[Docs][DirectX] Add relevant documentation of Root Signature (#149608)

This pr adds documentation of root signatures for reference of an LLVM
user.

The target audience is an LLVM user that wants to build a custom tool,
or front-end, that requires constructing a root signature part (RTS0)
for a `DXContainer`.

With that in mind, this pr adds documentation of the metadata
representation of root signatures implemented in LLVM and utilized in
Clang.

llvm/docs/DirectX/RootSignatures.rst

@@ -0,0 +1,245 @@
+===============
+Root Signatures
+===============
+
+.. contents::
+   :local:
+
+.. toctree::
+   :hidden:
+
+Overview
+========
+
+A root signature is used to describe what resources a shader needs access to
+and how they're organized and bound in the pipeline. The DirectX Container
+(DXContainer) contains a root signature part (RTS0), which stores this
+information in a binary format. To assist with the construction of, and
+interaction with, a root signature is represented as metadata
+(``dx.rootsignatures`` ) in the LLVM IR. The metadata can then be converted to
+its binary form, as defined in
+`llvm/include/llvm/llvm/Frontend/HLSL/RootSignatureMetadata.h
+<https://github.com/llvm/llvm-project/blob/main/llvm/include/llvm/Frontend/HLSL/RootSignatureMetadata.h>`_.
+This document serves as a reference for the metadata representation of a root
+signature for users to interface with.
+
+Metadata Representation
+=======================
+
+Consider the reference root signature, then the following sections describe the
+metadata representation of this root signature and the corresponding operands.
+
+.. code-block:: HLSL
+
+  RootFlags(ALLOW_INPUT_ASSEMBLER_INPUT_LAYOUT),
+  RootConstants(b0, space = 1, num32Constants = 3),
+  CBV(b1, flags = 0),
+  StaticSampler(
+    filter = FILTER_MIN_MAG_POINT_MIP_LINEAR,
+    addressU = TEXTURE_ADDRESS_BORDER,
+  ),
+  DescriptorTable(
+    visibility = VISIBILITY_ALL,
+    SRV(t0, flags = DATA_STATIC_WHILE_SET_AT_EXECUTE),
+    UAV(
+      numDescriptors = 5, u1, space = 10, offset = 5,
+      flags = DATA_VOLATILE
+    )
+  )
+
+.. note::
+
+  A root signature does not necessarily have a unique metadata representation.
+  Futher, a malformed root signature can be represented in the metadata format,
+  (eg. mixing Sampler and non-Sampler descriptor ranges), and so it is the
+  user's responsibility to verify that it is a well-formed root signature.
+
+Named Root Signature Table
+==========================
+
+.. code-block:: LLVM
+
+  !dx.rootsignatures = !{!0}
+
+A named metadata node, ``dx.rootsignatures``` is used to identify the root
+signature table. The table itself is a list of references to function/root
+signature pairs.
+
+Function/Root Signature Pair
+============================
+
+.. code-block:: LLVM
+
+  !1 = !{ptr @main, !2, i32 2 }
+
+The function/root signature associates a function (the first operand) with a
+reference to a root signature (the second operand). The root signature version
+(the third operand) used for validation logic and binary format follows.
+
+Root Signature
+==============
+
+.. code-block:: LLVM
+
+  !2 = !{ !3, !4, !5, !6, !7 }
+
+The root signature itself simply consists of a list of references to its root
+signature elements.
+
+Root Signature Element
+======================
+
+A root signature element is identified by the first operand, which is a string.
+The following root signature elements are defined:
+
+================= ======================
+Identifier String Root Signature Element
+================= ======================
+"RootFlags"       Root Flags
+"RootConstants"   Root Constants
+"RootCBV"         Root Descriptor
+"RootSRV"         Root Descriptor
+"RootUAV"         Root Descriptor
+"StaticSampler"   Static Sampler
+"DescriptorTable" Descriptor Table
+================= ======================
+
+Below is listed the representation for each type of root signature element.
+
+Root Flags
+==========
+
+.. code-block:: LLVM
+
+  !3 = { !"RootFlags", i32 1 }
+
+======================= ====
+Description             Type
+======================= ====
+`Root Signature Flags`_ i32
