http://d.puremagic.com/issues/show_bug.cgi?id=8745

           Summary: floating point comparison ("is" vs "==") inconsistency
                    (on 32)
           Product: D
           Version: unspecified
          Platform: x86
        OS/Version: All
            Status: NEW
          Severity: normal
          Priority: P2
         Component: DMD
        AssignedTo: nobody@puremagic.com
        ReportedBy: monarchdodra@gmail.com


--- Comment #0 from monarchdodra@gmail.com 2012-10-01 14:21:50 PDT ---
From http://forum.dlang.org/group/digitalmars.D.learn

//----
import std.stdio;
import std.math;

@property double getFloat()
{
   return sqrt(1.1);
}

void main()
{
    writeln(getFloat() == getFloat());
    writeln(getFloat() is getFloat());
}
//----

x32 Produces http://dpaste.dzfl.pl/e0e0bbae :
false
true

x64 Produces http://dpaste.dzfl.pl/c1c7a415 :
true
true

While one may argue that the comparison *may* return false, the difference in behavior between "==" and "is" not expected behavior (unless I'm missing something...? ).

PS: the behavior difference is consistent http://dpaste.dzfl.pl/2ee9ae4e :
//----
import std.stdio;
import std.math;

@property double getFloat()
{
   return sqrt(1.1);
}

void main()
{
    writeln(getFloat() == getFloat());
    writeln(getFloat() is getFloat());
    writeln(getFloat() is getFloat());
    writeln(getFloat() == getFloat());
    writeln(getFloat() is getFloat());
    writeln(getFloat() == getFloat());
    writeln(getFloat() == getFloat());
    writeln(getFloat() is getFloat());
}
//----
false
true
true
false
true
false
false
true
//----

On a side note, I find it strange that "==" return false in such a consistent manner. It might be over-sensitive to extended precision?

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Don <clugdbug@yahoo.com.au> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |clugdbug@yahoo.com.au


--- Comment #1 from Don <clugdbug@yahoo.com.au> 2012-10-02 01:03:51 PDT ---
The idea that for floating point, 'is' and '==' should be the same, is wrong. See bug 3632 for further discussion. I suspect this is a duplicate of the issues raised there.

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--- Comment #2 from monarchdodra@gmail.com 2012-10-02 01:19:17 PDT ---
(In reply to comment #1)
> The idea that for floating point, 'is' and '==' should be the same, is wrong. See bug 3632 for further discussion. I suspect this is a duplicate of the issues raised there.

I read the thread, it mostly relates to how comparison is done in regards to NaN and 0. Here, we are dealing with a "normal" float.

It mentions doing an "isIdentical" call if you really want binary comparison:

//----
void main()
{
    writeln(isIdentical(getFloat(), getFloat()));
    writeln(getFloat() is getFloat());
    writeln(getFloat() == getFloat());
}
//----
true
true
false
//----

:/

So, to get this straight, I got a and b. "a is b" and "a is is identical to b"
but "a != b" ? (!)

Is that really the intended behavior? If so, can you explain what I am seeing? I'm more confused than trying to request a change of behavior, really.

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Rainer Schuetze <r.sagitario@gmx.de> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |r.sagitario@gmx.de


--- Comment #3 from Rainer Schuetze <r.sagitario@gmx.de> 2012-10-02 11:16:06 PDT ---
I haven't checked the disassembly, but I guess the problem of the failing comparison is caused by storing one value to memory as double (and doing some rounding on it along the way), while the other is still available on the FPU stack with real-precision. You might get different results when enabling optimizations.

The "is" comparison is a bitwise comparison, so it will probably actually write both values to memory before the comparison, applying the same rounding to both results.

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Maxim Fomin <maxim@maxim-fomin.ru> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
                 CC|                            |maxim@maxim-fomin.ru
         Resolution|                            |DUPLICATE


--- Comment #4 from Maxim Fomin <maxim@maxim-fomin.ru> 2012-10-16 09:04:44 PDT ---
*** This issue has been marked as a duplicate of issue 8476 ***

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