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From: "Nasser M. Abbasi" <nma@12000.org>
Newsgroups: comp.lang.ada
Subject: Re: getting same output as gfortran, long_float
Date: Thu, 30 Apr 2015 20:12:47 -0500
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Date: 2015-04-30T20:12:47-05:00
List-Id: <comp.lang.ada>

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
x = 12.0D0 * 0.0001D0/(1.0D0 * (1.0D0 - 0.1D0)**4 )
PRINT *, x
END PROGRAM
------------------

gfortran -Wall  foo2.f90
./a.out
    1.82898948331047112025871115292829927E-0003

So, I wonder what it would take to integrate this
library to gnat to allow one to do

     type My_Float is digits 34;

and just have it work as is. I assume this requires
support and some changes at the compiler level
(system.max_digits has to change for example)? Now it
is set at 18 and the above gives compiler error.

--Nasser