Adding root constant documentation

Author: joaosaffran

llvm/docs/DirectX/DXContainer.rst

@@ -400,3 +400,75 @@ 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 (RST0) Part
+---------
+.. _RST0:
+
+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 to the array of root parameters header.
+- **NumStaticSamplers**: The number of static samplers defined in the root signature.
+- **StaticSamplerOffset**: Byte offset from the beginning 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 from the beginning 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
+~~~~~~~~~~~~~~~~~~~~~~~
+
+.. code-block:: c
+
+   union RootParameter {
+     RootConstants Constants;
+   };
+
+The `RootParameter` union represents the various types of parameters that can be specified in a root signature. Such includes:
+
+- **Constants**: Represents inline root constants that are directly embedded in the root signature and passed to the shader without requiring a constant buffer resource.
+
+Each specific parameter type will have its own structure with fields relevant to that parameter type. For example, `RootConstants` would include information about the register binding, count of 32-bit values, and other properties specific to constant parameters.
+
+When processing root parameters, readers should first check the `ParameterType` field in the corresponding header to determine which member of the union to access.