http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52053
--- Comment #3 from Tobias Burnus <burnus at gcc dot gnu.org> 2012-01-30 14:48:09 UTC --- (In reply to comment #0) > REAL(KIND=4) :: a(20,1000,1000) If you want to reduce problems due to the accumulation of rounds, you should increase the precision. Try kind=8, 10 or 16 (in this case "8" is enough). Note: Not all kind values are available on all systems. And depending on the algorithm and the required precision, even kind=16 might be not precise enough. In that case, one should find a better algorithm - for instance using integers. Note, additionally, that the result also depends on the optimization level: $ gfortran -O0 test.f90 && ./a.out 16777216.0 20000000.0 $ gfortran -Ofast test.f90 && ./a.out 20000000.0 20000000.0 And for a different compiler: $ ifort -O0 test.f90 && ./a.out 1.6777216E+07 2.0000000E+07 $ ifort -O3 test.f90 && ./a.out 2.0000000E+07 2.0000000E+07
