https://bugs.webkit.org/show_bug.cgi?id=86024

Reviewed by Oliver Hunt.

During garbage collection, each marking thread keeps a HashMap of
strings.  While visiting via MarkStack::copyAndAppend(), we check to
see if the string we are visiting is already in the HashMap.  If not
we add it. If so, we change the reference to the current string we're
visiting to the prior string.

To reduce the performance impact of this change, two throttles have
ben added.  1) We only try hash consting if a significant number of new 
strings have been created since the last hash const.  Currently this is
set at 100 strings.  2) If a string is unique at the end of a marking
it will not be checked during further GC phases. In some cases this
won't catch all duplicates, but we are trying to catch the growth of
duplicate strings.

* heap/Heap.cpp:
(JSC::Heap::markRoots):
* heap/MarkStack.cpp:
(JSC::MarkStackThreadSharedData::resetChildren):
(JSC::MarkStackThreadSharedData::MarkStackThreadSharedData):
(JSC::MarkStackThreadSharedData::reset):
(JSC::MarkStack::setup): Check to see if enough strings have been created
to hash const.
(JSC::MarkStack::reset): Added call to clear m_uniqueStrings.
(JSC::JSString::tryHashConstLock): New method to lock JSString for
hash consting.
(JSC::JSString::releaseHashConstLock): New unlock method.
(JSC::JSString::shouldTryHashConst): Set of checks to see if we should
try to hash const the string.
(JSC::MarkStack::internalAppend): New method that performs the hash consting.
(JSC::SlotVisitor::copyAndAppend): Changed to call the new hash
consting internalAppend().
* heap/MarkStack.h:
(MarkStackThreadSharedData):
(MarkStack):
* runtime/JSGlobalData.cpp:
(JSC::JSGlobalData::JSGlobalData):
* runtime/JSGlobalData.h:
(JSGlobalData):
(JSC::JSGlobalData::haveEnoughNewStringsToHashConst):
(JSC::JSGlobalData::resetNewStringsSinceLastHashConst):
* runtime/JSString.h:
(JSString): Changed from using bool flags to using an unsigned
m_flags field.  This works better with the weakCompareAndSwap in
JSString::tryHashConstLock(). Changed the 8bitness setting and
checking to use new accessors.
(JSC::JSString::JSString):
(JSC::JSString::finishCreation):
(JSC::JSString::is8Bit): Updated for new m_flags.
(JSC::JSString::setIs8Bit): New setter.
New hash const flags accessors:
(JSC::JSString::isHashConstSingleton):
(JSC::JSString::clearHashConstSingleton):
(JSC::JSString::setHashConstSingleton):
(JSC::JSRopeString::finishCreation):
(JSC::JSRopeString::append):


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@121806 268f45cc-cd09-0410-ab3c-d52691b4dbfc