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

[Alan Somers]

On Fri, May 6, 2016 at 8:45 AM, Kurt Lidl <lidl at pix.net> wrote:

> 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
>
>
UPDATING is updated as of r300539.  Any objection to merging this to
stable/10?