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From: "Stuart" <stuart@0.0>
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Subject: Re: conversions between fixed-point types
Date: Fri, 25 Sep 2009 09:47:24 +0100
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Date: 2009-09-25T09:47:24+01:00
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"Adam Beneschan" <adam@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