[llvm] [DirectX] Documenting Root Signature Binary representation (PR #131011)

[Justin Bogner via llvm-commits]

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@@ -400,3 +400,273 @@ 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 an array of root parameters 
+and an array of static samplers. The structure uses a versioned design with 
+offset-based references to allow for flexible serialization and deserialization. 
+One consequence of using an offset-based reference is that root parameters and 
+static samplers don't need to follow any specific ordering logic.
----------------
bogner wrote:

Calling the root parameters and static samplers "arrays" implies that they have fixed sizes, but at the very least root parameters do not. I also find the discussion of the versioning scheme and order independence here a bit random at this point in the document.

I think it might be clearer to say that the root signature part consists of three sections - a header that provides version information and offsets to the other parts, a parameters part that describes root parameters, and a samplers part that describes static samplers.

https://github.com/llvm/llvm-project/pull/131011