[llvm] [SelectionDAG] Use unaligned store to move AVX registers onto stack for `extractelement` (PR #78422)

[Matt Arsenault via llvm-commits]

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@@ -1377,6 +1378,21 @@ void SelectionDAGLegalize::LegalizeOp(SDNode *Node) {
   }
 }
 
+// Helper function that generates an MMO that considers the alignment of the
+// stack, and the size of the stack object
+static MachineMemOperand *getStackAlignedMMO(SDValue StackPtr,
+                                             MachineFunction &MF,
+                                             bool isObjectScalable) {
+  auto &MFI = MF.getFrameInfo();
+  int FI = cast<FrameIndexSDNode>(StackPtr)->getIndex();
+  MachinePointerInfo PtrInfo = MachinePointerInfo::getFixedStack(MF, FI);
+  uint64_t ObjectSize = isObjectScalable ? ~UINT64_C(0) : MFI.getObjectSize(FI);
+  MachineMemOperand *MMO = MF.getMachineMemOperand(
+      PtrInfo, MachineMemOperand::MOStore, ObjectSize, MFI.getObjectAlign(FI));
+
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arsenm wrote:

can fold to directly return MF.getMachineMemOperand 

https://github.com/llvm/llvm-project/pull/78422