QtScript crash after QTJSC::JSGlobalData::storeVPtrs(), when built with gcc-4.9.0

Distributions are experiencing various qt/kde related crashes (plasma-desktop in particular) since recent adoption of gcc-4.9.0 compiler.

Backtraces are fairly similar, and end up down in QTJSC::JSGlobalData::storeVPtrs() ... (backtrace appended)

See also downstream bugs:
https://bugs.kde.org/show_bug.cgi?id=333987
https://bugzilla.redhat.com/show_bug.cgi?id=1091482

In fedora, we tried some naive workarounds, including building with options:
-fno-delete-null-pointer-checks
-fno-tree-vrp
which didn't seem to help.

have not yet ruled out this may be a compiler bug.

– Backtrace:
Application: Plasma Desktop Shell (plasma-desktop), signal: Segmentation fault
Using host libthread_db library "/lib64/libthread_db.so.1".
81 T_PSEUDO (SYSCALL_SYMBOL, SYSCALL_NAME, SYSCALL_NARGS)
[Current thread is 1 (Thread 0x7f7c4e239880 (LWP 1481))]

Thread 2 (Thread 0x7f7c21ee0700 (LWP 1486)):
#0 pthread_cond_wait@@GLIBC_2.3.2 () at ../nptl/sysdeps/unix/sysv/linux/x86_64/pthread_cond_wait.S:185
#1 0x00007f7c365de21a in QTWTF::TCMalloc_PageHeap::scavengerThread (this=0x7f7c368e1f80 <QTWTF::pageheap_memory>) at ../3rdparty/javascriptcore/JavaScriptCore/wtf/FastMalloc.cpp:2359
#2 0x00007f7c365de249 in QTWTF::TCMalloc_PageHeap::runScavengerThread (context=<optimized out>) at ../3rdparty/javascriptcore/JavaScriptCore/wtf/FastMalloc.cpp:1464
#3 0x00007f7c4c804555 in start_thread (arg=0x7f7c21ee0700) at pthread_create.c:309
#4 0x00007f7c4adc7e2d in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:111

