common: hint the main branch of dout() accordingly to default verbosity.

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

src/common/likely.h

@@ -21,4 +21,6 @@
 #define likely(x)       __builtin_expect((x),1)
 #define unlikely(x)     __builtin_expect((x),0)
 
+#define expect(x, hint) __builtin_expect((x),(hint))
+
 #endif

src/common/subsys_types.h

@@ -53,6 +53,12 @@ ceph_subsys_get_as_array() {
 #undef DEFAULT_SUBSYS
 }
 
+constexpr static std::uint8_t
+ceph_subsys_get_max_default_level(const std::size_t subidx) {
+  const auto item = ceph_subsys_get_as_array()[subidx];
+  return std::max(item.log_level, item.gather_level);
+}
+
 // Compile time-capable version of std::strlen. Resorting to own
 // implementation only because C++17 doesn't mandate constexpr
 // on the standard one.

src/log/SubsystemMap.h

@@ -8,6 +8,7 @@
 #include <vector>
 #include <algorithm>
 
+#include "common/likely.h"
 #include "common/subsys_types.h"
 
 #include "include/assert.h"
@@ -74,11 +75,14 @@ class SubsystemMap {
 
     if constexpr (LvlV <= 0) {
       // handle the -1 and 0 levels entirely at compile-time.
-      // Such debugs are intended be gathered regardless even
+      // Such debugs are intended to be gathered regardless even
       // of the user configuration.
       return true;
     } else {
-      return LvlV <= static_cast<int>(m_gather_levels[SubV]);
+      // we expect that setting level different than the default
+      // is rather unusual.
+      return expect(LvlV <= static_cast<int>(m_gather_levels[SubV]),
+		    LvlV <= ceph_subsys_get_max_default_level(SubV));;
     }
   }
   bool should_gather(const unsigned sub, int level) {