diff --git a/src/gui/text/qtextengine.cpp b/src/gui/text/qtextengine.cpp
index febc364fb36..cb945b73ce7 100644
--- a/src/gui/text/qtextengine.cpp
+++ b/src/gui/text/qtextengine.cpp
@@ -2658,9 +2658,10 @@ QTextEngine::LayoutData::LayoutData(const QString &str, void **stack_memory, qsi
 {
     allocated = _allocated;
 
-    qsizetype space_charAttributes = sizeof(QCharAttributes) * string.size() / sizeof(void*) + 1;
-    qsizetype space_logClusters = sizeof(unsigned short) * string.size() / sizeof(void*) + 1;
-    available_glyphs = (allocated - space_charAttributes - space_logClusters) * sizeof(void*) / QGlyphLayout::SpaceNeeded;
+    constexpr qsizetype voidSize = sizeof(void*);
+    qsizetype space_charAttributes = sizeof(QCharAttributes) * string.size() / voidSize + 1;
+    qsizetype space_logClusters = sizeof(unsigned short) * string.size() / voidSize + 1;
+    available_glyphs = (allocated - space_charAttributes - space_logClusters) * voidSize / QGlyphLayout::SpaceNeeded;
 
     if (available_glyphs < str.size()) {
         // need to allocate on the heap
diff --git a/src/gui/text/qtextengine_p.h b/src/gui/text/qtextengine_p.h
index 7d5e2aa41ec..a829265a229 100644
--- a/src/gui/text/qtextengine_p.h
+++ b/src/gui/text/qtextengine_p.h
@@ -159,10 +159,8 @@ Q_DECLARE_TYPEINFO(QGlyphAttributes, Q_PRIMITIVE_TYPE);
 
 struct QGlyphLayout
 {
-    enum {
-        SpaceNeeded = sizeof(glyph_t) + sizeof(QFixed) + sizeof(QFixedPoint)
-                    + sizeof(QGlyphAttributes) + sizeof(QGlyphJustification)
-    };
+    static constexpr qsizetype SpaceNeeded = sizeof(glyph_t) + sizeof(QFixed) + sizeof(QFixedPoint)
+            + sizeof(QGlyphAttributes) + sizeof(QGlyphJustification);
 
     // init to 0 not needed, done when shaping
     QFixedPoint *offsets; // 8 bytes per element
diff --git a/tests/auto/gui/text/qfontmetrics/tst_qfontmetrics.cpp b/tests/auto/gui/text/qfontmetrics/tst_qfontmetrics.cpp
index 0c3b3fc86c1..678eb0393f5 100644
--- a/tests/auto/gui/text/qfontmetrics/tst_qfontmetrics.cpp
+++ b/tests/auto/gui/text/qfontmetrics/tst_qfontmetrics.cpp
@@ -34,6 +34,8 @@ private slots:
     void zeroWidthMetrics();
     void verticalMetrics_data();
     void verticalMetrics();
+    void largeText_data();
+    void largeText(); // QTBUG-123339
 };
 
 void tst_QFontMetrics::same()
@@ -388,5 +390,25 @@ void tst_QFontMetrics::verticalMetrics()
     QVERIFY(fm.ascent() != 0 || fm.descent() != 0);
 }
 
+void tst_QFontMetrics::largeText_data()
+{
+    QTest::addColumn<qsizetype>("size");
+    for (int i = 1; i < 20; ++i) {
+        qsizetype size = qsizetype(1) << i;
+        QByteArray rowText = QByteArray::number(size);
+        QTest::newRow(rowText.constData()) << size;
+    }
+}
+
+void tst_QFontMetrics::largeText()
+{
+    QFont font;
+    QFontMetrics fm(font);
+    QFETCH(qsizetype, size);
+    QString string(size, QLatin1Char('A'));
+    QRect boundingRect = fm.boundingRect(string);
+    QVERIFY(boundingRect.isValid());
+}
+
 QTEST_MAIN(tst_QFontMetrics)
 #include "tst_qfontmetrics.moc"