Hello all,
Over the last week I pushed several patches to enable prepend optimization in
some core containers, namely, QList, QString, QByteArray.
In the light of these changes, there may occur new subtle issues in your code.
New model
---
In a nutshell, prepend optimization code does some magic with the memory layout
and the stored elements location.
This, in turn, affects how you can and cannot use iterators/pointers/references
to container's elements and their invalidation.
As of today, please consider that any non-const member function of the
aforementioned container can invalidate "remembered" iterators, pointers or
references to elements of that container. For example, an iterator is returned
by QList::begin(), a pointer is returned by QString::data() (and
QString::constData()), a reference is returned by QList::at() - this is, of
course, not an exhaustive list.
The invalidation existed before for cases when a container could detach (due to
copy-on-write) or reallocate (due to growing or squeezing). Now this is also
true for non-detaching, non-reallocating, modifying operations. Examples of
such operations: append, prepend, insert, emplace, remove, etc.
How this affects existing code?
---
To the best of my knowledge, at least certain documentation pieces of QString
and QByteArray already explained such a thing. So, hopefully, diligent users of
these 2 containers should be safe.
QList's behavior changed with regards to the iterators/pointers: as described
above, using an iterator or a pointer to an element of a QList between
non-const modifying operations is an undefined behavior (same true for
references).
A more subtle issue concerns the users of QVector. Since QVector got unified
with QList and QList's implementation/behavior changed, old code that was valid
before may now be a subject to UB. Consider a simple example of the difference:
QVector<int> ints { 0, 1, 2, 3, 4, 5, ... }; // now: effectively QList<int>
auto thirdElemIt = ints.begin() + 2;
ints.remove(6); // removing element at position 6. before: thirdElemIt
remains valid
//
now: thirdElemIt may be invalidated
print(*thirdElemIt); // before: valid enough behavior; now: undefined
Aiding the debugging process
---
Below are some basic steps one can do to simplify the debugging of issues:
1) Enable assertions in the code. This should be possible even if you have a
release build, there's a macro definition QT_FORCE_ASSERTS that should
force-enable assertions (albeit not sure how exactly to specify it during build
stage).
2) Build with address sanitizer. Even basic "sanitize=address" should suffice
initially. It should be available both for GCC and Clang (guessing that any
recent GCC/Clang that supports C++17 also supports ASan). MSVC developers some
time ago also reported that ASan is now supported. To enable ASan locally with
CMake build toolchain, one can use "-DFEATURE_sanitize_address=ON" flag as part
of the CMake input arguments (when building Qt sources). Don't know an
alternative if you use configure script + qmake.
3) Valgrind's memory checker should cover roughly same issues as address
sanitizer.
Note that you do not need to rebuild Qt/your application to use valgrind which
may be a preferred option sometimes.
Feel free to contact me in case you find an issue caused by new changes and
need a help in resolving it.
Sorry for possible inconveniences!
--
Best Regards,
Andrei
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