[docs] Add a new tutorial that talk about how to make a change to llvm

[xgupta]

Summary

This tutorial will guide you through the process of making a change to LLVM, and contributing it back to the LLVM project.
We'll be making a change to Clang, but the steps for other parts of LLVM are the same. Even though the change we'll be making is simple,
we're going to cover steps like building LLVM, running the tests, and code review. This is good practice, and you'll be prepared for making larger changes.

Authors: @meikeb , @gribozavr
Commit: Zhiqian Xia

PS - This is a duplicate revision of https://reviews.llvm.org/D100714 which was actually used for patch review.

[kuhnel]

LGTM

[xgupta]

Thanks, Please also commit on my behalf as I don't have commit access.

[xgupta]

ping @kuhnel

[xgupta]

@kuhnel Please commit this patch.

[kuhnel]

Sorry, somehow the comments did not end up in my email inbox.
I just landed the patch.

[arichardson]

llvm/docs/MyFirstTypoFix.rst

71

This is no longer true.

[xgupta]

llvm/docs/MyFirstTypoFix.rst

71

fixed in https://reviews.llvm.org/rG387a8dea7234c94ddc7a1c14c04bb2d894947c53 but everyone is welcome to contribute to this doc.

llvm/docs/GettingStartedTutorials.rst

 Getting Started/Tutorials
 =========================
 
 For those new to the LLVM system.
 
 .. toctree::
    :hidden:
 
    CompilerWriterInfo
    Frontend/PerformanceTips
    GettingStarted
    GettingStartedVS
    ProgrammersManual
    tutorial/index
+   MyFirstTypoFix
 
 :doc:`GettingStarted`
    Discusses how to get up and running quickly with the LLVM infrastructure.
    Everything from unpacking and compilation of the distribution to execution
    of some tools.
 
 :doc:`tutorial/index`
    Tutorials about using LLVM. Includes a tutorial about making a custom
    language with LLVM.
 
 :doc:`ProgrammersManual`
   Introduction to the general layout of the LLVM sourcebase, important classes
   and APIs, and some tips & tricks.
 
 :doc:`Frontend/PerformanceTips`
    A collection of tips for frontend authors on how to generate IR
    which LLVM is able to effectively optimize.
 
 :doc:`GettingStartedVS`
    An addendum to the main Getting Started guide for those using Visual Studio
    on Windows.
 
 :doc:`CompilerWriterInfo`
-  A list of helpful links for compiler writers.
+  A list of helpful links for compiler writers.
+
+:doc:`MyFirstTypoFix`
+   This tutorial will guide you through the process of making a change to
+   LLVM, and contributing it back to the LLVM project.

