https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107753
--- Comment #12 from Steve Kargl <sgk at troutmask dot apl.washington.edu> --- On Sat, Nov 19, 2022 at 08:14:01PM +0000, anlauf at gcc dot gnu.org wrote: > 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. > The optimization level is irrelevant. gfortran unilaterally uses -fcx-fortran-rules, and there is no way to disable this option to user the slower, but stricter, evaluation. One will always get complex division computed by a+ib a + b(d/c) b - a(d/c) ---- = ---------- + i ------------ |c| > |d| c+id c + d(d/c) c + d(d/c) and similar for |d| > |c|. There are a few problems with this. d/c can trigger an invalid underflow exception. If d == c, you then have numerators of a + b and b - a, you can get a invalid overflow for a = huge() and b > 1e291_8.
