[Users] best way to maintain CCTK_COMPLEX precision with Fortran?

[Erik Schnetter]

Bernard

I usually define some constants in each routine or module:

CCTK_REAL, parameter :: zero = 0
integer, parameter :: rkind = kind(zero)

and then I can write "cmplx(x, kind=rk)".

-erik



On Mon, Jul 20, 2015 at 12:09 PM, Bernard Kelly <physicsbeany at gmail.com>
wrote:

> Thanks, Erik.
>
> I wasn't taking any magnitudes, but it looks like my problem was just
> blindly using the "cmplx" function to form the complex variable
> without a KIND argument, and it defaults to KIND=1 (i.e.
> single-precision) even if the real & imaginary components enter as
> double-precision.
>
> For now, I've switched to the GNU "complex" method, which seems to
> have fixed my problem on my Macbook.
>
> Bernard
>
> On 19 July 2015 at 14:52, Erik Schnetter <schnetter at cct.lsu.edu> wrote:
> > Bernard
> >
> > In Fortran, real(z) (where z is complex) does maintain precision. The
> > problem is likely caused by your using "-Ofast", "-ffast-math", or an
> Intel
> > compiler that uses the equivalent by default (so you have to explicitly
> > switch it off). With such an optimization, e.g. abs(z) is implemented as
> > sqrt(real(z)**2 + aimag(z)**2), which overflows the intermediate squares.
> > Without -ffast-math, compilers use a more expensive implementation that
> does
> > not overflow.
> >
> > In your case, I'd assume that the problem is no the call to real, but
> other
> > complex or real functions. Can you show us a larger part of the code, and
> > the compiler vendor, version, and options you are using?
> >
> > The C functions CCTK_Cmplx* are deprecated. These days, C compilers
> > implement complex numbers as defined by the standard, and you'd use the
> > respective C standard function instead.
> >
> > -erik
> >
> >
> > On Sun, Jul 19, 2015 at 2:16 PM, Bernard Kelly <physicsbeany at gmail.com>
> > wrote:
> >>
> >> Hi all.
> >>
> >> The Cactus documentation shows a set of C routines CCTK_CmplxXXXXXX
> >> that presumably preserve precision between CCTK_REAL and CCTK_COMPLEX
> >> types. Is there a "right" way to handle this in Cactus with Fortran
> >> (that isn't "recode in C")?
> >>
> >> Context: I have some Fortran code in a Cactus thorn that works with
> >> real & complex fields. I've noticed that when the numbers are very
> >> large in magnitude, taking a simple real(z) drops precision --- a
> >> finite but large real part ends up as +/- Inf instead. I suspect it's
> >> dropping to single precision at some stage. Switching to "dreal"
> >> doesn't help, and avoiding the use of CCTK_COMPLEX altogether ---
> >> while possible --- leads to longer, less readable code.
> >>
> >> Thanks,
> >>
> >> Bernard
> >> _______________________________________________
> >> Users mailing list
> >> Users at einsteintoolkit.org
> >> http://lists.einsteintoolkit.org/mailman/listinfo/users
> >
> >
> >
> >
> > --
> > Erik Schnetter <schnetter at cct.lsu.edu>
> > http://www.perimeterinstitute.ca/personal/eschnetter/
>



-- 
Erik Schnetter <schnetter at cct.lsu.edu>
http://www.perimeterinstitute.ca/personal/eschnetter/
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