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: https://gem5-review.googlesource.com/. 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: https://gem5-review.googlesource.com. 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.

Cloning the gem5 repo to contribute

If you plan on contributing, it is strongly encouraged for you to clone the repository directly, and checkout the develop branch from our gerrit instance at https://gem5.googlesource.com/.

To clone the gem5 repository:

 git clone https://gem5.googlesource.com/public/gem5

By default, the master branch is checked out. The master branch is stable, containing the latest released version of gem5. To obtain code still under-development (and which contributions can be made):

cd gem5
git checkout --track origin/develop

Changes should be made to this develop branch. Changes to the master branch will be blocked. Once a change on the develop branch is properly incorporated into the gem5 repo it will be merged into the master Branch upon the next release of gem5. New releases of gem5 occur three times a year. Ergo, changes made to the develop branch should appear on the master branch within three to four months as part of a stable release.

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.

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) [https://developercertificate.org/].
  • 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) [https://developercertificate.org/].

If your change relates to a Jira Issue, it is advised that you provide a link to the issue in the commit message (or messages if the Jira Issue relates to multiple commits). Though optional, doing this can help reviewers understand the context of a change.

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 TESTING.md for more information.

Posting a review

If you have not signed up for an account on the Gerrit review site (https://gem5-review.googlesource.com), you first have to create an account.

Setting up an account

  1. Go to https://gem5.googlesource.com/
  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 https://git-scm.com/docs/git-push.

There are three ways to push your changes to gerrit.

Push change to gerrit review

 git push origin HEAD:refs/for/develop

Assuming origin is https://gem5.googlesource.com/public/gem5 and you want to push the changeset at HEAD, this will create a new review request on top of the develop branch. More generally,

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

See https://gerrit-review.googlesource.com/Documentation/user-upload.html 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).

 curl -Lo `git rev-parse --git-dir`/hooks/commit-msg \
	https://gerrit-review.googlesource.com/tools/hooks/commit-msg ; \
 chmod +x `git rev-parse --git-dir`/hooks/commit-msg

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

 git commit --amend

Push change to gerrit as a Work In Progress

It is acceptable to push commits as “Work In Progress” (WIP) changes within gerrit. WIP changes are publicly visible though no one will be able to review the changes or be directly notified they have been submitted. WIP changes can be useful for backing up code currently under-development or for sharing incomplete code with the wider community (i.e., the link to the gerrit change may be shared, and others may download the change, comment on it, and track alterations over time).

See https://gerrit-review.googlesource.com/Documentation/intro-user.html#wip for details on WIP gerrit changes.

To push a change as a WIP:

 git push origin HEAD:refs/for/develop%wip

Once you have pushed your change as a WIP, you can log onto gerrit and view it. Once you're happy with the change you can add reviewers which shall move your change from WIP status to be considered for submission by the wider gem5 community. Switching from a WIP to a regular change does not notify the gem5 community, via the gem5-dev mailing-list, that a change has been submitted (as would occur if a change were submitted directly for review). It is therefore important to include reviewers and CC those who you wish to view the change (they will be notified automatically via email).

Push change bypassing gerrit

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

 git push origin HEAD:refs/heads/develop

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. https://gerrit-review.googlesource.com/Documentation/user-upload.html


By default, contributions to gem5 should be made on the develop branch. The master branch is maintained as a stable release branch (i.e., it can be pulled to obtain the latest official release of gem5). Creation of additional branches is generally discouraged due to their tendency to bloat git repositories with abandoned code. However, the creation of new branches is permitted for development of a specific feature or improvement if one or more of the following criteria are met:

  1. The feature/improvement is likely to be of a large size, consisting of many commits, with little logic in these commits being contributed separately.
  2. The feature/improvement will be developed over a long period of time.
  3. There is sufficient reason that a feature/improvement should not be part of the next gem5 release (e.g., the change should be held within a feature branch until ready for the next release, at which point it will be merged into the develop branch).

If a branch is required it can only be created by a project maintainer. Therefore, if a gem5 contributor desires a separate branch for their work, they should request one from the maintainer of the component the work relates to (see MAINTAINERS for the list of maintainers and the components they are responsible for). The maintainer shall use their discretion to determine whether the creation of a branch is necessary. If approved, the maintainer shall create the branch which the contributor may then use.

Development on a branch within Gerrit functions in exactly the same way as contributing to the develop branch. When contributors to a branch are satisfied, they should create a merge commit into the develop branch. The maintainer should then be notified that the branch they created can now be deleted.

Abandonment of changes within branches may result in these branches being removed from the repository. All branches within a repo should be under active development.

Reviewing patches

Reviewing patches is done on our gerrit instance at https://gem5-review.googlesource.com/.

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 (http://www.gem5.org/documentation/general_docs/development/coding_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/develop

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.

Review moderation and guidelines

Once a change is submitted, reviewers shall review the change. This may require several iterations before a merge. Comments from reviewers may include questions, and requests for alterations to the change prior to merging. The overarching philosophy in managing this process is that there should be politeness and clear communication between all parties at all times, and, whenever possible, permission should be asked before doing anything that may inconvenience another party. Included below are some guidelines we expect contributors and reviewers to follow.

