1 files changed, 15 insertions, 18 deletions

diff --git a/src/corelib/io/qiodevice.cpp b/src/corelib/io/qiodevice.cpp
index fd204b00de..36616a5236 100644
--- a/src/corelib/io/qiodevice.cpp
+++ b/src/corelib/io/qiodevice.cpp
@@ -1148,34 +1148,31 @@ QByteArray QIODevice::read(qint64 maxSize)
     Q_D(QIODevice);
     QByteArray result;
 
-    CHECK_MAXLEN(read, result);
-
 #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
 
+    // 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);
     if (maxSize >= MaxByteArraySize) {
         checkWarnMessage(this, "read", "maxSize argument exceeds QByteArray size limit");
         maxSize = MaxByteArraySize - 1;
     }
 
-    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());
-        }
-    }
+    result.resize(int(maxSize));
+    qint64 readBytes = read(result.data(), result.size());
 
     if (readBytes <= 0)
         result.clear();