Merge commit '06d6cbbb2e06' from apple/main into swift/next

Author: git apple-llvm automerger

mlir/lib/Dialect/Linalg/Transforms/DropUnitDims.cpp

@@ -459,7 +459,10 @@ getReassociationMapForFoldingUnitDims(ArrayRef<OpFoldResult> mixedSizes) {
     reassociation.emplace_back(ReassociationIndices{});
     std::swap(reassociation.back(), curr);
   }
-  if (!curr.empty())
+  // When the reassociations are not empty, then fold the remaining
+  // unit-dimensions into the last dimension.  If the reassociations so far is
+  // empty, then leave it emtpy. This will fold everything to a rank-0 tensor.
+  if (!curr.empty() && !reassociation.empty())
     reassociation.back().append(curr.begin(), curr.end());
   return reassociation;
 }

mlir/test/Dialect/Linalg/drop-unit-extent-dims.mlir

@@ -496,3 +496,27 @@ func @unit_dim_for_reduction_inner(%arg0: tensor<?x1x?x1xf32>) -> tensor<?x1xf32
 // CHECK-SAME:     outs(%[[FILL]] : tensor<?xf32>)
 //      CHECK:   %[[RESULT_RESHAPE:.+]] = linalg.tensor_reshape %[[RESULT]] {{\[}}[0, 1]]
 //      CHECK:   return %[[RESULT_RESHAPE]]
+
+// -----
+
+func @subtensor_unit_dims(%arg0: tensor<1x3xf32>) -> tensor<1x1xf32> {
+  %0 = subtensor %arg0[0, 2] [1, 1] [1, 1] : tensor<1x3xf32> to tensor<1x1xf32>
+  return %0 : tensor<1x1xf32>
+}
+// CHECK-LABEL: func @subtensor_unit_dims
+//       CHECK:   %[[SUBTENSOR:.+]] = subtensor
+//  CHECK-SAME:     tensor<1x3xf32> to tensor<f32>
+//       CHECK:   %[[RESULT:.+]] = linalg.tensor_reshape %[[SUBTENSOR]] []
+//       CHECK:   return %[[RESULT]]
+
+// -----
+
+func @subtensor_insert_unit_dims(%arg0: tensor<1x3xf32>, %arg1: tensor<1x1xf32>) -> tensor<1x3xf32> {
+  %0 = subtensor_insert %arg1 into %arg0[0, 2] [1, 1] [1, 1] : tensor<1x1xf32> into tensor<1x3xf32>
+  return %0 : tensor<1x3xf32>
+}
+// CHECK-LABEL: func @subtensor_insert_unit_dims
+//       CHECK:   %[[RESHAPE:.+]] = linalg.tensor_reshape %{{.+}} []
+//       CHECK:   %[[RESULT:.+]] = subtensor_insert %[[RESHAPE]]
+//  CHECK-SAME:     tensor<f32> into tensor<1x3xf32>
+//       CHECK:   return %[[RESULT]]