Index: head/UPDATING
===================================================================
--- head/UPDATING
+++ head/UPDATING
@@ -31,6 +31,12 @@
 	disable the most expensive debugging functionality run
 	"ln -s 'abort:false,junk:false' /etc/malloc.conf".)
 
+20160523:
+	The bitstring(3) API has been updated with new functionality and
+	improved performance.  But it is binary-incompatible with the old API.
+	Objects built with the new headers may not be linked against objects
+	built with the old headers.
+
 20160520:
 	The brk and sbrk functions have been removed from libc on arm64.
 	Binutils from ports has been updated to not link to these
Index: head/share/man/man3/bitstring.3
===================================================================
--- head/share/man/man3/bitstring.3
+++ head/share/man/man3/bitstring.3
@@ -27,7 +27,7 @@
 .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 .\" SUCH DAMAGE.
 .\"
-.\" Copyright (c) 2014 Spectra Logic Corporation
+.\" Copyright (c) 2014,2016 Spectra Logic Corporation
 .\" All rights reserved.
 .\"
 .\" Redistribution and use in source and binary forms, with or without
@@ -58,12 +58,13 @@
 .\"     @(#)bitstring.3	8.1 (Berkeley) 7/19/93
 .\" $FreeBSD$
 .\"
-.Dd May 4, 2016
+.Dd May 23, 2016
 .Dt BITSTRING 3
 .Os
 .Sh NAME
 .Nm bit_alloc ,
 .Nm bit_clear ,
+.Nm bit_count ,
 .Nm bit_decl ,
 .Nm bit_ffc ,
 .Nm bit_ffs ,
@@ -84,6 +85,8 @@
 .Ft void
 .Fn bit_clear "bitstr_t *name" "int bit"
 .Ft void
+.Fn bit_count "bitstr_t *name" "int count" "int nbits" "int *value"
+.Ft void
 .Fn bit_ffc "bitstr_t *name" "int nbits" "int *value"
 .Ft void
 .Fn bit_ffs "bitstr_t *name" "int nbits" "int *value"
@@ -225,6 +228,17 @@
 .Fa value
 is set to \-1.
 .Pp
+The
+.Fn bit_count
+function stores in the location referenced by
+.Fa value
+the number of bits set in the array of
+.Fa nbits
+bits referenced by
+.Fa name ,
+at or after the zero-based bit index
+.Fa start .
+.Pp
 The arguments in bit string macros are evaluated only once and may safely
 have side effects.
 .Sh EXAMPLES
Index: head/sys/kern/subr_unit.c
===================================================================
--- head/sys/kern/subr_unit.c
+++ head/sys/kern/subr_unit.c
@@ -224,7 +224,8 @@
 {
 	struct unr *up;
 	struct unrb *ub;
-	u_int x, y, z, w;
+	int w;
+	u_int y, z;
 
 	y = uh->first;
 	z = 0;
@@ -237,9 +238,7 @@
 			    up->len, NBITS, line));
 			z++;
 			w = 0;
-			for (x = 0; x < up->len; x++)
-				if (bit_test(ub->map, x))
-					w++;
+			bit_count(ub->map, 0, up->len, &w);
 			y += w;
 		} else if (up->ptr != NULL) 
 			y += up->len;
Index: head/sys/sys/bitstring.h
===================================================================
--- head/sys/sys/bitstring.h
+++ head/sys/sys/bitstring.h
@@ -65,6 +65,7 @@
 #ifdef _KERNEL
 #include <sys/libkern.h>
 #include <sys/malloc.h>
+#include <sys/types.h>
 #endif
 
 typedef	unsigned long bitstr_t;
@@ -202,7 +203,7 @@
 			_test &= _bit_make_mask(_start, _BITSTR_BITS - 1);
 		while (_test == 0 && _curbitstr < _stopbitstr)
 			_test = *(++_curbitstr);
-		
+
 		_offset = ffsl(_test);
 		_value = ((_curbitstr - _bitstr) * _BITSTR_BITS) + _offset - 1;
 		if (_offset == 0 || _value >= _nbits)
@@ -231,7 +232,7 @@
 			_test |= _bit_make_mask(0, _start - 1);
 		while (_test == _BITSTR_MASK && _curbitstr < _stopbitstr)
 			_test = *(++_curbitstr);
-		
+
 		_offset = ffsl(~_test);
 		_value = ((_curbitstr - _bitstr) * _BITSTR_BITS) + _offset - 1;
 		if (_offset == 0 || _value >= _nbits)
@@ -256,4 +257,40 @@
 	bit_ffc_at(_bitstr, /*start*/0, _nbits, _result);
 }
 
