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, 16 insertions, 7 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);