[clang] Use -Wshorten-64-to-32 to enable warnings about 64bit to 32bit truncations.

build/standalone.gypi

 # 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 225 matching lines @@
         ],
         'conditions': [
           ['os_posix == 1 and OS != "mac"', {
             # We don't want to get warnings from third-party code,
             # so remove any existing warning-enabling flags like -Wall.
             'cflags!': [
               '-pedantic',
               '-Wall',
               '-Werror',
               '-Wextra',
+              '-Wshorten-64-to-32',
             ],
             'cflags+': [
               # Clang considers the `register` keyword as deprecated, but
               # ICU uses it all over the place.
               '-Wno-deprecated-register',
               # ICU uses its own deprecated functions.
               '-Wno-deprecated-declarations',
               # ICU prefers `a && b || c` over `(a && b) || c`.
               '-Wno-logical-op-parentheses',
               # ICU has some `unsigned < 0` checks.
@@ skipping 126 matching lines @@
           '-Wno-long-long',
           '-pthread',
           '-fno-exceptions',
           '-pedantic',
           # Don't warn about the "struct foo f = {0};" initialization pattern.
           '-Wno-missing-field-initializers',
         ],
         'cflags_cc': [ '-Wnon-virtual-dtor', '-fno-rtti', '-std=gnu++0x' ],
         'ldflags': [ '-pthread', ],
         'conditions': [
+          # TODO(arm64): It'd be nice to enable this for arm64 as well,
+          # but the Assembler requires some serious fixing first.
+          [ 'clang==1 and v8_target_arch=="x64"', {
+            'cflags': [ '-Wshorten-64-to-32' ],
+          }],
           [ 'host_arch=="ppc64" and OS!="aix"', {
             'cflags': [ '-mminimal-toc' ],
           }],
           [ 'visibility=="hidden" and v8_enable_backtrace==0', {
             'cflags': [ '-fvisibility=hidden' ],
           }],
           [ 'component=="shared_library"', {
             'cflags': [ '-fPIC', ],
           }],
         ],
@@ skipping 228 matching lines @@
     ['use_goma==1 and ("<(GENERATOR)"=="ninja" or clang==1)', {
       'make_global_settings': [
        ['CC_wrapper', '<(gomadir)/gomacc'],
        ['CXX_wrapper', '<(gomadir)/gomacc'],
        ['CC.host_wrapper', '<(gomadir)/gomacc'],
        ['CXX.host_wrapper', '<(gomadir)/gomacc'],
       ],
     }],
   ],
 }