Clean up a couple of box gist helper functions.

The original idea of this patch was to make box picksplit run faster, by
eliminating unnecessary palloc() overhead, but that was obsoleted by the new
double-sorting split algorithm that doesn't call these functions so heavily
anymore. Nevertheless, the code looks better this way.

Original patch by me, reviewed and tidied up after the double-sorting patch
by Kevin Grittner.

src/backend/access/gist/gistproc.c

@@ -23,7 +23,7 @@
 
 static bool gist_box_leaf_consistent(BOX *key, BOX *query,
 						 StrategyNumber strategy);
-static double size_box(Datum dbox);
+static double size_box(BOX *box);
 static bool rtree_internal_consistent(BOX *key, BOX *query,
 						  StrategyNumber strategy);
 
@@ -35,21 +35,16 @@ static bool rtree_internal_consistent(BOX *key, BOX *query,
  * Box ops
  **************************************************/
 
-static Datum
-rt_box_union(PG_FUNCTION_ARGS)
+/*
+ * Calculates union of two boxes, a and b. The result is stored in *n.
+ */
+static void
+rt_box_union(BOX *n, BOX *a, BOX *b)
 {
-	BOX		   *a = PG_GETARG_BOX_P(0);
-	BOX		   *b = PG_GETARG_BOX_P(1);
-	BOX		   *n;
-
-	n = (BOX *) palloc(sizeof(BOX));
-
 	n->high.x = Max(a->high.x, b->high.x);
 	n->high.y = Max(a->high.y, b->high.y);
 	n->low.x = Min(a->low.x, b->low.x);
 	n->low.y = Min(a->low.y, b->low.y);
-
-	PG_RETURN_BOX_P(n);
 }
 
 /*
@@ -166,10 +161,12 @@ gist_box_penalty(PG_FUNCTION_ARGS)
 	GISTENTRY  *origentry = (GISTENTRY *) PG_GETARG_POINTER(0);
 	GISTENTRY  *newentry = (GISTENTRY *) PG_GETARG_POINTER(1);
 	float	   *result = (float *) PG_GETARG_POINTER(2);
-	Datum		ud;
+	BOX		   *origbox = DatumGetBoxP(origentry->key);
+	BOX		   *newbox = DatumGetBoxP(newentry->key);
+	BOX			unionbox;
 
-	ud = DirectFunctionCall2(rt_box_union, origentry->key, newentry->key);
-	*result = (float) (size_box(ud) - size_box(origentry->key));
+	rt_box_union(&unionbox, origbox, newbox);
+	*result = (float) (size_box(&unionbox) - size_box(origbox));
 	PG_RETURN_POINTER(result);
 }
 
@@ -937,11 +934,9 @@ gist_box_leaf_consistent(BOX *key, BOX *query, StrategyNumber strategy)
 }
 
 static double
-size_box(Datum dbox)
+size_box(BOX *box)
 {
-	BOX		   *box = DatumGetBoxP(dbox);
-
-	if (box == NULL || box->high.x <= box->low.x || box->high.y <= box->low.y)
+	if (box->high.x <= box->low.x || box->high.y <= box->low.y)
 		return 0.0;
 	return (box->high.x - box->low.x) * (box->high.y - box->low.y);
 }