[gn] Add gn configs for mips to match old behavior

BUILD.gn

Index: BUILD.gn
diff --git a/BUILD.gn b/BUILD.gn
index 4e2c2b9aacb7fb93d69c0ff84cce099e0e81a49a..5c5672c3a78aa28b4c46f305815678d0de79f29d 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -236,6 +236,13 @@ config("toolchain") {
     defines += [ "V8_TARGET_ARCH_ARM64" ]
   }
 
+  # Mips64el/mipsel simulators. Comparing target cpu with v8 target cpu to
+  # not affect simulator builds for making cross-compile snapshots.
+  if (target_cpu != v8_target_cpu &&

[Michael Achenbach, 2016/09/09 12:32:24]

CC also Dirk. Want to make sure that this expresses the right thing.

target_cpu != v8_target_cpu makes sure we have a simulator build as the target.
v8_current_cpu == "mipsel" e.g. makes sure we simulate mipsel.

I assume if I had used current_cpu != v8_current_cpu, it would also match a
mipsel simulator build in the snapshot toolchain on the host when we
crosscompile (which would be wrong).

[Dirk Pranke, 2016/09/09 20:00:45]

I'm not sure I understand the intent of this block. You want the define set when
doing a simulator build but *not* when generating the snapshot for the
simulator?

[Michael Achenbach, 2016/09/12 08:11:29]

Close. We want to set the define when the _target_ is a simulator build. We also
want to set it for creating the snapshot for such a build.

We don't want to set it when making the snapshot for a cross-compile target.

Making snapshot for cross-compile and other simulator builds can't be
distinguished by the toolchain. In both cases we'd have e.g. current_cpu="x86"
and v8_current_cpu="mipsel". The only way to distinguish I see is using target_X
for comparison.

This is simulating the old gyp behavior expressed with the _toolset=="target"
condition. E.g. here:
https://cs.chromium.org/chromium/src/v8/gypfiles/toolchain.gypi?q=_MIPS_TARGE...

[Dirk Pranke, 2016/09/12 21:19:44]

Got it, right.

Maybe create a separate top-level variable (not declare_arg(), just a regular
variable) called is_simulator_build or something, just to be a bit clearer.

[Michael Achenbach, 2016/09/13 07:12:56]

Done

+      (v8_current_cpu == "mipsel" || v8_current_cpu == "mips64el")) {

[Michael Achenbach, 2016/09/09 09:54:27]

We set this for target simulator build in gyp here:
https://cs.chromium.org/chromium/src/v8/gypfiles/toolchain.gypi?q=_MIPS_TARGE...

Does this matter at all for snapshot generation?

_MIPS_TARGET_HW seems to be dead code.

[vogelheim, 2016/09/09 10:56:37]

Might matter.

(This affects code generation, and thus might affect the code that ends up in
the snapshot. So if the snapshot code will later be run in the simulator (or
not), this might make a difference.)

+      defines += [ "_MIPS_TARGET_SIMULATOR" ]
+  }
+
   # TODO(jochen): Add support for mips.
   if (v8_current_cpu == "mipsel") {
     defines += [ "V8_TARGET_ARCH_MIPS" ]