Contributing to Eigen

Eigen is written and maintained by volunteers. Contributions — code, documentation, bug triage, tests on uncommon platforms, design feedback — are all welcome.

This document is the human-facing on-ramp. For the deeper “how this codebase actually works” reference (architecture, expression templates, SIMD layer, GPU stories, pitfalls), see AGENTS.md; it's written for AI coding agents but is also the most up-to-date single-file overview of the repo.

Where Eigen lives

Ways to contribute

You don't have to write C++ to help.

  • Answer questions on Discord. New users often ask things that point at gaps in our documentation — patches to fill those gaps are very welcome.
  • Triage bugs. Reproduce reports, ask reporters for missing details (compiler, OS, ISA, minimal example), and weigh in on issues labelled decision needed. Issues labelled contributions welcome are good entry points.
  • Run the test suite on rarely-tested compilers, OSes, or architectures and report failures. The nightly matrix is visible in the CI pipelines link above.
  • Improve documentation — both the Doxygen reference (comments in headers) and the prose pages at https://gitlab.com/libeigen/libeigen.gitlab.io.
  • Contribute code — bug fixes, new features, new tests, new benchmarks. See below.

Submitting a merge request

  1. Sign in to GitLab and fork https://gitlab.com/libeigen/eigen.
  2. Clone your fork and add libeigen/eigen as an upstream remote (use the https:// clone URL if you don't have an SSH key set up):
    git clone git@gitlab.com:<your-username>/eigen.git    # or: https://gitlab.com/<your-username>/eigen.git
cd eigen
git remote add upstream https://gitlab.com/libeigen/eigen.git
  3. Create a topic branch off master for your change:
    git fetch upstream
git checkout -b my-topic upstream/master
  4. Make your changes, with tests (and a benchmark if it's a performance-sensitive change — see Tests and benchmarks).
  5. Format with clang-format-17 (see Coding standards).
  6. Commit with an Area: short imperative subject message (see Commit messages). Stage files by path — don't use git add -A.
  7. Push to your fork and open a merge request against libeigen/eigen master.
  8. Be patient and responsive. Reviewers are volunteers; reviews can take a while. If your MR is being ignored for a long time, don't be shy about pinging it.

If you're planning a large change (new module, API redesign, significant refactor), open an issue or start a discussion first. Aligning on direction before you write code avoids wasted work for everyone.

Build and test

Eigen is header-only, so consumers don't need to build the library itself — but contributors do need to build the test suite. CMake configure happens out-of-source, and tests are intentionally not in the default all target — drive everything through the named targets.

cmake -S . -B build -G Ninja
cmake --build build --target buildtests        # build all unit tests
cmake --build build --target check             # build + run all tests
cmake --build build --target buildsmoketests   # the smoke set CI gates MRs on
ctest --test-dir build --parallel --output-on-failure

Other useful targets: BuildOfficial (just test/), BuildUnsupported (just unsupported/test/), buildtests_gpu, check_gpu. Once configured, the build dir also exposes ./buildtests.sh <regex> and ./check.sh <regex> for narrowing by test-name regex.

Full reference for CMake options (per-ISA test flags, GPU/SYCL toggles, external BLAS/LAPACK, etc.), the test-split mechanism, and the VERIFY_* macro family lives in AGENTS.md § “Build / test”.

In-flight migration: MR 2159 is migrating the test framework from Eigen's own EIGEN_DECLARE_TEST / CALL_SUBTEST_N macros to Google Test. Until it lands, write new tests with the existing framework (test/main.h, VERIFY_*, EIGEN_DECLARE_TEST, ei_add_test in the matching CMakeLists.txt).

In-flight rename: MR 2522 renames the top-level unsupported/ directory to contrib/. Paths in this document follow master (still unsupported/); substitute contrib/ mechanically once it lands.

Tests and benchmarks

  • Every new feature lands with tests. Bug fixes should add a regression test that fails before the fix and passes after.
  • Performance-sensitive changes land with a benchmark under benchmarks/ or unsupported/benchmarks/. Don't defer benchmarks to a follow-up MR.
  • Cover numerical corner cases, not just typical inputs: ±0, ±∞, NaN, subnormals, domain boundaries, near-overflow / underflow values. Decomposition and solver tests should exercise ill-conditioned and structured matrices (Hilbert, Vandermonde, rank-deficient, near-singular, Jordan blocks, …). The bar is matching LAPACK on conditioning, pivoting strategy, and backward stability. Standard references: Higham‘s Accuracy and Stability of Numerical Algorithms / Functions of Matrices, Golub & van Loan’s Matrix Computations.
  • New SIMD intrinsics need a test/packetmath.cpp entry (or unsupported/test/special_packetmath.cpp for special functions), and a specialization in every architecture backend that supports the type. Eigen/src/Core/arch/Default/ holds generic SIMD implementations shared across backends; scalar fallbacks live in Eigen/src/Core/GenericPacketMath.h and Eigen/src/Core/MathFunctions.h.
  • Benchmarking discipline. Benchmarks on a loaded system are meaningless. Before running one: check uptime shows a low load average, finish or cancel background builds, and never run two benchmark binaries in the same shell invocation (parallel or chained with &&). Run each in its own shell, take medians within a binary, and optionally alternate (A, B, A, B) across separate invocations to detect drift.

