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
