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
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