[DirectX] Documenting Root Signature Binary representation

llvm/docs/DirectX/DXContainer.rst

@@ -111,7 +111,7 @@ FXC are marked with \*.
 #. `PSV0`_ - Stores Pipeline State Validation data.
 #. RDAT† - Stores Runtime Data.
 #. RDEF\* - Stores resource definitions.
-#. RTS0 - Stores compiled root signature.
+#. `RTS0`_ - Stores compiled root signature.
 #. `SFI0`_ - Stores shader feature flags.
 #. SHDR\* - Stores compiled DXBC bytecode.
 #. SHEX\* - Stores compiled DXBC bytecode.
@@ -400,3 +400,260 @@ SFI0 Part
 The SFI0 part encodes a 64-bit unsigned integer bitmask of the feature flags.
 This denotes which optional features the shader requires. The flag values are
 defined in `llvm/include/llvm/BinaryFormat/DXContainerConstants.def <https://github.com/llvm/llvm-project/blob/main/llvm/include/llvm/BinaryFormat/DXContainerConstants.def>`_.
+
+
+Root Signature (RTS0) Part
+--------------------------
+.. _RTS0:
+
+The Root Signature defines the interface between the shader and the pipeline, 
+specifying which resources are bound to the shader and how they are accessed. 
+This structure serves as a contract between the application and the GPU, 
+establishing a layout for resource binding that both the shader compiler and 
+the runtime can understand.
+
+The Root Signature consists of a header followed by a collection of root parameters 
+and static samplers. The structure uses a versioned design with offset-based references 
+to allow for flexible serialization and deserialization.
+
+Root Signature Header
+~~~~~~~~~~~~~~~~~~~~~~~
+
+.. code-block:: c
+
+   struct RootSignatureHeader {
+     uint32_t Version;
+     uint32_t NumParameters;
+     uint32_t ParametersOffset;
+     uint32_t NumStaticSamplers;
+     uint32_t StaticSamplerOffset;
+     uint32_t Flags;
+   }
+
+
+The `RootSignatureHeader` structure contains the top-level information about a root signature:
+
+#. **Version**: Specifies the version of the root signature format. This allows for backward 
+   compatibility as the format evolves.
+#. **NumParameters**: The number of root parameters contained in this root signature.
+#. **ParametersOffset**: Byte offset from the beginning of RST0 section to the array of root 
+   parameters header.
+#. **NumStaticSamplers**: The number of static samplers defined in the root signature.
+#. **StaticSamplerOffset**: Byte offset to the array of static samplers.
+#. **Flags**: Bit flags that define global behaviors for the root signature, such as whether 
+   to deny vertex shader access to certain resources.
+
+This header allows readers to navigate the binary representation of the root signature by 
+providing counts and offsets to locate each component within the serialized data.
+
+Root Parameter Header
+~~~~~~~~~~~~~~~~~~~~~~~
+
+.. code-block:: c
+
+   struct RootParameterHeader {
+     dxbc::RootParameterType ParameterType;
+     dxbc::ShaderVisibility ShaderVisibility;
+     uint32_t ParameterOffset;
+   };
+
+
+Each root parameter in the signature is preceded by a `RootParameterHeader` that describes 
+the parameter's basic attributes:
+
+#. **ParameterType**: Enumeration indicating what type of parameter this is (e.g., descriptor 
+   table, constants, CBV, SRV, UAV).
+#. **ShaderVisibility**: Specifies which shader stages can access this parameter (e.g., all stages, 
+   vertex shader only, pixel shader only).
+#. **ParameterOffset**: Byte offset to the specific parameter data structure 
+   for this entry.
+
+The header uses a parameter type field rather than encoding the version of the parameter through 
+size, allowing for a more explicit representation of the parameter's nature.
+
+Root Parameters
+~~~~~~~~~~~~~~~
+
+The Root Parameters section contains structured definitions for each type of root parameter that can 
+be included in a root signature. Each structure corresponds to a specific parameter type as identified 
+by the ``ParameterType`` field in the ``RootParameterHeader``.
