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

[vincent-gcc at vinc17 dot net]

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

--- Comment #20 from Vincent Lefèvre <vincent-gcc at vinc17 dot net> ---
I suppose that for any 54-bit[*] odd integer multiplied by a power of two with
a large exponent (in absolute value), some decimal numbers close to this value
will be affected.

[*] 54-bit for double, 25-bit for float, 65-bit for long double if x87 extended
precision, 114-bit for long double if quadruple precision.