layout: page title: A beginners guide to contributing permalink: contributing author: Bobby R. Bruce

This document serves as a beginners guide to contributing to gem5. If questions arise while following this guide, we advise consulting which contains more details on how to contribute to gem5.

The following subsections outline, in order, the steps involved in contributing to the gem5 project.

Determining what you can contribute

The easiest way to see how you can contribute to gem5 is to check our Jira issue tracker: From Jira you can check open issues.

Browse these open issues and see if there are any which you are capable of handling. When you find a task you are happy to carry out, verify no one else is presently assigned, then leave a comment asking if you may assign yourself this task (this will involve creating a Jira account). Though not mandatory, we advise first-time contributors do this so developers more familiar with the task may give advice on how best to implement the necessary changes.

Once a developers has replied to your comment (and given any advice they may have), you may officially assign yourself the task. After this you should change the status of the task from Todo to In progress. This helps the gem5 development community understand which parts of the project are presently being worked on.

If, for whatever reason, you stop working on a task, please unassign yourself from the task and change the task's status back to Todo.

Obtaining the git repo

The gem5 git repository is hosted at Please note: contributions made to other gem5 repos (e.g., our GitHub mirror) will not be considered. Please contribute to exclusively.

To pull the gem5 git repo:

git clone

stable / develop branch

By default, the git repo will have the stable branch checked-out. The stable branch is the gem5 stable release branch. I.e., the HEAD of this branch contains the latest stable release of gem5. (execute git tag on the stable branch to see the list of stable releases. A particular release may be checked out by executing git checkout <release>). As the stable branch only contains officially released gem5 code contributors should not develop changes on top of the stable branch they should instead develop changes on top of the develop branch.

To checkout the develop branch:

git checkout --track origin/develop

Changes may be made on this branch to incorporate changes assigned to yourself.

As the develop branch is frequently updated, regularly obtain the latest develop branch by executing:

git pull --rebase

Conflicts may need resolved between your local changes and new changes on the develop branch.

Making modifications

Different tasks will require the project to be modified in different ways. Though, in all cases, our style-guide must be adhered to. The full style guide is outlined here.

As a high-level overview:

  • Lines must not exceed 79 characters in length.
  • There should be no trailing white-space on any line.
  • Indentations must be 4 spaces (no tab characters).
  • Class names must use upper camel case (e.g., ThisIsAClass).
  • Class member variables must use lower camel case (e.g., thisIsAMemberVariable).
  • Class member variables with their own public accessor must start with an underscore (e.g., _variableWithAccessor).
  • Local variables must use snake case (e.g., this_is_a_local_variable).
  • Functions must use lower camel case (e.g., thisIsAFunction)
  • Function parameters must use snake case.
  • Macros must be in all caps with underscores (e.g., THIS_IS_A_MACRO).
  • Function declaration return types must be on their own line.
  • Function brackets must be on their own line.
  • for/if/while branching operations must be followed by a white-space before the conditional statement (e.g., for (...)).
  • for/if/while branching operations' opening bracket must be on the same line, with the closing bracket on its own line (e.g., for (...) {\n ... \n}\n). There should be a space between the condition(s) and the opening bracket.
  • C++ access modifies must be indented by two spaces, with method/variables defined within indented by four spaces.

Below is a simple toy example of how a class should be formatted:

class ExampleClass
    int _fooBar;
    int barFoo;

        return _fooBar;

    aFunction(int parameter_one, int parameter_two)
        int local_variable = 0;
        if (true) {
            int local_variable = parameter_one + parameter_two + barFoo
                               + EXAMPLE_MACRO;
        return local_variable;


Compiling and running tests

The minimum criteria for a change to be submitted is that the code is compilable and the test cases pass.

The following command both compiles the project and runs our system-level checks:

cd tests
python run

Note: These tests can take several hours to build and execute. may be run on multiple threads with the -j flag. E.g.: python run -j6.

The unit tests should also pass. To run the unit tests:

scons build/NULL/unittests.opt

To compile an individual gem5 binary:

scons build/{ISA}/gem5.opt

where {ISA} is the target ISA. Common ISAs are ARM, MIPS, POWER, RISCV, SPARC, and X86. So, to build gem5 for X86:

scons build/X86/gem5.opt


When you feel your change is done, you may commit. Start by adding the changed files:

git add <changed files>

Then commit using:

git commit

The commit message must adhere to our style. The first line of the commit is the “header”. The header starts with a tag (or tags, separated by a comma), then a colon. Which tags are used depend on which components of gem5 you have modified. Please refer to the for a comprehensive list of accepted tags. After this colon a short description of the commit must be provided. This header line must not exceed 65 characters.

