[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 851 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") {
+  assert(!arm_use_neon)
+  script = "//components/cronet/tools/check_no_neon.py"
+  args = [
+    rebase_path("${android_tool_prefix}objdump", root_build_dir),
+
+    # libcronet.so may contain ARM Neon instructions from BoringSSL, but these
+    # are only used after checking whether the CPU supports NEON at runtime,
+    # so instead check base/ as it represents a large swath of code that only
+    # contains Neon instructions when Neon is enabled by default.
+    rebase_path("$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 20 matching lines @@
     "$root_out_dir/lib.unstripped/libcronet.so",
   ]
   outputs = [
     "$_package_dir/symbols/${android_app_abi}/libcronet.so",
   ]
   deps = [
     ":cronet",
   ]
 }
 
-group("cronet_package") {
-  deps = [
-    ":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",
-  ]
+# Enforce that arm_use_neon==false when building for ARMv7 by
+# not generating the cronet_package target otherwise.
+if (!(target_cpu == "arm" && arm_version == 7) || !arm_use_neon) {
+  group("cronet_package") {

[Dirk Pranke, 2016/07/27 00:01:49]

It seems like you generally want cronet_package to be defined and built, but you
want arm_use_neon=false when (target_cpu == "arm" && arm_version == 7), right?

So, the way I'd do this would be to leave the group defined unconditionally, and
conditionally add the dep as you're doing on lines 947-949.

You will also want :enforce_no_neon to only be defined if current_cpu == "arm"
&& arm_version == 7.

Does that make sense (and work)?

[pauljensen, 2016/07/27 12:11:46]

Done.

+    deps = [
+      ":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 == "arm" && arm_version == 7) {
+      deps += [ ":enforce_no_neon" ]
+    }
+  }
 }