Optimize thread variables in ServiceThread

Wraps thread-related member variables in preprocessor checks to ensure
they are excluded when multithreading is not supported. Previously,
these variables were declared unconditionally, leading to unnecessary
memory usage for platforms that don't support multithreading.

filament/backend/src/Driver.cpp

@@ -51,39 +51,39 @@ namespace filament::backend {
 
 DriverBase::DriverBase(const Platform::DriverConfig& driverConfig) noexcept
     : mDriverConfig(driverConfig) {
-    if constexpr (UTILS_HAS_THREADING) {
-        // This thread services user callbacks
-        mServiceThread = std::thread([this]() {
-            JobSystem::setThreadName("ServiceThread");
-            do {
-                auto& serviceThreadCondition = mServiceThreadCondition;
-                auto& serviceThreadCallbackQueue = mServiceThreadCallbackQueue;
+#if UTILS_HAS_THREADING
+    // This thread services user callbacks
+    mServiceThread = std::thread([this]() {
+        JobSystem::setThreadName("ServiceThread");
+        do {
+            auto& serviceThreadCondition = mServiceThreadCondition;
+            auto& serviceThreadCallbackQueue = mServiceThreadCallbackQueue;
 
-                // wait for some callbacks to dispatch
-                std::unique_lock<std::mutex> lock(mServiceThreadLock);
-                while (serviceThreadCallbackQueue.empty() && !mExitRequested) {
-                    serviceThreadCondition.wait(lock);
-                }
-                if (mExitRequested) {
-                    break;
-                }
-                // move the callbacks to a temporary vector
-                auto callbacks(std::move(serviceThreadCallbackQueue));
-                lock.unlock();
-                // and make sure to call them without our lock held
-                for (auto[handler, callback, user]: callbacks) {
-                    handler->post(user, callback);
-                }
-            } while (true);
-        });
-    }
+            // wait for some callbacks to dispatch
+            std::unique_lock<std::mutex> lock(mServiceThreadLock);
+            while (serviceThreadCallbackQueue.empty() && !mExitRequested) {
+                serviceThreadCondition.wait(lock);
+            }
+            if (mExitRequested) {
+                break;
+            }
+            // move the callbacks to a temporary vector
+            auto callbacks(std::move(serviceThreadCallbackQueue));
+            lock.unlock();
+            // and make sure to call them without our lock held
+            for (auto[handler, callback, user]: callbacks) {
+                handler->post(user, callback);
+            }
+        } while (true);
+    });
+#endif
 }
 
 DriverBase::~DriverBase() noexcept {
     assert_invariant(mCallbacks.empty());
-    if constexpr (UTILS_HAS_THREADING) {
-        stopServiceThread();
-    }
+#if UTILS_HAS_THREADING
+    stopServiceThread();
+#endif
 }
 
 // ------------------------------------------------------------------------------------------------
@@ -107,11 +107,14 @@ void DriverBase::CallbackData::release(CallbackData* data) {
 
 
 void DriverBase::scheduleCallback(CallbackHandler* handler, void* user, CallbackHandler::Callback callback) {
-    if (handler && UTILS_HAS_THREADING) {
+#if UTILS_HAS_THREADING
+    if (handler) {
         std::lock_guard<std::mutex> const lock(mServiceThreadLock);
         mServiceThreadCallbackQueue.emplace_back(handler, callback, user);
         mServiceThreadCondition.notify_one();
-    } else {
+    } else
+#endif
+    {
         std::lock_guard<std::mutex> const lock(mPurgeLock);
         mCallbacks.emplace_back(user, callback);
     }

filament/backend/src/DriverBase.h

@@ -238,9 +238,11 @@ protected:
     void debugCommandBegin(CommandStream* cmds, bool synchronous, const char* methodName) noexcept override;
     void debugCommandEnd(CommandStream* cmds, bool synchronous, const char* methodName) noexcept override;
 
+#if UTILS_HAS_THREADING
     // Stops the `ServiceThread`. This method is called during destruction but may be called
     // explicitly if earlier shutdown is needed. This method is idempotent.
     void stopServiceThread() noexcept;
+#endif
 
 private:
     const Platform::DriverConfig mDriverConfig;
@@ -248,11 +250,13 @@ private:
     std::mutex mPurgeLock;
     std::vector<std::pair<void*, CallbackHandler::Callback>> mCallbacks;
 
+#if UTILS_HAS_THREADING
     std::thread mServiceThread;
     std::mutex mServiceThreadLock;
     std::condition_variable mServiceThreadCondition;
     std::vector<std::tuple<CallbackHandler*, CallbackHandler::Callback, void*>> mServiceThreadCallbackQueue;
     bool mExitRequested = false;
+#endif
 };
 
 

filament/backend/src/opengl/OpenGLDriver.cpp

@@ -390,9 +390,9 @@ void OpenGLDriver::terminate() {
     if (getJobWorker()) {
         getJobWorker()->terminate();
     }
-    if constexpr (UTILS_HAS_THREADING) {
-        stopServiceThread();
-    }
+#if UTILS_HAS_THREADING
+    stopServiceThread();
+#endif
 
     mContext.terminate();
     mPlatform.terminate();