[Development] cast ... increases required alignment of target type [-Werror=cast-align]

[Atlant Schmidt]

Rohan:

  Because no one more-authoritative has answered you,
  I'll give it a try.

  I think that this occurs because QVariant must have
  sufficiently-strict alignment constraints to hold,
  properly aligned, the largest POD datum that can be
  stored in the variant. Other, more-specific objects
  may be known to not have such a large POD datum
  within them and so may have looser alignment
  requirements.

  When the more-loosely aligned object is cast to an
  object type with tighter alignment constraints, the
  compiler objects.

  In this exact case, it doesn't look like it will hurt
  you, but I think it would be good to code this in
  such a way as to avoid the warning (without disabling
  the warning). Unfortunately, I have no advice on how
  to do this.

                           Atlant


-----Original Message-----
From: development-bounces+aschmidt=dekaresearch.com at qt-project.org [mailto:development-bounces+aschmidt=dekaresearch.com at qt-project.org] On Behalf Of Rohan McGovern
Sent: Tuesday, March 06, 2012 18:35
To: development
Subject: [Development] cast ... increases required alignment of target type [-Werror=cast-align]

Does anyone have a suggestion on how to fix this warning?

This code in qlist.h:

  409: template <typename T>
  410: Q_INLINE_TEMPLATE void QList<T>::node_destruct(Node *from, Node *to)
  411: {
  412:    if (QTypeInfo<T>::isLarge || QTypeInfo<T>::isStatic)
  413:        while(from != to) --to, delete reinterpret_cast<T*>(to->v);
  414:    else if (QTypeInfo<T>::isComplex)
  415:        while (from != to) --to, reinterpret_cast<T*>(to)->~T();
  416: }

...when compiled for ARM, causes this warning (or error with -Werror):

  src/corelib/tools/qlist.h: In member function ‘void QList<T>::node_destruct(QList<T>::Node*, QList<T>::Node*) [with T = QVariant]’:
  src/corelib/tools/qlist.h:738:5:   instantiated from ‘void QList<T>::dealloc(QListData::Data*) [with T = QVariant]’
  src/corelib/tools/qlist.h:714:9:   instantiated from ‘QList<T>::~QList() [with T = QVariant]’
  src/corelib/statemachine/qstatemachine.h:81:59:   instantiated from here
  src/corelib/tools/qlist.h:415:28: error: cast from ‘QList<QVariant>::Node*’ to ‘QVariant*’ increases required alignment of target type [-Werror=cast-align]

"do not compile with -Wcast-align" may be a valid answer, but it would
be good to hear from someone who can confidently say whether or not this
issue can actually harm us in practice.
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