llvm/docs/MyFirstTypoFix.rst

+==============
+MyFirstTypoFix
+==============
+
+.. contents::
+   :local:
+
+Introduction
+============
+
+This tutorial will guide you through the process of making a change to
+LLVM, and contributing it back to the LLVM project. We'll be making a
+change to Clang, but the steps for other parts of LLVM are the same.
+Even though the change we'll be making is simple, we're going to cover
+steps like building LLVM, running the tests, and code review. This is
+good practice, and you'll be prepared for making larger changes.
+
+We'll assume you:
+
+-  know how to use an editor,
+
+-  have basic C++ knowledge,
+
+-  know how to install software on your system,
+
+-  are comfortable with the command line,
+
+-  have basic knowledge of git.
+
+
+The change we're making
+-----------------------
+
+Clang has a warning for infinite recursion:
+
+.. code:: console
+
+   $ echo "void foo() { foo(); }" > ~/test.cc
+   $ clang -c -Wall ~/test.cc
+   input.cc:1:14: warning: all paths through this function will call
+   itself [-Winfinite-recursion]
+
+This is clear enough, but not exactly catchy. Let's improve the wording
+a little:
+
+.. code:: console
+
+   input.cc:1:14: warning: to understand recursion, you must first
+   understand recursion [-Winfinite-recursion]
+
+
+Dependencies
+------------
+
+We're going to need some tools:
+
+-  git: to check out the LLVM source code,
+
+-  a C++ compiler: to compile LLVM source code. You'll want `a recent
+   version <https://llvm.org/docs/GettingStarted.html#host-c-toolchain-both-compiler-and-standard-library>`__
+   of Clang, GCC, or Visual Studio.
+
+-  CMake: used to configure how LLVM should be built on your system,
+
+-  ninja: runs the C++ compiler to (re)build specific parts of LLVM,
+
+-  python: to run the LLVM tests,
+
+-  arcanist: for uploading changes for review,
+
+-  subversion: to commit changes yourself (later).
+
+As an example, on Ubuntu:
+
+.. code:: console
+
+   $ sudo apt-get install git clang cmake ninja-build python arcanist subversion
+
+
+Building LLVM
+=============
+
+
+Checkout
+--------
+
+The source code is stored `on
+Github <https://github.com/llvm/llvm-project>`__ in one large repository
+("the monorepo").
+
+It may take a while to download!
+
+.. code:: console
+
+   $ git clone https://github.com/llvm/llvm-project.git
+
+This will create a directory "llvm-project" with all of the source
+code.(Checking out anonymously is OK - pushing commits uses a different
+mechanism, as we'll see later)
+
+Configure your workspace
+------------------------
+
+Before we can build the code, we must configure exactly how to build it
+by running CMake. CMake combines information from three sources:
+
+-  explicit choices you make (is this a debug build?)
+
+-  settings detected from your system (where are libraries installed?)
+
+-  project structure (which files are part of 'clang'?)
+
+First, create a directory to build in. Usually, this is
+llvm-project/build.
+
+.. code:: console
+
+   $ mkdir llvm-project/build
+   $ cd llvm-project/build
+
+Now, run CMake:
+
+.. code:: console
+
+   $ cmake -G Ninja ../llvm -DCMAKE_BUILD_TYPE=Release -DLLVM_ENABLE_PROJECTS=clang
+
+If all goes well, you'll see a lot of "performing test" lines, and
+finally:
+
+.. code:: console
+
+   Configuring done
+   Generating done
+   Build files have been written to: /path/llvm-project/build
+
+And you should see a build.ninja file.
+
+Let's break down that last command a little:
+
+-  **-G Ninja**: we're going to use ninja to build; please create
+   build.ninja
+
+-  **../llvm**: this is the path to the source of the "main" LLVM
+   project
+
+-  The two **-D** flags set CMake variables, which override
+   CMake/project defaults:
+
+-  **CMAKE\ BUILD\ TYPE=Release**: build in optimized mode, which is
+   (surprisingly) the fastest option.
+
+   If you want to run under a debugger, you should use the default Debug
+   (which is totally unoptimized, and will lead to >10x slower test
+   runs) or RelWithDebInfo which is a halfway point.
+   **CMAKE\ BUILD\ TYPE** affects code generation only, assertions are
+   on by default regardless! **LLVM\ ENABLE\ ASSERTIONS=Off** disables
+   them.
+
+-  **LLVM\ ENABLE\ PROJECTS=clang** : this lists the LLVM subprojects
+   you are interested in building, in addition to LLVM itself. Multiple
