it has already destroyed
https://bugs.webkit.org/show_bug.cgi?id=65556

Reviewed by Oliver Hunt.

This gets rid of dummy cells, and ensures that it's not necessary
to invoke a destructor on cells that have already been swept.  In
the common case, a block knows that either all of its free cells
still need to have destructors called, or none of them do, which
minimizes the amount of branching that needs to happen per cell
when performing a sweep.

This is performance neutral on SunSpider and V8.  It is meant as
a stepping stone to simplify the implementation of more
sophisticated sweeping algorithms.

* heap/Heap.cpp:
(JSC::CountFunctor::ClearMarks::operator()):
* heap/MarkedBlock.cpp:
(JSC::MarkedBlock::initForCellSize):
(JSC::MarkedBlock::callDestructor):
(JSC::MarkedBlock::specializedReset):
(JSC::MarkedBlock::reset):
(JSC::MarkedBlock::specializedSweep):
(JSC::MarkedBlock::sweep):
(JSC::MarkedBlock::produceFreeList):
(JSC::MarkedBlock::lazySweep):
(JSC::MarkedBlock::blessNewBlockForFastPath):
(JSC::MarkedBlock::blessNewBlockForSlowPath):
(JSC::MarkedBlock::canonicalizeBlock):
* heap/MarkedBlock.h:
(JSC::MarkedBlock::FreeCell::setNoObject):
(JSC::MarkedBlock::setDestructorState):
(JSC::MarkedBlock::destructorState):
(JSC::MarkedBlock::notifyMayHaveFreshFreeCells):
* runtime/JSCell.cpp:
* runtime/JSCell.h:
(JSC::JSCell::JSCell::JSCell):
* runtime/JSGlobalData.cpp:
(JSC::JSGlobalData::JSGlobalData):
(JSC::JSGlobalData::clearBuiltinStructures):
* runtime/JSGlobalData.h:
* runtime/Structure.h:


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@92233 268f45cc-cd09-0410-ab3c-d52691b4dbfc