From 3ffefcb95c73240e0681d25a9771de854c21ca0c Mon Sep 17 00:00:00 2001
From: Rasmus Munk Larsen <rmlarsen@google.com>
Date: Thu, 2 Dec 2021 23:55:25 +0000
Subject: [PATCH] Only include <atomic> if needed.

---
 Eigen/Core                             |  1 +
 Eigen/src/Core/products/Parallelizer.h | 12 +++++-------
 2 files changed, 6 insertions(+), 7 deletions(-)

diff --git a/Eigen/Core b/Eigen/Core
index 9f96eaa79..b8d8bc659 100644
--- a/Eigen/Core
+++ b/Eigen/Core
@@ -67,6 +67,7 @@
 #endif
 
 #ifdef EIGEN_HAS_OPENMP
+#include <atomic>
 #include <omp.h>
 #endif
 
diff --git a/Eigen/src/Core/products/Parallelizer.h b/Eigen/src/Core/products/Parallelizer.h
index d38d3da55..da4affb80 100644
--- a/Eigen/src/Core/products/Parallelizer.h
+++ b/Eigen/src/Core/products/Parallelizer.h
@@ -10,8 +10,6 @@
 #ifndef EIGEN_PARALLELIZER_H
 #define EIGEN_PARALLELIZER_H
 
-#include <atomic>
-
 #include "../InternalHeaderCheck.h"
 
 namespace Eigen {
@@ -78,14 +76,14 @@ namespace internal {
 
 template<typename Index> struct GemmParallelInfo
 {
-  GemmParallelInfo() : sync(-1), users(0), lhs_start(0), lhs_length(0) {}
 
-  // volatile is not enough on all architectures (see bug 1572)
-  // to guarantee that when thread A says to thread B that it is
-  // done with packing a block, then all writes have been really
-  // carried out... C++11 memory model+atomic guarantees this.
+#ifdef EIGEN_HAS_OPENMP
+  GemmParallelInfo() : sync(-1), users(0), lhs_start(0), lhs_length(0) {}
   std::atomic<Index> sync;
   std::atomic<int> users;
+#else
+  GemmParallelInfo() : lhs_start(0), lhs_length(0) {}
+#endif
 
   Index lhs_start;
   Index lhs_length;