[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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@@ -398,22 +398,24 @@ SDValue SelectionDAGLegalize::PerformInsertVectorEltInMemory(SDValue Vec,
   EVT EltVT = VT.getVectorElementType();
   SDValue StackPtr = DAG.CreateStackTemporary(VT);
 
-  int SPFI = cast<FrameIndexSDNode>(StackPtr.getNode())->getIndex();
+  MachineMemOperand *AlignedMMO = getStackAlignedMMO(
+      StackPtr, DAG.getMachineFunction(), EltVT.isScalableVector());
 
   // Store the vector.
-  SDValue Ch = DAG.getStore(
-      DAG.getEntryNode(), dl, Tmp1, StackPtr,
-      MachinePointerInfo::getFixedStack(DAG.getMachineFunction(), SPFI));
+  SDValue Ch = DAG.getStore(DAG.getEntryNode(), dl, Vec, StackPtr, AlignedMMO);
+
 
   SDValue StackPtr2 = TLI.getVectorElementPointer(DAG, StackPtr, VT, Tmp3);
 
   // Store the scalar value.
-  Ch = DAG.getTruncStore(
-      Ch, dl, Tmp2, StackPtr2,
-      MachinePointerInfo::getUnknownStack(DAG.getMachineFunction()), EltVT);
-  // Load the updated vector.
-  return DAG.getLoad(VT, dl, Ch, StackPtr, MachinePointerInfo::getFixedStack(
-                                               DAG.getMachineFunction(), SPFI));
+  Ch = DAG.getTruncStore(Ch, dl, Tmp2, StackPtr2, EltVT, AlignedMMO);
+
+  Align ElementAlignment = std::min(cast<StoreSDNode>(Ch)->getAlign(),
+                                    DAG.getDataLayout().getPrefTypeAlign(
+                                        VT.getTypeForEVT(*DAG.getContext())));
+
+  return DAG.getLoad(VT, dl, Ch, StackPtr, MachinePointerInfo(),
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arsenm wrote:

This is a reload of the entire vector, so there isn't any offset to consider. That is, the original MachinePointerInfo was correct. You should be able to use the MachinePointerInfo::getFixedStack as before 

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