Merge pull request gpuweb#29 from gpuweb/pipeline-objects

More detailed design of pipelines to start discussions

design/Pipelines.md

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+# Pipeline objects
+
+This is a document with pseudo-code for the parts of the API related to pipeline objects.
+Naming and the UX of the API are just an example and could change, the important parts are what’s available and what goes where.
+For example this document uses C structures when a C++ API might want to use builder objects, and a Javascript API could use dictionaries.
+
+## Creating pipeline objects
+
+For type safety, compute and graphics pipeline are separate types.
+To create a pipeline, a structure containing all the relevant information is passed to `DeviceCreate<TYPE>Pipeline`.
+
+## Compute pipeline creation
+
+To create a compute pipeline the only things needed are some shader code present in a `ShaderModule` object, and a `PipelineLayout` object describing how the pipeline interacts with the binding model.
+
+```cpp
+struct ComputePipelineDescriptor {
+    ShaderModule module;
+    const char* entryPoint;
+    PipelineLayout layout;
+};
+
+ComputePipeline CreateComputePipeline(Device device, const ComputePipelineDescriptor* descriptor);
+```
+
+Translation to the backing APIs would be the following:
+ - **D3D12**: Translates to `ID3D12::CreateComputePipelineState`, a `D3D12_SHADER_BYTECODE` is created from the `(module, entryPoint)` pair, and the `ID3D12RootSignature` is equivalent to the `PipelineLayout`.
+ - **Metal**: Translates to `MTLDevice::makeComputePipelineState`, the `MTLFunction` is created from the `(module, entryPoint, layout)` tuple by adapting the generated MSL to the resource slot allocation done in `layout`.
+ - **Vulkan**: Translates to `vkCreateComputePipelines` with one pipeline. The `vkShaderStageInfo` corresponds to `(module, entryPoint)` and the `vkPipelineLayout` corresponds to `layout`.
+
+Open questions:
+How do we take advantage of the pipeline caching present in D3D12 and Vulkan?
+Do we expose it to the application or is it done magically in the WebGPU implementation?
+
+## Render pipeline creation
+
+Render pipelines need `ShaderModule` and a `PipelineLayout` like compute pipelines and in addition require information about:
+ - Layout for vertex inputs
+ - Layout for fragment outputs
+ - All the fixed-function state
+
+For simplicity we assume most fixed-function state is created in separate object.
+For example a `DepthStencilState` object would be allocated and a pointer to it would be stored in the `RenderPipelineDescriptor`. This is part of the UX of the API and could be replaced with chained structure like Vulkan or member structure like D3D12.
+
+Mismatch:
+ - Metal has primitive restart always enabled.
+ - D3D12 needs to know whether the primitive restart index is `0xFFFF` or `0xFFFFFFFF` at pipeline creation time.
+ - Metal doesn’t have a sample mask.
+ - Vulkan can have some state like scissor and viewport set on the pipeline as an optimization on some GPUs.
+ - Vulkan allows creating pipelines in bulk, this is not only a UX things but allows reusing some results for faster creation.
+
+```cpp
+struct RenderPipelineDescriptor {
+    // Same translation as for compute pipelines
+    ShaderModule vsModule;
+    const char* vsEntryPoint;
+    ShaderModule fsModule;
+    const char* fsEntryPoint;
+    PipelineLayout layout;
+
+    // Pipeline input / outputs
+    InputState* inputState;
+    RenderPass* renderPass;
+    int subpassIndex;
+
+    // Fixed function state
+    DepthStencilState* depthStencil;
+    BlendState* blend[kMaxColorAttachments];
+    PrimitiveTopology topology;
+    // TODO other state: rasterizer state, “multisample state”
+};
+
+
+RenderPipeline CreateRenderPipeline(Device device, const RenderPipelineDescriptor* descriptor);
+```
+
+Translation to the backing APIs would be the following:
+ - **D3D12**: Translates to `ID3D12::CreateGraphicsPipelineState`. `IBStripCutValue` will always be set.
+ - **Metal**: Translates to `MTLDevice::makeRenderPipelineState`
+ - **Vulkan**: Translates to `vkCreateGraphicsPipelines`. `VkPipelineInputAssemblyStateCreateInfo`'s `primitiveRestartEnable` is always set to true. All dynamic states are set on all pipelines.
+
+Open question: should the type of the indices be set in `RenderPipelineDescriptor`? If not, how is the D3D12 `IBStripCutValue` chosen?
+
+The translation of individual members of RenderPipelineDescriptor is described below.
+
+### Input state
+
+This describes how the vertex buffers are stepped through (stride, instance vs. vertex, instance divisor), and how the attributes are extracted from the buffers (buffer index, format, offset).
