Force reuse of CUDA arches from thrust.

This only affects the test suite, examples, etc, and will not
affect consumers of CUB.

We haven't supported building CUB's test suite as a standalone
project for a while, preferring to build Thrust/CUB tests together.

This change prevents CUB from being successful configuring its
test suite unless it is inside the Thrust project.

The arch flags in both projectss will be removed soon and replaced
with the standard CUDA_ARCHITECTURE CMake properties during the switch
to a Thrust/CUB/libcu++ monorepo. This error will be removed when
that work is completed.

cmake/CubCudaConfig.cmake

@@ -1,132 +1,24 @@
 enable_language(CUDA)
 
-#
-# Architecture options:
-#
-
-set(all_archs 35 37 50 52 53 60 61 62 70 72 75 80 86 90)
-set(arch_message "CUB: Explicitly enabled compute architectures:")
-
-# Thrust sets up the architecture flags in CMAKE_CUDA_FLAGS already. Just
-# reuse them if possible. After we transition to CMake 3.18 CUDA_ARCHITECTURE
-# target properties this will need to be updated.
-if (CUB_IN_THRUST)
-  # Configure to use all flags from thrust. See ThrustCudaConfig.cmake for
-  # details.
-  set(CUB_CUDA_FLAGS_BASE "${THRUST_CUDA_FLAGS_BASE}")
-  set(CUB_CUDA_FLAGS_RDC "${THRUST_CUDA_FLAGS_RDC}")
-  set(CUB_CUDA_FLAGS_NO_RDC "${THRUST_CUDA_FLAGS_NO_RDC}")
-
-  # Update the enabled architectures list from thrust
-  foreach (arch IN LISTS all_archs)
-    if (THRUST_ENABLE_COMPUTE_${arch})
-      set(CUB_ENABLE_COMPUTE_${arch} True)
-      string(APPEND arch_message " sm_${arch}")
-    else()
-      set(CUB_ENABLE_COMPUTE_${arch} False)
-    endif()
-  endforeach()
-
-  # Otherwise create cache options and build the flags ourselves:
-else() # NOT CUB_IN_THRUST
-
-  # Split CUDA_FLAGS into 3 parts:
-  #
-  # CUB_CUDA_FLAGS_BASE: Common CUDA flags for all targets.
-  # CUB_CUDA_FLAGS_RDC: Additional CUDA flags for targets compiled with RDC.
-  # CUB_CUDA_FLAGS_NO_RDC: Additional CUDA flags for targets compiled without RDC.
-  #
-  # This is necessary because CUDA SMs 5.3, 6.2, and 7.2 do not support RDC, but
-  # we want to always build some targets (e.g. testing/cuda/*) with RDC.
-  # We work around this by building the "always RDC" targets without support for
-  # those SMs. This requires two sets of CUDA_FLAGS.
-  #
-  # Enabling any of those SMs along with the ENABLE_RDC options will result in a
-  # configuration error.
-  #
-  # Because of how CMake handles the CMAKE_CUDA_FLAGS variables, every target
-  # generated in a given directory will use the same value for CMAKE_CUDA_FLAGS,
-  # which is determined at the end of the directory's scope. This means caution
-  # should be used when trying to build different targets with different flags,
-  # since they might not behave as expected. This will improve with CMake 3.18,
-  # which add the DEVICE_LINK genex, fixing the issue with using per-target
-  # CUDA_FLAGS: https://gitlab.kitware.com/cmake/cmake/-/issues/18265
-  set(CUB_CUDA_FLAGS_BASE "${CMAKE_CUDA_FLAGS}")
-  set(CUB_CUDA_FLAGS_RDC)
-  set(CUB_CUDA_FLAGS_NO_RDC)
-
-  # Archs that don't support RDC:
-  set(no_rdc_archs 53 62 72)
-
-  # Find the highest arch:
-  list(SORT all_archs)
-  list(LENGTH all_archs max_idx)
-  math(EXPR max_idx "${max_idx} - 1")
-  list(GET all_archs ${max_idx} highest_arch)
-
-  set(option_init OFF)
-  if ("NVCXX" STREQUAL "${CMAKE_CUDA_COMPILER_ID}")
-    set(option_init ON)
-  endif()
-  option(CUB_DISABLE_ARCH_BY_DEFAULT
-    "If ON, then all compute architectures are disabled on the initial CMake run."
-    ${option_init}
-  )
-
-  set(option_init ON)
-  if (CUB_DISABLE_ARCH_BY_DEFAULT)
-    set(option_init OFF)
-  endif()
-
-  set(arch_flags)
-  set(num_archs_enabled 0)
-  foreach (arch IN LISTS all_archs)
-    option(CUB_ENABLE_COMPUTE_${arch}
-      "Enable code generation for sm_${arch}."
-      ${option_init}
-    )
-
-    if (CUB_ENABLE_COMPUTE_${arch})
-      math(EXPR num_archs_enabled "${num_archs_enabled} + 1")
-
-      if ("NVCXX" STREQUAL "${CMAKE_CUDA_COMPILER_ID}")
-        if (NOT ${num_archs_enabled} EQUAL 1)
-          message(FATAL_ERROR
-            "NVC++ does not support compilation for multiple device architectures "
-            "at once."
-          )
-        endif()
-        set(arch_flag "-gpu=cc${arch}")
-      elseif ("Clang" STREQUAL "${CMAKE_CUDA_COMPILER_ID}")
-        set(arch_flag "--cuda-gpu-arch=sm_${arch}")
-      else()
-        set(arch_flag "-gencode arch=compute_${arch},code=sm_${arch}")
-      endif()
-
-      string(APPEND arch_message " sm_${arch}")
-      string(APPEND CUB_CUDA_FLAGS_NO_RDC " ${arch_flag}")
-      if (NOT arch IN_LIST no_rdc_archs)
-        string(APPEND CUB_CUDA_FLAGS_RDC " ${arch_flag}")
-      endif()
-    endif()
-  endforeach()
+if (NOT CUB_IN_THRUST)
+  message(FATAL_ERROR
+    "Building CUB as a standalone project is no longer supported. "
+    "Use the Thrust repo instead.")
+endif()
 
