Messages in this thread | | | From | David Laight <> | Subject | RE: [PATCH v10 1/3] lib: Add strongly typed 64bit int_sqrt | Date | Thu, 21 Dec 2017 10:59:19 +0000 |
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From: Crt Mori > Sent: 20 December 2017 17:30 > >> OK, is there any more easy optimizations you see? > > > > I think this version works. > > It doesn't have the optimisation for small values. > > > > unsigned int sqrt64(unsigned long long x) > > { > > unsigned int x_hi = x >> 32; > > > > unsigned int b = 0; > > unsigned int y = 0; > > unsigned int i; > > > > for (i = 0; i < 32; i++) { > > b <<= 2; > > b |= x_hi >> 30; > > x_hi <<= 2; > > if (i == 15) > > x_hi = x; > > y <<= 1; > > if (b > y) > > b -= ++y; > > } > > return y; > > } .. > > I did a quick run through unit tests for the sensor and the results > are way off. On the sensor I had to convert double calculations to > integer calculations and target was to get end result within 0.02 degC > (with previous approximate sqrt implementation) in sensor working > range. This now gets into 3 degC delta at least and some are way off. > It might be off because of some scaling on the other hand during the > equation (not exactly comparing sqrt implementations here).
I didn't get it quite right... The last few lines need to be: if (b > y) { b -= ++y; y++; } } return y >> 1; }
Although that then fails for inputs larger than 2^62.
David
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