Re: [Qemu-devel] [PATCH v18 05/10] xbitmap: add more operations

[Wei Wang]

On 12/03/2017 09:50 AM, Tetsuo Handa wrote:
> Matthew Wilcox wrote:
> > On Fri, Dec 01, 2017 at 03:09:08PM +0000, Wang, Wei W wrote:
> > > On Friday, December 1, 2017 9:02 PM, Tetsuo Handa wrote:
> > > > If start == end is legal,
> > > >
> > > >     for (; start < end; start = (start | (IDA_BITMAP_BITS - 1)) + 1) {
> > > >
> > > > makes this loop do nothing because 10 < 10 is false.
> > > How about "start <= end "?
> > Don't ask Tetsuo for his opinion, write some userspace code that uses it.
> Please be sure to prepare for "end == -1UL" case, for "start < end" will become
> true when "start = (start | (IDA_BITMAP_BITS - 1)) + 1" made "start == 0" due to
> overflow.

I think there is one more corner case with this API: searching for bit 
"1" from [0, ULONG_MAX] while no bit is set in the range, there appear 
to be no possible value that we can return (returning "end + 1" will be 
"ULONG_MAX + 1", which is 0)
I plan to make the "end" be exclusive of the searching, that is, [start, 
end), and return "end" if no such bit is found.

For cases like [16, 16), returning 16 doesn't mean bit 16 is 1 or 0, it 
simply means there is no bits to search in the given range, since 16 is 
exclusive.

Please let me know if you have a different thought.

Best,
Wei