[Mono-dev] JIT floating point behaviour change between 2.4.2 and 2.6

[Rodrigo Kumpera]

I'm guessing that those changes are al on 32bits x86, right?
Those are caused by some fixes in float <> double precision conversions.
Those fixes
are not yet present on amd64 so if you guys want to support it over unity it
might be a good
idea to help us do so. ;)

Revisions with those changes are:

127557
127558
127559
127560
127585


On Tue, Jan 12, 2010 at 1:27 PM, Lucas Meijer <lucas at lucasmeijer.com> wrote:

> Hi,
>
> In our process to upgrade to mono 2.6, we've hit a snag where floating
> point behaviour of the JIT is slightly differently. This program prints
> "PASS" on mono2.6, and "FAIL" on mono2.4, on both osx, and win32.
>
> using System;
>
> public class Test
> {
>         static private float x;
>         static private float y;
>
>         private static void Main()
>         {
>                 //input
>                 float a = DecodeSingle(0x3070787f);
>                 float b = DecodeSingle(0x48a296ba);
>                 float c = DecodeSingle(0x3ddc0d93);
>
>                 x = DecodeSingle(0xbfc2f2e5);
>                 y = DecodeSingle(0xbd35e18b);
>
>                 // test
>                 float abc = (a * b * c);
>
>                 float s = ((a * b * c) / sqrMagnitude);
> //same calculation,just using a temporary variable, which seems to matter
>                 float t = ((abc) / sqrMagnitude);
>
>                 Console.WriteLine("Test: {0}", ((s == t) ? "PASSED" :
> "FAILED"));
>         }
>
>         static private float sqrMagnitude
>         {
>                 get { return ((x * x) + (y * y)); }
>         }
>
>         private static float DecodeSingle(uint value)
>         {
>                 return
> BitConverter.ToSingle(BitConverter.GetBytes(value), 0);
>         }
> }
>
> The good news is that mono2.6 behaviour looks correct, and Mono2.4
> behaviour wrong. I would really like to find out which svn revision has
> caused this change. We like the new behaviour, but want to temporarily
> get the old behaviour back, so we can succesfully run all our
> integrationtests, and feel confident that the mono upgrade didn't break
> anything. Because of the floating point difference, it's causing a lot
> of games in our regression suite to play back completely differently,
> which makes it hard to see if something else changed as well, or if the
> different playback is purely due to the floating point differences.
>
> I hope one of the JIT guys has a rough guess where this change might
> have been introduced,  if not I'll start a the big boring svn based
> binary search.
>
> Thanks in advance,
>
>   Lucas
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>
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