SERVER-18219 hint to GCC that WCE loops will actually return

3 files changed, 18 insertions, 0 deletions

diff --git a/src/mongo/platform/compiler.h b/src/mongo/platform/compiler.h
index f4868acf432..d261a1781e3 100644
--- a/src/mongo/platform/compiler.h
+++ b/src/mongo/platform/compiler.h
@@ -118,6 +118,20 @@
  *    shared libraries.
  *
  *    Same correct/incorrect usage as for MONGO_COMPILER_API_EXPORT.
+ *
+ * MONGO_COMPILER_UNREACHABLE
+ *
+ *   Instructs the compiler that a line of code will never be reached.
+ *
+ *
+ *   Example:
+ *       // code above checks that expr can only be FOO or BAR
+ *       switch (expr) {
+ *       case FOO: { ... }
+ *       case BAR: { ... }
+ *       default:
+ *           MONGO_COMPILER_UNREACHABLE;
+ *
  */
 
 #if defined(_MSC_VER)
diff --git a/src/mongo/platform/compiler_gcc.h b/src/mongo/platform/compiler_gcc.h
index 431465893c9..553f599b335 100644
--- a/src/mongo/platform/compiler_gcc.h
+++ b/src/mongo/platform/compiler_gcc.h
@@ -64,3 +64,5 @@
 
 #define MONGO_likely(x) static_cast<bool>(__builtin_expect(static_cast<bool>(x), 1))
 #define MONGO_unlikely(x) static_cast<bool>(__builtin_expect(static_cast<bool>(x), 0))
+
+#define MONGO_COMPILER_UNREACHABLE __builtin_unreachable()
diff --git a/src/mongo/platform/compiler_msvc.h b/src/mongo/platform/compiler_msvc.h
index fb4ab560f24..4a029fe40ed 100644
--- a/src/mongo/platform/compiler_msvc.h
+++ b/src/mongo/platform/compiler_msvc.h
@@ -56,3 +56,5 @@
 
 #define MONGO_likely(x) bool(x)
 #define MONGO_unlikely(x) bool(x)
+
+#define MONGO_COMPILER_UNREACHABLE __assume(0)