From 8e581ea6bbda961f8b85f6a7cc40b0d2d231e5f8 Mon Sep 17 00:00:00 2001
From: Jordan Woyak <jordan.woyak@gmail.com>
Date: Sun, 23 Mar 2025 16:46:21 -0500
Subject: [PATCH] CoreTiming: Limit throttling after Immediate XFB for better
 input latency.

---
 Source/Core/Core/CoreTiming.cpp | 26 ++++++++++++++++++++++++--
 Source/Core/Core/CoreTiming.h   |  4 ++++
 2 files changed, 28 insertions(+), 2 deletions(-)

diff --git a/Source/Core/Core/CoreTiming.cpp b/Source/Core/Core/CoreTiming.cpp
index 6d1ab99262..9c1954eeff 100644
--- a/Source/Core/Core/CoreTiming.cpp
+++ b/Source/Core/Core/CoreTiming.cpp
@@ -28,6 +28,7 @@
 #include "VideoCommon/PerformanceMetrics.h"
 #include "VideoCommon/VideoBackendBase.h"
 #include "VideoCommon/VideoConfig.h"
+#include "VideoCommon/VideoEvents.h"
 
 namespace CoreTiming
 {
@@ -104,6 +105,13 @@ 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()
@@ -113,6 +121,7 @@ void CoreTimingManager::Shutdown()
   ClearPendingEvents();
   UnregisterAllEvents();
   CPUThreadConfigCallback::RemoveConfigChangedCallback(m_registered_config_callback_id);
+  m_frame_hook.reset();
 }
 
 void CoreTimingManager::RefreshConfig()
@@ -403,6 +412,21 @@ 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);
@@ -422,8 +446,6 @@ 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)
   {
diff --git a/Source/Core/Core/CoreTiming.h b/Source/Core/Core/CoreTiming.h
index d6f652c35a..395eb71f3a 100644
--- a/Source/Core/Core/CoreTiming.h
+++ b/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"
@@ -223,6 +224,9 @@ private:
   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