From a20fc40e4ee2c50f635716ae844f44c5fdeb699a Mon Sep 17 00:00:00 2001
From: Rasmus Munk Larsen <4643818-rmlarsen1@users.noreply.gitlab.com>
Date: Mon, 3 Nov 2025 18:59:51 +0000
Subject: [PATCH] Revert "simplify squaredNorm"

This causes some subtle alignment-related bugs, which @chuckyschluz is currently investigating.

See merge request libeigen/eigen!2055
---
 Eigen/src/Core/Dot.h                    |  5 ++++-
 Eigen/src/Core/functors/UnaryFunctors.h | 20 ++++++++++++++++++++
 2 files changed, 24 insertions(+), 1 deletion(-)

diff --git a/Eigen/src/Core/Dot.h b/Eigen/src/Core/Dot.h
index a173306b4..059527c85 100644
--- a/Eigen/src/Core/Dot.h
+++ b/Eigen/src/Core/Dot.h
@@ -20,7 +20,10 @@ namespace internal {
 template <typename Derived, typename Scalar = typename traits<Derived>::Scalar>
 struct squared_norm_impl {
   using Real = typename NumTraits<Scalar>::Real;
-  static EIGEN_DEVICE_FUNC EIGEN_STRONG_INLINE Real run(const Derived& a) { return a.realView().cwiseAbs2().sum(); }
+  static EIGEN_DEVICE_FUNC EIGEN_STRONG_INLINE Real run(const Derived& a) {
+    Scalar result = a.unaryExpr(squared_norm_functor<Scalar>()).sum();
+    return numext::real(result) + numext::imag(result);
+  }
 };
 
 template <typename Derived>
diff --git a/Eigen/src/Core/functors/UnaryFunctors.h b/Eigen/src/Core/functors/UnaryFunctors.h
index d7fc7bb4d..202995ff0 100644
--- a/Eigen/src/Core/functors/UnaryFunctors.h
+++ b/Eigen/src/Core/functors/UnaryFunctors.h
@@ -106,6 +106,26 @@ struct functor_traits<scalar_abs2_op<Scalar>> {
   };
 };
 
+template <typename Scalar, bool IsComplex = NumTraits<Scalar>::IsComplex>
+struct squared_norm_functor {
+  typedef Scalar result_type;
+  EIGEN_DEVICE_FUNC EIGEN_STRONG_INLINE const Scalar operator()(const Scalar& a) const {
+    return Scalar(numext::real(a) * numext::real(a), numext::imag(a) * numext::imag(a));
+  }
+  template <typename Packet>
+  EIGEN_DEVICE_FUNC EIGEN_STRONG_INLINE const Packet packetOp(const Packet& a) const {
+    return Packet(pmul(a.v, a.v));
+  }
+};
+template <typename Scalar>
+struct squared_norm_functor<Scalar, false> : scalar_abs2_op<Scalar> {};
+
+template <typename Scalar>
+struct functor_traits<squared_norm_functor<Scalar>> {
+  using Real = typename NumTraits<Scalar>::Real;
+  enum { Cost = NumTraits<Real>::MulCost, PacketAccess = packet_traits<Real>::HasMul };
+};
+
 /** \internal
  * \brief Template functor to compute the conjugate of a complex value
  *