[Mono-dev] Simple floating maths problem

Fri Apr 30 16:15:37 EDT 2010

Sorry, somehow I got this wrong.

So in fact I also get the
47931 = 47932 = 47932 for x86 .Net 3.5 (and Mono) and
47932 = 47932 = 47932 for x64 .Net 3.5

Well there is a reason why you should never rely on floating point results
for producing exact (integer) values.

Andreas

-----Ursprüngliche Nachricht-----
Von: mono-devel-list-bounces at lists.ximian.com
[mailto:mono-devel-list-bounces at lists.ximian.com] Im Auftrag von Andreas
Nahr
Gesendet: Freitag, 30. April 2010 22:06
An: 'Rolf Bjarne Kvinge'; 'Stuart Fraser'; mono-devel-list at lists.ximian.com
Betreff: Re: [Mono-dev] Simple floating maths problem

I've been running your modified sample under x86 .Net 3.5 and my result is:
0,95864 * 50000 = 47932 = 47932 = 47932
0,89374 * 50000 = 44687 = 44687 = 44687
0,89092 * 50000 = 44546 = 44546 = 44546
0,89716 * 50000 = 44858 = 44858 = 44858
0,4191 * 50000 = 20955 = 20955 = 20955
0,79782 * 50000 = 39891 = 39891 = 39891
and running it under x64 .Net 3.5 is exactly the same as above.

Only running mono on the same machine results in:
0,95864 * 50000 = 47931 = 47932 = 47932
0,89374 * 50000 = 44686 = 44687 = 44687
0,89092 * 50000 = 44545 = 44546 = 44546
0,89716 * 50000 = 44857 = 44858 = 44858
0,4191 * 50000 = 20954 = 20955 = 20955
0,79782 * 50000 = 39890 = 39891 = 39891

So something is fishy *g*.
Maybe I'm wrong, but if it was a precision error, then how could compi be
different from comp?

Andreas

-----Ursprüngliche Nachricht-----
Von: mono-devel-list-bounces at lists.ximian.com
[mailto:mono-devel-list-bounces at lists.ximian.com] Im Auftrag von Rolf Bjarne
Kvinge
Gesendet: Freitag, 30. April 2010 15:22
An: 'Stuart Fraser'; mono-devel-list at lists.ximian.com
Betreff: Re: [Mono-dev] Simple floating maths problem

Hi,

Running this modified simple on x86 .NET:

			int fact = 50000;
			float [] tList = { 0.95864f, 0.89374f, 0.89092f,
0.89716f, 0.4191f, 0.79782f };

			foreach (float val in tList) {
				int comp = (int) (val * fact);
				float compf = (val * fact);
				int compi = (int) compf;
				Console.WriteLine ("{0} * {1} = {2} = {3} =
{4}", val, fact, comp, compf, compi);
			}

Gives this result:

0,95864 * 50000 = 47931 = 47932 = 47932
0,89374 * 50000 = 44686 = 44687 = 44687
0,89092 * 50000 = 44545 = 44546 = 44546
0,89716 * 50000 = 44857 = 44858 = 44858
0,4191 * 50000 = 20954 = 20955 = 20955
0,79782 * 50000 = 39890 = 39891 = 39891

I think it's a precision issue you're running into, as Robert said.

Rolf

> -----Mensaje original-----
> De: mono-devel-list-bounces at lists.ximian.com [mailto:mono-devel-list-
> bounces at lists.ximian.com] En nombre de Stuart Fraser
> Enviado el: viernes, 30 de abril de 2010 0:22
> Para: mono-devel-list at lists.ximian.com
> Asunto: [Mono-dev] Simple floating maths problem
> 
> HI All,
> 
> I'm currently working on some code for HPC benchmarking using a number
> of languages and runtimes but am primarily focussing on C#. During my
> coding I have come across some inconsistencies between results between
> calculated in Mono vs .Net (and C /Java), there may be a number of
> reasons for this but I was after a sanity check before I start looking
> into the mono runtime code...
> 
> Basically I have pulled out some of the internal values/working and
> problem is thus:
> 
>             int fact = 50000;
>             float[] tList = { 0.95864f, 0.89374f, 0.89092f, 0.89716f,
> 0.4191f, 0.79782f };
> 
>             foreach (float val in tList)
>             {
>                 int comp = (int)(val * fact);
>                 Console.WriteLine("{0:g} * {1:d} = {2:d}", val, fact,
> comp);
>             }
> 
> Mono results:
> 0.95864 * 50000 = 47931
> 0.89374 * 50000 = 44686
> 0.89092 * 50000 = 44545
> 0.89716 * 50000 = 44857
> 0.4191 * 50000 = 20954
> 0.79782 * 50000 = 39890
> 
> .Net results :
> 0.95864 * 50000 = 47932
> 0.89374 * 50000 = 44687
> 0.89092 * 50000 = 44546
> 0.89716 * 50000 = 44858
> 0.4191 * 50000 = 20955
> 0.79782 * 50000 = 39891
> 
> Obviously precision required is very important but this only appears to
> be a problem on Mono.
> I have tested this with Mono 2.6.2 and 2.6.4 on
>  - OpenSUSE 11.2 (x64) AMD Athlon x64
>  - Windows 7 (x64) Intel  i7
>  - Cray XT4h (x64) AMD Opteron :)
> I have tried the code being run checked and unchecked, I have compiled
> with .net and run the executable under mono and done a clean mono build
> and test,
> I have looked through the bugzilla but haven't seen anything that
> matches this, its late and I'm tired is this a bug in the mono
> jit/runtime?
> Suggestions on where to look for a fix?
> 
> Regards
> Stuart
> 
> 
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