diff --git a/returnn/frontend/array_.py b/returnn/frontend/array_.py
index e8efa3640f..348caffbb1 100644
--- a/returnn/frontend/array_.py
+++ b/returnn/frontend/array_.py
@@ -28,6 +28,7 @@
     "pad",
     "cum_concat_step",
     "masked_select",
+    "sequence_mask",
     "pack_padded",
     "gather",
     "slice",
@@ -352,6 +353,8 @@ def masked_select(
     tensor: Tensor, *, mask: Tensor, dims: Sequence[Dim], out_dim: Optional[Dim] = None
 ) -> Tuple[Tensor, Dim]:
     """
+    In TF, this is ``boolean_mask``.
+
     :param tensor:
     :param mask:
     :param dims: the order of the dims defines the format. those dims should be exactly the dims of the mask.
@@ -364,8 +367,19 @@ def masked_select(
     return tensor._raw_backend.masked_select(tensor, mask=mask, dims=dims, out_dim=out_dim)
 
 
+def sequence_mask(dims: Sequence[Dim], *, device: Optional[str] = None) -> Tensor:
+    """
+    :param dims:
+    :param device:
+    """
+    assert len(dims) > 0
+    dyn_dims = [d for d in dims if d.need_masking()]
+    assert len(dyn_dims) == 1  # not implemented otherwise yet...
+    return dyn_dims[0].get_mask(dim_order=dims, device=device)
+
+
 def pack_padded(
-    source: Tensor, *, dims: Sequence[Dim], enforce_sorted: bool = True, out_dim: Optional[Dim] = None
+    source: Tensor, *, dims: Sequence[Dim], enforce_sorted: bool = False, out_dim: Optional[Dim] = None
 ) -> Tuple[Tensor, Dim]:
     """
     Like pack_padded_sequence. Usually the sequences are padded when they have different lengths.
@@ -380,10 +394,7 @@ def pack_padded(
     :return: packed tensor, new packed dim
     """
     assert not enforce_sorted  # not implemented yet...
-    assert len(dims) > 0
-    dyn_dims = [d for d in dims if d.need_masking()]
-    assert len(dyn_dims) == 1  # not implemented otherwise yet...
-    mask = source.get_sequence_mask_tensor(source.get_axis_from_description(dyn_dims[0]))
+    mask = rf.sequence_mask(dims, device=source.device)
     return rf.masked_select(source, mask=mask, dims=dims, out_dim=out_dim)