From b86461ce94f7e6cab8dc00393a30624dbf778b54 Mon Sep 17 00:00:00 2001
From: Earl Ou <shunhsingou@google.com>
Date: Thu, 17 Sep 2020 14:31:44 +0800
Subject: [PATCH] systemc: avoid mutex lock in non async cases

Avoid acquiring a mutex lock in case there is no async update in the
scheduler. This helps increasing simulation speed by about 4%.

Change-Id: I971c7bf1a1eeb46208eeee6e5da6385c907092b3
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/34695
Reviewed-by: Earl Ou <shunhsingou@google.com>
Maintainer: Earl Ou <shunhsingou@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>
---
 src/systemc/core/scheduler.cc | 4 +++-
 src/systemc/core/scheduler.hh | 2 ++
 2 files changed, 5 insertions(+), 1 deletion(-)

diff --git a/src/systemc/core/scheduler.cc b/src/systemc/core/scheduler.cc
index 50a1e6bd3..cc0be7cd4 100644
--- a/src/systemc/core/scheduler.cc
+++ b/src/systemc/core/scheduler.cc
@@ -259,6 +259,7 @@ Scheduler::asyncRequestUpdate(Channel *c)
 {
     std::lock_guard<std::mutex> lock(asyncListMutex);
     asyncUpdateList.pushLast(c);
+    hasAsyncUpdate = true;
 }
 
 void
@@ -325,11 +326,12 @@ void
 Scheduler::runUpdate()
 {
     status(StatusUpdate);
-    {
+    if (hasAsyncUpdate) {
         std::lock_guard<std::mutex> lock(asyncListMutex);
         Channel *channel;
         while ((channel = asyncUpdateList.getNext()) != nullptr)
             updateList.pushLast(channel);
+        hasAsyncUpdate = false;
     }
 
     try {
diff --git a/src/systemc/core/scheduler.hh b/src/systemc/core/scheduler.hh
index 742f91638..13f35ed5f 100644
--- a/src/systemc/core/scheduler.hh
+++ b/src/systemc/core/scheduler.hh
@@ -28,6 +28,7 @@
 #ifndef __SYSTEMC_CORE_SCHEDULER_HH__
 #define __SYSTEMC_CORE_SCHEDULER_HH__
 
+#include <atomic>
 #include <functional>
 #include <list>
 #include <map>
@@ -529,6 +530,7 @@ class Scheduler
 
     ChannelList asyncUpdateList;
     std::mutex asyncListMutex;
+    std::atomic<bool> hasAsyncUpdate;
 
     std::map<::Event *, Tick> eventsToSchedule;
 
-- 
2.30.2