# Calculates offsets of fields in C structures for use in assembly.
template("generate_offsets") {
  # There are 3 targets involved:
  #  1. have the compiler calculate the offsets
  #  2. extract those offsets to a header file
  #  3. include the offsets in the build for validation
  obj_target = "${target_name}__obj"
  header_target = "${target_name}__header"
  validate_target = target_name

  # Have the compiler calculate the offsets and store that information in the
  # object file to extract later.
  source_set(obj_target) {
    forward_variables_from(invoker,
                           [
                             "cflags",
                             "cflags_c",
                             "defines",
                             "deps",
                             "public_deps",
                             "sources",
                             "testonly",
                           ])

    # Disable LTO so we get an object file.
    configs -= [ "//build:lto" ]
    defines = [ "GEN_OFFSETS" ]
    visibility = [ ":${header_target}" ]
  }

  # Extract the offset information we've had the compiler emit into the object
  # file.
  action_foreach("${header_target}") {
    forward_variables_from(invoker,
                           [
                             "sources",
                             "testonly",
                           ])
    script = "//build/image/extract_offsets.py"
    deps = [
      ":${obj_target}",
    ]
    args = [
      "--tool_prefix",
      arch_tool_prefix,
      "--input",
      rebase_path("${target_out_dir}/${obj_target}.{{source_name_part}}.o"),
      "--output",
      rebase_path("${root_gen_dir}/inc/{{source_name_part}}.h"),
    ]
    outputs = [
      "${root_gen_dir}/inc/{{source_name_part}}.h",
    ]
    visibility = [ ":${validate_target}" ]
  }

  # Include the offset source file in the build so the extracted offsets can be
  # validated.
  source_set(validate_target) {
    forward_variables_from(invoker,
                           [
                             "cflags",
                             "cflags_c",
                             "defines",
                             "deps",
                             "public_deps",
                             "sources",
                             "testonly",
                           ])

    deps = [
      ":${header_target}",
    ]
  }
}