https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107753
--- Comment #11 from anlauf at gcc dot gnu.org --- (In reply to Weslley da Silva Pereira from comment #7) > More data for the discussion: > 1. In a Ubuntu 18.04.5 LTS, using GNU Fortran 7.5.0, I tested optimization > flags `-O` but still reproduce the wrong result for complex divisions with > huge numbers. See It is possible that gfortran's dependence on optimization level depends on the version. If one wants to test run-time behavior and avoid compile-time simplification, it may be helpful to add: volatile :: x, y, z I then get consistent results for -O0 / -O1. > 4. My Ubuntu 20.04.5 LTS with compiler ifort 2021.7.1 computes the complex > division `x/x` accurately even for the case of huge numbers. Scenarios > tested: > - I tested the program in > https://github.com/Reference-LAPACK/lapack/blob/master/INSTALL/ > test_zcomplexdiv.f and the one in https://godbolt.org/z/b3WKWodvn. > - I tested ifort with flags -fp-model precise and -fp-model fast. The > latter enables more aggressive optimizations on floating-point data. > - I tested compilation with optimization flags -O0, -O, -O1, -O2, -O3. Intel might be fine, but at least some current llvm-based compilers (Nvidia, AMD flang) show more or less similar behavior to gfortran. E.g. nvfortran 22.11: (1.7976931348623157E+308,1.7976931348623157E+308) (8.9884656743115795E+307,8.9884656743115795E+307) (4.4942328371557898E+307,4.4942328371557898E+307) (NaN,0.000000000000000) (NaN,0.000000000000000) (1.000000000000000,0.000000000000000) As a sidenote: we are really discussing borderline cases here, valid but only rarely occuring in normal code execution. If I replace x = cmplx( huge(0.0d0), huge(0.0d0), dp ) y = cmplx( b**(E-1), b**(E-1), dp ) by x = cmplx( nearest(huge(0.0d0),-1.d0), nearest(huge(0.0d0),-1.d0), dp ) y = cmplx( nearest(b**(E-1), -1.d0), nearest(b**(E-1), -1.d0), dp ) then I get (1.0000000000000000,0.0000000000000000) (1.0000000000000000,0.0000000000000000) instead of NaN.
