Here is v6 of cube penalty patch.
There was a warning about unused edge function  under systems without
__STDC_IEC_559__ defined, this patch fixes it.

Regards, Andrey Borodin.
diff --git a/contrib/cube/cube.c b/contrib/cube/cube.c
index 3feddef..ad868ac 100644
--- a/contrib/cube/cube.c
+++ b/contrib/cube/cube.c
@@ -18,6 +18,7 @@
 
 #include "cubedata.h"
 
+
 PG_MODULE_MAGIC;
 
 /*
@@ -27,6 +28,15 @@ PG_MODULE_MAGIC;
 #define ARRNELEMS(x)  ArrayGetNItems( ARR_NDIM(x), ARR_DIMS(x))
 
 /*
+ * If IEEE754 floats are fully supported we use advanced penalty
+ * which takes into account cube volume, perimeter and tend to favor
+ * small nodes over big ones. For more info see g_cube_penalty implementation
+ */
+#ifdef __STDC_IEC_559__
+#define ADVANCED_PENALTY
+#endif
+
+/*
 ** Input/Output routines
 */
 PG_FUNCTION_INFO_V1(cube_in);
@@ -91,14 +101,18 @@ PG_FUNCTION_INFO_V1(cube_enlarge);
 /*
 ** For internal use only
 */
-int32          cube_cmp_v0(NDBOX *a, NDBOX *b);
-bool           cube_contains_v0(NDBOX *a, NDBOX *b);
-bool           cube_overlap_v0(NDBOX *a, NDBOX *b);
-NDBOX     *cube_union_v0(NDBOX *a, NDBOX *b);
-void           rt_cube_size(NDBOX *a, double *sz);
-NDBOX     *g_cube_binary_union(NDBOX *r1, NDBOX *r2, int *sizep);
-bool           g_cube_leaf_consistent(NDBOX *key, NDBOX *query, StrategyNumber 
strategy);
-bool           g_cube_internal_consistent(NDBOX *key, NDBOX *query, 
StrategyNumber strategy);
+static int32           cube_cmp_v0(NDBOX *a, NDBOX *b);
+static bool            cube_contains_v0(NDBOX *a, NDBOX *b);
+static bool            cube_overlap_v0(NDBOX *a, NDBOX *b);
+static NDBOX      *cube_union_v0(NDBOX *a, NDBOX *b);
+#ifdef ADVANCED_PENALTY
+static float           pack_float(float value, int realm);
+static void            rt_cube_edge(NDBOX *a, double *sz);
+#endif
+static void            rt_cube_size(NDBOX *a, double *sz);
+static NDBOX      *g_cube_binary_union(NDBOX *r1, NDBOX *r2, int *sizep);
+static bool            g_cube_leaf_consistent(NDBOX *key, NDBOX *query, 
StrategyNumber strategy);
+static bool            g_cube_internal_consistent(NDBOX *key, NDBOX *query, 
StrategyNumber strategy);
 
 /*
 ** Auxiliary funxtions
@@ -419,7 +433,36 @@ g_cube_decompress(PG_FUNCTION_ARGS)
        }
        PG_RETURN_POINTER(entry);
 }
+/*
+ * Function to pack floats of different realms
+ * This function serves to pack bit flags inside float type
+ * Resulted value represent can be from four different "realms"
+ * Every value from realm 3 is greater than any value from realms 2,1 and 0.
+ * Every value from realm 2 is less than every value from realm 3 and greater
+ * than any value from realm 1 and 0, and so on. Values from the same realm
+ * loose two bits of precision. This technique is possible due to floating
+ * point numbers specification according to IEEE 754: exponent bits are highest
+ * (excluding sign bits, but here penalty is always positive). If float a is
+ * greater than float b, integer A with same bit representation as a is greater
+ * than integer B with same bits as b.
+ */
+#ifdef ADVANCED_PENALTY
+static float
+pack_float(const float value, const int realm)
+{
+  union {
+    float f;
+    struct { unsigned value:31, sign:1; } vbits;
+    struct { unsigned value:29, realm:2, sign:1; } rbits;
+  } a;
+
+  a.f = value;
+  a.rbits.value = a.vbits.value >> 2;
+  a.rbits.realm = realm;
 
+  return a.f;
+}
+#endif
 
 /*
 ** The GiST Penalty method for boxes
@@ -441,9 +484,42 @@ g_cube_penalty(PG_FUNCTION_ARGS)
        rt_cube_size(DatumGetNDBOX(origentry->key), &tmp2);
        *result = (float) (tmp1 - tmp2);
 
