[clang] [clang][dataflow] Strengthen pointer comparison. (PR #75170)

[via cfe-commits]

martinboehme wrote:

> > In summary, if the storage locations for two pointers are the same, we can return a true literal for the comparison, but if the storage locations are different, we need to return an atom.
> 
> I am wondering, if it would make sense to "fix up" the state when we discover aliasing? I.e., "merging" the two storage locations. Consider:
> 
> ```
> void f(MyBoolPair* a, MyBoolPair *b) {
>   if (a == b && a->first && !b->second)
>   {
>     // Here, we should know: a->first == b->first == true and a->second == b->second == false
>   } 
> }
> ```

This sounds great -- but I think getting this to work wouldn't be trivial. At least, I don't yet have any good ideas on how to make it work.

I think we'd have to do something like this:

1.  When we evaluate `a == b`, associate this expression with an atom, then remember that this atom means that the storage locations for the two pointers are the same.

2.  When we enter a new block, if we can prove from the flow condition that the atom mentioned in step 1 is true, remember that the storage locations for `a` and `b` must alias.

3.  When we evaluate `b->first` -- I guess we iterate over all of the storage locations that must alias with the storage location for `b`, and if, for at least one of them, we can prove that `b->first` is true, we know it must be true? (I'm not super-conversant in alias analysis -- would have to read up more about this.)

Would require some more thought and definitely quite a bit of effort to implement, I think.

https://github.com/llvm/llvm-project/pull/75170