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/*
* Copyright (c) 2019 Samsung Electronics Co., Ltd. All Rights Reserved
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
#include <logo/RemoveDeadNodePass.h>
#include <loco/IR/Algorithm.h>
#include <loco/IR/CanonicalDialect.h>
#include <loco/IR/CanonicalNode.h>
#include <set>
namespace logo
{
bool RemoveDeadNodePass::run(loco::Graph *g)
{
// Let's enumerate nodes required to compute output nodes
auto active_nodes = loco::active_nodes(loco::output_nodes(g));
// Find dead(= non-active) nodes
std::set<loco::Node *> candidates;
for (auto node : loco::all_nodes(g))
{
if (active_nodes.find(node) == active_nodes.end())
{
candidates.insert(node);
}
}
// Let's drop the references from each dead node first and then remove these dead nodes
//
// Why?
//
// Let us consider the following example:
// %0 = Pull(...)
// %1 = ConstGen(...)
// %2 = Forward(input: %1)
// %3 = Push(from: %0) <- OUTPUT
//
// Forward (%2) is dead as it does not contribute to the final result (%3). However, it
// refers to another dead node (%1).
//
// This example indicates that naive implementation results in dangling references.
//
// There are two possible solutions:
// 1. Destroy nodes in topological order
// 2. Drop the reference first and then destroy them
//
// The current implementation takes the latter approach for the simplicity of implementation.
for (auto node : candidates)
{
node->drop();
}
for (auto node : candidates)
{
g->nodes()->destroy(node);
}
return candidates.size() > 0;
}
} // namespace logo
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