多线程架构下（GameThread -- RenderThread -- RHIThread）渲染时，不会直接调用图形API的上下文的接口
而是创建一个个FRHICommand对象（构成一个链表），并赋值给FExecuteRHIThreadTask对象的FRHICommandListBase* RHICmdList
接着FExecuteRHIThreadTask对象会被压入到TaskGraph[ENamedThreads::RHIThread]的Queue队列中
RHIThread依次处理TaskGraph[ENamedThreads::RHIThread]的Queue队列中任务，执行FExecuteRHIThreadTask时
将该对象中FRHICommandListBase* RHICmdList链表中的FRHICommand转换成图形API的上下文的接口并调用执行
FRHICommand
FRHICommand是RHI模块的渲染指令基类，这些指令通常由渲染线程通过命令队列Push到RHI线程，在合适的时机由RHI线程执行。
FRHICommand继承自FRHICommandBase，它们的定义如下：
// Engine\Source\Runtime\RHI\Public\RHICommandList.h // RHI命令基类. struct FRHICommandBase { // 下一个命令. (命令链表的节点) FRHICommandBase* Next = nullptr; // 执行命令后销毁. virtual void ExecuteAndDestruct(FRHICommandListBase& CmdList, FRHICommandListDebugContext& DebugContext) = 0; }; template<typename TCmd, typename NameType = FUnnamedRhiCommand> struct FRHICommand : public FRHICommandBase { // 执行命令后销毁. void ExecuteAndDestruct(FRHICommandListBase& CmdList, FRHICommandListDebugContext& Context) override final { TCmd *ThisCmd = static_cast<TCmd*>(this); ThisCmd->Execute(CmdList); ThisCmd->~TCmd(); } };
注：FRHICommandBase有指向下一个节点的Next变量，意味着FRHICommandBase是命令链表的节点。
FRHICommand拥有数量众多的子类，是通过特殊的宏来快速声明：
// 定义RHI命令子类的宏，会从FRHICommand上派生 #define FRHICOMMAND_MACRO(CommandName) \ struct PREPROCESSOR_JOIN(CommandName##String, __LINE__) \ { \ static const TCHAR* TStr() { return TEXT(#CommandName); } \ }; \ struct CommandName final : public FRHICommand<CommandName, PREPROCESSOR_JOIN(CommandName##String, __LINE__)>
// PREPROCESSOR_JOIN宏定义如下 详见： UnrealEngine\Engine\Source\Runtime\Core\Public\HAL\PreprocessorHelpers.h // Concatenates two preprocessor tokens, performing macro expansion on them first #define PREPROCESSOR_JOIN(x, y) PREPROCESSOR_JOIN_INNER(x, y) #define PREPROCESSOR_JOIN_INNER(x, y) x##y
有了以上的宏，就可以快速定义FRHICommand的子类（亦即具体的RHI命令），以FRHICommandSetStencilRef为例：
FRHICOMMAND_MACRO(FRHICommandSetStencilRef) { uint32 StencilRef; FORCEINLINE_DEBUGGABLE FRHICommandSetStencilRef(uint32 InStencilRef) : StencilRef(InStencilRef) { } RHI_API void Execute(FRHICommandListBase& CmdList); };
展开宏定义之后，代码如下：
struct FRHICommandSetStencilRefString853 { static const TCHAR* TStr() { return TEXT("FRHICommandSetStencilRef"); } }; // FRHICommandSetStencilRef继承了FRHICommand. struct FRHICommandSetStencilRef final : public FRHICommand<FRHICommandSetStencilRef, FRHICommandSetStencilRefString853> { uint32 StencilRef; FRHICommandSetStencilRef(uint32 InStencilRef) : StencilRef(InStencilRef) { } RHI_API void Execute(FRHICommandListBase& CmdList); };
利用FRHICOMMAND_MACRO声明的RHI命令数量众多，下面列举其中一部分：
FRHICOMMAND_MACRO(FRHISyncFrameCommand)
FRHICOMMAND_MACRO(FRHICommandStat)
FRHICOMMAND_MACRO(FRHICommandRHIThreadFence)
FRHICOMMAND_MACRO(FRHIAsyncComputeSubmitList)
FRHICOMMAND_MACRO(FRHICommandSubmitSubList)
FRHICOMMAND_MACRO(FRHICommandWaitForAndSubmitSubListParallel)
FRHICOMMAND_MACRO(FRHICommandWaitForAndSubmitSubList)
FRHICOMMAND_MACRO(FRHICommandWaitForAndSubmitRTSubList)
FRHICOMMAND_MACRO(FRHICommandWaitForTemporalEffect)
FRHICOMMAND_MACRO(FRHICommandBroadcastTemporalEffect)
FRHICOMMAND_MACRO(FRHICommandBeginUpdateMultiFrameResource)
FRHICOMMAND_MACRO(FRHICommandEndUpdateMultiFrameResource)
FRHICOMMAND_MACRO(FRHICommandBeginUpdateMultiFrameUAV)
FRHICOMMAND_MACRO(FRHICommandEndUpdateMultiFrameUAV)
FRHICOMMAND_MACRO(FRHICommandSetGPUMask)
FRHICOMMAND_MACRO(FRHICommandSetStencilRef)
FRHICOMMAND_MACRO(FRHICommandSetBlendFactor)
FRHICOMMAND_MACRO(FRHICommandSetStreamSource)
FRHICOMMAND_MACRO(FRHICommandSetStreamSource)
FRHICOMMAND_MACRO(FRHICommandSetViewport)
FRHICOMMAND_MACRO(FRHICommandSetScissorRect)
FRHICOMMAND_MACRO(FRHICommandBeginRenderPass)
FRHICOMMAND_MACRO(FRHICommandEndRenderPass)
FRHICOMMAND_MACRO(FRHICommandNextSubpass)
FRHICOMMAND_MACRO(FRHICommandBeginParallelRenderPass)
FRHICOMMAND_MACRO(FRHICommandEndParallelRenderPass)
FRHICOMMAND_MACRO(FRHICommandBeginRenderSubPass)
FRHICOMMAND_MACRO(FRHICommandEndRenderSubPass)
FRHICOMMAND_MACRO(FRHICommandDrawPrimitive)
FRHICOMMAND_MACRO(FRHICommandDrawIndexedPrimitive)
FRHICOMMAND_MACRO(FRHICommandDrawPrimitiveIndirect)
FRHICOMMAND_MACRO(FRHICommandDrawIndexedIndirect)
FRHICOMMAND_MACRO(FRHICommandDrawIndexedPrimitiveIndirect)
FRHICOMMAND_MACRO(FRHICommandSetGraphicsPipelineState)
FRHICOMMAND_MACRO(FRHICommandBeginUAVOverlap)
FRHICOMMAND_MACRO(FRHICommandEndUAVOverlap)
FRHICOMMAND_MACRO(FRHICommandSetDepthBounds)
FRHICOMMAND_MACRO(FRHICommandSetShadingRate)
FRHICOMMAND_MACRO(FRHICommandSetShadingRateImage)
FRHICOMMAND_MACRO(FRHICommandClearUAVFloat)
FRHICOMMAND_MACRO(FRHICommandCopyToResolveTarget)
FRHICOMMAND_MACRO(FRHICommandCopyTexture)
FRHICOMMAND_MACRO(FRHICommandBeginTransitions)
FRHICOMMAND_MACRO(FRHICommandEndTransitions)
FRHICOMMAND_MACRO(FRHICommandResourceTransition)
FRHICOMMAND_MACRO(FRHICommandClearColorTexture)
FRHICOMMAND_MACRO(FRHICommandClearDepthStencilTexture)
FRHICOMMAND_MACRO(FRHICommandClearColorTextures)
FRHICOMMAND_MACRO(FRHICommandSetGlobalUniformBuffers)
FRHICOMMAND_MACRO(FRHICommandBuildLocalUniformBuffer)
FRHICOMMAND_MACRO(FRHICommandBeginRenderQuery)
FRHICOMMAND_MACRO(FRHICommandEndRenderQuery)
FRHICOMMAND_MACRO(FRHICommandPollOcclusionQueries)
FRHICOMMAND_MACRO(FRHICommandBeginScene)
FRHICOMMAND_MACRO(FRHICommandEndScene)
FRHICOMMAND_MACRO(FRHICommandBeginFrame)
FRHICOMMAND_MACRO(FRHICommandEndFrame)
FRHICOMMAND_MACRO(FRHICommandBeginDrawingViewport)
FRHICOMMAND_MACRO(FRHICommandEndDrawingViewport)
FRHICOMMAND_MACRO(FRHICommandInvalidateCachedState)
FRHICOMMAND_MACRO(FRHICommandDiscardRenderTargets)
FRHICOMMAND_MACRO(FRHICommandUpdateTextureReference)
FRHICOMMAND_MACRO(FRHICommandUpdateRHIResources)
FRHICOMMAND_MACRO(FRHICommandBackBufferWaitTrackingBeginFrame)
FRHICOMMAND_MACRO(FRHICommandFlushTextureCacheBOP)
FRHICOMMAND_MACRO(FRHICommandCopyBufferRegion)
FRHICOMMAND_MACRO(FRHICommandCopyBufferRegions)
FRHICOMMAND_MACRO(FClearCachedRenderingDataCommand)
FRHICOMMAND_MACRO(FClearCachedElementDataCommand)
FRHICOMMAND_MACRO(FRHICommandRayTraceOcclusion)
FRHICOMMAND_MACRO(FRHICommandRayTraceIntersection)
FRHICOMMAND_MACRO(FRHICommandRayTraceDispatch)
FRHICOMMAND_MACRO(FRHICommandSetRayTracingBindings)
FRHICOMMAND_MACRO(FRHICommandClearRayTracingBindings)
这些RHI命令的void Execute(FRHICommandListBase& CmdList)函数大多实现在UnrealEngine\Engine\Source\Runtime\RHI\Public\RHICommandListCommandExecutes.inl文件中
void FRHICommandDrawPrimitive::Execute(FRHICommandListBase& CmdList)的实现体如下：
void FRHICommandDrawPrimitive::Execute(FRHICommandListBase& CmdList) { RHISTAT(DrawPrimitive); // 需开启RHI_STATS宏，才能统计 INTERNAL_DECORATOR(RHIDrawPrimitive)(BaseVertexIndex, NumPrimitives, NumInstances); //宏展开后为：CmdList.GetContext().RHIDrawPrimitive(BaseVertexIndex, NumPrimitives, NumInstances); }
FRHICommand的子类除了以上用FRHICOMMAND_MACRO声明的，还拥有以下直接派生的：
FRHICommandSetShaderParameter
FRHICommandSetShaderUniformBuffer
FRHICommandSetShaderTexture
FRHICommandSetShaderResourceViewParameter
FRHICommandSetUAVParameter
FRHICommandSetShaderSampler
FRHICommandSetComputeShader
FRHICommandSetComputePipelineState
FRHICommandDispatchComputeShader
FRHICommandDispatchIndirectComputeShader
FRHICommandSetAsyncComputeBudget
FRHICommandCopyToStagingBuffer
FRHICommandWriteGPUFence
FRHICommandSetLocalUniformBuffer
FRHICommandSubmitCommandsHint
FRHICommandPushEvent
FRHICommandPopEvent
FRHICommandBuildAccelerationStructure
FRHICommandBuildAccelerationStructures
......
