[llvm] [SelectionDAG] Use unaligned store/load to move AVX registers onto stack for `insertelement` (PR #82130)

[Matt Arsenault via llvm-commits]

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@@ -1489,24 +1491,27 @@ SDValue SelectionDAGLegalize::ExpandInsertToVectorThroughStack(SDValue Op) {
   EVT VecVT = Vec.getValueType();
   EVT SubVecVT = Part.getValueType();
   SDValue StackPtr = DAG.CreateStackTemporary(VecVT);
-  int FI = cast<FrameIndexSDNode>(StackPtr.getNode())->getIndex();
-  MachinePointerInfo PtrInfo =
-      MachinePointerInfo::getFixedStack(DAG.getMachineFunction(), FI);
+  MachineMemOperand *AlignedMMO = getStackAlignedMMO(
+      StackPtr, DAG.getMachineFunction(), VecVT.isScalableVector());
 
   // First store the whole vector.
-  SDValue Ch = DAG.getStore(DAG.getEntryNode(), dl, Vec, StackPtr, PtrInfo);
+  SDValue Ch = DAG.getStore(DAG.getEntryNode(), dl, Vec, StackPtr, AlignedMMO);
 
   // Then store the inserted part.
   SDValue SubStackPtr =
       TLI.getVectorSubVecPointer(DAG, StackPtr, VecVT, SubVecVT, Idx);
 
   // Store the subvector.
-  Ch = DAG.getStore(
-      Ch, dl, Part, SubStackPtr,
-      MachinePointerInfo::getUnknownStack(DAG.getMachineFunction()));
+  Ch = DAG.getStore(Ch, dl, Part, SubStackPtr, AlignedMMO);
+
+  Align ElementAlignment =
+      std::min(cast<StoreSDNode>(Ch)->getAlign(),
+               DAG.getDataLayout().getPrefTypeAlign(
+                   Op.getValueType().getTypeForEVT(*DAG.getContext())));
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arsenm wrote:

This should use the alignment of the actual frame index, not query the type preferred align 

https://github.com/llvm/llvm-project/pull/82130