[libc] Configure CMAKE_REQUIRED_FLAGS so the GPU can use flag checks (#95424)

Summary:
This patch adds `CMAKE_REQUIRED_FLAGS` for the GPU build so checks like
`check_cxx_compiler_flags` work as expected. This is required because we
need to hack around the potential lack of `nvlink` and `ptxas` for NVPTX
targets and the fact that the AMDGPU target needs `-nogpulib` to avoid
errors on lack of ROCm. This makes a few of the checks pass and also
allows us to just check `-mcpu=native` for architecture detection
instead of finding the tools manually.

Author: jhuber6

libc/cmake/modules/prepare_libc_gpu_build.cmake

@@ -16,6 +16,17 @@ if(NOT LLVM_LIBC_FULL_BUILD)
                       "GPU.")
 endif()
 
+# Set the required flags globally so standard CMake utilities can compile.
+if(LIBC_TARGET_TRIPLE)
+  set(CMAKE_REQUIRED_FLAGS "--target=${LIBC_TARGET_TRIPLE}")
+endif()
+if(LIBC_TARGET_ARCHITECTURE_IS_AMDGPU)
+  set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} -nogpulib")
+elseif(LIBC_TARGET_ARCHITECTURE_IS_NVPTX)
+  set(CMAKE_REQUIRED_FLAGS
+      "${CMAKE_REQUIRED_FLAGS} -flto -c -Wno-unused-command-line-argument")
+endif()
+
 # Identify the program used to package multiple images into a single binary.
 get_filename_component(compiler_path ${CMAKE_CXX_COMPILER} DIRECTORY)
 if(TARGET clang-offload-packager)
@@ -56,39 +67,9 @@ endif()
 
 set(LIBC_GPU_TEST_ARCHITECTURE "" CACHE STRING "Architecture for the GPU tests")
 if(LIBC_TARGET_ARCHITECTURE_IS_AMDGPU)
-  # Identify any locally installed AMD GPUs on the system using 'amdgpu-arch'.
-  if(TARGET amdgpu-arch)
-    get_target_property(LIBC_AMDGPU_ARCH amdgpu-arch LOCATION)
-  else()
-    find_program(LIBC_AMDGPU_ARCH
-                 NAMES amdgpu-arch NO_DEFAULT_PATH
-                 PATHS ${LLVM_BINARY_DIR}/bin ${compiler_path})
-  endif()
-  if(LIBC_AMDGPU_ARCH)
-    execute_process(COMMAND ${LIBC_AMDGPU_ARCH}
-                    OUTPUT_VARIABLE arch_tool_output
-                    ERROR_QUIET OUTPUT_STRIP_TRAILING_WHITESPACE)
-    if(arch_tool_output MATCHES "^gfx[0-9]+")
-      set(PLATFORM_HAS_GPU TRUE)
-    endif()
-  endif()
+  check_cxx_compiler_flag(-mcpu=native PLATFORM_HAS_GPU)
 elseif(LIBC_TARGET_ARCHITECTURE_IS_NVPTX)
-  # Identify any locally installed NVIDIA GPUs on the system using 'nvptx-arch'.
-  if(TARGET nvptx-arch)
-    get_target_property(LIBC_NVPTX_ARCH nvptx-arch LOCATION)
-  else()
-    find_program(LIBC_NVPTX_ARCH
-                 NAMES nvptx-arch NO_DEFAULT_PATH
-                 PATHS ${LLVM_BINARY_DIR}/bin ${compiler_path})
-  endif()
-  if(LIBC_NVPTX_ARCH)
-    execute_process(COMMAND ${LIBC_NVPTX_ARCH}
-                    OUTPUT_VARIABLE arch_tool_output
-                    ERROR_QUIET OUTPUT_STRIP_TRAILING_WHITESPACE)
-    if(arch_tool_output MATCHES "^sm_[0-9]+")
-      set(PLATFORM_HAS_GPU TRUE)
-    endif()
-  endif()
+  check_cxx_compiler_flag(-march=native PLATFORM_HAS_GPU)
 endif()
 
 set(gpu_test_architecture "")

llvm/runtimes/CMakeLists.txt

@@ -496,15 +496,15 @@ if(build_runtimes)
       if(TARGET amdhsa-loader)
         list(APPEND extra_cmake_args
              "-DRUNTIMES_amdgcn-amd-amdhsa_LIBC_GPU_LOADER_EXECUTABLE=$<TARGET_FILE:amdhsa-loader>")
-        list(APPEND extra_deps amdhsa-loader amdgpu-arch)
+        list(APPEND extra_deps amdhsa-loader)
       endif()
       list(APPEND extra_cmake_args "-DRUNTIMES_amdgcn-amd-amdhsa_LLVM_LIBC_FULL_BUILD=ON")
     endif()
     if("libc" IN_LIST RUNTIMES_nvptx64-nvidia-cuda_LLVM_ENABLE_RUNTIMES)
       if(TARGET nvptx-loader)
         list(APPEND extra_cmake_args
              "-DRUNTIMES_nvptx64-nvidia-cuda_LIBC_GPU_LOADER_EXECUTABLE=$<TARGET_FILE:nvptx-loader>")
-        list(APPEND extra_deps nvptx-loader nvptx-arch)
+        list(APPEND extra_deps nvptx-loader)
       endif()
       list(APPEND extra_cmake_args "-DRUNTIMES_nvptx64-nvidia-cuda_LLVM_LIBC_FULL_BUILD=ON")
     endif()