Replace ad-hoc weakness in transition array with WeakCell.

Replace ad-hoc weakness in transition array with WeakCell.

BUG=

Committed: https://crrev.com/885455e99de817f86a0b5df2dc0d932cfc179749
Cr-Commit-Position: refs/heads/master@{#29083}

[ulan, 2015-06-17 10:13:26]

ulan@chromium.org changed reviewers:
+ hpayer@chromium.org, jkummerow@chromium.org

[ulan, 2015-06-17 10:13:26]

PTAL

[Jakob Kummerow, 2015-06-17 10:38:51]

LGTM with comments.

https://codereview.chromium.org/1157943003/diff/60001/src/heap/mark-compact.cc
File src/heap/mark-compact.cc (right):

https://codereview.chromium.org/1157943003/diff/60001/src/heap/mark-compact.c...
src/heap/mark-compact.cc:2485:
DCHECK(TransitionArray::IsFullTransitionArray(transitions));
This DCHECK is unneeded, as it duplicates line 2475.

https://codereview.chromium.org/1157943003/diff/60001/src/transitions-inl.h
File src/transitions-inl.h (right):

https://codereview.chromium.org/1157943003/diff/60001/src/transitions-inl.h#n...
src/transitions-inl.h:81: DCHECK(!cell->cleared());
This relies on ClearMapTransitions() making sure that all transition arrays are
always properly compacted and right-trimmed, so there are no cleared cells in
the array during normal execution. Is that intentional? Seems fine, but I'd like
a comment here stating this explicitly.

[ulan, 2015-06-17 10:42:15]

Thanks, I removed the redundant dchecks.

[ulan, 2015-06-17 11:28:12]

The CQ bit was checked by ulan@chromium.org

[ulan, 2015-06-17 11:28:13]

The patchset sent to the CQ was uploaded after l-g-t-m from
jkummerow@chromium.org
Link to the patchset: https://codereview.chromium.org/1157943003/#ps80001
(title: "Remove redundant dchecks")

[commit-bot: I haz the power, 2015-06-17 11:28:24]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1157943003/80001

[commit-bot: I haz the power, 2015-06-17 12:07:50]

Committed patchset #5 (id:80001)

[commit-bot: I haz the power, 2015-06-17 12:08:04]

Patchset 5 (id:??) landed as
https://crrev.com/885455e99de817f86a0b5df2dc0d932cfc179749
Cr-Commit-Position: refs/heads/master@{#29083}

[ulan, 2015-06-18 15:51:28]

A revert of this CL (patchset #5 id:80001) has been created in
https://codereview.chromium.org/1194673002/ by ulan@chromium.org.

The reason for reverting is: Breaks descriptor array clearing..

[brucedawson, 2015-06-20 00:08:03]

brucedawson@chromium.org changed reviewers:
+ brucedawson@chromium.org

[brucedawson, 2015-06-20 00:08:04]

One post-commit comment, based on a /analyze warning. Just an FYI.

https://codereview.chromium.org/1157943003/diff/80001/src/heap/mark-compact.cc
File src/heap/mark-compact.cc (right):

https://codereview.chromium.org/1157943003/diff/80001/src/heap/mark-compact.c...
src/heap/mark-compact.cc:2490: WeakCell* target_cell = t->GetTargetCell(i);
This variable declaration shadows the target_cell variable of the same type in
the outer scope. There doesn't appear to be a bug here, but this can cause
confusion. Consider either reusing the outer variable or using a different name.