Code Reviews with Phabricator

If you prefer to use a web user interface for code reviews, you can now submit your patches for Clang and LLVM at LLVM’s Phabricator instance.

While Phabricator is a useful tool for some, the relevant -commits mailing list is the system of record for all LLVM code review. The mailing list should be added as a subscriber on all reviews, and Phabricator users should be prepared to respond to free-form comments in mail sent to the commits list.

Sign up

To get started with Phabricator, navigate to and click the power icon in the top right. You can register with a GitHub account, a Google account, or you can create your own profile.

Make sure that the email address registered with Phabricator is subscribed to the relevant -commits mailing list. If you are not subscribed to the commit list, all mail sent by Phabricator on your behalf will be held for moderation.

Note that if you use your Subversion user name as Phabricator user name, Phabricator will automatically connect your submits to your Phabricator user in the Code Repository Browser.

Requesting a review via the command line

Phabricator has a tool called Arcanist to upload patches from the command line. To get you set up, follow the Arcanist Quick Start instructions.

You can learn more about how to use arc to interact with Phabricator in the Arcanist User Guide.

Requesting a review via the web interface

The tool to create and review patches in Phabricator is called Differential.

Note that you can upload patches created through various diff tools, including git and svn. To make reviews easier, please always include as much context as possible with your diff! Don’t worry, Phabricator will automatically send a diff with a smaller context in the review email, but having the full file in the web interface will help the reviewer understand your code.

To get a full diff, use one of the following commands (or just use Arcanist to upload your patch):

  • git diff -U999999 other-branch
  • svn diff --diff-cmd=diff -x -U999999

To upload a new patch:

  • Click Differential.
  • Click + Create Diff.
  • Paste the text diff or browse to the patch file. Click Create Diff.
  • Leave the Repository field blank.
  • Leave the drop down on Create a new Revision... and click Continue.
  • Enter a descriptive title and summary. The title and summary are usually in the form of a commit message.
  • Add reviewers (see below for advice) and subscribe mailing lists that you want to be included in the review. If your patch is for LLVM, add llvm-commits as a Subscriber; if your patch is for Clang, add cfe-commits.
  • Leave the Repository and Project fields blank.
  • Click Save.

To submit an updated patch:

  • Click Differential.
  • Click + Create Diff.
  • Paste the updated diff or browse to the updated patch file. Click Create Diff.
  • Select the review you want to from the Attach To dropdown and click Continue.
  • Leave the Repository and Project fields blank.
  • Add comments about the changes in the new diff. Click Save.

Choosing reviewers: You typically pick one or two people as initial reviewers. This choice is not crucial, because you are merely suggesting and not requiring them to participate. Many people will see the email notification on cfe-commits or llvm-commits, and if the subject line suggests the patch is something they should look at, they will.

Here are a couple of ways to pick the initial reviewer(s):

  • Use svn blame and the commit log to find names of people who have recently modified the same area of code that you are modifying.
  • Look in CODE_OWNERS.TXT to see who might be responsible for that area.
  • If you’ve discussed the change on a dev list, the people who participated might be appropriate reviewers.

Even if you think the code owner is the busiest person in the world, it’s still okay to put them as a reviewer. Being the code owner means they have accepted responsibility for making sure the review happens.

Reviewing code with Phabricator

Phabricator allows you to add inline comments as well as overall comments to a revision. To add an inline comment, select the lines of code you want to comment on by clicking and dragging the line numbers in the diff pane. When you have added all your comments, scroll to the bottom of the page and click the Submit button.

You can add overall comments in the text box at the bottom of the page. When you’re done, click the Submit button.

Phabricator has many useful features, for example allowing you to select diffs between different versions of the patch as it was reviewed in the Revision Update History. Most features are self descriptive - explore, and if you have a question, drop by on #llvm in IRC to get help.

Note that as e-mail is the system of reference for code reviews, and some people prefer it over a web interface, we do not generate automated mail when a review changes state, for example by clicking “Accept Revision” in the web interface. Thus, please type LGTM into the comment box to accept a change from Phabricator.

Committing a change

Once a patch has been reviewed and approved on Phabricator it can then be committed to trunk. There are multiple workflows to achieve this. Whichever method you follow it is recommend that your commit message ends with the line:

Differential Revision: <URL>

where <URL> is the URL for the code review, starting with

This allows people reading the version history to see the review for context. This also allows Phabricator to detect the commit, close the review, and add a link from the review to the commit.

Note that if you use the Arcanist tool the Differential Revision line will be added automatically. If you don’t want to use Arcanist, you can add the Differential Revision line (as the last line) to the commit message yourself.

Using the Arcanist tool can simplify the process of committing reviewed code as it will retrieve reviewers, the Differential Revision, etc from the review and place it in the commit message. Several methods of using Arcanist to commit code are given below. If you do not wish to use Arcanist then simply commit the reviewed patch as you would normally.

Note that if you commit the change without using Arcanist and forget to add the Differential Revision line to your commit message then it is recommended that you close the review manually. In the web UI, under “Leap Into Action” put the SVN revision number in the Comment, set the Action to “Close Revision” and click Submit. Note the review must have been Accepted first.

Subversion and Arcanist

On a clean Subversion working copy run the following (where <Revision> is the Phabricator review number):

arc patch D<Revision>
arc commit --revision D<Revision>

The first command will take the latest version of the reviewed patch and apply it to the working copy. The second command will commit this revision to trunk.

git-svn and Arcanist

This presumes that the git repository has been configured as described in For developers to work with git-svn.

On a clean Git repository on an up to date master branch run the following (where <Revision> is the Phabricator review number):

arc patch D<Revision>

This will create a new branch called arcpatch-D<Revision> based on the current master and will create a commit corresponding to D<Revision> with a commit message derived from information in the Phabricator review.

Check you are happy with the commit message and amend it if necessary. Now switch to the master branch and add the new commit to it and commit it to trunk. This can be done by running the following:

git checkout master
git merge --ff-only arcpatch-D<Revision>
git svn dcommit

Abandoning a change

If you decide you should not commit the patch, you should explicitly abandon the review so that reviewers don’t think it is still open. In the web UI, scroll to the bottom of the page where normally you would enter an overall comment. In the drop-down Action list, which defaults to “Comment,” you should select “Abandon Revision” and then enter a comment explaining why. Click the Submit button to finish closing the review.


Please let us know whether you like it and what could be improved! We’re still working on setting up a bug tracker, but you can email klimek-at-google-dot-com and chandlerc-at-gmail-dot-com and CC the llvm-dev mailing list with questions until then. We also could use help implementing improvements. This sadly is really painful and hard because the Phabricator codebase is in PHP and not as testable as you might like. However, we’ve put exactly what we’re deploying up on an llvm-reviews GitHub project where folks can hack on it and post pull requests. We’re looking into what the right long-term hosting for this is, but note that it is a derivative of an existing open source project, and so not trivially a good fit for an official LLVM project.