[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.