bufferlist: pre-allocate buffer array in one go with the buffer list

We can now create and free a buffer list with one slice alloc/free
call in most cases, instead of one slice alloc/free for the list,
one slice alloc/free for the GArray, and one malloc/free for the
GArray array. In practice we know the max size of our buffer list
from the start, so can avoid reallocs.

https://bugzilla.gnome.org/show_bug.cgi?id=732284

gst/gstbufferlist.c

 /* GStreamer
  * Copyright (C) 2009 Axis Communications <dev-gstreamer at axis dot com>
  * @author Jonas Holmberg <jonas dot holmberg at axis dot com>
+ * Copyright (C) 2014 Tim-Philipp Müller <tim at centricular dot com>
  *
  * gstbufferlist.c: Buffer list
  *
@@ -38,9 +39,13 @@
 
 #include "gstbuffer.h"
 #include "gstbufferlist.h"
+#include "gstutils.h"
 
 #define GST_CAT_DEFAULT GST_CAT_BUFFER_LIST
 
+#define GST_BUFFER_LIST_IS_USING_DYNAMIC_ARRAY(list) \
+    ((list)->buffers != &(list)->arr[0])
+
 /**
  * GstBufferList:
  *
@@ -50,7 +55,16 @@ struct _GstBufferList
 {
   GstMiniObject mini_object;
 
-  GArray *array;
+  GstBuffer **buffers;
+  guint n_buffers;
+  guint n_allocated;
+
+  gsize slice_size;
+
+  /* one-item array, in reality more items are pre-allocated
+   * as part of the GstBufferList structure, and that
+   * pre-allocated array extends beyond the declared struct */
+  GstBuffer *arr[1];
 };
 
 GType _gst_buffer_list_type = 0;
@@ -69,15 +83,15 @@ _gst_buffer_list_copy (GstBufferList * list)
   GstBufferList *copy;
   guint i, len;
 
-  len = list->array->len;
-  copy = gst_buffer_list_new_sized (len);
+  len = list->n_buffers;
+  copy = gst_buffer_list_new_sized (list->n_allocated);
 
   /* add and ref all buffers in the array */
-  for (i = 0; i < len; i++) {
-    GstBuffer *buf = g_array_index (list->array, GstBuffer *, i);
-    buf = gst_buffer_ref (buf);
-    g_array_append_val (copy->array, buf);
-  }
+  for (i = 0; i < len; i++)
+    copy->buffers[i] = gst_buffer_ref (list->buffers[i]);
+
+  copy->n_buffers = len;
+
   return copy;
 }
 
@@ -85,25 +99,31 @@ static void
 _gst_buffer_list_free (GstBufferList * list)
 {
   guint i, len;
+
   GST_LOG ("free %p", list);
 
   /* unrefs all buffers too */
-  len = list->array->len;
+  len = list->n_buffers;
   for (i = 0; i < len; i++)
-    gst_buffer_unref (g_array_index (list->array, GstBuffer *, i));
-  g_array_free (list->array, TRUE);
+    gst_buffer_unref (list->buffers[i]);
+
+  if (GST_BUFFER_LIST_IS_USING_DYNAMIC_ARRAY (list))
+    g_free (list->buffers);
 
-  g_slice_free1 (sizeof (GstBufferList), list);
+  g_slice_free1 (list->slice_size, list);
 }
 
 static void
-gst_buffer_list_init (GstBufferList * list, guint asize)
+gst_buffer_list_init (GstBufferList * list, guint n_allocated, gsize slice_size)
 {
   gst_mini_object_init (GST_MINI_OBJECT_CAST (list), 0, _gst_buffer_list_type,
       (GstMiniObjectCopyFunction) _gst_buffer_list_copy, NULL,
       (GstMiniObjectFreeFunction) _gst_buffer_list_free);
 
-  list->array = g_array_sized_new (FALSE, FALSE, sizeof (GstBuffer *), asize);
+  list->buffers = &list->arr[0];
+  list->n_buffers = 0;
+  list->n_allocated = n_allocated;
+  list->slice_size = slice_size;
 
   GST_LOG ("init %p", list);
 }
@@ -125,12 +145,18 @@ GstBufferList *
 gst_buffer_list_new_sized (guint size)
 {
   GstBufferList *list;
+  gsize slice_size;
+  guint n_allocated;
 
-  list = g_slice_new0 (GstBufferList);
+  n_allocated = GST_ROUND_UP_16 (size);
+
+  slice_size = sizeof (GstBufferList) + (n_allocated - 1) * sizeof (gpointer);
+
+  list = g_slice_alloc0 (slice_size);
 
   GST_LOG ("new %p", list);
 
-  gst_buffer_list_init (list, size);
+  gst_buffer_list_init (list, n_allocated, slice_size);
 
   return list;
 }
@@ -165,7 +191,26 @@ gst_buffer_list_length (GstBufferList * list)
 {
   g_return_val_if_fail (GST_IS_BUFFER_LIST (list), 0);
 