无论是直接派生还是用FRHICOMMAND_MACRO宏，都是FRHICommand的子类，都可以提供给渲染线程操作的RHI层渲染命令。只是用FRHICOMMAND_MACRO会更简便，少写一些重复的代码罢了。
因此可知，RHI命令种类繁多，主要包含以下几大类：
- 数据和资源的设置、更新、清理、转换、拷贝、回读。
- 图元绘制。
- Pass、SubPass、场景、ViewPort等的开始和结束事件。
- 栅栏、等待、广播接口。
- 光线追踪。
- Slate、调试相关的命令。
下面绘制出FRHICommand的核心继承体系：
FRHICommandList
FRHICommandList是RHI的指令队列，用来管理、执行一组FRHICommand的对象。它和父类的定义如下：
// Engine\Source\Runtime\RHI\Public\RHICommandList.h // RHI命令列表基类. class FRHICommandListBase : public FNoncopyable // 不允许拷贝构造 { public: ~FRHICommandListBase(); // 附带了循环利用的自定义new/delete操作. void* operator new(size_t Size); void operator delete(void *RawMemory); // 刷新命令队列. inline void Flush(); // 是否立即模式. inline bool IsImmediate(); // 是否立即的异步计算. inline bool IsImmediateAsyncCompute(); // 获取已占用的内存. const int32 GetUsedMemory() const; // 入队异步命令队列的提交. void QueueAsyncCommandListSubmit(FGraphEventRef& AnyThreadCompletionEvent, class FRHICommandList* CmdList); // 入队并行的异步命令队列的提交. void QueueParallelAsyncCommandListSubmit(FGraphEventRef* AnyThreadCompletionEvents, bool bIsPrepass, class FRHICommandList** CmdLists, int32* NumDrawsIfKnown, int32 Num, int32 MinDrawsPerTranslate, bool bSpewMerge); // 入队渲染线程命令队列的提交. void QueueRenderThreadCommandListSubmit(FGraphEventRef& RenderThreadCompletionEvent, class FRHICommandList* CmdList); // 入队命令队列的提交. void QueueCommandListSubmit(class FRHICommandList* CmdList); // 增加派发前序任务. void AddDispatchPrerequisite(const FGraphEventRef& Prereq); // 等待接口. void WaitForTasks(bool bKnownToBeComplete = false); void WaitForDispatch(); void WaitForRHIThreadTasks(); void HandleRTThreadTaskCompletion(const FGraphEventRef& MyCompletionGraphEvent); // 分配接口. void* Alloc(int32 AllocSize, int32 Alignment); template <typename T> void* Alloc(); template <typename T> const TArrayView<T> AllocArray(const TArrayView<T> InArray); TCHAR* AllocString(const TCHAR* Name); // 分配指令. void* AllocCommand(int32 AllocSize, int32 Alignment); template <typename TCmd> void* AllocCommand(); bool HasCommands() const; bool IsExecuting() const; bool IsBottomOfPipe() const; bool IsTopOfPipe() const; bool IsGraphics() const; bool IsAsyncCompute() const; // RHI管线, ERHIPipeline::Graphics或ERHIPipeline::AsyncCompute. ERHIPipeline GetPipeline() const; // 是否忽略RHI线程而直接当同步执行. bool Bypass() const; // 交换命令队列. void ExchangeCmdList(FRHICommandListBase& Other); // 设置Context. void SetContext(IRHICommandContext* InContext); IRHICommandContext& GetContext(); void SetComputeContext(IRHIComputeContext* InComputeContext); IRHIComputeContext& GetComputeContext(); void CopyContext(FRHICommandListBase& ParentCommandList); void MaybeDispatchToRHIThread(); void MaybeDispatchToRHIThreadInner(); (......) private: // 命令链表的头. FRHICommandBase* Root; // 指向Root的指针. FRHICommandBase** CommandLink; bool bExecuting; uint32 NumCommands; uint32 UID; // 设备上下文. IRHICommandContext* Context; // 计算上下文. IRHIComputeContext* ComputeContext; FMemStackBase MemManager; FGraphEventArray RTTasks; // 重置. void Reset(); public: enum class ERenderThreadContext { SceneRenderTargets, Num }; // 渲染线程上下文. void *RenderThreadContexts[(int32)ERenderThreadContext::Num]; protected: //the values of this struct must be copied when the commandlist is split struct FPSOContext { uint32 CachedNumSimultanousRenderTargets = 0; TStaticArray<FRHIRenderTargetView, MaxSimultaneousRenderTargets> CachedRenderTargets; FRHIDepthRenderTargetView CachedDepthStencilTarget; ESubpassHint SubpassHint = ESubpassHint::None; uint8 SubpassIndex = 0; uint8 MultiViewCount = 0; bool HasFragmentDensityAttachment = false; } PSOContext; // 绑定的着色器输入. FBoundShaderStateInput BoundShaderInput; // 绑定的计算着色器RHI资源. FRHIComputeShader* BoundComputeShaderRHI; // 使绑定的着色器生效. void ValidateBoundShader(FRHIVertexShader* ShaderRHI); void ValidateBoundShader(FRHIPixelShader* ShaderRHI); (......) void CacheActiveRenderTargets(...); void CacheActiveRenderTargets(const FRHIRenderPassInfo& Info); void IncrementSubpass(); void ResetSubpass(ESubpassHint SubpassHint); public: void CopyRenderThreadContexts(const FRHICommandListBase& ParentCommandList); void SetRenderThreadContext(void* InContext, ERenderThreadContext Slot); void* GetRenderThreadContext(ERenderThreadContext Slot); // 通用数据. struct FCommonData { class FRHICommandListBase* Parent = nullptr; enum class ECmdListType { Immediate = 1, Regular, }; ECmdListType Type = ECmdListType::Regular; bool bInsideRenderPass = false; bool bInsideComputePass = false; }; bool DoValidation() const; inline bool IsOutsideRenderPass() const; inline bool IsInsideRenderPass() const; inline bool IsInsideComputePass() const; FCommonData Data; }; // 计算命令队列. class FRHIComputeCommandList : public FRHICommandListBase { public: FRHIComputeCommandList(FRHIGPUMask GPUMask) : FRHICommandListBase(GPUMask) {} void* operator new(size_t Size); void operator delete(void *RawMemory); // 着色器参数设置和获取. inline FRHIComputeShader* GetBoundComputeShader() const; void SetGlobalUniformBuffers(const FUniformBufferStaticBindings& UniformBuffers); void SetShaderUniformBuffer(FRHIComputeShader* Shader, uint32 BaseIndex, FRHIUniformBuffer* UniformBuffer); void SetShaderUniformBuffer(const FComputeShaderRHIRef& Shader, uint32 BaseIndex, FRHIUniformBuffer* UniformBuffer); void SetShaderParameter(FRHIComputeShader* Shader, uint32 BufferIndex, uint32 BaseIndex, uint32 NumBytes, const void* NewValue); void SetShaderParameter(FComputeShaderRHIRef& Shader, uint32 BufferIndex, uint32 BaseIndex, uint32 NumBytes, const void* NewValue); void SetShaderTexture(FRHIComputeShader* Shader, uint32 TextureIndex, FRHITexture* Texture); void SetShaderResourceViewParameter(FRHIComputeShader* Shader, uint32 SamplerIndex, FRHIShaderResourceView* SRV); void SetShaderSampler(FRHIComputeShader* Shader, uint32 SamplerIndex, FRHISamplerState* State); void SetUAVParameter(FRHIComputeShader* Shader, uint32 UAVIndex, FRHIUnorderedAccessView* UAV); void SetUAVParameter(FRHIComputeShader* Shader, uint32 UAVIndex, FRHIUnorderedAccessView* UAV, uint32 InitialCount); void SetComputeShader(FRHIComputeShader* ComputeShader); void SetComputePipelineState(FComputePipelineState* ComputePipelineState, FRHIComputeShader* ComputeShader); void SetAsyncComputeBudget(EAsyncComputeBudget Budget); // 派发计算着色器. void DispatchComputeShader(uint32 ThreadGroupCountX, uint32 ThreadGroupCountY, uint32 ThreadGroupCountZ); void DispatchIndirectComputeShader(FRHIVertexBuffer* ArgumentBuffer, uint32 ArgumentOffset); // 清理. void ClearUAVFloat(FRHIUnorderedAccessView* UnorderedAccessViewRHI, const FVector4& Values); void ClearUAVUint(FRHIUnorderedAccessView* UnorderedAccessViewRHI, const FUintVector4& Values); // 资源转换. void BeginTransitions(TArrayView<const FRHITransition*> Transitions); void EndTransitions(TArrayView<const FRHITransition*> Transitions); inline void Transition(TArrayView<const FRHITransitionInfo> Infos); void BeginTransition(const FRHITransition* Transition); void EndTransition(const FRHITransition* Transition); void Transition(const FRHITransitionInfo& Info) // ---- 旧有的API ---- void TransitionResource(ERHIAccess TransitionType, const FTextureRHIRef& InTexture); void TransitionResource(ERHIAccess TransitionType, FRHITexture* InTexture); inline void TransitionResources(ERHIAccess TransitionType, FRHITexture* const* InTextures, int32 NumTextures); void TransitionResourceArrayNoCopy(ERHIAccess TransitionType, TArray<FRHITexture*>& InTextures); inline void TransitionResources(ERHIAccess TransitionType, EResourceTransitionPipeline /* ignored TransitionPipeline */, FRHIUnorderedAccessView* const* InUAVs, int32 NumUAVs, FRHIComputeFence* WriteFence); void TransitionResource(ERHIAccess TransitionType, EResourceTransitionPipeline TransitionPipeline, FRHIUnorderedAccessView* InUAV, FRHIComputeFence* WriteFence); void