1 files changed, 5 insertions, 4 deletions
diff --git a/src/corelib/tools/qline.h b/src/corelib/tools/qline.h
index d4f62024d1..b2f8f491ed 100644
--- a/src/corelib/tools/qline.h
+++ b/src/corelib/tools/qline.h
@@ -378,10 +378,11 @@ Q_DECL_CONSTEXPR inline QPointF QLineF::center() const
 
 inline void QLineF::setLength(qreal len)
 {
-    if (isNull())
-        return;
-    QLineF v = unitVector();
-    pt2 = QPointF(pt1.x() + v.dx() * len, pt1.y() + v.dy() * len);
+    const qreal oldLength = length();
+    // Scale len by dx() / length() and dy() / length(), two O(1) quantities,
+    // rather than scaling dx() and dy() by len / length(), which might overflow.
+    if (oldLength > 0)
+        pt2 = QPointF(pt1.x() + len * (dx() / oldLength), pt1.y() + len * (dy() / oldLength));
 }
 
 Q_DECL_CONSTEXPR inline QPointF QLineF::pointAt(qreal t) const