From bab3ebec14c46835de8034f238f7723621c24f1c Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?R=C3=A9my=20Tassoux?= <contact@rt2.fr>
Date: Fri, 26 Sep 2025 16:07:38 +0200
Subject: [PATCH] Backends: DX12: Rework synchronization logic. (master)
 (#8961)

---
 docs/CHANGELOG.txt                        |  1 +
 examples/example_win32_directx12/main.cpp | 61 ++++++++---------------
 2 files changed, 22 insertions(+), 40 deletions(-)

diff --git a/docs/CHANGELOG.txt b/docs/CHANGELOG.txt
index 5dc538ec9..c8424cf5d 100644
--- a/docs/CHANGELOG.txt
+++ b/docs/CHANGELOG.txt
@@ -91,6 +91,7 @@ Other Changes:
 - Examples: SDL2+DirectX11: Try WARP software driver if hardware driver is
   not available. (#5924, #5562)
 - Examples: SDL3+DirectX11: Added SDL3+DirectX11 example. (#8956, #8957) [@tomaz82]
+- Examples: Win32+DirectX12: Rework synchronization logic. (#8961) [@RT2Code]
 - Examples: made examples's main.cpp consistent with returning 1 on error.
 
 
diff --git a/examples/example_win32_directx12/main.cpp b/examples/example_win32_directx12/main.cpp
index a5396d31b..665693462 100644
--- a/examples/example_win32_directx12/main.cpp
+++ b/examples/example_win32_directx12/main.cpp
@@ -102,8 +102,8 @@ bool CreateDeviceD3D(HWND hWnd);
 void CleanupDeviceD3D();
 void CreateRenderTarget();
 void CleanupRenderTarget();
-void WaitForLastSubmittedFrame();
-FrameContext* WaitForNextFrameResources();
+void WaitForPendingOperations();
+FrameContext* WaitForNextFrameContext();
 LRESULT WINAPI WndProc(HWND hWnd, UINT msg, WPARAM wParam, LPARAM lParam);
 
 // Main code
@@ -256,7 +256,7 @@ int main(int, char**)
         // Rendering
         ImGui::Render();
 
-        FrameContext* frameCtx = WaitForNextFrameResources();
+        FrameContext* frameCtx = WaitForNextFrameContext();
         UINT backBufferIdx = g_pSwapChain->GetCurrentBackBufferIndex();
         frameCtx->CommandAllocator->Reset();
 
@@ -282,19 +282,17 @@ int main(int, char**)
         g_pd3dCommandList->Close();
 
         g_pd3dCommandQueue->ExecuteCommandLists(1, (ID3D12CommandList* const*)&g_pd3dCommandList);
+        g_pd3dCommandQueue->Signal(g_fence, ++g_fenceLastSignaledValue);
+        frameCtx->FenceValue = g_fenceLastSignaledValue;
 
         // Present
         HRESULT hr = g_pSwapChain->Present(1, 0);   // Present with vsync
         //HRESULT hr = g_pSwapChain->Present(0, 0); // Present without vsync
         g_SwapChainOccluded = (hr == DXGI_STATUS_OCCLUDED);
-
-        UINT64 fenceValue = g_fenceLastSignaledValue + 1;
-        g_pd3dCommandQueue->Signal(g_fence, fenceValue);
-        g_fenceLastSignaledValue = fenceValue;
-        frameCtx->FenceValue = fenceValue;
+        g_frameIndex++;
     }
 
-    WaitForLastSubmittedFrame();
+    WaitForPendingOperations();
 
     // Cleanup
     ImGui_ImplDX12_Shutdown();
@@ -465,49 +463,33 @@ void CreateRenderTarget()
 
 void CleanupRenderTarget()
 {
-    WaitForLastSubmittedFrame();
+    WaitForPendingOperations();
 
     for (UINT i = 0; i < APP_NUM_BACK_BUFFERS; i++)
         if (g_mainRenderTargetResource[i]) { g_mainRenderTargetResource[i]->Release(); g_mainRenderTargetResource[i] = nullptr; }
 }
 
-void WaitForLastSubmittedFrame()
+void WaitForPendingOperations()
 {
-    FrameContext* frameCtx = &g_frameContext[g_frameIndex % APP_NUM_FRAMES_IN_FLIGHT];
+    g_pd3dCommandQueue->Signal(g_fence, ++g_fenceLastSignaledValue);
 
-    UINT64 fenceValue = frameCtx->FenceValue;
-    if (fenceValue == 0)
-        return; // No fence was signaled
-
-    frameCtx->FenceValue = 0;
-    if (g_fence->GetCompletedValue() >= fenceValue)
-        return;
-
-    g_fence->SetEventOnCompletion(fenceValue, g_fenceEvent);
-    WaitForSingleObject(g_fenceEvent, INFINITE);
+    g_fence->SetEventOnCompletion(g_fenceLastSignaledValue, g_fenceEvent);
+    ::WaitForSingleObject(g_fenceEvent, INFINITE);
 }
 
-FrameContext* WaitForNextFrameResources()
+FrameContext* WaitForNextFrameContext()
 {
-    UINT nextFrameIndex = g_frameIndex + 1;
-    g_frameIndex = nextFrameIndex;
-
-    HANDLE waitableObjects[] = { g_hSwapChainWaitableObject, nullptr };
-    DWORD numWaitableObjects = 1;
-
-    FrameContext* frameCtx = &g_frameContext[nextFrameIndex % APP_NUM_FRAMES_IN_FLIGHT];
-    UINT64 fenceValue = frameCtx->FenceValue;
-    if (fenceValue != 0) // means no fence was signaled
+    FrameContext* frame_context = &g_frameContext[g_frameIndex % APP_NUM_FRAMES_IN_FLIGHT];
+    if (g_fence->GetCompletedValue() < frame_context->FenceValue)
     {
-        frameCtx->FenceValue = 0;
-        g_fence->SetEventOnCompletion(fenceValue, g_fenceEvent);
-        waitableObjects[1] = g_fenceEvent;
-        numWaitableObjects = 2;
+        g_fence->SetEventOnCompletion(frame_context->FenceValue, g_fenceEvent);
+        HANDLE waitableObjects[] = { g_hSwapChainWaitableObject, g_fenceEvent };
+        ::WaitForMultipleObjects(2, waitableObjects, TRUE, INFINITE);
     }
+    else
+        ::WaitForSingleObject(g_hSwapChainWaitableObject, INFINITE);
 
-    WaitForMultipleObjects(numWaitableObjects, waitableObjects, TRUE, INFINITE);
-
-    return frameCtx;
+    return frame_context;
 }
 
 // Forward declare message handler from imgui_impl_win32.cpp
@@ -528,7 +510,6 @@ LRESULT WINAPI WndProc(HWND hWnd, UINT msg, WPARAM wParam, LPARAM lParam)
     case WM_SIZE:
         if (g_pd3dDevice != nullptr && wParam != SIZE_MINIMIZED)
         {
-            WaitForLastSubmittedFrame();
             CleanupRenderTarget();
             HRESULT result = g_pSwapChain->ResizeBuffers(0, (UINT)LOWORD(lParam), (UINT)HIWORD(lParam), DXGI_FORMAT_UNKNOWN, DXGI_SWAP_CHAIN_FLAG_FRAME_LATENCY_WAITABLE_OBJECT);
             assert(SUCCEEDED(result) && "Failed to resize swapchain.");