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

components/cronet/android/BUILD.gn

Index: components/cronet/android/BUILD.gn
diff --git a/components/cronet/android/BUILD.gn b/components/cronet/android/BUILD.gn
index 48b91046012f32cc836c44a5f4da01d75150ab00..104024b15c21098bd4de7cf03649dc11af3cbfc4 100644
--- a/components/cronet/android/BUILD.gn
+++ b/components/cronet/android/BUILD.gn
@@ -823,6 +823,27 @@ action("generate_javadoc") {
   ]
 }
 
+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",
@@ -875,4 +896,7 @@ group("cronet_package") {
     ":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" ]
+  }
 }