build CrOS chrome with gn

build/toolchain/cros/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/toolchain/gcc_toolchain.gni")
 
+# CrOS builds must cross-compile on a Linux host for the actual CrOS
+# device target. There are many different CrOS devices so the build
+# system provides configuration variables that permit a CrOS build to
+# control the cross-compilation tool chain. However, requiring such
+# fine-grain specification is tedious for build-bots and developers.
+# Consequently, the CrOS build system defaults to a convenience
+# compilation mode where the compilation host is also the build target.
+#
+# Chrome can be compiled in this way with the gn variable:
+#
+# target_os = "chromeos"
+#
+# To perform a board-specific build, first obtain the correct system
+# root (http://goo.gl/aFB4XH) for the board. Then configure GN to use it
+# by setting appropriate cross-compilation variables.
+#
+# For example, to compile a Chrome source tree in /g/src for an
+# auron_paine CrOS device with the system root cached in /g/.cros_cache,
+# the following GN arguments must be provided to configure
+# cross-compilation with Goma acceleration. (NB: additional variables
+# will be necessary to successfully compile a working CrOS Chrome. See
+# the definition of GYP_DEFINES inside a sysroot shell.)
+#
+# goma_dir = "/g/.cros_cache/common/goma+2"
+# target_sysroot= /g/.cros_cache/chrome-sdk/tarballs/auron_paine+7644.0.0+sysroot_chromeos-base_chromeos-chrome.tar.xz"
+# cros_target_cc = "x86_64-cros-linux-gnu-gcc -B/g/.cros_cache/chrome-sdk/tarballs/auron_paine+7657.0.0+target_toolchain/usr/x86_64-pc-linux-gnu/x86_64-cros-linux-gnu/binutils-bin/2.25.51-gold"
+# cros_target_cxx = "x86_64-cros-linux-gnu-g++ -B/g/.cros_cache/chrome-sdk/tarballs/auron_paine+7657.0.0+target_toolchain/usr/x86_64-pc-linux-gnu/x86_64-cros-linux-gnu/binutils-bin/2.25.51-gold"
+# cros_target_ar = "x86_64-cros-linux-gnu-gcc-ar"
+# target_cpu = "x64"
+
 declare_args() {
-  # The CrOS build system supports many different kinds of targets across
-  # many different architectures. Bringing your own toolchain is also supported,
-  # so it's actually impossible to enumerate all toolchains for all targets
-  # as GN toolchain specifications.
-  # These arguments provide a mechanism for specifying your CC, CXX and AR at
-  # buildfile-generation time, allowing the CrOS build system to always use
-  # the right tools for the current target.
+  # These must be specified for a board-specific build.
   cros_target_cc = ""
   cros_target_cxx = ""
   cros_target_ar = ""
 }
 
-gcc_toolchain("target") {
-  assert(cros_target_cc != "", "Must provide target CC.")
-  assert(cros_target_cxx != "", "Must provide target CXX.")
-  assert(cros_target_ar != "", "Must provide target AR.")
+clang_toolchain("clang_x64") {

[Dirk Pranke, 2015/12/01 20:13:48]

is there really a reason to support both the _x64 and _x86 toolchains? It seems
like a board-specific build would need to set target_cpu explicitly, no? Is
there much advantage to supporting a 32-bit linux compilation target if it isn't
also board-specific ? 

[rjkroege, 2015/12/02 00:22:39]

Probably not? I have simplified the file.

+  toolchain_cpu = "x64"
+  toolchain_os = "linux"
+}
 
-  cc = "${cros_target_cc}"
-  cxx = "${cros_target_cxx}"
+gcc_toolchain("x64") {
+  # These defaults permit building on a Linux host as described above.
+  cc = "gcc"
+  cxx = "g++"
+  ar = "ar"
 
-  ar = "${cros_target_ar}"
+  # But to build for a specific board, the cros_* args will need to be defined.
+  if (cros_target_cc != "") {
+    cc = "${cros_target_cc}"
+  }
+  if (cros_target_cxx != "") {
+    cxx = "${cros_target_cxx}"
+  }
+  if (cros_target_ar != "") {
+    ar = "${cros_target_ar}"
+  }
   ld = cxx
 
-  toolchain_cpu = "${target_cpu}"
+  toolchain_cpu = "x64"
   toolchain_os = "linux"
   is_clang = is_clang
   use_ccache = false
-  use_goma = false
 }
+
+clang_toolchain("clang_x86") {
+  toolchain_cpu = "x86"
+  toolchain_os = "linux"
+}
+
+gcc_toolchain("x86") {
+  cc = "gcc"
+  cxx = "g++"
+
+  readelf = "readelf"
+  nm = "nm"
+  ar = "ar"
+  ld = cxx
+
+  # To build for a specific board, the cros_* args will need to be defined.
+  if (cros_target_cc != "") {
+    cc = "${cros_target_cc}"
+  }
+  if (cros_target_cxx != "") {
+    cxx = "${cros_target_cxx}"
+  }
+  if (cros_target_ar != "") {
+    ar = "${cros_target_ar}"
+  }
+
+  toolchain_cpu = "x86"
+  toolchain_os = "linux"
+  is_clang = false
+}