Add bit_count to the bitstring(3) api

Add a bit_count function, which efficiently counts the number of bits set in
a bitstring.

sys/sys/bitstring.h
tests/sys/sys/bitstring_test.c
share/man/man3/bitstring.3
	Add bit_alloc

sys/kern/subr_unit.c
	Use bit_count instead of a naive counting loop in check_unrhdr, used
	when INVARIANTS are enabled. The userland test runs about 6x faster
	in a generic build, or 8.5x faster when built for Nehalem, which has
	the POPCNT instruction.

sys/sys/param.h
	Bump __FreeBSD_version due to the addition of bit_alloc

UPDATING
	Add a note about the ABI incompatibility of the bitstring(3)
	changes, as suggested by lidl.

Suggested by:	gibbs
Reviewed by:	gibbs, ngie
MFC after:	9 days
X-MFC-With:	299090, 300538
Relnotes:	yes
Sponsored by:	Spectra Logic Corp
Differential Revision:	https://reviews.freebsd.org/D6255

1 files changed, 6 insertions, 0 deletions

diff --git a/UPDATING b/UPDATING
index fd47ac0..df0542d 100644
--- a/UPDATING
+++ b/UPDATING
@@ -31,6 +31,12 @@ NOTE TO PEOPLE WHO THINK THAT FreeBSD 11.x IS SLOW:
 	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