Optimize QXcbScreen::visualForFormat()

... by stopping the work when finding the first RGB candidate,
and otherwise only remembering the first candidate as a fall-back.

This way, we don't need to allocate memory as we did when collecting
the candidates in a QVector.

Saves more than 800b in text size on optimized GCC 6.0 Linux AMD64
builds.

Change-Id: I7d0cae69fa421fed881dd5a0f1aa45035d8f7461
Reviewed-by: Olivier Goffart (Woboq GmbH) <ogoffart@woboq.com>

1 files changed, 11 insertions, 14 deletions

diff --git a/src/plugins/platforms/xcb/qxcbscreen.cpp b/src/plugins/platforms/xcb/qxcbscreen.cpp
index b4c772dd3b..23585ab511 100644
--- a/src/plugins/platforms/xcb/qxcbscreen.cpp
+++ b/src/plugins/platforms/xcb/qxcbscreen.cpp
@@ -387,10 +387,9 @@ QSurfaceFormat QXcbScreen::surfaceFormatFor(const QSurfaceFormat &format) const
 
 const xcb_visualtype_t *QXcbScreen::visualForFormat(const QSurfaceFormat &format) const
 {
-    QVector<const xcb_visualtype_t *> candidates;
+    const xcb_visualtype_t *candidate = nullptr;
 
-    for (auto ii = m_visuals.constBegin(); ii != m_visuals.constEnd(); ++ii) {
-        const xcb_visualtype_t &xcb_visualtype = ii.value();
+    for (const xcb_visualtype_t &xcb_visualtype : m_visuals) {
 
         const int redSize = qPopulationCount(xcb_visualtype.red_mask);
         const int greenSize = qPopulationCount(xcb_visualtype.green_mask);
@@ -409,19 +408,17 @@ const xcb_visualtype_t *QXcbScreen::visualForFormat(const QSurfaceFormat &format
         if (format.alphaBufferSize() != -1 && alphaSize != format.alphaBufferSize())
             continue;
 
-        candidates.append(&xcb_visualtype);
-    }
-
-    if (candidates.isEmpty())
-        return nullptr;
+        // Try to find a RGB visual rather than e.g. BGR or GBR
+        if (qCountTrailingZeroBits(xcb_visualtype.blue_mask) == 0)
+            return &xcb_visualtype;
 
-    // Try to find a RGB visual rather than e.g. BGR or GBR
-    for (const xcb_visualtype_t *candidate : qAsConst(candidates))
-        if (qCountTrailingZeroBits(candidate->blue_mask) == 0)
-            return candidate;
+        // In case we do not find anything we like, just remember the first one
+        // and hope for the best:
+        if (!candidate)
+            candidate = &xcb_visualtype;
+    }
 
-    // Did not find anything we like, just grab the first one and hope for the best
-    return candidates.first();
+    return candidate;
 }
 
 void QXcbScreen::sendStartupMessage(const QByteArray &message) const