diff -Nurp a/gcc/tree-vect-loop.c b/gcc/tree-vect-loop.c
--- a/gcc/tree-vect-loop.c	2020-07-09 10:42:35.824000000 +0800
+++ b/gcc/tree-vect-loop.c	2020-07-09 10:43:23.920000000 +0800
@@ -1143,7 +1143,9 @@ vect_compute_single_scalar_iteration_cos
              else
                kind = scalar_store;
             }
-          else
+	  else if (vect_nop_conversion_p (stmt_info))
+	    continue;
+	  else
             kind = scalar_stmt;
 
 	  record_stmt_cost (&LOOP_VINFO_SCALAR_ITERATION_COST (loop_vinfo),
diff -Nurp a/gcc/tree-vectorizer.h b/gcc/tree-vectorizer.h
--- a/gcc/tree-vectorizer.h	2020-07-09 10:42:35.824000000 +0800
+++ b/gcc/tree-vectorizer.h	2020-07-09 10:43:23.920000000 +0800
@@ -1645,6 +1645,7 @@ extern tree vect_get_vec_def_for_stmt_co
 extern bool vect_transform_stmt (stmt_vec_info, gimple_stmt_iterator *,
 				 slp_tree, slp_instance);
 extern void vect_remove_stores (stmt_vec_info);
+extern bool vect_nop_conversion_p (stmt_vec_info);
 extern opt_result vect_analyze_stmt (stmt_vec_info, bool *, slp_tree,
 				     slp_instance, stmt_vector_for_cost *);
 extern void vect_get_load_cost (stmt_vec_info, int, bool,
diff -Nurp a/gcc/tree-vect-slp.c b/gcc/tree-vect-slp.c
--- a/gcc/tree-vect-slp.c	2020-07-09 10:42:35.736000000 +0800
+++ b/gcc/tree-vect-slp.c	2020-07-09 10:43:23.920000000 +0800
@@ -2940,6 +2940,8 @@ vect_bb_slp_scalar_cost (basic_block bb,
           else
 	    kind = scalar_store;
         }
+      else if (vect_nop_conversion_p (stmt_info))
+	continue;
       else
 	kind = scalar_stmt;
       record_stmt_cost (cost_vec, 1, kind, stmt_info, 0, vect_body);
diff -Nurp a/gcc/tree-vect-stmts.c b/gcc/tree-vect-stmts.c
--- a/gcc/tree-vect-stmts.c	2020-07-09 10:42:35.732000000 +0800
+++ b/gcc/tree-vect-stmts.c	2020-07-09 10:43:23.920000000 +0800
@@ -5283,6 +5283,29 @@ vectorizable_conversion (stmt_vec_info s
   return true;
 }
 
+/* Return true if we can assume from the scalar form of STMT_INFO that
+   neither the scalar nor the vector forms will generate code.  STMT_INFO
+   is known not to involve a data reference.  */
+
+bool
+vect_nop_conversion_p (stmt_vec_info stmt_info)
+{
+  gassign *stmt = dyn_cast <gassign *> (stmt_info->stmt);
+  if (!stmt)
+    return false;
+
+  tree lhs = gimple_assign_lhs (stmt);
+  tree_code code = gimple_assign_rhs_code (stmt);
+  tree rhs = gimple_assign_rhs1 (stmt);
+
+  if (code == SSA_NAME || code == VIEW_CONVERT_EXPR)
+    return true;
+
+  if (CONVERT_EXPR_CODE_P (code))
+    return tree_nop_conversion_p (TREE_TYPE (lhs), TREE_TYPE (rhs));
+
+  return false;
+}
 
 /* Function vectorizable_assignment.
 
@@ -5398,7 +5421,9 @@ vectorizable_assignment (stmt_vec_info s
     {
       STMT_VINFO_TYPE (stmt_info) = assignment_vec_info_type;
       DUMP_VECT_SCOPE ("vectorizable_assignment");
-      vect_model_simple_cost (stmt_info, ncopies, dt, ndts, slp_node, cost_vec);
+      if (!vect_nop_conversion_p (stmt_info))
+	vect_model_simple_cost (stmt_info, ncopies, dt, ndts, slp_node,
+				cost_vec);
       return true;
     }