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Extend alignment of QArrayData to std::max_align_t in allocation

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Make stricter alignment requirements for the allocated header

This strict alignment allows reallocateUnaligned() to property account
for the padding occurring in cases when
alignof(QArrayData) < alignof(T) <= alignof(std::max_align_t), which
happens to be the case on e.g. 32-bit platforms with specific alignment
requirements.

This adds 4 bytes (the difference between alignof(std::max_align_t) and
sizeof(QArrayData)) of overhead for QString, QByteArray and certain QLists
on 32-bit systems.

Task-number: QTBUG-90359
Change-Id: I8176a4cc79f100ee772b09425e88fe8ff3ae226a
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
(cherry picked from commit 37e26d2b3011acc86bbda1e1f46114d7c8441915)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>
This commit is contained in:
Andrei Golubev 2021-01-22 09:26:09 +01:00 committed by Qt Cherry-pick Bot
parent 3947279d68
commit a6c6d0bb89
2 changed files with 45 additions and 8 deletions
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23
src/corelib/tools/qarraydata.cpp
View File

@ -1,6 +1,6 @@
/****************************************************************************
**
** Copyright (C) 2020 The Qt Company Ltd.
** Copyright (C) 2021 The Qt Company Ltd.
** Copyright (C) 2016 Intel Corporation.
** Contact: https://www.qt.io/licensing/
**
@ -184,6 +184,15 @@ static QArrayData *allocateData(qsizetype allocSize)
return header;
}
namespace {
// QArrayData with strictest alignment requirements supported by malloc()
struct alignas(std::max_align_t) AlignedQArrayData : QArrayData
{
};
}
void *QArrayData::allocate(QArrayData **dptr, qsizetype objectSize, qsizetype alignment,
qsizetype capacity, QArrayData::AllocationOption option) noexcept
{
@ -197,13 +206,13 @@ void *QArrayData::allocate(QArrayData **dptr, qsizetype objectSize, qsizetype al
return nullptr;
}
qsizetype headerSize = sizeof(QArrayData);
const qsizetype headerAlignment = alignof(QArrayData);
qsizetype headerSize = sizeof(AlignedQArrayData);
const qsizetype headerAlignment = alignof(AlignedQArrayData);
if (alignment > headerAlignment) {
// Allocate extra (alignment - Q_ALIGNOF(QArrayData)) padding bytes so we
// can properly align the data array. This assumes malloc is able to
// provide appropriate alignment for the header -- as it should!
// Allocate extra (alignment - Q_ALIGNOF(AlignedQArrayData)) padding
// bytes so we can properly align the data array. This assumes malloc is
// able to provide appropriate alignment for the header -- as it should!
headerSize += alignment - headerAlignment;
}
Q_ASSERT(headerSize > 0);
@ -233,7 +242,7 @@ QArrayData::reallocateUnaligned(QArrayData *data, void *dataPointer,
{
Q_ASSERT(!data || !data->isShared());
const qsizetype headerSize = sizeof(QArrayData);
const qsizetype headerSize = sizeof(AlignedQArrayData);
qsizetype allocSize = calculateBlockSize(capacity, objectSize, headerSize, option);
if (Q_UNLIKELY(allocSize < 0))
return qMakePair<QArrayData *, void *>(nullptr, nullptr);

30
tests/auto/corelib/tools/qlist/tst_qlist.cpp
View File

@ -1,6 +1,6 @@
/****************************************************************************
**
** Copyright (C) 2016 The Qt Company Ltd.
** Copyright (C) 2021 The Qt Company Ltd.
** Contact: https://www.qt.io/licensing/
**
** This file is part of the test suite of the Qt Toolkit.
@ -346,6 +346,8 @@ private slots:
void fromReadOnlyData() const;
void qtbug_90359() const;
private:
template<typename T> void copyConstructor() const;
template<typename T> void add() const;
@ -3180,5 +3182,31 @@ void tst_QList::fromReadOnlyData() const
}
}
struct alignas(8) CustomAligned
{
qint64 v = 0;
CustomAligned() = default;
CustomAligned(qint64 i) : v(i) { }
friend bool operator==(const CustomAligned &x, const CustomAligned &y) { return x.v == y.v; }
};
void tst_QList::qtbug_90359() const
{
// Note: a very special test that could only fail for specific alignments
constexpr bool canFail = (alignof(QArrayData) == 4) && (sizeof(QArrayData) == 12);
if constexpr (!canFail)
qWarning() << "This test will always succeed on this system.";
if constexpr (alignof(CustomAligned) > alignof(std::max_align_t))
QSKIP("The codepaths tested here wouldn't be executed.");
const QList<CustomAligned> expected({ 0, 1, 2, 3, 4, 5, 6 });
QList<CustomAligned> actual;
for (int i = 0; i < 7; ++i) {
actual.append(i);
QCOMPARE(actual.at(i), i);
}
QCOMPARE(actual, expected);
}
QTEST_MAIN(tst_QList)
#include "tst_qlist.moc"