Don't attempt to compute hints for unbacked expressions (#132060)

This breaks the inference we made that if you cat an N-D tensor with a 1-D tensor of size (u0,), the u0 must be zero, but no one really wanted that anyway...

Signed-off-by: Edward Z. Yang <ezyang@meta.com>

Pull Request resolved: https://github.com/pytorch/pytorch/pull/132060
Approved by: https://github.com/Skylion007

test/dynamo/test_misc.py

@@ -8755,7 +8755,9 @@ def ___make_guard_fn():
             z = y.item()
             return torch.cat([x, torch.ones(z)])
 
-        fn(torch.randn(2, 3), torch.tensor([0]))
+        self.assertRaises(
+            RuntimeError, lambda: fn(torch.randn(2, 3), torch.tensor([0]))
+        )
         self.assertRaises(
             RuntimeError, lambda: fn(torch.randn(2, 3), torch.tensor([1]))
         )

test/inductor/test_torchinductor.py

@@ -5411,13 +5411,17 @@ class CommonTemplate:
             z = y.item()
             return torch.cat([x, x.new_ones(z)])
 
-        self.common(
-            fn,
-            (
-                torch.randn([2, 3]),
-                torch.tensor([0]),
-            ),
-        )
+        with self.assertRaisesRegex(
+            RuntimeError,
+            "Expected 2-D tensors, but got 1-D for tensor number 1 in the list",
+        ):
+            self.common(
+                fn,
+                (
+                    torch.randn([2, 3]),
+                    torch.tensor([0]),
+                ),
+            )
 
     @torch._dynamo.config.patch(capture_scalar_outputs=True)
     def test_cat_unbacked_empty_1d(self):

torch/_refs/__init__.py

@@ -2780,7 +2780,17 @@ def cat(tensors: TensorSequenceType, dim: int = 0) -> TensorLikeType:
             assert tensor.ndim == 1  # we've already checked this above
             # Don't suggest the legacy behavior in the error message
             torch._check(
-                tensor.shape[0] == 0,
+                # NB: it is not enough to simply assert that tensor.shape[0] == 0;
+                # this MUST be true even under guard size oblivious.
+                # Effectively, we must actually know that the shape is zero,
+                # passing an unbacked SymInt which we will defer a runtime
+                # assert on won't cut it.  This is a policy decision (size
+                # oblivious semantics say that u0 tensors never are inferred
+                # to be zero size, even if they must be that for the cat to go
+                # through), and is load bearing for our Inductor lowerings
+                # (which assume that size oblivious tests are OK to determine
+                # if a shape is permissibly zero.)
+                guard_size_oblivious(tensor.shape[0] == 0),
                 lambda: f"Number of dimensions of tensors must match.  "
                 f"Expected {example.ndim}-D tensors, but got 1-D for "
                 f"tensor number {tensor_idx} in the list",

torch/fx/experimental/sym_node.py

@@ -110,9 +110,15 @@ class SymNode:
         # in sync, so we've deleted it for now.)
 
         def compute_hint():
+            from torch.fx.experimental.symbolic_shapes import free_unbacked_symbols
+
             # This occasionally gets exercised by, e.g.,
             # convert_shape_to_symint.  It's just a nicety so you don't HAVE
             # to have a correct hint on hand when making a SymNode.
+            # Don't attempt to compute for unbacked, this can be quite
+            # expensive.
+            if free_unbacked_symbols(self.expr):
+                return None
             hint = self.shape_env._maybe_evaluate_static(self.expr, compute_hint=True)
             if hint is not None:
                 hint = self.pytype(hint) if not isinstance(hint, SymTypes) else hint