[Docs] Add new docs on unified build workflow (#4890)

This document tries to capture the basics of a unified build and test
workflow for all platforms. It is complete enough to start using but not
yet ready to be the default build mode.

docs/BuildingAndTestingDXC.rst

@@ -0,0 +1,159 @@
+==========================================
+Building and Testing DirectXShaderCompiler
+==========================================
+
+.. contents::
+   :local:
+   :depth: 3
+
+.. warning::
+   This document describes a new and slightly experimental build process
+   building DXC and using LLVM's LIT tool to run DXC's tests. This workflow
+   is complete on Linux and Unix platforms, but is incomplete but usable on
+   Windows. Instructions for building on Windows are available in the repository
+   `readme <https://github.com/microsoft/DirectXShaderCompiler/blob/main/README.md>`_.
+   Instructions for the preexisting Linux and Unix workflow can be found here:
+   :doc:`DxcOnUnix`
+
+Introduction
+============
+
+DXC's build is configured using CMake and should be buildable with any preferred
+generator. In subsequent sections we will describe workflows using Ninja and
+Visual Studio. The Ninja workflow should also apply to makefile generators with
+minimal adaption.
+
+Basic CMake Usage
+-----------------
+
+When building DXC there are a bunch of CMake options which must be set. To
+simplify configuration those options are collected into a CMake cache script
+which can be passed into the CMake tool using the ``-C`` flag. Cache scripts run
+before any other CMake script evaluation and can be used to initialize variables
+for the configuration process.
+
+All of the most basic CMake configurations for DXC follow a similar format to:
+
+.. code:: sh
+  cmake <Repository Root> \
+    -C <Repository Root>/cmake/caches/PredefinedParams.cmake \
+    -DCMAKE_BUILD_TYPE=<Build Type> \
+    -G <Generator>
+
+Useful CMake Flags
+------------------
+
+By default CMake will place the generated build output files in the directory
+you run CMake in. Our build system disallows running CMake in the root of the
+repository so you either need to create an alternate directory to run CMake in,
+or you can use the CMake ``-B <path>`` flag to direct CMake to place the
+generated files in a different location.
+
+Generating a Visual Studio Solution
+-----------------------------------
+
+Open a Visual Stuido command prompt and run:
+
+.. code:: sh
+  cmake <Repository Root> \
+    -B <Path to Output> \
+    -DDXC_USE_LIT=On \
+    -C <Repository Root>/cmake/caches/PredefinedParams.cmake \
+    -DCMAKE_BUILD_TYPE=<Build Type> \
+    -G "Visual Studio 17 2022"
+
+Open the resulting LLVM.sln placed under the ``<Path to Output>``. DXC should
+build successfully with either the ``Visual Studio 17 2022`` or ``Visual Studio
+16 2019`` generators.
+
+Using Visual Studio's CMake Integration
+---------------------------------------
+
+Open Visual Studio and select "Open a local folder". Open the folder DXC is
+cloned into. If Visual Studio does not start automatically configuring the build
+you may need to go to the "Project" menu and select "CMake Workspace Settings"
+and add ``"enableCMake"; true`` to the JSON configuration file.
+
+After CMake configuration completes you should be able to toggle the "Solution
+Explorer" into "CMake Targets View" to see the available build targets.
+
+Generating Ninja or Makefiles
+-----------------------------
+
+In your preferred terminal run:
+
+.. code:: sh
+  cmake <Repository Root> \
+    -B <Path to Output> \
+    -DDXC_USE_LIT=On \
+    -C <Repository Root>/cmake/caches/PredefinedParams.cmake \
+    -DCMAKE_BUILD_TYPE=<Build Type> \
+    -G Ninja
+
+You may substitute ``Ninja`` for ``Unix Makefiles`` to generate a makefile
+build. After generation completes you can run ``ninja`` or ``make`` as
+appropriate.
+
+Building and Running Tests
+--------------------------
+
+With the LIT-based testing solution, builds and tests are all run through the
+generated build system. Regardless of which tool you use to build DXC you should
+have the following targets available:
+
+**llvm-test-depends** Builds all the binaries used by the tests.
+**clang-test-depends** Builds all the binaries used by the clang tests.
+**test-depends** Builds all the binaries used by all the tests.
+**check-llvm** Runs the LLVM tests after rebuilding any required out-of-date targets.
+**check-clang** Runs the Clang tests after rebuilding any required out-of-date targets.
+**check-all** Runs all available tests after rebuilding any out-of-date targets.
+
+Useful CMake Options
+--------------------
+
+By convention CMake options are all capital, underscore separated words, and the
+first word signifies what the option applies to. In the DXC codebase there are
+four commonly used option prefixes:
+
+#. CMAKE - For options defined by CMake itself which apply across the entire
+   configuration.
+#. LLVM - For options defined by LLVM which DXC has inherited. These apply
+   across the entire DXC codebase.
+#. CLANG - For options defined in the clang sub-project which DXC has inherited.
+   These options apply across just the tools/clang subdirectory.
+#. DXC - For DXC-specific options, which may apply across the entire codebase.
+
+**CMAKE_BUILD_TYPE**:STRING
+  Sets the build type for single-configuration generators (i.e. Ninja and
+  makefiles) Possible values are Release, Debug, RelWithDebInfo and MinSizeRel.
+  On systems like Visual Studio or Xcode the user sets the build type with the
+  IDE settings.
+
+**LLVM_USE_LINKER**:STRING
+  When building with Clang or GCC this option allows overriding the default
+  linker used by setting the ``-fuse-ld=`` flag. This may be important for Linux
+  users on systems where the system linker is ``ld.bfd`` as linking DXC with
+  debug information can be very memory intensive.
+
+**LLVM_PARALLEL_COMPILE_JOBS**:STRING
+  When building with Ninja, this option can be used to limit the number of
+  concurrent compilation jobs.
+
+**LLVM_PARALLEL_LINK_JOBS**:STRING
+  When building with Ninja, this option can be used to limit number of
+  concurrent link jobs.
+
+**DXC_COVERAGE**:BOOL
+  This option must be passed before the ``-C`` flag to set the PredefinedParams
+  cache script because it is handled by the cache script. This option enables
+  building DXC with code coverage instrumentation and build targets to generate
+  code coverage reports. With this setting enabled the
+  ``generate-coverage-report`` target is added to the build which produces a
+  static HTML page with code coverage analysis results.
+
+**DXC_USE_LIT**:BOOL
+  This option must be passed before the ``-C`` flag to set the PredefinedParams
+  cache script because it is handled by the cache script. This option enables
+  building DXC with the LLVM-LIT testing infrastructure enabled. This generates
+  check targets for each sub-project (i.e. ``check-llvm``, ``check-clang``...),
+  and a ``check-all`` target to build and run DXC's tests.