[Windows] Improve frame pacing by busy waiting as needed

platform/windows/os_windows.cpp

@@ -265,7 +265,15 @@ void OS_Windows::initialize() {
 
 	// set minimum resolution for periodic timers, otherwise Sleep(n) may wait at least as
 	//  long as the windows scheduler resolution (~16-30ms) even for calls like Sleep(1)
-	timeBeginPeriod(1);
+	TIMECAPS time_caps;
+	if (timeGetDevCaps(&time_caps, sizeof(time_caps)) == MMSYSERR_NOERROR) {
+		ticks_min_sleep = time_caps.wPeriodMin * 1000;
+		timeBeginPeriod(time_caps.wPeriodMin);
+	} else {
+		ERR_PRINT("Unable to detect sleep timer resolution.");
+		ticks_min_sleep = 1000;
+		timeBeginPeriod(1);
+	}
 
 	process_map = memnew((HashMap<ProcessID, ProcessInfo>));
 
@@ -2223,6 +2231,46 @@ String OS_Windows::get_system_ca_certificates() {
 	return certs;
 }
 
+void OS_Windows::add_frame_delay(bool p_can_draw) {
+	const uint32_t frame_delay = Engine::get_singleton()->get_frame_delay();
+	if (frame_delay) {
+		// Add fixed frame delay to decrease CPU/GPU usage. This doesn't take
+		// the actual frame time into account.
+		// Due to the high fluctuation of the actual sleep duration, it's not recommended
+		// to use this as a FPS limiter.
+		delay_usec(frame_delay * 1000);
+	}
+
+	// Add a dynamic frame delay to decrease CPU/GPU usage. This takes the
+	// previous frame time into account for a smoother result.
+	uint64_t dynamic_delay = 0;
+	if (is_in_low_processor_usage_mode() || !p_can_draw) {
+		dynamic_delay = get_low_processor_usage_mode_sleep_usec();
+	}
+	const int max_fps = Engine::get_singleton()->get_max_fps();
+	if (max_fps > 0 && !Engine::get_singleton()->is_editor_hint()) {
+		// Override the low processor usage mode sleep delay if the target FPS is lower.
+		dynamic_delay = MAX(dynamic_delay, (uint64_t)(1000000 / max_fps));
+	}
+
+	if (dynamic_delay > 0) {
+		target_ticks += dynamic_delay;
+		uint64_t current_ticks = get_ticks_usec();
+
+		// The minimum sleep resolution on windows is 1 ms on most systems.
+		if (current_ticks < (target_ticks - ticks_sleep_min)) {
+			delay_usec((target_ticks - ticks_sleep_min) - current_ticks);
+		}
+		// Busy wait for the remainder of time.
+		while (get_ticks_usec() < target_ticks) {
+			YieldProcessor();
+		}
+
+		current_ticks = get_ticks_usec();
+		target_ticks = MIN(MAX(target_ticks, current_ticks - dynamic_delay), current_ticks + dynamic_delay);
+	}
+}
+
 OS_Windows::OS_Windows(HINSTANCE _hInstance) {
 	hInstance = _hInstance;
 

platform/windows/os_windows.h

@@ -96,6 +96,7 @@ class JoypadWindows;
 class OS_Windows : public OS {
 	uint64_t ticks_start = 0;
 	uint64_t ticks_per_second = 0;
+	uint64_t ticks_sleep_min = 1000;
 
 	HINSTANCE hInstance;
 	MainLoop *main_loop = nullptr;
@@ -245,6 +246,8 @@ class OS_Windows : public OS {
 
 	virtual String get_system_ca_certificates() override;
 
+	void add_frame_delay(bool p_can_draw) override;
+
 	void set_main_window(HWND p_main_window) { main_window = p_main_window; }
 
 	HINSTANCE get_hinstance() { return hInstance; }