https://gcc.gnu.org/bugzilla/show_bug.cgi?id=117805
--- Comment #8 from Richard Biener <rguenth at gcc dot gnu.org> --- (In reply to kargls from comment #7) > (In reply to Richard Biener from comment #3) > > You can get the desired behavior with -fno-signed-zeros I think - it might > > be reasonable to add an additional -fcx-* flag specifying that just for > > _Complex > > operations signed zeros can be ignored (IIRC non-complex REAL have specified > > behavior wrt signed zeros, so a -fno-signed-zeros default for Fortran would > > be wrong). > > > > So, -fcx-no-signed-zeros or maybe simply alter -fcx-fortran-rules to cover > > signed zeros? > > It my opinion that -fcx-fortran-rules should be removed. It's exist > does match anything specified in the Fortran standard. But that's > for another PR. I think -fcx-fortran-rules was added for usage by C/C++ code and people wanting the behavior (and speed) they were used to from Fortran code. On the C/C++ side we only had strict conformance and -ffast-math while the gfortran (or was it g77?) default offered some in-between. > It is more than just adding a new option. > > program testprogram > > implicit none > > real, parameter :: rc = sign(0., -1.) > complex, parameter :: zc = nzero * (0, 1) > real r > complex z > > print *, rc, zc > > r = sign(0., -1.) > z = r * (0, 1) > print *, r, z > > end program > > % gfcx -o z -O a.f90 && ./z > -0.00000000 (-0.00000000,0.00000000) > -0.00000000 (-0.00000000,0.00000000) > % gfcx -o z -Ofast a.f90 && ./z > -0.00000000 (-0.00000000,0.00000000) > -0.00000000 (-0.00000000,-0.00000000) I think gfortrans position on how IEEE rules affect _Complex should be documented in the manual - the C manual has a section on Implementation Defined Behavior (but the C standard mandates this for items defered to implementations). Internally GCC uses flag_complex_method to indicate how _Complex multiplication and esp. division are implemented. On the user-facing side there's -fcx-limited-range and -fcx-fortran-rules controlling this flag. -ffast-math sets -fcx-limited-range.
