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

[Benjamin Maxwell via llvm-commits]

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@@ -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.
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MacDue wrote:

Comment needs updating

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