https://bugs.webkit.org/show_bug.cgi?id=70364

Reviewed by Gavin Barraclough.

~1% SunSpider speedup. Neutral elsewhere. Removes one cause for strings
having C++ destructors.

* heap/MarkStack.cpp:
(JSC::visitChildren): Call the JSString visitChildren function now,
since it's no longer a no-op.

* runtime/JSString.cpp:
(JSC::JSString::~JSString): Moved this destructor out of line because
it's called virtually, so there's no value to inlining.

(JSC::JSString::RopeBuilder::expand): Switched RopeBuilder to be a thin
initializing wrapper around JSString. JSString now represents ropes
directly, rather than relying on an underlying malloc object.

(JSC::JSString::visitChildren): Visit our rope fibers, since they're GC
objects now.

(JSC::JSString::resolveRope):
(JSC::JSString::resolveRopeSlowCase):
(JSC::JSString::outOfMemory): Updated for operating on JSStrings instead
of malloc objects.

(JSC::JSString::replaceCharacter): Removed optimizations for substringing
ropes and replacing subsections of ropes. We want to reimplement versions
of these optimizations in the future, but this patch already has good
performance without them.

* runtime/JSString.h:
(JSC::RopeBuilder::JSString):
(JSC::RopeBuilder::finishCreation):
(JSC::RopeBuilder::createNull):
(JSC::RopeBuilder::create):
(JSC::RopeBuilder::createHasOtherOwner):
(JSC::jsSingleCharacterString):
(JSC::jsSingleCharacterSubstring):
(JSC::jsNontrivialString):
(JSC::jsString):
(JSC::jsSubstring):
(JSC::jsOwnedString): Lots of mechanical changes here. The two important
things are: (1) The fibers in JSString::m_fibers are JSStrings now, not
malloc objects; (2) I simplified the JSString constructor interface to
only accept PassRefPtr<StringImpl>, instead of variations on that like
UString, reducing refcount churn.

* runtime/JSValue.h:
* runtime/JSValue.cpp:
(JSC::JSValue::toPrimitiveString): Updated this function to return a
JSString instead of a UString, since that's what clients want now.

* runtime/Operations.cpp:
(JSC::jsAddSlowCase):
* runtime/Operations.h:
(JSC::jsString):
* runtime/SmallStrings.cpp:
(JSC::SmallStrings::createEmptyString): Updated for interface changes above.

* runtime/StringConstructor.cpp:
(JSC::constructWithStringConstructor):
* runtime/StringObject.h:
(JSC::StringObject::create): Don't create a new JSString if we already
have a JSString.

* runtime/StringPrototype.cpp:
(JSC::stringProtoFuncConcat): Updated for interface changes above.


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@97827 268f45cc-cd09-0410-ab3c-d52691b4dbfc