how *not* to use denormalised numbers?

[FX]

[Anton]
> My understanding was that the use has to request
> the use of denormalised numbers explicitly, via USE statement,
> e.g. MFE, p.225.

Fortran 2008, 14.5: "The initial underflow mode is processor dependent”


[Nick]
> 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,

The default in gfortran is “do not change the FPU state”.


[Anton]
>  call ieee_get_underflow_mode( gradual )

Fortran 2008, 14.11.8: "IEEE_GET_UNDERFLOW_MODE shall not be invoked unless IEEE_SUPPORT_UNDERFLOW_CONTROL (X) is true for some X”


> So underflow control is not supported.

No. Underflow control is not "supported for all floating-point kinds". But it is supported, on your platform, for binary32 and binary64 floating-point types, i.e. real(kind=4) and real(kind=8).
Try ieee_support_underflow_control(0.)


FX