[glib/ebassi/object-allocator] gtype: Use the system allocator on UNIX

[Emmanuele Bassi <ebassi src gnome org>]

commit 3dc2901ff09f4dcedfa3c7e39c07bb68eff1b0a1
Author: Emmanuele Bassi <ebassi gnome org>
Date:   Wed Jul 6 20:43:45 2022 +0100

    gtype: Use the system allocator on UNIX
    
    Instead of replacing the slice allocator wholesale, we can start phasing
    it out by having GTypeInstance use the system allocator on operating
    systems where we can assume good performance profiles.
    
    We cannot commit to fully gutting GSlice in the cases where we might
    still need it, like the G(S)List allocator and small, similarly-sized
    data structures.
    
    The main user of GSlice is still GTypeInstance/GObject, and those have
    moved out of the sweet spot of GSlice's performance envelove over the
    years, with larger instance sizes and private data.
    
    See: #1079

 gobject/gtype.c | 20 ++++++++++++++++----
 1 file changed, 16 insertions(+), 4 deletions(-)
---
diff --git a/gobject/gtype.c b/gobject/gtype.c
index 177e446918..30ca7e30fb 100644
--- a/gobject/gtype.c
+++ b/gobject/gtype.c
@@ -1824,6 +1824,18 @@ type_iface_blow_holder_info_Wm (TypeNode *iface,
     }
 }
 
+/* We use the system allocator on UNIX-y systems, where we know we have
+ * access to a decent allocator. On other systems, we fall back to the
+ * slice allocator, as we know its performance profile
+ */
+#ifdef G_OS_UNIX
+# define instance_alloc(s)  g_malloc0 ((s))
+# define instance_free(s,p) g_free ((p))
+#else
+# define instance_alloc(s)  g_slice_alloc0 ((s))
+# define instance_free(s,p) g_slice_free1 ((s),(p))
+#endif
+
 /**
  * g_type_create_instance: (skip)
  * @type: an instantiatable type to create an instance for
@@ -1900,7 +1912,7 @@ g_type_create_instance (GType type)
       private_size += ALIGN_STRUCT (1);
 
       /* Allocate one extra pointer size... */
-      allocated = g_slice_alloc0 (private_size + ivar_size + sizeof (gpointer));
+      allocated = instance_alloc (private_size + ivar_size + sizeof (gpointer));
       /* ... and point it back to the start of the private data. */
       *(gpointer *) (allocated + private_size + ivar_size) = allocated + ALIGN_STRUCT (1);
 
@@ -1910,7 +1922,7 @@ g_type_create_instance (GType type)
     }
   else
 #endif
-    allocated = g_slice_alloc0 (private_size + ivar_size);
+    allocated = instance_alloc (private_size + ivar_size);
 
   instance = (GTypeInstance *) (allocated + private_size);
 
@@ -2000,14 +2012,14 @@ g_type_free_instance (GTypeInstance *instance)
       /* Clear out the extra pointer... */
       *(gpointer *) (allocated + private_size + ivar_size) = NULL;
       /* ... and ensure we include it in the size we free. */
-      g_slice_free1 (private_size + ivar_size + sizeof (gpointer), allocated);
+      instance_free (private_size + ivar_size + sizeof (gpointer), allocated);
 
       VALGRIND_FREELIKE_BLOCK (allocated + ALIGN_STRUCT (1), 0);
       VALGRIND_FREELIKE_BLOCK (instance, 0);
     }
   else
 #endif
-    g_slice_free1 (private_size + ivar_size, allocated);
+    instance_free (private_size + ivar_size, allocated);
 
 #ifdef G_ENABLE_DEBUG
   IF_DEBUG (INSTANCE_COUNT)