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Newsgroups: comp.lang.ada
Date: Sun, 3 May 2015 17:42:49 -0700 (PDT)
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Subject: Re: getting same output as gfortran, long_float
From: robin.vowels@gmail.com
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Xref: news.eternal-september.org comp.lang.ada:25715
Date: 2015-05-03T17:42:49-07:00
List-Id: <comp.lang.ada>

On Friday, May 1, 2015 at 11:12:51 AM UTC+10, Nasser M. Abbasi wrote:
> On 4/30/2015 5:08 PM, Dmitry A. Kazakov wrote:
> 
> > If GNAT allowed floating-point emulation, you could declare a custom type
> > of required precision:
> >
> >     type My_Float is digits 34; -- 112 * lg(2)
> >
> > Which is advisable anyway in order to make your program portable. Using
> > built-in types is a bad idea and poor taste in most cases.
> >
> > Since GNAT does support emulation, at least not with the standard compiler
> > switches, you need a library for doing this.
> >
> 
> I found that gfortran can do 128 bit floating point without
> the use of the compiler switch -fdefault-real-8. Which will
> map to similar thing as the above Ada construct:
> 
> -------------------
> PROGRAM foo
> IMPLICIT NONE
> REAL(KIND = 16) :: x  !-- kind=16 tells it is double quad

No it doesn't.  That form is non-standard, and won't compile with some
compilers.
You need to use SELECTED_REAL_KIND or something equivalent.

> x = 12.0D0 * 0.0001D0/(1.0D0 * (1.0D0 - 0.1D0)**4 )

And the above constants are double precision, not quadruple precision,
to the numerical result with have no more precision than double.

> PRINT *, x
> END PROGRAM
> ------------------
> 
> gfortran -Wall  foo2.f90
> ./a.out
>     1.82898948331047112025871115292829927E-0003