+   projects can be listed, separated by semicolons, such as "clang;
+   lldb".In this example, we'll be making a change to Clang, so we
+   should build it.
+
+Finally, create a symlink (or a copy) of
+llvm-project/build/compile-commands.json into llvm-project/:
+
+.. code:: console
+
+   $ ln -s build/compile_commands.json ../
+
+(This isn't strictly necessary for building and testing, but allows
+tools like clang-tidy, clang-query, and clangd to work in your source
+tree).
+
+
+Build and test
+--------------
+
+Finally, we can build the code! It's important to do this first, to
+ensure we're in a good state before making changes. But what to build?
+In ninja, you specify a **target**. If we just want to build the clang
+binary, our target name is "clang" and we run:
+
+.. code:: console
+
+   $ ninja clang
+
+The first time we build will be very slow - Clang + LLVM is a lot of
+code. But incremental builds are fast: ninja will only rebuild the parts
+that have changed. When it finally finishes you should have a working
+clang binary. Try running:
+
+.. code:: console
+
+   $ bin/clang --version
+
+There's also a target for building and running all the clang tests:
+
+.. code:: console
+
+   $ ninja check-clang
+
+This is a common pattern in LLVM: check-llvm is all the checks for core,
+other projects have targets like check-lldb.
+
+
+Making changes
+==============
+
+
+Edit
+----
+
+We need to find the file containing the error message.
+
+.. code:: console
+
+   $ git grep "all paths through this function" ..
+   ../clang/include/clang/Basic/DiagnosticSemaKinds.td:  "all paths through this function will call itself">,
+
+The string that appears in DiagnosticSemaKinds.td is the one that is
+printed by Clang. \*.td files define tables - in this case it's a list
+of warnings and errors clang can emit and their messages. Let's update
+the message in your favorite editor:
+
+.. code:: console
+
+   $ vi ../clang/include/clang/Basic/DiagnosticSemaKinds.td
+
+Find the message (it should be under
+warn\ *infinite*\ recursive_function)Change the message to "in order to
+understand recursion, you must first understand recursion".
+
+
+Test again
+----------
+
+To verify our change, we can build clang and manually check that it
+works.
+
+.. code:: console
+
+   $ ninja clang
+   $ bin/clang -Wall ~/test.cc
+
+   **/path/test.cc:1:124:** **warning****: in order to understand recursion, you must
+   first understand recursion [-Winfinite-recursion]**
+
+We should also run the tests to make sure we didn't break something.
+
+.. code:: console
+
+   $ ninja check-clang
+
+Notice that it is much faster to build this time, but the tests take
+just as long to run. Ninja doesn't know which tests might be affected,
+so it runs them all.
+
+.. code:: console
+
+   ********************
+   Testing Time: 408.84s
+   ********************
+   Failing Tests (1):
+       Clang :: SemaCXX/warn-infinite-recursion.cpp
+
+Well, that makes sense… and the test output suggests it's looking for
+the old string "call itself" and finding our new message instead.
+
+Let's fix it by updating the expectation in the test.
+
+.. code:: console
+
+   $ vi ../clang/test/SemaCXX/warn-infinite-recursion.cpp
+
+Everywhere we see // expected-warning{{call itself}}, let's replace it
+with // expected-warning{{to understand recursion}}.
+
+Now we could run **all** the tests again, but this is a slow way to
+iterate on a change! Instead, let's find a way to re-run just the
+specific test. There are two main types of tests in LLVM:
+
+-  **lit tests** (e.g. SemaCXX/warn-infinite-recursion.cpp).
+
+These are fancy shell scripts that run command-line tools and verify the
+output. They live in files like
+clang/**test**/FixIt/dereference-addressof.c. Re-run like this:
+
+.. code:: console
+
+   $ bin/llvm-lit -v ../clang/test/SemaCXX/warn-infinite-recursion.cpp
+
+-  **unit tests** (e.g. ToolingTests/ReplacementText.CanDeleteAllText)
+
+These are C++ programs that call LLVM functions and verify the results.
+They live in suites like ToolingTests. Re-run like this:
+
+.. code:: console
+
+   $ ninja ToolingTests && tools/clang/unittests/Tooling/ToolingTests
+   --gtest_filter=ReplacementText.CanDeleteAllText
+
+
+Commit locally
+--------------
+
+We'll save the change to a local git branch. This lets us work on other
