Extend alignment of QArrayData to std::max_align_t in allocation

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
Pick-to: 6.0
Change-Id: I8176a4cc79f100ee772b09425e88fe8ff3ae226a
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>

1 files changed, 29 insertions, 1 deletions

diff --git a/tests/auto/corelib/tools/qlist/tst_qlist.cpp b/tests/auto/corelib/tools/qlist/tst_qlist.cpp
index 3975bec678..64123c61a7 100644
--- a/tests/auto/corelib/tools/qlist/tst_qlist.cpp
+++ b/tests/auto/corelib/tools/qlist/tst_qlist.cpp
@@ -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.
@@ -347,6 +347,8 @@ private slots:
 
     void fromReadOnlyData() const;
 
+    void qtbug_90359() const;
+
 private:
     template<typename T> void copyConstructor() const;
     template<typename T> void add() const;
@@ -3181,5 +3183,31 @@ 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"