http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55539
--- Comment #4 from Tobias Burnus <burnus at gcc dot gnu.org> 2012-12-12
11:16:33 UTC ---
(In reply to comment #3)
> Strangely enough I needed to add some epsilon to 0.5 so that
> the second test passes, because the exact value 0.5 appears
> to get rounded down to 0 in formatted output.
That should depend on the rounding mode; you have the choice between
RD (round down), RC (compatible), RN (nearest), RP (processor dependent), RU
(round up), RZ (round towards zero).
The default ("RD") in gfortran is NEAREST, which has to match IEEE 754-1985
(alias IEC 60559:1989), namely (cf. Note 10.14 in Fortran 2008):
"On processors that support IEEE rounding on conversions, the I/O rounding
modes COMPATIBLE and NEAREST will produce the same results except when the
datum is halfway between the two representable values. In that case, NEAREST
will pick the even value, but COMPATIBLE will pick the value away from zero.
The I/O rounding modes UP, DOWN, and ZERO have the same e ect as those speci ed
in IEC 60559:1989 for round toward +oo, round toward -oo, and round toward 0,
respectively."
And 0.5 rounded to even rounds down to "0" and not up to "1". (Seemingly,
decimal "0.5" is exactly representable in binary FP while decimal "0.1" isn't.)
