On Mon, 2006-03-20 at 19:47 -0700, Jeffrey A Law wrote:
> 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).
A casual read of tree-vrp.c showed that symbolic_range_p is mostly
used to do nothing, did I miss something? May be it's in another file.
> Note that this is closely related to some of the bounds checking
> elimination we want to support for Java one day IIRC.
Pascal and Ada will benefit (may be Fortran too, I don't know enough to
say).
> 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.
Very likely to be all locals in typical Ada code.
Laurent
