Mark constructing an Unexpected as cold

[folly.git] / folly / CppAttributes.h

diff --git a/folly/CppAttributes.h b/folly/CppAttributes.h
index 4a001946a3cc27220880654a6940abc98bff253a..466aab91a81144527e968415207f666674d52d46 100644 (file)
--- a/folly/CppAttributes.h
+++ b/folly/CppAttributes.h
@@ -1,5 +1,5 @@
 /*
- * Copyright 2015 Facebook, Inc.
+ * 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.
@@ -20,8 +20,13 @@
  * @author Dominik Gabi
  */
 
-#ifndef FOLLY_BASE_ATTRIBUTES_H_
-#define FOLLY_BASE_ATTRIBUTES_H_
+#pragma once
+
+#ifndef __has_attribute
+#define FOLLY_HAS_ATTRIBUTE(x) 0
+#else
+#define FOLLY_HAS_ATTRIBUTE(x) __has_attribute(x)
+#endif
 
 #ifndef __has_cpp_attribute
 #define FOLLY_HAS_CPP_ATTRIBUTE(x) 0
@@ -29,6 +34,12 @@
 #define FOLLY_HAS_CPP_ATTRIBUTE(x) __has_cpp_attribute(x)
 #endif
 
+#ifndef __has_extension
+#define FOLLY_HAS_EXTENSION(x) 0
+#else
+#define FOLLY_HAS_EXTENSION(x) __has_extension(x)
+#endif
+
 /**
  * Fallthrough to indicate that `break` was left out on purpose in a switch
  * statement, e.g.
@@ -48,4 +59,59 @@
 #define FOLLY_FALLTHROUGH
 #endif
 
-#endif /* FOLLY_BASE_ATTRIBUTES_H_ */
+/**
+ *  Maybe_unused indicates that a function, variable or parameter might or
+ *  might not be used, e.g.
+ *
+ *  int foo(FOLLY_MAYBE_UNUSED int x) {
+ *    #ifdef USE_X
+ *      return x;
+ *    #else
+ *      return 0;
+ *    #endif
+ *  }
+ */
+#if FOLLY_HAS_CPP_ATTRIBUTE(maybe_unused)
+#define FOLLY_MAYBE_UNUSED [[maybe_unused]]
+#elif FOLLY_HAS_ATTRIBUTE(__unused__) || __GNUC__
+#define FOLLY_MAYBE_UNUSED __attribute__((__unused__))
+#else
+#define FOLLY_MAYBE_UNUSED
+#endif
+
+/**
+ * Nullable indicates that a return value or a parameter may be a `nullptr`,
+ * e.g.
+ *
+ * int* FOLLY_NULLABLE foo(int* a, int* FOLLY_NULLABLE b) {
+ *   if (*a > 0) {  // safe dereference
+ *     return nullptr;
+ *   }
+ *   if (*b < 0) {  // unsafe dereference
+ *     return *a;
+ *   }
+ *   if (b != nullptr && *b == 1) {  // safe checked dereference
+ *     return new int(1);
+ *   }
+ *   return nullptr;
+ * }
+ */
+#if FOLLY_HAS_EXTENSION(nullability)
+#define FOLLY_NULLABLE _Nullable
+#define FOLLY_NONNULL _Nonnull
+#else
+#define FOLLY_NULLABLE
+#define FOLLY_NONNULL
+#endif
+
+/**
+ * "Cold" indicates to the compiler that a function is only expected to be
+ * called from unlikely code paths. It can affect decisions made by the
+ * optimizer both when processing the function body and when analyzing
+ * call-sites.
+ */
+#if __GNUC__
+#define FOLLY_COLD __attribute__((__cold__))
+#else
+#define FOLLY_COLD
+#endif