Add cholesky of L.LT rewrite

pytensor/tensor/rewriting/linalg.py

@@ -109,6 +109,50 @@ def psd_solve_with_chol(fgraph, node):
             return [x]
 
 
+@register_canonicalize
+@register_stabilize
+@node_rewriter([Cholesky])
+def cholesky_ldotlt(fgraph, node):
+    """
+    rewrite cholesky(dot(L, L.T), lower=True) = L, where L is lower triangular,
+    or cholesky(dot(U.T, U), upper=True) = U where U is upper triangular.
+
+    This utilizes a boolean `lower_triangular` or `upper_triangular` tag on matrices.
+    """
+    if not isinstance(node.op, Cholesky):
+        return
+
+    A = node.inputs[0]
+    if not (A.owner and isinstance(A.owner.op, (Dot, Dot22))):
+        return
+
+    l, r = A.owner.inputs
+
+    # cholesky(dot(L,L.T)) case
+    if (
+        getattr(l.tag, "lower_triangular", False)
+        and r.owner
+        and isinstance(r.owner.op, DimShuffle)
+        and r.owner.op.new_order == (1, 0)
+        and r.owner.inputs[0] == l
+    ):
+        if node.op.lower:
+            return [l]
+        return [r]
+
+    # cholesky(dot(U.T,U)) case
+    if (
+        getattr(r.tag, "upper_triangular", False)
+        and l.owner
+        and isinstance(l.owner.op, DimShuffle)
+        and l.owner.op.new_order == (1, 0)
+        and l.owner.inputs[0] == r
+    ):
+        if node.op.lower:
+            return [l]
+        return [r]
+
+
 @register_stabilize
 @register_specialize
 @node_rewriter([Det])

tests/tensor/rewriting/test_linalg.py

 import numpy as np
 import numpy.linalg
+import pytest
+import scipy.linalg
 
 import pytensor
 from pytensor import function
 from pytensor import tensor as at
+from pytensor.compile import get_default_mode
 from pytensor.configdefaults import config
 from pytensor.tensor.elemwise import DimShuffle
 from pytensor.tensor.math import _allclose
@@ -105,3 +108,75 @@ def test_matrix_inverse_solve():
     node = matrix_inverse(A).dot(b).owner
     [out] = inv_as_solve.transform(None, node)
     assert isinstance(out.owner.op, Solve)
+
+
+@pytest.mark.parametrize("tag", ("lower", "upper", None))
+@pytest.mark.parametrize("cholesky_form", ("lower", "upper"))
+@pytest.mark.parametrize("product", ("lower", "upper", None))
+def test_cholesky_ldotlt(tag, cholesky_form, product):
+    cholesky = Cholesky(lower=(cholesky_form == "lower"))
+
+    transform_removes_chol = tag is not None and product == tag
+    transform_transposes = transform_removes_chol and cholesky_form != tag
+
+    A = matrix("L")
+    if tag:
+        setattr(A.tag, tag + "_triangular", True)
+
+    if product == "lower":
+        M = A.dot(A.T)
+    elif product == "upper":
+        M = A.T.dot(A)
+    else:
+        M = A
+
+    C = cholesky(M)
+    f = pytensor.function([A], C, mode=get_default_mode().including("cholesky_ldotlt"))
+
+    print(f.maker.fgraph.apply_nodes)
+
+    no_cholesky_in_graph = not any(
+        isinstance(node.op, Cholesky) for node in f.maker.fgraph.apply_nodes
+    )
+
+    assert no_cholesky_in_graph == transform_removes_chol
+
+    if transform_transposes:
+        assert any(
+            isinstance(node.op, DimShuffle) and node.op.new_order == (1, 0)
+            for node in f.maker.fgraph.apply_nodes
+        )
+
+    # Test some concrete value through f
+    # there must be lower triangular (f assumes they are)
+    Avs = [
+        np.eye(1, dtype=pytensor.config.floatX),
+        np.eye(10, dtype=pytensor.config.floatX),
+        np.array([[2, 0], [1, 4]], dtype=pytensor.config.floatX),
+    ]
+    if not tag:
+        # these must be positive def
+        Avs.extend(
+            [
+                np.ones((4, 4), dtype=pytensor.config.floatX)
+                + np.eye(4, dtype=pytensor.config.floatX),
+            ]
+        )
+
+    for Av in Avs:
+        if tag == "upper":
+            Av = Av.T
+
+        if product == "lower":
+            Mv = Av.dot(Av.T)
+        elif product == "upper":
+            Mv = Av.T.dot(Av)
+        else:
+            Mv = Av
+
+        assert np.all(
+            np.isclose(
+                scipy.linalg.cholesky(Mv, lower=(cholesky_form == "lower")),
+                f(Av),
+            )
+        )