How to optimise the power computation

[Fede Rico]

Posted on September 26, 2017 at 11:58

Hi there,

I'm working on STM32F103 MCU and i need to compute the powerof a value.

I use the math.h library but I'm looking for a more optimised library, if it's exists.

Is there a better way to compute the power?

Best Regards,

F.

#math.h

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[Danish1]

Posted on September 26, 2017 at 15:11

Am I right in thinking that you are trying to compute x^y but you find it takes too long on stm32f103?

If it is taking too long then that often means you want to do it many times in quick succession, perhaps with either x or y not changing between calculations.

In C, pow(x, y) does things in double-precision. If x and y are not double-precision (e.g. float - ) then it first has to convert them to double-precision, and if you only need the result to be float then it has to convert back to float. All 3 steps waste time. powf(x, y) is faster if you only need float.

And if x or y are integers (e.g you only want to calculate x^2) then it is much faster to explicitly do x*x

How else might things be speeded up?

pow calculates exp(y*log(x))

So if x doesn't change from call to call, then you can pre-calculate the log.

I'm sure there are other things to do to improve the speed of calculation, but only because you know something about the calculation or series-of-calculations that the compiler didn't guess.

I'd expect the math library to be pretty-well optimised and fast for double-precision numbers in general.

Hope this helps,

Danish

[AvaTar]

Posted on September 26, 2017 at 15:25

In C, pow(x, y) does things in double-precision.

As usual, there is a single-precision variant in C lib too, here called

powf()

.

Or, you are searching for some integer based algorithm. This function is not architecture-dependent, so you can try to port any algorithm.

Or, depending on your requirements, even a table-based approach might do, possibly with interpolation.

A lot depends on your ideas of performance, precision, and parameter range.

[Fede Rico]

Posted on September 26, 2017 at 15:27

Basically I want use the powf to read the distance from this distance sensor: GP2Y0A41SK0F

I found a libray (for arduino ) that uses the power