QRingBuffer: avoid allocation in c'tor

Reduces creation time and memory consumption when the buffer is unused.
Robin's benchmark:

  QBENCHMARK {
      QFile file(filename);
  }

reports the following results (run with -median 10 -iterations 524288):

before: 0.00028 msecs per iteration (total: 149, iterations: 524288)
after:  0.00017 msecs per iteration (total: 93, iterations: 524288).

Change-Id: Ied4e7caeca794b94260b8fc59b3ba656f4719c30
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@theqtcompany.com>

src/corelib/tools/qringbuffer.cpp

@@ -65,6 +65,8 @@ const char *QRingBuffer::readPointerAtPosition(qint64 pos, qint64 &length) const
 
 void QRingBuffer::free(qint64 bytes)
 {
+    Q_ASSERT(bytes <= bufferSize);
+
     while (bytes > 0) {
         const qint64 blockSize = buffers.first().size() - head;
 
@@ -100,20 +102,25 @@ char *QRingBuffer::reserve(qint64 bytes)
     if (bytes <= 0 || bytes >= MaxByteArraySize)
         return 0;
 
-    const qint64 newSize = bytes + tail;
-    // if need buffer reallocation
-    if (newSize > buffers.last().size()) {
-        if (newSize > buffers.last().capacity() && (tail >= basicBlockSize
-                || newSize >= MaxByteArraySize)) {
-            // shrink this buffer to its current size
-            buffers.last().resize(tail);
+    if (buffers.isEmpty()) {
+        buffers.append(QByteArray());
+        buffers.first().resize(qMax(basicBlockSize, int(bytes)));
+    } else {
+        const qint64 newSize = bytes + tail;
+        // if need buffer reallocation
+        if (newSize > buffers.last().size()) {
+            if (newSize > buffers.last().capacity() && (tail >= basicBlockSize
+                    || newSize >= MaxByteArraySize)) {
+                // shrink this buffer to its current size
+                buffers.last().resize(tail);
 
-            // create a new QByteArray
-            buffers.append(QByteArray());
-            ++tailBuffer;
-            tail = 0;
+                // create a new QByteArray
+                buffers.append(QByteArray());
+                ++tailBuffer;
+                tail = 0;
+            }
+            buffers.last().resize(qMax(basicBlockSize, tail + int(bytes)));
         }
-        buffers.last().resize(qMax(basicBlockSize, tail + int(bytes)));
     }
 
     char *writePtr = buffers.last().data() + tail;
@@ -134,9 +141,13 @@ char *QRingBuffer::reserveFront(qint64 bytes)
         return 0;
 
     if (head < bytes) {
-        buffers.first().remove(0, head);
-        if (tailBuffer == 0)
-            tail -= head;
+        if (buffers.isEmpty()) {
+            buffers.append(QByteArray());
+        } else {
+            buffers.first().remove(0, head);
+            if (tailBuffer == 0)
+                tail -= head;
+        }
 
         head = qMax(basicBlockSize, int(bytes));
         if (bufferSize == 0) {
@@ -155,6 +166,8 @@ char *QRingBuffer::reserveFront(qint64 bytes)
 
 void QRingBuffer::chop(qint64 bytes)
 {
+    Q_ASSERT(bytes <= bufferSize);
+
     while (bytes > 0) {
         if (tailBuffer == 0 || tail > bytes) {
             // keep a single block around if it does not exceed
@@ -185,6 +198,9 @@ void QRingBuffer::chop(qint64 bytes)
 
 void QRingBuffer::clear()
 {
+    if (buffers.isEmpty())
+        return;
+
     buffers.erase(buffers.begin() + 1, buffers.end());
     buffers.first().clear();
 
@@ -301,7 +317,10 @@ qint64 QRingBuffer::peek(char *data, qint64 maxLength, qint64 pos) const
 void QRingBuffer::append(const QByteArray &qba)
 {
     if (tail == 0) {
-        buffers.last() = qba;
+        if (buffers.isEmpty())
+            buffers.append(qba);
+        else
+            buffers.last() = qba;
     } else {
         buffers.last().resize(tail);
         buffers.append(qba);

src/corelib/tools/qringbuffer_p.h

@@ -54,9 +54,7 @@ class QRingBuffer
 {
 public:
     explicit inline QRingBuffer(int growth = 4096) :
-        head(0), tail(0), tailBuffer(0), basicBlockSize(growth), bufferSize(0) {
-        buffers.append(QByteArray());
-    }
+        head(0), tail(0), tailBuffer(0), basicBlockSize(growth), bufferSize(0) { }
 
     inline qint64 nextDataBlockSize() const {
         return (tailBuffer == 0 ? tail : buffers.first().size()) - head;

tests/auto/corelib/tools/qringbuffer/tst_qringbuffer.cpp

@@ -39,11 +39,11 @@ class tst_QRingBuffer : public QObject
 {
     Q_OBJECT
 private slots:
+    void constructing();
     void readPointerAtPositionWriteRead();
     void readPointerAtPositionEmptyRead();
     void readPointerAtPositionWithHead();
     void readPointerAtPositionReadTooMuch();
-    void sizeWhenEmpty();
     void sizeWhenReservedAndChopped();
     void sizeWhenReserved();
     void free();
@@ -57,6 +57,23 @@ private slots:
     void readLine();
 };
 
+void tst_QRingBuffer::constructing()
+{
+    QRingBuffer ringBuffer;
+
+    QCOMPARE(ringBuffer.size(), Q_INT64_C(0));
+    QVERIFY(ringBuffer.isEmpty());
+    QCOMPARE(ringBuffer.nextDataBlockSize(), Q_INT64_C(0));
+    QVERIFY(ringBuffer.readPointer() == Q_NULLPTR);
+    QCOMPARE(ringBuffer.skip(5), Q_INT64_C(0));
+    QCOMPARE(ringBuffer.read(), QByteArray());
+    QCOMPARE(ringBuffer.getChar(), -1);
+    QVERIFY(!ringBuffer.canReadLine());
+
+    char buf[5];
+    QCOMPARE(ringBuffer.peek(buf, sizeof(buf)), Q_INT64_C(0));
+}
+
 void tst_QRingBuffer::sizeWhenReserved()
 {
     QRingBuffer ringBuffer;
@@ -74,13 +91,6 @@ void tst_QRingBuffer::sizeWhenReservedAndChopped()
     QCOMPARE(ringBuffer.size(), Q_INT64_C(0));
 }
 
-void tst_QRingBuffer::sizeWhenEmpty()
-{
-    QRingBuffer ringBuffer;
-
-    QCOMPARE(ringBuffer.size(), Q_INT64_C(0));
-}
-
 void tst_QRingBuffer::readPointerAtPositionReadTooMuch()
 {
     QRingBuffer ringBuffer;