Authors: Jason Lowe-Power Andreas Sandberg Steve Reinhardt

If you've made changes to gem5 that might benefit others, we strongly encourage you to contribute those changes to the public gem5 repository. There are several reasons to do this:

  • Share your work with others, so that they can benefit from new functionality.
  • Support the scientific principle by enabling others to evaluate your suggestions without having to guess what you did.
  • Once your changes are part of the main repo, you no longer have to merge them back in every time you update your local repo. This can be a huge time saving!
  • Once your code is in the main repo, other people have to make their changes work with your code, and not the other way around.
  • Others may build on your contributions to make them even better, or extend them in ways you did not have time to do.
  • You will have the satisfaction of contributing back to the community.

The main method for contributing code to gem5 is via our code review website: This documents describes the details of how to create code changes, upload your changes, have your changes reviewed, and finally push your changes to gem5. More information can be found from the following sources:

High-level flow for submitting changes

| Make change |
|  Run tests  |<--------------+
       |                      |
       |                      |
       v                      |
| Post review |               |
       |                      |
       v                      |
| Wait for reviews |          |
       |                      |
       |                      |
       v                      |
  +----+----+   No     +------+------+
  |Reviewers+--------->+ Update code |
  |happy?   |          +------+------+
  +----+----+                 ^
       |                      |
       | Yes                  |
       v                      |
  +----+-----+   No           |
  |happy?    |
       | Yes
| Submit code |

After creating your change to gem5, you can post a review on our Gerrit code-review site: Before being able to submit your code to the mainline of gem5, the code is reviewed by others in the community. Additionally, the maintainer for that part of the code must sign off on it.

Contributing long-lived feature branches

Oftentimes users or institutions add features that are necessarily complex, and require many changes on long-lived feature branches. In this case, maintaining a perfect history where all changes work individually is infeasible. When contributing long-lived feature branches back to gem5‘s public repository users may merge entire long-lived branches into a single changeset and contribute their code back as long as 1) the changes have been reviewed by the maintainer 2) the maintainer agrees to allow such a change, and 3) the changes are passing the public tests. Changes that affect common code (outside of a specific maintainer’s purview) will still need to follow the standard gem5 protocol.

Cloning the gem5 repo to contribute

If you plan on contributing, it is strongly encouraged for you to clone the repository directly from our gerrit instance at

To clone the master gem5 repository:

 git clone

Other gem5 repositories

There are a few repositories other than the main gem5 development repository.

  • public/m5threads: The code for a pthreads implementation that works with gem5's syscall emulation mode.

Other gem5 branches

None right now.

Making changes to gem5

It is strongly encouraged to use git branches when making changes to gem5. Additionally, keeping changes small and concise and only have a single logical change per commit.

Unlike our previous flow with Mercurial and patch queues, when using git, you will be committing changes to your local branch. By using separate branches in git, you will be able to pull in and merge changes from mainline and simply keep up with upstream changes.

Requirements for change descriptions

To help reviewers and future contributors more easily understand and track changes, we require all change descriptions be strictly formatted.

A canonical commit message consists of three parts:

  • A short summary line describing the change. This line starts with one or more keywords (found in the MAINTAINERS file) separated by commas followed by a colon and a description of the change. This line should be no more than 65 characters long since version control systems usually add a prefix that causes line-wrapping for longer lines.
  • (Optional, but highly recommended) A detailed description. This describes what you have done and why. If the change isn't obvious, you might want to motivate why it is needed. Lines need to be wrapped to 75 characters or less.
  • Tags describing patch metadata. You are highly recommended to use tags to acknowledge reviewers for their work. Gerrit will automatically add most tags.

Tags are an optional mechanism to store additional metadata about a patch and acknowledge people who reported a bug or reviewed that patch. Tags are generally appended to the end of the commit message in the order they happen. We currently use the following tags:

  • Signed-off-by: Added by the author and the submitter (if different). This tag is a statement saying that you believe the patch to be correct and have the right to submit the patch according to the license in the affected files. Similarly, if you commit someone else's patch, this tells the rest of the world that you have have the right to forward it to the main repository. If you need to make any changes at all to submit the change, these should be described within hard brackets just before your Signed-off-by tag. By adding this line, the contributor certifies the contribution is made under the terms of the Developer Certificate of Origin (DCO) [].
  • Reviewed-by: Used to acknowledge patch reviewers. It's generally considered good form to add these. Added automatically.
  • Reported-by: Used to acknowledge someone for finding and reporting a bug.
  • Reviewed-on: Link to the review request corresponding to this patch. Added automatically.
  • Change-Id: Used by Gerrit to track changes across rebases. Added automatically with a commit hook by git.
  • Tested-by: Used to acknowledge people who tested a patch. Sometimes added automatically by review systems that integrate with CI systems.

