[GitHub] [tvm] Lunderberg commented on a diff in pull request #15507: [Unity] Remove unused local function definitions

["Lunderberg (via GitHub)" <gi...@apache.org>]

Lunderberg commented on code in PR #15507:
URL: https://github.com/apache/tvm/pull/15507#discussion_r1289473732


##########
tests/python/relax/test_analysis.py:
##########
@@ -117,6 +117,53 @@ def main(x: R.Tensor((32, 32), "float32")) -> R.Tensor:
     tvm.ir.assert_structural_equal(optimized, GroundTruth["main"])
 
 
+def test_binding_block_remove_all_unused_without_dataflow():
+    @tvm.script.ir_module
+    class IdentityUnused:
+        @R.function
+        def main(x: R.Tensor((32, 32), "float32")) -> R.Tensor:
+            lv0 = x
+            unused0 = R.call_dps_packed("my_sigmoid", (x,), R.Tensor((32, 32), dtype="float32"))
+            unused1 = R.call_dps_packed(
+                "my_dps_func", (unused0,), R.Tensor((32, 32), dtype="float32")
+            )
+            z = R.call_packed("vm.builtin.copy", lv0, sinfo_args=(R.Tensor((32, 32), "float32")))
+            return z
+
+    optimized = remove_all_unused(IdentityUnused["main"])
+
+    GroundTruth = IdentityUnused

Review Comment:
   For this specific change, because (to my understanding), the `call_dps_packed` doesn't guarantee that it has no side effects.  Within a `R.dataflow()` environment, the expression may not have side effects, so it is safe to remove.
   
   For newly-unused objects, I believe that is handled by the `GetUnusedVars` function, called during construction.  It iteratively walks from the function outputs to the predecessors required to generate those outputs, finding the set of all variables required to generate the output.  Anything not within that set is considered unused and can be removed, even it occurs within another expression (because that expression is also going to be removed).



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