[MLGO][Docs] Add documentation on corpus tooling (llvm#139362)

This adds some documentation on the three corpus tools, some examples,
and fixes the TODO telling me to get this done.

Author: boomanaiden154

llvm/docs/MLGO.rst

@@ -27,8 +27,180 @@ of models during training.
 Corpus Tooling
 ==============
 
-..
-    TODO(boomanaiden154): Write this section.
+Within the LLVM monorepo, there is the ``mlgo-utils`` python packages that
+lives at ``llvm/utils/mlgo-utils``. This package primarily contains tooling
+for working with corpora, or collections of LLVM bitcode. We use these corpora
+to train and evaluate ML models. Corpora consist of a description in JSON
+format at ``corpus_description.json`` in the root of the corpus, and then
+a bitcode file and command line flags file for each extracted module. The
+corpus structure is designed to contain sufficient information to fully
+compile the bitcode to bit-identical object files.
+
+.. program:: extract_ir.py
+
+Synopsis
+--------
+
+Extracts a corpus from some form of a structured compilation database. This
+tool supports a variety of different scenarios and input types.
+
+Options
+-------
+
+.. option:: --input
+
+  The path to the input. This should be a path to a supported structured
+  compilation database. Currently only ``compile_commands.json`` files, linker
+  parameter files, a directory containing object files (for the local
+  ThinLTO case only), or a JSON file containing a bazel aquery result are
+  supported.
+
+.. option:: --input_type
+
+  The type of input that has been passed to the ``--input`` flag.
+
+.. option:: --output_dir
+
+  The output directory to place the corpus in.
+
+.. option:: --num_workers
+
+  The number of workers to use for extracting bitcode into the corpus. This
+  defaults to the number of hardware threads available on the host system.
+
+.. option:: --llvm_objcopy_path
+
+  The path to the llvm-objcopy binary to use when extracting bitcode.
+
+.. option:: --obj_base_dir
+
+  The base directory for object files. Bitcode files that get extracted into
+  the corpus will be placed into the output directory based on where their
+  source object files are placed relative to this path.
+
+.. option:: --cmd_filter
+
+  Allows filtering of modules by command line. If set, only modules that much
+  the filter will be extracted into the corpus. Regular expressions are
+  supported in some instances.
+
+.. option:: --thinlto_build
+
+  If the build was performed with ThinLTO, this should be set to either
+  ``distributed`` or ``local`` depending upon how the build was performed.
+
+.. option:: --cmd_section_name
+
+  This flag allows specifying the command line section name. This is needed
+  on non-ELF platforms where the section name might differ.
+
+.. option:: --bitcode_section_name
+
+  This flag allows specifying the bitcode section name. This is needed on
+  non-ELF platforms where the section name might differ.
+
+Example: CMake
+--------------
+
+CMake can output a ``compilation_commands.json`` compilation database if the
+``CMAKE_EXPORT_COMPILE_COMMANDS`` switch is turned on at compile time. It is
+also necessary to enable bitcode embedding (done by passing 
+``-Xclang -fembed-bitcode=all`` to all C/C++ compilation actions in the
+non-ThinLTO case). For example, to extract a corpus from clang, you would
+run the following commands (assuming that the system C/C++ compiler is clang):
+
+.. code-block:: bash
+
+  cmake -GNinja \
+    -DCMAKE_BUILD_TYPE=Release \
+    -DCMAKE_EXPORT_COMPILE_COMMANDS=ON \
+    -DCMAKE_C_FLAGS="-Xclang -fembed-bitcode=all" \
+    -DCMAKE_CXX_FLAGS="-Xclang -fembed-bitcode-all"
+    ../llvm
+  ninja
+
+After running CMake and building the project, there should be a
+ ``compilation_commands.json`` file within the build directory. You can then
+ run the following command to create a corpus:
+
+.. code-block:: bash
+
+  python3 ./extract_ir.py \
+    --input=./build/compile_commands.json \
+    --input_type=json \
+    --output_dir=./corpus
+
+After running the above command, there should be a full
+corpus of bitcode within the ``./corpus`` directory.
+
+Example: Bazel Aquery
+---------------------
+
+This tool also supports extracting bitcode from bazel in multiple ways
+depending upon the exact configuration. For ThinLTO, a linker parameters file
+is preferred. For the non-ThinLTO case, the script will accept the output of
+``bazel aquery`` which it will use to find all the object files that are linked
+into a specific target and then extract bitcode from them. First, you need
+to generate the aquery output:
+
+.. code-block:: bash
+
+  bazel aquery --output=jsonproto //path/to:target > /path/to/aquery.json
+
+Afterwards, assuming that the build is already complete, you can run this
+script to create a corpus:
+
+.. code-block:: bash
+
+  python3 ./extract_ir.py \
+    --input=/path/to/aquery.json \
+    --input_type=bazel_aqeury \
+    --output_dir=./corpus \
+    --obj_base_dir=./bazel-bin
+
+This will again leave a corpus that contains all the bitcode files. This mode
+does not capture all object files in the build however, only the ones that
+are involved in the link for the binary passed to the ``bazel aquery``
+invocation.
+
+.. program:: make_corpus.py
+
+Synopsis
+--------
+
+Creates a corpus from a collection of bitcode files.
+
+Options
+-------
+
+.. option:: --input_dir
+
+  The input directory to search for bitcode files in.
+
+.. option:: --output_dir
+
+  The output directory to place the constructed corpus in.
+
+.. option:: --default_args
+
+  A list of space separated flags that are put into the corpus description.
+  These are used by some tooling when compiling the modules within the corpus.
+
+.. program:: combine_training_corpus.py
+
+Synopsis
+--------
+
+Combines two training corpora that share the same parent folder by generating
+a new ``corpus_description.json`` that contains all the modules in both corpora.
+
+Options
+-------
+
+.. option:: --root_dir
+
+  The root directory that contains subfolders consisting of the corpora that
+  should be combined.
 
 Interacting with ML models
 ==========================