cppunit-devel

[Cppunit-devel] assertDoubleEquals, NaN & Microsoft Visual Studio 6

[CppUnit d. m. l. <cpp...@li...>]

The detection of NaN in assertDoubleEquals failed on Microsoft Visual Studio 
6.

The implementation assumes that NaN != NaN (IEEE-754 says it should). The 
failure was caused by the way MSVC6 compares floating point number equality:
fld         qword ptr [nan]
fcomp       qword ptr [nan]
fnstsw      ax                        // copie fp (floating-point) status to 
register to ax
test        ah,40h                    // test bit 14 of ax (0x4000) => C3 of 
fp status register
je          $L18438+0B9h (00410be2)

According to this doc:
http://webster.cs.ucr.edu/AoA/Windows/HTML/RealArithmetic.html#1000117
C2 should be tested for "undefined" value (which I guess NaN fall into).

NaN comparison works under visual studio 2005. The equality test is done as 
follow:
fld         qword ptr [ebp+10h]
fcomp       qword ptr [ebp+8]
fnstsw      ax
test        ah,44h         // Visual Studio 2005 does test both C2 & C3...
jp          CppUnit::assertDoubleEquals+1B5h (4777A5h)

This seems to confirm that MSVC6 code generator is bug (does not respect 
IEEE-754 standard in spite of what the doc claim).

To work-around this, I added an explicit test for NaN value when testing 
non-finite value for equalities. Here is the function:
// To work around this, a NaN is assumed to be detected if no strict 
ordering is found.
inline bool floatingPointIsUnordered( double x )
{
   // x != x will detect a NaN on conformant platform
   // (2.0 < x  &&  x < 1.0) will detect a NaN on non conformant platform:
   // => no ordering can be found for x.
   return  (x != x) ||  (2.0 < x  &&  x < 1.0);
}

I'm not an expert on floating-point manipulation & NaN. Does any one see 
anything wrong on the above implementation or the comments? I modified the 
defined implementation for isfinite() test to:

/// \brief Tests if a floating-point is finite.
/// @return \c true if x is neither a NaN, nor +inf, nor -inf, \c false 
otherwise.
inline bool floatingPointIsFinite( double x )
{
   double testInf = x * 0.0;  // Produce 0.0 if x is finite, a NaN 
otherwise.
   return testInf == 0.0  &&  !floatingPointIsUnordered(testInf);
}

Feedbacks are welcome,
Baptiste.
---
Baptiste Lepilleur <bl...@us...>
CppUnit maintainer
OpenTest and CppUnit 2 developer. 

Re: [Cppunit-devel] assertDoubleEquals, NaN & Microsoft Visual Studio 6

[CppUnit d. m. l. <cpp...@li...>]

Hi Baptiste,

I'm unclear on what problem you are solving here.  My guess is that

     CPPUNIT_ASSERT_DOUBLES_EQUAL( nan, nan, 1.0 )

was passing when it should in fact fail.  Is that the case?


Also: why did you make the following change in =20
examples/cppunittest/TestAssertTest.cpp?  What does the comment mean?

-  CPPUNIT_ASSERT_ASSERTION_FAIL( CPPUNIT_ASSERT_DOUBLES_EQUAL( nan, =20
0.0, 1.0 ) );
+  CPPUNIT_ASSERT_ASSERTION_FAIL( CPPUNIT_ASSERT_DOUBLES_EQUAL( nan, =20
0.0, 1.0 ) ); // this one fails


Quoting CppUnit development mailing list =20
<cpp...@li...>:

> This seems to confirm that MSVC6 code generator is bug (does not respect
> IEEE-754 standard in spite of what the doc claim).
>
> To work-around this, I added an explicit test for NaN value when testing
> non-finite value for equalities. Here is the function:
> // To work around this, a NaN is assumed to be detected if no strict
> ordering is found.
> inline bool floatingPointIsUnordered( double x )
> {
>    // x !=3D x will detect a NaN on conformant platform
>    // (2.0 < x  &&  x < 1.0) will detect a NaN on non conformant platform:
>    // =3D> no ordering can be found for x.
>    return  (x !=3D x) ||  (2.0 < x  &&  x < 1.0);
> }

I don't understand why this works for you.  According to the standard =20
(see e.g. page 8 of =20
http://www.cs.berkeley.edu/~wkahan/ieee754status/ieee754.ps) all =20
comparisons with NaN are false except "x !=3D x", which is true.  You =20
state that MSVC6 gets "x !=3D x" wrong.  Does it also get "x < y" wrong =20
and return true?