Coding standards

Eigen has a few hard rules that catch contributors most often. The full discussion is in AGENTS.md § “Agent guidelines (read first)”; the short version:

  1. Header-only contract. Never #include anything under Eigen/src/... or unsupported/Eigen/src/... — InternalHeaderCheck.h makes that a hard compile error. User code reaches implementation only through the umbrella headers (Eigen/Core, Eigen/Dense, Eigen/SVD, …).
  2. Preserve EIGEN_DEVICE_FUNC on coefficient-level methods. Dropping it silently breaks CUDA / HIP / SYCL builds and rarely shows up in local testing.
  3. Format with clang-format-17 exactly — Google base, 120 columns, configured in .clang-format. Newer or older clang-format will diff against CI. Run scripts/format.sh (whole tree) or clang-format-17 -i <file> (one file) before pushing. Format failures are the single most common reason an MR is red.
  4. Don't apply general C++ “modernize” cleanups. No modernize-* / cppcoreguidelines-* clang-tidy fixes, no replacing EIGEN_STRONG_INLINE with inline, no reordering includes (.clang-format has SortIncludes: false). Eigen's conventions are encoded in .clang-format‘s StatementMacros and AttributeMacros lists — don’t “fix” their indentation.
  5. Tests use test/main.h, not gtest (today — see migration note above). VERIFY_* assertions, CALL_SUBTEST_N / EIGEN_TEST_PART_N for splitting, EIGEN_DECLARE_TEST(<name>) { ... } as the entry point.
  6. auto traps. auto x = A + B; captures a lazy expression holding references that can dangle. Use .eval() or an explicit type. Similarly, .noalias() is a promise from the caller — only use it when the destination doesn't appear on the right side (so mat.noalias() = mat * mat is wrong).
  7. Stage commits explicitly. Don't git add -A / git add . — the repo root accumulates .vscode/, .idea/, build*/, and other untracked files that must not enter commits. Add by path or with targeted globs.

Naming conventions:

  • Classes: PascalCase (Matrix, BDCSVD).
  • Methods: camelCase (coeffRef, applyOnTheLeft).
  • Macros and compile-time constants: EIGEN_UPPER_CASE (EIGEN_DEVICE_FUNC, EIGEN_STRONG_INLINE).
  • Internal implementation lives in the Eigen::internal namespace; public API stays in Eigen:: or module namespaces.
  • Use eigen_assert(cond) for runtime preconditions (not raw assert); EIGEN_STATIC_ASSERT(cond, MSG_TOKEN) for compile-time conditions; eigen_internal_assert for internal-only invariants gated on EIGEN_INTERNAL_DEBUGGING.

The canonical class layout is visible in recent additions such as unsupported/Eigen/src/GPU/DeviceMatrix.h. Template parameters that get re-exported as public aliases carry a trailing underscore (template <typename Scalar_, ...> paired with using Scalar = Scalar_; — prefer using over typedef in new code), member variables use an m_ prefix (m_matrix, m_isInitialized), implementation headers under Eigen/src/... begin with // IWYU pragma: private and #include "./InternalHeaderCheck.h", header guards follow EIGEN_<NAME>_H, and implementation detail lives inside nested namespace Eigen { namespace internal { ... } }. Public classes carry Doxygen blocks (\class, \brief, \tparam, \sa) and link to runnable snippets from doc/snippets/ via \include / \verbinclude. When in doubt, copy the style from a recent neighbouring file rather than inventing a new convention.

License and SPDX/REUSE headers

Every new source file must carry an inline copyright + license header. The checkformat:reuse CI job (reuse lint) blocks otherwise. Two header styles are in active use; pick whichever fits.

