# Is rsqrt the same as Fast inverse square root?

조회 수: 66(최근 30일)
Hongbo Zhu 2021년 9월 13일
댓글: Hongbo Zhu 2021년 9월 16일
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
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Hongbo Zhu 2021년 9월 13일
@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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### 채택된 답변

stozaki 2021년 9월 13일
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.
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Hongbo Zhu 2021년 9월 16일
@stozaki Great! Thank you very much.

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### 추가 답변(1개)

Jan 2021년 9월 13일
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.
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Hongbo Zhu 2021년 9월 13일
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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R2018b

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