Re: 4.3.0 strange problem with computation and optimization

[Andrew Haley <aph at redhat dot com>]

Andrew Haley wrote:
> Lasse Kliemann wrote:
>> Greetings,
>>
>> I have a C++ program that does some linear programming optimization. Out of 
>> this linear programming, I get a vector of doubles `x' and a double `opt' 
>> that should be the sum of the entries in `x' (`x' is the solution vector and 
>> `opt' is the optimal value). As a consistency chek, I compute the sum of the 
>> elements of `x' and then compute the difference to `opt'. It should be close 
>> to zero. Here is a snippet from the code:
>>
>>    double sum = 0;
>>    const double *x = model->getColSolution();
>>    for (unsigned int j=0; j<m; ++j) sum += x[j];
>>    double d=opt-sum;
>>    cout << "difference: " << d << endl;
>>
>> If I compile that with g++ version 4.3.0 and -O0, -O1, or -Os, everything 
>> looks OK; in rare cases the difference is zero, and in most cases it is a 
>> very small number.  Now, if I compile that with -O2 or -O3, the difference is 
>> *always* printed to be zero, which I presume is wrong.
> 
> http://gcc.gnu.org/bugzilla/show_bug.cgi?id=323\

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=323