Re: Access keyword

[Adam Beneschan <adam@irvine.com>]

On Apr 29, 6:55 am, Sébastien <seb.mor...@gmail.com> wrote:
> Hi,
>
> I'd like to know if there is performance issue using the access keyword?
>
> for instance the following code:
>
> package MonTest is
>
>    type Vecteur is
>      record
>        x: Integer;
>        y: Integer;
>      end record;
>
>    function Norme(v: in Vecteur) return integer;
>    function NormeAccess(v: access Vecteur) return integer;
>
> end MonTest;
>
> package body MonTest is
>
>    function Norme(v: in Vecteur) return integer is
>      n: Integer := v.x*v.x + v.y*v.y;
>    begin
>      return n;
>    end Norme;
>
>    function NormeAccess(v: access Vecteur) return integer is
>      n: Integer := v.x*v.x + v.y*v.y;
>    begin
>      return n;
>    end NormeAccess;
>
> end MonTest;
>
> Is NormeAccess faster because I'm using access keyword?

Something to keep in mind is that "access" parameters require
additional information to be passed, in order to do accessibility
level checking.  For example, suppose the body looked like this:

   type Vec_Acc is access all Vecteur;
   Save_Vec_Acc : Vec_Acc;
   function NormeAccess(v: access Vecteur) return integer is
     n : Integer := v.x*v.x + v.y*v.y;
   begin
     Save_Vec_Acc := Vec_Acc(v);
   end NormeAccess;

Since you can't have a global access variable pointing to a local
variable belonging to some procedure, the program will have to check,
at run time, whether the "v" you passed in points to a global or
local; and this means that whoever calls NormeAccess will also have to
tell it the "accessibility level" of the access that it's passing in.
Now I realize that your NormeAccess doesn't actually do anything like
this with the access, but the compiler isn't going to know that when
it compiles the specification of MonTest, so the accessibility level
will need to be passed in somehow, regardless of whether it will be
used.  This will almost certainly use up a few extra cycles.

                                  -- Adam