Copy update and reset gates in GRU op before applying sigmoid activation

This is a workaround until #192 is
solved more generally.

src/ops/rnn.rs

@@ -7,8 +7,8 @@ use rten_tensor::{Tensor, TensorView};
 use crate::check_dims;
 use crate::gemm::{GemmExecutor, GemmInputA, GemmInputB};
 use crate::ops::{
-    add_in_place, mul_in_place, sigmoid_in_place, tanh, tanh_in_place, InputList, IntoOpResult,
-    OpError, Operator, Output,
+    add_in_place, mul_in_place, sigmoid, sigmoid_in_place, tanh, tanh_in_place, InputList,
+    IntoOpResult, OpError, Operator, Output,
 };
 use crate::tensor_pool::{AutoReturn, TensorPool};
 
@@ -265,26 +265,30 @@ pub fn gru(
                 hidden_scratch_reset_update_gates.as_dyn(),
             );
 
-            // nb. This is slower than it should be because it falls back to
-            // the slow path for non-contiguous tensors.
-            sigmoid_in_place(update_reset_gates.as_dyn_mut());
+            // Copy gates before applying activation because `sigmoid_in_place`
+            // and `tanh_in_place` are slow with non-contiguous tensors.
+            // See https://github.com/robertknight/rten/issues/192.
+            //
+            // Note `gate_range` can be still used because the update and reset
+            // gates are in the same positions in the `update_reset_gates` slice
+            // as `gates`.
+            let update_reset_gates = sigmoid(pool, update_reset_gates.as_dyn()).auto_return(pool);
+            let update_gate = update_reset_gates.slice::<2, _>((.., gate_range(UPDATE_GATE)));
+            let reset_gate = update_reset_gates.slice::<2, _>((.., gate_range(RESET_GATE)));
 
             // Combine inputs for hidden gate and apply activation.
-            let reset_gate = gates.slice::<2, _>((.., gate_range(RESET_GATE)));
             let mut hidden_gate_recurrent =
                 hidden_scratch.slice_mut::<2, _>((.., gate_range(HIDDEN_GATE)));
             mul_in_place(hidden_gate_recurrent.as_dyn_mut(), reset_gate.as_dyn());
 
             let mut hidden_gate = gates.slice_mut::<2, _>((.., gate_range(HIDDEN_GATE)));
             add_in_place(hidden_gate.as_dyn_mut(), hidden_gate_recurrent.as_dyn());
 
-            // Copy the hidden gate because `tanh_in_place` is slow with
-            // non-contiguous tensors.
+            // See note above about `sigmoid_in_place`.
             let hidden_gate = tanh(pool, hidden_gate.as_dyn()).auto_return(pool);
 
             // Compute next hidden state
             let mut hidden_item = hidden.slice_mut::<2, _>([dir]);
-            let update_gate = gates.slice::<2, _>((.., gate_range(UPDATE_GATE)));
 
             for (hidden, update, hidden_gate) in zip3(
                 hidden_item.iter_mut(),