Auto-detect whether 64-bit std::atomic really works

The C++ standard says it must, but some badly-configured toolchains seem
to be lacking support.

In particular, for some 32-bit platforms without native support for
them, GCC implements 64-bit atomics via out-of-line functions in
libatomic. If that library is missing... well, then std::atomic 64-bit
doesn't work and we mustn't try to use it.

This was found when trying to compile Qt 5.6 for MIPS 32-bit:

Linking library libQt5Core.so.5.6.0
.obj/qsimd.o: In function `std::__atomic_base<unsigned long long>::load(std::memory_order) const':
/opt/poky/1.7/sysroots/mips32r2-poky-linux/usr/include/c++/4.9.1/bits/atomic_base.h:500: undefined reference to `__atomic_load_8'
.obj/qsimd.o: In function `std::__atomic_base<unsigned long long>::store(unsigned long long, std::memory_order)':
/opt/poky/1.7/sysroots/mips32r2-poky-linux/usr/include/c++/4.9.1/bits/atomic_base.h:478: undefined reference to `__atomic_store_8'

Yocto bug report: https://bugzilla.yoctoproject.org/show_bug.cgi?id=8274

Change-Id: I42e7ef1a481840699a8dffff140224d6614e5c36
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@theqtcompany.com>
Reviewed-by: Lars Knoll <lars.knoll@theqtcompany.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

2 files changed, 57 insertions, 0 deletions

diff --git a/config.tests/common/atomic64/atomic64.cpp b/config.tests/common/atomic64/atomic64.cpp
new file mode 100644
index 0000000000..8dbea96c8b
--- /dev/null
+++ b/config.tests/common/atomic64/atomic64.cpp
@@ -0,0 +1,54 @@
+/****************************************************************************
+**
+** Copyright (C) 2015 Intel Corporation.
+** Contact: http://www.qt.io/licensing/
+**
+** This file is part of the FOO module of the Qt Toolkit.
+**
+** $QT_BEGIN_LICENSE:LGPL21$
+** Commercial License Usage
+** Licensees holding valid commercial Qt licenses may use this file in
+** accordance with the commercial license agreement provided with the
+** Software or, alternatively, in accordance with the terms contained in
+** a written agreement between you and The Qt Company. For licensing terms
+** and conditions see http://www.qt.io/terms-conditions. For further
+** information use the contact form at http://www.qt.io/contact-us.
+**
+** GNU Lesser General Public License Usage
+** Alternatively, this file may be used under the terms of the GNU Lesser
+** General Public License version 2.1 or version 3 as published by the Free
+** Software Foundation and appearing in the file LICENSE.LGPLv21 and
+** LICENSE.LGPLv3 included in the packaging of this file. Please review the
+** following information to ensure the GNU Lesser General Public License
+** requirements will be met: https://www.gnu.org/licenses/lgpl.html and
+** http://www.gnu.org/licenses/old-licenses/lgpl-2.1.html.
+**
+** As a special exception, The Qt Company gives you certain additional
+** rights. These rights are described in The Qt Company LGPL Exception
+** version 1.1, included in the file LGPL_EXCEPTION.txt in this package.
+**
+** $QT_END_LICENSE$
+**
+****************************************************************************/
+
+#include <atomic>
+#include <cstdint>
+
+void test(volatile std::atomic<std::int64_t> &a)
+{
+    std::int64_t v = a.load(std::memory_order_acquire);
+    while (!a.compare_exchange_strong(v, v + 1,
+                                      std::memory_order_acq_rel,
+                                      std::memory_order_acquire)) {
+        v = a.exchange(v - 1);
+    }
+
+    a.store(v + 1, std::memory_order_release);
+}
+
+int main(int, char **)
+{
+    void *ptr = (void*)0xffffffc0; // any random pointer
+    test(*reinterpret_cast<std::atomic<std::int64_t> *>(ptr));
+    return 0;
+}
diff --git a/config.tests/common/atomic64/atomic64.pro b/config.tests/common/atomic64/atomic64.pro
new file mode 100644
index 0000000000..c9a85817ca
--- /dev/null
+++ b/config.tests/common/atomic64/atomic64.pro
@@ -0,0 +1,3 @@
+SOURCES = atomic64.cpp
+CONFIG += c++11 console
+CONFIG -= qt