--- linux-2.5/include/linux/mman.h	Sun Sep  7 10:05:18 2003
+++ optimizing-macro-2.6.0-t6/include/linux/mman.h	Tue Sep 30 09:47:19 2003
@@ -28,8 +28,13 @@
 	vm_acct_memory(-pages);
 }
 
-/* Optimisation macro. */
-#define _calc_vm_trans(x,bit1,bit2) \
+/* 
+ * Optimisation macro.  It is equivalent to: 
+ *      (x & bit1) ? bit2 : 0
+ * but this version is faster.  
+ * ("bit1" and "bit2" must be single bits). 
+ */
+#define _calc_vm_trans(x, bit1, bit2) \
   ((bit1) <= (bit2) ? ((x) & (bit1)) * ((bit2) / (bit1)) \
    : ((x) & (bit1)) / ((bit1) / (bit2)))