Allow D3D12Device to use an existing device handle (#1940)

* Added a new field for an existing device handle to IDevice::Desc; Modified D3D12Device::initialize to set the device stored in desc if it already exists instead of creating a new one

* Turned existingDeviceHandle into a struct containing an array of two elements; Updated D3D12Device::initialize to match changes to existingDeviceHandle; Updated comments

* Fixed style error for ExistingDeviceHandles struct

2 files changed, 33 insertions, 18 deletions

diff --git a/slang-gfx.h b/slang-gfx.h
index f4a70d25c..40f0985b9 100644
--- a/slang-gfx.h
+++ b/slang-gfx.h
@@ -1424,6 +1424,13 @@ public:
         DeviceType deviceType = DeviceType::Default;
         // Name to identify the adapter to use
         const char* adapter = nullptr;
+        // Device handles (if they already exist)
+        struct ExistingDeviceHandles
+        {
+            // For D3D12, this only contains a single value for the ID3D12Device.
+            // For Vulkan, the first value is the VkInstance and the second is the VkDevice.
+            uint64_t values[2] = { 0 };
+        } existingDeviceHandles;
         // Number of required features.
         int requiredFeatureCount = 0;
         // Array of required feature names, whose size is `requiredFeatureCount`.
diff --git a/tools/gfx/d3d12/render-d3d12.cpp b/tools/gfx/d3d12/render-d3d12.cpp
index feacc624d..d0423044f 100644
--- a/tools/gfx/d3d12/render-d3d12.cpp
+++ b/tools/gfx/d3d12/render-d3d12.cpp
@@ -4287,32 +4287,40 @@ Result D3D12Device::initialize(const Desc& desc)
         return SLANG_FAIL;
     }
 
-    FlagCombiner combiner;
-    // TODO: we should probably provide a command-line option
-    // to override UseDebug of default rather than leave it
-    // up to each back-end to specify.
+    if (desc.existingDeviceHandles.values[0] == 0)
+    {
+        FlagCombiner combiner;
+        // TODO: we should probably provide a command-line option
+        // to override UseDebug of default rather than leave it
+        // up to each back-end to specify.
 #if ENABLE_DEBUG_LAYER
-    combiner.add(DeviceCheckFlag::UseDebug, ChangeType::OnOff);                 ///< First try debug then non debug
+        combiner.add(DeviceCheckFlag::UseDebug, ChangeType::OnOff);                 ///< First try debug then non debug
 #else
-    combiner.add(DeviceCheckFlag::UseDebug, ChangeType::Off);                   ///< Don't bother with debug
+        combiner.add(DeviceCheckFlag::UseDebug, ChangeType::Off);                   ///< Don't bother with debug
 #endif
-    combiner.add(DeviceCheckFlag::UseHardwareDevice, ChangeType::OnOff);        ///< First try hardware, then reference
-    
-    const D3D_FEATURE_LEVEL featureLevel = D3D_FEATURE_LEVEL_11_0;
+        combiner.add(DeviceCheckFlag::UseHardwareDevice, ChangeType::OnOff);        ///< First try hardware, then reference
 
-    const int numCombinations = combiner.getNumCombinations();
-    for (int i = 0; i < numCombinations; ++i)
-    {
-        if (SLANG_SUCCEEDED(_createDevice(combiner.getCombination(i), UnownedStringSlice(desc.adapter), featureLevel, m_deviceInfo)))
+        const D3D_FEATURE_LEVEL featureLevel = D3D_FEATURE_LEVEL_11_0;
+
+        const int numCombinations = combiner.getNumCombinations();
+        for (int i = 0; i < numCombinations; ++i)
         {
-            break;
+            if (SLANG_SUCCEEDED(_createDevice(combiner.getCombination(i), UnownedStringSlice(desc.adapter), featureLevel, m_deviceInfo)))
+            {
+                break;
+            }
         }
-    }
 
-    if (!m_deviceInfo.m_adapter)
+        if (!m_deviceInfo.m_adapter)
+        {
+            // Couldn't find an adapter
+            return SLANG_FAIL;
+        }
+    }
+    else
     {
-        // Couldn't find an adapter
-        return SLANG_FAIL;
+        // Store the existing device handle in desc in m_deviceInfo
+        m_deviceInfo.m_device = (ID3D12Device*)desc.existingDeviceHandles.values[0];
     }
 
     // Set the device