Logically empty WeakBlocks should not pin down their MarkedBlocks indefinitely.
<https://webkit.org/b/143210>
Reviewed by Geoffrey Garen.
Since a MarkedBlock cannot be destroyed until all the WeakBlocks pointing into it are gone,
we had a little problem where WeakBlocks with only null pointers would still keep their
MarkedBlock alive.
This patch fixes that by detaching WeakBlocks from their MarkedBlock once a sweep discovers
that the WeakBlock contains no pointers to live objects. Ownership of the WeakBlock is passed
to the Heap, which will sweep the list of these detached WeakBlocks as part of a full GC,
destroying them once they're fully dead.
This allows the garbage collector to reclaim the 64kB MarkedBlocks much sooner, and resolves
a mysterious issue where doing two full garbage collections back-to-back would free additional
memory in the second collection.
Management of detached WeakBlocks is implemented as a Vector<WeakBlock*> in Heap, along with
an index of the next block in that vector that needs to be swept. The IncrementalSweeper then
calls into Heap::sweepNextLogicallyEmptyWeakBlock() to sweep one block at a time.
- heap/Heap.h:
- heap/Heap.cpp:
(JSC::Heap::collectAllGarbage): Add a final pass where we sweep the logically empty WeakBlocks
owned by Heap, after everything else has been swept.
(JSC::Heap::notifyIncrementalSweeper): Set up an incremental sweep of logically empty WeakBlocks
after a full garbage collection ends. Note that we don't do this after Eden collections, since
they are unlikely to cause entire WeakBlocks to go empty.
(JSC::Heap::addLogicallyEmptyWeakBlock): Added. Interface for passing ownership of a WeakBlock
to the Heap when it's detached from a WeakSet.
(JSC::Heap::sweepAllLogicallyEmptyWeakBlocks): Helper for collectAllGarbage() that sweeps all
of the logically empty WeakBlocks owned by Heap.
(JSC::Heap::sweepNextLogicallyEmptyWeakBlock): Sweeps one logically empty WeakBlock if needed
and updates the next-logically-empty-weak-block-to-sweep index.
(JSC::Heap::lastChanceToFinalize): call sweepAllLogicallyEmptyWeakBlocks() here, since there
won't be another chance after this.
- heap/IncrementalSweeper.h:
(JSC::IncrementalSweeper::hasWork): Deleted.
- heap/IncrementalSweeper.cpp:
(JSC::IncrementalSweeper::fullSweep):
(JSC::IncrementalSweeper::doSweep):
(JSC::IncrementalSweeper::sweepNextBlock): Restructured IncrementalSweeper a bit to simplify
adding a new sweeping stage for the Heap's logically empty WeakBlocks. sweepNextBlock() is
changed to return a bool (true if there's more work to be done.)
(JSC::WeakBlock::sweep): This now figures out if the WeakBlock is logically empty, i.e doesn't
contain any pointers to live objects. The answer is stored in a new SweepResult member.
(JSC::WeakBlock::isLogicallyEmptyButNotFree): Added. Can be queried after a sweep to determine
if the WeakBlock could be detached from the MarkedBlock.
(JSC::WeakBlock::SweepResult::SweepResult): Deleted in favor of initializing member variables
when declaring them.
