buffer: add buffer_shrink

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Peter Lieven <pl@kamp.de>
Reviewed-by: Daniel P. Berrange <berrange@redhat.com>
Message-id: 1446203414-4013-6-git-send-email-kraxel@redhat.com

include/qemu/buffer.h

@@ -51,6 +51,16 @@ struct Buffer {
 void buffer_init(Buffer *buffer, const char *name, ...)
         GCC_FMT_ATTR(2, 3);
 
+/**
+ * buffer_shrink:
+ * @buffer: the buffer object
+ *
+ * Try to shrink the buffer.  Checks current buffer capacity and size
+ * and reduces capacity in case only a fraction of the buffer is
+ * actually used.
+ */
+void buffer_shrink(Buffer *buffer);
+
 /**
  * buffer_reserve:
  * @buffer: the buffer object

util/buffer.c

@@ -20,7 +20,8 @@
 
 #include "qemu/buffer.h"
 
-#define BUFFER_MIN_INIT_SIZE 4096
+#define BUFFER_MIN_INIT_SIZE     4096
+#define BUFFER_MIN_SHRINK_SIZE  65536
 
 void buffer_init(Buffer *buffer, const char *name, ...)
 {
@@ -31,6 +32,23 @@ void buffer_init(Buffer *buffer, const char *name, ...)
     va_end(ap);
 }
 
+void buffer_shrink(Buffer *buffer)
+{
+    /*
+     * Only shrink in case the used size is *much* smaller than the
+     * capacity, to avoid bumping up & down the buffers all the time.
+     * realloc() isn't exactly cheap ...
+     */
+    if (buffer->offset < (buffer->capacity >> 3) &&
+        buffer->capacity > BUFFER_MIN_SHRINK_SIZE) {
+        return;
+    }
+
+    buffer->capacity = pow2ceil(buffer->offset);
+    buffer->capacity = MAX(buffer->capacity, BUFFER_MIN_SHRINK_SIZE);
+    buffer->buffer = g_realloc(buffer->buffer, buffer->capacity);
+}
+
 void buffer_reserve(Buffer *buffer, size_t len)
 {
     if ((buffer->capacity - buffer->offset) < len) {