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// Copyright 2015 the V8 project authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
#include "src/base/compiler-specific.h"
#include "src/compiler/graph-reducer.h"
#include "src/globals.h"
#include "src/machine-type.h"
namespace v8 {
namespace internal {
namespace compiler {
// Forward declarations.
class CommonOperatorBuilder;
// Propagates {Dead} control and {DeadValue} values through the graph and
// thereby removes dead code.
// We detect dead values based on types, replacing uses of nodes with
// {Type::None()} with {DeadValue}. A pure node (other than a phi) using
// {DeadValue} is replaced by {DeadValue}. When {DeadValue} hits the effect
// chain, a crashing {Unreachable} node is inserted and the rest of the effect
// chain is collapsed. We wait for the {EffectControlLinearizer} to connect
// {Unreachable} nodes to the graph end, since this is much easier if there is
// no floating control.
// {DeadValue} has an input, which has to have {Type::None()}. This input is
// important to maintain the dependency on the cause of the unreachable code.
// {Unreachable} has a value output and {Type::None()} so it can be used by
// {DeadValue}.
// {DeadValue} nodes track a {MachineRepresentation} so they can be lowered to a
// value-producing node. {DeadValue} has the runtime semantics of crashing and
// behaves like a constant of its representation so it can be used in gap moves.
// Since phi nodes are the only remaining use of {DeadValue}, this
// representation is only adjusted for uses by phi nodes.
// In contrast to {DeadValue}, {Dead} can never remain in the graph.
class V8_EXPORT_PRIVATE DeadCodeElimination final
: public NON_EXPORTED_BASE(AdvancedReducer) {
DeadCodeElimination(Editor* editor, Graph* graph,
CommonOperatorBuilder* common, Zone* temp_zone);
~DeadCodeElimination() final {}
const char* reducer_name() const override { return "DeadCodeElimination"; }
Reduction Reduce(Node* node) final;
Reduction ReduceEnd(Node* node);
Reduction ReduceLoopOrMerge(Node* node);
Reduction ReduceLoopExit(Node* node);
Reduction ReduceNode(Node* node);
Reduction ReducePhi(Node* node);
Reduction ReducePureNode(Node* node);
Reduction ReduceUnreachableOrIfException(Node* node);
Reduction ReduceEffectNode(Node* node);
Reduction ReduceDeoptimizeOrReturnOrTerminate(Node* node);
Reduction ReduceBranchOrSwitch(Node* node);
Reduction RemoveLoopExit(Node* node);
Reduction PropagateDeadControl(Node* node);
void TrimMergeOrPhi(Node* node, int size);
Node* DeadValue(Node* none_node,
MachineRepresentation rep = MachineRepresentation::kNone);
Graph* graph() const { return graph_; }
CommonOperatorBuilder* common() const { return common_; }
Node* dead() const { return dead_; }
Graph* const graph_;
CommonOperatorBuilder* const common_;
Node* const dead_;
Zone* zone_;
} // namespace compiler
} // namespace internal
} // namespace v8