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