Index: sys/kern/subr_blist.c
===================================================================
--- sys/kern/subr_blist.c
+++ sys/kern/subr_blist.c
@@ -170,6 +170,35 @@
 }
 
 /*
+ * Use binary search to find the 1 bit in a u_daddr_t.
+ * Assumes that the argument has only one bit set.  ffs(x)
+ * could be implemented as return (x==0? 0: 1+bitpos(x&-x)).
+ */
+static inline int
+bitpos(u_daddr_t mask)
+{
+	int hi, lo, mid;
+
+	switch (sizeof(u_daddr_t)) {
+#ifdef HAVE_INLINE_FFSLL
+	case sizeof(long long):
+		return (ffsll(mask) - 1);
+#endif
+	default:
+		lo = 0;
+		hi = BLIST_BMAP_RADIX;
+		while (lo + 1 < hi) {
+			mid = (lo + hi) >> 1;
+			if ((mask >> mid) != 0)
+				lo = mid;
+			else
+				hi = mid;
+		}
+		return (lo);
+	}
+}
+
+/*
  * blist_create() - create a blist capable of handling up to the specified
  *		    number of blocks
  *
@@ -355,13 +384,13 @@
  *
  *	This is the core of the allocator and is optimized for the
  *	BLIST_BMAP_RADIX block allocation case.  Otherwise, execution
- *	time is proportional to log2(count) + log2(BLIST_BMAP_RADIX).
+ *	time is proportional to log2(count) + bitpos time.
  */
 static daddr_t
 blst_leaf_alloc(blmeta_t *scan, daddr_t blk, int count, daddr_t cursor)
 {
 	u_daddr_t mask;
-	int count1, hi, lo, mid, num_shifts, range1, range_ext;
+	int count1, lo, num_shifts, range1, range_ext;
 
 	if (count == BLIST_BMAP_RADIX) {
 		/*
@@ -419,17 +448,10 @@
 	/*
 	 * The least significant set bit in mask marks the start of the first
 	 * available range of sufficient size.  Clear all the bits but that one,
-	 * and then perform a binary search to find its position.
+	 * and find its position.
 	 */
 	mask &= -mask;
-	hi = BLIST_BMAP_RADIX - count1;
-	while (lo + 1 < hi) {
-		mid = (lo + hi) >> 1;
-		if ((mask >> mid) != 0)
-			lo = mid;
-		else
-			hi = mid;
-	}
+	lo = bitpos(mask);
 
 	/*
 	 * Set in mask exactly the bits being allocated, and clear them from