[Bug c++/66346] New: GCC computes log10(2.L) constant wrongly

["b7.10110111 at gmail dot com" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66346

            Bug ID: 66346
           Summary: GCC computes log10(2.L) constant wrongly
           Product: gcc
           Version: 5.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: b7.10110111 at gmail dot com
  Target Milestone: ---

Consider the following code:

#include <iostream>
#include <cmath>
int main()
{
    volatile long double two=2.L;
    long double vol=log10(two);
    long double con=log10(2.L);
    long double ref=0.3010299956639811952L;
    std::cout.precision(19);
    std::cout << "computed constant: " << con << '\n';
    std::cout << "computed volatile: " << vol << '\n';
    std::cout << "reference value  : " << ref << "\n";
}

Here reference value was computed using Wolfram Mathematica via N[Log10[2],19]
command.
On x86 system it this code, compiled with gcc 4.5, 4.8, 4.9 and 5.1, gives me
this output:

computed constant: 0.301029995663981198
computed volatile: 0.3010299956639811952
reference value  : 0.3010299956639811952

On x86_64 system it prints this:

computed constant: 0.301029995663981198
computed volatile: 0.301029995663981198
reference value  : 0.3010299956639811952

This appears to be wrong result. The same code compiled by clang++ 3.0.6ubuntu3
gives all values equal to reference value.