+/* Count the number of bits set in a bitstr of size _nbits at or after _start */
+static inline void
+bit_count(bitstr_t *_bitstr, int _start, int _nbits, int *_result)
+{
+	bitstr_t *_curbitstr, mask;
+	int _value = 0, curbitstr_len;
+
+	if (_start >= _nbits)
+		goto out;
+
+	_curbitstr = _bitstr + _bit_idx(_start);
+	_nbits -= _BITSTR_BITS * _bit_idx(_start);
+	_start -= _BITSTR_BITS * _bit_idx(_start);
+
+	if (_start > 0) {
+		curbitstr_len = (int)_BITSTR_BITS < _nbits ?
+				(int)_BITSTR_BITS : _nbits;
+		mask = _bit_make_mask(_start, _bit_offset(curbitstr_len - 1));
+		_value += __bitcountl(*_curbitstr & mask);
+		_curbitstr++;
+		_nbits -= _BITSTR_BITS;
+	}
+	while (_nbits >= (int)_BITSTR_BITS) {
+		_value += __bitcountl(*_curbitstr);
+		_curbitstr++;
+		_nbits -= _BITSTR_BITS;
+	}
+	if (_nbits > 0) {
+		mask = _bit_make_mask(0, _bit_offset(_nbits - 1));
+		_value += __bitcountl(*_curbitstr & mask);
+	}
+
+out:
+	*_result = _value;
+}
+
 #endif	/* _SYS_BITSTRING_H_ */
Index: head/sys/sys/param.h
===================================================================
--- head/sys/sys/param.h
+++ head/sys/sys/param.h
@@ -58,7 +58,7 @@
  *		in the range 5 to 9.
  */
 #undef __FreeBSD_version
-#define __FreeBSD_version 1100111	/* Master, propagated to newvers */
+#define __FreeBSD_version 1100112	/* Master, propagated to newvers */
 
 /*
  * __FreeBSD_kernel__ indicates that this system uses the kernel of FreeBSD,
Index: head/tests/sys/sys/bitstring_test.c
===================================================================
--- head/tests/sys/sys/bitstring_test.c
+++ head/tests/sys/sys/bitstring_test.c
@@ -342,6 +342,67 @@
 	}
 }
 
+BITSTRING_TC_DEFINE(bit_count)
+/* bitstr_t *bitstr, int nbits, const char *memloc */
+{
+	int result, s, e, expected;
+
+	/* Empty bitstr */
+	memset(bitstr, 0, bitstr_size(nbits));
+	bit_count(bitstr, 0, nbits, &result);
+	ATF_CHECK_MSG(0 == result,
+			"bit_count_%d_%s_%s: Failed with result %d",
+			nbits, "clear", memloc, result);
+
+	/* Full bitstr */
+	memset(bitstr, 0xFF, bitstr_size(nbits));
+	bit_count(bitstr, 0, nbits, &result);
+	ATF_CHECK_MSG(nbits == result,
+			"bit_count_%d_%s_%s: Failed with result %d",
+			nbits, "set", memloc, result);
+
+	/* Invalid _start value */
+	memset(bitstr, 0xFF, bitstr_size(nbits));
+	bit_count(bitstr, nbits, nbits, &result);
+	ATF_CHECK_MSG(0 == result,
+			"bit_count_%d_%s_%s: Failed with result %d",
+			nbits, "invalid_start", memloc, result);
+	
+	/* Alternating bitstr, starts with 0 */
+	memset(bitstr, 0xAA, bitstr_size(nbits));
+	bit_count(bitstr, 0, nbits, &result);
+	ATF_CHECK_MSG(nbits / 2 == result,
+			"bit_count_%d_%s_%d_%s: Failed with result %d",
+			nbits, "alternating", 0, memloc, result);
+
+	/* Alternating bitstr, starts with 1 */
+	memset(bitstr, 0x55, bitstr_size(nbits));
+	bit_count(bitstr, 0, nbits, &result);
+	ATF_CHECK_MSG((nbits + 1) / 2 == result,
+			"bit_count_%d_%s_%d_%s: Failed with result %d",
+			nbits, "alternating", 1, memloc, result);
+
+	/* Varying start location */
+	memset(bitstr, 0xAA, bitstr_size(nbits));
+	for (s = 0; s < nbits; s++) {
+		expected = s % 2 == 0 ? (nbits - s) / 2 : (nbits - s + 1) / 2;
+		bit_count(bitstr, s, nbits, &result);
+		ATF_CHECK_MSG(expected == result,
+				"bit_count_%d_%s_%d_%s: Failed with result %d",
+				nbits, "vary_start", s, memloc, result);
+	}
+
+	/* Varying end location */
+	memset(bitstr, 0xAA, bitstr_size(nbits));
+	for (e = 0; e < nbits; e++) {
+		bit_count(bitstr, 0, e, &result);
+		ATF_CHECK_MSG(e / 2 == result,
+				"bit_count_%d_%s_%d_%s: Failed with result %d",
+				nbits, "vary_end", e, memloc, result);
+	}
+
+}
+
 ATF_TP_ADD_TCS(tp)
 {
 
@@ -354,6 +415,7 @@
 	BITSTRING_TC_ADD(tp, bit_ffc_at);
 	BITSTRING_TC_ADD(tp, bit_nclear);
 	BITSTRING_TC_ADD(tp, bit_nset);
+	BITSTRING_TC_ADD(tp, bit_count);
 
 	return (atf_no_error());
 }