Fix CUDA 13 LTO architecture selection with older CMake

cpp/CMakeLists.txt

@@ -186,21 +186,11 @@ set(CUDF_CUDA_DEFINITIONS "")
 # override must therefore not exceed the architecture of any target GPU on which it will be linked.
 set(CUDF_LTO_ARCHITECTURE
     ""
-    CACHE STRING "LTO fragment architecture; empty selects the minimum supported by the toolkit"
+    CACHE STRING "LTO fragment architecture; empty selects the minimum configured architecture"
 )
 
-if(CUDF_LTO_ARCHITECTURE STREQUAL "")
-  foreach(architecture IN LISTS CMAKE_CUDA_ARCHITECTURES_ALL)
-    string(REGEX MATCH "^[0-9]+" architecture "${architecture}")
-    if(architecture AND (NOT CUDF_LTO_ARCHITECTURE OR architecture LESS CUDF_LTO_ARCHITECTURE))
-      set(CUDF_LTO_ARCHITECTURE "${architecture}")
-    endif()
-  endforeach()
-endif()
-
-if(NOT CUDF_LTO_ARCHITECTURE MATCHES "^[0-9]+$")
-  message(FATAL_ERROR "CUDF_LTO_ARCHITECTURE must be a numeric architecture")
-endif()
+include(cmake/Modules/SelectLtoArchitecture.cmake)
+cudf_select_lto_architecture(CUDF_LTO_ARCHITECTURE)
 message(VERBOSE "CUDF: Using ${CUDF_LTO_ARCHITECTURE} as the common LTO architecture")
 
 # For now, disable CMake's automatic module scanning for C++ files. There is an sccache bug in the

cpp/cmake/Modules/SelectLtoArchitecture.cmake

@@ -0,0 +1,48 @@
+# =============================================================================
+# cmake-format: off
+# SPDX-FileCopyrightText: Copyright (c) 2026, NVIDIA CORPORATION & AFFILIATES. All rights reserved.
+# SPDX-License-Identifier: Apache-2.0
+# cmake-format: on
+# =============================================================================
+
+include_guard(GLOBAL)
+
+# Select the common LTO base architecture and return it through output_variable.
+function(cudf_select_lto_architecture output_variable)
+  set(selected_architecture "${CUDF_LTO_ARCHITECTURE}")
+
+  if(selected_architecture STREQUAL "")
+    # CMAKE_CUDA_ARCHITECTURES is resolved against the active compiler by rapids-cmake. Prefer it
+    # over CMAKE_CUDA_ARCHITECTURES_ALL, whose value depends on the CMake version and can therefore
+    # contain architectures that the active compiler no longer supports.
+    foreach(architecture IN LISTS CMAKE_CUDA_ARCHITECTURES)
+      string(REGEX MATCH "^[0-9]+" numeric_architecture "${architecture}")
+      if(numeric_architecture AND (NOT selected_architecture OR numeric_architecture LESS
+                                                                selected_architecture)
+      )
+        set(selected_architecture "${numeric_architecture}")
+      endif()
+    endforeach()
+  endif()
+
+  if(selected_architecture STREQUAL "")
+    # Preserve support for symbolic CMake values such as `all` and `all-major`.
+    foreach(architecture IN LISTS CMAKE_CUDA_ARCHITECTURES_ALL)
+      string(REGEX MATCH "^[0-9]+" numeric_architecture "${architecture}")
+      if(numeric_architecture AND (NOT selected_architecture OR numeric_architecture LESS
+                                                                selected_architecture)
+      )
+        set(selected_architecture "${numeric_architecture}")
+      endif()
+    endforeach()
+  endif()
+
+  if(NOT selected_architecture MATCHES "^[0-9]+$")
+    message(FATAL_ERROR "CUDF_LTO_ARCHITECTURE must be a numeric architecture")
+  endif()
+
+  set(${output_variable}
+      "${selected_architecture}"
+      PARENT_SCOPE
+  )
+endfunction()

cpp/cmake/tests/CMakeLists.txt

@@ -34,3 +34,9 @@ add_test(NAME cudf_export_consumer COMMAND ${CMAKE_COMMAND} --build
 
 set_tests_properties(cudf_export_consumer_configure PROPERTIES FIXTURES_SETUP cudf_export_consumer)
 set_tests_properties(cudf_export_consumer PROPERTIES FIXTURES_REQUIRED cudf_export_consumer)
+
+add_test(
+  NAME cudf_lto_architecture_selection
+  COMMAND ${CMAKE_COMMAND} "-DCUDF_REPOSITORY_DIR=${CUDF_REPOSITORY_DIR}" -P
+          "${CMAKE_CURRENT_LIST_DIR}/select_lto_architecture.cmake"
+)

cpp/cmake/tests/select_lto_architecture.cmake

@@ -0,0 +1,38 @@
+# =============================================================================
+# cmake-format: off
+# SPDX-FileCopyrightText: Copyright (c) 2026, NVIDIA CORPORATION & AFFILIATES. All rights reserved.
+# SPDX-License-Identifier: Apache-2.0
+# cmake-format: on
+# =============================================================================
+
+include("${CUDF_REPOSITORY_DIR}/cpp/cmake/Modules/SelectLtoArchitecture.cmake")
+
+# Verify that the selected LTO architecture matches the expected value.
+function(assert_lto_architecture expected configured_architectures all_architectures override)
+  set(CMAKE_CUDA_ARCHITECTURES "${configured_architectures}")
+  set(CMAKE_CUDA_ARCHITECTURES_ALL "${all_architectures}")
+  set(CUDF_LTO_ARCHITECTURE "${override}")
+
+  cudf_select_lto_architecture(actual)
+
+  if(NOT actual STREQUAL expected)
+    message(FATAL_ERROR "Expected LTO architecture ${expected}, got ${actual}")
+  endif()
+endfunction()
+
+# CMake 4.0's static list still contains SM50 when paired with CUDA 13, while rapids-cmake's
+# compiler-aware configured list correctly begins at SM75.
+assert_lto_architecture(
+  75 "75-real;80-real;86-real;90a-real;100f-real;120a-real;120" "50;52;60;61;70;75;80;86;90" ""
+)
+
+# CUDA 12 continues to use SM70 as its common LTO base.
+assert_lto_architecture(
+  70 "70-real;75-real;80-real;86-real;90a-real;90-virtual" "50;52;60;61;70;75;80;86;90" ""
+)
+
+# An explicit user override remains authoritative.
+assert_lto_architecture(80 "75-real;80-real" "50;52;60;61;70;75;80" 80)
+
+# Preserve the existing fallback for symbolic CMake architecture values.
+assert_lto_architecture(70 all "70;75;80;86;90" "")