Other than the “Signed-off-by”, “Reported-by”, and “Tested-by” tags, you generally don't need to add these manually as they are added automatically by Gerrit.

It is encouraged for the author of the patch and the submitter to add a Signed-off-by tag to the commit message. By adding this line, the contributor certifies the contribution is made under the terms of the Developer Certificate of Origin (DCO) [].

It is imperative that you use your real name and your real email address in both tags and in the author field of the changeset.

For significant changes, authors are encouraged to add copyright information and their names at the beginning of the file. The main purpose of the author names on the file is to track who is most knowledgeable about the file (e.g., who has contributed a significant amount of code to the file).

Note: If you do not follow these guidelines, the gerrit review site will automatically reject your patch. If this happens, update your changeset descriptions to match the required style and resubmit. The following is a useful git command to update the most recent commit (HEAD).

 git commit --amend

Running tests

Before posting a change to the code review site, you should always run the quick tests! See for more information.

Posting a review

If you have not signed up for an account on the Gerrit review site (, you first have to create an account.

Setting up an account

  1. Go to
  2. Click “Sign In” in the upper right corner. Note: You will need a Google account to contribute.
  3. After signing in, click “Generate Password” and follow the instructions.

Submitting a change

In gerrit, to submit a review request, you can simply push your git commits to a special named branch. For more information on git push see

There are three ways to push your changes to gerrit.

Push change to gerrit review

 git push origin HEAD:refs/for/master

Assuming origin is and you want to push the changeset at HEAD, this will create a new review request on top of the master branch. More generally,

 git push <gem5 gerrit instance> <changeset>:refs/for/<branch>

See for more information.

Pushing your first change

The first time you push a change you may get the following error:

 remote: ERROR: [fb1366b] missing Change-Id in commit message footer

Within the error message, there is a command line you should run. For every new clone of the git repo, you need to run the following command to automatically insert the change id in the the commit (all on one line).


If you receive the above error, simply run this command and then amend your changeset.

 git commit --amend

Push change to gerrit as a draft/private

See for details on private gerrit changes.

 git push origin HEAD:refs/for/master%private

Once you have pushed your change as “private”, you can log onto [gerrit] ( and once you're happy with the commit click the “unmark private” which may be hidden in the “more options” dropdown in the upper right corner.

Push change bypassing gerrit

Only maintainers can bypass gerrit review. This should very rarely be used.

 git push origin HEAD:refs/heads/master

Other gerrit push options

There are a number of options you can specify when uploading your changes to gerrit (e.g., reviewers, labels). The gerrit documentation has more information.

Reviewing patches

Reviewing patches is done on our gerrit instance at

After logging in with your Google account, you will be able to comment, review, and push your own patches as well as review others' patches. All gem5 users are encouraged to review patches. The only requirement to review patches is to be polite and respectful of others.

There are multiple labels in Gerrit that can be applied to each review detailed below.

  • Code-review: This is used by any gem5 user to review patches. When reviewing a patch you can give it a score of -2 to +2 with the following semantics.
    • -2: This blocks the patch. You believe that this patch should never be committed. This label should be very rarely used.
    • -1: You would prefer this is not merged as is
    • 0: No score
    • +1: This patch seems good, but you aren't 100% confident that it should be pushed.
    • +2: This is a good patch and should be pushed as is.
  • Maintainer: Currently only PMC members are maintainers. At least one maintainer must review your patch and give it a +1 before it can be merged.
  • Verified: This is automatically generated from the continuous integrated (CI) tests. Each patch must receive at least a +1 from the CI tests before the patch can be merged. The patch will receive a +1 if gem5 builds and runs, and it will receive a +2 if the stats match.
  • Style-Check: This is automatically generated and tests the patch against the gem5 code style ( The patch must receive a +1 from the style checker to be pushed.

Note: Whenever the patch creator updates the patch all reviewers must re-review the patch. There is no longer a “Fix it, then Ship It” option.

Once you have received reviews for your patch, you will likely need to make changes. To do this, you should update the original git changeset. Then, you can simply push the changeset again to the same Gerrit branch to update the review request.

 git push origin HEAD:refs/for/master

Note: If you have posted a patch and don't receive any reviews, you may need to prod the reviewers. You can do this by adding a reply to your changeset review on gerrit. It is expected that at least the maintainer will supply a review for your patch.

Committing changes

Each patch must meet the following criteria to be merged:

  • At least one review with +2
  • At least one maintainer with +1
  • At least +1 from the CI tests (gem5 must build and run)
  • At least +1 from the style checker

Once a patch meets the above criteria, the submitter of the patch will be able to merge the patch by pressing the “Submit” button on Gerrit. When the patch is submitted, it is merged into the public gem5 branch.