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From: sjw <simon.j.wright@mac.com>
Newsgroups: comp.lang.ada
Subject: Re: conversions between fixed-point types
Date: Fri, 25 Sep 2009 13:41:15 -0700 (PDT)
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Date: 2009-09-25T13:41:15-07:00
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On Sep 25, 9:47=A0am, "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). =A04.9(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. =A0But 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. =A0It also rounds values like -1.59999 down =
to
> -1.2 (which is a logical consequence). =A0Technically this is still withi=
n the
> error bounds declared for FpB (delta 0.4) so it is not "wrong" within the
> rules of the language definition. =A0But 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.
>
> =A0 =A0-1.5 =A0 =A0 =A0 =A0 -> -1.6
> =A0 =A0-1.40001 -> -1.6
> =A0 =A0-1.4 =A0 =A0 =A0 =A0 -> -1.2
>
> So although there does look to be a language "gotcha" here, it depends up=
on
> 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.