Added branch predictor hint.

Author: mjp41

src/ds/bits.h

@@ -9,7 +9,11 @@
 #  define ALWAYSINLINE __forceinline
 #  define NOINLINE __declspec(noinline)
 #  define HEADER_GLOBAL __declspec(selectany)
+#  define likely(x)   !!(x)
+#  define unlikely(x) !!(x)
 #else
+#  define likely(x)   __builtin_expect(!!(x), 1)
+#  define unlikely(x) __builtin_expect(!!(x), 0)
 #  include <cpuid.h>
 #  include <emmintrin.h>
 #  define ALWAYSINLINE __attribute__((always_inline))

src/mem/alloc.h

@@ -872,7 +872,7 @@ namespace snmalloc
     inline void handle_message_queue()
     {
       // Inline the empty check, but not necessarily the full queue handling.
-      if (message_queue().is_empty())
+      if (likely(message_queue().is_empty()))
         return;
 
       handle_message_queue_inner();