For building v8 using gn on aix_ppc64, linux_s390x and linux_ppc64.

build/config/features.gni

 # 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.
 
 # =============================================
 #   PLEASE DO NOT ADD MORE FLAGS TO THIS FILE
 # =============================================
 #
 # These flags are effectively global. Your feature flag should go near the
 # code it controls. Most of these items are here now because they control
 # legacy global #defines passed to the compiler (now replaced with generated
 # buildflag headers -- see //build/buildflag_header.gni).
 #
 # There is more advice on where to put build flags in the "Build flag" section
 # of //build/config/BUILDCONFIG.gn.
 
 import("//build/config/chrome_build.gni")
 import("//build/config/chromecast_build.gni")
 if (is_android) {
   import("//build/config/android/config.gni")
 }
 
 declare_args() {
   # Enables Native Client support.
-  # Temporarily disable nacl on arm64 linux to get rid of compilation errors.
+  # Temporarily disable nacl on arm64 linux, ppc64 and s390x

[brettw, 2017/04/26 20:07:42]

I thought the changes you're doing is to support V8 and Node stuff. How are you
hitting NaCl stuff? Maybe I don't understand your overall goals for what you're
doing with the build.

[rayb, 2017/04/27 19:29:45]

Hmm, I see what you mean. Since IIUC, I shouldn't even be checking whether to
disable or enable nacl for building v8. But this (features.gni) gets imported
regardless of what I'm building via 'build/config/BUILD.gn'. And if I don't
disable this here, then this tries to  define a nacl_toolchain inside
'build/config/nacl/config.gni' and of course there is no such nacl_toolchain for
the aforementioned platforms (and pretty sure it doesn't make sense for us to
have one as well). That's why I'm disabling it so that the whole logic
reagarding nacl never kicks in. Even though,  'build/config/nacl/config.gni'
again does get imported regardless of what I'm building through
'build/config/compiler/BUILD.gn'. Notice that none of the mentioned '*.gni'
imports are guarded by anything.

Also yes, all the changes here are to build v8 using GN. Currently on the
mentioned platforms, gyp is used to build v8. I have already ported GN (on the
mentioned platforms) and this (along with the rest of the linked cls) are so
that we can build v8 using that GN. And of course GN relies on the already
existing infrastructure in place inside //build to create v8.

Sorry for the rather long reply. Please let me know if something is still
unclear or in case I am misunderstanding something. 

[brettw, 2017/04/27 20:45:52]

I don't quite follow.

build/config/compiler/BUILD.gn references build/config;nacl/config.gni but that
.gni file doesn't reference any targets or actually instantiate any toolchains.
Just having the nacl_toolchain variable defined shouldn't affect you at all.

I agree defining this is theoretically yucky, but shouldn't actually be blocking
you. For cleanliness I did https://codereview.chromium.org/2845943005/ which
should only do this import when compiling with the nacl toolchain.

But from what I'm understanding, not changing this file should still make the
build work OK. If it doesn't can you explain precisely what's broken? We should
fix that regardless.

+  # to get rid of compilation errors.
   # TODO(mcgrathr): When mipsel-nacl-clang is available, drop the exclusion.
-  enable_nacl = !is_ios && !is_android && !is_chromecast &&
-                current_cpu != "mipsel" && !(is_linux && target_cpu == "arm64")
+  enable_nacl =
+      !is_ios && !is_android && !is_chromecast && current_cpu != "mipsel" &&
+      !(is_linux && target_cpu == "arm64") && !is_power_or_z
 
   # Non-SFI is not yet supported on mipsel
   enable_nacl_nonsfi = current_cpu != "mipsel"
 
   # Enables proprietary codecs and demuxers; e.g. H264, AAC, MP3, and MP4.
   # We always build Google Chrome and Chromecast with proprietary codecs.
   #
   # Note: this flag is used by WebRTC which is DEPSed into Chrome. Moving it
   # out of //build will require using the build_overrides directory.
   proprietary_codecs = is_chrome_branded || is_chromecast
@@ skipping 25 matching lines @@
   use_gconf = is_linux && !is_chromeos && !is_chromecast
 
   use_gio = is_linux && !is_chromeos && !is_chromecast
 }
 #
 # =============================================
 #   PLEASE DO NOT ADD MORE FLAGS TO THIS FILE
 # =============================================
 #
 # See comment at the top.