[CodeGen] Remove calls to getVectorNumElements in DAGTypeLegalizer::SplitVecOp_EXTRACT_SUBVECTOR

[david-arm]

Summary

In DAGTypeLegalizer::SplitVecOp_EXTRACT_SUBVECTOR I have replaced
calls to getVectorNumElements with getVectorMinNumElements, since
this code path works for both fixed and scalable vector types. For
scalable vectors the index will be multiplied by VSCALE.

Fixes warnings in this test:

sve-sext-zext.ll

[sdesmalen]

llvm/lib/CodeGen/SelectionDAG/LegalizeVectorTypes.cpp

2183

I think this assert is saying that this is not *yet* supported, because if extracting a fixed-width vector from a scalable vector is allowed then it should (at some point) also support extracting it from an illegal scalable vector.

So maybe better to change the wording to "Extracting a fixed-length vector from a scalable vector is not yet supported", and perhaps you can wrap it in a report_fatal_error(..) instead of an assert, because this is more a is-unsupported-error.

[david-arm]

ping

[sdesmalen]

LGTM!

llvm/lib/CodeGen/SelectionDAG/LegalizeVectorTypes.cpp

   SDValue Idx = N->getOperand(1);
   SDLoc dl(N);
   SDValue Lo, Hi;
+
+  assert(SubVT.isScalableVector() ==
+             N->getOperand(0).getValueType().isScalableVector() &&
+         "We only support extracting fixed length vectors from legal scalable "
+         "vector types");
   GetSplitVector(N->getOperand(0), Lo, Hi);
 
-  uint64_t LoElts = Lo.getValueType().getVectorNumElements();
+  uint64_t LoElts = Lo.getValueType().getVectorMinNumElements();
   uint64_t IdxVal = cast<ConstantSDNode>(Idx)->getZExtValue();
 
   if (IdxVal < LoElts) {
-    assert(IdxVal + SubVT.getVectorNumElements() <= LoElts &&
+    assert(IdxVal + SubVT.getVectorMinNumElements() <= LoElts &&
            "Extracted subvector crosses vector split!");
     return DAG.getNode(ISD::EXTRACT_SUBVECTOR, dl, SubVT, Lo, Idx);
   } else {