[llvm] [DirectX] Adding root constant documentation (PR #129569)

[Finn Plummer via llvm-commits]

================
@@ -400,3 +400,100 @@ 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.
----------------
inbelic wrote:

nit: do we also enforce 80 char lines here?

https://github.com/llvm/llvm-project/pull/129569