Scripts for unity/jumbo (default disabled) compilation

build/config/jumbo.gni

+# Copyright 2017 The Chromium Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+import("//build/split_static_library.gni")  # When someone uses that target_type
+
+declare_args() {
+  # If true, use a jumbo build (files compiled together) to speed up
+  # compilation.
+  use_jumbo_build = false
+
+  # A target to exclude from jumbo builds, for optimal round trip time
+  # when frequently changing a single cpp file.
+  jumbo_build_excluded = ""
+
+  # How many files to group at most. Smaller numbers give more
+  # parallellism, higher numbers give less total CPU usage. Higher
+  # numbers also give longer single-file recompilation times.
+  #
+  # Recommendations:
+  # Higher numbers than 200 does not reduce wall clock compile times
+  # for 4 cores or less.
+  # 200 uses 8% less total CPU than 100 when compiling content and 10%
+  # less wall clock when compiling with 4 cores.
+  jumbo_file_merge_limit = 200
+}
+
+# Use this to generate a target which merges sources if possible to
+# compile much faster.
+#
+# Special values.
+#
+#   target_type
+#      The kind of target to build. For example the string
+#      "static_library".
+#
+#   always_build_jumbo
+#      If set and set to true, then use jumbo compile even when it is
+#      globally disabled. Otherwise it has no effect.
+#
+#   never_build_jumbo
+#      If set and set to true, then do not jumbo compile even if it is
+#      globally enabled. Otherwise it has no effect.
+#
+#   jumbo_excluded_sources
+#      If set to a list of files, those files will not be merged with
+#      the rest. This can be necessary if merging the files causes
+#      compilation issues and fixing the issues is impractical.
+template("jumbo_target") {
+  use_jumbo_build_for_target = use_jumbo_build
+  if (defined(invoker.always_build_jumbo) && invoker.always_build_jumbo) {
+    use_jumbo_build_for_target = true
+  }
+  if (defined(invoker.never_build_jumbo) && invoker.never_build_jumbo) {
+    use_jumbo_build_for_target = false
+  }
+  if (target_name == jumbo_build_excluded) {
+    use_jumbo_build_for_target = false
+  }
+
+  excluded_sources = []
+  if (defined(invoker.jumbo_excluded_sources)) {
+    excluded_sources += invoker.jumbo_excluded_sources
+  }
+
+  invoker_sources = invoker.sources
+  gen_target_dir = get_path_info(invoker_sources[0], "gen_dir")
+  assert(excluded_sources != [] || true)  # Prevent "unused variable".
+  assert(gen_target_dir != "")  # Prevent "unused variable".
+
+  if (use_jumbo_build_for_target) {
+    jumbo_files = []
+
+    # Split the sources list into chunks that are not excessively large
+    files_per_chunk = jumbo_file_merge_limit
+    current_file_index = 0
+    next_chunk_start = 0
+    next_chunk_number = 1
+    foreach(source_file, invoker.sources) {
+      if (current_file_index == next_chunk_start) {
+        jumbo_files += [ "$gen_target_dir/" + target_name + "_jumbo_" +
+                         next_chunk_number + ".cc" ]
+        next_chunk_number += 1
+        next_chunk_start += files_per_chunk
+      }
+      current_file_index += 1
+    }
+
+    merge_action_name = target_name + "__jumbo_merge"
+
+    # Create an action that calls a script that merges all the source files.
+    action(merge_action_name) {
+      script = "//build/config/merge_for_jumbo.py"
+      response_file_contents =
+          rebase_path(invoker.sources - excluded_sources, gen_target_dir)
+      outputs = jumbo_files
+      args = [ "--outputs" ] + rebase_path(outputs, root_build_dir) +
+             [ "--file-list={{response_file_name}}" ]
+    }
+  }
+
+  target_type = invoker.target_type
+  if (use_jumbo_build_for_target && target_type == "split_static_library") {
+    # Meaningless and also impossible if split_count > len(jumbo_files)
+    target_type = "static_library"
+
+    # Prevent "unused variable" warning.
+    assert(!defined(invoker.split_count) || invoker.split_count > 0)
+  }
+
+  # Perform the actual operation, either on the original sources or
+  # the sources post-jumbo merging.
+  target(target_type, target_name) {
+    deps = []
+    if (defined(invoker.deps)) {
+      deps += invoker.deps
+    }
+
+    # Take everything else not handled above from the invoker.
+    variables_to_not_forward = [ "deps" ]
+    if (use_jumbo_build_for_target) {
+      deps += [ ":" + merge_action_name ]
+      variables_to_not_forward += [ "sources" ]
+      assert(jumbo_files != [])
+      sources = jumbo_files + excluded_sources
+
+      # Need to keep the headers in sources so that dependency checks
+      # work, and we need to keep Objective-C code since they
+      # cannot be merged into a cc file (FIXME).
+      foreach(source_file, invoker.sources) {
+        source_ext = get_path_info(source_file, "extension")
+        if (source_ext == "h" || source_ext == "mm") {
+          sources += [ source_file ]
+        }
+      }
+    }
+    forward_variables_from(invoker, "*", variables_to_not_forward)
+  }
+}
+
+set_defaults("jumbo_target") {
+  # This sets the default list of configs when the content_source_set target
+  # is defined. The default_compiler_configs comes from BUILDCONFIG.gn and
+  # is the list normally applied to static libraries and source sets.
+  configs = default_compiler_configs
+}