https://gcc.gnu.org/bugzilla/show_bug.cgi?id=87036
Bug ID: 87036
Summary: -O1/-O2 affects floating point precision on at least
i686 and ppc64el
Product: gcc
Version: 6.4.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: other
Assignee: unassigned at gcc dot gnu.org
Reporter: doko at debian dot org
Target Milestone: ---
[forwarded from http://bugs.debian.org/906753]
no difference in the results on x86_64-linux-gnu, the bug submitter claims that
clang 6.0 is working as expected. Seen on at least i686 and ppc64el. Same
behaviour down to GCC 5 on both archs.
on ppc64el:
$ g++ -O0 tst.cc && ./a.out
$ g++ -O2 tst.cc && ./a.out
different mass sq values: 2.42 2.42
difference is : 3.55271e-15
on i686:
$ g++ -O0 tst.cc && ./a.out
$ g++ -O2 tst.cc && ./a.out
different mass sq values: 2.42 2.42
difference is : 1.77636e-15
$cat tst.cc
#include <cmath>
#include <algorithm> // for swap
#include <iostream>
namespace HepMC {
class FourVector {
public:
double m_x, m_y, m_z, m_t;
FourVector( double xin, double yin, double zin, double tin=0) : m_x(xin),
m_y(yin), m_z(zin), m_t(tin) {}
inline double m2() const {
return m_t*m_t - (m_x*m_x + m_y*m_y + m_z*m_z);
}
inline double m() const {
double mm = m2();
return mm < 0.0 ? -std::sqrt(-mm) : std::sqrt(mm);
}
};
} // HepMC
int main()
{
double eps = 1.e-15; // allowed differnce between doubles
// FourVector
HepMC::FourVector vector(1.1,2.2,3.3,4.4);
HepMC::FourVector v4 (1.1,2.2,3.3,4.4);
//vector = v4;
double masssq1 = v4.m2();
double mass1 = v4.m();
double masssq2 = vector.m2();
double mass2 = vector.m();
if( fabs( masssq1 - masssq2 ) > eps ) {
std::cout << "different mass sq values: " << masssq1 << " " << masssq2 <<
std::endl;
std::cout << "difference is : " << ( masssq1 - masssq2 ) << std::endl;
}
return 0;
}
