From ee56e9f8a8202bfa02c2d9ae3cfa07d4a41ab567 Mon Sep 17 00:00:00 2001
From: Nikita Shulga <nshulga@meta.com>
Date: Fri, 20 Jun 2025 15:49:57 -0700
Subject: [PATCH] [BE] Make Eigen an optional dependency (#155955)

Whose version is controlled by `eigen_pin.txt`, but which will be installed only if BLAS providers could not be found.
Why this is good for CI: we don't really build with Eigen ever and gitlab can be down when github is up, which causes spurious CI failures in the past, for example.

Remove eigen submodule and replace it with eigen_pin.txt

Fixes https://github.com/pytorch/pytorch/issues/108773
Pull Request resolved: https://github.com/pytorch/pytorch/pull/155955
Approved by: https://github.com/atalman
---
 .ci/pytorch/build.sh         |  1 +
 .gitmodules                  |  4 ----
 cmake/Dependencies.cmake     | 42 ++++++++++++++++++++----------------
 third_party/eigen            |  1 -
 third_party/eigen_pin.txt    |  1 +
 tools/optional_submodules.py | 18 +++++++++++++++-
 6 files changed, 42 insertions(+), 25 deletions(-)
 delete mode 160000 third_party/eigen
 create mode 100644 third_party/eigen_pin.txt

diff --git a/.ci/pytorch/build.sh b/.ci/pytorch/build.sh
index 05d74153eef2..3ba2d36624ea 100755
--- a/.ci/pytorch/build.sh
+++ b/.ci/pytorch/build.sh
@@ -257,6 +257,7 @@ if [[ "$BUILD_ENVIRONMENT" == *-bazel-* ]]; then
   set -e -o pipefail
 
   get_bazel
+  python3 tools/optional_submodules.py checkout_eigen
 
   # Leave 1 CPU free and use only up to 80% of memory to reduce the change of crashing
   # the runner
diff --git a/.gitmodules b/.gitmodules
index bf1fc38c6797..b22e1026d6a7 100644
--- a/.gitmodules
+++ b/.gitmodules
@@ -2,10 +2,6 @@
     ignore = dirty
     path = third_party/pybind11
     url = https://github.com/pybind/pybind11.git
-[submodule "third_party/eigen"]
-    ignore = dirty
-    path = third_party/eigen
-    url = https://gitlab.com/libeigen/eigen.git
 [submodule "third_party/googletest"]
     ignore = dirty
     path = third_party/googletest
diff --git a/cmake/Dependencies.cmake b/cmake/Dependencies.cmake
index 5e31f60a4b10..55055f444821 100644
--- a/cmake/Dependencies.cmake
+++ b/cmake/Dependencies.cmake
@@ -778,25 +778,6 @@ elseif(NOT TARGET fp16 AND USE_SYSTEM_FP16)
 endif()
 list(APPEND Caffe2_DEPENDENCY_LIBS fp16)
 
-# ---[ EIGEN
-# Due to license considerations, we will only use the MPL2 parts of Eigen.
-set(EIGEN_MPL2_ONLY 1)
-if(USE_SYSTEM_EIGEN_INSTALL)
-  find_package(Eigen3)
-  if(EIGEN3_FOUND)
-    message(STATUS "Found system Eigen at " ${EIGEN3_INCLUDE_DIR})
-  else()
-    message(STATUS "Did not find system Eigen. Using third party subdirectory.")
-    set(EIGEN3_INCLUDE_DIR ${CMAKE_CURRENT_LIST_DIR}/../third_party/eigen)
-    caffe2_update_option(USE_SYSTEM_EIGEN_INSTALL OFF)
-  endif()
-else()
-  message(STATUS "Using third party subdirectory Eigen.")
-  set(EIGEN3_INCLUDE_DIR ${CMAKE_CURRENT_LIST_DIR}/../third_party/eigen)
-endif()
-include_directories(SYSTEM ${EIGEN3_INCLUDE_DIR})
-
-
 # ---[ Python Interpreter
 # If not given a Python installation, then use the current active Python
 if(NOT Python_EXECUTABLE)
@@ -810,6 +791,29 @@ if(NOT Python_EXECUTABLE)
   endif()
 endif()
 
+
+# ---[ EIGEN
+# Due to license considerations, we will only use the MPL2 parts of Eigen.
+set(EIGEN_MPL2_ONLY 1)
+if(USE_SYSTEM_EIGEN_INSTALL)
+  find_package(Eigen3)
+  if(EIGEN3_FOUND)
+    message(STATUS "Found system Eigen at " ${EIGEN3_INCLUDE_DIR})
+  else()
+    message(STATUS "Did not find system Eigen. Using third party subdirectory.")
+    execute_process(COMMAND ${Python_EXECUTABLE} ../tools/optional_modules.py checkout_eigen
+                    WORKING_DIRECTORY ${CMAKE_CURRENT_LIST_DIR})
+
+    set(EIGEN3_INCLUDE_DIR ${CMAKE_CURRENT_LIST_DIR}/../third_party/eigen)
+    caffe2_update_option(USE_SYSTEM_EIGEN_INSTALL OFF)
+  endif()
+else()
+  message(STATUS "Using third party subdirectory Eigen.")
+  set(EIGEN3_INCLUDE_DIR ${CMAKE_CURRENT_LIST_DIR}/../third_party/eigen)
+endif()
+include_directories(SYSTEM ${EIGEN3_INCLUDE_DIR})
+
+
 if(BUILD_PYTHON)
   set(PYTHON_COMPONENTS Development.Module)
   if(USE_NUMPY)
diff --git a/third_party/eigen b/third_party/eigen
deleted file mode 160000
index 3147391d946b..000000000000
--- a/third_party/eigen
+++ /dev/null
@@ -1 +0,0 @@
-Subproject commit 3147391d946bb4b6c68edd901f2add6ac1f31f8c
diff --git a/third_party/eigen_pin.txt b/third_party/eigen_pin.txt
new file mode 100644
index 000000000000..18091983f59d
--- /dev/null
+++ b/third_party/eigen_pin.txt
@@ -0,0 +1 @@
+3.4.0
diff --git a/tools/optional_submodules.py b/tools/optional_submodules.py
index b8fa1b8e8168..1e7589edf2fb 100644
--- a/tools/optional_submodules.py
+++ b/tools/optional_submodules.py
@@ -43,5 +43,21 @@ def checkout_nccl() -> None:
         _checkout_by_tag("https://github.com/NVIDIA/nccl", release_tag)
 
 
+def checkout_eigen() -> None:
+    eigen_tag = _read_file(third_party_path / "eigen_pin.txt")
+    print(f"-- Checkout Eigen release tag: {eigen_tag}")
+    eigen_basedir = third_party_path / "eigen"
+    if not eigen_basedir.exists():
+        _checkout_by_tag("https://gitlab.com/libeigen/eigen", eigen_tag)
+
+
 if __name__ == "__main__":
-    checkout_nccl()
+    import sys
+
+    if len(sys.argv) == 1:
+        # If no arguments are given checkout all optional dependency
+        checkout_nccl()
+        checkout_eigen()
+    else:
+        # Otherwise just call top-level function of choice
+        globals()[sys.argv[1]]()