Forbid runtime broadcasting in Alloc

pytensor/link/jax/dispatch/tensor_basic.py

@@ -41,9 +41,10 @@ def jax_funcify_AllocEmpty(op, **kwargs):
 
 
 @jax_funcify.register(Alloc)
-def jax_funcify_Alloc(op, **kwargs):
+def jax_funcify_Alloc(op, node, **kwargs):
     def alloc(x, *shape):
         res = jnp.broadcast_to(x, shape)
+        Alloc._check_runtime_broadcast(node, jnp.asarray(x), res.shape)
         return res
 
     return alloc

pytensor/link/numba/dispatch/tensor_basic.py

@@ -77,16 +77,24 @@ def numba_funcify_Alloc(op, node, **kwargs):
         " " * 4,
     )
 
+    check_runtime_broadcast = []
+    for i, val_static_dim in enumerate(node.inputs[0].type.shape[::-1]):
+        if val_static_dim is None:
+            check_runtime_broadcast.append(
+                f'if val.shape[{-i - 1}] == 1 and scalar_shape[{-i - 1}] != 1: raise ValueError("{Alloc._runtime_broadcast_error_msg}")'
+            )
+    check_runtime_broadcast_src = indent("\n".join(check_runtime_broadcast), " " * 4)
+
     alloc_def_src = f"""
 def alloc(val, {", ".join(shape_var_names)}):
     val_np = np.asarray(val)
 {shapes_to_items_src}
     scalar_shape = {create_tuple_string(shape_var_item_names)}
+{check_runtime_broadcast_src}
     res = np.empty(scalar_shape, dtype=val_np.dtype)
     res[...] = val_np
     return res
     """
-
     alloc_fn = compile_function_src(alloc_def_src, "alloc", {**globals(), **global_env})
 
     return numba_basic.numba_njit(alloc_fn)

pytensor/tensor/basic.py

@@ -1431,6 +1431,12 @@ class Alloc(COp):
 
     __props__ = ()
 
+    _runtime_broadcast_error_msg = (
+        "Runtime broadcasting not allowed. "
+        "The output of Alloc requires broadcasting a dimension of the input value, which was not marked as broadcastable. "
+        "If broadcasting was intended, use `specify_broadcastable` on the relevant input."
+    )
+
     def make_node(self, value, *shape):
         value = as_tensor_variable(value)
         shape, static_shape = infer_static_shape(shape)
@@ -1468,10 +1474,21 @@ class Alloc(COp):
         otype = TensorType(dtype=value.dtype, shape=combined_static_shape)
         return Apply(self, [value] + shape, [otype()])
 
+    @staticmethod
+    def _check_runtime_broadcast(node, value, shape):
+        value_static_shape = node.inputs[0].type.shape
+        for v_static_dim, value_dim, out_dim in zip(
+            value_static_shape[::-1], value.shape[::-1], shape[::-1]
+        ):
+            if v_static_dim is None and value_dim == 1 and out_dim != 1:
+                raise ValueError(Alloc._runtime_broadcast_error_msg)
+
     def perform(self, node, inputs, out_):
         (out,) = out_
         v = inputs[0]
         sh = tuple([int(i) for i in inputs[1:]])
+        self._check_runtime_broadcast(node, v, sh)
+
         if out[0] is None or out[0].shape != sh:
             if v.size == 1 and v.item() == 0:
                 out[0] = np.zeros(sh, dtype=v.dtype)
@@ -1484,12 +1501,19 @@ class Alloc(COp):
 
     def c_code(self, node, name, inp, out, sub):
         vv = inp[0]
-        ndim = len(inp[1:])
         (zz,) = out
         fail = sub["fail"]
 
+        v_static_shape = node.inputs[0].type.shape
+        o_static_shape = node.outputs[0].type.shape
+        v_ndim = len(v_static_shape)
+        o_ndim = len(o_static_shape)
+        assert o_ndim == len(inp[1:])
+
+        # Declare variables
         code = f"""
-            npy_intp shape[{ndim}];
+            npy_intp shape[{o_ndim}];
+            int need_new_out;
             """
 
         # Initialize shape
@@ -1498,15 +1522,26 @@ class Alloc(COp):
                 shape[{i}] = ((dtype_{shp_i}*) PyArray_DATA({shp_i}))[0];
             """
 
+        # Add checks for runtime broadcasting
+        for i, v_static_dim in enumerate(v_static_shape[::-1]):
+            if v_static_dim is None:
                 code += f"""
-            int need_new_out = (NULL == {zz});
-            for (int i = 0; i < {ndim}; i++)
+                if (PyArray_DIMS({vv})[{v_ndim - i - 1}] == 1 && shape[{o_ndim - i - 1}] != 1)
+                {{
+                    PyErr_Format(PyExc_ValueError, "{self._runtime_broadcast_error_msg}");
+                    {fail}
+                }}
+                """
+
+        code += f"""
+            need_new_out = (NULL == {zz});
+            for (int i = 0; i < {o_ndim}; i++)
                 need_new_out = (need_new_out || (PyArray_DIMS({zz})[i] != shape[i]));
 
