[Docs][DirectX] Add relevant documentation of Root Signature

[inbelic]

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.

[llvmbot]

@llvm/pr-subscribers-backend-directx

Author: Finn Plummer (inbelic)

Changes

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.

---

Full diff: https://github.com/llvm/llvm-project/pull/149608.diff

1 Files Affected:

• (added) llvm/docs/DirectX/RootSignatures.rst (+231)

diff --git a/llvm/docs/DirectX/RootSignatures.rst b/llvm/docs/DirectX/RootSignatures.rst
new file mode 100644
index 0000000000000..7a5e4d3e8fc27
--- /dev/null
+++ b/llvm/docs/DirectX/RootSignatures.rst
@@ -0,0 +1,231 @@
+===============
+Root Signatures
+===============
+
+.. contents::
+   :local:
+
+.. toctree::
+   :hidden:
+
+Overview
+========
+
+A root signature is used to describe what resources a shader needs to 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:: C
+
+  #define DemoRootSignature                                                    \
+    "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:
+
+.. csv-table::
+
+  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 }
+
+.. csv-table::
+
+  Description, Type
+
+  `Root Signature Flags <https://learn.microsoft.com/en-us/windows/win32/api/d3d12/ne-d3d12-d3d12_root_signature_flags>`_, i32
+
+
+Root Constants
+==============
+
+.. code-block:: LLVM
+
+  !4 = { !"RootConstants", i32 0, i32 1, i32 2, i32 3 }
+
+.. csv-table::
+
+  Description, Type
+
+  `Shader Visibility <https://learn.microsoft.com/en-us/windows/win32/api/d3d12/ne-d3d12-d3d12_shader_visibility>`_, i32
+  Shader Register, i32
+  Register Space, i32
+  Number 32-bit Values, i32
+
+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 }
+
+.. csv-table::
+
+  Description, Type
+
+  `Shader Visibility`_, i32
+  Shader Register, i32
+  Register Space, i32
+  `Root Descriptor Flags <https://learn.microsoft.com/en-us/windows/win32/api/d3d12/ne-d3d12-d3d12_root_descriptor_flags>`_, i32
+
+Static Sampler
+==============
+
+.. code-block:: LLVM
+
+  !6 = !{ !"StaticSampler", i32 1, i32 4, ... }; remaining operands omitted for space
+
+.. csv-table::
+
+  Description, Type
+
+  `Filter <https://learn.microsoft.com/en-us/windows/win32/api/d3d12/ne-d3d12-d3d12_filter>`_, i32
+  `AddressU <https://learn.microsoft.com/en-us/windows/win32/api/d3d12/ne-d3d12-d3d12_texture_address_mode>`_, i32
+  `AddressV <https://learn.microsoft.com/en-us/windows/win32/api/d3d12/ne-d3d12-d3d12_texture_address_mode>`_, i32
+  `AddressW <https://learn.microsoft.com/en-us/windows/win32/api/d3d12/ne-d3d12-d3d12_texture_address_mode>`_, i32
+  MipLODBias, float
+  MaxAnisotropy, i32
+  `ComparisonFunc <https://learn.microsoft.com/en-us/windows/win32/api/d3d12/ne-d3d12-d3d12_comparison_func>`_, i32
+  `BorderColor <https://learn.microsoft.com/en-us/windows/win32/api/d3d12/ne-d3d12-d3d12_static_border_color>`_, i32
+  MinLOD, float
+  MaxLOD, float
+  ShaderRegister, i32
+  RegisterSpace, i32
+  `Shader Visibility`_, i32
+
+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 }
+
+.. csv-table::
+
+  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 }
+
+.. csv-table::
+
+  Description, Type
+
+  Number of Descriptors in Range, i32
+  Shader Register, i32
+  Register Space, i32
+  `Offset <https://learn.microsoft.com/en-us/windows/win32/api/d3d12/ns-d3d12-d3d12_descriptor_range>`_, i32
+  `Descriptor Range Flags <https://learn.microsoft.com/en-us/windows/win32/api/d3d12/ne-d3d12-d3d12_descriptor_range_flags>`_, i32

[inbelic]

If there is any other documentation pertaining to an LLVM user that wants to construct a root signature, that should be added here. Please bring this up!

[V-FEXrt]

Is there a way to view the rendered version? This change seems alright but I think seeing it rendered would help a lot

[V-FEXrt]

Suggested change

	A root signature is used to describe what resources a shader needs to access to
	A root signature is used to describe what resources a shader needs access to

[inbelic]

Is there a way to view the rendered version? This change seems alright but I think seeing it rendered would help a lot

You can view the "rich diff" in github to get the markdown view, which is kind of along the lines. But I am not sure about the rts rendered view. Maybe if the docs build suceeds it will produce an artifact?