TransitionResource(ERHIAccess TransitionType, EResourceTransitionPipeline TransitionPipeline, FRHIUnorderedAccessView* InUAV); void TransitionResources(ERHIAccess TransitionType, EResourceTransitionPipeline TransitionPipeline, FRHIUnorderedAccessView* const* InUAVs, int32 NumUAVs); void WaitComputeFence(FRHIComputeFence* WaitFence); void BeginUAVOverlap(); void EndUAVOverlap(); void BeginUAVOverlap(FRHIUnorderedAccessView* UAV); void EndUAVOverlap(FRHIUnorderedAccessView* UAV); void BeginUAVOverlap(TArrayView<FRHIUnorderedAccessView* const> UAVs); void EndUAVOverlap(TArrayView<FRHIUnorderedAccessView* const> UAVs); void PushEvent(const TCHAR* Name, FColor Color); void PopEvent(); void BreakPoint(); void SubmitCommandsHint(); void CopyToStagingBuffer(FRHIVertexBuffer* SourceBuffer, FRHIStagingBuffer* DestinationStagingBuffer, uint32 Offset, uint32 NumBytes); void WriteGPUFence(FRHIGPUFence* Fence); void SetGPUMask(FRHIGPUMask InGPUMask); (......) }; // RHI命令队列. class FRHICommandList : public FRHIComputeCommandList { public: FRHICommandList(FRHIGPUMask GPUMask) : FRHIComputeCommandList(GPUMask) {} bool AsyncPSOCompileAllowed() const; void* operator new(size_t Size); void operator delete(void *RawMemory); // 获取绑定的着色器. inline FRHIVertexShader* GetBoundVertexShader() const; inline FRHIHullShader* GetBoundHullShader() const; inline FRHIDomainShader* GetBoundDomainShader() const; inline FRHIPixelShader* GetBoundPixelShader() const; inline FRHIGeometryShader* GetBoundGeometryShader() const; // 更新多帧资源. void BeginUpdateMultiFrameResource(FRHITexture* Texture); void EndUpdateMultiFrameResource(FRHITexture* Texture); void BeginUpdateMultiFrameResource(FRHIUnorderedAccessView* UAV); void EndUpdateMultiFrameResource(FRHIUnorderedAccessView* UAV); // Uniform Buffer接口. FLocalUniformBuffer BuildLocalUniformBuffer(const void* Contents, uint32 ContentsSize, const FRHIUniformBufferLayout& Layout); template <typename TRHIShader> void SetLocalShaderUniformBuffer(TRHIShader* Shader, uint32 BaseIndex, const FLocalUniformBuffer& UniformBuffer); template <typename TShaderRHI> void SetLocalShaderUniformBuffer(const TRefCountPtr<TShaderRHI>& Shader, uint32 BaseIndex, const FLocalUniformBuffer& UniformBuffer); void SetShaderUniformBuffer(FRHIGraphicsShader* Shader, uint32 BaseIndex, FRHIUniformBuffer* UniformBuffer); template <typename TShaderRHI> FORCEINLINE void SetShaderUniformBuffer(const TRefCountPtr<TShaderRHI>& Shader, uint32 BaseIndex, FRHIUniformBuffer* UniformBuffer); // 着色器参数. void SetShaderParameter(FRHIGraphicsShader* Shader, uint32 BufferIndex, uint32 BaseIndex, uint32 NumBytes, const void* NewValue); template <typename TShaderRHI> void SetShaderParameter(const TRefCountPtr<TShaderRHI>& Shader, uint32 BufferIndex, uint32 BaseIndex, uint32 NumBytes, const void* NewValue); void SetShaderTexture(FRHIGraphicsShader* Shader, uint32 TextureIndex, FRHITexture* Texture); template <typename TShaderRHI> void SetShaderTexture(const TRefCountPtr<TShaderRHI>& Shader, uint32 TextureIndex, FRHITexture* Texture); void SetShaderResourceViewParameter(FRHIGraphicsShader* Shader, uint32 SamplerIndex, FRHIShaderResourceView* SRV); template <typename TShaderRHI> void SetShaderResourceViewParameter(const TRefCountPtr<TShaderRHI>& Shader, uint32 SamplerIndex, FRHIShaderResourceView* SRV); void SetShaderSampler(FRHIGraphicsShader* Shader, uint32 SamplerIndex, FRHISamplerState* State); template <typename TShaderRHI> void SetShaderSampler(const TRefCountPtr<TShaderRHI>& Shader, uint32 SamplerIndex, FRHISamplerState* State); void SetUAVParameter(FRHIPixelShader* Shader, uint32 UAVIndex, FRHIUnorderedAccessView* UAV); void SetUAVParameter(const TRefCountPtr<FRHIPixelShader>& Shader, uint32 UAVIndex, FRHIUnorderedAccessView* UAV); void SetBlendFactor(const FLinearColor& BlendFactor = FLinearColor::White); // 图元绘制. void DrawPrimitive(uint32 BaseVertexIndex, uint32 NumPrimitives, uint32 NumInstances); void DrawIndexedPrimitive(FRHIIndexBuffer* IndexBuffer, int32 BaseVertexIndex, uint32 FirstInstance, uint32 NumVertices, uint32 StartIndex, uint32 NumPrimitives, uint32 NumInstances); void DrawPrimitiveIndirect(FRHIVertexBuffer* ArgumentBuffer, uint32 ArgumentOffset); void DrawIndexedIndirect(FRHIIndexBuffer* IndexBufferRHI, FRHIStructuredBuffer* ArgumentsBufferRHI, uint32 DrawArgumentsIndex, uint32 NumInstances); void DrawIndexedPrimitiveIndirect(FRHIIndexBuffer* IndexBuffer, FRHIVertexBuffer* ArgumentsBuffer, uint32 ArgumentOffset); // 设置数据. void SetStreamSource(uint32 StreamIndex, FRHIVertexBuffer* VertexBuffer, uint32 Offset); void SetStencilRef(uint32 StencilRef); void SetViewport(float MinX, float MinY, float MinZ, float MaxX, float MaxY, float MaxZ); void SetStereoViewport(float LeftMinX, float RightMinX, float LeftMinY, float RightMinY, float MinZ, float LeftMaxX, float RightMaxX, float LeftMaxY, float RightMaxY, float MaxZ); void SetScissorRect(bool bEnable, uint32 MinX, uint32 MinY, uint32 MaxX, uint32 MaxY); void ApplyCachedRenderTargets(FGraphicsPipelineStateInitializer& GraphicsPSOInit); void SetGraphicsPipelineState(class FGraphicsPipelineState* GraphicsPipelineState, const FBoundShaderStateInput& ShaderInput, bool bApplyAdditionalState); void SetDepthBounds(float MinDepth, float MaxDepth); void SetShadingRate(EVRSShadingRate ShadingRate, EVRSRateCombiner Combiner); void SetShadingRateImage(FRHITexture* RateImageTexture, EVRSRateCombiner Combiner); // 拷贝纹理. void CopyToResolveTarget(FRHITexture* SourceTextureRHI, FRHITexture* DestTextureRHI, const FResolveParams& ResolveParams); void CopyTexture(FRHITexture* SourceTextureRHI, FRHITexture* DestTextureRHI, const FRHICopyTextureInfo& CopyInfo); void ResummarizeHTile(FRHITexture2D* DepthTexture); // 渲染查询. void BeginRenderQuery(FRHIRenderQuery* RenderQuery) void EndRenderQuery(FRHIRenderQuery* RenderQuery) void CalibrateTimers(FRHITimestampCalibrationQuery* CalibrationQuery); void PollOcclusionQueries() /* LEGACY API */ void TransitionResource(FExclusiveDepthStencil DepthStencilMode, FRHITexture* DepthTexture); void BeginRenderPass(const FRHIRenderPassInfo& InInfo, const TCHAR* Name); void EndRenderPass(); void NextSubpass(); // 下面接口需要在立即模式的命令队列调用. void BeginScene(); void EndScene(); void BeginDrawingViewport(FRHIViewport* Viewport, FRHITexture* RenderTargetRHI); void EndDrawingViewport(FRHIViewport* Viewport, bool bPresent, bool bLockToVsync); void BeginFrame(); void EndFrame(); void RHIInvalidateCachedState(); void DiscardRenderTargets(bool Depth, bool Stencil, uint32 ColorBitMask); void CopyBufferRegion(FRHIVertexBuffer* DestBuffer, uint64 DstOffset, FRHIVertexBuffer* SourceBuffer, uint64 SrcOffset, uint64 NumBytes); (......) };
FRHICommandListBase定义了命令队列所需的基本数据（命令列表、设备上下文）和接口（命令的刷新、等待、入队、派发等，内存分配）。
FRHIComputeCommandList定义了计算着色器相关的接口、GPU资源状态转换和着色器部分参数的设置。
FRHICommandList定义了普通渲染管线的接口，与IRHICommandContext的接口高度相似且重叠。包含VS、PS、GS的绑定，图元绘制，更多着色器参数的设置和资源状态转换，资源创建、更新和等待等等。
FRHICommandList还有数个子类，定义如下：