+things while the change is being reviewed. Changes should have a
+description, to explain to reviewers and future readers of the code why
+the change was made.
+
+.. code:: console
+
+   $ git checkout -b myfirstpatch
+   $ git commit -am "[Diagnostic] Clarify -Winfinite-recursion message"
+
+Now we're ready to send this change out into the world! By the way,
+There is a unwritten convention of using tag for your commit. Tags
+usually represent modules that you intend to modify. If you don't know
+the tags for your modules, you can look at the commit history :
+https://github.com/llvm/llvm-project/commits/main.
+
+
+Code review
+===========
+
+
+Finding a reviewer
+------------------
+
+Changes can be reviewed by anyone in the LLVM community who has commit
+access.For larger and more complicated changes, it's important that the
+reviewer has experience with the area of LLVM and knows the design goals
+well. The author of a change will often assign a specific reviewer (git
+blame and git log can be useful to find one).
+
+As our change is fairly simple, we'll add the cfe-commits mailing list
+as a subscriber; anyone who works on clang can likely pick up the
+review. (For changes outside clang, llvm-commits is the usual list. See
+`http://lists.llvm.org/ <http://lists.llvm.org/mailman/listinfo>`__ for
+all the \*-commits mailing lists).
+
+
+Uploading a change for review
+-----------------------------
+
+LLVM code reviews happen at https://reviews.llvm.org. The web interface
+is called Phabricator, and the code review part is Differential. You
+should create a user account there for reviews (click "Log In" and then
+"Register new account").
+
+Now you can upload your change for review:
+
+.. code:: console
+
+   $ arc diff HEAD^
+
+This creates a review for your change, comparing your current commit
+with the previous commit. You will be prompted to fill in the review
+details. Your commit message is already there, so just add cfe-commits
+under the "subscribers" section. It should print a code review URL:
+https://reviews.llvm.org/D58291 You can always find your active reviews
+on Phabricator under "My activity".
+
+
+Review process
+--------------
+
+When you upload a change for review, an email is sent to you, the
+cfe-commits list, and anyone else subscribed to these kinds of changes.
+Within a few days, someone should start the review. They may add
+themselves as a reviewer, or simply start leaving comments. You'll get
+another email any time the review is updated. The details are in the
+`https://llvm.org/docs/CodeReview/ <https://llvm.org/docs/CodeReview.html>`__.
+
+
+Comments
+~~~~~~~~
+
+The reviewer can leave comments on the change, and you can reply. Some
+comments are attached to specific lines, and appear interleaved with the
+code. You can either reply to these, or address them and mark them as
+"done". Note that in-line replies are **not** sent straight away! They
+become "draft" comments and you must click "Submit" at the bottom of the
+page.
+
+
+Updating your change
+~~~~~~~~~~~~~~~~~~~~
+
+If you make changes in response to a reviewer's comments, simply run
+
+.. code:: console
+
+   $ arc diff
+
+again to update the change and notify the reviewer. Typically this is a
+good time to send any draft comments as well.
+
+
+Accepting a revision
+~~~~~~~~~~~~~~~~~~~~
+
+When the reviewer is happy with the change, they will **Accept** the
+revision. They may leave some more minor comments that you should
+address, but at this point the review is complete. It's time to get it
+committed!
+
+
+Commit by proxy
+---------------
+
+As this is your first change, you won't have access to commit it
+yourself yet. The reviewer **doesn't know this**, so you need to tell
+them! Leave a message on the review like:
+
+   Thanks @somellvmdev. I don't have commit access, can you land this
+   patch for me? Please use "My Name my@email" to commit the change.
+
+The review will be updated when the change is committed.
+
+
+Review expectations
+-------------------
+
+In order to make LLVM a long-term sustainable effort, code needs to be
+maintainable and well tested. Code reviews help to achieve that goal.
+Especially for new contributors, that often means many rounds of reviews
+and push-back on design decisions that do not fit well within the
