1 files changed, 34 insertions, 12 deletions

diff --git a/src/corelib/tools/qringbuffer_p.h b/src/corelib/tools/qringbuffer_p.h
index 2279f1bc2a..58227d6755 100644
--- a/src/corelib/tools/qringbuffer_p.h
+++ b/src/corelib/tools/qringbuffer_p.h
@@ -91,11 +91,20 @@ public:
             int blockSize = buffers.first().size() - head;
 
             if (tailBuffer == 0 || blockSize > bytes) {
-                bufferSize -= bytes;
-                if (bufferSize <= 0)
-                    clear(); // try to minify/squeeze us
-                else
+                // keep a single block around if it does not exceed
+                // the basic block size, to avoid repeated allocations
+                // between uses of the buffer
+                if (bufferSize <= bytes) {
+                    if (buffers.first().size() <= basicBlockSize) {
+                        bufferSize = 0;
+                        head = tail = 0;
+                    } else {
+                        clear(); // try to minify/squeeze us
+                    }
+                } else {
                     head += bytes;
+                    bufferSize -= bytes;
+                }
                 return;
             }
 
@@ -113,7 +122,7 @@ public:
 
         // if need buffer reallocation
         if (tail + bytes > buffers.last().size()) {
-            if (tail >= basicBlockSize) {
+            if (tail + bytes > buffers.last().capacity() && tail >= basicBlockSize) {
                 // shrink this buffer to its current size
                 buffers.last().resize(tail);
 
@@ -139,11 +148,20 @@ public:
     inline void chop(int bytes) {
         while (bytes > 0) {
             if (tailBuffer == 0 || tail > bytes) {
-                bufferSize -= bytes;
-                if (bufferSize <= 0)
-                    clear(); // try to minify/squeeze us
-                else
+                // keep a single block around if it does not exceed
+                // the basic block size, to avoid repeated allocations
+                // between uses of the buffer
+                if (bufferSize <= bytes) {
+                    if (buffers.first().size() <= basicBlockSize) {
+                        bufferSize = 0;
+                        head = tail = 0;
+                    } else {
+                        clear(); // try to minify/squeeze us
+                    }
+                } else {
                     tail -= bytes;
+                    bufferSize -= bytes;
+                }
                 return;
             }
 
@@ -156,7 +174,7 @@ public:
     }
 
     inline bool isEmpty() const {
-        return tailBuffer == 0 && tail == 0;
+        return bufferSize == 0;
     }
 
     inline int getChar() {
@@ -175,10 +193,14 @@ public:
     inline void ungetChar(char c) {
         --head;
         if (head < 0) {
-            buffers.prepend(QByteArray());
+            if (bufferSize != 0) {
+                buffers.prepend(QByteArray());
+                ++tailBuffer;
+            } else {
+                tail = basicBlockSize;
+            }
             buffers.first().resize(basicBlockSize);
             head = basicBlockSize - 1;
-            ++tailBuffer;
         }
         buffers.first()[head] = c;
         ++bufferSize;