Re: Use aggregates (Was: Allocation question)

[sjw <simon.j.wright@mac.com>]

On May 11, 1:16 pm, Ludovic Brenta <ludo...@ludovic-brenta.org> wrote:
> Jacob Sparre Andersen wrote on comp.lang.ada:
>
>
>
> > Olivier Scalbert wrote:
> > > procedure Fill(Image: in out Image_T; Color: Color_T) is
> > > begin
> > >       for x in Image'range(1) loop
> > >           for y in Image'range(2) loop
> > >               Image(x,y) := Color;
> > >           end loop;
> > >       end loop;
> > > end Fill;
>
> > I would sugges that you rather wrote this:
>
> >    Image := (others => (others => Color));
>
> > It is a more precise expression of what (I guess) you want done, and
> > thus it gives the compiler more precise information to work with.
>
> This is not always a good idea. On some targets, GNAT places the
> aggregate on the stack and this can raise a Storage_Error. In avionics
> programs, I remember replacing aggregates with loops for this reason.

Someone recently found it was quicker to test an array for some
condition by looping through it rather than by comparing with an
aggregate [eg X = (others => 0)]. I had naively assumed that GNAT
would just do the comparisons of the elements of X against 0, as the
hand-crafted loop does, rather than construct a whole array of zeros
on the stack and then loop through that ...