[idd] d3d12: signal reusable command slots

Signal a per-slot event after callbacks finish and the command slot has
returned to the free state. This removes the one millisecond polling delay
when candidate state becomes visible just before its slot is reusable.

idd/LGIdd/CD3D12CommandQueue.cpp

@@ -99,6 +99,14 @@ bool CD3D12CommandSlot::Init(ID3D12Device3 * device,
     return false;
   }
 
+  m_availableEvent.Attach(CreateEvent(NULL, FALSE, FALSE, NULL));
+  if (!m_availableEvent.Get())
+  {
+    DEBUG_ERROR_HR(GetLastError(),
+      "Failed to create the availability event (%ls)", name);
+    return false;
+  }
+
   if (!RegisterWaitForSingleObject(
     &m_waitHandle,
     m_event.Get(),
@@ -130,6 +138,7 @@ void CD3D12CommandSlot::DeInit()
   }
 
   m_event.Close();
+  m_availableEvent.Close();
   m_cmdList.Reset();
   m_gfxList.Reset();
   m_allocator.Reset();
@@ -224,6 +233,7 @@ void CD3D12CommandSlot::Cancel()
   m_completionParams[1] = nullptr;
   m_submitted.store(false, std::memory_order_release);
   m_state.store(STATE_FREE, std::memory_order_release);
+  SetEvent(m_availableEvent.Get());
 }
 
 bool CD3D12CommandSlot::Execute()
@@ -250,7 +260,10 @@ bool CD3D12CommandSlot::Execute()
     return true;
 
   if (!m_submitted.load(std::memory_order_acquire))
+  {
     m_state.store(STATE_FREE, std::memory_order_release);
+    SetEvent(m_availableEvent.Get());
+  }
   return false;
 }
 
@@ -343,6 +356,7 @@ void CD3D12CommandSlot::OnCompletion(bool timeout)
   m_completionParams[1] = nullptr;
   m_submitted.store(false, std::memory_order_release);
   m_state.store(STATE_FREE, std::memory_order_release);
+  SetEvent(m_availableEvent.Get());
 }
 
 bool CD3D12CommandQueue::InitTiming(ID3D12Device3 * device, UINT slotCount)
@@ -493,13 +507,23 @@ CD3D12CommandSlot * CD3D12CommandQueue::Acquire(UINT slotIndex)
   if (slotIndex >= m_slotCount)
     return nullptr;
 
-  for (int i = 0; i < 100; ++i)
+  const ULONGLONG deadline = GetTickCount64() + 100;
+  for (;;)
   {
     if (m_slots[slotIndex].Acquire())
       return &m_slots[slotIndex];
     if (m_failed.load(std::memory_order_acquire))
       break;
-    Sleep(1);
+
+    const ULONGLONG now = GetTickCount64();
+    if (now >= deadline)
+      break;
+
+    const DWORD result =
+      WaitForSingleObject(m_slots[slotIndex].m_availableEvent.Get(),
+        static_cast<DWORD>(deadline - now));
+    if (result != WAIT_OBJECT_0)
+      break;
   }
 
   DEBUG_ERROR("Failed to acquire CommandSlot(%ls:%u)", m_name, slotIndex);