Add ThreadBarrier; use in GCMarker and unit test

Add ThreadBarrier; use in GCMarker and unit test

Thread barrier with:
* fixed (at construction) number n of participating threads {T1,T2,T3,...,Tn}
* unknown number of rounds.
Requirements:
* there is some R such that each participating thread makes
  R calls to Sync() followed by its one and only call to Exit().
Guarantees:
* for any two threads Ti and Tj and round number r <= R,
  everything done by Ti before its r'th call to Sync() happens before
  everything done by Tj after its r'th call to Sync().
Note:
* it's not required that the thread that constructs the barrier participates.


BUG=

Committed: https://github.com/dart-lang/sdk/commit/17a6b944d16aeefd0e4e8374d958465f62b8d8e6

[koda, 2015-09-11 21:27:21]

koda@google.com changed reviewers:
+ iposva@google.com

[koda, 2015-09-11 21:28:09]

I can move the implementation into a .cc file later if you prefer, but I figured
the review would be easier with everything in the header.

[koda, 2015-09-14 19:34:54]

On 2015/09/11 21:28:09, koda wrote:
> I can move the implementation into a .cc file later if you prefer, but I
figured
> the review would be easier with everything in the header.

There is now another CL that builds on top of this one:
https://codereview.chromium.org/1336383003/

[koda, 2015-09-15 20:03:04]

Added usage example, PTAL.

[Ivan Posva, 2015-09-16 04:36:11]

LGTMwC

-Ivan

https://codereview.chromium.org/1337943004/diff/100001/runtime/vm/thread_barr...
File runtime/vm/thread_barrier.h (right):

https://codereview.chromium.org/1337943004/diff/100001/runtime/vm/thread_barr...
runtime/vm/thread_barrier.h:45: // guarantee for Exit().
Thanks for the example. It makes the use a lot clearer seeing it aligned here.

Also an empty line or two before "class ThreadBarrier" would help adding a
visual break.

https://codereview.chromium.org/1337943004/diff/100001/runtime/vm/thread_barr...
runtime/vm/thread_barrier.h:83: MonitorLocker ml(&done_monitor_);
I am wondering whether this could be achieved with a single monitor, but that is
a future optimization.

[koda, 2015-09-16 19:11:52]

Committed patchset #8 (id:140001) manually as
17a6b944d16aeefd0e4e8374d958465f62b8d8e6 (presubmit successful).

[koda, 2015-09-16 19:12:15]

https://codereview.chromium.org/1337943004/diff/100001/runtime/vm/thread_barr...
File runtime/vm/thread_barrier.h (right):

https://codereview.chromium.org/1337943004/diff/100001/runtime/vm/thread_barr...
runtime/vm/thread_barrier.h:45: // guarantee for Exit().
On 2015/09/16 04:36:11, Ivan Posva wrote:
> Thanks for the example. It makes the use a lot clearer seeing it aligned here.
> 
> Also an empty line or two before "class ThreadBarrier" would help adding a
> visual break.

Done.

https://codereview.chromium.org/1337943004/diff/100001/runtime/vm/thread_barr...
runtime/vm/thread_barrier.h:83: MonitorLocker ml(&done_monitor_);
On 2015/09/16 04:36:11, Ivan Posva wrote:
> I am wondering whether this could be achieved with a single monitor, but that
is
> a future optimization.

Acknowledged.