[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..e08f2368ed63ef4318f9b1b711d5358b4188b6a2 100644
--- a/components/cronet/android/BUILD.gn
+++ b/components/cronet/android/BUILD.gn
@@ -909,16 +909,46 @@ copy("cronet_package_copy_native_lib_unstripped") {
   ]
 }
 
+# Enforce that ARM Neon is not used when building for ARMv7
+if (target_cpu == "arm" && arm_version == 7 && !arm_use_neon) {
+  action("enforce_no_neon") {
+    assert(!arm_use_neon)

[Dirk Pranke, 2016/07/27 16:43:33]

Given the if conditions, this assert is no longer useful.

[pauljensen, 2016/08/19 11:52:05]

Done.

+    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",
+    ]
+  }
+}
+
 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 including any deps in cronet_package target otherwise.
+  if (!(target_cpu == "arm" && arm_version == 7) || !arm_use_neon) {
+    deps = [

[Dirk Pranke, 2016/07/27 16:43:33]

Don't you want these deps to be set unconditionally?

[pauljensen, 2016/08/18 16:12:33]

No, that would allow building Cronet for ARMv7 with Neon, which is what I want
to disallow.

I need to allow generation of ninja files for cronet_package with arm_use_neon
set to true, but I need the builds to fail or produce nothing.  It was
recommended I add the assert(!arm_use_neon) to enforce_no_neon, which forced me
to choose the "produce nothing" option.

[Dirk Pranke, 2016/08/18 17:10:38]

Ah, got it.

+      ":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" ]
+    }
+  }
 }