The first step might be to use GHDL and explore its possibilities in your own project. If you are new to VHDL, see the Quick Start Guide for an introduction. Furthermore, we encourage you to read Invoking GHDL, where the most commonly used options are explained. You can also check the complete Command Reference.

If you are more familiar with GHDL, you might start asking yourself how it works internally. If so, you might find Implementation of VHDL and Implementation of VITAL interesting.

While using GHDL, you might find flaws, such as bugs, missing features, typos in the documentation, or topics which still are not covered. In order to improve GHDL, we welcome bug reports, suggestions, and contributions for any aspect of GHDL. Whether it’s a bug or an enhancement, have a look at the Open issues and Closed issues to see if someone already told us about it. You might find a solution there.

If you found no information on your topic, please, report so that we are aware! You can reach us through various ways: Talk to us on Gitter or open a Open new issue at GitHub.


Since the development of GHDL started fifteen years ago, multiple platforms have been used as a support for both distribution and getting feedback. However, the development is now centralized in github.

Reporting bugs


  • If the compiler crashes, this is a bug. Reliable tools never crash.
  • If the compiler emits an error message for a perfectly valid input or does not emit an error message for an invalid input, this may be a bug.
  • If the executable created from your VHDL sources crashes, this may be a bug at runtime or the code itself may be wrong. However, since VHDL has a notion of pointers, an erroneous VHDL program (using invalid pointers for example) may crash.
  • If a compiler message is not clear enough, please tell us. The error messages can be improved, but we do not have enough experience with them.

Please, report issues of this kind through Open new bug report at GitHub, as this allows us to categorize issues into groups and to assign developers to them. You can track the issue’s state and see how it’s getting solved.


As suggested in the bug report template, please elaborate a Minimal (non) Working Example (MWE) prior to sending the report, so that the possible bug source is isolated. Should it fulfill the format requirements of issue-runner, you would be able to test your bug with the latest GHDL version. Please do so in order to ensure that the bug is not solved already.

Also, please include enough information in the bug report, for the maintainers to reproduce the problem. The template includes:

  • Operating system and version of GHDL (you can get it with ghdl --version).
  • Whether you have built GHDL from sources (provide short SHA of the used commit) or used the binary distribution (note which release/tag).
    • If you cannot compile, please report which compiler you are using and the version.
  • Content of the input files which comprise the MWE
  • Description of the problem:
    • Comment explaining whether the MWE should compile or not; if yes, whether or not is should run until the assertion.
    • What you expect to happen and what you actually get. If you know the LRM well enough, please specify which paragraph might not be implemented well.
    • Samples of any log.
    • Anything else that you think would be helpful.


If you don’t know the LRM, be aware that an issue claimed as a bug report may be rejected because there is no bug according to it. GHDL aims at implementing VHDL as defined in IEEE 1076. However, some other tools allow constructs which do not fully follow the standard revisions. Therefore, comparisons with other VHDL is not a solid argument. Some of them are supported by GHDL (see IEEE library pitfalls), but any such enhancement will have very low priority.

Requesting enhancements

Open new feature request at GitHub Talk to us on Gitter

All enhancements and feature requests are welcome. Please open a new issue to report any, so you can track the request’s status and implementation. Depending on the complexity of the request, you may want to chat on Gitter, to polish it before opening an issue.

Improving the documentation

If you found a mistake in the documentation, please send a comment. If you didn’t understand some parts of this manual, please tell us. English is not our mother tongue, so this documentation may not be well-written.

Likewise, rewriting part of the documentation or missing content (such as examples) is a good way to improve it. Since it automatically is built from reStructuredText and Markdown sources, you can fork, modify and request the maintainers to pull your copy. See Fork, modify and pull-request.

Fork, modify and pull-request


  • Before starting any modification, you might want to have a look at Open pull requests and Closed pull requests, to check which other contributions are being made or have been made. If you observe that the modifications you are about to start might conflict with any other, please Talk to us on Gitter or open a Open new Pull Request (PR) at GitHub to coordinate.
  • See section Directory structure to faster find the location of the sources you need to modify, and/or to know where to place new ones.

Contributing source code/documentation via Git is very easy. Although we don’t provide direct write access to our repositories, the project is hosted at GitHub, which follows a fork, edit and pull-request flow . That is:

  1. Make a copy (fork) of the project.
  2. Do the changes you wish (edit, add, rename, move and/or delete).
  3. When you think that the changes are ready to be merged, notify the maintainers by opening a Pull Request (PR).
  4. The maintainers will review the proposed changes and will reply in the corresponding thread if any further modification is required. If so, you can keep adding commits to the same branch, and the PR will be automatically updated.
  5. Last, the maintainers will merge your branch. You will be notified, the PR will be closed, and you’ll be allowed to delete the branch, if you want.


  • It is recommended to read A successful Git branching model for a reference on how maintainers expect to handle multiple branches. However, our actual model is not as exhaustive as explained there.
  • Some commit messages can automatically close issues. This is a very useful feature, which you are not required to use. However beware that using fix anywhere in the commit message can have side effects. If you closed any issue unexpectedly, just reply to it (even if it’s closed) so that maintainers can check it.
  • It is recommended to read Coding Style before contributing modifications to Ada sources.