+
+Mismatches:
+ - D3D12 takes the stride along with the vertex buffers in `ID3D12GraphicsCommandList::IASetVertexBuffers` whereas Vulkan and Metal take it at pipeline compilation time.
+ - Vulkan doesn’t support a divisor for its step rate.
+
+```cpp
+enum StepRate {
+    StepRateVertex,
+    StepRateInstance,
+};
+
+Enum VertexFormat {
+    // TODO make a list of portable vertex formats
+};
+
+struct InputStateDescriptor {
+    struct {
+        bool enabled;
+        VertexFormat format;
+        int offsetInBuffer;
+        int bufferIndex;
+    } attributes[MAX_ATTRIBUTES];
+
+    struct {
+        StepRate rate;
+        int stride;
+    } buffers[MAX_VERTEX_BUFFERS];
+};
+
+InputState* CreateInputState(Device* device, InputStateDescriptor* descriptor);
+```
+
+Translation to the backing APIs would be the following:
+ - **D3D12**: Translates to a `D3D12_INPUT_DESC`.
+   Each enabled attribute corresponds to a `D3D12_INPUT_ELEMENT_DESC` with `InputSlot` being the index of the attribute.
+   Other members of the `D3D12_INPUT_ELEMENT_DESC` are translated trivially.
+   The stride is looked up in the pipeline state before calls to `ID3D12GraphicsCommandList::IASetVertexBuffers`.
+   `IASetVertexBuffers` might be deferred until before a draw and vertex buffers might be invalidated by pipeline changes.
+ - **Metal**: Translates to a `MTLVertexDescriptor`, with attributes corresponding to `MTLVertexDescriptor::attributes` and buffers corresponding to `MTLVertexDescriptor::layouts`.
+   Attributes translate trivially to `MTLVertexAttributeDescriptor` structures and buffers to `MTLVertexBufferLayoutDescriptor` structures.
+   Extra care only needs to be taken to translate a zero stride to a constant step rate.
+ - **Vulkan**: Translates to a `VkPipelineVertexInputStateCreateInfo`.
+   Buffers translate trivially to `VkVertexInputBindingDescription` and attributes to `VkVertexInputAttributeDescription`.
+
+Open questions:
+Should the vertex attributes somehow be included in the `PipelineLayout` so vertex buffers are treated as other resources and changed in bulk with them?
+
+### Render pass / render target format
+
+The `RenderPass` will contain for each subpass a list of the attachment formats for color attachments and depth-stencil attachments.
+Information from the `RenderPass` is used to fill the following:
+ - **D3D12**: `RTVFormats`, `DSVFormats` and `NumRenderTargets` in `D3D12_GRAPHICS_PIPELINE_STATE_DESC`.
+ - **Metal**: `colorAttachments[N].pixelFormat`, `depthAttachmentPixelFormat` and `stencilAttachmentPixelFormat` in `MTLRenderPipelineDescriptor`.
+ - **Vulkan**: `renderPass` and `subpass` in `VkGraphicsPipelineCreateInfo`.
+
+Open question:
+does the sample count of the pipeline state come from the RenderPass too?
+
+### Primitive topology
+
+Mismatch:
+ - Metal and D3D12 only require “point vs. line vs. triangle” at pipeline compilation time, the exact topology is set via `ID3D12GraphicsCommandList::IASetPrimitiveTopology` or passed in the `MTLRenderCommandEncoder::draw*`.
+   Vulkan requires the exact topology at compilation time.
+ - Vulkan supports triangle fans but Metal and D3D12 don’t.
+
+```cpp
+enum PrimitiveTopology {
+    PrimitiveTopologyPoints,
+    PrimitiveTopologyLineList,
+    PrimitiveTopologyLineStrip,
+    PrimitiveTopologyTriangleList,
+    PrimitiveTopologyTriangleStrip,
+};
+```
+
+Translation to the backing APIs would be the following:
+ - **D3D12 and Metal**: The primitive topology type is set on the `D3D12_GRAPHICS_PIPELINE_STATE_DESC` and `MTLRenderPipelineDescriptor`.
+   At draw-time, the exact topology is queried from the pipeline.
+ - **Vulkan**: The primitive topology type is set in the `VkGraphicsPipelineCreateInfo`.
+
+### Blend state
+
+Mismatch:
+ - In Vulkan per-attachment blending and dual source blending are exposed as optional features.
+   `independentBlend` is supported almost everywhere but Adreno 4XX while `dualSrcBlend` is also not supported on Mali GPUs.
+ - Metal doesn’t have logic ops.
