https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107753
kargl at gcc dot gnu.org changed: What |Removed |Added ---------------------------------------------------------------------------- Priority|P3 |P4 --- Comment #10 from kargl at gcc dot gnu.org --- (In reply to Steve Kargl from comment #9) > On Fri, Nov 18, 2022 at 11:24:29PM +0000, sgk at troutmask dot > apl.washington.edu wrote: > > > > Does anyone know what is meant by "Fortran rules"? F66 does not > > have any particular algorithm specified. I'll look at F77 shortly. > > > > Well, I hunted down the origins of -fcx-fortran-rules. > > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=29549 > > So, it appears to be an optimization, where Smith's algorithm > will fail for extreme values of the real and imaginary parts > of the complex number. So, I wrong a dirty little program to time complex division. program foo use timerm, only : rdtsc implicit none integer, parameter :: n = 1024*1024, dp = kind(1.d0) real(dp) re(n), im(n) complex(dp) x(n) integer i integer(8) t0, t t = 0 do i = 1, 10 call random_number(re) call random_number(im) x = cmplx(4 * re, 10 * im,8) t0 = rdtsc() x = x / x t = t + (rdtsc() - t0) end do print '(G0,1X,G0)', x(1) print *, real(t,10) / 10 / n end program foo Compiled with gfortran with its current method of doing division (i.e., -fcx-fortran-rules), I see roughly 44.5 clock ticks per division. If run with a patched gfortran that uses the method that the C compiler uses, I get about 62 ticks per division. So, using the stricter method impacts performance. I'll note that gfortran unilaterally enforces -fcx-fortran-rules, i.e., -fno-cx-fortran-rules has no effect. Perhaps, gfortran could be given a new -fcx-division=XXX option, where XXX is one of naive --> what -ffast-math does (flags_complex_method = 0) smith --> what -fcx-fortran-rules (flags_complex_method = 1) strict -> default C behavior (flags_complex_method = 2)