[llvm] SelectionDAG store merging default implementations are unreasonable #90714 (PR #131424)

[Matt Arsenault via llvm-commits]

arsenm wrote:

The hook needs more parameters, at a minimum needs to consider both the register and memory type and the address space.

> This predicate just checks whether store merging is attempted at all, there are more checks (isTypeLegal, canMergeStoresTo, allowsMemoryAccess) before actually doing the merge. Possibly your target is not handling some of these correctly?

The predicate is a hack around the fact that the DAG's expressiveness is severely lacking in what is considered legal. We have the side channel for the alignment, the side channel for truncating stores, and nothing for the address space. If we had an accurate legality query this hook probably wouldn't need to exist in the first place 



https://github.com/llvm/llvm-project/pull/131424