[llvm] [SelectionDAG]Salvage debuginfo when combining load and sext instrs. (PR #169779)

[Orlando Cazalet-Hyams via llvm-commits]

https://github.com/OCHyams commented:

Hi 🙂 

> The DBG_VALUE needs to be transferred correctly to the new combined instruction, and it needs to be appended with a DIExpression which contains a DW_OP_LLVM_fragment, describing that the lower bits of the virtual register contain the value.

I don't think we typically use fragments to describe exts, they're normally used to describe when source variables are chopped up. It could be worth checking what the IR-level salvaging does for casts?

https://github.com/llvm/llvm-project/pull/169779