// 立即模式的命令队列. class FRHICommandListImmediate : public FRHICommandList { // 命令匿名函数. template <typename LAMBDA> struct TRHILambdaCommand final : public FRHICommandBase { LAMBDA Lambda; void ExecuteAndDestruct(FRHICommandListBase& CmdList, FRHICommandListDebugContext&) override final; }; FRHICommandListImmediate(); ~FRHICommandListImmediate(); public: // 立即刷新命令. void ImmediateFlush(EImmediateFlushType::Type FlushType); // 阻塞RHI线程. bool StallRHIThread(); // 取消阻塞RHI线程. void UnStallRHIThread(); // 是否阻塞中. static bool IsStalled(); void SetCurrentStat(TStatId Stat); static FGraphEventRef RenderThreadTaskFence(); static FGraphEventArray& GetRenderThreadTaskArray(); static void WaitOnRenderThreadTaskFence(FGraphEventRef& Fence); static bool AnyRenderThreadTasksOutstanding(); FGraphEventRef RHIThreadFence(bool bSetLockFence = false); // 将给定的异步计算命令列表按当前立即命令列表的顺序排列. void QueueAsyncCompute(FRHIComputeCommandList& RHIComputeCmdList); bool IsBottomOfPipe(); bool IsTopOfPipe(); template <typename LAMBDA> void EnqueueLambda(LAMBDA&& Lambda); // 资源创建. FSamplerStateRHIRef CreateSamplerState(const FSamplerStateInitializerRHI& Initializer) FRasterizerStateRHIRef CreateRasterizerState(const FRasterizerStateInitializerRHI& Initializer) FDepthStencilStateRHIRef CreateDepthStencilState(const FDepthStencilStateInitializerRHI& Initializer) FBlendStateRHIRef CreateBlendState(const FBlendStateInitializerRHI& Initializer) FPixelShaderRHIRef CreatePixelShader(TArrayView<const uint8> Code, const FSHAHash& Hash) FVertexShaderRHIRef CreateVertexShader(TArrayView<const uint8> Code, const FSHAHash& Hash) FHullShaderRHIRef CreateHullShader(TArrayView<const uint8> Code, const FSHAHash& Hash) FDomainShaderRHIRef CreateDomainShader(TArrayView<const uint8> Code, const FSHAHash& Hash) FGeometryShaderRHIRef CreateGeometryShader(TArrayView<const uint8> Code, const FSHAHash& Hash) FComputeShaderRHIRef CreateComputeShader(TArrayView<const uint8> Code, const FSHAHash& Hash) FComputeFenceRHIRef CreateComputeFence(const FName& Name) FGPUFenceRHIRef CreateGPUFence(const FName& Name) FStagingBufferRHIRef CreateStagingBuffer() FBoundShaderStateRHIRef CreateBoundShaderState(...) FGraphicsPipelineStateRHIRef CreateGraphicsPipelineState(const FGraphicsPipelineStateInitializer& Initializer) TRefCountPtr<FRHIComputePipelineState> CreateComputePipelineState(FRHIComputeShader* ComputeShader) FUniformBufferRHIRef CreateUniformBuffer(...) FIndexBufferRHIRef CreateAndLockIndexBuffer(uint32 Stride, uint32 Size, EBufferUsageFlags InUsage, ERHIAccess InResourceState, FRHIResourceCreateInfo& CreateInfo, void*& OutDataBuffer) FIndexBufferRHIRef CreateAndLockIndexBuffer(uint32 Stride, uint32 Size, uint32 InUsage, FRHIResourceCreateInfo& CreateInfo, void*& OutDataBuffer) // 顶点/索引接口. void* LockIndexBuffer(FRHIIndexBuffer* IndexBuffer, uint32 Offset, uint32 SizeRHI, EResourceLockMode LockMode); void UnlockIndexBuffer(FRHIIndexBuffer* IndexBuffer); void* LockStagingBuffer(FRHIStagingBuffer* StagingBuffer, FRHIGPUFence* Fence, uint32 Offset, uint32 SizeRHI); void UnlockStagingBuffer(FRHIStagingBuffer* StagingBuffer); FVertexBufferRHIRef CreateAndLockVertexBuffer(uint32 Size, EBufferUsageFlags InUsage, ...); FVertexBufferRHIRef CreateAndLockVertexBuffer(uint32 Size, uint32 InUsage, FRHIResourceCreateInfo& CreateInfo, void*& OutDataBuffer); void* LockVertexBuffer(FRHIVertexBuffer* VertexBuffer, uint32 Offset, uint32 SizeRHI, EResourceLockMode LockMode); void UnlockVertexBuffer(FRHIVertexBuffer* VertexBuffer); void CopyVertexBuffer(FRHIVertexBuffer* SourceBuffer, FRHIVertexBuffer* DestBuffer); void* LockStructuredBuffer(FRHIStructuredBuffer* StructuredBuffer, uint32 Offset, uint32 SizeRHI, EResourceLockMode LockMode); void UnlockStructuredBuffer(FRHIStructuredBuffer* StructuredBuffer); // UAV/SRV创建. FUnorderedAccessViewRHIRef CreateUnorderedAccessView(FRHIStructuredBuffer* StructuredBuffer, bool bUseUAVCounter, bool bAppendBuffer) FUnorderedAccessViewRHIRef CreateUnorderedAccessView(FRHITexture* Texture, uint32 MipLevel) FUnorderedAccessViewRHIRef CreateUnorderedAccessView(FRHITexture* Texture, uint32 MipLevel, uint8 Format) FUnorderedAccessViewRHIRef CreateUnorderedAccessView(FRHIVertexBuffer* VertexBuffer, uint8 Format) FUnorderedAccessViewRHIRef CreateUnorderedAccessView(FRHIIndexBuffer* IndexBuffer, uint8 Format) FShaderResourceViewRHIRef CreateShaderResourceView(FRHIStructuredBuffer* StructuredBuffer) FShaderResourceViewRHIRef CreateShaderResourceView(FRHIVertexBuffer* VertexBuffer, uint32 Stride, uint8 Format) FShaderResourceViewRHIRef CreateShaderResourceView(const FShaderResourceViewInitializer& Initializer) FShaderResourceViewRHIRef CreateShaderResourceView(FRHIIndexBuffer* Buffer) uint64 CalcTexture2DPlatformSize(...); uint64 CalcTexture3DPlatformSize(...); uint64 CalcTextureCubePlatformSize(...); // 纹理操作. void GetTextureMemoryStats(FTextureMemoryStats& OutStats); bool GetTextureMemoryVisualizeData(...); void CopySharedMips(FRHITexture2D* DestTexture2D, FRHITexture2D* SrcTexture2D); void TransferTexture(FRHITexture2D* Texture, FIntRect Rect, uint32 SrcGPUIndex, uint32 DestGPUIndex, bool PullData); void TransferTextures(const TArrayView<const FTransferTextureParams> Params); void GetResourceInfo(FRHITexture* Ref, FRHIResourceInfo& OutInfo); FShaderResourceViewRHIRef CreateShaderResourceView(FRHITexture* Texture, const FRHITextureSRVCreateInfo& CreateInfo); FShaderResourceViewRHIRef CreateShaderResourceView(FRHITexture* Texture, uint8 MipLevel); FShaderResourceViewRHIRef CreateShaderResourceView(FRHITexture* Texture, uint8 MipLevel, uint8 NumMipLevels, uint8 Format); FShaderResourceViewRHIRef CreateShaderResourceViewWriteMask(FRHITexture2D* Texture2DRHI); FShaderResourceViewRHIRef CreateShaderResourceViewFMask(FRHITexture2D* Texture2DRHI); uint32 ComputeMemorySize(FRHITexture* TextureRHI); FTexture2DRHIRef AsyncReallocateTexture2D(...); ETextureReallocationStatus FinalizeAsyncReallocateTexture2D(FRHITexture2D* Texture2D, bool bBlockUntilCompleted); ETextureReallocationStatus CancelAsyncReallocateTexture2D(FRHITexture2D* Texture2D, bool bBlockUntilCompleted); void* LockTexture2D(...); void UnlockTexture2D(FRHITexture2D* Texture, uint32 MipIndex, bool bLockWithinMiptail, bool bFlushRHIThread = true); void* LockTexture2DArray(...); void UnlockTexture2DArray(FRHITexture2DArray* Texture, uint32 TextureIndex, uint32 MipIndex, bool bLockWithinMiptail); void UpdateTexture2D(...); void UpdateFromBufferTexture2D(...); FUpdateTexture3DData BeginUpdateTexture3D(...); void EndUpdateTexture3D(FUpdateTexture3DData& UpdateData); void EndMultiUpdateTexture3D(TArray<FUpdateTexture3DData>& UpdateDataArray); void UpdateTexture3D(...); void* LockTextureCubeFace(...); void UnlockTextureCubeFace(FRHITextureCube* Texture, ...); // 读取纹理表面数据. void ReadSurfaceData(FRHITexture* Texture, ...); void ReadSurfaceData(FRHITexture* Texture, ...); void MapStagingSurface(FRHITexture* Texture, void*& OutData, int32& OutWidth, int32& OutHeight); void MapStagingSurface(FRHITexture* Texture, ...); void UnmapStagingSurface(FRHITexture* Texture); void ReadSurfaceFloatData(FRHITexture* Texture, ...); void ReadSurfaceFloatData(FRHITexture* Texture, ...); void Read3DSurfaceFloatData(FRHITexture* Texture,...); // 渲染线程的资源状态转换. void AcquireTransientResource_RenderThread(FRHITexture* Texture); void DiscardTransientResource_RenderThread(FRHITexture* Texture); void AcquireTransientResource_RenderThread(FRHIVertexBuffer* Buffer); void DiscardTransientResource_RenderThread(FRHIVertexBuffer* Buffer); void AcquireTransientResource_RenderThread(FRHIStructuredBuffer* Buffer); void DiscardTransientResource_RenderThread(FRHIStructuredBuffer* Buffer); // 获取渲染查询结果. bool GetRenderQueryResult(FRHIRenderQuery* RenderQuery, ...); void PollRenderQueryResults(); // 视口 FViewportRHIRef CreateViewport(void* WindowHandle, ...); uint32 GetViewportNextPresentGPUIndex(FRHIViewport* Viewport); FTexture2DRHIRef GetViewportBackBuffer(FRHIViewport* Viewport); void AdvanceFrameForGetViewportBackBuffer(FRHIViewport* Viewport); void ResizeViewport(FRHIViewport* Viewport, ...); void AcquireThreadOwnership(); void ReleaseThreadOwnership(); // 提交命令并刷新到GPU. void SubmitCommandsAndFlushGPU(); // 执行命令队列. void ExecuteCommandList(FRHICommandList* CmdList); // 更新资源. void UpdateTextureReference(FRHITextureReference* TextureRef, FRHITexture* NewTexture); void UpdateRHIResources(FRHIResourceUpdateInfo* UpdateInfos, int32 Num, bool