diff --git a/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp b/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
--- a/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
@@ -9639,14 +9639,22 @@
 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);
 }