[Bug c++/19675] New: wrong double comparison result if taken from function result.

[cognot at earthdecision dot com]

Hi,

The small example below gives an incorrect result on 32 bit platforms. Both
tests should lead to the same result, but one is false, the other is true.

tested with the following compilers, all of them exhibiting the bug:

Reading specs from /usr/lib/gcc-lib/i386-redhat-linux/2.96/specs
gcc version 2.96 20000731 (Red Hat Linux 7.1 2.96-98)

Reading specs from /usr/lib/gcc-lib/i386-redhat-linux/3.2.3/specs
Configured with: ../configure --prefix=/usr --mandir=/usr/share/man
--infodir=/usr/share/info --enable-shared --enable-threads=posix
--disable-checking --with-system-zlib --enable-__cxa_atexit --host=i386-redhat-linux
Thread model: posix
gcc version 3.2.3 20030502 (Red Hat Linux 3.2.3-34)

Reading specs from /usr/lib/gcc-lib/x86_64-redhat-linux/3.2.3/specs
Configured with: ../configure --prefix=/usr --mandir=/usr/share/man
--infodir=/usr/share/info --enable-shared --enable-threads=posix
--disable-checking --with-system-zlib --enable-__cxa_atexit
--host=x86_64-redhat-linux
Thread model: posix
gcc version 3.2.3 20030502 (Red Hat Linux 3.2.3-42)

Reading specs from /usr/lib/gcc-lib/i586-mandrake-linux-gnu/3.3.1/specs
Configured with: ../configure --prefix=/usr --libdir=/usr/lib
--with-slibdir=/lib --mandir=/usr/share/man --infodir=/usr/share/info
--enable-shared --enable-threads=posix --disable-checking --enable-long-long
--enable-__cxa_atexit --enable-languages=c,c++,ada,f77,objc,java,pascal
--host=i586-mandrake-linux-gnu --with-system-zlib
Thread model: posix
gcc version 3.3.1 (Mandrake Linux 9.2 3.3.1-2mdk)

Reading specs from /usr/lib/gcc/x86_64-redhat-linux/3.4.2/specs
Configured with: ../configure --prefix=/usr --mandir=/usr/share/man
--infodir=/usr/share/info --enable-shared --enable-threads=posix
--disable-checking --with-system-zlib --enable-__cxa_atexit
--disable-libunwind-exceptions --enable-languages=c,c++,objc,java,f77
--enable-java-awt=gtk --host=x86_64-redhat-linux
Thread model: posix
gcc version 3.4.2 20041017 (Red Hat 3.4.2-6.fc3)

Compilation command, on 32 bits systems:

gcc -o testgcc testgcc.cpp -lstdc++

Compilation command, on 64 bits systems:

gcc -m32 -o testgcc testgcc.cpp -lstdc++

NOTE: on 64 bits systems, 64bit executable run as expected.

Regards,
Richard.

------------------testgcc.cpp---------------------------------------
#include <iostream>

using std::cerr ;

class BugMe {
public:
    BugMe( double start, double step ) ;

    double value( int i ) const ;

    bool bugme( const BugMe* other ) ;
private:
    double start_ ;
    double step_ ;
} ;

BugMe::BugMe(double start, double step) : start_(start), step_(step) {
}

double BugMe::value(int i) const {
    return start_ + i*step_ ;
}

bool BugMe::bugme( const BugMe* other ) {
    bool b1 = (value(44) == other->value(44)) ;
    double v1 = value(44) ;
    double v2 = other->value(44) ;
    bool b2 = (v1==v2) ;

    return b1 != b2 ;
}

main() {
    BugMe* b1 = new BugMe(123, 2345.54) ;
    BugMe* b2 = new BugMe(123, 2345.54) ;

    if( b1->bugme(b2) ) {
        cerr << "BUG!\n" ;
    }
}
------------------testgcc.cpp---------------------------------------

-- 
           Summary: wrong double comparison result if taken from function
                    result.
           Product: gcc
           Version: 3.4.2
            Status: UNCONFIRMED
          Severity: critical
          Priority: P2
         Component: c++
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: cognot at earthdecision dot com
                CC: gcc-bugs at gcc dot gnu dot org


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19675