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Fix shape issues in jax tridiagonal solve #1414

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merged 5 commits into from
May 24, 2025
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Fix shape issues in jax tridiagonal solve #1414

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a95e1e1
fix shape issues in jax tridiagonal solve; close #1413
benmaier May 22, 2025
d0239a3
added tests for tridiagonal solve
benmaier May 22, 2025
a093550
Update pytensor/link/jax/dispatch/slinalg.py
benmaier May 23, 2025
6924f4e
incorporate changes as asked for
benmaier May 23, 2025
4ee85c2
overwrite ruff
benmaier May 23, 2025
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16 changes: 15 additions & 1 deletion pytensor/link/jax/dispatch/slinalg.py
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Original file line number Diff line number Diff line change
Expand Up @@ -54,7 +54,21 @@ def solve(a, b):
dl = jax.numpy.diagonal(a, offset=-1, axis1=-2, axis2=-1)
d = jax.numpy.diagonal(a, offset=0, axis1=-2, axis2=-1)
du = jax.numpy.diagonal(a, offset=1, axis1=-2, axis2=-1)
return jax.lax.linalg.tridiagonal_solve(dl, d, du, b, lower=lower)
# jax requires dl and du to have the same shape as d
dl = jax.numpy.pad(dl, (1, 0))
du = jax.numpy.pad(du, (0, 1))
# if b is a vector, broadcast it to be a matrix
b_is_vec = len(b.shape) == 1
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You don't need to check this at runtime. The Solve Op has a property b_ndim, so you can do:

b_is_vec = op.b_ndim

if assume_a == 'tridiagonal':
    ... # carry on

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The check as written will also fail in the batched case (that's why we have it at the Op level)

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awesome

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you'd have to do b_is_vec = op.b_ndim == 1 though, no? because bool(op.b_ndim) -> True for b_ndim > 0

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yes exactly, my code has an error

if b_is_vec:
b = jax.numpy.expand_dims(b, -1)

res = jax.lax.linalg.tridiagonal_solve(dl, d, du, b)

if b_is_vec:
# if b is a vector, return a vector
return res.flatten()
else:
return res

else:
if assume_a not in ("gen", "sym", "her", "pos"):
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32 changes: 32 additions & 0 deletions tests/link/jax/test_slinalg.py
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Expand Up @@ -122,6 +122,38 @@ def test_jax_solve():
)


def test_jax_tridiagonal_solve():
N = 10
A = pt.matrix("A", shape=(N, N))
b = pt.vector("b", shape=(N,))

out = pt.linalg.solve(A, b, assume_a="tridiagonal")

A_val = np.eye(N)
for i in range(N - 1):
A_val[i, i + 1] = np.random.randn()
A_val[i + 1, i] = np.random.randn()

b_val = np.random.randn(N)

compare_jax_and_py(
[A, b],
[out],
[A_val, b_val],
)

b_ = pt.matrix("b", shape=(N, 2))

out = pt.linalg.solve(A, b_, assume_a="tridiagonal")
b_val = np.random.randn(N, 2)

compare_jax_and_py(
[A, b_],
[out],
[A_val, b_val],
)


def test_jax_SolveTriangular():
rng = np.random.default_rng(utt.fetch_seed())

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