1 files changed, 28 insertions, 18 deletions

diff --git a/src/corelib/io/qiodevice.cpp b/src/corelib/io/qiodevice.cpp
index c4fc040685..52a78ad1c4 100644
--- a/src/corelib/io/qiodevice.cpp
+++ b/src/corelib/io/qiodevice.cpp
@@ -747,6 +747,18 @@ void QIODevicePrivate::setWriteChannelCount(int count)
 }
 
 /*!
+    \internal
+*/
+bool QIODevicePrivate::allWriteBuffersEmpty() const
+{
+    for (const QRingBuffer &ringBuffer : writeBuffers) {
+        if (!ringBuffer.isEmpty())
+            return false;
+    }
+    return true;
+}
+
+/*!
     Opens the device and sets its OpenMode to \a mode. Returns \c true if successful;
     otherwise returns \c false. This function should be called from any
     reimplementations of open() or other functions that open the device.
@@ -1156,31 +1168,29 @@ QByteArray QIODevice::read(qint64 maxSize)
     Q_D(QIODevice);
     QByteArray result;
 
-    CHECK_MAXLEN(read, result);
-    CHECK_MAXBYTEARRAYSIZE(read);
-
 #if defined QIODEVICE_DEBUG
     printf("%p QIODevice::read(%lld), d->pos = %lld, d->buffer.size() = %lld\n",
            this, maxSize, d->pos, d->buffer.size());
 #endif
 
-    qint64 readBytes = 0;
-    if (maxSize) {
-        result.resize(int(maxSize));
-        if (!result.size()) {
-            // If resize fails, read incrementally.
-            qint64 readResult;
-            do {
-                result.resize(int(qMin(maxSize, qint64(result.size() + d->readBufferChunkSize))));
-                readResult = read(result.data() + readBytes, result.size() - readBytes);
-                if (readResult > 0 || readBytes == 0)
-                    readBytes += readResult;
-            } while (readResult == d->readBufferChunkSize);
-        } else {
-            readBytes = read(result.data(), result.size());
-        }
+    // Try to prevent the data from being copied, if we have a chunk
+    // with the same size in the read buffer.
+    if (maxSize == d->buffer.nextDataBlockSize() && !d->transactionStarted
+        && (d->openMode & (QIODevice::ReadOnly | QIODevice::Text)) == QIODevice::ReadOnly) {
+        result = d->buffer.read();
+        if (!d->isSequential())
+            d->pos += maxSize;
+        if (d->buffer.isEmpty())
+            readData(nullptr, 0);
+        return result;
     }
 
+    CHECK_MAXLEN(read, result);
+    CHECK_MAXBYTEARRAYSIZE(read);
+
+    result.resize(int(maxSize));
+    qint64 readBytes = read(result.data(), result.size());
+
     if (readBytes <= 0)
         result.clear();
     else