QCborValue: store US-ASCII strings as 8-bit

They're easy to convert back to UTF-16, their length is the same, they
occupy half the memory and they're easy to encode into CBOR (no
transformation necessary).

The code was copied from QJsonPrivate::Latin1String::operator=().

Change-Id: I56b444f9d6274221a3b7fffd150c52bcb6c97f37
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

1 files changed, 19 insertions, 0 deletions

diff --git a/src/corelib/serialization/qcborvalue.cpp b/src/corelib/serialization/qcborvalue.cpp
index 96d3feaf1b..970ae23352 100644
--- a/src/corelib/serialization/qcborvalue.cpp
+++ b/src/corelib/serialization/qcborvalue.cpp
@@ -48,6 +48,7 @@
 #include <qlocale.h>
 #include <private/qnumeric_p.h>
 #include <qscopedvaluerollback.h>
+#include <private/qsimd_p.h>
 #include <qstack.h>
 
 #include <new>
@@ -1082,6 +1083,24 @@ void QCborContainerPrivate::replaceAt_complex(Element &e, const QCborValue &valu
     }
 }
 
+// in qstring.cpp
+void qt_to_latin1_unchecked(uchar *dst, const ushort *uc, qsizetype len);
+
+Q_NEVER_INLINE void QCborContainerPrivate::appendAsciiString(const QString &s)
+{
+    qsizetype len = s.size();
+    QtCbor::Element e;
+    e.value = addByteData(nullptr, len);
+    e.type = QCborValue::String;
+    e.flags = Element::HasByteData | Element::StringIsAscii;
+    elements.append(e);
+
+    char *ptr = data.data() + e.value + sizeof(ByteData);
+    uchar *l = reinterpret_cast<uchar *>(ptr);
+    const ushort *uc = (const ushort *)s.unicode();
+    qt_to_latin1_unchecked(l, uc, len);
+}
+
 QT_WARNING_DISABLE_MSVC(4146)   // unary minus operator applied to unsigned type, result still unsigned
 static int compareContainer(const QCborContainerPrivate *c1, const QCborContainerPrivate *c2);
 static int compareElementNoData(const Element &e1, const Element &e2)