[gtk/css-lookup-caching: 3/3] Keep an emtpy lookup singleton

[Matthias Clasen <matthiasc src gnome org>]

commit 8e46f324a2f6a9b7f6a11b236dd8da36d1987b2d
Author: Matthias Clasen <mclasen redhat com>
Date:   Tue Feb 11 19:06:24 2020 -0500

    Keep an emtpy lookup singleton
    
    This is a simple optimization to avoid wasting a good
    chunk of memory (~16k in widget-factory) on identical
    structures. This is the one case that it easy to handle.

 gtk/gtkcssstaticstyle.c | 14 ++++++++++++--
 1 file changed, 12 insertions(+), 2 deletions(-)
---
diff --git a/gtk/gtkcssstaticstyle.c b/gtk/gtkcssstaticstyle.c
index 8ccc1fe9f0..dc3263abdb 100644
--- a/gtk/gtkcssstaticstyle.c
+++ b/gtk/gtkcssstaticstyle.c
@@ -336,6 +336,8 @@ gtk_css_static_style_get_static_style (GtkCssStyle *style)
   return (GtkCssStaticStyle *)style;
 }
 
+static GtkCssLookup *the_empty_lookup;
+
 static void
 gtk_css_static_style_class_init (GtkCssStaticStyleClass *klass)
 {
@@ -360,6 +362,8 @@ gtk_css_static_style_class_init (GtkCssStaticStyleClass *klass)
   gtk_css_other_values_init ();
 
   verify_style_groups ();
+
+  the_empty_lookup = gtk_css_lookup_new ();
 }
 
 static void
@@ -980,6 +984,8 @@ gtk_css_static_style_new_compute (GtkStyleProvider             *provider,
   GtkCssStaticStyle *result;
   GtkCssNode *parent;
 
+  result = g_object_new (GTK_TYPE_CSS_STATIC_STYLE, NULL);
+
   if (lookup != NULL && change != 0)
     {
       gtk_css_lookup_ref (lookup);
@@ -993,9 +999,13 @@ gtk_css_static_style_new_compute (GtkStyleProvider             *provider,
                                    node,
                                    lookup,
                                    change == 0 ? &change : NULL);
-    }
 
-  result = g_object_new (GTK_TYPE_CSS_STATIC_STYLE, NULL);
+      if (_gtk_bitmask_is_empty (gtk_css_lookup_get_set_values (lookup)))
+        {
+          gtk_css_lookup_unref (lookup);
+          lookup = gtk_css_lookup_ref (the_empty_lookup);
+        }
+    }
 
   result->lookup = lookup;
   result->change = change;