Fix performance regression in QUuid::createUuid()

After commit 4363d4eebce4afd2ce3b3d6da205f8037357099a, createUuid()
would create a QFile, open it, read from it, and destroy it. This
resulted in a pretty serious performance regression (report says around
2000 times slower, I measured 3600 times slower on my machine).

This change creates one QFile per thread (since QFile is not thread
safe) and leaves the file open until the thread exits (at which point
QThreadStorage deletes the QFile).

This approach is around 600 times faster than the current code, which is
around 6 times slower than the original. Unfortunately, it is not
possible to restore the original performance without reintroducing the
problems that the commit mentioned above had fixed.

Benchmark is included.

Task-number: QTBUG-13877
Reviewed-by: thiago
Reviewed-by: mgoetz

1 files changed, 42 insertions, 25 deletions

diff --git a/src/corelib/plugin/quuid.cpp b/src/corelib/plugin/quuid.cpp
index 6c1b6e7..983d249 100644
--- a/src/corelib/plugin/quuid.cpp
+++ b/src/corelib/plugin/quuid.cpp
@@ -582,48 +582,65 @@ QT_BEGIN_INCLUDE_NAMESPACE
 #include <stdlib.h> // for RAND_MAX
 QT_END_INCLUDE_NAMESPACE
 
+#if !defined(QT_BOOTSTRAPPED) && defined(Q_OS_UNIX)
+Q_GLOBAL_STATIC(QThreadStorage<QFile *>, devUrandomStorage);
+#endif
+
 QUuid QUuid::createUuid()
 {
     QUuid result;
     uint *data = &(result.data1);
 
-#ifdef Q_OS_UNIX
-    QFile devUrandom;
-    devUrandom.setFileName(QLatin1String("/dev/urandom"));
-    if (devUrandom.open(QIODevice::ReadOnly)) {
-        qint64 numToRead = 4 * sizeof(uint);
-        devUrandom.read((char *) data, numToRead); // should read 128-bits of data
+#if defined(Q_OS_UNIX)
+    QFile *devUrandom;
+#  if !defined(QT_BOOTSTRAPPED)
+    devUrandom = devUrandomStorage()->localData();
+    if (!devUrandom) {
+        devUrandom = new QFile(QLatin1String("/dev/urandom"));
+        devUrandom->open(QIODevice::ReadOnly | QIODevice::Unbuffered);
+        devUrandomStorage()->setLocalData(devUrandom);
+    }
+# else
+    QFile file(QLatin1String("/dev/urandom"));
+    devUrandom = &file;
+    devUrandom->open(QIODevice::ReadOnly | QIODevice::Unbuffered);
+# endif
+    enum { AmountToRead = 4 * sizeof(uint) };
+    if (devUrandom->isOpen()
+        && devUrandom->read((char *) data, AmountToRead) == AmountToRead) {
+        // we got what we wanted, nothing more to do
+        ;
     } else
 #endif
     {
         static const int intbits = sizeof(int)*8;
         static int randbits = 0;
         if (!randbits) {
-        int r = 0;
+            int r = 0;
             int max = RAND_MAX;
             do { ++r; } while ((max=max>>1));
             randbits = r;
         }
 
-    // Seed the PRNG once per thread with a combination of current time, a
-    // stack address and a serial counter (since thread stack addresses are
-    // re-used).
+        // Seed the PRNG once per thread with a combination of current time, a
+        // stack address and a serial counter (since thread stack addresses are
+        // re-used).
 #ifndef QT_BOOTSTRAPPED
-    static QThreadStorage<int *> uuidseed;
-    if (!uuidseed.hasLocalData())
-    {
-        int *pseed = new int;
-        static QBasicAtomicInt serial = Q_BASIC_ATOMIC_INITIALIZER(2);
-        qsrand(*pseed = QDateTime::currentDateTime().toTime_t()
-                        + quintptr(&pseed)
-                        + serial.fetchAndAddRelaxed(1));
-        uuidseed.setLocalData(pseed);
-    }
+        static QThreadStorage<int *> uuidseed;
+        if (!uuidseed.hasLocalData())
+        {
+            int *pseed = new int;
+            static QBasicAtomicInt serial = Q_BASIC_ATOMIC_INITIALIZER(2);
+            qsrand(*pseed = QDateTime::currentDateTime().toTime_t()
+                   + quintptr(&pseed)
+                   + serial.fetchAndAddRelaxed(1));
+            uuidseed.setLocalData(pseed);
+        }
 #else
-    static bool seeded = false;
-    if (!seeded)
-        qsrand(QDateTime::currentDateTime().toTime_t()
-               + quintptr(&seeded));
+        static bool seeded = false;
+        if (!seeded)
+            qsrand(QDateTime::currentDateTime().toTime_t()
+                   + quintptr(&seeded));
 #endif
 
         int chunks = 16 / sizeof(uint);
@@ -631,7 +648,7 @@ QUuid QUuid::createUuid()
             uint randNumber = 0;
             for (int filled = 0; filled < intbits; filled += randbits)
                 randNumber |= qrand()<<filled;
-             *(data+chunks) = randNumber;
+            *(data+chunks) = randNumber;
         }
     }