[CodeGen][SVE] Avoid scalarizing zero splat stores on scalable vectors.

[hkao13]

Summary

Implemented in replaceZeroVectorStore(). Fixes several warnings in AArch64 SVE unit tests.

[sdesmalen]

After D80720 this function will no longer be called from splitStores for scalable vectors, but it still makes sense to bail out early in case that ever changes.

llvm/lib/Target/AArch64/AArch64ISelLowering.cpp

12018–12022

Probably better to bail out directly if VT is scalable and leave the call to getVectorNumElements() intact.

[hkao13]

llvm/lib/Target/AArch64/AArch64ISelLowering.cpp

12018–12022

Thanks for the review. I reverted the change to use getVectorNumElements(), and added a check above.

[sdesmalen]

LGTM

llvm/lib/Target/AArch64/AArch64ISelLowering.cpp

12020

I think you can remove the condition || !VT.isFixedLengthVector(), because this function would otherwise break if it isn't.

[hkao13]

llvm/lib/Target/AArch64/AArch64ISelLowering.cpp

12020

Fixed, thanks for feedback.

llvm/lib/Target/AArch64/AArch64ISelLowering.cpp

 /// instead of:
 ///
 ///   movi v0.2d, #0
 ///   str q0, [x0]
 ///
 static SDValue replaceZeroVectorStore(SelectionDAG &DAG, StoreSDNode &St) {
   SDValue StVal = St.getValue();
   EVT VT = StVal.getValueType();
 
+  // Avoid scalarizing zero splat stores for scalable vectors.
+  if (VT.isScalableVector())
+    return SDValue();
+
   // It is beneficial to scalarize a zero splat store for 2 or 3 i64 elements or
   // 2, 3 or 4 i32 elements.
   int NumVecElts = VT.getVectorNumElements();
   if (!(((NumVecElts == 2 || NumVecElts == 3) &&
          VT.getVectorElementType().getSizeInBits() == 64) ||
         ((NumVecElts == 2 || NumVecElts == 3 || NumVecElts == 4) &&
          VT.getVectorElementType().getSizeInBits() == 32)))
     return SDValue();