Clarify arm gn configuration

BUILD.gn

 # Copyright 2014 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/config/android/config.gni")
 import("//build/config/arm.gni")
 import("//build/config/mips.gni")
 
 # Because standalone V8 builds are not supported, assume this is part of a
 # Chromium build.
@@ skipping 106 matching lines @@
   # TODO(jochen): Add support for arm subarchs, mips, mipsel.
 
   if (v8_target_arch == "arm") {
     defines += [ "V8_TARGET_ARCH_ARM" ]
     if (current_cpu == "arm") {
       if (arm_version == 7) {
         defines += [ "CAN_USE_ARMV7_INSTRUCTIONS" ]
       }
       if (arm_fpu == "vfpv3-d16") {
         defines += [ "CAN_USE_VFP3_INSTRUCTIONS" ]
-      }
-      if (arm_fpu == "vfpv3") {
+      } else if (arm_fpu == "vfpv3") {
         defines += [
           "CAN_USE_VFP3_INSTRUCTIONS",
           "CAN_USE_VFP32DREGS",
         ]
-      }
-      if (arm_fpu == "neon") {
+      } else if (arm_fpu == "neon") {
         defines += [
           "CAN_USE_VFP3_INSTRUCTIONS",
           "CAN_USE_VFP32DREGS",
           "CAN_USE_NEON",
         ]
       }
     } else {
+      # These defines ares used for the ARM simulator.

[jochen (gone - plz use gerrit), 2015/03/13 07:42:51]

it's not just for the snapshot toolchain - we could target_cpu == x86 and
v8_target_arch == arm

[Dirk Pranke, 2015/03/13 20:47:17]

Um, okay ... I guess I don't understand what this is fully yet, and need to look
into where else (or how else) v8_target_arch is used.

       defines += [
         "CAN_USE_ARMV7_INSTRUCTIONS",
         "CAN_USE_VFP3_INSTRUCTIONS",
         "CAN_USE_VFP32DREGS",
         "USE_EABI_HARDFLOAT=0",
       ]
     }
 
     # TODO(jochen): Add support for arm_test_noprobe.
   }
@@ skipping 1430 matching lines @@
       sources += [
         "src/d8-debug.cc",
         "$target_gen_dir/d8-js.cc",
       ]
     }
     if (v8_enable_i18n_support) {
       deps += [ "//third_party/icu" ]
     }
   }
 }