[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 170bb4c7f3885d6d1c56b6e34efb54a2bdee56f5..170812c5f84b0ef5bf5c863d849af74a0d4a8c2e 100644
--- a/components/cronet/android/BUILD.gn
+++ b/components/cronet/android/BUILD.gn
@@ -869,6 +869,26 @@ action("generate_javadoc") {
   ]
 }
 
+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",
@@ -909,16 +929,23 @@ copy("cronet_package_copy_native_lib_unstripped") {
   ]
 }
 
-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" ]
+    }
+  }
 }