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SubjectRe: [PATCH] bitmap: optimize bitmap_remap()
On Thu, Aug 17, 2023 at 12:38:28PM +0300, Andy Shevchenko wrote:
> On Thu, Aug 17, 2023 at 12:37:05PM +0300, Andy Shevchenko wrote:
> > On Tue, Aug 15, 2023 at 04:59:34PM -0700, Yury Norov wrote:
>
> ...
>
> > > int n = bitmap_pos_to_ord(old, oldbit, nbits);
> > >
> > > + bit = (n < 0) ? oldbit : /* identity map */
> >
> > Can't you also optimize this case?
> >
> > Something like
> >
> > bitmap_xor(tmp, old, new) // maybe even better approach, dunno
>
> > bitmap_empty(tmp) // can be replaced by find first bit
>
> Or reuse bitmap_weight()...

That way it wouldn't work, but I think something like this would:

if (dst == src) /* following doesn't handle inplace remaps */
return;

w = bitmap_weight(new, nbits);
if (w == 0) {
bitmap_copy(dst, src, nbits);
return;
}

/* Identity part */
bitmap_andnot(dst, src, old, nbits);
for_each_and_bit(oldbit, src, old, nbits) {
int n = bitmap_weight(old, oldbit);
__set_bit(find_nth_bit(new, nbits, n % w), dst);
}

And it needs bitmap_weight_from() and find_nth_bit_from() to avoid
Schlemiel the Painter's problem.

You're right. This has a room for more optimizations. Thanks for
discussion. Need to give it a run.

Thanks,
Yury

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