[Bug middle-end/21718] real.c rounding not perfect

[exploringbinary at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=21718

--- Comment #19 from Rick Regan <exploringbinary at gmail dot com> ---
I've looked into this and found that real.c/real.h use a precision of
SIGNIFICAND_BITS, which is dependent on an architecture-dependent value called
HOST_BITS_PER_LONG. In addition, SIGNIFICAND_BITS limits the precision to 192
bits on a 64-bit system, and I was able to find an example of an incorrect
conversion there:
5.0216813883093451685872615018317116712748411717802652598273e58.

(Please see my article
http://www.exploringbinary.com/gcc-conversions-are-incorrect-architecture-or-otherwise/
for details.)