Re: conversions between fixed-point types

[Dirk Herrmann <fight_spam@invalid.invalid>]

Dirk Herrmann wrote:
>> And, while experimenting and trying out Adam Beneschan's solution for 
>> rounding (thanks for your answer, Alan) I got totally confused 
>> because of the following code (I am using GNAT 4.3.4 with the 
>> following command line options: gnatmake -f -gnatVa -gnata 
>> -gnatwadhl.o -save-temps conversion.adb):
> [...]
>> Is this confusing behaviour allowed?  I will submit a bug report if 
>> some expert confirms it is a GNAT bug.

John B. Matthews wrote:
> I'm no expert, and I had some trouble following the conversions in your 
> example.  [...]

Sorry, I should have given more details about why I am confused.

In particular I am disturbed by the fact that the following two lines produce 
different results with GNAT, as has been confirmed for GNAT 3.15p (thanks, 
tmoran):
       FIO.Put(Float(FpB(Float(-1.5)))); TIO.Put(" ");      --> -1.20000E+00
       FIO.Put(Float(MakeB(-1.5))); TIO.Put(" ");           --> -1.60000E+00
In the first line, the conversion to FpB is done from a Float value, which is 
a direct cast from -1.5.  In the second line, the conversion to FpB is also 
done from a Float value, but in this case -1.5 is passed as a Float argument 
to MakeB.  What I don't understand is, why it should make a difference whether 
I cast -1.5 to Float and then convert it to FpB compared to the situation 
where -1.5 is converted to a Float argument, which is then converted to FpB.

The same question arises for the following three lines:
       FIO.Put(Float(FpB(FpA(Float(-1.5))))); TIO.Put(" "); --> -1.20000E+00
       FIO.Put(Float(MakeBFromA(FpA(-1.5)))); TIO.Put(" "); --> -1.60000E+00
       FIO.Put(Float(MakeBViaA(-1.5))); TIO.Put(" ");       --> -1.60000E+00
Again, the difference lies in whether a sequence of casts is used compared to 
whether the very same value is passed as an argument.

Thanks for your help,

Dirk Herrmann
(email address gladly given on request)