set unbacked bindings in reinplace pass for newly created nodes during generalize_scatter decomp (#164948)

Two fixes:
1. in rein_place pass, set unbacked bindings for newly created nodes.
2. In inductor, ComputeBuffer used to miss detecting some used symbols, fixed that.

Pull Request resolved: https://github.com/pytorch/pytorch/pull/164948
Approved by: https://github.com/bobrenjc93
ghstack dependencies: #164341

test/test_dynamic_shapes.py

@@ -4301,6 +4301,34 @@ def forward(self, arg0_1: "i64[1][1]cpu", arg1_1: "Sym(u1)", arg2_1: "i64[u1][1]
             accumulate(X0, torch.tensor([1])), compiled(X0, torch.tensor([1]))
         )
 
+    @torch._dynamo.config.patch("capture_scalar_outputs", True)
+    def test_unbacked_item_set_item3(self):
+        def func(x, y):
+            u0 = y.item()
+            x[u0] = 0
+            return x
+
+        compiled = torch.compile(func, fullgraph=True, disable=False)
+        b = torch.tensor([0])
+        a = torch.ones(9, dtype=torch.int32)
+
+        compiled(a, b)
+        self.assertEqual(compiled(a, b), func(a, b))
+
+    @torch._dynamo.config.patch("capture_scalar_outputs", True)
+    def test_select_scatter_unbacked_index(self):
+        def func(x, y):
+            u0 = y.item()
+            # Create a scalar tensor to scatter into the selected index
+            scalar_src = torch.tensor(42, dtype=x.dtype)
+            return x.select_scatter(scalar_src, 0, u0)
+
+        compiled = torch.compile(func, fullgraph=True, dynamic=True, backend="inductor")
+        b = torch.tensor([0])
+        a = torch.ones(9, dtype=torch.int32)
+
+        self.assertEqual(compiled(a, b), func(a, b))
+
 
 instantiate_parametrized_tests(TestUnbacked)
 

torch/_inductor/fx_passes/reinplace.py

@@ -24,7 +24,10 @@ from torch._inductor.lowering import (
     inplaceable_foreach_ops as inplaceable_foreach_ops_lowerings,
 )
 from torch._inductor.virtualized import V
-from torch.fx.experimental.symbolic_shapes import GuardOnDataDependentSymNode
+from torch.fx.experimental.symbolic_shapes import (
+    compute_unbacked_bindings,
+    GuardOnDataDependentSymNode,
+)
 from torch.fx.immutable_collections import immutable_dict, immutable_list
 from torch.fx.passes.reinplace import _is_view_op
 from torch.utils import _pytree as pytree
@@ -60,7 +63,9 @@ def graph_call_function(graph: torch.fx.Graph, fn, *args, **kwargs):
         fake_result = fn(*fake_args, **fake_kwargs)
 
     node = graph.call_function(fn, args, kwargs)
+
     node.meta["val"] = fake_result
+
     return node
 
 
@@ -171,6 +176,13 @@ def _decompose_scatter_mutating(
     tmp = inp
     for view in view_ops:  # type: ignore[union-attr]
         tmp = graph_call_function(graph, view.target, tmp, *view.args, **view.kwargs)  # type: ignore[union-attr]
+        # we need to set unbacked bindings that could have been created in the view ops.
+        if (V.fake_mode.shape_env) and (
+            symbol_to_path := compute_unbacked_bindings(
+                V.fake_mode.shape_env, tmp.meta["val"]
+            )
+        ):
+            tmp.meta["unbacked_bindings"] = symbol_to_path
 
     graph_call_function(graph, aten.copy_.default, tmp, src)
     return inp  # type: ignore[return-value]

torch/_inductor/ir.py

@@ -4542,9 +4542,7 @@ class ComputedBuffer(OperationBuffer):
             unbacked_only
         ) | self.data.get_free_symbol_uses(unbacked_only)
 
-        if self.has_store_function() and isinstance(
+        if self.has_store_function():
-            self.get_store_function(), LoopBody
-        ):
             result |= self.get_read_writes().get_free_symbol_uses(unbacked_only)
         return result