+
+Root Constants
+~~~~~~~~~~~~~~
+
+.. code-block:: cpp
+
+  struct RootConstants {
+    uint32_t ShaderRegister;
+    uint32_t RegisterSpace;
+    uint32_t Num32BitValues;
+  };
+
+The ``RootConstants`` structure represents inline root constants that are directly embedded in the root 
+signature and passed to the shader without requiring a constant buffer resource:
+
+#. **ShaderRegister**: The shader register (b#) where these constants are bound.
+#. **RegisterSpace**: The register space used for the binding.
+#. **Num32BitValues**: The number of 32-bit values included in this constant buffer.
+
+Root constants provide a fast way to pass small amounts of data directly to the shader without the overhead of creating and binding a constant buffer resource.
+
+Root Descriptor
+~~~~~~~~~~~~~~~
+
+Root descriptors provide a direct mechanism for binding individual resources to shader stages in the Direct3D 12 
+rendering pipeline. They represent a critical interface for efficient resource management, allowing applications 
+to specify how shader stages access specific GPU resources.
+
+.. code-block:: cpp
+
+   // Version 1.0 Root Descriptor
+   struct RootDescriptor_V1_0 {
+      uint32_t ShaderRegister;
+      uint32_t RegisterSpace;
+   };
+
+   // Version 1.1 Root Descriptor
+   struct RootDescriptor_V1_1 {
+      uint32_t ShaderRegister;
+      uint32_t RegisterSpace;
+      // New flags for Version 1.1
+      enum Flags {
+        None                = 0x0,
+        DATA_STATIC         = 0x1,
+        DATA_STATIC_WHILE_SET_AT_EXECUTE = 0x2,
+        DATA_VOLATILE       = 0x4
+      };
+
+      // Bitfield of flags from the Flags enum
+      uint32_t Flags;
+   };
+
+The Root Descriptor structure has evolved to support two versions, providing enhanced flexibility and 
+performance optimization capabilities.
+
+Version 1.0 Root Descriptor
+'''''''''''''''''''''''''''
+The Version 1.0 RootDescriptor_V1_0 provides basic resource binding:
+
+#. **ShaderRegister**: The shader register where the descriptor is bound.
+#. **RegisterSpace**: The register space used for the binding.
+
+Version 1.1 Root Descriptor
+'''''''''''''''''''''''''''
+The Version 1.1 RootDescriptor_V1_1 extends the base structure with the following additional fields:
+
+#. **Flags**: Provides additional metadata about the descriptor's usage pattern.
+
+Root Descriptor Table
+~~~~~~~~~~~~~~~~~~~~~
+
+Descriptor tables function as containers that hold references to descriptors in descriptor heaps. 
+They allow multiple descriptors to be bound to the pipeline through a single root signature parameter.
+
+.. code-block:: cpp
+
+   struct DescriptorRange_V1_0 {
+      dxbc::DescriptorRangeType RangeType;
+      uint32_t NumDescriptors;
+      uint32_t BaseShaderRegister;
+      uint32_t RegisterSpace;
+      uint32_t OffsetInDescriptorsFromTableStart;
+   };
+
+   struct DescriptorRange_V1_1 {
+      dxbc::DescriptorRangeType RangeType;
+      uint32_t NumDescriptors;
+      uint32_t BaseShaderRegister;
+      uint32_t RegisterSpace;
+      uint32_t OffsetInDescriptorsFromTableStart;
+      // New flags for Version 1.1
+      enum Flags {
+        None                        = 0x0,
+        // Descriptors are static and known at root signature creation
+        DESCRIPTORS_STATIC          = 0x1,
+        // Descriptors remain constant during command list execution
+        DESCRIPTORS_STATIC_KEEPING_BUFFER_BOUNDS_CHECKS = 0x2,
+        // Descriptors may change frequently
+        DESCRIPTORS_VOLATILE        = 0x4
+      };
+
+      // Bitfield of flags from the Flags enum
+      uint32_t Flags;
+   };
+
+   struct RootDescriptorTable {
+      uint32_t NumDescriptorRanges;      
+      uint32_t DescriptorRangesOffset;  
+   };
+
+
+Descriptor Range Version 1.0
+''''''''''''''''''''''''''''
+The Version 1.0 ``DescriptorRange_V1_0`` provides basic descriptor range definition:
+
+#. **RangeType**: Type of descriptors (CBV, SRV, UAV, or Sampler)
+#. **NumDescriptors**: Number of descriptors in the range
+#. **BaseShaderRegister**: First shader register in the range
+#. **RegisterSpace**: Register space for the range
+#. **OffsetInDescriptorsFromTableStart**: Offset from the descriptor heap start
+
+Descriptor Range Version 1.1
+''''''''''''''''''''''''''''
+The Version 1.1 DescriptorRange_V1_1 extends the base structure with performance optimization flags.
