perf: optimize negative time_task

src/library/time_task.cpp

@@ -43,17 +43,21 @@ void finalize_time_task() {
 
 time_task::time_task(std::string const & category, options const & opts, name decl) :
         m_category(category) {
-    if (get_profiler(opts)) {
+    if (g_current_time_task && !m_category.size()) {
+        // exclude given block from surrounding task
+        // avoid somewhat costly accesses on `opts`
+        m_timeit = optional<xtimeit>([](second_duration _) {});
+        m_parent_task = g_current_time_task;
+        g_current_time_task = this;
+    } else if (get_profiler(opts)) {
         m_timeit = optional<xtimeit>(get_profiling_threshold(opts), [=](second_duration duration) mutable {
-            if (m_category.size()) {
-                sstream ss;
-                ss << m_category;
-                if (decl)
-                    ss << " of " << decl;
-                ss << " took " << display_profiling_time{duration} << "\n";
-                // output atomically, like IO.print
-                tout() << ss.str();
-            }
+            sstream ss;
+            ss << m_category;
+            if (decl)
+                ss << " of " << decl;
+            ss << " took " << display_profiling_time{duration} << "\n";
+            // output atomically, like IO.print
+            tout() << ss.str();
         });
         m_parent_task = g_current_time_task;
         g_current_time_task = this;