difference in calculation result when using gcc vs Visual studio and optimisation flag

[Игорь Сотниченко]

Hello!

1.       There is a decent difference between calculations on Visual studio
compiler and gcc 7.1 in  calculation result of my program.

2.       When optimization in gcc  are disabled, then there is no
difference.

3.       When I have replaced O2 with keys from O1 and O2 (
https://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html), then there is no
difference in results, but there is a calculation speed degradation
compared to O2.