Enable export lookup by two threads (mutator and background compiler) by carying a copy of exports_…

Enable export lookup by two threads (mutator and background compiler) by carying a copy of exports_ in exports2_.

BUG=
R=hausner@google.com

Committed: https://github.com/dart-lang/sdk/commit/7ed2b851a9098fe7f4b238b3fbac1550866b1092

[srdjan, 2016-03-29 16:27:30]

srdjan@google.com changed reviewers:
+ asiva@google.com, hausner@google.com

[hausner, 2016-03-29 17:48:50]

LGTM.

For later reference: we discussed another idea: storing a special sentinel into
the exports_ array during lookup. If the background compiler encounters it, it
aborts compilation. As you pointed out, that doesn't work because if the
background compiler mutates and then the foreground compiler encounters the
sentinel, it cannot abort. (It would have to wait for the background compiler to
restore the original value.)

[srdjan, 2016-03-29 17:57:52]

Description was changed from

==========
Enable export lookup by two threads (mutator and background compiler) by carying
a copy of exports_ in exports2_.

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

to

==========
Enable export lookup by two threads (mutator and background compiler) by carying
a copy of exports_ in exports2_.

BUG=
R=hausner@google.com

Committed:
https://github.com/dart-lang/sdk/commit/7ed2b851a9098fe7f4b238b3fbac1550866b1092
==========

[srdjan, 2016-03-29 17:57:54]

Committed patchset #2 (id:20001) manually as
7ed2b851a9098fe7f4b238b3fbac1550866b1092 (presubmit successful).

[hausner, 2016-03-29 19:15:23]

PS

https://codereview.chromium.org/1839123002/diff/20001/runtime/vm/object.h
File runtime/vm/object.h (right):

https://codereview.chromium.org/1839123002/diff/20001/runtime/vm/object.h#new...
runtime/vm/object.h:3557: RawArray* exports2() const { return
raw_ptr()->exports_; }
Siva found this bug:

you should return exports2_ here.

[siva, 2016-03-29 19:17:08]

DBC, how does this solution scale if we decide to open the compiler task queue
to the thread pool ?

[srdjan, 2016-03-29 19:38:21]

On 2016/03/29 19:17:08, siva wrote:
> DBC, how does this solution scale if we decide to open the compiler task queue
> to the thread pool ?

This does not scale for more than two threads. I understand this is not an
immediate goal.