Add support for multi-architecture application bundle on iOS.

build/config/BUILDCONFIG.gn

Index: build/config/BUILDCONFIG.gn
diff --git a/build/config/BUILDCONFIG.gn b/build/config/BUILDCONFIG.gn
index 4f3a223c32d23ab7b79007add5081deac14bdf7e..a6018a95a10aeea0b0e7e9532ad4d5e849358818 100644
--- a/build/config/BUILDCONFIG.gn
+++ b/build/config/BUILDCONFIG.gn
@@ -2,6 +2,15 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
+if (target_os == "ios") {
+  declare_args() {
+    # If defined, then the build is a multi-architecture build (aka fat build)
+    # and target_cpu must not be defined. Any value supported by target_cpu can
+    # be used in this list.
+    multi_arch_target_cpus = []

[brettw, 2016/07/11 23:57:17]

[read the next comment first]

I'm trying *really* hard to keep stuff out of this file. This case I'm more
sympathetic to than many because this does seem like global build configuration,
but the only reason it's needed here (if you take my advice below to move the
toolchain list) is for the target_cpu variable.

At first I was thinking of making this variable in rules.gni, and then asserting
that multi_arch_target_cpus[0] == target_cpu. But that's annoying.

What do you think about making "additional_target_cpus" and then using
target_cpu + these to define the multi_arch_toolchains. This means you don't
have the annoying logic for making sure only one is used, target_cpu is always
well-defined to be the default main thing you set, and we can remove this code
from this file :)

[sdefresne, 2016/07/12 09:13:33]

Done.

+  }
+}
+
 # =============================================================================
 # PLATFORM SELECTION
 # =============================================================================
@@ -45,6 +54,18 @@ if (target_cpu == "") {
     # This allows us to not have to specify both target_os and target_cpu
     # on the command line.
     target_cpu = "arm"
+  } else if (target_os == "ios") {
+    # If we're building for iOS, we respect the multi_arch_target_cpus if
+    # it is defined. In that case, we pick the first architecture listed in
+    # multi_arch_target_cpus and use it as target_cpu, the others will be
+    # build as secondary toolchain by the ios_app_bundle target.
+    if (multi_arch_target_cpus != []) {
+      assert(target_cpu == "",
+             "cannot define both multi_arch_target_cpus and target_cpu")
+      target_cpu = multi_arch_target_cpus[0]
+    } else {
+      target_cpu = host_cpu
+    }
   } else {
     target_cpu = host_cpu
   }
@@ -214,6 +235,20 @@ if (target_os == "android") {
   }
 } else if (target_os == "ios") {
   _default_toolchain = "//build/toolchain/mac:ios_clang_$target_cpu"
+  assert(custom_toolchain == "" || multi_arch_target_cpus == [],
+         "cannot define both multi_arch_target_cpus and custom_toolchain")
+
+  # Initialize multi_arch_toolchains from multi_arch_target_cpus. The
+  # list will not contain the default_toolchain and may be empty while
+  # multi_arch_target_cpus is not. This allows the build to default to
+  # regular build if multi_arch_target_cpus contains a single cpu.
+  multi_arch_toolchains = []

[brettw, 2016/07/11 23:57:17]

Can this code by put into build/config/ios/rules.gni or perhaps a new .gni file
for this purpose?> I want to keep all non-essential logic out of BUILDCONFIG.

[sdefresne, 2016/07/12 09:13:33]

This will duplicate the "//build/toolchain/mac:ios_clang_$_cpu" string, but
probably minor regarding modifying this file. Done.

+  foreach(_multi_arch_target_cpu, multi_arch_target_cpus) {
+    if (_multi_arch_target_cpu != target_cpu) {
+      multi_arch_toolchains +=
+          [ "//build/toolchain/mac:ios_clang_$_multi_arch_target_cpu" ]
+    }
+  }
 } else if (target_os == "linux") {
   if (is_clang) {
     _default_toolchain = "//build/toolchain/linux:clang_$target_cpu"