[ABANDONED] Disable AVX2 in libvpx for the SyzyAsan builds of Chrome.

Disable AVX2 in libvpx for the SyzyAsan builds of Chrome.

The SyzyAsan instrumentation pipeline doesn't support these instructions
and so we should avoid using them in the SyzyAsan build (where performance
isn't such a concern anyway).

BUG=670664

[Sébastien Marchand, 2016-12-01 21:13:28]

Description was changed from

==========
Disable AVX2 for the SyzyAsan builds of Chrome.

The SyzyAsan instrumentation pipeline doesn't support these instructions
and so we should avoid using them in the SyzyAsan build (where performance
isn't such a concern anyway).
==========

to

==========
Disable AVX2 in libvpx for the SyzyAsan builds of Chrome.

The SyzyAsan instrumentation pipeline doesn't support these instructions
and so we should avoid using them in the SyzyAsan build (where performance
isn't such a concern anyway).
==========

[Sébastien Marchand, 2016-12-01 21:13:28]

sebmarchand@chromium.org changed reviewers:
+ tomfinegan@chromium.org

[Sébastien Marchand, 2016-12-01 21:14:45]

Hey Tom,

is it enough to guarantee that there won't be any AVX2 instruction in the libvpx
code for the SyzyAsan builds of Chrome?

[Sébastien Marchand, 2016-12-01 21:15:03]

The CQ bit was checked by sebmarchand@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2016-12-01 21:15:47]

Dry run: CQ is trying da patch. Follow status at
 
https://chromium-cq-status.appspot.com/v2/patch-status/codereview.chromium.or...

[commit-bot: I haz the power, 2016-12-01 22:10:35]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2016-12-01 22:10:36]

Dry run: This issue passed the CQ dry run.

[Sébastien Marchand, 2016-12-02 13:05:55]

Doesn't seem to work (I need to provide an alternate definition for these
functions).

[chromium-reviews, 2016-12-02 13:07:49]

If you look through the build file for other mentions of msan you should be
able to sort it. Sorry I can't be more helpful at the moment, OOO until
Monday.

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[chromium-reviews, 2016-12-02 13:09:06]

But you can't disable just avx, you have to disable all the optimizations
(sorry we don't have a more granular switch)

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[chromium-reviews, 2016-12-02 13:10:38]

And it would be good to have a bug tracking avx fixes for syzyasan so it
can be turned back on.

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[Sébastien Marchand, 2016-12-02 13:12:13]

Ha, thanks for the reply while OOO. I'm trying to disable AVX2 and F16C, not
AVX. Will Chrome still be usable (for our small SyzyAsan Canary user population)
if we disable the optimizations (the SyzyAsan builds are already 2-3x slower
than a 'normal' build).

I'll open a crbug.

Thanks!

[chromium-reviews, 2016-12-02 13:22:06]

They probably won't be able to play back video at more than 240p, maybe
360p, and will have to set that manually for video sites because they don't
detect performance. WebRTC will likely not work at all.

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[Sébastien Marchand, 2016-12-02 13:38:49]

Ack, in this case I'll check if there's another approach that we can take.

I've opened crbug.com/670664

[Sébastien Marchand, 2016-12-02 13:39:56]

Description was changed from

==========
Disable AVX2 in libvpx for the SyzyAsan builds of Chrome.

The SyzyAsan instrumentation pipeline doesn't support these instructions
and so we should avoid using them in the SyzyAsan build (where performance
isn't such a concern anyway).
==========

to

==========
Disable AVX2 in libvpx for the SyzyAsan builds of Chrome.

The SyzyAsan instrumentation pipeline doesn't support these instructions
and so we should avoid using them in the SyzyAsan build (where performance
isn't such a concern anyway).

BUG=670664
==========

[Johann, 2016-12-05 23:50:53]

On 2016/12/02 13:38:49, Sébastien Marchand wrote:
> Ack, in this case I'll check if there's another approach that we can take.
> 
> I've opened crbug.com/670664

A possible approach would be to disable AVX2 instructions at runtime. Can you
set an environment variable?

I believe chrome still uses the x86_simd_caps function to check:
https://chromium.googlesource.com/webm/libvpx/+/master/vpx_ports/x86.h#178

I *think* this would look like:
VPX_SIMD_CAPS_MASK=HAS_AVX2

This would prevent the library from calling any of the avx2 functions. The code
will still exist, but it sounds like this is a runtime analyzer not a static
analyzer.

[Sébastien Marchand, 2016-12-07 18:50:24]

The SyzyAsan builds get shipped to the Canary channel, so we don't control the
environment.