https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107753
--- Comment #5 from Steve Kargl <sgk at troutmask dot apl.washington.edu> --- On Fri, Nov 18, 2022 at 10:05:21PM +0000, kargl at gcc dot gnu.org wrote: > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107753 > > --- Comment #4 from kargl at gcc dot gnu.org --- > (In reply to anlauf from comment #3) > > I guess the reporter assumes that gcc uses a clever algorithm like Smith's > > to handle such extreme cases of complex division. Not sure if that one is > > available by some compilation flag, and I think it would impact performance. > > > > In any case, if the reporter wants to get robust results and in a portable > > way, I would advise him to change/fix his algorithm accordingly. It appears > > that a few other compilers behave here like gfortran. > > It's likely coming from the middle-end where gcc.info has > the option > > '-fcx-fortran-rules' > Complex multiplication and division follow Fortran rules. Range > reduction is done as part of complex division, but there is no > checking whether the result of a complex multiplication or division > is 'NaN + I*NaN', with an attempt to rescue the situation in that > case. Does anyone know what is meant by "Fortran rules"? F66 does not have any particular algorithm specified. I'll look at F77 shortly. Tracking down what -fcx-fortran-rules does, one finds the eventually flag_complex_method is set to 1. The lower of complex division occurs in gcc/tree-complex.cc (expand_complex_division). If I use this patch % git diff gcc/tree-complex.cc | cat diff --git a/gcc/tree-complex.cc b/gcc/tree-complex.cc index ea9df6114a1..8051b7a3843 100644 --- a/gcc/tree-complex.cc +++ b/gcc/tree-complex.cc @@ -1501,6 +1501,7 @@ expand_complex_division (gimple_stmt_iterator *gsi, tree type, break; case 2: + case 1: if (SCALAR_FLOAT_TYPE_P (inner_type)) { expand_complex_libcall (gsi, type, ar, ai, br, bi, code, true); @@ -1508,7 +1509,6 @@ expand_complex_division (gimple_stmt_iterator *gsi, tree type, } /* FALLTHRU */ - case 1: /* wide ranges of inputs must work for complex divide. */ expand_complex_div_wide (gsi, inner_type, ar, ai, br, bi, code); break; to force gfortran through the C language code path, I get void doit (complex(kind=8) & restrict z) { complex(kind=8) _1; complex(kind=8) _2; complex(kind=8) _3; real(kind=8) _7; real(kind=8) _8; real(kind=8) _9; real(kind=8) _10; real(kind=8) _11; real(kind=8) _12; <bb 2> : _7 = REALPART_EXPR <*z_5(D)>; _8 = IMAGPART_EXPR <*z_5(D)>; _1 = COMPLEX_EXPR <_7, _8>; _9 = REALPART_EXPR <*z_5(D)>; _10 = IMAGPART_EXPR <*z_5(D)>; _2 = COMPLEX_EXPR <_9, _10>; _3 = __divdc3 (_7, _8, _9, _10); _11 = REALPART_EXPR <_3>; _12 = IMAGPART_EXPR <_3>; REALPART_EXPR <*z_5(D)> = _11; IMAGPART_EXPR <*z_5(D)> = _12; return; } with the result % gfcx -o z -fdump-tree-all a.f90 && ./z (1.79769313486231571E+308,1.79769313486231571E+308) (1.0000000000000000,0.0000000000000000) So, is -fcx-fortran-rules a relic of g77 past?
