[svn-r7372] add likely/unlikely macros to mpiutil.h

On Blue Gene (and some other platforms) the compiler occasionally generates
suboptimal code if it guesses the likelihood of a branch incorrectly.
These macros allow us to help the compiler out in cases that it guesses
poorly.

No reviewer.

src/include/mpiutil.h

@@ -172,4 +172,27 @@ int MPIR_Assert_fail_fmt(const char *cond, const char *file_name, int line_num,
 #define MPIU_UNIQUE_IMPL2_(prefix_,line_) MPIU_UNIQUE_IMPL3_(prefix_,line_)
 #define MPIU_UNIQUE_IMPL3_(prefix_,line_) prefix_##line_
 
+/* These likely/unlikely macros provide static branch prediction hints to the
+ * compiler, if such hints are available.  Simply wrap the relevant expression in
+ * the macro, like this:
+ *
+ * if (unlikely(ptr == NULL)) {
+ *     // ... some unlikely code path ...
+ * }
+ *
+ * They should be used sparingly, especially in upper-level code.  It's easy to
+ * incorrectly estimate branching likelihood, while the compiler can often do a
+ * decent job if left to its own devices.
+ *
+ * These macros are not namespaced because the namespacing is cumbersome.
+ */
+/* safety guard for now, add a configure check in the future */
+#if defined(__GNUC__) && (__GNUC__ >= 3)
+#  define unlikely(x_) __builtin_expect(!!(x_),0)
+#  define likely(x_)   __builtin_expect(!!(x_),1)
+#else
+#  define unlikely(x_) (x_)
+#  define likely(x_)   (x_)
+#endif
+
 #endif /* !defined(MPIUTIL_H_INCLUDED) */