Deprecated Submitting Contributions

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All of the contribution details have been moved into the gem5 source tree. The information below is out of date!

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:

  • 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 savings!
  • 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.

In the common case, the process is fairly simple:

  1. Organize your changes into one or more self-contained, documented patches with appropriate commit messages.
  2. Test your patches using the regression tests, as well as any other specific tests required to exercise your code.
  3. Post your patch(es) on our Gerrit server.
  4. Wait for reviews. Reviewers may ask you to modify your patch, or may engage you in some discussion.
  5. If necessary, update your patch based on initial reviews and wait for re-reviews.
  6. Once you've resolved any outstanding reviewer concerns and received a few 'ship it!' reviews, someone with commit access should volunteer to commit your code. If they don't, please make an explicit request on the gem5-dev mailing list. If you submit enough high-quality patches that people find it annoying to keep committing your patches for you, you will likely be given commit access yourself :).

The sections below provide more detail on these steps.

Creating/Submitting Patches to Gerrit

Please see the contributing documentation for gem5-specific contributing notes and the gerrit documentation for gerrit-specific instructions.

Creating Patches

See Managing Local Changes with Mercurial Queues to learn how to set up an use MQ to manage your code changes. The rest of this section will assume that you are using MQ to manage the patches you wish to contribute.

  • When you've got something that you want to commit, think about what it does and who it might affect. Is it self contained? Can it be broken up into more logical segments? gem5 is a big project and there are a lot of people working on it. No one person knows everything, so we rely on the revision history and on comments to understand what is going on in the code. It is very important that you make change sets self contained. It is far easier to understand and review a series of ten 200 line changesets and what they do compared to a single 2000 change set that does a whole lot of stuff. This will also help you as a developer. If you keep your changesets small and self contained and you read them regularly, you will find your own bugs before you commit them.
  • Please don't forget to put commit messages on your patches. The -e, -l, or -m options to hg qref will allow you to do this (run hg help qref for details).
  • We want to be able to identify everyone who makes a change to the repository, so we need your name to show up in a consistent manner in the repository. Please add this to your $HOME/.hgrc file before you start making changesets (hg commit or hg qfinish)
username=Nathan Binkert <>
  • Please try to follow the Coding Style. They make search and replace much more effective. It also makes it easier for developers to follow code if it all has a similar look. I'm sure that there are things that you hate about the style. I've never worked on a project where everyone liked the style, unless they've all worked on the project for a long time and just gotten used to it.

Commit Messages

A canonical commit message consists of three parts:

  • A short summary line describing the change. This line starts with one or more keywords 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.
  • (Optional) Tags describing patch metadata. You are highly recommended to use tags to acknowledge reviewers for their work.

For example:

scons, arch: Fixed the way the build system handled ISAs

Note the keyword and colon on the first line, and that it's < 65 chars.
Please keep your detailed commit comments to 75 columns or less.  These
comments will likely have to be wrapped manually, but that's not so bad
a thing to have to do.

Reported-by: Dedicated gem5 User <email>
Signed-off-by: Original Developer <email>
Reviewed-by: Grey Beard <email>
[Committer: Rebased patch onto latest gem5]
Signed-off-by: Committer <email>

The keyword should be one or more of the following separated by commas:

  • Architecture name in lower case (e.g., arm or x86): Anything that is target-architecture specific.
  • base
  • ext
  • stats
  • sim
  • syscall_emul
  • config:
  • mem: Classic memory system. Ruby uses its own keyword.
  • ruby: Ruby memory models.
  • cpu: CPU-model specific (except for kvm)
  • kvm: KVM-specific. Changes to host architecture specific components should include an architecture keyword (e.g., arm or x86) as well.
  • gpu-compute
  • energy
  • dev
  • arch: General architecture support (src/arch/)
  • scons: Build-system related. Trivial changes as a side effect of doing something unrelated (e.g., adding a source file to a SConscript) don't require this.
  • tests
  • style: Changes to the style checkers of style fixes.
  • misc

