Canonicalize subtensor negative integer indices

pytensor/tensor/rewriting/subtensor.py

@@ -724,6 +724,44 @@ def local_useless_subtensor(fgraph, node):
     return [node.inputs[0]]
 
 
+@register_canonicalize
+@node_rewriter([Subtensor])
+def local_convert_negative_indices(fgraph, node):
+    """Convert negative indices in `Subtensor` with static length to positive indices."""
+    x, *raw_idxs = node.inputs
+    idxs = indices_from_subtensor(raw_idxs, node.op.idx_list)
+
+    new_idxs = None
+    for i, (dim_length, idx) in enumerate(zip(x.type.shape, idxs)):
+        if (
+            dim_length is None
+            or isinstance(idx, slice)
+            or not isinstance(idx, Constant)
+        ):
+            continue
+
+        val = idx.data
+        if val >= 0:
+            continue
+
+        new_val = val + dim_length
+        if new_val < 0:
+            # This is an invalid index, keep original to not confuse the user
+            return None
+
+        if new_idxs is None:
+            new_idxs = list(idxs)
+        new_idxs[i] = new_val
+
+    if new_idxs is None:
+        # No negative indices to convert
+        return None
+
+    new_subtensor = x[tuple(new_idxs)]
+    copy_stack_trace(node.outputs, new_subtensor)
+    return [new_subtensor]
+
+
 @register_canonicalize
 @register_specialize
 @node_rewriter([AdvancedSubtensor1])

tests/tensor/rewriting/test_subtensor.py

@@ -1992,3 +1992,20 @@ def test_extract_diag_of_diagonal_set_subtensor():
     expected_outs.append(outs[-1])
 
     assert equal_computations(rewritten_outs, expected_outs)
+
+
+def test_local_convert_negative_indices():
+    x = pt.tensor("x", shape=(None, 3, 1))
+
+    # Dim length is unknown rewrite can't be applied
+    rewritten_out = rewrite_graph(x[-2])
+    assert equal_computations([rewritten_out], [x[-2]])
+
+    # Rewrite applies
+    rewritten_out = rewrite_graph(x[:, -2])
+    assert equal_computations([rewritten_out], [x[:, 1]])
+
+    # Rewrite doesn't apply because index is invalid
+    # TODO: If Subtensor decides to raise on make_node, this test can be removed
+    rewritten_out = rewrite_graph(x[:, :, -2])
+    assert equal_computations([rewritten_out], [x[:, :, -2]])

tests/tensor/rewriting/test_subtensor_lift.py

@@ -202,7 +202,7 @@ def test_local_subtensor_of_reduce(original_fn, expected_fn):
 
     out = original_fn(x)
     expected_opt_out = expected_fn(x)
-    opt_out = rewrite_graph(out)
+    opt_out = rewrite_graph(out, exclude=("local_convert_negative_indices",))
     assert equal_computations([opt_out], [expected_opt_out]), debugprint(
         [expected_opt_out, opt_out], print_type=True
     )