[Bug ada/41416] Conversion Float to fixed-point behaves differently for static expressions

["dirk dot herrmann-privat at gmx dot de" <gcc-bugzilla at gcc dot gnu dot org>]

------- Comment #2 from dirk dot herrmann-privat at gmx dot de  2009-10-05 16:48 -------
Hi,

thanks for considering my report.

The relevant sections seem to be (these were pointed out to me on comp.lang.ada
by Adam Beneschan):

4.9(38) : "For a real static expression that is not part of a larger static
expression ... the implementation shall round or truncate the value (according
to the Machine_Rounds attribute of the expected type) to the nearest machine
number of the expected type...".

4.9(38.1/2): "The rounding of static floating point expressions is
implementation-defined. It is recommended that the rounding be the same as the
default rounding on the target system."

A.5.4(3): S'Machine_Rounds: Yields the value True if rounding is performed on
inexact results of every predefined operation that yields a result of the type
T; yields the value False otherwise. The value of this attribute is of the
predefined type Boolean.

Here's the interpretation (for which I also used some input from comp.lang.ada
by Adam Beneschan):

4.9(38) demands rounding or truncation for real static expressions, depending
on the value of Machine_Rounds.  A.5.4(3) is not limited to static expressions,
but relates "Machine_Rounds = True" to rounding.  The behaviour in case of
"Machine_Rounds = False" is left open by A.5.4(3), though.  The implementation
advice in 4.9(38.1/2) recommends that static rounding should be the same as the
default rounding.

There is no explicit statement with respect to how non-static expressions
should be handled in case of "Machine_Rounds = False".  But, 4.9(38.1/2) can be
interpreted such that it is generally desired that static and non-static
behaviour should match.  Therefore: 4.9(38) demands truncation for static
expressions in case of "Machine_Rounds = False", and it seems plausible to me
that this should then also apply for non-static expressions.

Thanks again,
Dirk


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=41416