I have already written about approximations of `e^x`, `log(x)` and `pow(a, b)` in my post Optimized Exponential Functions for Java. Now I have more 🙂 In particular, the `pow()` function is now even faster, simpler, and more accurate. Without further ado, I proudly give you the brand new approximation:

Contents

# Approximation of pow() in Java

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public static double pow(final double a, final double b) { final int x = (int) (Double.doubleToLongBits(a) >> 32); final int y = (int) (b * (x - 1072632447) + 1072632447); return Double.longBitsToDouble(((long) y) << 32); } |

This is really very compact. The calculation only requires 2 shifts, 1 mul, 2 add, and 2 register operations. That’s it! In my tests it usually within an error margin of 5% to 12%, in extreme cases sometimes up to 25%. A careful analysis is left as an exercise for the reader. This is very usable for in e.g. metaheuristics or neural nets.

## UPDATE, December 10, 2011

I just managed to make the above code about 30% faster than the one above on my machine. The error is a tiny fraction different (not better or worse).

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public static double pow(final double a, final double b) { final long tmp = Double.doubleToLongBits(a); final long tmp2 = (long)(b * (tmp - 4606921280493453312L)) + 4606921280493453312L; return Double.longBitsToDouble(tmp2); } |

This new approximation is about **23 times** as fast as Math.pow() on my machine (Intel Core2 Quad, Q9550, Java 1.7.0_01-b08, 64-Bit Server VM). Unfortunately, microbenchmarks are difficult to do in Java, so your mileage may vary. You can download the benchmark PowBench.java and have a look, I have tried to prevent overoptimization, and substract the overhead introduced due to this preventation.

# Approximation of pow() in C and C++

## UPDATE, January 25, 2012

The code below is updated with using union, you do not need `-fno-strict-aliasing` any more for compiling. Also, here is a more precise version of the approximation.

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double fastPow(double a, double b) { union { double d; int x[2]; } u = { a }; u.x[1] = (int)(b * (u.x[1] - 1072632447) + 1072632447); u.x[0] = 0; return u.d; } |

Compiled on my Pentium-M with gcc 4.1.2:

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gcc -O3 -march=pentium-m -fomit-frame-pointer |

This version is **7.8 times** faster than pow() from the standard library.

# Approximation of pow() in C#

Jason Jung has posted a port of the this code to C#:

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public static double PowerA(double a, double b) { int tmp = (int)(BitConverter.DoubleToInt64Bits(a) >> 32); int tmp2 = (int)(b * (tmp - 1072632447) + 1072632447); return BitConverter.Int64BitsToDouble(((long)tmp2) << 32); } |

# How the Approximation was Developed

It is quite impossible to understand what is going on in this function, it just magically works. To shine a bit more light on it, here is a detailed description how I have developed this.

## Approximation of e^x

As described here, the paper “A Fast, Compact Approximation of the Exponential Function” develops a C macro that does a good job at exploiting the IEEE 754 floating-point representation to calculate `e^x`. This macro can be transformed into Java code straightforward, which looks like this:

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public static double exp(double val) { final long tmp = (long) (1512775 * val + (1072693248 - 60801)); return Double.longBitsToDouble(tmp << 32); } |

## Use Exponential Functions for a^b

Thanks to the power of math, we know that `a^b` can be transformed like this:

- Take exponential
1a^b = e^(ln(a^b))
- Extract b
1a^b = e^(ln(a)*b)

Now we have expressed the pow calculation with `e^x` and `ln(x)`. We already have the `e^x` approximation, but no good `ln(x)`. The old approximation is very bad, so we need a better one. So what now?

## Approximation of ln(x)

Here comes the big trick: Rember that we have the nice `e^x` approximation? Well, `ln(x)` is exactly the inverse function! That means we just need to transform the above approximation so that the output of `e^x` is transformed back into the original input.

That’s not too difficult. Have a look at the above code, we now take the output and move backwards to undo the calculation. First reverse the shift:

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final double tmp = (Double.doubleToLongBits(val) >> 32); |

Now solve the equation

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tmp = (1512775 * val + (1072693248 - 60801)) |

for val:

- The original formula
1tmp = (1512775 * val + (1072693248 - 60801))
- Perform subtraction
1tmp = 1512775 * val + 1072632447
- Bring value to other side
1tmp - 1072632447 = 1512775 * val
- Divide by factor
1(tmp - 1072632447) / 1512775 = val
- Finally, val on the left side
1val = (tmp - 1072632447) / 1512775

Voíla, now we have a nice approximation of `ln(x)`:

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public double ln(double val) { final double x = (Double.doubleToLongBits(val) >> 32); return (x - 1072632447) / 1512775; } |

## Combine Both Approximations

Finally we can combine the two approximations into `e^(ln(a) * b)`:

