[Relax] Improve CanonicalizeBindings in DataflowVar edge case (#16783)

* [Relax] Improve CanonicalizeBindings in DataflowVar edge case

If there is a trivial binding of `Var = DataflowVar`, but the
non-dataflow variable is never used outside the dataflow block in
which is is declared, then we should keep the name of the upstream
`DataflowVar`, as it is more likely to be the human-readable
name (e.g. a function parameter).

* Update comment for used/not used Var

* ci bump

Author: Lunderberg

src/relax/transform/canonicalize_bindings.cc

@@ -91,18 +91,21 @@ class CanonicalizePlanner : public ExprVisitor {
         bound_to = opt.value();
       }
 
-      if (bound_var.as<DataflowVarNode>() || !bound_to.as<DataflowVarNode>()) {
+      if (bound_var.as<DataflowVarNode>() || !bound_to.as<DataflowVarNode>() ||
+          !visitor.used_outside_home_dataflow_.count(bound_var)) {
         // Case 1: Var = Var
         // Case 2: DataflowVar = Var
         // Case 3: DataflowVar = DataflowVar
+        // Case 4a: Var = DataflowVar, where the Var is not used
+        //          outside the DataflowBlock containing the binding
         //
-        // For these three cases, the trivial binding can be
-        // unwrapped, using the bound variable directly at the point
-        // of use.
+        // For these four cases, the trivial binding can be unwrapped,
+        // using the bound variable directly at the point of use.
         plan.replace_usage.Set(bound_var->vid, bound_to);
         plan.bindings_to_remove.insert(bound_var->vid);
       } else {
-        // Case 4: Var = DataflowVar
+        // Case 4b: Var = DataflowVar, where the Var is used somewhere
+        //          outside the DataflowBlock containing the binding
         //
         // Replacing a Var with a DataflowVar could result in illegal
         // use of a DataflowVar outside of a DataflowBlock.  Instead,

tests/python/relax/test_transform_canonicalize_bindings.py

@@ -977,5 +977,39 @@ def main():
     verify(TestChainAssignments, Expected)
 
 
+def test_trivial_binding_of_replaced_non_dataflow_var():
+    @I.ir_module
+    class Before:
+        @R.function
+        def main(param_tuple: R.Tuple([R.Tensor])):
+            with R.dataflow():
+                A = param_tuple[0]
+                B = A
+                C = R.add(A, B)
+                R.output(A, B, C)
+            return C
+
+    @I.ir_module
+    class Expected:
+        @R.function
+        def main(param_tuple: R.Tuple([R.Tensor])):
+            with R.dataflow():
+                A = param_tuple[0]
+                C = R.add(A, A)
+                R.output(C)
+            return C
+
+    After = CanonicalizeBindings()(Before)
+    tvm.ir.assert_structural_equal(After, Expected)
+
+    def _get_binding_names(mod):
+        return [binding.var.name_hint for binding in mod["main"].body.blocks[0].bindings]
+
+    expected_names = _get_binding_names(Expected)
+    after_names = _get_binding_names(After)
+
+    assert after_names == expected_names
+
+
 if __name__ == "__main__":
     tvm.testing.main()