[libc] Simplify enabling the GPU build for libc

[jhuber6]

Summary

Currently the GPU build requires the LLVM_LIBC_FULL_BUILD option to be
set. This patch changes the logic so that it is always enabled when
targeting the GPU. Also, this patch allows LIBC_GPU_BUILD and
LIBC_GPU_ARCHITECTURES to both enable a GPU build. Now, enabling the
GPU support should only require the following CMake:

-DLLVM_ENABLE_RUNTIMES=libc -DLIBC_GPU_ARCHITECTURES=gfx1030

[jdoerfert]

LG

libc/CMakeLists.txt

 list(APPEND CMAKE_MODULE_PATH "${CMAKE_CURRENT_SOURCE_DIR}/cmake/modules")
 
 # The top-level sourse and binary directories.
 set(LIBC_SOURCE_DIR ${CMAKE_CURRENT_SOURCE_DIR})
 set(LIBC_BINARY_DIR ${CMAKE_CURRENT_BINARY_DIR})
 # The top-level directory in which libc is being built.
 set(LIBC_BUILD_DIR ${CMAKE_CURRENT_BINARY_DIR})
 
-if(LLVM_LIBC_FULL_BUILD)
+if(LLVM_LIBC_FULL_BUILD OR LIBC_GPU_BUILD OR LIBC_GPU_ARCHITECTURES)
   if(NOT LIBC_HDRGEN_EXE)
     # We need to set up hdrgen first since other targets depend on it.
     add_subdirectory(utils/LibcTableGenUtil)
     add_subdirectory(utils/HdrGen)
   else()
     message(STATUS "Will use ${LIBC_HDRGEN_EXE} for libc header generation.")
   endif()
 endif()

libc/cmake/modules/LLVMLibCArchitectures.cmake

 # ------------------------------------------------------------------------------
 # Architecture and OS definitions.
 #
 # The correct target OS and architecture to build the libc for is deduced here.
 # When possible, we also setup appropriate compile options for the target
 # platform.
 # ------------------------------------------------------------------------------
 
-if(LIBC_GPU_BUILD)
+if(LIBC_GPU_BUILD OR LIBC_GPU_ARCHITECTURES)
   # We set the generic target and OS to "gpu" here. More specific defintions
   # for the exact target GPU are set up in prepare_libc_gpu_build.cmake.
   set(LIBC_TARGET_OS "gpu")
   set(LIBC_TARGET_ARCHITECTURE_IS_GPU TRUE)
   set(LIBC_TARGET_ARCHITECTURE "gpu")
   if(LIBC_TARGET_TRIPLE)
     message(WARNING "LIBC_TARGET_TRIPLE is ignored as LIBC_GPU_BUILD is on. ")
   endif()

libc/cmake/modules/prepare_libc_gpu_build.cmake

 set(req_ver "${LLVM_VERSION_MAJOR}.${LLVM_VERSION_MINOR}.${LLVM_VERSION_PATCH}")
 if(NOT (CMAKE_CXX_COMPILER_ID MATCHES "[Cc]lang" AND
         ${CMAKE_CXX_COMPILER_VERSION} VERSION_EQUAL "${req_ver}"))
   message(FATAL_ERROR "Cannot build libc for GPU. CMake compiler "
                       "'${CMAKE_CXX_COMPILER_ID} ${CMAKE_CXX_COMPILER_VERSION}' "
                       " is not `Clang ${req_ver}.")
 endif()
 if(NOT LLVM_LIBC_FULL_BUILD)
-  message(FATAL_ERROR "LLVM_LIBC_FULL_BUILD must be enabled to build libc for "
-                      "GPU.")
+  message(STATUS "LLVM_LIBC_FULL_BUILD must be enabled to build libc for GPU. "
+                 "Overriding LLVM_LIBC_FULL_BUILD to ON.")
+  set(LLVM_LIBC_FULL_BUILD ON FORCE)
 endif()
 
 # Identify the program used to package multiple images into a single binary.
 find_program(LIBC_CLANG_OFFLOAD_PACKAGER
              NAMES clang-offload-packager
              PATHS ${LLVM_BINARY_DIR}/bin)
 if(NOT LIBC_CLANG_OFFLOAD_PACKAGER)
   message(FATAL_ERROR "Cannot find the 'clang-offload-packager' for the GPU "

llvm/runtimes/CMakeLists.txt

if(runtimes)
   if("openmp" IN_LIST LLVM_ENABLE_RUNTIMES)
     if(TARGET opt)
       list(APPEND extra_deps opt)
     endif()
     if(TARGET llvm-link)
       list(APPEND extra_deps llvm-link)
     endif()
   endif()
-  if("libc" IN_LIST LLVM_ENABLE_RUNTIMES AND LLVM_LIBC_FULL_BUILD)
+  if("libc" IN_LIST LLVM_ENABLE_RUNTIMES AND 
+      (LLVM_LIBC_FULL_BUILD OR LIBC_GPU_BUILD OR LIBC_GPU_ARCHITECTURES))
     if(TARGET libc-hdrgen)
       set(libc_tools libc-hdrgen)
       set(libc_cmake_args "-DLIBC_HDRGEN_EXE=$<TARGET_FILE:libc-hdrgen>"
                           "-DLLVM_LIBC_FULL_BUILD=ON")
       list(APPEND extra_deps ${libc_tools})
     else()
       # We want to build the libc build tools before we can build the libc
       # itself. So, the libc project should be included in LLVM_ENABLE_PROJECTS.