Add fast-path in QLibraryStore::findOrCreate() for !instance()

If there's no QLibraryStore::instance(), then there's no LibraryMap,
so no need to construct a mapName we know we'll just throw away.

We must keep locking the qt_library_mutex to check instance(), but we
can drop it immediately if we find there isn't, and construct the
QLibraryPrivate object outside the critical section (the hope is that
the compiler sees this as an opportunity to tail-call).

The final goal of this exercise is to get rid of the double-lookup in
LibraryMap.

Pick-to: 6.7 6.6 6.5
Change-Id: I181dd2657e831a37e2d6f1c5d4df7e2a25ac48cd
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

1 files changed, 12 insertions, 11 deletions

diff --git a/src/corelib/plugin/qlibrary.cpp b/src/corelib/plugin/qlibrary.cpp
index ff1f4513ab..d584dce644 100644
--- a/src/corelib/plugin/qlibrary.cpp
+++ b/src/corelib/plugin/qlibrary.cpp
@@ -407,23 +407,24 @@ inline QLibraryPrivate *QLibraryStore::findOrCreate(const QString &fileName, con
     QMutexLocker locker(&qt_library_mutex);
     QLibraryStore *data = instance();
 
+    if (Q_UNLIKELY(!data)) {
+        locker.unlock();
+        return lazyNewLib();
+    }
+
     QString mapName = version.isEmpty() ? fileName : fileName + u'\0' + version;
 
     // check if this library is already loaded
-    QLibraryPrivate *lib = nullptr;
-    if (Q_LIKELY(data)) {
-        lib = data->libraryMap.value(mapName);
-        if (lib) {
-            lib->libraryRefCount.ref();
-            lib->mergeLoadHints(loadHints);
-        }
-    }
-    if (!lib)
+    QLibraryPrivate *lib = data->libraryMap.value(mapName);
+    if (lib) {
+        lib->libraryRefCount.ref();
+        lib->mergeLoadHints(loadHints);
+    } else {
         lib = lazyNewLib();
+    }
 
     // track this library
-    if (Q_LIKELY(data))
-        data->libraryMap.insert(mapName, lib);
+    data->libraryMap.insert(mapName, lib);
 
     return lib;
 }