D3D12: Only enable toggle 'use_dxc' when Dawn can load DXC DLLs

This patch adds an additional check in the initialization of device
on D3D12 backend to ensure that 'use_dxc' is enabled only when Dawn
can load DXC DLLs (dxil.dll and dxcompiler.dll).

BUG=dawn:402

Change-Id: I85bc8698cf54b612244efc3673241f60a9cee209
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/33060
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Austin Eng <enga@chromium.org>
Commit-Queue: Jiawei Shao <jiawei.shao@intel.com>

src/dawn_native/Toggles.cpp

@@ -126,7 +126,9 @@ namespace dawn_native {
               "default. This setting is used to test bindgroup encoding.",
               "https://crbug.com/dawn/155"}},
             {Toggle::UseDXC,
-             {"use_dxc", "Use DXC instead of FXC for compiling HLSL",
+             {"use_dxc",
+              "Use DXC instead of FXC for compiling HLSL when both dxcompiler.dll and dxil.dll "
+              "is available.",
               "https://crbug.com/dawn/402"}},
             {Toggle::DisableRobustness,
              {"disable_robustness", "Disable robust buffer access", "https://crbug.com/dawn/480"}},

src/dawn_native/d3d12/DeviceD3D12.cpp

@@ -153,6 +153,10 @@ namespace dawn_native { namespace d3d12 {
         // Device shouldn't be used until after DeviceBase::Initialize so we must wait until after
         // device initialization to call NextSerial
         DAWN_TRY(NextSerial());
+
+        // The environment can only use DXC when it's available. Override the decision if it is not
+        // applicable.
+        ApplyUseDxcToggle();
         return {};
     }
 
@@ -188,6 +192,12 @@ namespace dawn_native { namespace d3d12 {
         return ToBackend(GetAdapter())->GetBackend()->GetFactory();
     }
 
+    void Device::ApplyUseDxcToggle() {
+        if (!ToBackend(GetAdapter())->GetBackend()->GetFunctions()->IsDXCAvailable()) {
+            ForceSetToggle(Toggle::UseDXC, false);
+        }
+    }
+
     ResultOrError<IDxcLibrary*> Device::GetOrCreateDxcLibrary() const {
         return ToBackend(GetAdapter())->GetBackend()->GetOrCreateDxcLibrary();
     }

src/dawn_native/d3d12/DeviceD3D12.h

@@ -178,6 +178,8 @@ namespace dawn_native { namespace d3d12 {
 
         MaybeError CheckDebugLayerAndGenerateErrors();
 
+        void ApplyUseDxcToggle();
+
         ComPtr<ID3D12Fence> mFence;
         HANDLE mFenceEvent = nullptr;
         ExecutionSerial CheckAndUpdateCompletedSerials() override;

src/tests/end2end/ShaderFloat16Tests.cpp

@@ -108,4 +108,4 @@ TEST_P(ShaderFloat16Tests, Basic16BitFloatFeaturesTest) {
     EXPECT_BUFFER_U16_RANGE_EQ(expected, bufferOut, 0, 2);
 }
 
-DAWN_INSTANTIATE_TEST(ShaderFloat16Tests, MetalBackend(), VulkanBackend());
+DAWN_INSTANTIATE_TEST(ShaderFloat16Tests, D3D12Backend(), MetalBackend(), VulkanBackend());