-  if (NOT "NVCXX" STREQUAL "${CMAKE_CUDA_COMPILER_ID}")
-    option(CUB_ENABLE_COMPUTE_FUTURE
-      "Enable code generation for tests for compute_${highest_arch}"
-      ${option_init}
-    )
-    if (CUB_ENABLE_COMPUTE_FUTURE)
-      string(APPEND THRUST_CUDA_FLAGS_BASE
-        " -gencode arch=compute_${highest_arch},code=compute_${highest_arch}"
-      )
-      string(APPEND arch_message " compute_${highest_arch}")
-    endif()
+set(CUB_CUDA_FLAGS_BASE "${THRUST_CUDA_FLAGS_BASE}")
+set(CUB_CUDA_FLAGS_RDC "${THRUST_CUDA_FLAGS_RDC}")
+set(CUB_CUDA_FLAGS_NO_RDC "${THRUST_CUDA_FLAGS_NO_RDC}")
+
+# Update the enabled architectures list from thrust
+foreach (arch IN LISTS THRUST_KNOWN_COMPUTE_ARCHS)
+  if (THRUST_ENABLE_COMPUTE_${arch})
+    set(CUB_ENABLE_COMPUTE_${arch} True)
+    string(APPEND arch_message " sm_${arch}")
+  else()
+    set(CUB_ENABLE_COMPUTE_${arch} False)
   endif()
-
-  # TODO Once CMake 3.18 is required, use the CUDA_ARCHITECTURE target props
-  string(APPEND CMAKE_CUDA_FLAGS "${arch_flags}")
-endif()
+endforeach()
 
 message(STATUS ${arch_message})