Bug#906753: Acknowledgement (GCC's -O2 optimization breaks floating point precision or something else)

[Matthias Klose <doko@debian.org>]

Control: tags -1 important
Control: forwarded -1 https://gcc.gnu.org/PR87036

On 20.08.2018 21:53, Lumin wrote:
> This is the minimal code for repro #906753:
> 
> OK with -O0, FAIL with -O2 on i386, ppc64el, ...

that always failed on i386 and ppc64el with -O2, so not a regression. You talked
about other architectures, but don't name these with the reduced test case.

> 
> masssq1 and masssq2 are computed from the same vector [1.1, 2.2, 3.3, 4.4],
> but the results are different!
> 
> ==========================================================
> 
> #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;
> }
>