Index: sys/kern/subr_pctrie.c
===================================================================
--- sys/kern/subr_pctrie.c
+++ sys/kern/subr_pctrie.c
@@ -53,6 +53,7 @@
 
 #include <sys/param.h>
 #include <sys/systm.h>
+#include <sys/libkern.h>
 #include <sys/kernel.h>
 #include <sys/pctrie.h>
 #include <sys/proc.h>	/* smr.h depends on struct thread. */
@@ -265,15 +266,11 @@
 static __inline uint16_t
 pctrie_keydiff(uint64_t index1, uint64_t index2)
 {
-	uint16_t clev;
 
 	KASSERT(index1 != index2, ("%s: passing the same key value %jx",
 	    __func__, (uintmax_t)index1));
 
-	index1 ^= index2;
-	for (clev = PCTRIE_LIMIT;; clev--)
-		if (pctrie_slot(index1, clev) != 0)
-			return (clev);
+	return ((flsll(index1 ^ index2) - 1) / PCTRIE_WIDTH);
 }
 
 /*
Index: sys/vm/vm_radix.c
===================================================================
--- sys/vm/vm_radix.c
+++ sys/vm/vm_radix.c
@@ -57,6 +57,7 @@
 
 #include <sys/param.h>
 #include <sys/systm.h>
+#include <sys/libkern.h>
 #include <sys/kernel.h>
 #include <sys/proc.h>
 #include <sys/vmmeter.h>
@@ -291,15 +292,11 @@
 static __inline uint16_t
 vm_radix_keydiff(vm_pindex_t index1, vm_pindex_t index2)
 {
-	uint16_t clev;
 
 	KASSERT(index1 != index2, ("%s: passing the same key value %jx",
 	    __func__, (uintmax_t)index1));
 
-	index1 ^= index2;
-	for (clev = VM_RADIX_LIMIT;; clev--)
-		if (vm_radix_slot(index1, clev) != 0)
-			return (clev);
+	return ((flsll(index1 ^ index2) - 1) / VM_RADIX_WIDTH);
 }
 
 /*