John Fine wrote: > Andrew Haley wrote: >> >> We obviously don't want to start having a no-strict-alias attribute for >> pointers. I think it's pointless because it would only apply to new >> code, and the whole point of no-strict-alias is to make old code work. > > I sure disagree with that. > > How about making old code repairable. > > It would be a monster task to rewrite old code to not need the problem > casts. The hard part IME is finding the bad casts. Once you've done that it's not so awfully hard to convert pointer casts to use unions instead. Adding a no-strict-alias attribute solves the easy part, leaving the hard part. > It is a much more reasonable task to find all the pointers that > are used with problem casts. If you could just tack some attribute onto > the declaration of each such pointer, you could take a giant application > you don't even understand and repair it for this issue in under a day. > > I think I've tricked the compiler into generating working code. I don't > think my correction is correct (I think I just confused the optimizer). > But I don't know enough about the exact rules of aliasing to be sure. > > I made a union of void* and int*. And my basic pointer now points to > that union instead of being void**. > Then I never use the int* member of the union. What used to directly > use the void** now uses the void* member of the union. What used to use > the Datatye** cast of the void** now uses a Datatype** cast of the > address of the void* member of the union. That still means I'm casting > a void** to a Datatype**, but it seems to have changed the compiler's > understanding of it. Why not make it a union of Datatype* and void* ? Then you wouldn't need to cast at all. Andrew.
