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
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>

2 files changed, 45 insertions, 8 deletions

diff --git a/src/corelib/tools/qarraydata.cpp b/src/corelib/tools/qarraydata.cpp
index 8ca315024d..3297b51cfe 100644
--- a/src/corelib/tools/qarraydata.cpp
+++ b/src/corelib/tools/qarraydata.cpp
@@ -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);
diff --git a/tests/auto/corelib/tools/qlist/tst_qlist.cpp b/tests/auto/corelib/tools/qlist/tst_qlist.cpp
index 67f3849f94..3c4befe3ed 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.
@@ -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"