+overall architecture of the project.
+
+For your first patches, this means:
+
+-  be kind, and expect reviewers to be kind in return - LLVM has a `Code
+   of Conduct <https://llvm.org/docs/CodeOfConduct.html>`__;
+
+-  be patient - understanding how a new feature fits into the
+   architecture of the project is often a time consuming effort, and
+   people have to juggle this with other responsibilities in their
+   lives; **ping the review once a week** when there is no response;
+
+-  if you can't agree, generally the best way is to do what the reviewer
+   asks; we optimize for readability of the code, which the reviewer is
+   in a better position to judge; if this feels like it's not the right
+   option, you can contact the cfe-dev mailing list to get more feedback
+   on the direction;
+
+
+Commit access
+=============
+
+Once you've contributed a handful of patches to LLVM, start to think
+about getting commit access yourself. It's probably a good idea if:
+
+-  you've landed 3-5 patches of larger scope than "fix a typo"
+
+-  you'd be willing to review changes that are closely related to yours
+
+-  you'd like to keep contributing to LLVM.
+
+
+Getting commit access
+---------------------
+
+LLVM uses Git for committing changes. The details are in the `developer
+policy
+document <https://llvm.org/docs/DeveloperPolicy.html#obtaining-commit-access>`__.
+
+
+With great power
+----------------
+
+Actually, this would be a great time to read the rest of the `developer
+policy <https://llvm.org/docs/DeveloperPolicy.html>`__, too. At minimum,
+you need to be subscribed to the relevant commits list before landing
+changes (e.g. [e-mail]), as discussion often happens
+there if a new patch causes problems.
+
+
+Commit
+------
+
+Let's say you have a change on a local git branch, reviewed and ready to
+commit. Things to do first:
+
+-  if you used multiple fine-grained commits locally, squash them into a
+   single commit. LLVM prefers commits to match the code that was
+   reviewed. (If you created one commit and then used "arc diff", you're
+   fine)
+
+-  rebase your patch against the latest LLVM code. LLVM uses a linear
+   history, so everything should be based on an up-to-date origin/main.
+
+.. code:: console
+
+   $ git pull --rebase https://github.com/llvm/llvm-project.git main
+
+-  ensure the patch looks correct.
+
+.. code:: console
+
+   $ git show
+
+-  run the tests one last time, for good luck
+
+At this point git show should show a single commit on top of
+origin/main.
+
+Now you can push your commit with
+
+.. code:: console
+
+   $ git push https://github.com/llvm/llvm-project.git HEAD:main
+
+You should see your change `on
+GitHub <https://github.com/llvm/llvm-project/commits/main>`__ within
+minutes.
+
+
+Post-commit errors
+------------------
+
+Once your change is submitted it will be picked up by automated build
+bots that will build and test your patch in a variety of configurations.
+
+You can see all configurations and their current state in a waterfall
+view at http://lab.llvm.org:8011/waterfall. The waterfall view is good
+to get a general overview over the tested configurations and to see
+which configuration have been broken for a while.
+
+The console view at http://lab.llvm.org:8011/console helps to get a
+better understanding of the build results of a specific patch. If you
+want to follow along how your change is affecting the build bots, **this
+should be the first place to look at** - the colored bubbles correspond
+to projects in the waterfall.
+
+If you see a broken build, do not despair - some build bots are
+continuously broken; if your change broke the build, you will see a red
+bubble in the console view, while an already broken build will show an
+orange bubble. Of course, even when the build was already broken, a new
+change might introduce a hidden new failure.
+
+| When you want to see more details how a specific build is broken,
+  click the red bubble.
+| If post-commit error logs confuse you, do not worry too much -
+  everybody on the project is aware that this is a bit unwieldy, so
+  expect people to jump in and help you understand what's going on!
+
+buildbots, overview of bots, getting error logs.
+
+
+Reverts
+-------
+
+if in doubt, revert and re-land.
+
+
+Conclusion
+==========
+
+llvm is a land of contrasts.