function(collect_object_file_deps target result)
  # NOTE: This function does add entrypoint targets to |result|.
  # It is expected that the caller adds them separately.
  set(all_deps "")
  get_target_property(target_type ${target} "TARGET_TYPE")
  if(NOT target_type)
    return()
  endif()

  if(${target_type} STREQUAL ${OBJECT_LIBRARY_TARGET_TYPE})
    list(APPEND all_deps ${target})
    get_target_property(deps ${target} "DEPS")
    foreach(dep IN LISTS deps)
      collect_object_file_deps(${dep} dep_targets)
      list(APPEND all_deps ${dep_targets})
    endforeach(dep)
    list(REMOVE_DUPLICATES all_deps)
    set(${result} ${all_deps} PARENT_SCOPE)
    return()
  endif()

  if(${target_type} STREQUAL ${ENTRYPOINT_OBJ_TARGET_TYPE})
    set(entrypoint_target ${target})
    get_target_property(is_alias ${entrypoint_target} "IS_ALIAS")
    if(is_alias)
      get_target_property(aliasee ${entrypoint_target} "DEPS")
      if(NOT aliasee)
        message(FATAL_ERROR
                "Entrypoint alias ${entrypoint_target} does not have an aliasee.")
      endif()
      set(entrypoint_target ${aliasee})
    endif()
    get_target_property(deps ${target} "DEPS")
    foreach(dep IN LISTS deps)
      collect_object_file_deps(${dep} dep_targets)
      list(APPEND all_deps ${dep_targets})
    endforeach(dep)
    list(REMOVE_DUPLICATES all_deps)
    set(${result} ${all_deps} PARENT_SCOPE)
    return()
  endif()

  if(${target_type} STREQUAL ${ENTRYPOINT_EXT_TARGET_TYPE})
    # It is not possible to recursively extract deps of external dependencies.
    # So, we just accumulate the direct dep and return.
    get_target_property(deps ${target} "DEPS")
    set(${result} ${deps} PARENT_SCOPE)
    return()
  endif()
endfunction(collect_object_file_deps)

function(get_all_object_file_deps result fq_deps_list)
  set(all_deps "")
  foreach(dep ${fq_deps_list})
    get_target_property(dep_type ${dep} "TARGET_TYPE")
    if(NOT ((${dep_type} STREQUAL ${ENTRYPOINT_OBJ_TARGET_TYPE}) OR
            (${dep_type} STREQUAL ${ENTRYPOINT_EXT_TARGET_TYPE})))
      message(FATAL_ERROR "Dependency '${dep}' of 'add_entrypoint_collection' is "
                          "not an 'add_entrypoint_object' or 'add_entrypoint_external' target.")
    endif()
    collect_object_file_deps(${dep} recursive_deps)
    list(APPEND all_deps ${recursive_deps})
    # Add the entrypoint object target explicitly as collect_object_file_deps
    # only collects object files from non-entrypoint targets.
    if(${dep_type} STREQUAL ${ENTRYPOINT_OBJ_TARGET_TYPE})
      set(entrypoint_target ${dep})
      get_target_property(is_alias ${entrypoint_target} "IS_ALIAS")
      if(is_alias)
        get_target_property(aliasee ${entrypoint_target} "DEPS")
        if(NOT aliasee)
          message(FATAL_ERROR
                  "Entrypoint alias ${entrypoint_target} does not have an aliasee.")
        endif()
        set(entrypoint_target ${aliasee})
      endif()
    endif()
    list(APPEND all_deps ${entrypoint_target})
  endforeach(dep)
  list(REMOVE_DUPLICATES all_deps)
  set(${result} ${all_deps} PARENT_SCOPE)
endfunction()

# A rule to build a library from a collection of entrypoint objects and bundle
# it in a single LLVM-IR bitcode file.
# Usage:
#     add_gpu_entrypoint_library(
#       DEPENDS <list of add_entrypoint_object targets>
#     )
function(add_bitcode_entrypoint_library target_name base_target_name)
  cmake_parse_arguments(
    "ENTRYPOINT_LIBRARY"
    "" # No optional arguments
    "" # No single value arguments
    "DEPENDS" # Multi-value arguments
    ${ARGN}
  )
  if(NOT ENTRYPOINT_LIBRARY_DEPENDS)
    message(FATAL_ERROR "'add_entrypoint_library' target requires a DEPENDS list "
                        "of 'add_entrypoint_object' targets.")
  endif()

  get_fq_deps_list(fq_deps_list ${ENTRYPOINT_LIBRARY_DEPENDS})
  get_all_object_file_deps(all_deps "${fq_deps_list}")

  set(objects "")
  foreach(dep IN LISTS all_deps)
    set(object $<$<STREQUAL:$<TARGET_NAME_IF_EXISTS:${dep}>,${dep}>:$<TARGET_OBJECTS:${dep}>>)
    list(APPEND objects ${object})
  endforeach()

  add_executable(${target_name} ${objects})
  target_link_options(${target_name} PRIVATE "${LIBC_COMPILE_OPTIONS_DEFAULT}"
                      "-r" "-nostdlib" "-flto" "-Wl,--lto-emit-llvm")
endfunction(add_bitcode_entrypoint_library)

