Throttle after IXFB for improved input latency

Technical explanation: dolphin-emu#13449

Author: Motobug

Source/Core/Core/CoreTiming.cpp

@@ -29,6 +29,7 @@
 #include "VideoCommon/PerformanceMetrics.h"
 #include "VideoCommon/VideoBackendBase.h"
 #include "VideoCommon/VideoConfig.h"
+#include "VideoCommon/VideoEvents.h"
 
 namespace CoreTiming
 {
@@ -105,6 +106,14 @@ void CoreTimingManager::Init()
 
   m_last_oc_factor = m_config_oc_factor;
   m_globals.last_OC_factor_inverted = m_config_oc_inv_factor;
+  
+  m_throttled_since_presentation = false;
+  m_frame_hook = AfterPresentEvent::Register(
+    [this](const PresentInfo&) {
+      m_throttled_since_presentation.store(false, std::memory_order_relaxed);
+    },
+    "CoreTiming AfterPresentEvent"
+  );
 }
 
 void CoreTimingManager::Shutdown()
@@ -406,6 +415,22 @@ void CoreTimingManager::SleepUntil(TimePoint time_point)
 
 void CoreTimingManager::Throttle(const s64 target_cycle)
 {
+  const TimePoint time = Clock::now();
+
+  const bool already_throttled =
+      m_throttled_since_presentation.exchange(true, std::memory_order_relaxed);
+
+  // When Immediate XFB is enabled, try to Throttle just once since each presentation.
+  //  This lowers latency by speeding through to the next presentation after grabbing input.
+  // Make sure we don't get too far ahead of proper timing though,
+  //  otherwise the emulator unreasonably speeds through loading screens that don't have XFB copies.
+
+  const bool skip_throttle = already_throttled && g_ActiveConfig.bImmediateXFB &&
+                             ((GetTargetHostTime(target_cycle) - time) < (m_max_fallback / 2));
+
+  if (skip_throttle)
+    return;
+  
   if (IsSpeedUnlimited())
   {
     ResetThrottle(target_cycle);
@@ -425,8 +450,6 @@ void CoreTimingManager::Throttle(const s64 target_cycle)
 
   TimePoint target_time = CalculateTargetHostTimeInternal(target_cycle);
 
-  const TimePoint time = Clock::now();
-
   const TimePoint min_target = time - m_max_fallback;
   if (target_time < min_target)
   {

Source/Core/Core/CoreTiming.h

@@ -23,6 +23,7 @@
 #include <vector>
 
 #include "Common/CommonTypes.h"
+#include "Common/HookableEvent.h"
 #include "Common/SPSCQueue.h"
 #include "Common/Timer.h"
 #include "Core/CPUThreadConfigCallback.h"
@@ -225,6 +226,8 @@ class CoreTimingManager
   std::atomic_bool m_use_precision_timer = false;
   Common::PrecisionTimer m_precision_cpu_timer;
   Common::PrecisionTimer m_precision_gpu_timer;
+  Common::EventHook m_frame_hook;
+  std::atomic<bool> m_throttled_since_presentation = false;
 };
 
 }  // namespace CoreTiming