[docs] Adding DirectX target usage doc

This document is a work in progress to begin fleshing out documentation
for the DirectX backend and related changes in the LLVM project.

This is not intended to be exhaustive or complete, it is intended as a
starting
point so taht future changes have a place for documentation to land.

Differential Revision: https://reviews.llvm.org/D127640

llvm/docs/DirectXUsage.rst

@@ -0,0 +1,89 @@
+=================================
+User Guide for the DirectX Target
+=================================
+
+.. warning::
+   Disclaimer: The DirectX backend is experimental and under active development.
+   It is not yet feature complete or ready to be used outside of experimental or
+   demonstration contexts.
+
+.. contents::
+   :local:
+
+.. toctree::
+   :hidden:
+
+Introduction
+============
+
+The DirectX target implements the DirectX programmability interfaces. These
+interfaces are documented in the `DirectX Specifications. <https://github.com/Microsoft/DirectX-Specs>`_
+
+Initially the backend is aimed at supporting DirectX 12, and support for DirectX
+11 is planned at a later date.
+
+The DirectX backend is currently experimental and is not shipped with any
+release builds of LLVM tools. To enable building the DirectX backend locally add
+``DirectX`` to the ``LLVM_EXPERIMENTAL_TARGETS_TO_BUILD`` CMake option. For more
+information on building LLVM see the :doc:`CMake` documentation.
+
+.. _dx-target-triples:
+
+Target Triples
+==============
+
+At present the DirectX target only supports the ``dxil`` architecture, which
+generates code for the
+`DirectX Intermediate Language. <https://github.com/microsoft/DirectXShaderCompiler/blob/main/docs/DXIL.rst>`_
+
+In addition to target architecture, the DirectX backend also needs to know the
+target runtime version and pipeline stage. These are expressed using the OS and
+Environment triple component.
+
+Presently the DirectX backend requires targeting the ``shadermodel`` OS, and
+supports versions 6.0+ (at time of writing the latest announced version is 6.7).
+
+.. table:: DirectX Environments
+
+     ================== ========================================================
+     Environment         Description
+     ================== ========================================================
+     ``pixel``           Pixel shader
+     ``vertex``          Vertex shader
+     ``geometry``        Geometry shader
+     ``hull``            Hull shader (tesselation)
+     ``domain``          Domain shader (tesselation)
+     ``compute``         Compute kernel
+     ``library``         Linkable ``dxil`` library
+     ``raygeneration``   Ray generation (ray tracing)
+     ``intersection``    Ray intersection (ray tracing)
+     ``anyhit``          Ray any collision (ray tracing)
+     ``closesthit``      Ray closest collision (ray tracing)
+     ``miss``            Ray miss (ray tracing)
+     ``callable``        Callable shader (ray tracing)
+     ``mesh``            Mesh shader
+     ``amplification``   Amplification shader
+     ================== ========================================================
+
+Output Binaries
+===============
+
+The DirectX runtime APIs read a file format based on the
+`DirectX Specification. <https://github.com/Microsoft/DirectX-Specs>`_. In
+different codebases the file format is referred to by different names
+(specifically ``DXBC`` and ``DXILContainer``). Since the format is used to store
+both ``DXBC`` and ``DXIL`` outputs, and the ultimate goal is to support both as
+code generation targets in LLVM, the LLVM codebase uses a more neutral name,
+``DXContainer``.
+
+The ``DXContainer`` format is sparsely documented in the functional
+specification, but a reference implementation exists in the
+`DirectXShaderCompiler. <https://github.com/microsoft/DirectXShaderCompiler>`_.
+
+Support for generating ``DXContainer`` files in LLVM, is being added to the LLVM
+MC layer for object streamers and writers, and to the Object and ObjectYAML
+libraries for testing and object file tooling.
+
+For ``dxil`` targeting, bitcode emission into ``DXContainer`` files follows a
+similar model to the ``-fembed-bitcode`` flag supported by clang for other
+targets.

llvm/docs/UserGuides.rst

@@ -29,6 +29,7 @@ intermediate LLVM representation.
    CoverageMappingFormat
    CycleTerminology
    DebuggingJITedCode
+   DirectXUsage
    Docker
    ExtendingLLVM
    GoldPlugin
@@ -247,3 +248,7 @@ Additional Topics
 
 :doc:`SPIRVUsage`
    This document describes using the SPIR-V target to compile GPU kernels.
+
+:doc:`DirectXUsage`
+   This document describes using the DirectX target to compile GPU code for the
+   DirectX runtime.