glib/gvariant: Avoid extraneous GBytes ref counting

Right now we create a bunch of GBytes which then get their reference count
incremented and immediately decremented. This causes quite a bit of
disruption for cacheline re-use.

Instead, this change creates an internal helper to transfer ownership of
GBytes to the new GVariant directly.

Surprisingly, this reduced wallclock time by about 6% for a contrived
benchmark of building "as" variant with GVariantBuilder.

glib/gvariant-core.c

@@ -593,6 +593,26 @@ GVariant *
 g_variant_new_from_bytes (const GVariantType *type,
                           GBytes             *bytes,
                           gboolean            trusted)
+{
+  return g_variant_new_take_bytes (type, g_bytes_ref (bytes), trusted);
+}
+
+/* -- internal -- */
+
+/* < internal >
+ * g_variant_new_take_bytes:
+ * @bytes: (transfer full): a #GBytes
+ * @trusted: if the contents of @bytes are trusted
+ *
+ * The same as g_variant_new_from_bytes() but takes ownership
+ * of @bytes.
+ *
+ * Returns: a new #GVariant with a floating reference
+ */
+GVariant *
+g_variant_new_take_bytes (const GVariantType *type,
+                          GBytes             *bytes,
+                          gboolean            trusted)
 {
   GVariant *value;
   guint alignment;
@@ -639,21 +659,23 @@ g_variant_new_from_bytes (const GVariantType *type,
       if (aligned_size != 0)
         memcpy (aligned_data, g_bytes_get_data (bytes, NULL), aligned_size);
 
-      bytes = owned_bytes = g_bytes_new_with_free_func (aligned_data,
-                                                        aligned_size,
-                                                        free, aligned_data);
+      owned_bytes = bytes;
+      bytes = g_bytes_new_with_free_func (aligned_data,
+                                          aligned_size,
+                                          free, aligned_data);
       aligned_data = NULL;
 #else
       /* NOTE: there may be platforms that lack posix_memalign() and also
        * have malloc() that returns non-8-aligned.  if so, we need to try
        * harder here.
        */
-      bytes = owned_bytes = g_bytes_new (g_bytes_get_data (bytes, NULL),
-                                         g_bytes_get_size (bytes));
+      owned_bytes = bytes;
+      bytes = g_bytes_new (g_bytes_get_data (bytes, NULL),
+                           g_bytes_get_size (bytes));
 #endif
     }
 
-  value->contents.serialised.bytes = g_bytes_ref (bytes);
+  value->contents.serialised.bytes = bytes;
 
   if (size && g_bytes_get_size (bytes) != size)
     {
@@ -682,8 +704,6 @@ g_variant_new_from_bytes (const GVariantType *type,
   return value;
 }
 
-/* -- internal -- */
-
 /* < internal >
  * g_variant_new_from_children:
  * @type: a #GVariantType

glib/gvariant-core.h

@@ -27,6 +27,9 @@
 
 /* gvariant-core.c */
 
+GVariant *              g_variant_new_take_bytes                        (const GVariantType  *type,
+                                                                         GBytes              *bytes,
+                                                                         gboolean             trusted);
 GVariant *              g_variant_new_from_children                     (const GVariantType  *type,
                                                                          GVariant           **children,
                                                                          gsize                n_children,

glib/gvariant.c

@@ -321,14 +321,7 @@ g_variant_new_from_trusted (const GVariantType *type,
                             gconstpointer       data,
                             gsize               size)
 {
-  GVariant *value;
-  GBytes *bytes;
-
-  bytes = g_bytes_new (data, size);
-  value = g_variant_new_from_bytes (type, bytes, TRUE);
-  g_bytes_unref (bytes);
-
-  return value;
+  return g_variant_new_take_bytes (type, g_bytes_new (data, size), TRUE);
 }
 
 /**
@@ -1311,14 +1304,8 @@ g_variant_new_take_string (gchar *string)
 
   if G_LIKELY (g_utf8_validate (string, -1, &end))
     {
-      GVariant *value;
-      GBytes *bytes;
-
-      bytes = g_bytes_new_take (string, end - string + 1);
-      value = g_variant_new_from_bytes (G_VARIANT_TYPE_STRING, bytes, TRUE);
-      g_bytes_unref (bytes);
-
-      return value;
+      GBytes *bytes = g_bytes_new_take (string, end - string + 1);
+      return g_variant_new_take_bytes (G_VARIANT_TYPE_STRING, g_steal_pointer (&bytes), TRUE);
     }
 
   g_critical ("g_variant_new_take_string(): requires valid UTF-8");
@@ -1358,8 +1345,7 @@ g_variant_new_printf (const gchar *format_string,
   va_end (ap);
 
   bytes = g_bytes_new_take (string, strlen (string) + 1);
-  value = g_variant_new_from_bytes (G_VARIANT_TYPE_STRING, bytes, TRUE);
-  g_bytes_unref (bytes);
+  value = g_variant_new_take_bytes (G_VARIANT_TYPE_STRING, g_steal_pointer (&bytes), TRUE);
 
   return value;
 }
@@ -6167,8 +6153,7 @@ g_variant_byteswap (GVariant *value)
       g_variant_serialised_byteswap (serialised);
 
       bytes = g_bytes_new_take (serialised.data, serialised.size);
-      new = g_variant_ref_sink (g_variant_new_from_bytes (g_variant_get_type (value), bytes, TRUE));
-      g_bytes_unref (bytes);
+      new = g_variant_ref_sink (g_variant_new_take_bytes (g_variant_get_type (value), g_steal_pointer (&bytes), TRUE));
     }
   else if (alignment)
     /* (potentially) contains multi-byte numeric data */
@@ -6233,7 +6218,6 @@ g_variant_new_from_data (const GVariantType *type,
                          GDestroyNotify      notify,
                          gpointer            user_data)
 {
-  GVariant *value;
   GBytes *bytes;
 
   g_return_val_if_fail (g_variant_type_is_definite (type), NULL);
@@ -6244,10 +6228,7 @@ g_variant_new_from_data (const GVariantType *type,
   else
     bytes = g_bytes_new_static (data, size);
 
-  value = g_variant_new_from_bytes (type, bytes, trusted);
-  g_bytes_unref (bytes);
-
-  return value;
+  return g_variant_new_take_bytes (type, g_steal_pointer (&bytes), trusted);
 }
 
 /* Epilogue {{{1 */