Inefficiency of `mod'

[Maurice Lombardi]

Le 25/10/2010 12:13, Prof A Olowofoyeku (The African Chief) a écrit :
> On 25 Oct 2010 at 11:21, Jonas Maebe wrote:
>
>> Actually, it's rather strange: "shr" is a logical shift right (both in
>> FPC and Borland Pascal, and I assume also in GPC) as opposed to an
>> arithmetic shift right. So I guess that Kylix and GPC also perform a  64
>> bit logical shift right on the value, but then unlike FPC truncate  the
>> result back to 32 bits. Otherwise I'm not sure where the "-1"  comes
>> from when doing "(-2)shr 1".
>
> I don't much understand what all this is about, but for what it's
> worth, here are my results with Delphi (3, 4, and 7) and BP 7.0.
>
> program unary;
> begin
>     writeln( -1 ); { -1 }
>     writeln( 2 shr 1 ); { 1 }
>     writeln((-2)shr 1 ); { -1; BP7=2147483647 }
>     writeln(-(2 shr 1)); { -1 }
>     writeln( -2 shr 1 ); { -1; BP7=2147483647 }
>     writeln( -2 div 2 ); { -1 }
> end.

This means that GPC does an arithmetical right shift (i.e. propagating 
the 1 (or 0) in the leftmost (highest weight) position), while BP does a 
logical right shift.
It seems more sound to use an arithmetical shift for integer and a 
logical shift for cardinal. Standard does not help since it does not 
define the shift operators. C (and BP?) are too mind confused to enforce 
this point.

Maurice

-- 
        Maurice Lombardi
Laboratoire de  Spectrometrie Physique,
Universite Joseph Fourier de Grenoble, BP87
38402 Saint Martin d'Heres Cedex     FRANCE
Tel: 33 (0)4 76 51 47 51
Fax: 33 (0)4 76 63 54 95
mailto:Maurice.Lombardi at ujf-grenoble.fr