build/toolchain/offset_size_header.gni

# Copyright 2019 The Hafnium Authors.
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
#     https://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.

# This template auto-generate a C header file with "#define" constants, e.g.
# struct sizes and member offsets.
#
# It uses a trick similar to other projects, e.g. Linux, where the integer
# constant is used as an immediate in an inline assembly block. The source file
# is compiled and the constant extracted by a script which generates the header
# file. For easy grep-ing, the constant is compiled into a '.ascii' string,
# surrounded by magic strings, and extracted using the 'strings' binutils tool.
#
# To guarantee correctness, the same source file is compiled again as part
# of the parent target but this time the declarations are converted to
# static_asserts to check the values at its compile-time.
template("offset_size_header") {
  target_lib = "${target_name}__lib"
  target_header = "${target_name}__header"

  # Compile source files into binaries that contain strings with definitions
  # of constants.
  static_library(target_lib) {
    forward_variables_from(invoker,
                           [
                             "sources",
                             "deps",
                             "test_only",
                           ])
    defines = [ "GENERATE_BINARY" ]

    # Disable LTO to force emitting assembly.
    cflags = [ "-fno-lto" ]
  }

  # Extract strings from the static library, parse definitions and generate
  # a header file.
  action(target_header) {
    forward_variables_from(invoker, [ "test_only" ])
    lib_file = "${target_out_dir}/${target_lib}.a"
    out_file = "${root_gen_dir}/offset_size_header/${invoker.path}"

    script = "//build/toolchain/gen_offset_size_header.py"
    args = [
      rebase_path(lib_file, root_build_dir),
      rebase_path(out_file, root_build_dir),
    ]
    deps = [
      ":$target_lib",
    ]
    outputs = [
      out_file,
    ]
  }

  # This source_set will be compiled into the target that depends on this one.
  # This generates static_asserts which check the constants in the generated
  # header against compile-time structs.
  source_set(target_name) {
    forward_variables_from(invoker,
                           [
                             "sources",
                             "test_only",
                           ])
    cflags = [
      "-include",
      invoker.path,
    ]
    defines = [ "VERIFY_HEADER" ]
    deps = [
      ":$target_header",
    ]
  }
}