[Bug c++/55787] New: Comparisons of double values don't work correctly

["tasin at hm dot edu" <gcc-bugzilla@gcc.gnu.org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55787

             Bug #: 55787
           Summary: Comparisons of double values don't work correctly
    Classification: Unclassified
           Product: gcc
           Version: 4.7.2
            Status: UNCONFIRMED
          Severity: critical
          Priority: P3
         Component: c++
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: tasin@hm.edu


Created attachment 29027
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=29027
example code (as .ii)

A simple comparison shows that floating point operations with the c++-type
double doesn't set the floating status flag correctly.

Changing the type of the members of the class Load to float or long double
makes the application work correctly.

In the attached example the free function 
bool operator<(const Load &, const Load &); 
doesn't work correctly.

The example returns the output:
Compare: true
instead of
Compare: false

This example was compiled in different configurations and on different machines
with the same wrong result:

System configuration 1:
---
Intel(R) Core(TM)2 CPU 6420 @ 2.13GHz
3 GB RAM (pae)
Windows XP Professional Version 2002 SP3
32 bit arch
---
Es werden eingebaute Spezifikationen verwendet.
COLLECT_GCC=g++
COLLECT_LTO_WRAPPER=c:/mingw/bin/../libexec/gcc/mingw32/4.7.2/lto-wrapper.exe
Ziel: mingw32
Konfiguriert mit: ../gcc-4.7.2/configure
--enable-languages=c,c++,ada,fortran,objc,obj-c++ --disable-sjlj-exceptions
--with-dwarf2 --enable-shared --enable-libgomp --disable-win32-registry
--enable-libstdcxx-debug --disable-build-poststage1 -with-cxx
--enable-version-specific-runtime-libs --build=mingw32 --prefix=/mingw

Thread-Modell: win32
gcc-Version 4.7.2 (GCC)
---
The build process call was:
g++ -v -save-temps -Wall -pedantic -Wextra -o derror.exe src\derror.cpp

--- derror.cpp ---
// test routine to show the double error
#include <iostream>
#include <stdexcept>
using namespace std;

// class definition
class Load
{
    double m_dPowerOut, m_dEfficiency;
public:
    Load(double powerOut, double eff=1) :
      m_dPowerOut(powerOut), m_dEfficiency(eff) { };
      virtual double consumption() const;
      virtual ~Load() { }
};

bool operator<(const Load &a, const Load &b);

// class method definition
double Load::consumption(void) const
{
    if (m_dEfficiency==0.0)
            throw logic_error("Efficiency must not be 0");
    return m_dPowerOut/m_dEfficiency;
}

/// free function
// ... doesn't work correctly
bool operator<(const Load &a, const Load &b)
{
    return a.consumption() < b.consumption();
}
/*
// ... seems to work correctly, but the float-status-flag IMHO 
//     still isn't correct
bool operator<(const Load &a, const Load &b)
{
    double bcon=b.consumption();
    return a.consumption() < bcon;
}
*/

int main()
{
    try
    {
        Load pump(53.0, 0.43);
        bool cmp=(pump < pump);  // should be false !!!
        cout << "Compare: " << (cmp ? "true" : "false") << endl;
    }
    catch (const exception &ex)
    {
        cerr << "Exception: " << ex.what() << endl;
    }
    return 0;
}