From 0ff1cf0dd5342335d61bf8403c57c68f816a4a2d Mon Sep 17 00:00:00 2001
From: Cameron Desrochers <cdesrochers4@bloomberg.net>
Date: Fri, 20 Dec 2024 16:14:03 -0500
Subject: [PATCH] Optimize getLogger() for common case of single singleton
 (saves ~45 ns) (#5138)

---
 groups/bal/ball/ball_loggermanager.cpp |  8 +++++++-
 groups/bal/ball/ball_loggermanager.h   | 20 ++++++++++++++++++++
 2 files changed, 27 insertions(+), 1 deletion(-)

diff --git a/groups/bal/ball/ball_loggermanager.cpp b/groups/bal/ball/ball_loggermanager.cpp
index 8d29eadb9e..39309c653d 100644
--- a/groups/bal/ball/ball_loggermanager.cpp
+++ b/groups/bal/ball/ball_loggermanager.cpp
@@ -739,6 +739,7 @@ LoggerManager::LoggerManager(
 , d_userFieldsPopulator(configuration.userFieldsPopulatorCallback())
 , d_attributeCollectors(bslma::Default::globalAllocator(globalAllocator))
 , d_logger_p(0)
+, d_defaultLoggerCount(0)
 , d_categoryManager(bslma::Default::globalAllocator(globalAllocator))
 , d_maxNumCategoriesMinusOne((unsigned int)-1)
 , d_loggers(bslma::Default::globalAllocator(globalAllocator))
@@ -1143,6 +1144,7 @@ LoggerManager::LoggerManager(
 , d_userFieldsPopulator(configuration.userFieldsPopulatorCallback())
 , d_attributeCollectors(bslma::Default::globalAllocator(globalAllocator))
 , d_logger_p(0)
+, d_defaultLoggerCount(0)
 , d_categoryManager(bslma::Default::globalAllocator(globalAllocator))
 , d_maxNumCategoriesMinusOne((unsigned int)-1)
 , d_loggers(bslma::Default::globalAllocator(globalAllocator))
@@ -1334,13 +1336,15 @@ void LoggerManager::deallocateLogger(Logger *logger)
             ++itr;
         }
     }
+    d_defaultLoggerCount.storeRelease(
+                               static_cast<unsigned>(d_defaultLoggers.size()));
     d_defaultLoggersLock.unlock();
 
     logger->~Logger();
     d_allocator_p->deallocate(logger);
 }
 
-Logger& LoggerManager::getLogger()
+Logger& LoggerManager::getLoggerSlow()
 {
     // TBD: optimize it using thread local storage
 
@@ -1363,6 +1367,8 @@ void LoggerManager::setLogger(Logger *logger)
     else {
         d_defaultLoggers[id] = logger;
     }
+    d_defaultLoggerCount.storeRelease(
+                               static_cast<unsigned>(d_defaultLoggers.size()));
 }
 
                              // Category Management
diff --git a/groups/bal/ball/ball_loggermanager.h b/groups/bal/ball/ball_loggermanager.h
index 0cc8a5dc64..555dd97adc 100644
--- a/groups/bal/ball/ball_loggermanager.h
+++ b/groups/bal/ball/ball_loggermanager.h
@@ -900,7 +900,9 @@ BSLS_IDENT("$Id: $")
 #include <bslmt_mutex.h>
 #include <bslmt_readerwritermutex.h>
 
+#include <bsls_atomic.h>
 #include <bsls_compilerfeatures.h>
+#include <bsls_performancehint.h>
 #include <bsls_util.h>     // 'forward<T>(V)'
 
 #include <bsl_functional.h>
@@ -1289,6 +1291,9 @@ class LoggerManager {
     Logger                *d_logger_p;           // holds default logger
                                                  // (owned)
 
+    bsls::AtomicUint       d_defaultLoggerCount; // number of thread-specific
+                                                 // default loggers
+
     CategoryManager        d_categoryManager;    // category manager
 
     unsigned int           d_maxNumCategoriesMinusOne;
@@ -1375,6 +1380,11 @@ class LoggerManager {
     /// publication `cause`.
     void publishAllImp(Transmission::Cause cause);
 
+    /// Return a non-`const` reference to a logger managed by this logger
+    /// manager suitable for performing logging operations for this thread
+    /// of execution.
+    Logger& getLoggerSlow();
+
   public:
     // CLASS METHODS
 #ifndef BDE_OMIT_INTERNAL_DEPRECATED
@@ -2349,6 +2359,16 @@ void LoggerManager::visitObservers(
                    BSLS_COMPILERFEATURES_FORWARD(t_OBSERVER_VISITOR, visitor));
 }
 
+inline
+Logger& LoggerManager::getLogger()
+{
+    if (BSLS_PERFORMANCEHINT_PREDICT_UNLIKELY(
+                                         d_defaultLoggerCount.loadAcquire())) {
+        return getLoggerSlow();
+    }
+    return *d_logger_p;
+}
+
 // ACCESSORS
 inline
 bslma::Allocator *LoggerManager::allocator() const