QRingBuffer: cache the last released block

A typical ring buffer usage is a sequence of reserve()->chop()->read()
cycles. Usually, between these cycles, the buffer doesn't contain data and
all blocks are released. To reduce reallocations, keep the most recently
used block while the buffer is empty.

Change-Id: I8128f1f04649ae005fd0a480f17f95de01a9a135
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@theqtcompany.com>

1 files changed, 26 insertions, 8 deletions

diff --git a/src/corelib/tools/qringbuffer_p.h b/src/corelib/tools/qringbuffer_p.h
index 8fe56323a3..0066d3d1b6 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;
             }
 
@@ -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;
             }