \
 	ApplyTuple.h \
 	Arena.h \
 	Arena-inl.h \
+	Assume.h \
 	AtomicBitSet.h \
 	AtomicHashArray.h \
 	AtomicHashArray-inl.h \

folly/Portability.h

@@ -116,7 +116,7 @@
 #elif defined(__clang__) || defined(__GNUC__)
 # define FOLLY_ALWAYS_INLINE inline __attribute__((__always_inline__))
 #else
-# define FOLLY_ALWAYS_INLINE
+# define FOLLY_ALWAYS_INLINE inline
 #endif
 
 // detection for 64 bit