[libc] Fix building bitcode library for GPU (#100491)

Summary:
The GPU build provides a bitcode version of the library to mimic how
NVIDIA and AMD provide their libraries. Previously we had the fatbinary
archive that created the necessary dependency for this to actually
build. Since it was removed we no longer had anything triggering this to
build.

I have no idea if there's a better way to force a custom command to
actually be run in the same space as libraries, but the only thing I
could come up with was a dummy target.

Author: jhuber6

libc/cmake/modules/LLVMLibCLibraryRules.cmake

@@ -111,19 +111,9 @@ function(add_bitcode_entrypoint_library target_name base_target_name)
     list(APPEND objects ${object})
   endforeach()
 
-  set(output ${CMAKE_CURRENT_BINARY_DIR}/${target_name}.bc)
-  add_custom_command(
-    OUTPUT ${output}
-    COMMAND ${LIBC_LLVM_LINK} ${objects} -o ${output}
-    DEPENDS ${all_deps} ${base_target_name}
-    COMMENT "Linking LLVM-IR bitcode for ${base_target_name}"
-    COMMAND_EXPAND_LISTS
-  )
-  add_custom_target(${target_name} DEPENDS ${output} ${all_deps})
-  set_target_properties(${target_name} PROPERTIES TARGET_OBJECT ${output})
-  if(TARGET llvm-link)
-    add_dependencies(${target_name} llvm-link)
-  endif()
+  add_executable(${target_name} ${objects})
+  target_link_options(${target_name} PRIVATE
+                      "-r" "-nostdlib" "-flto" "-Wl,--lto-emit-llvm" "-march= ")
 endfunction(add_bitcode_entrypoint_library)
 
 # A rule to build a library from a collection of entrypoint objects.

libc/cmake/modules/prepare_libc_gpu_build.cmake

@@ -21,36 +21,10 @@ 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")
+  set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} -nogpulib -nostdlib")
 elseif(LIBC_TARGET_ARCHITECTURE_IS_NVPTX)
   set(CMAKE_REQUIRED_FLAGS
-      "${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)
-  get_target_property(LIBC_CLANG_OFFLOAD_PACKAGER clang-offload-packager LOCATION)
-else()
-  find_program(LIBC_CLANG_OFFLOAD_PACKAGER
-               NAMES clang-offload-packager NO_DEFAULT_PATH
-               PATHS ${LLVM_BINARY_DIR}/bin ${compiler_path})
-  if(NOT LIBC_CLANG_OFFLOAD_PACKAGER)
-    message(FATAL_ERROR "Cannot find the 'clang-offload-packager' for the GPU "
-                        "build")
-  endif()
-endif()
-
-# Identify llvm-link program so we can merge the output IR into a single blob.
-if(TARGET llvm-link)
-  get_target_property(LIBC_LLVM_LINK llvm-link LOCATION)
-else()
-  find_program(LIBC_LLVM_LINK
-               NAMES llvm-link NO_DEFAULT_PATH
-               PATHS ${LLVM_BINARY_DIR}/bin ${compiler_path})
-  if(NOT LIBC_LLVM_LINK)
-    message(FATAL_ERROR "Cannot find 'llvm-link' for the GPU build")
-  endif()
+      "${CMAKE_REQUIRED_FLAGS} -flto -c -Wno-unused-command-line-argument -nostdlib")
 endif()
 
 # Optionally set up a job pool to limit the number of GPU tests run in parallel.

libc/lib/CMakeLists.txt

@@ -51,7 +51,7 @@ foreach(archive IN ZIP_LISTS
       PROPERTIES
         OUTPUT_NAME ${archive_1}.bc
     )
-    list(APPEND added_gpu_bitcode_targets ${archive_1}bitcode)
+    list(APPEND added_bitcode_targets ${archive_1}bitcode)
   endif()
 endforeach()
 
@@ -61,24 +61,13 @@ install(
   COMPONENT libc
 )
 
-if(LIBC_TARGET_OS_IS_GPU)
-  set(gpu_install_dir lib${LLVM_LIBDIR_SUFFIX})
-  if(LLVM_ENABLE_PER_TARGET_RUNTIME_DIR)
-    set(gpu_install_dir lib${LLVM_LIBDIR_SUFFIX}/${LLVM_HOST_TRIPLE})
-  endif()
-  install(
-    TARGETS ${added_gpu_archive_targets}
-    ARCHIVE DESTINATION ${gpu_install_dir}
-    COMPONENT libc
+foreach(file ${added_bitcode_targets})
+  install(FILES $<TARGET_FILE:${file}>
+          DESTINATION ${LIBC_INSTALL_LIBRARY_DIR}
+          RENAME $<TARGET_PROPERTY:${file},OUTPUT_NAME>
+          COMPONENT libc
   )
-  foreach(file ${added_gpu_bitcode_targets})
-    install(FILES $<TARGET_PROPERTY:${file},TARGET_OBJECT>
-            DESTINATION ${LIBC_INSTALL_LIBRARY_DIR}
-            RENAME $<TARGET_PROPERTY:${file},OUTPUT_NAME>
-            COMPONENT libc
-    )
-  endforeach()
-endif()
+endforeach()
 
 if(NOT LIBC_TARGET_OS_IS_BAREMETAL)
   # For now we will disable libc-startup installation for baremetal. The
@@ -93,13 +82,15 @@ endif()
 
 add_custom_target(install-libc
                   DEPENDS ${added_archive_targets}
+                          ${added_bitcode_targets}
                           ${startup_target}
                           ${header_install_target}
                   COMMAND "${CMAKE_COMMAND}"
                           -DCMAKE_INSTALL_COMPONENT=libc
                           -P "${CMAKE_BINARY_DIR}/cmake_install.cmake")
 add_custom_target(install-libc-stripped
                   DEPENDS ${added_archive_targets}
+                          ${added_bitcode_targets}
                           ${startup_target}
                           ${header_install_target}
                   COMMAND "${CMAKE_COMMAND}"