ARM: Make use of d16-d31 when available.

tools/gyp/v8.gyp

 # Copyright 2012 the V8 project authors. All rights reserved.
 # Redistribution and use in source and binary forms, with or without
 # modification, are permitted provided that the following conditions are
 # met:
 #
 #     * Redistributions of source code must retain the above copyright
 #       notice, this list of conditions and the following disclaimer.
 #     * Redistributions in binary form must reproduce the above
 #       copyright notice, this list of conditions and the following
 #       disclaimer in the documentation and/or other materials provided
@@ skipping 140 matching lines @@
                 ['v8_target_arch=="arm"', {
                   # The following rules should be consistent with chromium's
                   # common.gypi and V8's runtime rule to ensure they all generate
                   # the same correct machine code. The following issue is about
                   # V8's runtime rule about vfpv3 and neon:
                   # http://code.google.com/p/v8/issues/detail?id=914
                   'conditions': [
                     ['armv7==1', {
                       # The ARM Architecture Manual mandates VFPv3 if NEON is
                       # available.
-                      # The current V8 doesn't use d16-d31, so for vfpv3-d16, we can
-                      # also enable vfp3 for the better performance.
+                      # V8 does not use d16-d31 unless explicitly enabled
+                      # (--enable_32dregs) or detected at run-time, so for vfpv3-d16,
+                      # we can also enable vfp3 for the better performance.
                       'conditions': [
                         ['arm_neon!=1 and arm_fpu!="vfpv3" and arm_fpu!="vfpv3-d16"', {
                           'variables': {
                             'mksnapshot_flags': [
                               '--noenable_vfp3',
                             ],
                           },
                         }],
                       ],
                     },{ # else: armv7!=1
@@ skipping 871 matching lines @@
             }],
           ],
           'dependencies': [
             'v8'
           ],
         },
       ],
     }],
   ],
 }