+
+#. **Flags**: Provide additional information about the descriptors and enable further driver optimizations.
+   For details, check `Direct X documentation <https://learn.microsoft.com/en-us/windows/win32/direct3d12/root-signature-version-1-1#static-and-volatile-flags>`_.
+
+Root Descriptor Table
+'''''''''''''''''''''
+RootDescriptorTable provides basic table structure:
+
+#. **NumDescriptorRanges**: Number of descriptor ranges
+#. **DescriptorRangesOffset**: Offset to descriptor range array
+
+Static Samplers
+~~~~~~~~~~~~~~~
+
+Static samplers provide a way to define fixed sampler states within the root signature itself.
+
+.. code-block:: cpp
+
+   struct StaticSamplerDesc {
+      FilterMode Filter;
+      TextureAddressMode AddressU;
+      TextureAddressMode AddressV;
+      TextureAddressMode AddressW;
+      float MipLODBias;
+      uint32_t MaxAnisotropy;
+      ComparisonFunc ComparisonFunc;
+      StaticBorderColor BorderColor;
+      float MinLOD;
+      float MaxLOD;
+      uint32_t ShaderRegister;
+      uint32_t RegisterSpace;
+      ShaderVisibility ShaderVisibility;
+   };
+
+
+The StaticSamplerDesc structure defines all properties of a static sampler:
+
+#. Filter: The filtering mode (e.g., point, linear, anisotropic) used for texture sampling. 
+   For details, check `Static Sampler Fileters definition. <https://learn.microsoft.com/en-us/windows/win32/api/d3d12/ne-d3d12-d3d12_filter#syntax>`_. 
+#. AddressU: The addressing mode for the U texture coordinate.
+   For details, check `Texture address mode definition. <https://learn.microsoft.com/en-us/windows/win32/api/d3d12/ne-d3d12-d3d12_texture_address_mode>`_. 
+#. AddressV: The addressing mode for the V texture coordinate.
+#. AddressW: The addressing mode for the W texture coordinate.
+#. MipLODBias: Bias value applied to mipmap level of detail calculations.
+#. MaxAnisotropy: Maximum anisotropy level when using anisotropic filtering.
+#. ComparisonFunc: Comparison function used for comparison samplers.
+   For details, check `Comparison Function definition. <https://learn.microsoft.com/en-us/windows/win32/api/d3d12/ne-d3d12-d3d12_comparison_func>`_. 
+#. BorderColor: Predefined border color used when address mode is set to border.
+   For details, check `Static border color <https://learn.microsoft.com/en-us/windows/win32/api/d3d12/ne-d3d12-d3d12_static_border_color>`_. 
+#. MinLOD: Minimum level of detail to use for sampling.
+#. MaxLOD: Maximum level of detail to use for sampling.
+#. ShaderRegister: The shader sampler register (s#) where this sampler is bound.
+#. RegisterSpace: The register space used for the binding.
+#. ShaderVisibility: Specifies which shader stages can access this sampler.
+   For details, check `Sahder Visibility definition. <https://learn.microsoft.com/en-us/windows/win32/api/d3d12/ne-d3d12-d3d12_shader_visibility>`_. 
+