Rewrite concatenate([x, x]) as repeat(x, 2)

pytensor/tensor/rewriting/basic.py

@@ -910,6 +910,52 @@ def local_join_make_vector(fgraph, node):
         return [ret]
 
 
+@register_canonicalize
+@node_rewriter([Join])
+def local_join_to_repeat(fgraph, node):
+    """Join(axis, x, x, x, ...) -> repeat(x, n, axis)
+
+    When the same tensor is concatenated multiple times along an axis
+    where it has size 1, replace with a repeat operation which is more efficient.
+
+    Examples
+    --------
+    concatenate([x[None], x[None], x[None]], axis=0) -> repeat(x[None], 3, axis=0)
+    """
+    # Extract axis and the tensors being joined
+    axis, *tensors = node.inputs
+
+    # Optimization only applies when axis is constant
+    if not isinstance(axis, Constant):
+        return None
+
+    # Extract the Python integer from the constant
+    axis_val = axis.data
+
+    # Need at least 2 tensors to consider optimization
+    if len(tensors) <= 1:
+        return
+
+    # Check if all tensors are identical
+    if not all(t == tensors[0] for t in tensors[1:]):
+        return
+
+    # Only optimize if the tensor has size 1 along the join axis
+    first_tensor = tensors[0]
+    if first_tensor.type.shape[axis_val] != 1:
+        return
+
+    # Replace with repeat operation
+    from pytensor.tensor.extra_ops import repeat
+
+    result = repeat(first_tensor, len(tensors), axis_val)
+
+    # Preserve debugging information
+    copy_stack_trace(node.outputs[0], result)
+
+    return [result]
+
+
 @register_specialize
 @register_canonicalize
 @register_useless

tests/tensor/rewriting/test_basic.py

@@ -1247,6 +1247,106 @@ def test_local_join_1():
     assert f.maker.fgraph.outputs[0].dtype == config.floatX
 
 
+def test_local_join_to_repeat():
+    """Test that Join(axis, x, x, ...) gets rewritten to repeat(x, n, axis)
+
+    This optimization applies when joining the same tensor multiple times
+    along an axis where it has size 1 (e.g., after ExpandDims).
+    """
+
+    # Test with vector expanded to (1, n) - concatenate along axis 0
+    x = vector("x")
+    x_expanded = x[None]  # Shape: (1, n)
+    s = join(0, x_expanded, x_expanded, x_expanded)  # Shape: (3, n)
+    f = function([x], s, mode=rewrite_mode)
+
+    # Check numerical correctness
+    test_val = np.array([1.0, 2.0, 3.0], dtype=config.floatX)
+    result = f(test_val)
+    expected = np.array(
+        [[1.0, 2.0, 3.0], [1.0, 2.0, 3.0], [1.0, 2.0, 3.0]], dtype=config.floatX
+    )
+    assert np.allclose(result, expected)
+
+    # Check that Join was replaced with Alloc (repeat with scalar repeats becomes Alloc)
+    ops = f.maker.fgraph.toposort()
+    assert len([n for n in ops if isinstance(n.op, Join)]) == 0
+    assert len([n for n in ops if isinstance(n.op, Alloc)]) >= 1
+
+    # Test with matrix - add dimension and concatenate along new axis
+    a = matrix("a")  # Shape: (m, n)
+    a_expanded = a[None, :, :]  # Shape: (1, m, n)
+    s = join(0, a_expanded, a_expanded, a_expanded, a_expanded)  # Shape: (4, m, n)
+    f = function([a], s, mode=rewrite_mode)
+
+    test_mat = np.array([[1.0, 2.0], [3.0, 4.0]], dtype=config.floatX)
+    result = f(test_mat)
+    expected = np.array([test_mat, test_mat, test_mat, test_mat])
+    assert np.allclose(result, expected)
+
+    # Check optimization applied
+    ops = f.maker.fgraph.toposort()
+    assert len([n for n in ops if isinstance(n.op, Join)]) == 0
+    assert len([n for n in ops if isinstance(n.op, Alloc)]) >= 1
+
+    # Test with matrix - expand along axis 1 and concatenate
+    a_expanded_ax1 = a[:, None, :]  # Shape: (m, 1, n)
+    s = join(1, a_expanded_ax1, a_expanded_ax1)  # Shape: (m, 2, n)
+    f = function([a], s, mode=rewrite_mode)
+
+    result = f(test_mat)
+    expected = np.array([[[1.0, 2.0], [1.0, 2.0]], [[3.0, 4.0], [3.0, 4.0]]])
+    assert np.allclose(result, expected)
+
+    # Check optimization applied
+    ops = f.maker.fgraph.toposort()
+    assert len([n for n in ops if isinstance(n.op, Join)]) == 0
+    assert len([n for n in ops if isinstance(n.op, Alloc)]) >= 1
+
+    # Test that it does NOT apply when tensors are different
+    y = vector("y")
+    s = join(0, x[None], y[None])
+    f = function([x, y], s, mode=rewrite_mode)
+
+    test_vec1 = np.array([1.0, 2.0], dtype=config.floatX)
+    test_vec2 = np.array([3.0, 4.0], dtype=config.floatX)
+    result = f(test_vec1, test_vec2)
+    expected = np.array([[1.0, 2.0], [3.0, 4.0]])
+    assert np.allclose(result, expected)
+
+    # Join should still be present (not optimized)
+    ops = f.maker.fgraph.toposort()
+    assert len([n for n in ops if isinstance(n.op, Join)]) == 1
+
+    # Test that it does NOT apply when tensor doesn't have size 1 along join axis
+    # (regular concatenation without ExpandDims)
+    s = join(0, x, x, x)  # Shape: (3n,) not using ExpandDims
+    f = function([x], s, mode=rewrite_mode)
+
+    test_val = np.array([1.0, 2.0], dtype=config.floatX)
+    result = f(test_val)
+    expected = np.array([1.0, 2.0, 1.0, 2.0, 1.0, 2.0], dtype=config.floatX)
+    assert np.allclose(result, expected)
+
+    # Join should still be present (optimization doesn't apply)
+    ops = f.maker.fgraph.toposort()
+    assert len([n for n in ops if isinstance(n.op, Join)]) == 1
+
+    # Test with 5 repetitions to ensure it works with larger counts
+    s = join(0, x[None], x[None], x[None], x[None], x[None])
+    f = function([x], s, mode=rewrite_mode)
+
+    test_val = np.array([1.0, 2.0], dtype=config.floatX)
+    result = f(test_val)
+    expected = np.array([[1.0, 2.0]] * 5, dtype=config.floatX)
+    assert np.allclose(result, expected)
+
+    # Check optimization applied
+    ops = f.maker.fgraph.toposort()
+    assert len([n for n in ops if isinstance(n.op, Join)]) == 0
+    assert len([n for n in ops if isinstance(n.op, Alloc)]) >= 1
+
+
 def test_local_join_empty():
     # Vector case
     empty_vec = np.asarray([], dtype=config.floatX)