Re: Feature or Bug? 2#1111# shifted by 4 is 224

[Tero Koskinen <tero.koskinen@iki.fi>]

On Mon, 02 Jun 2008 17:17:19 +0200 Dennis Hoppe wrote:
> Hi,
> 
> we probably found a bug in Ada or we cannot realize, that this is indeed 
> a feature ;)
[snip]
> 
> In Example B, we limit the parameter N to Positive, because we do not 
> want "negative" shifts. The result of Shift_Left(2#1111#, 4) is 224
> (2#11100000#) !
> 
> -- Example B
> type Modular_Type is mod 2**4;
> function Shift_Left (Item : in Modular_Type;
>                       N    : in Positive) return Modular_Type is
> begin
>    return Item * 2**N;
> end Shift_Left;
> 
> Why does this happen?

It is a bug in GCC on i386 platform. I can repeat it with GNAT GPL 2007
and GCC 4.3.0 (OpenBSD/i386). With GCC 4.3.0 optimization flags
(-O2 vs. -O0) seem to affect to the result.

On Solaris (5.10 Generic_127111-05 sun4u sparc SUNW,Sun-Fire-V240) with
GCC 4.2.1 the example returns 0 always.

Here is more complete test case:
with Ada.Text_IO; use Ada.Text_IO;
procedure Shift is

type Modular_Type is mod 2**4;
function Shift_Left (Item : in Modular_Type;
                     N    : in Positive) return Modular_Type is
begin
   return Item * (2**N);
end Shift_Left;

    X : Modular_Type;
begin
    X := Shift_Left(2#1111#, 4);
    Put_Line (X'Img);
end Shift;

-- 
Tero Koskinen - http://iki.fi/tero.koskinen/