After this, a more detailed description of the commit can be added. This is inserted below the header, separated by an empty line. Including a description is optional but it's strongly recommended. The description may span multiple lines, and multiple paragraphs. No line in the description may exceed 72 characters.

To improve the navigability of the gem5 project we would appreciate if commit messages include a link to the relevant Jira issue/issues.

Below is an example of how a gem5 commit message should be formatted:

test,base: This commit tests some classes in the base component

This is a more detailed description of the commit. This can be as long
as is necessary to adequately describe the change.

A description may spawn multiple paragraphs if desired.

Jira Issue:

If you feel the need to change your commit, add the necessary files then amend the changes to the commit using:

git commit --amend

This will give you opportunity to edit the commit message.

Pushing to Gerrit

Pushing to Gerrit will allow others in the gem5 project to review the change to be fully merged into the gem5 source.

To start this process, execute:

git push origin HEAD:refs/for/develop

At this stage you may receive an error if you're not registered to contribute to our Gerrit. To resolve this issue:

  1. Create an account at
  2. Go to User Settings.
  3. Select Obtain password (under HTTP Credentials).
  4. A new tab shall open, explaining how to authenticate your machine to make contributions to Gerrit. Follow these instructions and try pushing again.

Gerrit will amend your commit message with a Change-ID. Any commit pushed to Gerrit with this Change-ID is assumed to be part of this change.

Code review

Now, at, you can view the change you have submitted (Your -> Changes -> Outgoing reviews). We suggest that, at this stage, you mark the corresponding Jira issue as In Review. Adding a link to the change on Gerrit as a comment to the issue is also helpful.

Through the Gerrit portal we strongly advise you add reviewers. Gerrit will automatically notify those you assign. The “maintainers” of the components you have modified should be added as reviewers. These should correspond to the tags you included in the commit header. Please consult to see who maintains which component. As an example, for a commit with a header of tests,arch : This is testing the arch component then the maintainers for both tests and arch should be included as reviewers.

Reviewers will then review this change. There are three scores which the commit shall be evaluated: “Code-Review”, “Maintainer”, and “Verified”.

Each reviewer can give a score from -2 to +2 to the “Code-Review” score, where +2 indicates the reviewer is 100% okay with the patch in its current state and -2 when the reviewer is certain they do not want the patch merged in its current state.

Maintainers can add +1 or -1 to the “Maintainer” score. A +1 score indicates that the maintainer is okay with the patch.

When a Maintainer gives a +1 our continuous integration system will process the change. At the time of writing, the continuous integration system will run:

scons build/NULL/unittests.opt
cd tests
python run

If this executes successfully (i.e. the project builds and the tests pass) the continuous integration system will give a +1 to the “Verifier” score, and a -1 if it did not execute successfully.

Gerrit will permit a commit to be merged if at least one reviewer has given a +2 to the “Reviewer” score, one maintainer has given a +1 to the “Maintainer” score, and the continuous integration system has given a +1 to the “Verifier” score.

For non-trivial changes, it is not unusual for a change to receive feedback from reviewers that they will want incorporated before giving the commit a score necessary for it to be merged. This leads to an iterative process.

Making iterative improvements based on feedback

A reviewer will ask questions and post suggestions on Gerrit. You should read these comments and answer these questions. All communications between reviewers and contributors should be done in a polite manner. Rude and/or dismissive remarks will not be tolerated.

When you understand what changes are required, using the same workspace as before, make the necessary modifications to the gem5 repo, and amend the changes to the commit:

git commit --amend

Then push the new changes to Gerrit:

git push original HEAD:refs/for/develop

If for some reason you no longer have your original workspace, you may pull the change by going to your change in Gerrit, clicking Download and executing one of the listed commands.

When your new change is uploaded via the git push command, the reviewers will re-review the change to ensure you have incorporated their suggested improvements. The reviewers may suggest more improvements and, in this case, you will have to incorporate them using the same process as above. This process is therefore iterative, and it may therefore take several cycles until the patch is in a state in which the reviewers are happy. Please do not be deterred, it is very common for a change to require several iterations.

Submit and merge

Once this iterative process is complete. The patch may be merged. This is done via Gerrit (Simply click Submit within the relevant Gerrit page).

As one last step, you should change the corresponding Jira issue status to Done then link the Gerrit page as a comment on Jira as to provide evidence that the task has been completed.

Stable releases of gem5 are published three times per year. Therefore, a change successfully submitted to the develop branch will be merged into the stable branch within three to four months after submission.