[Mono-list] More .NET and mono floating point inconsistencies
Kornél Pál
kornelpal at gmail.com
Sat Feb 21 03:30:10 EST 2009
David,
I've evaluated your test cases and found that the behavior exposed by
your tests cases is not a bug.
For the first sight this seems to be a bug is MS.NET, but it isn't a bug
of MS.NET either.
ECMA specs (part3, 1.1.1 Numeric data types) explicitly state the following:
When a floating-point value ... is put in a storage location, it is
automatically coerced to the type of the storage location. ... However,
the value might be retained in the internal representation for future
use, if it is reloaded from the storage location without having been
modified. ... This freedom to carry extra precision is not permitted,
however, following the execution of an explicit conversion (conv.r4 or
conv.r8) ...
This means that unlike integer variables, floating point store/load
pairs are not (necessarily) cause conversion.
On the other hand if you need deterministic behavior, you should issue
an explicit conv.r4 (x = (float)y; in C#) because this is an
implementation detail of the current MS JIT compiler that may change in
the future even in that compiler.
Although ECMA specs permit the native float type to have additional
precision, you will most likely never notice the same behavior with
double (float64), because both Mono and MS.NET configure the FPU to
round each arithmetic operations to 64-bits.
Also note that there is no performance gain from using float (float32),
because the FPU still operates in 64-bit mode that has to be converted
to 32-bits. As a result if you want performance you shouldn't use
float32 at all.
A simplified test case:
float f1=200;
float f2=162.980057f;
float f3 = (1 - (f2 / f1));
float f4 = f3*f3;
Console.WriteLine(f4.ToString("R", CultureInfo.InvariantCulture));
Adding an extra conversion you will get the behavior of Mono on MS.NET
as well:
float f1=200;
float f2=162.980057f;
float f3 = (float)(1 - (f2 / f1));
float f4 = f3*f3;
Console.WriteLine(f4.ToString("R", CultureInfo.InvariantCulture));
Kornél
Rodrigo Kumpera wrote:
> I have commited the fixed from Kornél for all bugs that have tests.
>
> On Fri, Feb 20, 2009 at 12:14 AM, ddambro <ddambro at gmail.com
> <mailto:ddambro at gmail.com>> wrote:
>
>
> Hi,
>
> Thanks for looking into my issues. I hope my post didn't come across as
> rude or anything, I was really just looking to ask if it was better
> to post
> issues here or directly to Bugzilla. I'm sure there are far more
> important
> issues than my weird floating point inconsistencies.
>
> For the curious, I am an AI researcher working on Evolutionary
> Computation.
> I tend to use Windows and .NET to run my code, but my research group has
> several large computing clusters that could massively speed up my
> experiments, but they only run Linux. Also academia tends to shy
> away from
> "Windows Only" software, so if I can say "Runs in Linux" when I
> release my
> code to the public, it's a pretty big boon for me as well as the
> people who
> want to run it. However, because of the floating point issues
> described,
> experiments run on mono are not compatible with experiments run on
> .NET, as
> over the course of a simulation, the small errors propagate over
> thousands
> of time steps into large differences in the final AI behavior.
> Thus, if I
> used both mono and .NET, not only would I have to be mindful of which
> experiment was run on which platform when I do analysis and
> demonstrations,
> but when I release my results to the public I would also have to mark
> arbitrary sets of experiments as "mono only" or ".NET only."
>
>
> Kornél Pál wrote:
> >
> > Hi,
> >
> > Thanks for the test cases, I'll invetigate these as well and try
> to fix
> > them.
> >
> > The best way to get bugs fixed is to produce test cases and
> report them
> > in buzilla.
> >
> > You can help more if you are able to provide a patch to fix the bugs.
> >
> > Note that for some reasons Novell guys seem to ignore this bug mostly
> > because they have other things to do and their approval is
> required for
> > these changes.
> >
> > Could you please describe your scenario (application name,
> purpose, etc.
> > if they are public) where you need these floating point roundings and
> > describe why these bugs are critical for you.
> >
> > Providing some description may raise component owners' attention.
