[llvm] [DebugInfo][LoopLoadElim] Fix missing debug location updates (PR #91839)

[J. Ryan Stinnett via llvm-commits]

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@@ -462,14 +466,20 @@ class LoadEliminationForLoop {
            "The type sizes should match!");
 
     Value *StoreValue = Cand.Store->getValueOperand();
-    if (LoadType != StoreType)
+    if (LoadType != StoreType) {
       StoreValue = CastInst::CreateBitOrPointerCast(StoreValue, LoadType,
                                                     "store_forward_cast",
                                                     Cand.Store->getIterator());
+      // Because it casts the old `load` value and is used by the new `phi`
+      // which replaces the old `load`, we give the `load`'s debug location
+      // to it.
+      cast<Instruction>(StoreValue)->setDebugLoc(Cand.Load->getDebugLoc());
+    }
 
     PHI->addIncoming(StoreValue, L->getLoopLatch());
 
     Cand.Load->replaceAllUsesWith(PHI);
+    PHI->setDebugLoc(Cand.Load->getDebugLoc());
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jryans wrote:

Using the load's location here does make intuitive sense to me as well. I'm less sure what we've done for such cases historically though, so I'm interested to hear what others think as well.

https://github.com/llvm/llvm-project/pull/91839