http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54159
Dominique d'Humieres <dominiq at lps dot ens.fr> changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|UNCONFIRMED |RESOLVED
Resolution| |INVALID
--- Comment #3 from Dominique d'Humieres <dominiq at lps dot ens.fr> 2012-08-02
11:35:32 UTC ---
What you see is the "round to even" effect as shown with the simpler following
test:
program quad_test
implicit none
integer, parameter :: dp = kind(0d0)
integer, parameter :: qp = selected_real_kind(32)
real(qp) :: y
y = 1.0_qp+0.5*real(epsilon(1.0_dp), kind=qp)
print *, y, nearest(y,1.0_qp)
print *, real(y, kind=dp), real(nearest(y,1.0_qp), kind=dp)
end program quad_test
which outputs
1.00000000000000011102230246251565404
1.00000000000000011102230246251565423
1.0000000000000000 1.0000000000000002
i.e., y is rounded to 1.0_dp (the closest even binary representation) while
nearest(y,1.0_qp) is rounded to the closest double: nearest(1.0_dp,1.0_dp). For
your numbers the hexa representations are:
400E4A0CCB6E8DB58800000000000001
400E4A0CCB6E8DB58800000000000000
40E4A0CCB6E8DB59
40E4A0CCB6E8DB58
and if I am not mistaken, the (default) rounding to "even" are correct.
Your comments "Two quad prec numbers that differ by last 3 digits" and
"truncated output" make me suspect that you are not fully aware of the detailed
behavior of floating-point representations and their default rounding
properties.
