From 30128de0e3a6bf40f6b8937dde9000d1c028d8a6 Mon Sep 17 00:00:00 2001
From: Pavel Guzenfeld <pavelguzenfeld@gmail.com>
Date: Fri, 20 Mar 2026 19:03:13 +0000
Subject: [PATCH] Guard eigen_fill_helper on trivially copyable scalars

libeigen/eigen!2313

Closes #2956
---
 Eigen/src/Core/Fill.h | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/Eigen/src/Core/Fill.h b/Eigen/src/Core/Fill.h
index cacf921c2..ccbeb50f0 100644
--- a/Eigen/src/Core/Fill.h
+++ b/Eigen/src/Core/Fill.h
@@ -20,11 +20,14 @@ namespace internal {
 template <typename Xpr>
 struct eigen_fill_helper : std::false_type {};
 
+// Only enable std::fill_n for trivially copyable scalars.  GCC's libstdc++
+// fill_n pessimizes non-trivially-copyable types (extra moves per iteration),
+// causing measurable regressions for types like AutoDiffScalar (issue #2956).
 template <typename Scalar, int Rows, int Cols, int Options, int MaxRows, int MaxCols>
-struct eigen_fill_helper<Matrix<Scalar, Rows, Cols, Options, MaxRows, MaxCols>> : std::true_type {};
+struct eigen_fill_helper<Matrix<Scalar, Rows, Cols, Options, MaxRows, MaxCols>> : std::is_trivially_copyable<Scalar> {};
 
 template <typename Scalar, int Rows, int Cols, int Options, int MaxRows, int MaxCols>
-struct eigen_fill_helper<Array<Scalar, Rows, Cols, Options, MaxRows, MaxCols>> : std::true_type {};
+struct eigen_fill_helper<Array<Scalar, Rows, Cols, Options, MaxRows, MaxCols>> : std::is_trivially_copyable<Scalar> {};
 
 template <typename Xpr, int BlockRows, int BlockCols>
 struct eigen_fill_helper<Block<Xpr, BlockRows, BlockCols, /*InnerPanel*/ true>> : eigen_fill_helper<Xpr> {};