[mlir][sparse][NFC] Disallow writing into sparse_tensor.to_... results

[springerm]

Summary

Writing into the memrefs returned by sparse_tensor.to_values etc. can cause additional copies during bufferization.

[aartbik]

mlir/include/mlir/Dialect/SparseTensor/IR/SparseTensorOps.td

121–123

Here too?

[springerm]

address comments

mlir/include/mlir/Dialect/SparseTensor/IR/SparseTensorOps.td

def SparseTensor_ToPointersOp : SparseTensor_Op<"pointers", [Pure]>,
   let description = [{
     Returns the pointers array of the sparse storage format at the
     given dimension for the given sparse tensor. This is similar to the
     `bufferization.to_memref` operation in the sense that it provides a bridge
     between a tensor world view and a bufferized world view. Unlike the
     `bufferization.to_memref` operation, however, this sparse operation actually
     lowers into code that extracts the pointers array from the sparse storage
     scheme (either by calling a support library or through direct code).
 
+    Writing into the result of this operation is undefined behavior.
+
     Example:
 
     ```mlir
     %1 = sparse_tensor.pointers %0 { dimension = 1 : index }
        : tensor<64x64xf64, #CSR> to memref<?xindex>
     ```
   }];
   let assemblyFormat = "$tensor attr-dict `:` type($tensor) `to` type($result)";
   let hasVerifier = 1;
 }
 
 def SparseTensor_ToIndicesOp : SparseTensor_Op<"indices", [Pure]>,
     Arguments<(ins AnySparseTensor:$tensor, IndexAttr:$dimension)>,
     Results<(outs AnyStridedMemRefOfRank<1>:$result)> {
   let summary = "Extracts indices array at given dimension from a tensor";
   let description = [{
     Returns the indices array of the sparse storage format at the
     given dimension for the given sparse tensor. This is similar to the
     `bufferization.to_memref` operation in the sense that it provides a bridge
     between a tensor world view and a bufferized world view. Unlike the
     `bufferization.to_memref` operation, however, this sparse operation actually
     lowers into code that extracts the indices array from the sparse storage
     scheme (either by calling a support library or through direct code).
 
+    Writing into the result of this operation is undefined behavior.
+
     Example:
 
     ```mlir
     %1 = sparse_tensor.indices %0 { dimension = 1 : index }
        : tensor<64x64xf64, #CSR> to memref<?xindex>
     ```
   }];
   let assemblyFormat = "$tensor attr-dict `:` type($tensor) `to` type($result)";
let description = [{
     and a bufferized world view. Unlike the `bufferization.to_memref` operation,
     however, this sparse operation actually lowers into code that extracts the
     linear indices array from the sparse storage scheme that stores the indices
     for the COO region as an array of structures. For example, a 2D COO sparse
     tensor with two non-zero elements at coordinates (1, 3) and (4, 6) are
     stored in a linear buffer as (1, 4, 3, 6) instead of two buffer as (1, 4)
     and (3, 6).
 
+    Writing into the result of this operation is undefined behavior.
+
     Example:
 
     ```mlir
     %1 = sparse_tensor.indices_buffer %0
     : tensor<64x64xf64, #COO> to memref<?xindex>
     ```
   }];
   let assemblyFormat = "$tensor attr-dict `:` type($tensor) `to` type($result)";
   let hasVerifier = 1;
 }
 
 def SparseTensor_ToValuesOp : SparseTensor_Op<"values", [Pure]>,
     Arguments<(ins AnySparseTensor:$tensor)>,
     Results<(outs AnyStridedMemRefOfRank<1>:$result)> {
   let summary = "Extracts numerical values array from a tensor";
   let description = [{
     Returns the values array of the sparse storage format for the given
     sparse tensor, independent of the actual dimension. This is similar to
     the `bufferization.to_memref` operation in the sense that it provides a bridge
     between a tensor world view and a bufferized world view. Unlike the
     `bufferization.to_memref` operation, however, this sparse operation actually
     lowers into code that extracts the values array from the sparse storage
     scheme (either by calling a support library or through direct code).
 
+    Writing into the result of this operation is undefined behavior.
+
     Example:
 
     ```mlir
     %1 = sparse_tensor.values %0 : tensor<64x64xf64, #CSR> to memref<?xf64>
     ```
   }];
   let assemblyFormat = "$tensor attr-dict `:` type($tensor) `to` type($result)";
   let hasVerifier = 1;