[gn] Switch arm sim to gn

[gn] Switch arm sim to gn

BUG=chromium:474921

[Michael Achenbach, 2016-07-22 09:50:40]

Description was changed from

==========
[gn] Switch arm sim to gn

BUG=
==========

to

==========
[gn] Switch arm sim to gn

BUG=chromium:474921
==========

[Michael Achenbach, 2016-07-22 09:50:54]

machenbach@chromium.org changed reviewers:
+ jochen@chromium.org, vogelheim@chromium.org

[Michael Achenbach, 2016-07-22 09:53:49]

Observations from
https://build.chromium.org/p/client.v8.ports/builders/V8%20Linux%20-%20arm%20...
for v8_base:

  defines differ:
    In gyp, but not in GN:
      -DUSE_EABI_HARDFLOAT=0

    In GN, but not in gyp:
      -mfpmath=sse
      -mmmx
      -momit-leaf-frame-pointer
      -msse2

What is the impact of those?

[jochen (gone - plz use gerrit), 2016-07-22 12:37:23]

gyp used to default to softfp, but gn defaults on non-android to hardfp for arm
7 (it defaults to softfp to arm6)

not sure what to do here: change gn's default to softfp? override the arm_eabi
for the simulator build explicitly? just go with hardfp?

[Michael Achenbach, 2016-07-25 09:30:28]

On 2016/07/22 12:37:23, jochen wrote:
> gyp used to default to softfp, but gn defaults on non-android to hardfp for
arm
> 7 (it defaults to softfp to arm6)
> 
> not sure what to do here: change gn's default to softfp? override the arm_eabi
> for the simulator build explicitly? just go with hardfp?

Is it essential to get simualtor coverage for hard and soft? Then we probably
need separate bots... We set hardfp explicitly on our arm cros tests, so this
looks like we might loose coverage for softfp.

[jochen (gone - plz use gerrit), 2016-07-25 09:34:30]

On 2016/07/25 at 09:30:28, machenbach wrote:
> On 2016/07/22 12:37:23, jochen wrote:
> > gyp used to default to softfp, but gn defaults on non-android to hardfp for
arm
> > 7 (it defaults to softfp to arm6)
> > 
> > not sure what to do here: change gn's default to softfp? override the
arm_eabi
> > for the simulator build explicitly? just go with hardfp?
> 
> Is it essential to get simualtor coverage for hard and soft? Then we probably
need separate bots... We set hardfp explicitly on our arm cros tests, so this
looks like we might loose coverage for softfp.

So far we only tested one with the simulator, no?

not sure whether it supports hardfp at all

[Michael Achenbach, 2016-07-25 10:15:31]

> So far we only tested one with the simulator, no?
Yes. With simulator we only tested softfp and with real arm hardware we tested
hardfp.

> not sure whether it supports hardfp at all
The trybot above is green. No idea if it really made use of the defines?

[Michael Achenbach, 2016-07-26 08:00:18]

Current differences:
https://build.chromium.org/p/client.v8.ports/builders/V8%20Linux%20-%20arm%20...

We use softfp now like in GYP. The flags
      -mfpmath=sse
      -mmmx
      -msse2
come from
https://cs.chromium.org/chromium/src/build/config/compiler/BUILD.gn?q=%22-mss...
. I assume they are not harmful as they don't target our generated code. Other
opinions?

Looking deeper I also found this:
    In GN, but not in gyp:
      -DCAN_USE_NEON

This seems to be the default in GN. Should we also here have an effort to unset
it?

[jochen (gone - plz use gerrit), 2016-07-27 07:42:04]

we should unset can_use_neon