[llvm] [DebugInfo][RemoveDIs] Support maintaining DPValues in CodeGenPrepare (PR #73660)

[Orlando Cazalet-Hyams via llvm-commits]

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@@ -2829,34 +2836,46 @@ class TypePromotionTransaction {
       Instruction *PrevInst;
       BasicBlock *BB;
     } Point;
+    std::optional<DPValue::self_iterator> beforeDPValue = std::nullopt;
 
     /// Remember whether or not the instruction had a previous instruction.
     bool HasPrevInstruction;
 
   public:
     /// Record the position of \p Inst.
     InsertionHandler(Instruction *Inst) {
-      BasicBlock::iterator It = Inst->getIterator();
-      HasPrevInstruction = (It != (Inst->getParent()->begin()));
-      if (HasPrevInstruction)
-        Point.PrevInst = &*--It;
-      else
-        Point.BB = Inst->getParent();
+      HasPrevInstruction = (Inst != &*(Inst->getParent()->begin()));
+      BasicBlock *BB = Inst->getParent();
+
+      // Record where we would have to re-insert the instruction in the sequence
+      // of DPValues, if we ended up reinserting.
+      if (BB->IsNewDbgInfoFormat) {
+        DPMarker *DPM = BB->createMarker(Inst);
+        beforeDPValue = DPM->getReinsertionPosition();
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OCHyams wrote:

Looking at the implementation of `DPMarker::getReinsertionPosition`, it looks like creating a marker just for this call is possibly a waste - I don't think any Marker-specific state is used in `getReinsertPosition`? I might be reading it wrong -- either way, I'll drop a comment on the other review.

https://github.com/llvm/llvm-project/pull/73660