             if (need_new_out)
             {{
                 Py_XDECREF({zz});
-                {zz} = (PyArrayObject*) PyArray_SimpleNew({ndim}, shape, PyArray_TYPE({vv}));
+                {zz} = (PyArrayObject*) PyArray_SimpleNew({o_ndim}, shape, PyArray_TYPE({vv}));
                 if (!{zz})
                 {{
                     PyErr_SetString(PyExc_MemoryError, "alloc failed");
@@ -1522,7 +1557,7 @@ class Alloc(COp):
         return code
 
     def c_code_cache_version(self):
-        return (3,)
+        return (4,)
 
     def infer_shape(self, fgraph, node, input_shapes):
         return [node.inputs[1:]]

tests/link/jax/test_tensor_basic.py

 import numpy as np
 import pytest
 
+from pytensor.compile import get_mode
+
 
 jax = pytest.importorskip("jax")
 import jax.errors
@@ -12,6 +14,7 @@ from pytensor.graph.fg import FunctionGraph
 from pytensor.graph.op import get_test_value
 from pytensor.tensor.type import iscalar, matrix, scalar, vector
 from tests.link.jax.test_basic import compare_jax_and_py
+from tests.tensor.test_basic import TestAlloc
 
 
 def test_jax_Alloc():
@@ -50,6 +53,10 @@ def test_jax_Alloc():
     compare_jax_and_py(x_fg, [np.ones(10, dtype=config.floatX)])
 
 
+def test_alloc_runtime_broadcast():
+    TestAlloc.check_runtime_broadcast(get_mode("JAX"))
+
+
 def test_jax_MakeVector():
     x = at.make_vector(1, 2, 3)
     x_fg = FunctionGraph([], [x])

tests/link/numba/test_tensor_basic.py

@@ -5,6 +5,7 @@ import pytensor.scalar as aes
 import pytensor.tensor as at
 import pytensor.tensor.basic as atb
 from pytensor import config, function
+from pytensor.compile import get_mode
 from pytensor.compile.sharedvalue import SharedVariable
 from pytensor.graph.basic import Constant
 from pytensor.graph.fg import FunctionGraph
@@ -15,6 +16,7 @@ from tests.link.numba.test_basic import (
     compare_shape_dtype,
     set_test_value,
 )
+from tests.tensor.test_basic import TestAlloc
 
 
 pytest.importorskip("numba")
@@ -49,6 +51,10 @@ def test_Alloc(v, shape):
     assert numba_res.shape == shape
 
 
+def test_alloc_runtime_broadcast():
+    TestAlloc.check_runtime_broadcast(get_mode("NUMBA"))
+
+
 def test_AllocEmpty():
     x = at.empty((2, 3), dtype="float32")
     x_fg = FunctionGraph([], [x])

tests/tensor/test_basic.py

@@ -719,6 +719,38 @@ class TestAlloc:
     shared = staticmethod(pytensor.shared)
     allocs = [Alloc()] * 3
 
+    @staticmethod
+    def check_allocs_in_fgraph(fgraph, n):
+        assert (
+            len([node for node in fgraph.apply_nodes if isinstance(node.op, Alloc)])
+            == n
+        )
+
+    @staticmethod
+    def check_runtime_broadcast(mode):
+        """Check we emmit a clear error when runtime broadcasting would occur according to Numpy rules."""
+        floatX = config.floatX
+        x_v = vector("x", shape=(None,))
+
+        out = alloc(x_v, 5, 3)
+        f = pytensor.function([x_v], out, mode=mode)
+        TestAlloc.check_allocs_in_fgraph(f.maker.fgraph, 1)
+
+        np.testing.assert_array_equal(
+            f(x=np.zeros((3,), dtype=floatX)),
+            np.zeros((5, 3), dtype=floatX),
+        )
+        with pytest.raises(ValueError, match="Runtime broadcasting not allowed"):
+            f(x=np.zeros((1,), dtype=floatX))
+
+        out = alloc(specify_shape(x_v, (1,)), 5, 3)
+        f = pytensor.function([x_v], out, mode=mode)
+        TestAlloc.check_allocs_in_fgraph(f.maker.fgraph, 1)
+
+        np.testing.assert_array_equal(
+            f(x=np.zeros((1,), dtype=floatX)),
+            np.zeros((5, 3), dtype=floatX),
+        )
 
     def setup_method(self):
         self.rng = np.random.default_rng(seed=utt.fetch_seed())
@@ -853,6 +885,8 @@ class TestAlloc:
 
     def test_alloc_of_view_linker(self):
         """Check we can allocate a new array properly in the C linker when input is a view."""
+        floatX = config.floatX
+
         x_v = vector("x", shape=(None,))
         dim_len = scalar("dim_len", dtype=int)
         out = alloc(specify_shape(x_v, (1,)), 5, dim_len)
@@ -862,7 +896,14 @@ class TestAlloc:
             f.maker.fgraph.outputs, [alloc(specify_shape(x_v, (1,)), 5, dim_len)]
         )
 
-        np.testing.assert_array_equal(f(x=np.zeros((1,)), dim_len=3), np.zeros((5, 3)))
+        np.testing.assert_array_equal(
+            f(x=np.zeros((1,), dtype=floatX), dim_len=3),
+            np.zeros((5, 3), dtype=floatX),
+        )
+
+    @pytest.mark.parametrize("mode", (Mode("py"), Mode("c")))
+    def test_runtime_broadcast(self, mode):
+        self.check_runtime_broadcast(mode)
 
 
 def test_infer_shape():