-       /*
-        * fprintf(stderr, "penalty\n"); fprintf(stderr, "\t%g\n", *result);
-        */
+#ifdef ADVANCED_PENALTY
+       /* Realm tricks are used only in case of IEEE754 support(IEC 60559) */
+
+       /* REALM 0: No extension is required, volume is zero, return edge       
*/
+       /* REALM 1: No extension is required, return nonzero volume             
        */
+       /* REALM 2: Volume extension is zero, return nonzero edge extension     
*/
+       /* REALM 3: Volume extension is nonzero, return it                      
                */
+
+       if( *result == 0 )
+       {
+               double tmp3 = tmp1; /* remember entry volume */
+               rt_cube_edge(ud, &tmp1);
+               rt_cube_edge(DatumGetNDBOX(origentry->key), &tmp2);
+               *result = (float) (tmp1 - tmp2);
+               if( *result == 0 )
+               {
+                       if( tmp3 != 0 )
+                       {
+                               *result = pack_float(tmp3, 1); /* REALM 1 */
+                       }
+                       else
+                       {
+                               *result = pack_float(tmp1, 0); /* REALM 0 */
+                       }
+               }
+               else
+               {
+                       *result = pack_float(*result, 2); /* REALM 2 */
+               }
+       }
+       else
+       {
+               *result = pack_float(*result, 3); /* REALM 3 */
+       }
+#endif
+
        PG_RETURN_FLOAT8(*result);
 }
 
@@ -626,7 +702,7 @@ g_cube_same(PG_FUNCTION_ARGS)
 /*
 ** SUPPORT ROUTINES
 */
-bool
+static bool
 g_cube_leaf_consistent(NDBOX *key,
                                           NDBOX *query,
                                           StrategyNumber strategy)
@@ -658,7 +734,7 @@ g_cube_leaf_consistent(NDBOX *key,
        return (retval);
 }
 
-bool
+static bool
 g_cube_internal_consistent(NDBOX *key,
                                                   NDBOX *query,
                                                   StrategyNumber strategy)
@@ -688,7 +764,7 @@ g_cube_internal_consistent(NDBOX *key,
        return (retval);
 }
 
-NDBOX *
+static NDBOX *
 g_cube_binary_union(NDBOX *r1, NDBOX *r2, int *sizep)
 {
        NDBOX      *retval;
@@ -701,7 +777,7 @@ g_cube_binary_union(NDBOX *r1, NDBOX *r2, int *sizep)
 
 
 /* cube_union_v0 */
-NDBOX *
+static NDBOX *
 cube_union_v0(NDBOX *a, NDBOX *b)
 {
        int                     i;
@@ -875,25 +951,42 @@ cube_size(PG_FUNCTION_ARGS)
        PG_RETURN_FLOAT8(result);
 }
 
-void
+static void
 rt_cube_size(NDBOX *a, double *size)
 {
        int                     i;
+       double          result = 0;
 
-       if (a == (NDBOX *) NULL)
-               *size = 0.0;
-       else
+       if (a != (NDBOX *) NULL)
        {
-               *size = 1.0;
+               result = 1.0;
                for (i = 0; i < DIM(a); i++)
-                       *size = (*size) * Abs(UR_COORD(a, i) - LL_COORD(a, i));
+                       result *= UR_COORD(a, i) - LL_COORD(a, i);
        }
+       *size = Abs(result);
        return;
 }
 
+#ifdef ADVANCED_PENALTY
+static void
+rt_cube_edge(NDBOX *a, double *size)
+{
+       int                     i;
+       double          result = 0;
+
+       if (a != (NDBOX *) NULL)
+       {
+               for (i = 0; i < DIM(a); i++)
+                       result += Abs(UR_COORD(a, i) - LL_COORD(a, i));
+       }
+       *size = result;
+       return;
+}
+#endif
+
 /* make up a metric in which one box will be 'lower' than the other
    -- this can be useful for sorting and to determine uniqueness */
-int32
+static int32
 cube_cmp_v0(NDBOX *a, NDBOX *b)
 {
        int                     i;
@@ -1080,7 +1173,7 @@ cube_ge(PG_FUNCTION_ARGS)
 
 /* Contains */
 /* Box(A) CONTAINS Box(B) IFF pt(A) < pt(B) */
-bool
+static bool
 cube_contains_v0(NDBOX *a, NDBOX *b)
 {
        int                     i;
@@ -1150,7 +1243,7 @@ cube_contained(PG_FUNCTION_ARGS)
 
 /* Overlap */
 /* Box(A) Overlap Box(B) IFF (pt(a)LL < pt(B)UR) && (pt(b)LL < pt(a)UR) */
-bool
+static bool
 cube_overlap_v0(NDBOX *a, NDBOX *b)
 {
        int                     i;
-- 
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