Optimize QXcbKeyboard::lookupString.

For the common case of strings smaller than 32 chars this removes a
temporary QByteArray allocation and the second call to
xkb_state_key_get_utf8.

Change-Id: I81cbbf2df683476b38c2ffb96119293cd5b09a90
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@theqtcompany.com>
Reviewed-by: Marc Mutz <marc.mutz@kdab.com>

1 files changed, 5 insertions, 3 deletions

diff --git a/src/plugins/platforms/xcb/qxcbkeyboard.cpp b/src/plugins/platforms/xcb/qxcbkeyboard.cpp
index 3b356525e3..6c80c1b30e 100644
--- a/src/plugins/platforms/xcb/qxcbkeyboard.cpp
+++ b/src/plugins/platforms/xcb/qxcbkeyboard.cpp
@@ -1515,10 +1515,12 @@ void QXcbKeyboard::handleKeyEvent(xcb_window_t sourceWindow, QEvent::Type type,
 
 QString QXcbKeyboard::lookupString(struct xkb_state *state, xcb_keycode_t code) const
 {
-    QByteArray chars;
-    chars.resize(1 + xkb_state_key_get_utf8(state, code, 0, 0));
-    // equivalent of XLookupString
+    QVarLengthArray<char, 32> chars(32);
     const int size = xkb_state_key_get_utf8(state, code, chars.data(), chars.size());
+    if (Q_UNLIKELY(size + 1 > chars.size())) { // +1 for NUL
+        chars.resize(size + 1);
+        xkb_state_key_get_utf8(state, code, chars.data(), chars.size());
+    }
     return QString::fromUtf8(chars.constData(), size);
 }