[llvm] [BranchFolding] Kill common hoisted debug instructions (PR #140091)

Thu May 15 09:08:05 PDT 2025

https://github.com/OCHyams created https://github.com/llvm/llvm-project/pull/140091

Please ignore the first 2 commits - they're from #140063

---

branch-folder hoists common instructions from TBB and FBB into their
pred. Without this patch it achieves this by splicing the instructions from TBB
and deleting the common ones in FBB. That moves the debug locations and debug
instructions from TBB into the pred without modification, which is not
ideal. Debug locations are handled in https://github.com/llvm/llvm-project/pull/140063.

This patch handles debug instructions - in the simplest way possible, which is
to just kill (undef) them. We kill and hoist the ones in FBB as well as TBB
because otherwise the fact there's an assignment on the code path is deleted
(which might lead to a prior location extending further than it should).

There's possibly something we could do to preserve some variable locations in
some cases, but this is the easiest not-incorrect thing to do.

Note I had to replace the constant DBG_VALUEs to use registers in the test- it
turns out setDebugValueUndef doesn't undef constant DBG_VALUEs... which feels
wrong to me, but isn't something I want to touch right now.

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