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From: "kargl at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug fortran/107753] gfortran returns NaN in complex divisions (x+x*I)/(x+x*I) and (x+x*I)/(x-x*I)
Date: Sat, 19 Nov 2022 19:11:41 +0000 [thread overview]
Message-ID: <bug-107753-4-aiTb8H4N0e@http.gcc.gnu.org/bugzilla/> (raw)
In-Reply-To: <bug-107753-4@http.gcc.gnu.org/bugzilla/>
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)
next prev parent reply other threads:[~2022-11-19 19:11 UTC|newest]
Thread overview: 16+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-11-18 19:32 [Bug fortran/107753] New: " weslley.pereira at ucdenver dot edu
2022-11-18 19:50 ` [Bug fortran/107753] " anlauf at gcc dot gnu.org
2022-11-18 20:50 ` kargl at gcc dot gnu.org
2022-11-18 21:26 ` anlauf at gcc dot gnu.org
2022-11-18 22:05 ` kargl at gcc dot gnu.org
2022-11-18 23:24 ` sgk at troutmask dot apl.washington.edu
2022-11-18 23:32 ` sgk at troutmask dot apl.washington.edu
2022-11-18 23:45 ` weslley.pereira at ucdenver dot edu
2022-11-18 23:47 ` weslley.pereira at ucdenver dot edu
2022-11-19 0:25 ` sgk at troutmask dot apl.washington.edu
2022-11-19 19:11 ` kargl at gcc dot gnu.org [this message]
2022-11-19 20:14 ` anlauf at gcc dot gnu.org
2022-11-20 0:54 ` sgk at troutmask dot apl.washington.edu
2022-12-07 21:16 ` anlauf at gcc dot gnu.org
2022-12-07 21:50 ` kargl at gcc dot gnu.org
2022-12-07 22:31 ` anlauf at gcc dot gnu.org
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