> >
> > Kornél
> >
> > ddambro wrote:
> >> I previously posted about some differences I noticed between the
> floating
> >> point math in .NET and mono
> >>
> http://www.nabble.com/Mono-and-.Net-Floating-Point-Inconsistencies-to21428695ef1367.html
> >> here . The bug report can be found
> >> https://bugzilla.novell.com/show_bug.cgi?id=467201 here . The patch
> >> provided (Thank you!) fixes several issues I was encountering, but
> >> unfortunately it led me to discover a couple more. The
> following two
> >> code
> >> samples were tested x86 machines using Linux mono 2.2 with the patch
> >> found
> >> in the bug report, a clean Windows mono 2.0.1, and the latest
> version of
> >> .NET targeting 3.0 framework and x86.
> >>
> >> ---
> >> 1.
> >>
> >> using System;
> >> using System.Runtime.CompilerServices;
> >>
> >> class Testing
> >> {
> >> [MethodImpl(MethodImplOptions.NoInlining)]
> >> public static void Main()
> >> {
> >> float f1=200;
> >>
> >> float distance=Distance(300, 500, 387.5f, 362.5f);
> >>
> >> float dist = 1 - (distance / f1);
> >>
> >> float distSqud = dist * dist;
> >>
> >> Console.WriteLine(distSqud.ToString("R"));
> >>
> >> foreach (byte b in BitConverter.GetBytes(distSqud))
> >> Console.WriteLine(b);
> >>
> >> }
> >>
> >> public static float Distance(float x1, float y1, float x2,
> float y2)
> >> {
> >> float xDist = x1 - x2;
> >> float yDist = y1 - y2;
> >> float dist = (float)Math.Sqrt(xDist * xDist + yDist *
> yDist);
> >> return dist;
> >> }
> >>
> >> On .NET this code produces:
> >> 0.0342619047
> >> 54
> >> 86
> >> 12
> >> 61
> >>
> >> and on mono it produces:
> >> 0.03426191
> >> 55
> >> 86
> >> 12
> >> 61
> >>
> >> ---
> >> 2.
> >>
> >> using System;
> >> using System.Runtime.CompilerServices;
> >>
> >> class Testing
> >> {
> >> [MethodImpl(MethodImplOptions.NoInlining)]
> >> public static void Main()
> >> {
> >> float distance = Distance(616.161255f, 391.2928f, 550.8382f,
> >> 131.006973f);
> >> Console.WriteLine(distance.ToString("R"));
> >>
> >> foreach (byte b in BitConverter.GetBytes(distance))
> >> Console.WriteLine(b);
> >> }
> >>
> >> public static float Distance(float x1, float y1, float x2,
> float y2)
> >> {
> >> float xDist = x1 - x2;
> >> float yDist = y1 - y2;
> >> float dist = (float)Math.Sqrt(xDist * xDist + yDist *
> yDist);
> >>
> >> Console.WriteLine(dist.ToString("R"));
> >>
> >> foreach (byte b in BitConverter.GetBytes(dist))
> >> Console.WriteLine(b);
> >>
> >> return dist;
> >> }
> >> }
> >>
> >> On .NET this code produces:
> >> 268.3576
> >> 198
> >> 45
> >> 134
> >> 67
> >> 268.3576
> >> 198
> >> 45
> >> 134
> >> 67
> >>
> >> and on mono it produces:
> >> 268.357635
> >> 199
> >> 45
> >> 134
> >> 67
> >> 268.357635
> >> 199
> >> 45
> >> 134
> >> 67
> >>
> >> ---
> >> These seem to be very similar to the issues I was encountering
> before,
> >> but
> >> they are not fixed by the patch. What is the best way to
> resolve these
> >> inconsistencies?
> >>
> >> Thanks,
> >> David
> > _______________________________________________
> > Mono-list maillist - Mono-list at lists.ximian.com
> <mailto:Mono-list at lists.ximian.com>
> > http://lists.ximian.com/mailman/listinfo/mono-list
> >
> >
>
> --
> View this message in context:
> http://www.nabble.com/More-.NET-and-mono-floating-point-inconsistencies-tp22018718p22114104.html
> Sent from the Mono - General mailing list archive at Nabble.com.
>
> _______________________________________________
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