bNeedReleaseRefs); // 刷新资源. void FlushResources(); // 帧更新. void Tick(float DeltaTime); // 阻塞直到GPU空闲. // 强制把当前的所有rhi中指令执行完毕，并且把commandbuffer发送给gpu，并且等待gpu执行完成，相当于一个强制同步到GPU的过程. void BlockUntilGPUIdle(); // 暂停/开启渲染. void SuspendRendering(); void ResumeRendering(); bool IsRenderingSuspended(); // 压缩/解压数据. bool EnqueueDecompress(uint8_t* SrcBuffer, uint8_t* DestBuffer, int CompressedSize, void* ErrorCodeBuffer); bool EnqueueCompress(uint8_t* SrcBuffer, uint8_t* DestBuffer, int UnCompressedSize, void* ErrorCodeBuffer); // 其它接口. bool GetAvailableResolutions(FScreenResolutionArray& Resolutions, bool bIgnoreRefreshRate); void GetSupportedResolution(uint32& Width, uint32& Height); void VirtualTextureSetFirstMipInMemory(FRHITexture2D* Texture, uint32 FirstMip); void VirtualTextureSetFirstMipVisible(FRHITexture2D* Texture, uint32 FirstMip); // 获取原生的数据. void* GetNativeDevice(); void* GetNativeInstance(); // 获取立即模式的命令上下文. IRHICommandContext* GetDefaultContext(); // 获取命令上下文容器. IRHICommandContextContainer* GetCommandContextContainer(int32 Index, int32 Num); uint32 GetGPUFrameCycles(); }; // 在RHI实现中标记命令列表的递归使用的类型定义. class FRHICommandList_RecursiveHazardous : public FRHICommandList { public: FRHICommandList_RecursiveHazardous(IRHICommandContext *Context, FRHIGPUMask InGPUMask = FRHIGPUMask::All()); }; // RHI内部使用的工具类，以更安全地使用FRHICommandList_RecursiveHazardous template <typename ContextType> class TRHICommandList_RecursiveHazardous : public FRHICommandList_RecursiveHazardous { template <typename LAMBDA> struct TRHILambdaCommand final : public FRHICommandBase { LAMBDA Lambda; TRHILambdaCommand(LAMBDA&& InLambda); void ExecuteAndDestruct(FRHICommandListBase& CmdList, FRHICommandListDebugContext&) override final; }; public: TRHICommandList_RecursiveHazardous(ContextType *Context, FRHIGPUMask GPUMask = FRHIGPUMask::All()); template <typename LAMBDA> void RunOnContext(LAMBDA&& Lambda); };
FRHICommandListImmediate封装了立即模式的图形API接口，在UE渲染体系中被应用得非常广泛。它额外定义了资源的操作、创建、更新、读取和状态转换接口，也增加了线程同步和GPU同步的接口。
下面对FRHICommandList核心继承体系来个UML图总结一下：
RHI体系总览
若抛开图形API的具体实现细节和众多的RHI具体子类，将RHI Context/CommandList/Command/Resource等的顶层概念汇总成UML关系图，则是如下模样：
下图是在上面的基础上细化了子类的UML：
RHI命令执行
FRHICommandListExecutor
FRHICommandListExecutor负责将Renderer层的RHI中间指令转译（或直接调用）到目标平台的图形API，它在RHI体系中起着举足轻重的作用，定义如下：
// Engine\Source\Runtime\RHI\Public\RHICommandList.h class RHI_API FRHICommandListExecutor { public: enum { DefaultBypass = PLATFORM_RHITHREAD_DEFAULT_BYPASS }; FRHICommandListExecutor() : bLatchedBypass(!!DefaultBypass) , bLatchedUseParallelAlgorithms(false) { } // 静态接口, 获取立即命令列表. static inline FRHICommandListImmediate& GetImmediateCommandList(); // 静态接口, 获取立即异步计算命令列表. static inline FRHIAsyncComputeCommandListImmediate& GetImmediateAsyncComputeCommandList(); // 执行命令列表. void ExecuteList(FRHICommandListBase& CmdList); void ExecuteList(FRHICommandListImmediate& CmdList); void LatchBypass(); // 等待RHI线程栅栏. static void WaitOnRHIThreadFence(FGraphEventRef& Fence); // 是否绕过命令生成模式, 如果是, 则直接调用目标平台的图形API. FORCEINLINE_DEBUGGABLE bool Bypass() { #if CAN_TOGGLE_COMMAND_LIST_BYPASS return bLatchedBypass; #else return !!DefaultBypass; #endif } // 是否使用并行算法. FORCEINLINE_DEBUGGABLE bool UseParallelAlgorithms() { #if CAN_TOGGLE_COMMAND_LIST_BYPASS return bLatchedUseParallelAlgorithms; #else return FApp::ShouldUseThreadingForPerformance() && !Bypass() && (GSupportsParallelRenderingTasksWithSeparateRHIThread || !IsRunningRHIInSeparateThread()); #endif } static void CheckNoOutstandingCmdLists(); static bool IsRHIThreadActive(); static bool IsRHIThreadCompletelyFlushed(); private: // 内部执行. void ExecuteInner(FRHICommandListBase& CmdList); // 内部执行, 真正执行转译. static void ExecuteInner_DoExecute(FRHICommandListBase& CmdList); bool bLatchedBypass; bool bLatchedUseParallelAlgorithms; // 同步变量. FThreadSafeCounter UIDCounter; FThreadSafeCounter OutstandingCmdListCount; // 立即模式的命令队列. FRHICommandListImmediate CommandListImmediate; // 立即模式的异步计算命令队列. FRHIAsyncComputeCommandListImmediate AsyncComputeCmdListImmediate; };
下面是FRHICommandListExecutor部分重要接口的实现代码：
// Engine\Source\Runtime\RHI\Private\RHICommandList.cpp // 检测RHI线程是否激活状态. bool FRHICommandListExecutor::IsRHIThreadActive() { // 是否异步提交. bool bAsyncSubmit = CVarRHICmdAsyncRHIThreadDispatch.GetValueOnRenderThread() > 0; // r.RHICmdAsyncRHIThreadDispatch // 1. 先检测是否存在未完成的子命令列表提交任务. if (bAsyncSubmit) { if (RenderThreadSublistDispatchTask.GetReference() && RenderThreadSublistDispatchTask->IsComplete()) { RenderThreadSublistDispatchTask = nullptr; } if (RenderThreadSublistDispatchTask.GetReference()) { return true; // it might become active at any time } // otherwise we can safely look at RHIThreadTask } // 2. 再检测是否存在未完成的RHI线程任务. if (RHIThreadTask.GetReference() && RHIThreadTask->IsComplete()) { RHIThreadTask = nullptr; PrevRHIThreadTask = nullptr; } return !!RHIThreadTask.GetReference(); } // 检测RHI线程是否完全刷新了数据. bool FRHICommandListExecutor::IsRHIThreadCompletelyFlushed() { if (IsRHIThreadActive() || GetImmediateCommandList().HasCommands()) { return false; } if (RenderThreadSublistDispatchTask.GetReference() && RenderThreadSublistDispatchTask->IsComplete()) { #if NEEDS_DEBUG_INFO_ON_PRESENT_HANG bRenderThreadSublistDispatchTaskClearedOnRT = IsInActualRenderingThread(); bRenderThreadSublistDispatchTaskClearedOnGT = IsInGameThread(); #endif RenderThreadSublistDispatchTask = nullptr; } return !RenderThreadSublistDispatchTask; } void FRHICommandListExecutor::ExecuteList(FRHICommandListImmediate& CmdList) { { SCOPE_CYCLE_COUNTER(STAT_ImmedCmdListExecuteTime); ExecuteInner(CmdList); } } void FRHICommandListExecutor::ExecuteList(FRHICommandListBase& CmdList) { // 执行命令队列转换之前先刷新已有的命令. if (IsInRenderingThread() && !GetImmediateCommandList().IsExecuting()) { GetImmediateCommandList().ImmediateFlush(EImmediateFlushType::DispatchToRHIThread); } // 内部执行. ExecuteInner(CmdList); } void FRHICommandListExecutor::ExecuteInner(FRHICommandListBase& CmdList) { // 是否在渲染线程中. bool bIsInRenderingThread = IsInRenderingThread(); // 是否在游戏线程中. bool bIsInGameThread = IsInGameThread(); // 开启了专用的RHI线程. if (IsRunningRHIInSeparateThread()) { bool bAsyncSubmit = false; ENamedThreads::Type RenderThread_Local = ENamedThreads::GetRenderThread_Local(); if (bIsInRenderingThread) { if (!bIsInGameThread && !FTaskGraphInterface::Get().IsThreadProcessingTasks(RenderThread_Local)) { // 把所有需要传递的东西都处理掉. FTaskGraphInterface::Get().ProcessThreadUntilIdle(RenderThread_Local); } // 检测子命令列表任务是否完成. bAsyncSubmit = CVarRHICmdAsyncRHIThreadDispatch.GetValueOnRenderThread() > 0; // r.RHICmdAsyncRHIThreadDispatch if (RenderThreadSublistDispatchTask.GetReference() && RenderThreadSublistDispatchTask->IsComplete()) { RenderThreadSublistDispatchTask = nullptr; if (bAsyncSubmit && RHIThreadTask.GetReference() && RHIThreadTask->IsComplete()) { RHIThreadTask = nullptr; PrevRHIThreadTask = nullptr; } } // 检测RHI线程任务是否完成. if (!bAsyncSubmit && RHIThreadTask.GetReference() && RHIThreadTask->IsComplete()) { RHIThreadTask = nullptr; PrevRHIThreadTask = nullptr; } } if (CVarRHICmdUseThread.GetValueOnRenderThread() > 0 && bIsInRenderingThread && !bIsInGameThread) // r.RHICmdUseThread { // 交换前序和RT线程任务的列表. FRHICommandList* SwapCmdList; FGraphEventArray Prereq; Exchange(Prereq, CmdList.RTTasks); { QUICK_SCOPE_CYCLE_COUNTER(STAT_FRHICommandListExecutor_SwapCmdLists); SwapCmdList = new FRHICommandList(CmdList.GetGPUMask()); static_assert(sizeof(FRHICommandList) == sizeof(FRHICommandListImmediate), "We are memswapping FRHICommandList and FRHICommandListImmediate; they need to be swappable."); SwapCmdList->ExchangeCmdList(CmdList); CmdList.CopyContext(*SwapCmdList); CmdList.GPUMask = SwapCmdList->GPUMask; CmdList.InitialGPUMask = SwapCmdList->GPUMask; CmdList.PSOContext = SwapCmdList->PSOContext; CmdList.Data.bInsideRenderPass = SwapCmdList->Data.bInsideRenderPass; CmdList.Data.bInsideComputePass = SwapCmdList->Data.bInsideComputePass; } // 提交任务. QUICK_SCOPE_CYCLE_COUNTER(STAT_FRHICommandListExecutor_SubmitTasks); // 创建FDispatchRHIThreadTask, 并将AllOutstandingTasks和RenderThreadSublistDispatchTask作为它的前序任务. if (AllOutstandingTasks.Num() || RenderThreadSublistDispatchTask.GetReference()) { Prereq.Append(AllOutstandingTasks); AllOutstandingTasks.Reset(); if (RenderThreadSublistDispatchTask.GetReference()) { Prereq.Add(RenderThreadSublistDispatchTask); } RenderThreadSublistDispatchTask = TGraphTask<FDispatchRHIThreadTask>::CreateTask(&Prereq, ENamedThreads::GetRenderThread()).ConstructAndDispatchWhenReady(SwapCmdList, bAsyncSubmit); } // 创建FExecuteRHIThreadTask, 并将RHIThreadTask作为它的前序任务. else { if (RHIThreadTask.GetReference()) { Prereq.Add(RHIThreadTask); } PrevRHIThreadTask = RHIThreadTask; RHIThreadTask = TGraphTask<FExecuteRHIThreadTask>::CreateTask(&Prereq, ENamedThreads::GetRenderThread()).ConstructAndDispatchWhenReady(SwapCmdList); } if (CVarRHICmdForceRHIFlush.GetValueOnRenderThread() > 0 ) // r.RHICmdForceRHIFlush { // 检测渲染线程是否死锁. if (FTaskGraphInterface::Get().IsThreadProcessingTasks(RenderThread_Local)) { // this is a deadlock. RT tasks must be done by now or they won't be done. We could add a third queue... UE_LOG(LogRHI, Fatal, TEXT("Deadlock in FRHICommandListExecutor::ExecuteInner 2.")); } // 检测RenderThreadSublistDispatchTask是否完成. if (RenderThreadSublistDispatchTask.GetReference()) { FTaskGraphInterface::Get().WaitUntilTaskCompletes(RenderThreadSublistDispatchTask, RenderThread_Local); RenderThreadSublistDispatchTask = nullptr; } // 等待RHIThreadTask完成. while (RHIThreadTask.GetReference()) { FTaskGraphInterface::Get().WaitUntilTaskCompletes(RHIThreadTask, RenderThread_Local); if (RHIThreadTask.GetReference() && RHIThreadTask->IsComplete()) { RHIThreadTask = nullptr; PrevRHIThreadTask = nullptr; } } } return; } // 执行RTTasks/RenderThreadSublistDispatchTask/RHIThreadTask等任务. if (bIsInRenderingThread) { if (CmdList.RTTasks.Num()) { if (FTaskGraphInterface::Get().IsThreadProcessingTasks(RenderThread_Local)) { UE_LOG(LogRHI, Fatal, TEXT("Deadlock in FRHICommandListExecutor::ExecuteInner (RTTasks).")); } FTaskGraphInterface::Get().WaitUntilTasksComplete(CmdList.RTTasks, RenderThread_Local); CmdList.RTTasks.Reset(); } if (RenderThreadSublistDispatchTask.GetReference()) { if (FTaskGraphInterface::Get().IsThreadProcessingTasks(RenderThread_Local)) { // this is a deadlock. RT tasks must be done by now or they won't be done. We could add a third queue... UE_LOG(LogRHI, Fatal, TEXT("Deadlock in FRHICommandListExecutor::ExecuteInner (RenderThreadSublistDispatchTask).")); } FTaskGraphInterface::Get().WaitUntilTaskCompletes(RenderThreadSublistDispatchTask, RenderThread_Local); #if NEEDS_DEBUG_INFO_ON_PRESENT_HANG bRenderThreadSublistDispatchTaskClearedOnRT = IsInActualRenderingThread(); bRenderThreadSublistDispatchTaskClearedOnGT = bIsInGameThread; #endif RenderThreadSublistDispatchTask = nullptr; } while (RHIThreadTask.GetReference()) { if (FTaskGraphInterface::Get().IsThreadProcessingTasks(RenderThread_Local)) { // this is a deadlock. RT tasks must be done by now or they won't be done. We could add a third queue... UE_LOG(LogRHI, Fatal, TEXT("Deadlock in FRHICommandListExecutor::ExecuteInner (RHIThreadTask).")); } FTaskGraphInterface::Get().WaitUntilTaskCompletes(RHIThreadTask, RenderThread_Local); if (RHIThreadTask.GetReference() && RHIThreadTask->IsComplete()) { RHIThreadTask = nullptr; PrevRHIThreadTask = nullptr; } } } } // 非RHI专用线程. else { if (bIsInRenderingThread && CmdList.RTTasks.Num()) { ENamedThreads::Type RenderThread_Local = ENamedThreads::GetRenderThread_Local(); if (FTaskGraphInterface::Get().IsThreadProcessingTasks(RenderThread_Local)) { // this is a deadlock. RT tasks must be done by now or they won't be done. We could add a third queue... UE_LOG(LogRHI, Fatal, TEXT("Deadlock in FRHICommandListExecutor::ExecuteInner (RTTasks).")); } FTaskGraphInterface::Get().WaitUntilTasksComplete(CmdList.RTTasks, RenderThread_Local); CmdList.RTTasks.Reset(); } } // 内部执行命令. ExecuteInner_DoExecute(CmdList); } void FRHICommandListExecutor::ExecuteInner_DoExecute(FRHICommandListBase& CmdList) { FScopeCycleCounter ScopeOuter(CmdList.ExecuteStat); CmdList.bExecuting = true; check(CmdList.Context || CmdList.ComputeContext); FMemMark Mark(FMemStack::Get()); // 设置多GPU的Mask. #if WITH_MGPU if (CmdList.Context != nullptr) { CmdList.Context->RHISetGPUMask(CmdList.InitialGPUMask); } if (CmdList.ComputeContext != nullptr && CmdList.ComputeContext != CmdList.Context) { CmdList.ComputeContext->RHISetGPUMask(CmdList.InitialGPUMask); } #endif FRHICommandListDebugContext DebugContext; FRHICommandListIterator Iter(CmdList); // 统计执行信息. #if STATS bool bDoStats = CVarRHICmdCollectRHIThreadStatsFromHighLevel.GetValueOnRenderThread() > 0 && FThreadStats::IsCollectingData() && (IsInRenderingThread() || IsInRHIThread()); //r.RHICmdCollectRHIThreadStatsFromHighLevel if (bDoStats) { while (Iter.HasCommandsLeft()) { TStatIdData const* Stat = GCurrentExecuteStat.GetRawPointer(); FScopeCycleCounter Scope(GCurrentExecuteStat); while (Iter.HasCommandsLeft() && Stat == GCurrentExecuteStat.GetRawPointer()) { FRHICommandBase* Cmd = Iter.NextCommand(); Cmd->ExecuteAndDestruct(CmdList, DebugContext); } } } else // 统计指定事件. #elif ENABLE_STATNAMEDEVENTS bool bDoStats = CVarRHICmdCollectRHIThreadStatsFromHighLevel.GetValueOnRenderThread() > 0 && GCycleStatsShouldEmitNamedEvents && (IsInRenderingThread() || IsInRHIThread()); //r.RHICmdCollectRHIThreadStatsFromHighLevel if (bDoStats) { while (Iter.HasCommandsLeft()) { PROFILER_CHAR const* Stat = GCurrentExecuteStat.StatString; FScopeCycleCounter Scope(GCurrentExecuteStat); while (Iter.HasCommandsLeft() && Stat == GCurrentExecuteStat.StatString) { FRHICommandBase* Cmd = Iter.NextCommand(); Cmd->ExecuteAndDestruct(CmdList, DebugContext); } } } else #endif // 不调试或不统计信息的版本. { // 循环所有命令, 执行并销毁之. while (Iter.HasCommandsLeft()) { FRHICommandBase* Cmd = Iter.NextCommand(); GCurrentCommand = Cmd; Cmd->ExecuteAndDestruct(CmdList, DebugContext); } } // 充值命令列表. CmdList.Reset(); }
由此可知，FRHICommandListExecutor处理了复杂的各类任务，并且要判定任务的前序、等待、依赖关系，还有各个线程之间的依赖和等待关系。上述代码中涉及到了两个重要的任务类型：
// 派发RHI线程任务. class FDispatchRHIThreadTask { FRHICommandListBase* RHICmdList; // 待派发的命令列表. bool bRHIThread; // 是否在RHI线程中派发. public: FDispatchRHIThreadTask(FRHICommandListBase* InRHICmdList, bool bInRHIThread) : RHICmdList(InRHICmdList) , bRHIThread(bInRHIThread) { } FORCEINLINE TStatId GetStatId() const; static ESubsequentsMode::Type GetSubsequentsMode() { return ESubsequentsMode::TrackSubsequents; } // 预期的线程由是否在RHI线程/是否在独立的RHI线程等变量决定. ENamedThreads::Type GetDesiredThread() { return bRHIThread ? (IsRunningRHIInDedicatedThread() ? ENamedThreads::RHIThread : CPrio_RHIThreadOnTaskThreads.Get()) : ENamedThreads::GetRenderThread_Local(); } void DoTask(ENamedThreads::Type CurrentThread, const FGraphEventRef& MyCompletionGraphEvent) { // 前序任务是RHIThreadTask. FGraphEventArray Prereq; if (RHIThreadTask.GetReference()) { Prereq.Add(RHIThreadTask); } // 将当前任务放到PrevRHIThreadTask中. PrevRHIThreadTask = RHIThreadTask; // 创建FExecuteRHIThreadTask任务并赋值到RHIThreadTask. RHIThreadTask = TGraphTask<FExecuteRHIThreadTask>::CreateTask(&Prereq, CurrentThread).ConstructAndDispatchWhenReady(RHICmdList); } }; // 执行RHI线程任务. class FExecuteRHIThreadTask { FRHICommandListBase* RHICmdList; public: FExecuteRHIThreadTask(FRHICommandListBase* InRHICmdList) : RHICmdList(InRHICmdList) { } FORCEINLINE TStatId GetStatId() const; static ESubsequentsMode::Type GetSubsequentsMode() { return ESubsequentsMode::TrackSubsequents; } // 根据是否在专用的RHI线程而选择RHI或渲染线程. ENamedThreads::Type GetDesiredThread() { return IsRunningRHIInDedicatedThread() ? ENamedThreads::RHIThread : CPrio_RHIThreadOnTaskThreads.Get(); } void DoTask(ENamedThreads::Type CurrentThread, const FGraphEventRef& MyCompletionGraphEvent) { // 设置全局变量GRHIThreadId if (IsRunningRHIInTaskThread()) { GRHIThreadId = FPlatformTLS::GetCurrentThreadId(); } // 执行RHI命令队列. { // 临界区, 保证线程访问安全. FScopeLock Lock(&GRHIThreadOnTasksCritical); FRHICommandListExecutor::ExecuteInner_DoExecute(*RHICmdList); delete RHICmdList; } // 清空全局变量GRHIThreadId if (IsRunningRHIInTaskThread()) { GRHIThreadId = 0; } } };
