Re: how *not* to use denormalised numbers?

[Anton Shterenlikht <mexas@bris.ac.uk>]

From nmm1@hermes.cam.ac.uk Tue Nov 10 15:51:26 2015
>
>On Nov 10 2015, Steve Kargl wrote:
>>
>>Although I haven't checked, you'll probably need to use the
>>IEEE arithmetic modules to control subnormal numbers or use
>>C interop and fenv features to manipulate the FPU.
>
>I agree.  I would hope that the default is to use the 'native' mode,
>whatever that is, on the grounds that changing the mode is bad news
>for interoperability with C and some libraries,

I'm confused:

$ cat z.f90 
use ieee_arithmetic
logical :: gradual
  write (*,*) "Support underflow control:", &
              ieee_support_underflow_control()
  call ieee_get_underflow_mode( gradual )
  write (*,*) "Gradual underflow:", gradual
  write (*,*) "Support denormals:", ieee_support_denormal()
! flip underflow mode
  call ieee_set_underflow_mode( .not. gradual )
  call ieee_get_underflow_mode( gradual )
  write (*,*) "Gradual underflow:", gradual
  write (*,*) "Support denormals:", ieee_support_denormal()
end
$ gfortran6 -Wl,-rpath="/usr/local/lib/gcc6" z.f90
$ ./a.out 
 Support underflow control: F
 Gradual underflow: T
 Support denormals: T
 Gradual underflow: F
 Support denormals: T
$

So underflow control is not supported.
Why then ieee_set_underflow_mode pretends to
switch gradual underflow off?

Another compiler gives:

 Support underflow control: T
 Gradual underflow: F
 Support denormals: F
 Gradual underflow: T
 Support denormals: T

which makes sense.

So the impression I get is that denormals are
enabled by default in gfortran and cannot be switched off.
I read that denormals processing could be substantially
slower that just flash to zero. I think my code might
be affected.

Anton