+======================= ====
+
+.. _Root Signature Flags: https://learn.microsoft.com/en-us/windows/win32/api/d3d12/ne-d3d12-d3d12_root_signature_flags
+
+Root Constants
+==============
+
+.. code-block:: LLVM
+
+  !4 = { !"RootConstants", i32 0, i32 1, i32 2, i32 3 }
+
+==================== ====
+Description          Type
+==================== ====
+`Shader Visibility`_ i32
+Shader Register      i32
+Register Space       i32
+Number 32-bit Values i32
+==================== ====
+
+.. _Shader Visibility: https://learn.microsoft.com/en-us/windows/win32/api/d3d12/ne-d3d12-d3d12_shader_visibility
+
+Root Descriptor
+===============
+
+As noted in the table above, the first operand will denote the type of
+root descriptor.
+
+.. code-block:: LLVM
+
+  !5 = { !"RootCBV", i32 0, i32 1, i32 0, i32 0 }
+
+======================== ====
+Description              Type
+======================== ====
+`Shader Visibility`_     i32
+Shader Register          i32
+Register Space           i32
+`Root Descriptor Flags`_ i32
+======================== ====
+
+.. _Root Descriptor Flags: https://learn.microsoft.com/en-us/windows/win32/api/d3d12/ne-d3d12-d3d12_root_descriptor_flags
+
+Static Sampler
+==============
+
+.. code-block:: LLVM
+
+  !6 = !{ !"StaticSampler", i32 1, i32 4, ... }; remaining operands omitted for space
+
+==================== =====
+Description          Type
+==================== =====
+`Filter`_            i32
+`AddressU`_          i32
+`AddressV`_          i32
+`AddressW`_          i32
+MipLODBias           float
+MaxAnisotropy        i32
+`ComparisonFunc`_    i32
+`BorderColor`_       i32
+MinLOD               float
+MaxLOD               float
+ShaderRegister       i32
+RegisterSpace        i32
+`Shader Visibility`_ i32
+==================== =====
+
+.. _Filter: https://learn.microsoft.com/en-us/windows/win32/api/d3d12/ne-d3d12-d3d12_filter
+.. _AddressU: https://learn.microsoft.com/en-us/windows/win32/api/d3d12/ne-d3d12-d3d12_texture_address_mode
+.. _AddressV: https://learn.microsoft.com/en-us/windows/win32/api/d3d12/ne-d3d12-d3d12_texture_address_mode
+.. _AddressW: https://learn.microsoft.com/en-us/windows/win32/api/d3d12/ne-d3d12-d3d12_texture_address_mode
+.. _ComparisonFunc: https://learn.microsoft.com/en-us/windows/win32/api/d3d12/ne-d3d12-d3d12_comparison_func>
+.. _BorderColor: https://learn.microsoft.com/en-us/windows/win32/api/d3d12/ne-d3d12-d3d12_static_border_color>
+
+Descriptor Table
+================
+
+A descriptor table consists of a visibility and the remaining operands are a
+list of references to its descriptor ranges.
+
+.. note::
+
+  The term Descriptor Table Clause is synonymous with Descriptor Range when
+  referencing the implementation details.
+
+.. code-block:: LLVM
+
+  !7 = { !"DescriptorTable", i32 0, !8, !9 }
+
+========================= ================
+Description               Type
+========================= ================
+`Shader Visibility`_      i32
+Descriptor Range Elements Descriptor Range
+========================= ================
+
+
+Descriptor Range
+================
+
+Similar to a root descriptor, the first operand will denote the type of
+descriptor range. It is one of the following types:
+
+- "CBV"
+- "SRV"
+- "UAV"
+- "Sampler"
+
+.. code-block:: LLVM
+
+  !8 = !{ !"SRV", i32 1, i32 0, i32 0, i32 -1, i32 4 }
+  !9 = !{ !"UAV", i32 5, i32 1, i32 10, i32 5, i32 2 }
+
+============================== ====
+Description                    Type
+============================== ====
+Number of Descriptors in Range i32
+Shader Register                i32
+Register Space                 i32
+`Offset`_                      i32
+`Descriptor Range Flags`_      i32
+============================== ====
+
+.. _Offset: https://learn.microsoft.com/en-us/windows/win32/api/d3d12/ns-d3d12-d3d12_descriptor_range
+.. _Descriptor Range Flags: https://learn.microsoft.com/en-us/windows/win32/api/d3d12/ne-d3d12-d3d12_descriptor_range_flags

llvm/docs/DirectXUsage.rst

@@ -17,6 +17,7 @@ User Guide for the DirectX Target
    DirectX/DXILArchitecture
    DirectX/DXILOpTableGenDesign
    DirectX/DXILResources
+   DirectX/RootSignatures
 
 Introduction
 ============