Index: sys/sys/libkern.h
===================================================================
--- sys/sys/libkern.h
+++ sys/sys/libkern.h
@@ -186,6 +186,31 @@
 	    8 * sizeof(mask) - __builtin_clzll((unsigned long long)mask));
 }
 
+static __inline __pure2 int
+ilog2(int mask)
+{
+
+	MPASS(mask != 0);
+	return (8 * sizeof(mask) - 1 - __builtin_clz((u_int)mask));
+}
+
+static __inline __pure2 int
+ilog2l(long mask)
+{
+
+	MPASS(mask != 0);
+	return (8 * sizeof(mask) - 1 - __builtin_clzl((u_long)mask));
+}
+
+static __inline __pure2 int
+ilog2ll(long long mask)
+{
+
+	MPASS(mask != 0);
+	return (8 * sizeof(mask) - 1 -
+	    __builtin_clzll((unsigned long long)mask));
+}
+
 #define	bitcount64(x)	__bitcount64((uint64_t)(x))
 #define	bitcount32(x)	__bitcount32((uint32_t)(x))
 #define	bitcount16(x)	__bitcount16((uint16_t)(x))