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

build/config/features.gni

Index: build/config/features.gni
diff --git a/build/config/features.gni b/build/config/features.gni
index d143dac4b89ce082835f225866a0e9f164ccc019..7bb8b6397cb30b9953c63fa32cbcb440b5f70ab6 100644
--- a/build/config/features.gni
+++ b/build/config/features.gni
@@ -22,10 +22,12 @@ if (is_android) {
 
 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"