core: hint the dout()'s message crafting as a cold code.

The idea is to:
  1. Do not put the dout()'s crafting stuff on the hot, fall-through
     path. Cheapest branches are those that are forward and never taken.
  2. Move it to separated sections placed far away from the main path
     to be more friendly to ICache and ITLB. That is, dout_impl constructs
     a function now.

Signed-off-by: Radoslaw Zarzynski <rzarzyns@redhat.com>

src/common/compiler_extensions.h

@@ -22,9 +22,15 @@
 #ifdef __GNUC__
 // GCC
 #define WARN_UNUSED_RESULT __attribute__((warn_unused_result))
+
+#define HINT_COLD_CODE __attribute__((cold))
+#define HINT_NO_INLINE __attribute__((noinline))
 #else
 // some other compiler - just make it a no-op
 #define WARN_UNUSED_RESULT
+
+#define HINT_COLD_CODE
+#define HINT_NO_INLINE
 #endif
 
 #endif

src/common/dout.h

@@ -20,6 +20,7 @@
 
 #include "global/global_context.h"
 #include "common/config.h"
+#include "common/compiler_extensions.h"
 #include "common/likely.h"
 #include "common/Clock.h"
 #include "log/Log.h"
@@ -78,7 +79,8 @@ struct is_dynamic<dynamic_marker_t<T>> : public std::true_type {};
     }									\
   }(cct);								\
 									\
-  if (should_gather) {							\
+  if (unlikely(should_gather)) {					\
+    [&]() HINT_NO_INLINE HINT_COLD_CODE {				\
     static size_t _log_exp_length = 80; 				\
     ceph::logging::Entry *_dout_e = 					\
       cct->_log->create_entry(v, sub, &_log_exp_length);		\
@@ -108,7 +110,8 @@ struct is_dynamic<dynamic_marker_t<T>> : public std::true_type {};
 // /usr/include/assert.h clobbers our fancier version.
 #define dendl_impl std::flush;				\
   _ASSERT_H->_log->submit_entry(_dout_e);		\
-    }						\
+    }();						\
+    }							\
   } while (0)
 
 #define dendl dendl_impl

src/common/likely.h

@@ -18,7 +18,7 @@
 /*
  * Likely / Unlikely macros
  */
-#define likely(x)       __builtin_expect((x),1)
-#define unlikely(x)     __builtin_expect((x),0)
+#define likely(x)       __builtin_expect((const bool)(x),1)
+#define unlikely(x)     __builtin_expect((const bool)(x),0)
 
 #endif