refactor(logger): make timed_loggers more efficient if unused

include/dwarfs/logger.h

@@ -22,9 +22,7 @@
 #pragma once
 
 #include <atomic>
-#include <chrono>
 #include <cstddef>
-#include <ctime>
 #include <iosfwd>
 #include <memory>
 #include <mutex>
@@ -37,10 +35,7 @@
 #include <type_traits>
 #include <utility>
 
-#include <boost/chrono/thread_clock.hpp>
-
 #include <dwarfs/detail/logging_class_factory.h>
-#include <dwarfs/util.h>
 
 namespace dwarfs {
 
@@ -162,53 +157,26 @@ class level_log_entry {
 
 class timed_level_log_entry {
  public:
-  using thread_clock = boost::chrono::thread_clock;
-
   timed_level_log_entry(logger& lgr, logger::level_type level,
-                        std::source_location loc, bool with_cpu = false)
-      : lgr_(lgr)
-      , level_(level)
-      , start_time_(std::chrono::high_resolution_clock::now())
-      , with_cpu_(with_cpu)
-      , loc_(loc) {
-    if (with_cpu) {
-      cpu_start_time_ = thread_clock::now();
-    }
-  }
-
+                        std::source_location loc, bool with_cpu = false);
   timed_level_log_entry(timed_level_log_entry const&) = delete;
-
-  ~timed_level_log_entry() {
-    if (output_) {
-      std::chrono::duration<double> sec =
-          std::chrono::high_resolution_clock::now() - start_time_;
-      oss_ << " [" << time_with_unit(sec.count());
-      if (with_cpu_) {
-        boost::chrono::duration<double> cpu_time_sec =
-            thread_clock::now() - cpu_start_time_;
-        oss_ << ", " << time_with_unit(cpu_time_sec.count()) << " CPU";
-      }
-      oss_ << "]";
-      lgr_.write(level_, oss_.str(), loc_);
-    }
-  }
+  ~timed_level_log_entry();
 
   template <typename T>
   timed_level_log_entry& operator<<(const T& val) {
-    output_ = true;
-    oss_ << val;
+    if (state_) {
+      output_ = true;
+      oss_ << val;
+    }
     return *this;
   }
 
  private:
-  logger& lgr_;
+  class state;
+
   std::ostringstream oss_;
-  logger::level_type const level_;
-  std::chrono::time_point<std::chrono::high_resolution_clock> start_time_;
-  thread_clock::time_point cpu_start_time_;
   bool output_{false};
-  bool const with_cpu_;
-  std::source_location const loc_;
+  std::unique_ptr<state const> state_;
 };
 
 class no_log_entry {

src/logger.cpp

@@ -19,6 +19,7 @@
  * along with dwarfs.  If not, see <https://www.gnu.org/licenses/>.
  */
 
+#include <chrono>
 #include <cstring>
 #include <exception>
 #include <iterator>
@@ -27,6 +28,8 @@
 #include <folly/Conv.h>
 #include <folly/small_vector.h>
 
+#include <boost/chrono/thread_clock.hpp>
+
 #include <dwarfs/config.h>
 
 #ifdef DWARFS_STACKTRACE_ENABLED
@@ -288,6 +291,61 @@ void stream_logger::set_threshold(level_type threshold) {
   }
 }
 
+class timed_level_log_entry::state {
+ public:
+  using thread_clock = boost::chrono::thread_clock;
+
+  state(logger& lgr, logger::level_type level, std::source_location loc,
+        bool with_cpu)
+      : lgr_{lgr}
+      , level_{level}
+      , start_time_{std::chrono::high_resolution_clock::now()}
+      , cpu_start_time_{cpu_now(with_cpu)}
+      , loc_{loc} {}
+
+  void log(std::ostringstream& oss) const {
+    std::chrono::duration<double> sec =
+        std::chrono::high_resolution_clock::now() - start_time_;
+    oss << " [" << time_with_unit(sec.count());
+    if (cpu_start_time_) {
+      boost::chrono::duration<double> cpu_time_sec =
+          thread_clock::now() - cpu_start_time_.value();
+      oss << ", " << time_with_unit(cpu_time_sec.count()) << " CPU";
+    }
+    oss << "]";
+    lgr_.write(level_, oss.str(), loc_);
+  }
+
+ private:
+  static std::optional<thread_clock::time_point> cpu_now(bool with_cpu) {
+    if (with_cpu) {
+      return thread_clock::now();
+    }
+    return std::nullopt;
+  }
+
+  logger& lgr_;
+  logger::level_type const level_;
+  std::chrono::time_point<std::chrono::high_resolution_clock> const start_time_;
+  std::optional<thread_clock::time_point> const cpu_start_time_;
+  std::source_location const loc_;
+};
+
+timed_level_log_entry::timed_level_log_entry(logger& lgr,
+                                             logger::level_type level,
+                                             std::source_location loc,
+                                             bool with_cpu) {
+  if (level <= lgr.threshold()) {
+    state_ = std::make_unique<state>(lgr, level, loc, with_cpu);
+  }
+}
+
+timed_level_log_entry::~timed_level_log_entry() {
+  if (state_ && output_) {
+    state_->log(oss_);
+  }
+}
+
 namespace detail {
 
 bool logging_class_factory::is_policy_name(logger const& lgr,

src/utility/rewrite_filesystem.cpp

@@ -22,6 +22,7 @@
 #include <dwarfs/history.h>
 #include <dwarfs/logger.h>
 #include <dwarfs/reader/filesystem_v2.h>
+#include <dwarfs/util.h>
 #include <dwarfs/utility/rewrite_options.h>
 #include <dwarfs/writer/category_resolver.h>
 #include <dwarfs/writer/filesystem_writer.h>