# A rule to build a library from a collection of entrypoint objects.
# Usage:
#     add_entrypoint_library(
#       DEPENDS <list of add_entrypoint_object targets>
#     )
#
# NOTE: If one wants an entrypoint to be available in a library, then they will
# have to list the entrypoint target explicitly in the DEPENDS list. Implicit
# entrypoint dependencies will not be added to the library.
function(add_entrypoint_library target_name)
  cmake_parse_arguments(
    "ENTRYPOINT_LIBRARY"
    "" # No optional arguments
    "" # No single value arguments
    "DEPENDS" # Multi-value arguments
    ${ARGN}
  )
  if(NOT ENTRYPOINT_LIBRARY_DEPENDS)
    message(FATAL_ERROR "'add_entrypoint_library' target requires a DEPENDS list "
                        "of 'add_entrypoint_object' targets.")
  endif()

  get_fq_deps_list(fq_deps_list ${ENTRYPOINT_LIBRARY_DEPENDS})
  get_all_object_file_deps(all_deps "${fq_deps_list}")

  set(objects "")
  foreach(dep IN LISTS all_deps)
    list(APPEND objects $<$<STREQUAL:$<TARGET_NAME_IF_EXISTS:${dep}>,${dep}>:$<TARGET_OBJECTS:${dep}>>)
  endforeach(dep)

  add_library(
    ${target_name}
    STATIC
    ${objects}
  )
  set_target_properties(${target_name} PROPERTIES ARCHIVE_OUTPUT_DIRECTORY ${LIBC_LIBRARY_DIR})
endfunction(add_entrypoint_library)

# Rule to build a shared library of redirector objects.
function(add_redirector_library target_name)
  cmake_parse_arguments(
    "REDIRECTOR_LIBRARY"
    ""
    ""
    "DEPENDS"
    ${ARGN}
  )

  set(obj_files "")
  foreach(dep IN LISTS REDIRECTOR_LIBRARY_DEPENDS)
    # TODO: Ensure that each dep is actually a add_redirector_object target.
    list(APPEND obj_files $<TARGET_OBJECTS:${dep}>)
  endforeach(dep)

  # TODO: Call the linker explicitly instead of calling the compiler driver to
  # prevent DT_NEEDED on C++ runtime.
  add_library(
    ${target_name}
    EXCLUDE_FROM_ALL
    SHARED
    ${obj_files}
  )
  set_target_properties(${target_name}  PROPERTIES LIBRARY_OUTPUT_DIRECTORY ${LIBC_LIBRARY_DIR})
  target_link_libraries(${target_name}  -nostdlib -lc -lm)
  set_target_properties(${target_name}  PROPERTIES LINKER_LANGUAGE "C")
endfunction(add_redirector_library)

set(HDR_LIBRARY_TARGET_TYPE "HDR_LIBRARY")

# Internal function, used by `add_header_library`.
function(create_header_library fq_target_name)
  cmake_parse_arguments(
    "ADD_HEADER"
    "" # Optional arguments
    "" # Single value arguments
    "HDRS;DEPENDS;FLAGS;COMPILE_OPTIONS" # Multi-value arguments
    ${ARGN}
  )

  if(NOT ADD_HEADER_HDRS)
    message(FATAL_ERROR "'add_header_library' target requires a HDRS list of .h files.")
  endif()

  if(SHOW_INTERMEDIATE_OBJECTS)
    message(STATUS "Adding header library ${fq_target_name}")
    if(${SHOW_INTERMEDIATE_OBJECTS} STREQUAL "DEPS")
      foreach(dep IN LISTS ADD_HEADER_DEPENDS)
        message(STATUS "  ${fq_target_name} depends on ${dep}")
      endforeach()
    endif()
  endif()

  add_library(${fq_target_name} INTERFACE)
  target_sources(${fq_target_name} INTERFACE ${ADD_HEADER_HDRS})
  if(ADD_HEADER_DEPENDS)
    add_dependencies(${fq_target_name} ${ADD_HEADER_DEPENDS})

    # `*.__copied_hdr__` is created only to copy the header files to the target
    # location, not to be linked against.
    set(link_lib "")
    foreach(dep ${ADD_HEADER_DEPENDS})
      if (NOT dep MATCHES "__copied_hdr__")
        list(APPEND link_lib ${dep})
      endif()
    endforeach()

    target_link_libraries(${fq_target_name} INTERFACE ${link_lib})
  endif()
  if(ADD_HEADER_COMPILE_OPTIONS)
    target_compile_options(${fq_target_name} INTERFACE ${ADD_HEADER_COMPILE_OPTIONS})
  endif()
  set_target_properties(
    ${fq_target_name}
    PROPERTIES
      INTERFACE_FLAGS "${ADD_HEADER_FLAGS}"
      TARGET_TYPE "${HDR_LIBRARY_TARGET_TYPE}"
      DEPS "${ADD_HEADER_DEPENDS}"
      FLAGS "${ADD_HEADER_FLAGS}"
  )
endfunction(create_header_library)

# Rule to add header only libraries.
# Usage
#    add_header_library(
#      <target name>
#      HDRS  <list of .h files part of the library>
#      DEPENDS <list of dependencies>
#      FLAGS <list of flags>
#    )

function(add_header_library target_name)
  add_target_with_flags(
    ${target_name}
    CREATE_TARGET create_header_library
    ${ARGN}
  )
endfunction(add_header_library)