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

diff --git a/folly/Assume.h b/folly/Assume.h
new file mode 100644 (file)
index 0000000..1da9bb0
--- /dev/null
+++ b/folly/Assume.h
@@ -0,0 +1,49 @@
+/*
+ * 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

diff --git a/folly/Makefile.am b/folly/Makefile.am
index b7f69dc6ca1000434752f929036dffaa3107b31e..49c22ef6e2481d3e2dc2cc3e016ddd3620c572f3 100644 (file)
--- a/folly/Makefile.am
+++ b/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 \

diff --git a/folly/Portability.h b/folly/Portability.h
index 77baf95ab6cd58583ea013b99944edcde6a008be..b990b302938c138d5b82a34d382c99c2372df119 100644 (file)
--- a/folly/Portability.h
+++ b/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