Fix decoding of bytecode instructions on ARM generated on x86-64

Due to the difference of sign of the char type, byte code is encoded
differently on x86-64 than if it was on ARM and it also is decoded
differently. The problem at hand here was that negative jumps were
encoded as two-byte instructions (opcode and negative offset as byte) on
x86-64 when qmlcachegen is run. At run-time the negative offset was read
into a char type and consequently interpreted as a positive jump,
leading to crashes.

The explicit use of qint8 as signed byte type in encoding/decoding
sensitive parts avoids the decoding issue and should also result in
consistent encoding.

The added auto-test is (among other configurations) run in the CI under
qemu, which means the x86-64 host-built qmlcachegen will generate byte
code and the tst_qmlcachegen ARM binary will run under qemu and
reproduce this scenario.

Task-number: QTBUG-65521
Change-Id: I615628f53475dad38a41095c6e7ffea0c34d58ac
Reviewed-by: Lars Knoll <lars.knoll@qt.io>

1 files changed, 34 insertions, 0 deletions

diff --git a/tests/auto/qml/qmlcachegen/tst_qmlcachegen.cpp b/tests/auto/qml/qmlcachegen/tst_qmlcachegen.cpp
index 7e81df93b9..fb1072e67f 100644
--- a/tests/auto/qml/qmlcachegen/tst_qmlcachegen.cpp
+++ b/tests/auto/qml/qmlcachegen/tst_qmlcachegen.cpp
@@ -45,6 +45,7 @@ private slots:
     void translationExpressionSupport();
     void signalHandlerParameters();
     void errorOnArgumentsInSignalHandler();
+    void aheadOfTimeCompilation();
 };
 
 // A wrapper around QQmlComponent to ensure the temporary reference counts
@@ -225,6 +226,39 @@ void tst_qmlcachegen::errorOnArgumentsInSignalHandler()
     QVERIFY2(errorOutput.contains("error: The use of the arguments object in signal handlers is"), errorOutput);
 }
 
+void tst_qmlcachegen::aheadOfTimeCompilation()
+{
+    QTemporaryDir tempDir;
+    QVERIFY(tempDir.isValid());
+
+    const auto writeTempFile = [&tempDir](const QString &fileName, const char *contents) {
+        QFile f(tempDir.path() + '/' + fileName);
+        const bool ok = f.open(QIODevice::WriteOnly | QIODevice::Truncate);
+        Q_ASSERT(ok);
+        f.write(contents);
+        return f.fileName();
+    };
+
+    const QString testFilePath = writeTempFile("test.qml", "import QtQml 2.0\n"
+                                                           "QtObject {\n"
+                                                           "    function runTest() { var x = 0; while (x < 42) { ++x }; return x; }\n"
+                                                           "}");
+
+    QVERIFY(generateCache(testFilePath));
+
+    const QString cacheFilePath = testFilePath + QLatin1Char('c');
+    QVERIFY(QFile::exists(cacheFilePath));
+    QVERIFY(QFile::remove(testFilePath));
+
+    QQmlEngine engine;
+    CleanlyLoadingComponent component(&engine, QUrl::fromLocalFile(testFilePath));
+    QScopedPointer<QObject> obj(component.create());
+    QVERIFY(!obj.isNull());
+    QVariant result;
+    QMetaObject::invokeMethod(obj.data(), "runTest", Q_RETURN_ARG(QVariant, result));
+    QCOMPARE(result.toInt(), 42);
+}
+
 QTEST_GUILESS_MAIN(tst_qmlcachegen)
 
 #include "tst_qmlcachegen.moc"