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

[Orlando Cazalet-Hyams 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());
----------------
OCHyams wrote:

This is an interesting case. When promoting store/loads I think we usually give phis a debugloc from the store (or if it's from stores, plural, it's a merged location). At least that's what we do in mem2reg. I think in this case using the `load`'s DebugLoc makes intuitive sense - we're just eliminating the load here after all, but I'd be interested to hear what others think about this case too (cc @SLTozer , @adrian-prantl , @dwblaikie, @jryans - there's a comment at the top of the function that explains the transformation) and what you make of this @Apochens?





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