From 95e8bc3267b7ffb8672e14fe88166127d38ea3fa Mon Sep 17 00:00:00 2001
From: Rasmus Munk Larsen <4643818-rmlarsen1@users.noreply.gitlab.com>
Date: Fri, 20 Feb 2026 21:28:47 -0800
Subject: [PATCH] Add EIGEN_LAMBDA_ALWAYS_INLINE macro for MSVC lambda inlining

libeigen/eigen!2170

Closes #3033

Co-authored-by: Rasmus Munk Larsen <rmlarsen@gmail.com>
---
 Eigen/src/Core/products/GeneralBlockPanelKernel.h |  3 +--
 Eigen/src/Core/util/Macros.h                      | 12 ++++++++++++
 2 files changed, 13 insertions(+), 2 deletions(-)

diff --git a/Eigen/src/Core/products/GeneralBlockPanelKernel.h b/Eigen/src/Core/products/GeneralBlockPanelKernel.h
index fc3a81c63..18cd96016 100644
--- a/Eigen/src/Core/products/GeneralBlockPanelKernel.h
+++ b/Eigen/src/Core/products/GeneralBlockPanelKernel.h
@@ -1403,8 +1403,7 @@ EIGEN_DONT_INLINE void gebp_kernel<LhsScalar, RhsScalar, Index, DataMapper, mr,
   // template instantiation of this generic lambda as a separate function,
   // adding call overhead that causes 10-17 % regressions in LLT/TRSM
   // for small-to-medium matrix sizes.
-  auto micro_panel = [&](auto mrp_tag, auto nrc_tag, auto& local_traits, Index i, Index j2)
-      __attribute__((always_inline)) {
+  auto micro_panel = [&](auto mrp_tag, auto nrc_tag, auto& local_traits, Index i, Index j2) EIGEN_LAMBDA_ALWAYS_INLINE {
     constexpr int MrP = decltype(mrp_tag)::value;
     constexpr int NrC = decltype(nrc_tag)::value;
     using LTraits = std::remove_reference_t<decltype(local_traits)>;
diff --git a/Eigen/src/Core/util/Macros.h b/Eigen/src/Core/util/Macros.h
index e76c74cca..b73f2ac8f 100644
--- a/Eigen/src/Core/util/Macros.h
+++ b/Eigen/src/Core/util/Macros.h
@@ -906,6 +906,18 @@
 #define EIGEN_ALWAYS_INLINE EIGEN_STRONG_INLINE
 #endif
 
+// EIGEN_LAMBDA_ALWAYS_INLINE forces inlining of lambda functions.
+// On GCC/Clang, __attribute__((always_inline)) works on lambdas.
+// On MSVC, __forceinline cannot be applied to lambdas, but the
+// [[msvc::forceinline]] attribute (available since VS 2022 17.1) can.
+#if EIGEN_COMP_GNUC && !defined(SYCL_DEVICE_ONLY)
+#define EIGEN_LAMBDA_ALWAYS_INLINE __attribute__((always_inline))
+#elif EIGEN_COMP_MSVC >= 1931
+#define EIGEN_LAMBDA_ALWAYS_INLINE [[msvc::forceinline]]
+#else
+#define EIGEN_LAMBDA_ALWAYS_INLINE
+#endif
+
 #if EIGEN_COMP_GNUC
 #define EIGEN_DONT_INLINE __attribute__((noinline))
 #elif EIGEN_COMP_MSVC