# Is rsqrt the same as Fast inverse square root?

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Hongbo Zhu on 13 Sep 2021
Edited: Walter Roberson on 31 Jul 2022
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
Rik on 13 Sep 2021
It might be, although it doesn't have to be. Implementing this C code as a mex function might speed up what you want to do, or might slow it down.
If you don't know what you're working with well enough to know this code is not Matlab but C, I am not sure you would be able to write something faster than the engineers working for Mathworks. This algorithm is famous, I would expect them to know it. It would highly surprise me if they haven't tried implementing it to see the relative performance.
What is the reason for your concern about performance? If performance is so important, why aren't you doing everything in C anyway (or in a language even closer to your bare metal of choice)? I personally have the view that Matlab (and Simulink) are great tools for rapid prototyping and doing complex things. However, they tend to be the slower choice if performance is absolutely critical (it often isn't).
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.

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.
Hongbo Zhu on 16 Sep 2021
@stozaki Great! Thank you very much.

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.
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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