[Development] Pointer aliasing problem in optimized gcc builds

[Soroush Rabiei]

Hi everybody


I'm using a custom build of Qt 5.1.1 compiled with GCC 4.8.1 on
Windows (MinGW builds x86_64). For some performance reasons I have to
enable "-O3" flag until my project is ported out of Qt. The Qt was
built with C++11 support.


When program tries to append an item to a container it crashes.
Debuggers stops at this line in qlist.h:


template <typename T>

Q_OUTOFLINE_TEMPLATE void QList<T>::append(const T &t)

{

    if (d->ref.isShared()) { // <== Here

        Node *n = detach_helper_grow(INT_MAX, 1);

        QT_TRY {

            node_construct(n, t);

        } QT_CATCH(...) {

            --d->end;

            QT_RETHROW;

        }

// ...


Looking into functions that append calls, I found that node_construct
contains some unsafe code:


template <typename T>

Q_INLINE_TEMPLATE void QList<T>::node_construct(Node *n, const T &t)

{

    if (QTypeInfo<T>::isLarge || QTypeInfo<T>::isStatic) n->v = new T(t);

    else if (QTypeInfo<T>::isComplex) new (n) T(t);

#if (defined(__GNUC__) || defined(__INTEL_COMPILER) ||
defined(__IBMCPP__)) && !defined(__OPTIMIZE__)

    // This violates pointer aliasing rules, but it is known to be
safe (and silent)

    // in unoptimized GCC builds (-fno-strict-aliasing). The other
compilers which

    // set the same define are assumed to be safe.

    else *reinterpret_cast<T*>(n) = t;

#else

    // This is always safe, but penaltizes unoptimized builds a lot.

    else ::memcpy(n, static_cast<const void *>(&t), sizeof(T));

#endif

}


I guess problem is same in detach_helper_grow function. Tried adding
-fno-strict-aliasing to compile flags and defined __OPTIMIZE__. Also
removed -O3 from compile flags, but no hope. So I have a couple of
questions:
* How bad are -O3 compiles? I've read that it's completely safe for
well-written code but using optimization aggressively, may cause undefined
behavior or even runtime crashes for bad codes. How is Qt code?
* How can I solve this problem? (Using official builds is not preferred)
* Do I need to recompile Qt with "-fno-strict-aliasing" flag or defining
__OPTIMIZE__ ? Since templated classes are header-only and I already tried
both in my project, I guess the answer is no.
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