[Cronet] Enforce ARMv7 Cronet doesn't inadvertently use ARM Neon instructions

components/cronet/android/BUILD.gn

 # Copyright 2015 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/buildflag_header.gni")
 import("//build/config/android/config.gni")
 import("//build/config/android/rules.gni")
 import("//build/util/version.gni")
 import("//chrome/version.gni")
 import("//testing/test.gni")
@@ skipping 805 matching lines @@
     rebase_path(depfile, root_build_dir),
     "--lib-java-dir",
     rebase_path("$root_build_dir/lib.java/components/cronet/android",
                 root_build_dir),
   ]
   deps = [
     ":cronet_javadoc_classpath",
   ]
 }
 
+action("enforce_no_neon") {
+  script = "//components/cronet/tools/enforce_no_neon.py"
+  args = [
+    rebase_path(
+        "$android_ndk_root/toolchains/arm-linux-androideabi-*/prebuilt/$host_os-x86_64/",

[mef, 2016/06/24 20:18:08]

this seems quite fragile. I wonder if there is a better way to specify current
toolchain.

[pauljensen, 2016/06/27 12:58:42]

Done, I fixed it up.

+        root_build_dir),
+
+    # libcronet.so may contain ARM Neon instructions from BoringSSL, but these
+    # are only used after checking whether the CPU supports NEON at runtime,
+    # so instead check base/ as it represents a large swath of code that only
+    # contains Neon instructions when Neon is enabled by default.
+    rebase_path("$root_out_dir/obj/base/base/*.o", root_build_dir),
+  ]
+  outputs = [
+    "$root_out_dir/cronet_$target_name.d",
+  ]
+  deps = [
+    "//base:base",
+  ]
+}
+
 copy("cronet_package_copy") {
   sources = [
     "$root_out_dir/lib.java/components/cronet/android/cronet_api.jar",
     "//AUTHORS",
     "//chrome/VERSION",
     "//components/cronet/android/proguard.cfg",
   ]
   outputs = [
     "$_package_dir/{{source_file_part}}",
   ]
@@ skipping 32 matching lines @@
     ":cronet_package_copy",
     ":cronet_package_copy_native_lib",
     ":cronet_package_copy_native_lib_unstripped",
     ":generate_javadoc",
     ":generate_licenses",
     ":jar_cronet_api_source",
     ":jar_cronet_other_source",
     ":jar_cronet_sample_source",
     ":repackage_extracted_jars",
   ]
+  if (current_cpu == "armv7" || current_cpu == "arm") {

[mef, 2016/06/24 20:18:08]

does arm (v6, not armv7) ever get neon?

[pauljensen, 2016/06/27 12:58:42]

No, NEON was introduced with ARMv7:
http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.dht0002a/BABIIFHA...

[mef, 2016/06/28 16:46:24]

So, why do we need to do it for current_cpu == "arm"?

[pauljensen, 2016/06/29 12:47:42]

Actually "arm" means all versions of ARM.  "armv7" looks like some old relic. 
I've redone this line.

+    deps += [ ":enforce_no_neon" ]
+  }
 }