Fix the toolchain used to build the snapshots in GN.

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.
 
 # Because standalone V8 builds are not supported, assume this is part of a
 # Chromium build.
 import("//build/module_args/v8.gni")
 
 # TODO(jochen): These will need to be user-settable to support standalone V8
 # builds.
 v8_deprecation_warnings = false
 v8_enable_disassembler = false
 v8_enable_gdbjit = false
 v8_enable_handle_zapping = true
 v8_enable_i18n_support = true
 v8_enable_verify_heap = false
 v8_interpreted_regexp = false
 v8_object_print = false
 v8_postmortem_support = false
 v8_use_snapshot = true
+
+# TODO(GYP): We should be just target_cpu and not need v8_target_arch.

[Nico, 2015/03/11 13:47:48]

this reads grammared incorrectly

[jochen (gone - plz use gerrit), 2015/03/11 16:10:30]

also, we want to be able to e.g. compile for linux x64 as target_cpu but use
v8_target_arch arm64. This will run v8 with a simulator, allowing for better
asan coverage.

[Dirk Pranke, 2015/03/11 16:53:30]

Will fix :).
Sure.

 v8_target_arch = current_cpu
+
 v8_random_seed = "314159265"
 v8_toolset_for_d8 = "host"
 
+# The snapshot needs to be compiled for the host, but compiled with
+# a toolchain that matches the bit-width of the target.
+# TODO(GYP): For now we only support 32-bit little-endian builds from
+# an x86/x64 Linux host. We need to support the other architectures as well.
+if (target_os == "android" || target_os == "chromeos") {

[jochen (gone - plz use gerrit), 2015/03/11 16:10:30]

why this restriction?

[Dirk Pranke, 2015/03/11 16:53:30]

I'm not sure I understand the question. Which restriction are you referring to? 
The host must be linux restriction is there mostly to be sure we understand what
configurations are actually being called (i.e., I don't expect trying to build
android v8 from windows to work at this point, so I don't want people to try it
and spend time debugging weird things).

I'm happy to restructure it if you like.

[jochen (gone - plz use gerrit), 2015/03/11 18:32:28]

ok, maybe add a comment then that this is supposed to work for all target and
host os combinations that are supported by v8 (or chrome for that matter)

[Dirk Pranke, 2015/03/11 18:56:10]

That was the intent of the comment on lines 30-31, but perhaps it's not
completely clear.

I will reword it a bit.

[jochen (gone - plz use gerrit), 2015/03/11 18:58:14]

ah, that's probably enough. Maybe architectures/OSs?

+  assert(host_os == "linux" &&
+         (host_cpu == "x86" || host_cpu == "x64") &&
+         (target_cpu == "arm" || target_cpu == "mipsel" || target_cpu == "x86"))
+  snapshot_toolchain = "//build/toolchain/linux:clang_x86"
+} else {
+  snapshot_toolchain = default_toolchain
+}
+
 ###############################################################################
 # Configurations
 #
 config("internal_config") {
   visibility = [ ":*" ]  # Only targets in this file can depend on this.
 
   include_dirs = [ "." ]
 
   if (component_mode == "shared_library") {
     defines = [
@@ skipping 280 matching lines @@
   ]
 
   args = rebase_path(outputs, root_build_dir) +
          rebase_path(sources, root_build_dir)
 }
 
 action("run_mksnapshot") {
   visibility = [ ":*" ]  # Only targets in this file can depend on this.
 
   deps = [
-    ":mksnapshot($host_toolchain)",
+    ":mksnapshot($snapshot_toolchain)",
   ]
 
   script = "tools/run.py"
 
   outputs = [
     "$target_gen_dir/snapshot.cc",
   ]
 
   args = [
-    "./" + rebase_path(get_label_info(":mksnapshot($host_toolchain)",
+    "./" + rebase_path(get_label_info(":mksnapshot($snapshot_toolchain)",
                                       "root_out_dir") + "/mksnapshot",
                        root_build_dir),
     "--log-snapshot-positions",
     "--logfile",
     rebase_path("$target_gen_dir/snapshot.log", root_build_dir),
     rebase_path("$target_gen_dir/snapshot.cc", root_build_dir),
   ]
 
   if (v8_random_seed != "0") {
     args += [
@@ skipping 1075 matching lines @@
 
   deps = [
     ":v8_libbase",
   ]
 }
 
 ###############################################################################
 # Executables
 #
 
-if (current_toolchain == host_toolchain) {
+if (current_toolchain == snapshot_toolchain) {
   executable("mksnapshot") {
     visibility = [ ":*" ]  # Only targets in this file can depend on this.
 
     sources = [
       "src/mksnapshot.cc",
     ]
 
     configs -= [ "//build/config/compiler:chromium_code" ]
     configs += [ "//build/config/compiler:no_chromium_code" ]
     configs += [
@@ skipping 115 matching lines @@
       sources += [
         "src/d8-debug.cc",
         "$target_gen_dir/d8-js.cc",
       ]
     }
     if (v8_enable_i18n_support) {
       deps += [ "//third_party/icu" ]
     }
   }
 }