Update tag bits atomically

Use compare and swap when updating tag word in all C++ runtime code that might run concurrently.

This was done back in r39731, but then partially reverted in r39734, and has been broken ever since.

Also distinguish between sync/unsync versions. While old-space GC marker is non-concurrent, it uses unsync versions for efficiency.

Add a template to share code between sync versions and avoid unintentional discrepancies.

R=iposva@google.com

Committed: https://code.google.com/p/dart/source/detail?r=42409

[koda, 2014-12-16 00:22:20]

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

[koda, 2014-12-16 17:37:24]

Updated CL to provide both sync/unsync versions and use unsync where possible.
Performance eval is underway.

PTAL

[Ivan Posva, 2014-12-16 20:33:23]

LGTM

-Ivan

https://codereview.chromium.org/804673003/diff/100001/runtime/vm/gc_marker.cc
File runtime/vm/gc_marker.cc (right):

https://codereview.chromium.org/804673003/diff/100001/runtime/vm/gc_marker.cc...
runtime/vm/gc_marker.cc:192: if (raw_obj->IsWatched()) {
How about:

bool is_watched = raw_obj->IsWatched();
raw_obj->SetMarkBitUnsynchronized();
raw_obj->ClearRememberedBitUnsynchronized();
raw_obj->ClearWatchedBitUnsynchronized();
if (is_watched) {
...

https://codereview.chromium.org/804673003/diff/100001/runtime/vm/gc_marker.cc...
runtime/vm/gc_marker.cc:204: raw_obj->ClearWatchedBitUnsynchronized();
Can we at least move this up to immediately clear the watched bit after the
if-statement?

[koda, 2014-12-16 20:49:20]

https://codereview.chromium.org/804673003/diff/100001/runtime/vm/gc_marker.cc
File runtime/vm/gc_marker.cc (right):

https://codereview.chromium.org/804673003/diff/100001/runtime/vm/gc_marker.cc...
runtime/vm/gc_marker.cc:192: if (raw_obj->IsWatched()) {
On 2014/12/16 20:33:23, Ivan Posva wrote:
> How about:
> 
> bool is_watched = raw_obj->IsWatched();
> raw_obj->SetMarkBitUnsynchronized();
> raw_obj->ClearRememberedBitUnsynchronized();
> raw_obj->ClearWatchedBitUnsynchronized();
> if (is_watched) {
> ...

Done (after removing ASSERT from ClearWatched...).

Compiles to:
...
 842244f:	89 d0                	mov    %edx,%eax
 8422451:	83 e0 ec             	and    $0xffffffec,%eax
 8422454:	83 c8 02             	or     $0x2,%eax
 8422457:	83 e2 01             	and    $0x1,%edx
...

https://codereview.chromium.org/804673003/diff/100001/runtime/vm/gc_marker.cc...
runtime/vm/gc_marker.cc:204: raw_obj->ClearWatchedBitUnsynchronized();
On 2014/12/16 20:33:23, Ivan Posva wrote:
> Can we at least move this up to immediately clear the watched bit after the
> if-statement?

Acknowledged.

[koda, 2014-12-16 21:31:33]

Committed patchset #7 (id:120001) manually as r42409 (presubmit successful).