Re: Ada subtypes and base types

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

On Fri, 2006-02-24 at 18:42 +0100, Sebastian Pop wrote:
> Richard Kenner wrote:
> >     Just to make sure I've dotted the i's and crossed the t's, this is not
> >     what's happening when we hang in VRP when compiling a-textio.
> > 
> >     We convert the incoming object from natural___XDLU_0___2147483647
> >     into its base type, perform the addition in the base type, then
> >     convert back to XDLU_0_2147483647.
> > 
> > The above is exactly what I thought everybody agrees is and should be
> > happening, so I'm confused by your "this is not what's happening"
> > comment above.
> 
> So if I understand correctly, if we can prove that the operation does
> not overflow in natural___XDLU_0___2147483647, then there is no need
> of a cast to the base type and back.
> 
> chrec_convert is checking for non overflowing sequences before
> removing a cast, and that is missing from the aggressive convert.
> Aggressive convert has been intentionally implemented this way because
> the conservative chrec_convert has caused performance regressions (see
> sixtrack slowdowns from last August).
> 
> A patch like the following would solve the problem too, but will
> introduce performance regressions... so I'm not sure that it is a good
> solution.
Another possibility is to simply not allow conversions between a
subtype and basetype.  Again, that might not be as drastic as
your change.

jeff