Description
We have an amazing amount of information that we're not passing along to LLVM. We should be attaching metadata to pointers and using it to respond to alias analysis queries. A good start would be to figure out how to make a shim responding MayAlias to all queries.
The NoAlias response may be used when there is never an immediate dependence between any memory reference based on one pointer and any memory reference based the other. The most obvious example is when the two pointers point to non-overlapping memory ranges. Another is when the two pointers are only ever used for reading memory. Another is when the memory is freed and reallocated between accesses through one pointer and accesses through the other — in this case, there is a dependence, but it’s mediated by the free and reallocation.
For Const objects, assuming no incorrect unsafe blocks, we can respond NoAlias for every &mut, & and ~ pointer because they either only read memory or are the only handle able to mutate the memory while they exist. We have the potential to beat C/C++ on a lot of benchmarks here...