https://bugs.webkit.org/show_bug.cgi?id=70258

Reviewed by Filip Pizlo.
        
No perf. change.
        
This is a first step toward GC allocating string backing stores, starting
with ropes. It also enables future simplifications and optimizations.
        
- Replaced some complex mark stack logic with a simple linear stack of
JSCell pointers.
        
- Replaced logic for short-circuiting marking based on JSType and/or
Structure flags with special cases for object, array, and string.
        
- Fiddled with inlining for better codegen.

* JavaScriptCore.exp:
* heap/HandleStack.cpp: Build!

* heap/Heap.cpp:
(JSC::Heap::Heap): Provide more vptrs to SlotVisitor, for use in marking.

* heap/HeapRootVisitor.h: Removed unused functions that no longer build.

* heap/MarkStack.cpp:
(JSC::MarkStackArray::MarkStackArray):
(JSC::MarkStackArray::~MarkStackArray):
(JSC::MarkStackArray::expand):
(JSC::MarkStackArray::shrinkAllocation):
(JSC::MarkStack::reset):
(JSC::visitChildren):
(JSC::SlotVisitor::drain):
* heap/MarkStack.h:
(JSC::MarkStack::MarkStack):
(JSC::MarkStack::~MarkStack):
(JSC::MarkStackArray::append):
(JSC::MarkStackArray::removeLast):
(JSC::MarkStackArray::isEmpty):
(JSC::MarkStack::append):
(JSC::MarkStack::appendUnbarrieredPointer):
(JSC::MarkStack::internalAppend): Replaced complex mark set logic with
simple linear stack.

* heap/SlotVisitor.h:
(JSC::SlotVisitor::SlotVisitor): Updated for above changes.

* runtime/JSArray.cpp:
(JSC::JSArray::visitChildren):
* runtime/JSArray.h:
* runtime/JSObject.cpp:
(JSC::JSObject::visitChildren):
* runtime/JSObject.h: Don't inline visitChildren; it's too big.

* runtime/Structure.h:
(JSC::MarkStack::internalAppend): Nixed the short-circuit for CompoundType
because it prevented strings from owning GC pointers.

* runtime/WriteBarrier.h:
(JSC::MarkStack::appendValues): No need to validate; internalAppend will
do that for us.


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@97642 268f45cc-cd09-0410-ab3c-d52691b4dbfc