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.
  • Reviewed-by: Used to acknowledge patch reviewers. It's generally considered good form to add these. Some code review systems add them automatically.
  • Reported-by: Used to acknowledge someone for finding and reporting a bug.
  • Reviewed-on: Link to the review request corresponding to this patch. Some review systems add these automatically.
  • Change-Id: Used by Gerrit to track changes across rebases.
  • Tested-by: Used to acknowledge people who tested a patch. Sometimes added automatically by review systems that integrate with CI systems.

Testing Patches

  • Before you circulate your patch for review, ask yourself: Have I compiled this code for all ISAs? Have I run the regression tests that are relevant? (You should run at least the quick regressions and the long too if you think you're going to change any results.) You should avoid embarrassing yourself, annoying reviewers, or worst of all breaking the tree with trivial things that can be caught by these two steps.

Posting Patches

  • Now that you've made small, self contained changesets, you should seek feedback from people. We have our own Reviewboard server for this process. You can use the Reviewboard Extension which provides the hg postreview command to send out your changesets for review. Some Tips:
    • If you want to post a review for the changeset at the tip, use "hg postreview -o"
    • If you want to update a review for the changeset at the tip, use "hg postreview -o -u -e <number>", where <number> is the review number on reviewboard.
    • Everything is easier if you are using Mercurial Queues as you can use "hg qpush" and "hg qpop" to get your diff to the tip and then use the postreview commands above.

If your patch touches Ruby code, please get a review from Brad or Nilay before committing. If your patch touches ARM code please get a review from Ali or Andreas before committing.

Responding to Reviews

  • Your reviews may request some changes; if so, make those and (unless the changes are trivial) re-post the modified patch for review, updating the existing review as described above.

Committing Patches

  • Once the reviewers are satisfied with your change, make sure your change is based on the up-to-date head of the tree. If you're using mq, this is pretty trivial: hg qpop -a; hg pull -u; hg qpush (assuming the tree hasn't changed too much out from under you)
  • If you made any changes in the last two steps, recompile and re-run the quick regression tests. Even if your changes were totally trivial, you should at least recompile and run the "hello world" test to make sure you haven't done something dumb (speaking from experience).
  • If you're a new developer and don't have commit access, you'll have to get someone with commit access to push the patch to the repository on your behalf. If you do have commit access, follow these steps:
    • Now that you're ready to commit, you can hg qfinish the relevant patches to turn them into changesets.
    • Ok, so you're ready to push your changesets to the repository. One last step. Run hg outgoing. Are you about to push what you think you are? Do the commit messages all make sense? Ok, go ahead and hg push.

If your patch has been on reviewboard for a while without getting any reviews (or re-revires after you've posted changes), please email the gem5-dev list. If you have commit access, it is fair to give warning via email that you intend to commit the changes at some future date (e.g., a week out from the date of the email) if you do not hear any objections. Please do not simply commit a patch without giving warning on the gem5-dev list.

Committing Patches for other contributors

  1. Download patch from reviewboard
  2. Import the patch (hg qimport)
  3. Push the patch (hg qpush)
  4. Fix any style errors, etc.
  5. Copy description from reviewboard (hg qref -e OR vi .hg/patches/<patch>)
  6. Update the patch user with reviewboard info for the person who wrote the patch (hg qref -u “Name <email>”).
  7. Put your ID as “Signed-off-by: Name <email>”
  8. Check that it looks right (hg log -l1 -v)
  9. Test the patch(es) (something like scons build/ARM/tests/opt/quick/se/). Do what’s appropriate, the submitter should have already tested!
  11. Commit the patches locally (hg qfinish -a)
  12. Make sure the commit messages still look right (hg log)
  13. Push the patches (hg push ssh://
  14. Ask the submitters to close the reviews on reviewboard