[llvm] [SelectionDAG] Use unaligned store to legalize `EXTRACT_VECTOR_ELT` type (PR #98176)

[Phoebe Wang via llvm-commits]

================
@@ -3531,14 +3548,14 @@ SDValue DAGTypeLegalizer::SplitVecOp_EXTRACT_VECTOR_ELT(SDNode *N) {
   // Store the vector to the stack.
   // In cases where the vector is illegal it will be broken down into parts
   // and stored in parts - we should use the alignment for the smallest part.
-  Align SmallestAlign = DAG.getReducedAlign(VecVT, /*UseABI=*/false);
+  Align SmallestAlign =
+      std::min(DAG.getSubtarget().getFrameLowering()->getStackAlign(),
+               DAG.getReducedAlign(VecVT, /*UseABI=*/false));
   SDValue StackPtr =
       DAG.CreateStackTemporary(VecVT.getStoreSize(), SmallestAlign);
-  auto &MF = DAG.getMachineFunction();
-  auto FrameIndex = cast<FrameIndexSDNode>(StackPtr.getNode())->getIndex();
-  auto PtrInfo = MachinePointerInfo::getFixedStack(MF, FrameIndex);
-  SDValue Store = DAG.getStore(DAG.getEntryNode(), dl, Vec, StackPtr, PtrInfo,
-                               SmallestAlign);
+  MachineMemOperand *StoreMMO = getStackAlignedMMO(
+      StackPtr, DAG.getMachineFunction(), VecVT.isScalableVector());
+  SDValue Store = DAG.getStore(DAG.getEntryNode(), dl, Vec, StackPtr, StoreMMO);
----------------
phoebewang wrote:

The changes in lit test are functionally correct, but I'm not sure if we prefer to align them or not.

Revisit the original change, I think we also realigned the stack in the `foo2`, but it's not used. I guess some later optimizations may schedule the order of mov instructions. So that approach is a bit suboptimal to me.

@RKSimon which solution do you prefer?

https://github.com/llvm/llvm-project/pull/98176