[gn] Use external startup data by default

BUILD.gn

 # Copyright 2014 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/config/android/config.gni")
 import("//build/config/arm.gni")
 import("//build/config/mips.gni")
 import("//build/config/sanitizers/sanitizers.gni")
 
 if (is_android) {
   import("//build/config/android/rules.gni")
 }
 
 # Because standalone V8 builds are not supported, assume this is part of a
 # Chromium build.
 import("//build_overrides/v8.gni")
 
 import("snapshot_toolchain.gni")
 
 declare_args() {
   # Enable the snapshot feature, for fast context creation.
   # http://v8project.blogspot.com/2015/09/custom-startup-snapshots.html
   v8_use_snapshot = true
 
+  # Use external files for startup data blobs:
+  # the JS builtins sources and the start snapshot.
+  v8_use_external_startup_data = true

[Michael Achenbach, 2016/05/12 08:06:33]

Couldn't find where chromium sets this. Are we maybe not running with external
snapshot at all?

[Michael Achenbach, 2016/05/12 09:24:31]

OK. Found it:
https://code.google.com/p/chromium/codesearch#chromium/src/build_overrides/v8...

But we should still set it to true so that we also use the default in
stand-alone.

[vogelheim, 2016/05/12 12:35:10]

Agreed!

+
   # Sets -DVERIFY_HEAP.
   v8_enable_verify_heap = false
 
   # Enable compiler warnings when using V8_DEPRECATED apis.
   v8_deprecation_warnings = false
 
   # Enable compiler warnings when using V8_DEPRECATE_SOON apis.
   v8_imminent_deprecation_warnings = false
 
   # Sets -dENABLE_DISASSEMBLER.
@@ skipping 567 matching lines @@
 action("run_mksnapshot") {
   visibility = [ ":*" ]  # Only targets in this file can depend on this.
 
   deps = [
     ":mksnapshot($snapshot_toolchain)",
   ]
 
   script = "tools/run.py"
 
   outputs = [
     "$target_gen_dir/snapshot.cc",

[Michael Achenbach, 2016/05/12 08:06:33]

Isn't it wrong to have this output spec when v8_use_external_startup_data ==
false?

[vogelheim, 2016/05/12 12:35:10]

Not wrong, but superflous? The way the rule is written (see the args section
below) it always instructs mksnapshot to write the snapshot.cc file, so I guess
it's correct to unconditionally declare it as the output.

I guess the rule _could_ be written differently, so that it will only generate
the file that is used. E.g. by turning the if(v8_use_external_startup_data)...
into if-else.

(Wouldn't make a performance difference, though. The work done my mksnapshot is
mostly the same, regardless out output file.)

[Michael Achenbach, 2016/05/12 14:09:16]

Acknowledged.

   ]
 
   args = [
     "./" + rebase_path(get_label_info(":mksnapshot($snapshot_toolchain)",
                                       "root_out_dir") + "/mksnapshot",
                        root_build_dir),
     "--startup_src",
     rebase_path("$target_gen_dir/snapshot.cc", root_build_dir),
   ]
 
@@ skipping 1580 matching lines @@
   ]
 
   deps = [
     ":fuzzer_support",
   ]
 
   configs = [
     ":internal_config",
   ]
 }