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Adapt to MSVC difference in behavior in initializing non-aggregates

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Testcase:

 struct A
 {
     A() = default;
     constexpr A(int v) :i(v) {}
     int i;
 };
 extern const A y[1] = {};

In our case, A = std::atomic<int> and y = shared_null.

With GCC, ICC, and Clang that "y" variable is value-initialized at
static initialization time, and no dynamic initialization code is
generated. However, with MSVC, because A is not an aggregate, the
default constructor isn't constexpr (it leaves A::i uninitialized) so
"y" must be dynamically initialized. That leads to Static Initialization
Order Fiasco.

This seems to be a regression in the MSVC 2019 16.6 STL:
https://github.com/microsoft/STL/issues/661

The solution is simple: call the constexpr constructor.

Code is different in 6.0 so sending separately from 5.x.

Fixes: QTBUG-71548
Task-number: QTBUG-59721
Change-Id: I3d4f433ff6e94fd390a9fffd161b4a7e8c1867b1
Reviewed-by: Kai Koehne <kai.koehne@qt.io>
(cherry picked from commit b619f2a26ea2ab56e18a3f9a8b635f96fc479563)
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
This commit is contained in:
Thiago Macieira 2020-06-23 13:51:32 -07:00
parent 4bb4512649
commit 9115c84103
1 changed files with 6 additions and 9 deletions
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15
src/corelib/tools/qarraydata.cpp
View File

@ -1,7 +1,7 @@
/****************************************************************************
**
** Copyright (C) 2016 The Qt Company Ltd.
** Copyright (C) 2016 Intel Corporation.
** Copyright (C) 2020 The Qt Company Ltd.
** Copyright (C) 2020 Intel Corporation.
** Contact: https://www.qt.io/licensing/
**
** This file is part of the QtCore module of the Qt Toolkit.
@ -46,19 +46,16 @@
QT_BEGIN_NAMESPACE
QT_WARNING_PUSH
QT_WARNING_DISABLE_GCC("-Wmissing-field-initializers")
const QArrayData QArrayData::shared_null[2] = {
{ Q_REFCOUNT_INITIALIZE_STATIC, 0, 0, 0, sizeof(QArrayData) }, // shared null
/* zero initialized terminator */};
{ { Q_BASIC_ATOMIC_INITIALIZER(0) }, 0, 0, 0, 0 } /* zero initialized terminator */
};
static const QArrayData qt_array[3] = {
{ Q_REFCOUNT_INITIALIZE_STATIC, 0, 0, 0, sizeof(QArrayData) }, // shared empty
{ { Q_BASIC_ATOMIC_INITIALIZER(0) }, 0, 0, 0, sizeof(QArrayData) }, // unsharable empty
/* zero initialized terminator */};
QT_WARNING_POP
{ { Q_BASIC_ATOMIC_INITIALIZER(0) }, 0, 0, 0, 0 } /* zero initialized terminator */
};
static const QArrayData &qt_array_empty = qt_array[0];
static const QArrayData &qt_array_unsharable_empty = qt_array[1];