[llvm] [SelectionDAG] Fix unsafe cases for loop.dependence.{war/raw}.mask (PR #168565)

[Benjamin Maxwell via llvm-commits]

================
@@ -1692,23 +1699,32 @@ void DAGTypeLegalizer::SplitVecRes_BITCAST(SDNode *N, SDValue &Lo,
   Hi = DAG.getNode(ISD::BITCAST, dl, HiVT, Hi);
 }
 
+/// Split a loop dependence mask.
+/// This is done by creating a high and low mask, each of half the vector
+/// length. A select of the high mask and a predicate of all zeroes is needed to
+/// guarantee that the high mask is safe. A case where simply producing a high
+/// mask without the select is unsafe, is when the difference between the two
+/// pointers is less than half the vector length, e.g. ptrA = 0 and ptrB 3 when
+/// the vector length is 32.
+///     The full 32xi1 mask should be three active lanes and the rest inactive,
+///     however when half the vector length is added to ptrA to produce the high
+///     mask, the difference between ptrA and ptrB is now -13, which will result
+///     in a mask with all lanes active. The select will guard against this case
+///     by choosing a mask of all inactive lanes when ptrA + VL/2 >= ptrB.
 void DAGTypeLegalizer::SplitVecRes_LOOP_DEPENDENCE_MASK(SDNode *N, SDValue &Lo,
                                                         SDValue &Hi) {
   SDLoc DL(N);
   EVT LoVT, HiVT;
   std::tie(LoVT, HiVT) = DAG.GetSplitDestVTs(N->getValueType(0));
   SDValue PtrA = N->getOperand(0);
   SDValue PtrB = N->getOperand(1);
-  Lo = DAG.getNode(N->getOpcode(), DL, LoVT, PtrA, PtrB, N->getOperand(2));
-
-  unsigned EltSize = N->getConstantOperandVal(2);
-  unsigned Offset = EltSize * HiVT.getVectorMinNumElements();
-  SDValue Addend = HiVT.isScalableVT()
-                       ? DAG.getVScale(DL, MVT::i64, APInt(64, Offset))
-                       : DAG.getConstant(Offset, DL, MVT::i64);
 
-  PtrA = DAG.getNode(ISD::ADD, DL, MVT::i64, PtrA, Addend);
-  Hi = DAG.getNode(N->getOpcode(), DL, HiVT, PtrA, PtrB, N->getOperand(2));
+  Lo = DAG.getNode(N->getOpcode(), DL, LoVT, PtrA, PtrB, N->getOperand(2),
+                   N->getOperand(3));
----------------
MacDue wrote:

nit: Can you label the unnamed parameters 
```suggestion
  Lo = DAG.getNode(N->getOpcode(), DL, LoVT, PtrA, PtrB, /*ElementSizeInBytes=*/N->getOperand(2),
                   /*LaneOffset=*/N->getOperand(3));
```

https://github.com/llvm/llvm-project/pull/168565