Index: sys/sys/_bitset.h
===================================================================
--- sys/sys/_bitset.h
+++ sys/sys/_bitset.h
@@ -58,4 +58,15 @@
 #define	BITSET_FSET(n)							\
 	[ 0 ... ((n) - 1) ] = (-1L)
 
+/*
+ * Helpers to allocate a bitset without it's size being a constant.
+ *
+ * Sadly we cannot allocate a bitset struct with '__bits[]', because it's
+ * the only member of the struct.
+ */
+#define BITSET_DEFINE_VAR(t)	BITSET_DEFINE(t, 1)
+#define BITSET_FREE(p, mt)	free(p, mt)
+#define BITSET_ALLOC(p, _s, mt, mf)					\
+	p = malloc(__bitset_words(_s) * sizeof(long), mt, mf)
+
 #endif /* !_SYS__BITSET_H_ */