  • In all forms of communication, contributors and reviewers must be polite. Comments seen as being needlessly hostile or dismissive will not be tolerated.
  • Change contributors should respond to, or act upon, each item of feedback given by reviewers. If there is disagreement with a piece of feedback, a sufficiently detailed reason for this disagreement should be given. Polite discussion, and sharing of information and expertise is strongly encouraged.
  • Contributors are advised to assign reviewers when submitting a change. Anyone who contributes to gem5 can be assigned as a reviewer. However, all changes must be accepted by at least one maintainer prior to a merge, ergo assigning of at least one maintainer as a reviewer is strongly recommended. Please see MAINTAINERS for a breakdown of gem5 maintainers and which components they claim responsibility for. Maintainers should be chosen based on which components the change is targeting. Assigning of reviewers is not strictly enforced, though not assigning reviewers may slow the time in which a change is reviewed.
  • If a contributor posts a change and does not receive any reviews after two working days (excluding regional holidays), it is acceptable to “prod” reviewers. This can be done by adding a reply to the changeset review (e.g., “Would it be possible for someone to review my change?”). If the contributor has yet to assign reviewers, they are strongly advised to do so. Reviewers will get notified when assigned to referee a change.
  • By default, the original contributor is assumed to own a change. I.e., they are assumed to be the sole party to submit patchsets. If someone other than the original contributor wishes to submit patchsets to a change on the original contributor's behalf, they should first ask permission. If two working days pass without a response, a patchset may be submitted without permission. Permission does not need to be asked to submit a patchset consisting of minor, inoffensive, changes such a typo and format fixes.
  • Once a change is ready to merge, it enters a “Ready to Submit” state. The original contributor should merge their change at this point, assuming they are content with the commit in its present form. After two working days, a reviewer may message a contributor to remind them of the change being in a “Ready to Submit” state and ask if they can merge the change on the contributors behalf. If a further two working days elapse without a response, the reviewer may merge without permission. A contributor may keep a change open for whatever reason though this should be communicated to the reviewer when asked.
  • After a month of inactivity from a contributor on an active change, a reviewer may post a message on the change reminding the submitter, and anyone else watching the change, of its active status and ask if they are still interested in eventually merging the change. After two weeks of no response the reviewer reserves the right to abandon the change under the assumption there is no longer interest.
  • The final arbiter in any dispute between reviewers and/or contributors is the PMC (PMC members are highlighted in MAINTAINERS). Disputes requiring intervention by the PMC are undesirable. Attempts should be made to resolve disagreements via respectful and polite discourse before being escalated to this level.


gem5 releases occur 3 times per year. The procedure for releasing gem5 is as follows:

  1. Developers will be notified, via the gem5-dev mailing list, that a new release of gem5 will occur. This should be no sooner than 2 weeks prior to the creation of the staging branch (the first step in releasing a new version of gem5). This gives time for developers to ensure their changes for the next release are submitted to the develop branch.
  2. When a release is ready, a new staging branch shall be created by a project maintainer, from develop, with the name “release-staging-{VERSION}”. The gem5-dev mailing list will be notified that the staging branch will be merged into the master branch after two weeks, thus marking the new release.
  3. The staging branch will have the full suite of gem5 tests run on it to ensure all tests pass and the to-be-released code is in a decent state.
  4. If a user submits a changeset to the staging branch, it will be considered and undergo the standard Gerrit review process. However, only alterations that cannot wait until the following release will be accepted for submission into the branch (i.e., submissions to the staging branch for “last minute” inclusions to the release should be of a high priority, such as a critical bug fix). The project maintainers will use their discretion in deciding whether a change may be submitted directly to the staging branch. All other submissions to gem5 will continue to be made to the develop branch. Patches submitted into the staging branch do not need to be re-added to the develop branch.
  5. Once signed off by members of the PMC the staging branch shall be merged into the master and develop branch. The staging branch will then be deleted.
  6. The master branch shall be tagged with the correct version number for that release. gem5 conforms to a “v{YY}.{MAJOR}.{MINOR}.{HOTFIX}” versioning system. E.g., the first major release of 2022 will be “v22.0.0.0”, followed by “v22.1.0.0”. All the releases (with the exception of hotfixes) are considered major releases. For the meantime, there are no minor releases though we keep the minor release numbers in case this policy changes in the future.
  7. The gem5-dev and gem5-user mailing lists shall be notified of the new gem5 release.


There may be circumstances in which a change to gem5 is deemed critical and cannot wait for an official release (e.g., a high-priority bug fix). In these circumstances a hotfix shall be made.

First, if a developer suspects a hotfix may be necessary then the issue should be discussed on the gem5-dev mailing list. The community will decide whether the issue is worthy of a hotfix, and the final decision should be made by members of the PMC if there is no consensus. Assuming the hotfix is permitted, the following steps will be taken:

  1. A new branch with the prefix “hotfix-” will be created from the master branch. Only gem5 maintainers can create branches. If a non-maintainer requires the creation of a hotfix branch then they should contact a gem5 maintainer.
  2. The change shall be submitted to the hotfix branch via gerrit. Full review, as with any other change, will be required.
  3. Once fully submitted, the hotfix branch shall be merged into both the develop and the master branch by a gem5 maintainer.
  4. The master branch will be tagged with the new version number; the same as the last but with an incremented hotfix number (e.g., “v20.2.0.0” would transition to “v20.2.0.1”).
  5. The hotfix branch will then be deleted.
  6. The gem5-dev and the gem5-user mailing lists shall be notified of this hotfix.