svn commit: r299090 - in head: etc/mtree include lib/libbluetooth sbin/hastd share/man/man3 sys/dev/xen/blkback sys/kern sys/net sys/sys tests/sys tests/sys/sys usr.sbin/bluetooth/hccontrol

[Kurt Lidl]

On 5/5/16 12:31 PM, John Baldwin wrote:
> On Wednesday, May 04, 2016 10:34:11 PM Alan Somers wrote:
>> Author: asomers
>> Date: Wed May  4 22:34:11 2016
>> New Revision: 299090
>> URL: https://svnweb.freebsd.org/changeset/base/299090
>>
>> Log:
>>   Improve performance and functionality of the bitstring(3) api
>>
>>   Two new functions are provided, bit_ffs_at() and bit_ffc_at(), which allow
>>   for efficient searching of set or cleared bits starting from any bit offset
>>   within the bit string.
>>
>>   Performance is improved by operating on longs instead of bytes and using
>>   ffsl() for searches within a long. ffsl() is a compiler builtin in both
>>   clang and gcc for most architectures, converting what was a brute force
>>   while loop search into a couple of instructions.
>>
>>   All of the bitstring(3) API continues to be contained in the header file.
>>   Some of the functions are large enough that perhaps they should be uninlined
>>   and moved to a library, but that is beyond the scope of this commit.
>
> Doesn't switching from bytes to longs break the ABI?  That is, setting bit 9
> now has a different representation on big-endian systems (0x00 0x01 before,
> now 0x00 0x00 0x01 0x00 on 32-bit BE, and 4 more leading 0 bytes on 64-bit).
> This means you can't have an object file compiled against the old header
> pass a bitstring to an object file compiled against the new header on big-endian
> systems.
>
> Even on little-endian systems if an old object file allocates storage for a
> bitstring the new code might read off the end of it and fault (or return
> garbage if bits are set in the extra bytes it reads off the end)?
>
> Is the API is so little used we don't care?
>

Just as a note - at my prior job (Pi-Coral, now defunct) we used this
API everywhere in the dataplane code of our product.  Since the company
is gone, that particular use-case doesn't matter anymore.

At the very least, this deserves a mention in the release notes, and
also UPDATING!

-Kurt