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

[Manish Kausik H via llvm-commits]

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@@ -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);
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Nirhar wrote:

If we consider modifying `SelectionDAG::getReducedAlign` I see two locations where we can teach it to consider the Stack alignment:
1. Teach `TLI::getVectorTypeBreakdown` to clamp down to Stack Alignment. I see that we dont consider Stack Alignment inside the function. 
https://github.com/llvm/llvm-project/blob/462d084241616627be1ac2b967a7fcba9b0facfe/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp#L2479
2.  [My Preference] Modify these lines in `getReducedAlign`:
https://github.com/llvm/llvm-project/blob/462d084241616627be1ac2b967a7fcba9b0facfe/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp#L2482-L2487
and clamp down RedAlign2 to StackAlign if it exceeds it.

Which one is a better solution?

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