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public static double pow1(final double a, final double b) { // calculate ln(a) final double x = (Double.doubleToLongBits(a) >> 32); final double ln_a = (x - 1072632447) / 1512775; // ln(a) * b final double tmp1 = ln_a * b; // e^(ln(a) * b) final long tmp2 = (long) (1512775 * tmp1 + (1072693248 - 60801)); return Double.longBitsToDouble(tmp2 << 32); } |

Between the two shifts, we can simply insert the `tmp1` calculation into the tmp2 calculation to get

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public static double pow2(final double a, final double b) { final double x = (Double.doubleToLongBits(a) >> 32); final long tmp2 = (long) (1512775 * (x - 1072632447) / 1512775 * b + (1072693248 - 60801)); return Double.longBitsToDouble(tmp2 << 32); } |

Now simplify `tmp2` calculation:

- The original formula
1tmp2 = (1512775 * (x - 1072632447) / 1512775 * b + (1072693248 - 60801))
- We can drop the factor
`1512775`1tmp2 = (x - 1072632447) * b + (1072693248 - 60801) - And finally, calculate the substraction
1tmp2 = b * (x - 1072632447) + 1072632447

## The Result

That’s it! Add some casts, and the complete function is the same as above.

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public static double pow(final double a, final double b) { final int tmp = (int) (Double.doubleToLongBits(a) >> 32); final int tmp2 = (int) (b * (tmp - 1072632447) + 1072632447); return Double.longBitsToDouble(((long) tmp2) << 32); } |

This concludes my little tutorial on microoptimization of the pow() function. If you have come this far, I congratulate your presistence 🙂

**UPDATE** Recently there several other approximative `pow` calculation methods have been developed, here are some others that I have found through reddit:

- Fast pow() With Adjustable Accuracy — This looks quite a bit more sophisticated and precise than my approximation. Written in C and for float values. A Java port should not be too difficult.
- Fast SSE2 pow: tables or polynomials? — Uses SSE operation and seems to be a bit faster than the table approach from the link above with the potential to scale better when due to less cache usage.

Please post what you think about this!

46 Comments on "Optimized pow() approximation for Java, C / C++, and C#"

Would it be possible to use this with float value instead of doubles?

I dont think so, the whole trick is based on the bit layout of the double numbers. So you need to find a very fast e^x approximation that works with floats, then you can do the rest again.

Another benchmark: on a Pentium 4, Windows, Java 1.6.0 -server, the approximation is about 41 times faster than Math.pow() 🙂

Not bad! But: if I want to compute a pow(), in which cases it might be ok to get an error up to 25%? Could you provide us some examples?

In my experience the error gets bigger when the exponent gets bigger. I have written a short program that uses random numbers to try to find a combination that gets very bad errors. Base is a random double between 0 and 1000, exponent random double between 0 and 5. The biggest error after 100 million evaluations is:

a between 0 and 1000, b between 0 and 5:

Worst case:

a=512.0125338006894

b=4.914054794454942

a^b approximation: 2.5571362865152E13

a^b Math.pow(): 2.0585114388503066E13

Errors is 19.499345822682237 %

Average error is 4.021374964371438 %

—–

a between 0 and 100, b between 0 and 3:

Worst case:

a=64.00103767757574

b=2.8915318496742626

a^b approximation: 191223.125

a^b Math.pow(): 166973.39656298532

Error is 12.681378592162784 %

Average error is 2.7778168699408558 %

Mr Skeptic, I am using this (a modified form) in an Ant Colony Optimization algorithm. There the core algorithms needs to calculate a^b * c^d very often. I have done comparison of the optimization quality when using this optimization and when using Math.pow(), the error does not have any measurable negative effect in the examples I am using. And the optimization got a hell of a lot faster, so even if there is a slight decrease in optimization quality this is more than compensated by the higher number of iterations I am able to do now.

[…] Optimized pow() approximation for Java and C / C++ by Martin Ankerl – […]

Hi,

I need have a procedure which can handle 50! and 1/50! , that is 50 factorial and 1 divided by 50 factorial….

Is there a way to handle these huge numbers

In Java use BigInteger and BigDecimal:

http://javadoc.ankerl.com/results.html?cx=006156709672261707051%3Atrd_lbkwu2y&cof=FORID%3A9&q=bigdecimal&sa=Search#1356

Hi seejayjames, the behaviour of the error is really a bit strange. I think it is a bit off because of the way the constants were selected (read the paper for how they did it). This should minimize the average error.

But the 19.5% error does not look like such a big problem to me, because 512^4.9 is about 2*10^13, which is a very large number that might be out of the useful range for most usages.

I have used this a^b version as the core part in a research project at work, and it gives a huge speedup.

[…] UPDATE This pow() approximation is obsolete. I have a much faster and more accurate approximation version here. […]

The code in .net for the approximation:

Thanks Jason! I have integrated your code into the article.

