Refactor aesara.tensor.basic_opt.local_fill_to_alloc

aesara/tensor/basic_opt.py

@@ -1652,50 +1652,42 @@ def local_fill_sink(fgraph, node):
 
 @register_specialize
 @register_stabilize
-# @register_canonicalize  # We make full pass after the canonizer phase.
 @local_optimizer([fill])
 def local_fill_to_alloc(fgraph, node):
-    """fill(s,v) -> alloc(v, shape(s))
+    r"""Remove `fill`\s or replace them with `Alloc`\s.
 
-    This is an important optimization because with the shape_to_shape_i
-    optimization, the dependency on 's' is often removed.
+    `Alloc`\s are preferable because they replace explicit tensor dependencies
+    with their dependencies on those tensors' shapes, and sometimes those
+    shapes can be computed without needing to compute the tensors themselves.
 
+    XXX: This rewrite can produce inconsistent results, so do *not* consider
+    making it a canonicalization until those inconsistencies are
+    resolved/justified.
     """
-    if node.op == fill:
-        r, v = node.inputs
-        if v.type == node.outputs[0].type:
-            # this is a useless fill, erase it.
-            rval = [v]
-        elif v.type.broadcastable == node.outputs[0].type.broadcastable:
-            # this is a cast
-            rval = [cast(v, node.outputs[0].type.dtype)]
-        elif r.type.broadcastable == node.outputs[0].type.broadcastable:
-            # we are broadcasting v somehow, but not r
-            o = broadcast_like(v, r, fgraph, dtype=v.dtype)
-            copy_stack_trace(node.outputs[0], o)
-            rval = [o]
-        else:
-            # we are broadcasting both v and r,
-            # the output shape must be computed
-            #
-            # TODO: implement this case (including a test!)
-            #
-            #  I think the strategy should be to extend the shorter
-            #  shape vector with 1s (how?) and then take the
-            #  elementwise max of the two.  - how to flag an error of
-            #  shape mismatch where broadcasting should be illegal?
-            return
-            # TODO: cut out un-necessary dimshuffles of v
-
-        assert rval[0].type == node.outputs[0].type, (
-            "rval",
-            rval[0].type,
-            "orig",
-            node.outputs[0].type,
-            "node",
-            node,
-        )  # aesara.printing.debugprint(node.outputs[0], file='str'))
-        return rval
+    shape_ref, values_ref = node.inputs
+    out_type = node.outputs[0].type
+
+    if values_ref.type.broadcastable == out_type.broadcastable:
+        # The assumption here is that `values_ref` already has the same shape
+        # as `shape_ref`, so a `fill`/`Alloc` is unnecessary.
+
+        # XXX FIXME TODO: The only way this can be determined is if one
+        # absolutely knows that the shapes of `shape_ref` and `values_ref` are
+        # equal.
+        # This is an old rewrite, and it's only a
+        # "specialization/stabilization", so we're going to leave it be for
+        # now.
+        return [values_ref]
+
+    if shape_ref.type.broadcastable == out_type.broadcastable:
+        # In this case, we assume that some broadcasting is needed (otherwise
+        # the condition above would've been true), so we replace the `fill`
+        # with an `Alloc`.
+        o = broadcast_like(values_ref, shape_ref, fgraph, dtype=values_ref.dtype)
+        copy_stack_trace(node.outputs[0], o)
+        return [o]
+
+    return
 
 
 # Register this after stabilize at 1.5 to make sure stabilize don't

tests/tensor/test_basic_opt.py

@@ -1292,12 +1292,10 @@ def test_local_fill_useless():
     x = dvector()
     y = dvector()
     z = lvector()
-    m = dmatrix()
 
     x_ = np.random.random((5,))
     y_ = np.random.random((5,))
     z_ = (np.random.random((5,)) * 5).astype("int64")
-    m_ = np.random.random((5, 5))
 
     # basic case
     f = function([x], at.fill(x, x) * 2, mode=mode_opt)
@@ -1329,12 +1327,35 @@ def test_local_fill_useless():
     assert [node.op for node in f.maker.fgraph.toposort()] == [mul]
     f(x_, y_)
 
-    # Test with different number of dimensions
-    # The fill is not useless, so it should stay
-    f = function([m, x], at.fill(m, x) * 2, mode=mode_opt)
-    ops = [node.op.__class__ for node in f.maker.fgraph.toposort()]
-    assert Alloc in ops
-    f(m_, x_)
+
+def test_local_fill_to_alloc():
+    x = dvector()
+    m = dmatrix()
+
+    x_ = np.random.random((5,))
+    m_ = np.random.random((5, 5))
+
+    y = at.fill(m, x)
+
+    mode = mode_opt.including("stabilize", "local_fill_to_alloc").excluding(
+        "useless", "local_useless_fill"
+    )
+
+    f = function([m, x], y, mode=mode)
+    assert Alloc in [node.op.__class__ for node in f.maker.fgraph.toposort()]
+
+    res = f(m_, x_)
+    exp_res = np.broadcast_to(x_, m_.shape)
+    assert np.array_equal(res, exp_res)
+
+    y = at.fill(x, m)
+
+    f = function([m, x], y, mode=mode)
+
+    assert Alloc not in [node.op.__class__ for node in f.maker.fgraph.toposort()]
+
+    res = f(m_, x_)
+    assert np.array_equal(res, m_)
 
 
 class TestLocalCanonicalizeAlloc: