Improve the loading performance of QLibrary

If an absolute path is specified we try that first. Otherwise we first
try the most likely system-specific format (e.g. libfoo.so) on Unix.
This improves performance especially on systems with slow flash devices.

For example, prior to this commit loading the Xcursor library (in the
xcb plugin) results in attempts to dlopen:

"Xcursor"
"Xcursor.so.1"
"libXcursor"
"libXcursor.so.1"

With this commit this is reduced to a single attempt of:

"libXcursor.so.1"

Plugin loading uses absolute paths with QLibrary so there is no
performance penalty for plugins with this commit.

This is however a behavioural change with respect to Qt4 but one
that I believe is justified and wanted.

Change-Id: I7813afa335f9bf515e87934c2f8f97888818c69c
Reviewed-by: Lars Knoll <lars.knoll@nokia.com>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@nokia.com>

1 files changed, 50 insertions, 23 deletions

diff --git a/src/corelib/plugin/qlibrary_win.cpp b/src/corelib/plugin/qlibrary_win.cpp
index 1e8ab150b5..10ca17cd2d 100644
--- a/src/corelib/plugin/qlibrary_win.cpp
+++ b/src/corelib/plugin/qlibrary_win.cpp
@@ -45,6 +45,7 @@
 #include "qdir.h"
 #include "qfileinfo.h"
 #include "qdir.h"
+#include <private/qfilesystementry_p.h>
 
 #if defined(QT_NO_LIBRARY) && defined(Q_OS_WIN)
 #undef QT_NO_LIBRARY
@@ -59,46 +60,72 @@ extern QString qt_error_string(int code);
 
 bool QLibraryPrivate::load_sys()
 {
-#ifdef Q_OS_WINCE
-    QString attempt = QFileInfo(fileName).absoluteFilePath();
-#else
-    QString attempt = fileName;
-#endif
-
     //avoid 'Bad Image' message box
     UINT oldmode = SetErrorMode(SEM_FAILCRITICALERRORS|SEM_NOOPENFILEERRORBOX);
-    pHnd = LoadLibrary((wchar_t*)QDir::toNativeSeparators(attempt).utf16());
 
-    if (pluginState != IsAPlugin) {
+    // We make the following attempts at locating the library:
+    //
+    // WinCE
+    // if (absolute)
+    //     fileName
+    //     fileName + ".dll"
+    // else
+    //     fileName + ".dll"
+    //     fileName
+    //     QFileInfo(fileName).absoluteFilePath()
+    //
+    // Windows
+    // if (absolute)
+    //     fileName
+    //     fileName + ".dll"
+    // else
+    //     fileName + ".dll"
+    //     fileName
+    //
+    // NB If it's a plugin we do not ever try the ".dll" extension
+    QStringList attempts;
+
+    if (pluginState != IsAPlugin)
+        attempts.append(fileName + QLatin1String(".dll"));
+
+    // If the fileName is an absolute path we try that first, otherwise we
+    // use the system-specific suffix first
+    QFileSystemEntry fsEntry(fileName);
+    if (fsEntry.isAbsolute()) {
+        attempts.prepend(fileName);
+    } else {
+        attempts.append(fileName);
 #if defined(Q_OS_WINCE)
-        if (!pHnd && ::GetLastError() == ERROR_MOD_NOT_FOUND) {
-            QString secondAttempt = fileName;
-            pHnd = LoadLibrary((wchar_t*)QDir::toNativeSeparators(secondAttempt).utf16());
-        }
+        attempts.append(QFileInfo(fileName).absoluteFilePath());
 #endif
-        if (!pHnd && ::GetLastError() == ERROR_MOD_NOT_FOUND) {
-            attempt += QLatin1String(".dll");
-            pHnd = LoadLibrary((wchar_t*)QDir::toNativeSeparators(attempt).utf16());
-        }
+    }
+
+    Q_FOREACH (const QString &attempt, attempts) {
+        pHnd = LoadLibrary((wchar_t*)QDir::toNativeSeparators(attempt).utf16());
+
+        // If we have a handle or the last error is something other than "unable
+        // to find the module", then bail out
+        if (pHnd || ::GetLastError() != ERROR_MOD_NOT_FOUND)
+            break;
     }
 
     SetErrorMode(oldmode);
     if (!pHnd) {
         errorString = QLibrary::tr("Cannot load library %1: %2").arg(fileName).arg(qt_error_string());
-    }
-    if (pHnd) {
+    } else {
+        // Query the actual name of the library that was loaded
         errorString.clear();
 
         wchar_t buffer[MAX_PATH];
         ::GetModuleFileName(pHnd, buffer, MAX_PATH);
-        attempt = QString::fromWCharArray(buffer);
 
-        const QDir dir =  QFileInfo(fileName).dir();
-        const QString realfilename = attempt.mid(attempt.lastIndexOf(QLatin1Char('\\')) + 1);
+        QString moduleFileName = QString::fromWCharArray(buffer);
+        moduleFileName.remove(0, 1 + moduleFileName.lastIndexOf(QLatin1Char('\\')));
+        const QDir dir(fsEntry.path());
         if (dir.path() == QLatin1String("."))
-            qualifiedFileName = realfilename;
+            qualifiedFileName = moduleFileName;
         else
-            qualifiedFileName = dir.filePath(realfilename);
+            qualifiedFileName = dir.filePath(moduleFileName);
     }
     return (pHnd != 0);
 }