[CodeGen] Fix warnings in SelectionDAG::SplitVector

[david-arm]

Summary

Fixes warnings in this test:

sve-sext-zext.ll

[kmclaughlin]

LGTM

[sdesmalen]

Can you please clarify the title/summary a bit more before committing the patch?

llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp

9657

nit: can you add a comment explaining why getVectorMinNumElements() is valid here?

llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp

SelectionDAG::GetDependentSplitDestVTs(const EVT &VT, const EVT &EnvVT,
   return std::make_pair(LoVT, HiVT);
 }
 
 /// SplitVector - Split the vector with EXTRACT_SUBVECTOR and return the
 /// low/high part.
 std::pair<SDValue, SDValue>
 SelectionDAG::SplitVector(const SDValue &N, const SDLoc &DL, const EVT &LoVT,
                           const EVT &HiVT) {
-  assert(LoVT.getVectorNumElements() + HiVT.getVectorNumElements() <=
-         N.getValueType().getVectorNumElements() &&
+  assert(LoVT.isScalableVector() == HiVT.isScalableVector() &&
+         LoVT.isScalableVector() == N.getValueType().isScalableVector() &&
+         "Splitting vector with an invalid mixture of fixed and scalable "
+         "vector types");
+  assert(LoVT.getVectorMinNumElements() + HiVT.getVectorMinNumElements() <=
+             N.getValueType().getVectorMinNumElements() &&
          "More vector elements requested than available!");
   SDValue Lo, Hi;
   Lo =
       getNode(ISD::EXTRACT_SUBVECTOR, DL, LoVT, N, getVectorIdxConstant(0, DL));
+  // For scalable vectors it is safe to use LoVT.getVectorMinNumElements()
+  // (rather than having to use ElementCount), because EXTRACT_SUBVECTOR scales
+  // IDX with the runtime scaling factor of the result vector type. For
+  // fixed-width result vectors, that runtime scaling factor is 1.
   Hi = getNode(ISD::EXTRACT_SUBVECTOR, DL, HiVT, N,
-               getVectorIdxConstant(LoVT.getVectorNumElements(), DL));
+               getVectorIdxConstant(LoVT.getVectorMinNumElements(), DL));
   return std::make_pair(Lo, Hi);
 }
 
 /// Widen the vector up to the next power of two using INSERT_SUBVECTOR.
 SDValue SelectionDAG::WidenVector(const SDValue &N, const SDLoc &DL) {
   EVT VT = N.getValueType();
   EVT WideVT = EVT::getVectorVT(*getContext(), VT.getVectorElementType(),
                                 NextPowerOf2(VT.getVectorNumElements()));