> I'm not an expert on floating-point manipulation & NaN. Does any one see
> anything wrong on the above implementation or the comments?

The section of the paper referenced above also states that all =20
comparisons with NaN, except =3D=3D and !=3D, are *invalid* and thus signal.=
 =20
  So the code you wrote is going to cause a problem if the FPU signals =20
are enabled.


If we need this function, can we rename it floatingPointIsNan()?  Then =20
I can create suitable automakery to use the standard isnan() on =20
systems that have it.  On that note, does MSVC6 have something like =20
isnan(), maybe called _isnan()?
http://msdn2.microsoft.com/en-us/library/tzthab44(VS.80).aspx


Cheers,
-Steve

Re: [Cppunit-devel] assertDoubleEquals, NaN & Microsoft Visual Studio 6

[CppUnit d. m. l. <cpp...@li...>]

CppUnit development mailing list wrote:
> Hi Baptiste,
>
> I'm unclear on what problem you are solving here.  My guess is that
>
>     CPPUNIT_ASSERT_DOUBLES_EQUAL( nan, nan, 1.0 )
>
> was passing when it should in fact fail.  Is that the case?

Yes. I should have put this first. All the following unit tests were 
failing:
  CPPUNIT_ASSERT_ASSERTION_FAIL( CPPUNIT_ASSERT_DOUBLES_EQUAL( nan, 0.0, 
1.0 ) );
  CPPUNIT_ASSERT_ASSERTION_FAIL( CPPUNIT_ASSERT_DOUBLES_EQUAL( nan, nan, 
1.0 ) );
  CPPUNIT_ASSERT_ASSERTION_FAIL( CPPUNIT_ASSERT_DOUBLES_EQUAL( nan, inf, 
1.0 ) );
  CPPUNIT_ASSERT_ASSERTION_FAIL( CPPUNIT_ASSERT_DOUBLES_EQUAL( inf, nan, 
1.0 ) );

This was caused by the fact that
       equal = expected == actual;
always evaluated to true whenever expected or actual was a NaN.

> Also: why did you make the following change in
> examples/cppunittest/TestAssertTest.cpp?  What does the comment mean?
>
> -  CPPUNIT_ASSERT_ASSERTION_FAIL( CPPUNIT_ASSERT_DOUBLES_EQUAL( nan,
> 0.0, 1.0 ) );
> +  CPPUNIT_ASSERT_ASSERTION_FAIL( CPPUNIT_ASSERT_DOUBLES_EQUAL( nan,
> 0.0, 1.0 ) ); // this one fails

Ignore the comment, I forgot to remove it. It's done now.

I hope it's clearer now.
Baptiste.

> Quoting CppUnit development mailing list
> <cpp...@li...>:
>
>> This seems to confirm that MSVC6 code generator is bug (does not
>> respect IEEE-754 standard in spite of what the doc claim).
>>
>> To work-around this, I added an explicit test for NaN value when
>> testing non-finite value for equalities. Here is the function:
>> // To work around this, a NaN is assumed to be detected if no strict
>> ordering is found.
>> inline bool floatingPointIsUnordered( double x )
>> {
>>    // x != x will detect a NaN on conformant platform
>>    // (2.0 < x  &&  x < 1.0) will detect a NaN on non conformant
>>    platform: // => no ordering can be found for x.
>>    return  (x != x) ||  (2.0 < x  &&  x < 1.0);
>> }
>
> I don't understand why this works for you.  According to the standard
> (see e.g. page 8 of
> http://www.cs.berkeley.edu/~wkahan/ieee754status/ieee754.ps) all
> comparisons with NaN are false except "x != x", which is true.  You
> state that MSVC6 gets "x != x" wrong.  Does it also get "x < y" wrong
> and return true?
>
>
>> I'm not an expert on floating-point manipulation & NaN. Does any one
>> see anything wrong on the above implementation or the comments?
>
> The section of the paper referenced above also states that all
> comparisons with NaN, except == and !=, are *invalid* and thus signal.
>  So the code you wrote is going to cause a problem if the FPU signals
> are enabled.
>
>
> If we need this function, can we rename it floatingPointIsNan()?  Then
> I can create suitable automakery to use the standard isnan() on
> systems that have it.  On that note, does MSVC6 have something like
> isnan(), maybe called _isnan()?
> http://msdn2.microsoft.com/en-us/library/tzthab44(VS.80).aspx
>
>
> Cheers,
> -Steve
>
>
>
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