-  return list->array->len;
+  return list->n_buffers;
+}
+
+static inline void
+gst_buffer_list_remove_range_internal (GstBufferList * list, guint idx,
+    guint length, gboolean unref_old)
+{
+  guint i;
+
+  if (unref_old) {
+    for (i = idx; i < idx + length; ++i)
+      gst_buffer_unref (list->buffers[i]);
+  }
+
+  if (idx + length != list->n_buffers) {
+    memmove (&list->buffers[idx], &list->buffers[idx + length],
+        (list->n_buffers - (idx + length)) * sizeof (void *));
+  }
+
+  list->n_buffers -= length;
 }
 
 /**
@@ -193,20 +238,20 @@ gst_buffer_list_foreach (GstBufferList * list, GstBufferListFunc func,
   g_return_val_if_fail (GST_IS_BUFFER_LIST (list), FALSE);
   g_return_val_if_fail (func != NULL, FALSE);
 
-  len = list->array->len;
+  len = list->n_buffers;
   for (i = 0; i < len;) {
     GstBuffer *buf, *buf_ret;
 
-    buf = buf_ret = g_array_index (list->array, GstBuffer *, i);
+    buf = buf_ret = list->buffers[i];
     ret = func (&buf_ret, i, user_data);
 
     /* Check if the function changed the buffer */
     if (buf != buf_ret) {
       if (buf_ret == NULL) {
-        g_array_remove_index (list->array, i);
-        len--;
+        gst_buffer_list_remove_range_internal (list, i, 1, FALSE);
+        --len;
       } else {
-        g_array_index (list->array, GstBuffer *, i) = buf_ret;
+        list->buffers[i] = buf_ret;
       }
     }
 
@@ -234,14 +279,10 @@ gst_buffer_list_foreach (GstBufferList * list, GstBufferListFunc func,
 GstBuffer *
 gst_buffer_list_get (GstBufferList * list, guint idx)
 {
-  GstBuffer *buf;
-
   g_return_val_if_fail (GST_IS_BUFFER_LIST (list), NULL);
-  g_return_val_if_fail (idx < list->array->len, NULL);
+  g_return_val_if_fail (idx < list->n_buffers, NULL);
 
-  buf = g_array_index (list->array, GstBuffer *, idx);
-
-  return buf;
+  return list->buffers[idx];
 }
 
 /**
@@ -265,15 +306,42 @@ gst_buffer_list_get (GstBufferList * list, guint idx)
 void
 gst_buffer_list_insert (GstBufferList * list, gint idx, GstBuffer * buffer)
 {
+  guint want_alloc;
+
   g_return_if_fail (GST_IS_BUFFER_LIST (list));
   g_return_if_fail (buffer != NULL);
 
-  if (idx == -1)
-    g_array_append_val (list->array, buffer);
-  else {
-    g_return_if_fail (idx < list->array->len);
-    g_array_insert_val (list->array, idx, buffer);
+  if (idx == -1 && list->n_buffers < list->n_allocated) {
+    list->buffers[list->n_buffers++] = buffer;
+    return;
   }
+
+  if (idx == -1 || idx > list->n_buffers)
+    idx = list->n_buffers;
+
+  want_alloc = list->n_buffers + 1;
+
+  if (want_alloc > list->n_allocated) {
+    want_alloc = MAX (GST_ROUND_UP_16 (want_alloc), list->n_allocated * 2);
+
+    if (GST_BUFFER_LIST_IS_USING_DYNAMIC_ARRAY (list)) {
+      list->buffers = g_renew (GstBuffer *, list->buffers, want_alloc);
+    } else {
+      list->buffers = g_new0 (GstBuffer *, want_alloc);
+      memcpy (list->buffers, &list->arr[0], list->n_buffers * sizeof (void *));
+      GST_CAT_LOG (GST_CAT_PERFORMANCE, "exceeding pre-alloced array");
+    }
+
+    list->n_allocated = want_alloc;
+  }
+
+  if (idx < list->n_buffers) {
+    memmove (&list->buffers[idx + 1], &list->buffers[idx],
+        (list->n_buffers - idx) * sizeof (void *));
+  }
+
+  ++list->n_buffers;
+  list->buffers[idx] = buffer;
 }
 
 /**
@@ -288,15 +356,9 @@ gst_buffer_list_insert (GstBufferList * list, gint idx, GstBuffer * buffer)
 void
 gst_buffer_list_remove (GstBufferList * list, guint idx, guint length)
 {
-  GstBuffer *buf;
-  gint i;
-
   g_return_if_fail (GST_IS_BUFFER_LIST (list));
-  g_return_if_fail (idx < list->array->len);
+  g_return_if_fail (idx < list->n_buffers);
+  g_return_if_fail (idx + length <= list->n_buffers);
 
-  for (i = idx; i < idx + length; ++i) {
-    buf = g_array_index (list->array, GstBuffer *, i);
-    gst_buffer_unref (buf);
-  }
-  g_array_remove_range (list->array, idx, length);
+  gst_buffer_list_remove_range_internal (list, idx, length, TRUE);
 }