[llvm] [LLVM][DAGCombiner] Look through freeze when combining extensions of loads (PR #175022)

[David Sherwood via llvm-commits]

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@@ -14552,35 +14556,49 @@ static SDValue tryToFoldExtOfLoad(SelectionDAG &DAG, DAGCombiner &Combiner,
   // TODO: isFixedLengthVector() should be removed and any negative effects on
   // code generation being the result of that target's implementation of
   // isVectorLoadExtDesirable().
-  if ((LegalOperations || VT.isFixedLengthVector() ||
-       !cast<LoadSDNode>(N0)->isSimple()) &&
-      !TLI.isLoadExtLegal(ExtLoadType, VT, N0.getValueType()))
+  if ((LegalOperations || VT.isFixedLengthVector() || !Load->isSimple()) &&
+      !TLI.isLoadExtLegal(ExtLoadType, VT, Load->getValueType(0)))
     return {};
 
   bool DoXform = true;
   SmallVector<SDNode *, 4> SetCCs;
-  if (!N0.hasOneUse())
-    DoXform = ExtendUsesToFormExtLoad(VT, N, N0, ExtOpc, SetCCs, TLI);
+  if (!Load->hasOneUse() || (Frozen && !N0->hasOneUse()))
+    DoXform = ExtendUsesToFormExtLoad(VT, N, Frozen ? N0 : SDValue(Load, 0),
+                                      ExtOpc, SetCCs, TLI);
   if (VT.isVector())
     DoXform &= TLI.isVectorLoadExtDesirable(SDValue(N, 0));
   if (!DoXform)
     return {};
 
-  LoadSDNode *LN0 = cast<LoadSDNode>(N0);
-  SDValue ExtLoad = DAG.getExtLoad(ExtLoadType, SDLoc(LN0), VT, LN0->getChain(),
-                                   LN0->getBasePtr(), N0.getValueType(),
-                                   LN0->getMemOperand());
-  Combiner.ExtendSetCCUses(SetCCs, N0, ExtLoad, ExtOpc);
+  SDLoc DL(Load);
+  SDValue ExtLoad =
+      DAG.getExtLoad(ExtLoadType, DL, VT, Load->getChain(), Load->getBasePtr(),
+                     Load->getValueType(0), Load->getMemOperand());
   // If the load value is used only by N, replace it via CombineTo N.
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david-arm wrote:

Following your suggestion above about initialising `Res` as `ExtLoad` it's not possible to move this any lower than just above the declaration of `Res`.

https://github.com/llvm/llvm-project/pull/175022