feature or a bug with long double (quad-precision) in gcc (6.2.1)

[Vincent Lefevre]

On 2016-12-03 22:10:52 +0800, Vladimir Lomov wrote:
> I faced strange problem with long double (C) in comparison with fortran
> (gfortran) on my system:
> Archlinux x86_64, gcc 6.2.1 20160830.

So, in C, long double = x86 extended precision (80 bits + padding).

> I prepared two small programs on C and Fortran (see attached files):
> c-double-ldouble.c, gfortran-real-quad.f95.
[...]
> and in accordance with
> https://en.wikipedia.org/wiki/Double-precision_floating-point_format
> (except the order, but I suppose this is expected), but for long double
> and real(kind=16) and different
> 
>   $ hexdump bin-ldouble.bin
>   0000000 aaab aaaa aaaa aaaa 3ffd 99fb 7ffe 0000
>   0000010

The first five 16-bit words correspond to the value on 80 bits.
The other three 16-bit words are padding and contain garbage.

>   $ hexdump bin-ldouble.dat
>   0000000 5555 5555 5555 5555 5555 5555 5555 3ffd
>   0000010

This is quadruple precision.

> I suppose that hexdump for fortran case is correct, according to
> https://en.wikipedia.org/wiki/Quadruple-precision_floating-point_format
> so result of long double is wrong, besides it depends on options passed
> to compiler.

No, this is just a different format. See:

  https://en.wikipedia.org/wiki/Long_double

for the various formats depending on the platform.

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Vincent Lefèvre <vincent@vinc17.net> - Web: <https://www.vinc17.net/>
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