[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())));
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arsenm wrote:

Instead of querying the preferred type alignment, this should use the alignment for the real stack object you have. It just happens that today DAG.CreateStackTemporary uses getPrefTypeAlign, but you should check what the MachineFrameInfo says the alignment is for the frame index 

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