Thread 1 (Thread 0x7f7c4e239880 (LWP 1481)):
[KCrash Handler]
#6 derefBase (this=0x0) at ../3rdparty/javascriptcore/JavaScriptCore/wtf/RefCounted.h:69
#7 deref (this=0x0) at ../3rdparty/javascriptcore/JavaScriptCore/wtf/RefCounted.h:108
#8 derefIfNotNull<QTJSC::Structure> (ptr=0x0) at ../3rdparty/javascriptcore/JavaScriptCore/wtf/PassRefPtr.h:42
#9 ~NonNullPassRefPtr (this=0x7fffebb1f8a0, __in_chrg=<optimized out>) at ../3rdparty/javascriptcore/JavaScriptCore/wtf/PassRefPtr.h:129
#10 QTJSC::JSGlobalData::storeVPtrs () at ../3rdparty/javascriptcore/JavaScriptCore/runtime/JSGlobalData.cpp:85
#11 0x00007f7c3656f143 in initializeThreadingOnce () at ../3rdparty/javascriptcore/JavaScriptCore/runtime/InitializeThreading.cpp:52
#12 QTJSC::initializeThreading () at ../3rdparty/javascriptcore/JavaScriptCore/runtime/InitializeThreading.cpp:66
#13 0x00007f7c366222b8 in QScriptEnginePrivate::QScriptEnginePrivate (this=this@entry=0x24e1ef0) at api/qscriptengine.cpp:977
#14 0x00007f7c3662312f in QScriptEngine::QScriptEngine (this=0x7fffebb1fc70, parent=0x0) at api/qscriptengine.cpp:1976
#15 0x00007f7c2c82832c in WorkspaceScripting::ScriptEngine::ScriptEngine(Plasma::Corona*, QObject*) () from /lib64/libplasmagenericshell.so.4
#16 0x00007f7c2d3ad1d3 in WorkspaceScripting::DesktopScriptEngine::DesktopScriptEngine(Plasma::Corona*, bool, QObject*) () from /usr/lib64/libkdeinit4_plasma-desktop.so
#17 0x00007f7c2d3b8c1c in DesktopCorona::evaluateScripts(QStringList const&, bool) () from /usr/lib64/libkdeinit4_plasma-desktop.so
#18 0x00007f7c2d3b902a in DesktopCorona::loadDefaultLayout() () from /usr/lib64/libkdeinit4_plasma-desktop.so
#19 0x00007f7c38e5b04f in Plasma::Corona::initializeLayout (this=0x246ae90, configName=...) at /usr/src/debug/kdelibs-4.13.0/plasma/corona.cpp:185
#20 0x00007f7c2d3db2f7 in PlasmaApp::corona(bool) () from /usr/lib64/libkdeinit4_plasma-desktop.so
#21 0x00007f7c2d3db794 in PlasmaApp::setupDesktop() () from /usr/lib64/libkdeinit4_plasma-desktop.so
#22 0x00007f7c2d3e0d85 in PlasmaApp::qt_static_metacall(QObject*, QMetaObject::Call, int, void**) [clone .part.83] () from /usr/lib64/libkdeinit4_plasma-desktop.so
#23 0x00007f7c4cbb8b51 in QObject::event (this=this@entry=0x2319310, e=e@entry=0x2463f10) at kernel/qobject.cpp:1222
#24 0x00007f7c4cba3cbc in QCoreApplication::event (this=this@entry=0x2319310, e=e@entry=0x2463f10) at kernel/qcoreapplication.cpp:1765
#25 0x00007f7c4bcdd323 in QApplication::event (this=0x2319310, e=0x2463f10) at kernel/qapplication.cpp:2544
#26 0x00007f7c4bcd7fbc in QApplicationPrivate::notify_helper (this=this@entry=0x2333670, receiver=receiver@entry=0x2319310, e=e@entry=0x2463f10) at kernel/qapplication.cpp:4565
#27 0x00007f7c4bcdea58 in QApplication::notify (this=this@entry=0x2319310, receiver=receiver@entry=0x2319310, e=e@entry=0x2463f10) at kernel/qapplication.cpp:4351
#28 0x00007f7c4dbffc7a in KApplication::notify (this=0x2319310, receiver=0x2319310, event=0x2463f10) at /usr/src/debug/kdelibs-4.13.0/kdeui/kernel/kapplication.cpp:311
#29 0x00007f7c4cb9f7ed in QCoreApplication::notifyInternal (this=0x2319310, receiver=receiver@entry=0x2319310, event=event@entry=0x2463f10) at kernel/qcoreapplication.cpp:953
#30 0x00007f7c4cba2ab1 in sendEvent (event=0x2463f10, receiver=0x2319310) at kernel/qcoreapplication.h:231
#31 QCoreApplicationPrivate::sendPostedEvents (receiver=receiver@entry=0x0, event_type=event_type@entry=0, data=0x22511d0) at kernel/qcoreapplication.cpp:1577
#32 0x00007f7c4cba2f43 in QCoreApplication::sendPostedEvents (receiver=receiver@entry=0x0, event_type=event_type@entry=0) at kernel/qcoreapplication.cpp:1470
#33 0x00007f7c4cbceaa3 in sendPostedEvents () at kernel/qcoreapplication.h:236
#34 postEventSourceDispatch (s=0x2331520) at kernel/qeventdispatcher_glib.cpp:280
#35 0x00007f7c474a69aa in g_main_context_dispatch () from /lib64/libglib-2.0.so.0
#36 0x00007f7c474a6cf8 in g_main_context_iterate.isra () from /lib64/libglib-2.0.so.0
#37 0x00007f7c474a6dac in g_main_context_iteration () from /lib64/libglib-2.0.so.0
#38 0x00007f7c4cbcebed in QEventDispatcherGlib::processEvents (this=0x2331de0, flags=...) at kernel/qeventdispatcher_glib.cpp:425
#39 0x00007f7c4bd7b556 in QGuiEventDispatcherGlib::processEvents (this=<optimized out>, flags=...) at kernel/qguieventdispatcher_glib.cpp:207
#40 0x00007f7c4cb9e321 in QEventLoop::processEvents (this=this@entry=0x7fffebb20610, flags=...) at kernel/qeventloop.cpp:149
#41 0x00007f7c4cb9e685 in QEventLoop::exec (this=this@entry=0x7fffebb20610, flags=...) at kernel/qeventloop.cpp:204
#42 0x00007f7c4cba3d77 in QCoreApplication::exec () at kernel/qcoreapplication.cpp:1225
#43 0x00007f7c2d3c1612 in kdemain () from /usr/lib64/libkdeinit4_plasma-desktop.so
#44 0x000000000040822b in launch (argc=argc@entry=1, _name=_name@entry=0x22d9368 "/usr/bin/plasma-desktop", args=args@entry=0x22d9380 "", cwd=cwd@entry=0x0, envc=envc@entry=0, envs=<optimized out>, envs@entry=0x22d9388 "", reset_env=false, tty=0x0, avoid_loops=false, startup_id_str=0x40ab25 "0") at /usr/src/debug/kdelibs-4.13.0/kinit/kinit.cpp:726
#45 0x00000000004090c9 in handle_launcher_request (sock=8, who=<optimized out>) at /usr/src/debug/kdelibs-4.13.0/kinit/kinit.cpp:1218
#46 0x00000000004097a7 in handle_requests (waitForPid=waitForPid@entry=0) at /usr/src/debug/kdelibs-4.13.0/kinit/kinit.cpp:1411
#47 0x000000000040537d in main (argc=4, argv=<optimized out>, envp=0x7fffebb211f0) at /usr/src/debug/kdelibs-4.13.0/kinit/kinit.cpp:1899