I’m currently wrestling with an application that uses exp a lot, and am looking at your approximation. Will the smae magic numbers work on 64bit linux, or do they need to be redetermined?

I forgot to mention that we are using C++ and the g++ compiler.

Hi Stephen, if I am not mistaken the magic numbers should be the same, but you have to rewrite the function because the casts like

rely on 32 bit boundaries. I’m not sure how to best do this, I am not a C guy.

If you manage to get a working version of the code, please share it! May I ask what for you need the exp function?

Great Routines for J2ME!

[…] use floating point tricks based on my pow() approximation. Basically I just took the pow() formula and for a^b I substitued b with 0.5, then simplified this […]

Hi,

Your aproximation seems to be really usefor for slow embeddede devices.Besides,I have no RAM free space enough to load math library. However,i tried to used your aproximation for pow in an embedded micro(JN5139) 32-bit RISC CPU using Big-Endian format and using gcc compiler and it doesn’t work. Any idea of what kind of changes can be done to make it work ?

Thanks.

Hi Ralf, sorry but I have no idea. big-endian and 32 bit sounds like it should work. I have never programmed anything for embedded devices. double should be 64 bit, int should be 32 bit.

Well, code looks OK, but I’ve Cut and Paste it into J2ME (on Windows XP, SP3),

run it like pow(1.026f, 0.077f) an the result is: 0.97423… !?

The result is [b]less[/b] than 1.0 !.

OK, I decided to use ‘powTaylor(double, double)’

from http://today.java.net/pub/a/today/2007/11/06/creating-java-me-math-pow-method.html

The results are quite accurate.

The link to the paper referenced (“A Fast, Compact Approximation of the Exponential Function”) appears to be broken. This appears to be better:

http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.57.1569

I am having trouble implementing this in Microsoft Visual C++ any tips?

I have tracked the problem down to both instances of pointer operations

ex:

(*(1 + (int *)&a));

neither operation returns the value it should how do I fix this?

Hi I would like to implement this for ActionScript 3. As3 only has 32 bit floats and ints so could you give me a pointer on how to implement this for floats instead of doubles.

The Double.longBitsToDouble does that just take the bits in a long and put it in a double?

Thanks in advance

i love you! this babies work SOOO fast… still have some problems since the numbers are indeed quite a way off, but that should be manageable.

thanks a lot!

If you replace *(1 + (int * )&p) = tmp2; with union { double d; int x[2]; } u = { p }; u.x[1] = tmp2; p = u.d; you get standards-compliant C/C++ and you can drop the ugly requirement for -fno-strict-aliasing. I think the compiler will generate similar code.

thanks!

@Sam thanks! I will give it a try. I will update the post with some new findings when I have time.

On my machine I found little difference in cpu time between the native exact pow and this fast pow function. I was doing x^5 with x from 10 to 500, and the x^1/5 with larger numbers

Maybe the native pow is better optimized on my 6-core?

Further more careful testing suggest that this is actually quite a bit slower than the native pow function for C (didn’t test C++). I did 100 million pow calculations and this “fast” pow(double,double) method took 5 times longer than the native C pow(double,double). Here was the test code section:

x=.01;

y=.1;

for(i=0;i<100000000;i++)

{

x += .00001;

y += .0000001;

pow(x,y);

}

It took 26 seconds using the above pow code, and ony 6 seconds with the native C pow function.

So…I would definitely do some testing on your machine before jumping onto this. Perhaps Java is quite different.

Hi bob, are you sure that the compiler does not just optimize the pow(x,y) call away? Could you try to change the benchmark into something like this, and try again:

Same results as before (~ 4.5 sec). Largest error is 33% or so at the upper end of x and y. I’m not using optimization so that may change things.

I get a speedup of a factor of about 4.2 when I enable optimization, fast floating point, and SSE2.

At http://pastebin.com/DRvPJL2K I have also added a new method

fastPrecisePowthat is much more precise for large exponents, but a bit slower. It is 3.3 times faster than pow on my PC.I’m using Visual Studio 2008’s compiler. It is at least somewhat faster and perhaps more so with optimization so I am going to use it in my fast max filtering. This does not depend strongly on small errors in pow so it looks indistinguishable (to my eyes). Thanks!

Nice to hear that this is of use for you. I have just wtriten a new blog post with union version, and a more precise version:

http://martin.ankerl.com/2012/01/25/optimized-approximative-pow-in-c-and-cpp/

[…] taking a look at this: optimized pow() approximation BBCode tags | How to ask questions the smart way | Common Java Mistakes | Official Forum […]

[…] high accuracy require black magic. To see what I mean by “black magic,” take a look at this blog post by Martin Ankerl and an associated paper he links to in Neural Computation. Also see the CORDIC […]

Thanks for your post. I needed to optimize my source code seems I am carrying out an evolutionary algorithm using “pow” (a feature weighting approach in Data Mining). It works perfect!