[llvm] [NVPTX] Lower invalid `ISD::ADDRSPACECAST` (PR #125607)

[Artem Belevich via llvm-commits]

Artem-B wrote:

> > Ideally we'd do it as soon as we create such an invalid ASC, but for that we must have target info available.
> 
> However, I'm unsure of the best place to do this. e.g., `InstCombine` would not want to contain this optimization because it's target-dependent, correct?

InstCombine does happen to have access to the TargetTransformInfo, so we *may* be able to do it there but...

The thing is that we must work correctly regardless of whether we ran any optimizations. That kind of paints us into the corner and means that we can't really do it in the IR. That means that it must be done during lowering.
We may consider replacing bad casts with undef or poison, or figure out better up-front diagnostics, but that would have to be done **in addition** to the back-end being able to handle those bad casts without crashing, unless we can guarantee that bad casts never get to the back-end.

Here's an idea, instead of laundering the pointer, what if we return an obviously broken pointer with the value that would provide a strong hint that things went wrong. E.g. `0xebadca51`  ("Error BAD CAST") for 32-bit pointers and `(0xebadca51 | dst_as<<16 | src_as)`. This will most likely crash and the distinct constant value would be reasonably easy to search for in the sources.


https://github.com/llvm/llvm-project/pull/125607