Attributed to an individual contributor:

// This file is part of Eigen, a lightweight C++ template library
// for linear algebra.
//
// Copyright (C) <year> <Your Name> <your.email@example.com>
//
// This Source Code Form is subject to the terms of the Mozilla
// Public License v. 2.0. If a copy of the MPL was not distributed
// with this file, You can obtain one at http://mozilla.org/MPL/2.0/.
// SPDX-License-Identifier: MPL-2.0

Attributed collectively (common for files written by multiple contributors, or where individual attribution isn't useful):

// This file is part of Eigen, a lightweight C++ template library
// for linear algebra.
//
// Copyright (C) <year> The Eigen Authors.
//
// This Source Code Form is subject to the terms of the Mozilla
// Public License v. 2.0. If a copy of the MPL was not distributed
// with this file, You can obtain one at http://mozilla.org/MPL/2.0/.
// SPDX-License-Identifier: MPL-2.0

Markdown docs, generated files (*.in), and binary assets that can‘t carry inline headers are covered by path annotations in REUSE.toml — add your path there if you’re creating one.

Provenance and attribution

This is non-negotiable.

  • Eigen code must be original, or derived from publicly published MPL-2.0-compatible material.
  • Do not copy — verbatim, paraphrased, or “translated” to another syntax — from incompatibly-licensed sources (proprietary, NDA-encumbered, prior-employer internal, GPL/AGPL). Paraphrasing is still a derivative work. The same discipline applies to AI-suggested code: cite the source, or rewrite from a known reference and cite that, or drop it. Ideas aren't copyrightable; specific expressions are — learn from the source, write your own, and credit it.
  • Cite published references inline when they inform an implementation: LAPACK / LAWN, ACM TOMS / SIAM papers, Higham, Golub & van Loan, textbook algorithms, Boost components, vendor application notes — by name (author, year, identifier). A comment or Doxygen \note block is enough.
  • When copying permissively-licensed code into an MPL-2.0 file, follow Mozilla's Guidelines for Developers.

Copyright and credit

  • No copyright assignment. Everyone keeps copyright on their own contributions. There is no CLA.
  • Commit under your own identity. Make sure user.name and user.email in your git config are set to credit you correctly.
  • When contributing on behalf of your employer, commit in your own name where possible rather than your employer's.
  • For substantial improvements to a source file, feel free to add yourself to its copyright line — but it isn‘t required; you get copyright implicitly on the code you wrote. The collective The Eigen Authors attribution is fine when individual credit isn’t useful.

Commit messages

Format: Area: short imperative subject.

Examples (from recent history):

  • BDCSVD: fix edge case
  • GPU: Drop direct LruCache.h include from CuBlasSupport.h
  • ci: drop CodeRabbit summary
  • TriangularView: alias-aware fallback for structured-diagonal product fast path

Keep the subject under ~72 characters; use the body for the why, not the what — the diff already shows what changed. Reference issues by #<n> when relevant.

Quality bar

Eigen aspires to state-of-the-art on two axes — performance and numerical accuracy / IEEE-754 conformance — and treats them as separate goals that are sometimes in tension.

  • Performance. Eigen Core is mostly optimized for single-core throughput via per-architecture packet backends (SIMD) and cache-aware blocking. Multi-core in Core is opt-in (OpenMP or EIGEN_GEMM_THREADPOOL) and covers a subset of operations. The Tensor module is multi-core by design via ThreadPoolDevice.
  • Numerical accuracy. LAPACK-level for linear algebra (decompositions, solvers — backward stability, pivoting, conditioning) and C++ standard-library level for standard math functions on scalars. On regular inputs, a few ULPs of error in vectorized math is the long-standing trade-off for SIMD throughput; larger deviations need explicit justification.
  • IEEE-754 / special values. For special values (NaN, ±0, ±∞, subnormals, singularities, branch boundaries — e.g. log(0), pow(0, 0)) the bar is exact conformance to IEEE 754 / ISO C / C++ specifications, with cppreference as the authoritative spec. Special-value handling is not subject to the few-ULPs trade-off.

For decompositions and solvers specifically, the bar is matching LAPACK on conditioning, pivoting strategy, and backward stability. Don't trade numerical robustness for speed in those code paths without explicit sign-off.

Load-bearing modules

Some “unsupported” modules carry stability guarantees beyond what the name suggests:

  • unsupported/Eigen/Tensor and Eigen/ThreadPool together form TensorFlow's core compute backend. “Unsupported” here means looser API-stability guarantees — it does not mean low-traffic or low-stakes. Breaking changes to header layout, signatures, semantics, or contraction / reduction kernel performance ripple into every TensorFlow build. Prefer additive changes, keep header paths stable, run unsupported/test/tensor_* before submitting, and call out any behaviour change prominently in the MR description.

Communication

  • Discord is the primary chat channel (link above). Most informal design discussion happens there.
  • GitLab issues are the place for bug reports, feature requests, and threaded design discussions tied to a specific topic.
  • For larger contributions, discuss the plan first to avoid duplicated effort and to align on API decisions before implementation.

Further reading

Thank you for contributing to Eigen!