Re: getting same output as gfortran, long_float

["Nasser M. Abbasi" <nma@12000.org>]

On 4/30/2015 5:08 PM, Dmitry A. Kazakov wrote:
>
> I don't know what FORTRAN does, but assuming, you are using Intel, the
> longest floating-point available there is not, as you seem assume, of 16
> bytes (quadruple-precision), but so-called extended precision, which is not
> that long (10 bytes, namely).
>

Yes, I do not know either how gfortran does it. I am on intel,
using

>gfortran --version
GNU Fortran (Ubuntu 4.8.2-19ubuntu1) 4.8.2

It might be linked to libquadmath? I see article on this

http://glennklockwood.blogspot.com/2014/02/linux-perf-libquadmath-and-gfortrans.html

"Executive Summary: libquadmath was introduced in GFortran 4.6
which fundamentally changed what the -fdefault-real-8 switch does"

thanks for your input below, will look at it:

> AFAIK, GNAT on x86 maps:
>
> Float to single precision float (4 bytes)
> Long_Float to double precision float (8 bytes)
> Long_Long_Float to extended precision float (10 bytes, effectively)
>
> You can try this program to see how many binary mantissa bits are available
> for each type:
> -----------------------------
> with Text_IO; use Text_IO;
> procedure Test is
> begin
>     Put_Line ("Float mantissa:" & Integer'Image (Float'Machine_Mantissa));
>     Put_Line ("Long Float mantissa:" & Integer'Image
> (Long_Float'Machine_Mantissa));
>     Put_Line ("Long Long Float mantissa:" & Integer'Image
> (Long_Long_Float'Machine_Mantissa));
> end Test;
> --------------------------
> On an Intel machine it will print:
>
> Float mantissa: 24
> Long_Float mantissa: 53
> Long_Long_Float mantissa: 64
>
> If FORTRAN supports longer floating point numbers (e.g.
> quadruple-precision) then it must emulate them since Intel hardware does
> not have them. That would make computations quite slow and you should
> consider if you really need that many significant digits.
>
> 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.
>