QString: optimize compare_helper

For a long time now we have fast mixed UTF-16/UTF-8 comparisons. But
no-one told this ol' relic, which still converted UTF-8 to UTF-16 for
comparison.

Fix by using QtPrivate::compareStrings(QSV, QU8SV), which, as the
central entry point, uses the fast-path.

Consequently, compare_helper can now be noexcept.

Pick-to: 6.3 6.2
Change-Id: I4cc9f07d9bc48628f1fe695e80015a9a07b79d6f
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

2 files changed, 4 insertions, 7 deletions

diff --git a/src/corelib/text/qstring.cpp b/src/corelib/text/qstring.cpp
index 8fd9dc96cf..9b13d4a276 100644
--- a/src/corelib/text/qstring.cpp
+++ b/src/corelib/text/qstring.cpp
@@ -6267,7 +6267,7 @@ int QString::compare(QLatin1String other, Qt::CaseSensitivity cs) const noexcept
     \since 5.0
 */
 int QString::compare_helper(const QChar *data1, qsizetype length1, const char *data2, qsizetype length2,
-                            Qt::CaseSensitivity cs)
+                            Qt::CaseSensitivity cs) noexcept
 {
     Q_ASSERT(length1 >= 0);
     Q_ASSERT(data1 || length1 == 0);
@@ -6275,11 +6275,8 @@ int QString::compare_helper(const QChar *data1, qsizetype length1, const char *d
         return length1;
     if (Q_UNLIKELY(length2 < 0))
         length2 = qsizetype(strlen(data2));
-    // ### make me nothrow in all cases
-    QVarLengthArray<ushort> s2(length2);
-    const auto beg = reinterpret_cast<QChar *>(s2.data());
-    const auto end = QUtf8::convertToUnicode(beg, QByteArrayView(data2, length2));
-    return QtPrivate::compareStrings(QStringView(data1, length1), QStringView(beg, end - beg), cs);
+    return QtPrivate::compareStrings(QStringView(data1, length1),
+                                     QUtf8StringView(data2, length2), cs);
 }
 
 /*!
diff --git a/src/corelib/text/qstring.h b/src/corelib/text/qstring.h
index 8eb0dd630f..80139205af 100644
--- a/src/corelib/text/qstring.h
+++ b/src/corelib/text/qstring.h
@@ -1106,7 +1106,7 @@ private:
                               Qt::CaseSensitivity cs = Qt::CaseSensitive) noexcept;
     static int compare_helper(const QChar *data1, qsizetype length1,
                               const char *data2, qsizetype length2,
-                              Qt::CaseSensitivity cs = Qt::CaseSensitive);
+                              Qt::CaseSensitivity cs = Qt::CaseSensitive) noexcept;
     static int localeAwareCompare_helper(const QChar *data1, qsizetype length1,
                                          const QChar *data2, qsizetype length2);
     static QString toLower_helper(const QString &str);