Re: conversions between fixed-point types

[sjw <simon.j.wright@mac.com>]

On Sep 25, 9:47 am, "Stuart" <stu...@0.0> wrote:
> "Adam Beneschan" <a...@irvine.com> wrote in message
>
> news:c8af7daf-9f00-4dbb-ad0d-9ca65e0a83d6@f18g2000prf.googlegroups.com...
>
>
>
> > Dirk Herrmann wrote:
>
> > > FpB'Image(FpB'(-1.5)) evaluates to -1.2
> > > FpB'Image(FpB(FpA'(-1.5))) evaluates to -1.2
> > > FpB'Image(MakeBFromA(FpA'(-1.5))) evaluates to -1.6
> > The answer is curious (and I'm not sure if it's what the language
> > designers intended).  4.9(38) says, ...
> ...
> > If FpB'Machine_Rounds is FALSE, though, the situation is interesting.
> > 4.9(38) requires that static expressions get *truncated* toward zero,
> > as I read it.  But there's no similar requirement for expressions
> > computed at runtime.
>
> Having played with this on GNATPro 6.1.2 (to PC target) it does report
> FpB'Machine_Rounds as false.  It also rounds values like -1.59999 down to
> -1.2 (which is a logical consequence).  Technically this is still within the
> error bounds declared for FpB (delta 0.4) so it is not "wrong" within the
> rules of the language definition.  But it is very counter-intuitive and is
> the sort of thing I find frustrating.
>
> I also tested the problem on GreenHills AdaMULTI (4.2.3 to PowerPC target);
> this gave FpB'Machine_Rounds as true and gave the more intuitive results.
>
>    -1.5         -> -1.6
>    -1.40001 -> -1.6
>    -1.4         -> -1.2
>
> So although there does look to be a language "gotcha" here, it depends upon
> the compiler's choice of 'Machine_Rounds.
>
> --
> Stuart

This is all very weird. Can I point you to my earlier post at
http://groups.google.com/group/comp.lang.ada/msg/f79a71b9abe7e287
which (unless I'm deluded) shows GNAT rounding away from zero for
dynamic values even though 'Machine_Rounds is False!! (and I get the
same results on Ubuntu 8.04 with GNAT GPL 2008.