dicrease the complexity of CalcDep from exponential to linear (apache…

…#4053)

src/relay/pass/dead_code.cc

@@ -110,7 +110,15 @@ class CalcDep : private ExprVisitor {
   VarMap<size_t> use_map_;
 
   void VisitExpr(const Expr& e) final {
-    return ExprFunctor<void(const Expr& e)>::VisitExpr(e);
+    visit_counter_[e.get()]++;
+    // The dce code seprate variable into three parts:
+    // used 0 times (remove)
+    // used 1 times (inline)
+    // used 2 times (dont do anything).
+    if (visit_counter_[e.get()] <= 2) {
+      using TParent = ExprFunctor<void(const Expr&)>;
+      TParent::VisitExpr(e);
+    }
   }
 
   void VisitExpr_(const LetNode* l) final {

tests/python/relay/test_pass_dead_code_elimination.py

@@ -19,7 +19,9 @@
 from tvm.relay import Function, transform
 from tvm.relay.analysis import alpha_equal, graph_equal, free_vars, assert_alpha_equal
 from tvm.relay.op import log, add, equal, subtract
+from tvm.relay.testing import inception_v3
 
+import pytest
 
 class env:
     def __init__(self):
@@ -129,6 +131,12 @@ def test_tuple_get_item():
     assert alpha_equal(Function(free_vars(dced), dced), Function(free_vars(g), g))
 
 
+@pytest.mark.timeout(timeout=10, method="thread")
+def test_complexity():
+    g = inception_v3.get_net(1, 1000, (3, 299, 299), 'float32')
+    run_opt_pass(g, transform.DeadCodeElimination())
+
+
 if __name__ == "__main__":
     test_let()
     test_used_let()
@@ -138,3 +146,4 @@ def test_tuple_get_item():
     test_recursion_dead()
     test_op_let()
     test_tuple_get_item()
+    test_complexity()