[BUG] ToBasicBlockNormalForm immutability

src/relay/transforms/to_basic_block_normal_form.cc

@@ -51,8 +51,11 @@ Expr ToBasicBlockNormalFormAux(const Expr& e) {
 IRModule ToBasicBlockNormalForm(const IRModule& mod) {
   DLOG(INFO) << "ToBBlock:" << std::endl << mod;
 
+  // Create a new module by shallow copy.
+  auto mod_ = IRModule(mod->functions, mod->type_definitions, mod->Imports(), mod->source_map);
+
   tvm::Map<GlobalVar, Function> updates;
-  auto funcs = mod->functions;
+  auto funcs = mod_->functions;
   for (const auto& it : funcs) {
     ICHECK_EQ(FreeVars(it.second).size(), 0) << "Expected no free variables";
     if (const auto* n = it.second.as<FunctionNode>()) {
@@ -63,12 +66,12 @@ IRModule ToBasicBlockNormalForm(const IRModule& mod) {
   }
 
   for (auto pair : updates) {
-    mod->Add(pair.first, pair.second, true);
+    mod_->Add(pair.first, pair.second, true);
   }
 
-  DLOG(INFO) << "ToBBlock: transformed" << std::endl << mod;
+  DLOG(INFO) << "ToBBlock: transformed" << std::endl << mod_;
 
-  return mod;
+  return mod_;
 }
 
 bool BasicBlockNormalFormCheck(const Expr& e) {

tests/python/relay/test_pass_to_basic_block_normal_form.py

@@ -22,7 +22,7 @@
 from tvm.relay.analysis import detect_feature
 from tvm.relay import op, create_executor, transform
 from tvm.relay.prelude import Prelude
-from tvm.relay.testing import count
+from tvm.relay.testing import count, create_workload
 from tvm.relay.analysis import Feature
 from tvm.relay.analysis import check_basic_block_normal_form
 
@@ -489,5 +489,27 @@ def test_higher_order_nested():
     check_basic_block_normal_form(bblock)
 
 
+def test_immutability():
+    simple_net = relay.nn.conv2d(
+        data=relay.var("data", relay.TensorType((1, 3, 224, 224), "float32")),
+        weight=relay.var("weight"),
+        kernel_size=(5, 5),
+        channels=3,
+        padding=(1, 1),
+    )
+    simple_net = relay.Function(relay.analysis.free_vars(simple_net), simple_net)
+    mod, _ = create_workload(simple_net)
+
+    old_mod = mod
+
+    with tvm.transform.PassContext(opt_level=4):
+        with tvm.target.Target("llvm"):
+            seq = tvm.transform.Sequential(passes=[transform.ToBasicBlockNormalForm()], opt_level=4)
+            new_mod = seq(mod)
+
+    assert old_mod.astext() == mod.astext()
+    assert old_mod.astext() != new_mod.astext()
+
+
 if __name__ == "__main__":
     pytest.main([__file__])