Re: Ada subtypes and base types

[Jeffrey A Law <law at redhat dot com>]

On Sat, 2006-03-18 at 10:24 +0100, Laurent GUERBY wrote:
> On Fri, 2006-03-17 at 12:51 -0700, Jeffrey A Law wrote:
> > I'm not suggesting the FEs deduce more types and track ranges;
> > that would be rather absurd.  What I'm saying is that exposing
> > these types outside the FE is most likely costing you both on
> > the compile-time side and on the run-time side.
> 
> About optimization, in most languages with array bounds and
> range checks, the FE will build a structure with bounds
> and code like the following for a typical array loop (sorry
> for poor C):
> 
> struct {
>    int low,high; /* no alias */
>    double *data;
> } X;
> 
> int first_i=X.low+2; 
> int last_i=X.high-3;
> int i;
> 
> if (first_i<=last_i) {
>    for(i=first_i;i<=last_i;i++) {
>       if (i<X.low||i>X.high) raise_bound_error(); /* CHECK */
>       do_something(array_with_bounds[i]);
>    }
> }
> 
> The interesting thing performance wise would be to be able to remove the
> CHECK in the BE. 
> 
> Is there some optimization pass able to do that?
> 
> And when "+2" and "-3" are replaced by "+x" and "-y" and we
> know through typing that x>=0 and y>=0?
Not sure, mostly because of the structure accesses and my lack
of knowledge about the symbolic range support.  I know we have
symbolic range support, but haven't looked to see how good it
really is.  What we're doing now is certainly better than what
DOM did for symbolic ranges (nothing).

Note that this is closely related to some of the bounds checking
elimination we want to support for Java one day IIRC.

Note also that if i, first_i and/or last_i are globals, then the
odds of these kind of tests being deleted go down considerably
as they're likely call-clobbered by the call to do_something.

Jeff