Re: conversions between fixed-point types

[Dirk Herrmann <fight_spam@invalid.invalid>]

Adam Beneschan wrote:
> So I guess whether this is a GNAT bug or not depends on
> FpB'Machine_Rounds.  If it's TRUE, the first two expressions are
> computed incorrectly, but if it's FALSE, this seemingly anomalous
> result is allowed.

I checked that T'Machine_Rounds is FALSE for FpA and FpB.  Thus, according to 
your analysis the result should be OK with respect to the RM.

> Also, it's clear that static expressions need to round or truncate to
> a multiple of the 'Small if they're not part of a larger static
> expression.  But I'm not sure what this should do:
> 
>    FpA'Image (FpA (FpB' (-1.5)))
> 
> Here, FpB' (-1.5) *is* part of a larger static expression, so the rule
> in 4.9(38) doesn't apply.  But I'm not sure just what does apply---is
> the value of FpB'(-1.5) still assumed to be -1.5 (so that the type
> conversion to FpA still results in -1.5), or should the type
> conversion take place (which could result in -1.0 if truncation is
> used for both type conversions), or is the result unspecified by the
> language?

The following statement is made in the annotated RM (4.9(38.d/2)):
   {AI95-00100-01} Note that the only machine numbers values of a fixed point
   type are the multiples of the small, so a static conversion to a fixed-point
   type, or division by an integer, must do truncation to a multiple of small.
   It is not correct for the implementation to do all static calculations in
   infinite precision.
To me this makes it clear that conversions to a fixed point type within a 
larger static expression have to convert the value to a multiple of the small. 
  What surprises me is that it says "truncation" in contrast to 4.9(38) itself.

--
Dirk