Is rsqrt the same as Fast inverse square root?

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Hongbo Zhu
Hongbo Zhu on 13 Sep 2021
Commented: Hongbo Zhu on 16 Sep 2021
Hello,
I would like to us sqrt() and run it on a embeded controller. I was wondering sqrt() is the same as Fast inverse square root (https://en.wikipedia.org/wiki/Fast_inverse_square_root). if not, is it efficent as Fast inverse square root. Does it support code generation? Can I use it in embeded control software?
Thanks,
Hongbo
  4 Comments
Hongbo Zhu
Hongbo Zhu on 13 Sep 2021
@Rik Thank you very much for your quick response and explanation. It will be great if someone from MathWork can confirm rsqrt() is efficient for embeded control system.

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Accepted Answer

stozaki
stozaki on 13 Sep 2021
Hi,
The Simulink sqrt block supports rsqrt.
The algorithm is Newton-Raphson, so it can be considered the same as the one described on the wiki.
With that setting, C code generation is possible.

More Answers (1)

Jan
Jan on 13 Sep 2021
i = * ( long * ) &y
This is equivalent to:
y = single(pi);
i = typecast(y, 'int32');
The shown code of Q_rsqrt is an approximation of 1/sqrt(x) for single precision floating point values. It might have a fair speed on CPUs without a floating point unit, but it is less accurate then the direct calculation.
  1 Comment
Hongbo Zhu
Hongbo Zhu on 13 Sep 2021
Hi @Jan, Thank you very much. I noticed there is also a simulink block named Float Typecast. Does it supported by code generation for embeded controller?

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