On 9/3/07, Daniel Berlin <[EMAIL PROTECTED]> wrote: > On 9/2/07, Paul Brook <[EMAIL PROTECTED]> wrote: > > > > That said, second, my understanding of restrict, from reading the c99 > > > > standard, is that it is perfectly valid for restrict pointers to alias > > > > each other during *loads*.. IE you can guarantee any restricted > > > > pointer that is stored into can't alias the other restricted pointers. > > > > Those only used for loads can alias each other. > > > > > > How does it make a difference? If for two pointers that are only > > > loaded from we say they don't alias I cannot imagine a transformation > > > that would get anything wrong. > > Easy answer: Dependence testing and then loop transforms. > > Given p[i] = a[i] + b[i], if you claim a and b can't alias, you will > now claim that a[i] and b[i] don't access the same memory on the same > iteration. > > We could easily use this and some profit estimation to decide to say, > change the iteration space of a but not b,which will break since they > really do alias, and breaking this is bad because they are allowed to.
Eh? Maybe I'm blind, but how can a change in iteration space that is valid for the non-aliasing case be invalid for the aliasing case _if we do not modify any data_? Ok, Marks example is the only one we could get wrong. But we simply can not derive value ranges from restrict qualification. Richard.
