Messages in this thread | | | Date | Thu, 17 Aug 2023 07:21:59 -0700 | From | Yury Norov <> | Subject | Re: [PATCH] bitmap: optimize bitmap_remap() |
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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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