+
+```cpp
+enum BlendOperation {
+    BlendOperationAdd,
+    BlendOperationSubtract,
+    BlendOperationReverseSubtract,
+    BlendOperationMin,
+    BlendOperationMax,
+};
+
+enum BlendFactor {
+    BlendFactorOne,
+    BlendFactorSrcColor,
+    BlendFactorOneMinusSrcColor,
+    BlendFactorSrcAlpha,
+    BlendFactorOneMinusSrcAlpha,
+    BlendFactorDstColor,
+    BlendFactorOneMinusDstColor,
+    BlendFactorDstAlpha,
+    BlendFactorOneMinusDstAlphe,
+    BlendFactorSrcAlphaSaturated,
+    BlendFactorBlendColor,
+    BlendFactorOneMinusBlendColor,
+};
+
+struct BlendStateDescriptor {
+    bool enabled;
+    BlendFactor srcColorFactor;
+    BlendFactor dstColorFactor;
+    BlendFactor srcAlphaFactor;
+    BlendFactor dstAlphaFactor;
+    BlendOperation colorOperation;
+    BlendOperation alphaOperation;
+    int writeMask;
+};
+
+BlendState* CreateBlendState(Device* device, BlendStateDescriptor* descriptor);
+```
+
+Translation to backing APIs would be the following:
+ - **D3D12**: when filling the `D3D12_GRAPHICS_PIPELINE_DESC`, `BlendState` will be filled with data coming from the `BlendStates` referenced in the `RenderPipelineDescriptor`.
+   Translation from a `BlendState` to a `D3D12_RENDER_TARGET_BLEND_DESC` is trivial.
+ - **Metal**: the `BlendStates` will be used to fill all of the data for a `MTLRenderPipelineColorAttachmentDescriptor` but `pixelFormat`.
+   Translation of individual members is trivial.
+ - **Vulkan**: the `BlendStates` will be translated to elements of `pAttachments` in the `VkPipelineColorBlendStateCreateInfo`.
+   Translation of individual members is trivial.
+
+Open question:
+should enablement of independent attachment blend state be explicit like in D3D12 or explicit? Should alpha to coverage be part of the multisample state or the blend state?
+
+### Depth stencil state
+
+Mismatch:
+ - D3D12 doesn’t have per-face stencil read and write masks.
+ - In Metal the depth stencil state is built and bound separately from the pipeline state.
+
+```cpp
+enum CompareFunction {
+    CompareFunctionNever,
+    CompareFunctionLess,
+    CompareFunctionLessEqual,
+    CompareFunctionGreater,
+    CompareFunctionGreaterEqual,
+    CompareFunctionEqual,
+    CompareFunctionNotEqual,
+    CompareFunctionAlways,
+};
+
+enum StencilOperation {
+    StencilOperationKeep,
+    StencilOperationZero,
+    StencilOperationReplace,
+    StencilOperationInvert,
+    StencilOperationIncrementClamp,
+    StencilOperationDecrementClamp,
+    StencilOperationIncrementWrap,
+    StencilOperationDecrementWrap,
+};
+
+struct StencilFaceDescriptor {
+    CompareFunction stencilCompare;
+    StencilOperation stencilPass;
+    StencilOperation stencilFail;
+    StencilOperation depthFail;
+};
+
+struct DepthStencilStateDescriptor {
+    CompareFunction depthCompare;
+    StencilFaceDescriptor front;
+    StencilFaceDescriptor back;
+    int stencilReadMask;
+    Int stencilWriteMask;
+};
+
+DepthStencilState* CreateDepthStencilState(Device* device, DepthStencilDescriptor* descriptor);
+```
+
+Translation to backing APIs would be the following:
+ - **D3D12**: `DepthStencilState` translates trivially to a `D3D12_DEPTH_STENCIL_DESC`.
+   `DepthEnable` would be set as `depthCompare != Always`.
+ - **Metal**: `DepthStencilState` translates trivially to `MTLDepthStencilDescriptor` except that front and back stencil masks have to be set to the single stencil mask value from WebGPU.
+   When a pipeline is bound, the corresponding depth-stencil state is bound at the same time.
+ - **Vulkan**: `DepthStencilState` translates trivially to `VkPipelineDepthStencilStateCreateInfoxcept` except that front and back stencil masks have to be set to the single stencil mask value from WebGPU.
+   `depthTestEnable` would be set to `depthCompare != Always`.
+
+Open question:
+what about Vulkan’s `VkPipelineDepthStencilStateCreateInfo::depthBoundTestEnable` and D3D12's `D3D12_DEPTH_STENCIL_DESC1::DepthBoundsTestEnable`?
+Should “depth test enable” be implicit or explicit?