由上可知，在派发和转译命令队列时，可能在专用的RHI线程执行，也可能在渲染线程或工作线程执行。
GRHICommandList
GRHICommandList乍一看以为是FRHICommandListBase的实例，但实际类型是FRHICommandListExecutor。它的声明和实现如下：
// Engine\Source\Runtime\RHI\Public\RHICommandList.h extern RHI_API FRHICommandListExecutor GRHICommandList; // Engine\Source\Runtime\RHI\Private\RHICommandList.cpp RHI_API FRHICommandListExecutor GRHICommandList;
有关GRHICommandList的全局或静态接口如下：
FRHICommandListImmediate& FRHICommandListExecutor::GetImmediateCommandList() { return GRHICommandList.CommandListImmediate; } FRHIAsyncComputeCommandListImmediate& FRHICommandListExecutor::GetImmediateAsyncComputeCommandList() { return GRHICommandList.AsyncComputeCmdListImmediate; }
在UE的渲染模块和RHI模块中拥有大量的GRHICommandList使用案例，取其中之一：
// Engine\Source\Runtime\Renderer\Private\DeferredShadingRenderer.cpp void ServiceLocalQueue() { FTaskGraphInterface::Get().ProcessThreadUntilIdle(ENamedThreads::GetRenderThread_Local()); if (IsRunningRHIInSeparateThread()) { FRHICommandListExecutor::GetImmediateCommandList().ImmediateFlush(EImmediateFlushType::DispatchToRHIThread); } }
在RHI命令队列模块，除了涉及GRHICommandList，还涉及诸多全局的任务变量：
// Engine\Source\Runtime\RHI\Private\RHICommandList.cpp static FGraphEventArray AllOutstandingTasks; static FGraphEventArray WaitOutstandingTasks; static FGraphEventRef RHIThreadTask; static FGraphEventRef PrevRHIThreadTask; static FGraphEventRef RenderThreadSublistDispatchTask;
它们的创建或添加任务的代码如下：
void FRHICommandListBase::QueueParallelAsyncCommandListSubmit(FGraphEventRef* AnyThreadCompletionEvents, ...) { (......) if (Num && IsRunningRHIInSeparateThread()) { (......) // 创建FParallelTranslateSetupCommandList任务. FGraphEventRef TranslateSetupCompletionEvent = TGraphTask<FParallelTranslateSetupCommandList>::CreateTask(&Prereq, ENamedThreads::GetRenderThread()).ConstructAndDispatchWhenReady(CmdList, &RHICmdLists[0], Num, bIsPrepass); QueueCommandListSubmit(CmdList); // 添加到AllOutstandingTasks. AllOutstandingTasks.Add(TranslateSetupCompletionEvent); (......) FGraphEventArray Prereq; FRHICommandListBase** RHICmdLists = (FRHICommandListBase**)Alloc(sizeof(FRHICommandListBase*) * (1 + Last - Start), alignof(FRHICommandListBase*)); // 将所有外部任务AnyThreadCompletionEvents加入到对应的列表中. for (int32 Index = Start; Index <= Last; Index++) { FGraphEventRef& AnyThreadCompletionEvent = AnyThreadCompletionEvents[Index]; FRHICommandList* CmdList = CmdLists[Index]; RHICmdLists[Index - Start] = CmdList; if (AnyThreadCompletionEvent.GetReference()) { Prereq.Add(AnyThreadCompletionEvent); AllOutstandingTasks.Add(AnyThreadCompletionEvent); WaitOutstandingTasks.Add(AnyThreadCompletionEvent); } } (......) // 并行转译任务FParallelTranslateCommandList. FGraphEventRef TranslateCompletionEvent = TGraphTask<FParallelTranslateCommandList>::CreateTask(&Prereq, ENamedThreads::GetRenderThread()).ConstructAndDispatchWhenReady(&RHICmdLists[0], 1 + Last - Start, ContextContainer, bIsPrepass); AllOutstandingTasks.Add(TranslateCompletionEvent); (......) } void FRHICommandListBase::QueueAsyncCommandListSubmit(FGraphEventRef& AnyThreadCompletionEvent, class FRHICommandList* CmdList) { (......) // 处理外部任务AnyThreadCompletionEvent if (AnyThreadCompletionEvent.GetReference()) { if (IsRunningRHIInSeparateThread()) { AllOutstandingTasks.Add(AnyThreadCompletionEvent); } WaitOutstandingTasks.Add(AnyThreadCompletionEvent); } (......) } class FDispatchRHIThreadTask { void DoTask(ENamedThreads::Type CurrentThread, const FGraphEventRef& MyCompletionGraphEvent) { (......) // 创建RHI线程任务FExecuteRHIThreadTask. PrevRHIThreadTask = RHIThreadTask; RHIThreadTask = TGraphTask<FExecuteRHIThreadTask>::CreateTask(&Prereq, CurrentThread).ConstructAndDispatchWhenReady(RHICmdList); } }; class FParallelTranslateSetupCommandList { void DoTask(ENamedThreads::Type CurrentThread, const FGraphEventRef& MyCompletionGraphEvent) { (......) // 创建并行转译任务FParallelTranslateCommandList. FGraphEventRef TranslateCompletionEvent = TGraphTask<FParallelTranslateCommandList>::CreateTask(nullptr, ENamedThreads::GetRenderThread()).ConstructAndDispatchWhenReady(&RHICmdLists[Start], 1 + Last - Start, ContextContainer, bIsPrepass); MyCompletionGraphEvent->DontCompleteUntil(TranslateCompletionEvent); // 利用RHICmdList的接口FRHICommandWaitForAndSubmitSubListParallel提交任务, 最终会进入AllOutstandingTasks和WaitOutstandingTasks. ALLOC_COMMAND_CL(*RHICmdList, FRHICommandWaitForAndSubmitSubListParallel)(TranslateCompletionEvent, ContextContainer, EffectiveThreads, ThreadIndex++); }; void FRHICommandListExecutor::ExecuteInner(FRHICommandListBase& CmdList) { (......) if (IsRunningRHIInSeparateThread()) { (......) if (AllOutstandingTasks.Num() || RenderThreadSublistDispatchTask.GetReference()) { (......) // 创建渲染线程子命令派发(提交)任务FDispatchRHIThreadTask. RenderThreadSublistDispatchTask = TGraphTask<FDispatchRHIThreadTask>::CreateTask(&Prereq, ENamedThreads::GetRenderThread()).ConstructAndDispatchWhenReady(SwapCmdList, bAsyncSubmit); } else { (......) PrevRHIThreadTask = RHIThreadTask; // 创建渲染线程子命令转译任务FExecuteRHIThreadTask. RHIThreadTask = TGraphTask<FExecuteRHIThreadTask>::CreateTask(&Prereq, ENamedThreads::GetRenderThread()).ConstructAndDispatchWhenReady(SwapCmdList); } (......) }
总结一下这些任务变量的作用：
| 任务变量 | 执行线程 | 描述 |
|---|---|---|
| AllOutstandingTasks | 渲染、RHI、工作 | 所有在处理或待处理的任务列表。类型是FParallelTranslateSetupCommandList、FParallelTranslateCommandList。 |
| WaitOutstandingTasks | 渲染、RHI、工作 | 待处理的任务列表。类型是FParallelTranslateSetupCommandList、FParallelTranslateCommandList。 |
| RHIThreadTask | RHI、工作 | 正在处理的RHI线程任务。类型是FExecuteRHIThreadTask。 |
| PrevRHIThreadTask | RHI、工作 | 上一次处理的RHIThreadTask。类型是FExecuteRHIThreadTask。 |
| RenderThreadSublistDispatchTask | 渲染、RHI、工作 | 正在派发（提交）的任务。类型是FDispatchRHIThreadTask。注：与并行渲染AllOutstandingTasks、WaitOutstandingTasks有关 |
D3D11命令执行
本节将研究UE4.26在PC平台的通用RHI及D3D11命令运行过程和机制。由于UE4.26在PC平台默认的RHI是D3D11，并且关键的几个控制台变量的默认值如下：
也就是说开启了命令跳过模式，并且禁用了RHI线程。在此情况下，FRHICommandList的某个接口被调用时，不会生成单独的FRHICommand，而是直接调用Context的方法。以FRHICommandList::DrawPrimitive为例：
class RHI_API FRHICommandList : public FRHIComputeCommandList { void DrawPrimitive(uint32 BaseVertexIndex, uint32 NumPrimitives, uint32 NumInstances) { // 默认情况下Bypass为1, 进入此分支. if (Bypass()) { // 直接调用图形API的上下文的对应方法. GetContext().RHIDrawPrimitive(BaseVertexIndex, NumPrimitives, NumInstances); return; } // 分配单独的FRHICommandDrawPrimitive命令. ALLOC_COMMAND(FRHICommandDrawPrimitive)(BaseVertexIndex, NumPrimitives, NumInstances);
// 展开后变为：
// new(AllocCommand(sizeof(FRHICommandDrawPrimitive), alignof(FRHICommandDrawPrimitive))) FRHICommandDrawPrimitive(
// BaseVertexIndex, NumPrimitives, NumInstances);
}
}
因此，在PC的默认图形API（D3D11）下，r.RHICmdBypass 1且r.RHIThread.Enable 0，FRHICommandList将直接调用图形API的上下文的接口，相当于同步调用图形API，此时的图形API运行于渲染线程（如果开启）。
接着将r.RHICmdBypass设为0，但保持r.RHIThread.Enable为0，此时不再直接调用Context的方法，而是通过生成一条条单独的FRHICommand，然后由FRHICommandList相关的对象执行。还是以FRHICommandList::DrawPrimitive为例，调用堆栈如下所示：
