generator/dawn_generator.gni

# Copyright 2019 The Dawn 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
#
#     http://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.

import("../scripts/dawn_overrides_with_defaults.gni")

###############################################################################
# Template to wrap the Dawn code generator
###############################################################################

# Template to help with invocation of the Dawn code generator, looks like this:
#
#   dawn_generator("my_target_gen") {
#     # Which generator target to output
#     target = "my_target"
#     # The list of expected outputs, generation fails if there's a mismatch
#     outputs = [
#       "MyTarget.cpp",
#       "MyTarget.h",
#     ]
#
#     # Optional, use a custom generated file directory.
#     custom_gen_dir = "${target_gen_dir}/.."
#   }
#
# Using the generated files is done like so:
#
#   shared_library("my_target") {
#     deps = [ ":my_target_gen "]
#     sources = get_target_outputs(":my_target_gen")
#   }
#
template("dawn_generator") {
  # The base arguments for the generator: from this dawn.json, generate this
  # target using templates in this directory.
  generator_args = [
    rebase_path("${dawn_root}/dawn.json", root_build_dir),
    "--wire-json",
    rebase_path("${dawn_root}/dawn_wire.json", root_build_dir),
    "--template-dir",
    rebase_path("${dawn_root}/generator/templates", root_build_dir),
    "--targets",
    invoker.target,
  ]

  # Use the Jinja2 version pulled from the DEPS file. We do it so we don't
  # have version problems, and users don't have to install Jinja2.
  jinja2_python_path = rebase_path("${dawn_jinja2_dir}/..")
  generator_args += [
    "--extra-python-path",
    jinja2_python_path,
  ]

  # Chooses either the default gen_dir or the custom one required by the
  # invoker. This allows moving the definition of code generators in different
  # BUILD.gn files without changing the location of generated file. Without
  # this generated headers could cause issues when old headers aren't removed.
  gen_dir = target_gen_dir
  if (defined(invoker.custom_gen_dir)) {
    gen_dir = invoker.custom_gen_dir
  }

  # For build parallelism GN wants to know the exact inputs and outputs of
  # action targets like we use for our code generator. We avoid asking the
  # generator about its inputs by using the "depfile" feature of GN/Ninja.
  #
  # A ninja limitation is that the depfile is a subset of Makefile that can
  # contain a single target, so we output a single "JSON-tarball" instead.
  json_tarball = "${gen_dir}/${target_name}.json_tarball"
  json_tarball_depfile = "${json_tarball}.d"

  generator_args += [
    "--output-json-tarball",
    rebase_path(json_tarball, root_build_dir),
    "--depfile",
    rebase_path(json_tarball_depfile, root_build_dir),
  ]

  # After the JSON tarball is created we need an action target to extract it
  # with a list of its outputs. The invoker provided a list of expected
  # outputs. To make sure the list is in sync between the generator and the
  # build files, we write it to a file and ask the generator to assert it is
  # correct.
  expected_outputs_file = "${gen_dir}/${target_name}.expected_outputs"
  write_file(expected_outputs_file, invoker.outputs)

  generator_args += [
    "--expected-outputs-file",
    rebase_path(expected_outputs_file, root_build_dir),
  ]

  # The code generator invocation that will write the JSON tarball, check the
  # outputs are what's expected and write a depfile for Ninja.
  action("${target_name}_json_tarball") {
    script = "${dawn_root}/generator/main.py"
    outputs = [
      json_tarball,
    ]
    depfile = json_tarball_depfile
    args = generator_args
  }

  # Extract the JSON tarball into the gen_dir
  action("${target_name}") {
    script = "${dawn_root}/generator/extract_json.py"
    args = [
      rebase_path(json_tarball, root_build_dir),
      rebase_path(gen_dir, root_build_dir),
    ]

    deps = [
      ":${target_name}_json_tarball",
    ]
    inputs = [
      json_tarball,
    ]

    # The expected output list is relative to the gen_dir but action
    # target outputs are from the root dir so we need to rebase them.
    outputs = []
    foreach(source, invoker.outputs) {
      outputs += [ "${gen_dir}/${source}" ]
    }
  }
}