class RHI_API FRHICommandList : public FRHIComputeCommandList { void FRHICommandList::DrawPrimitive(uint32 BaseVertexIndex, uint32 NumPrimitives, uint32 NumInstances) { // 默认情况下Bypass为1, 进入此分支. if (Bypass()) { // 直接调用图形API的上下文的对应方法. GetContext().RHIDrawPrimitive(BaseVertexIndex, NumPrimitives, NumInstances); return; } // 分配单独的FRHICommandDrawPrimitive命令. // ALLOC_COMMAND宏会调用AllocCommand接口. ALLOC_COMMAND(FRHICommandDrawPrimitive)(BaseVertexIndex, NumPrimitives, NumInstances); } template <typename TCmd> void* AllocCommand() { return AllocCommand(sizeof(TCmd), alignof(TCmd)); } void* AllocCommand(int32 AllocSize, int32 Alignment) { FRHICommandBase* Result = (FRHICommandBase*) MemManager.Alloc(AllocSize, Alignment); ++NumCommands; // CommandLink指向了上一个命令节点的Next. *CommandLink = Result; // 将CommandLink赋值为当前节点的Next. CommandLink = &Result->Next; return Result; } }
利用ALLOC_COMMAND宏分配的命令实例会进入FRHICommandListBase的命令链表，但此时并未执行，而是等待其它合适的时机执行，例如在FRHICommandListImmediate::ImmediateFlush。下面是执行FRHICommandList的调用堆栈：
由调用堆栈可以得知，在此情况下，命令执行的过程变得复杂起来，多了很多中间执行步骤。还是以FRHICommandList::DrawPrimitive为例，调用流程示意图如下：
上图的使用了宏INTERNAL_DECORATOR，其和相关宏的定义如下：
// Engine\Source\Runtime\RHI\Public\RHICommandListCommandExecutes.inl #define INTERNAL_DECORATOR(Method) CmdList.GetContext().Method #define INTERNAL_DECORATOR_COMPUTE(Method) CmdList.GetComputeContext().Method
相当于通过宏来调用CommandList的Context接口。
在RHI禁用（r.RHIThread.Enable 0）情况下，以上的调用在渲染线程执行：
接下来将r.RHIThread.Enable设为1，以开启RHI线程。此时运行命令的线程变成了RHI：
并且调用堆栈是从TaskGraph的RHI线程发起任务：
此时，命令执行的流程图如下：
上面流程图中，方角表示在渲染线程执行，而圆角在RHI线程执行（绿框中）。
开启RHI线程后，将出现它的统计数据：
左：未开启RHI线程的统计数据；右：开启RHI线程后的统计数据。
下面绘制出开启或关闭Bypass和RHI线程的流程图（以调用D3D11的DrawPrimitive为例）：
上面流程图中，方角表示在渲染线程中执行，圆角表示在RHI线程中执行（绿框中）。
ImmediateFlush
在FDynamicRHI中，提及了刷新类型（FlushType），是指EImmediateFlushType定义的类型：
// Engine\Source\Runtime\RHI\Public\RHICommandList.h namespace EImmediateFlushType { enum Type { WaitForOutstandingTasksOnly = 0, // 等待仅正在处理的任务完成. DispatchToRHIThread, // 派发到RHI线程. 注：创建一个FExecuteRHIThreadTask任务并投递到RHIThread的TaskGraph任务队列中 WaitForDispatchToRHIThread, // 等待派发到RHI线程. FlushRHIThread, // 刷新RHI线程. FlushRHIThreadFlushResources, // 刷新RHI线程和资源 FlushRHIThreadFlushResourcesFlushDeferredDeletes // 刷新RHI线程/资源和延迟删除. }; };
EImmediateFlushType中各个值的区别在FRHICommandListImmediate::ImmediateFlush的实现代码中体现出来：
// Engine\Source\Runtime\RHI\Public\RHICommandList.inl void FRHICommandListImmediate::ImmediateFlush(EImmediateFlushType::Type FlushType) { switch (FlushType) { // 等待任务完成. case EImmediateFlushType::WaitForOutstandingTasksOnly: { WaitForTasks(); } break; // 派发RHI线程(执行命令队列) case EImmediateFlushType::DispatchToRHIThread: { if (HasCommands()) { GRHICommandList.ExecuteList(*this); } } break; // 等待RHI线程派发. case EImmediateFlushType::WaitForDispatchToRHIThread: { if (HasCommands()) { GRHICommandList.ExecuteList(*this); } WaitForDispatch(); } break; // 刷新RHI线程. case EImmediateFlushType::FlushRHIThread: { // 派发并等待RHI线程. if (HasCommands()) { GRHICommandList.ExecuteList(*this); } WaitForDispatch(); // 等待RHI线程任务. if (IsRunningRHIInSeparateThread()) { WaitForRHIThreadTasks(); } // 重置正在处理的任务列表. WaitForTasks(true); } break; case EImmediateFlushType::FlushRHIThreadFlushResources: case EImmediateFlushType::FlushRHIThreadFlushResourcesFlushDeferredDeletes: { if (HasCommands()) { GRHICommandList.ExecuteList(*this); } WaitForDispatch(); WaitForRHIThreadTasks(); WaitForTasks(true); // 刷新管线状态缓存的资源. PipelineStateCache::FlushResources(); // 刷新将要删除的资源. FRHIResource::FlushPendingDeletes(FlushType == EImmediateFlushType::FlushRHIThreadFlushResourcesFlushDeferredDeletes); } break; } }
上面代码中涉及到了若干种处理和等待任务的接口，它们的实现如下：
// 等待任务完成. void FRHICommandListBase::WaitForTasks(bool bKnownToBeComplete) { if (WaitOutstandingTasks.Num()) { // 检测是否存在未完成的等待任务. bool bAny = false; for (int32 Index = 0; Index < WaitOutstandingTasks.Num(); Index++) { if (!WaitOutstandingTasks[Index]->IsComplete()) { bAny = true; break; } } // 存在就利用TaskGraph的接口开启线程等待. if (bAny) { ENamedThreads::Type RenderThread_Local = ENamedThreads::GetRenderThread_Local(); FTaskGraphInterface::Get().WaitUntilTasksComplete(WaitOutstandingTasks, RenderThread_Local); } // 重置等待任务列表. WaitOutstandingTasks.Reset(); } } // 等待渲染线程派发完成. void FRHICommandListBase::WaitForDispatch() { // 如果RenderThreadSublistDispatchTask已完成, 则置空. if (RenderThreadSublistDispatchTask.GetReference() && RenderThreadSublistDispatchTask->IsComplete()) { RenderThreadSublistDispatchTask = nullptr; } // RenderThreadSublistDispatchTask有未完成的任务. while (RenderThreadSublistDispatchTask.GetReference()) { ENamedThreads::Type RenderThread_Local = ENamedThreads::GetRenderThread_Local(); FTaskGraphInterface::Get().WaitUntilTaskCompletes(RenderThreadSublistDispatchTask, RenderThread_Local); if (RenderThreadSublistDispatchTask.GetReference() && RenderThreadSublistDispatchTask->IsComplete()) { RenderThreadSublistDispatchTask = nullptr; } } } // 等待RHI线程任务完成. void FRHICommandListBase::WaitForRHIThreadTasks() { bool bAsyncSubmit = CVarRHICmdAsyncRHIThreadDispatch.GetValueOnRenderThread() > 0; //r.RHICmdAsyncRHIThreadDispatch ENamedThreads::Type RenderThread_Local = ENamedThreads::GetRenderThread_Local(); // 相当于执行FRHICommandListBase::WaitForDispatch() if (bAsyncSubmit) { if (RenderThreadSublistDispatchTask.GetReference() && RenderThreadSublistDispatchTask->IsComplete()) { RenderThreadSublistDispatchTask = nullptr; } while (RenderThreadSublistDispatchTask.GetReference()) { if (FTaskGraphInterface::Get().IsThreadProcessingTasks(RenderThread_Local)) { while (!RenderThreadSublistDispatchTask->IsComplete()) { FPlatformProcess::SleepNoStats(0); } } else { FTaskGraphInterface::Get().WaitUntilTaskCompletes(RenderThreadSublistDispatchTask, RenderThread_Local); } if (RenderThreadSublistDispatchTask.GetReference() && RenderThreadSublistDispatchTask->IsComplete()) { RenderThreadSublistDispatchTask = nullptr; } } // now we can safely look at RHIThreadTask } // 如果RHI线程任务已完成, 则置空任务. if (RHIThreadTask.GetReference() && RHIThreadTask->IsComplete()) { RHIThreadTask = nullptr; PrevRHIThreadTask = nullptr; } // 如果RHI线程有任务未完成, 则执行并等待. while (RHIThreadTask.GetReference()) { // 如果已在处理, 则用sleep(0)跳过此时间片. if (FTaskGraphInterface::Get().IsThreadProcessingTasks(RenderThread_Local)) { while (!RHIThreadTask->IsComplete()) { FPlatformProcess::SleepNoStats(0); } } // 任务尚未处理, 开始并等待之. else { FTaskGraphInterface::Get().WaitUntilTaskCompletes(RHIThreadTask, RenderThread_Local); } // 如果RHI线程任务已完成, 则置空任务. if (RHIThreadTask.GetReference() && RHIThreadTask->IsComplete()) { RHIThreadTask = nullptr; PrevRHIThreadTask = nullptr; } } }
RHI控制台变量
前面章节的代码也显示RHI体系涉及的控制台变量非常多，下面列出部分控制台变量，以便调试、优化RHI渲染效果或效率：
| 名称 | 描述 |
|---|---|
| r.RHI.Name | 显示当前RHI的名字，如D3D11。 |
| r.RHICmdAsyncRHIThreadDispatch | 实验选项，是否执行RHI调度异步。可使数据更快地刷新到RHI线程，避免帧末尾出现卡顿。 |
| r.RHICmdBalanceParallelLists | 允许启用DrawList的预处理，以尝试在命令列表之间均衡负载。0：关闭，1：开启，2：实验选项，使用上一帧的结果（在分屏等不做任何事情）。 |
| r.RHICmdBalanceTranslatesAfterTasks | 实验选项，平衡并行翻译后的渲染任务完成。可最小化延迟上下文的数量，但会增加启动转译的延迟。 |
| r.RHICmdBufferWriteLocks | 仅与RHI线程相关。用于诊断缓冲锁问题的调试选项。 |
| r.RHICmdBypass | 是否绕过RHI命令列表，立即发送RHI命令。0：禁用（需开启多线程渲染），1：开启。 |
| r.RHICmdCollectRHIThreadStatsFromHighLevel | 这将在执行的RHI线程上推送统计信息，这样就可以确定它们来自哪个高层级的Pass。对帧速率有不利影响。默认开启。 |
| r.RHICmdFlushOnQueueParallelSubmit | 在提交后立即等待并行命令列表的完成。问题诊断。只适用于部分RHI。 |
| r.RHICmdFlushRenderThreadTasks | 如果为真，则每次调用时都刷新渲染线程任务。问题诊断。这是一个更细粒度cvars的主开关。 |
| r.RHICmdForceRHIFlush | 对每个任务强制刷新发送给RHI线程。问题诊断。 |
| r.RHICmdMergeSmallDeferredContexts | 合并小的并行转译任务，基于r.RHICmdMinDrawsPerParallelCmdList。 |
| r.RHICmdUseDeferredContexts | 使用延迟上下文并行执行命令列表。只适用于部分RHI。 |
| r.RHICmdUseParallelAlgorithms | True使用并行算法。如果r.RHICmdBypass为1则忽略。 |
| r.RHICmdUseThread | 使用RHI线程。问题诊断。 |
| r.RHICmdWidth | 控制并行渲染器中大量事物的任务粒度。 |
| r.RHIThread.Enable | 启用/禁用RHI线程，并确定RHI工作是否在专用线程上运行。 |
| RHI.GPUHitchThreshold | GPU上检测卡顿的阈值(毫秒)。 |
| RHI.MaximumFrameLatency | 可以排队进行渲染的帧数。 |
| RHI.SyncThreshold | 在垂直同步功能启用前的连续“快速”帧数。 |
| RHI.TargetRefreshRate | 如果非零，则显示的更新频率永远不会超过目标刷新率(以Hz为单位)。 |
注：以上只列出部分RHI相关的变量，还有很多未列出。
参考