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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/compiler/dead-code-elimination.h"
#include "src/compiler/common-operator.h"
#include "src/compiler/graph.h"
#include "src/compiler/node-properties.h"
#include "src/compiler/operator-properties.h"
namespace v8 {
namespace internal {
namespace compiler {
DeadCodeElimination::DeadCodeElimination(Editor* editor, Graph* graph,
CommonOperatorBuilder* common)
: AdvancedReducer(editor),
graph_(graph),
common_(common),
dead_(graph->NewNode(common->Dead())) {
NodeProperties::SetType(dead_, Type::None());
}
Reduction DeadCodeElimination::Reduce(Node* node) {
switch (node->opcode()) {
case IrOpcode::kEnd:
return ReduceEnd(node);
case IrOpcode::kLoop:
case IrOpcode::kMerge:
return ReduceLoopOrMerge(node);
case IrOpcode::kLoopExit:
return ReduceLoopExit(node);
default:
return ReduceNode(node);
}
UNREACHABLE();
return NoChange();
}
Reduction DeadCodeElimination::ReduceEnd(Node* node) {
DCHECK_EQ(IrOpcode::kEnd, node->opcode());
Node::Inputs inputs = node->inputs();
DCHECK_LE(1, inputs.count());
int live_input_count = 0;
for (int i = 0; i < inputs.count(); ++i) {
Node* const input = inputs[i];
// Skip dead inputs.
if (input->opcode() == IrOpcode::kDead) continue;
// Compact live inputs.
if (i != live_input_count) node->ReplaceInput(live_input_count, input);
++live_input_count;
}
if (live_input_count == 0) {
return Replace(dead());
} else if (live_input_count < inputs.count()) {
node->TrimInputCount(live_input_count);
NodeProperties::ChangeOp(node, common()->End(live_input_count));
return Changed(node);
}
DCHECK_EQ(inputs.count(), live_input_count);
return NoChange();
}
Reduction DeadCodeElimination::ReduceLoopOrMerge(Node* node) {
DCHECK(IrOpcode::IsMergeOpcode(node->opcode()));
Node::Inputs inputs = node->inputs();
DCHECK_LE(1, inputs.count());
// Count the number of live inputs to {node} and compact them on the fly, also
// compacting the inputs of the associated {Phi} and {EffectPhi} uses at the
// same time. We consider {Loop}s dead even if only the first control input
// is dead.
int live_input_count = 0;
if (node->opcode() != IrOpcode::kLoop ||
node->InputAt(0)->opcode() != IrOpcode::kDead) {
for (int i = 0; i < inputs.count(); ++i) {
Node* const input = inputs[i];
// Skip dead inputs.
if (input->opcode() == IrOpcode::kDead) continue;
// Compact live inputs.
if (live_input_count != i) {
node->ReplaceInput(live_input_count, input);
for (Node* const use : node->uses()) {
if (NodeProperties::IsPhi(use)) {
DCHECK_EQ(inputs.count() + 1, use->InputCount());
use->ReplaceInput(live_input_count, use->InputAt(i));
}
}
}
++live_input_count;
}
}
if (live_input_count == 0) {
return Replace(dead());
} else if (live_input_count == 1) {
// Due to compaction above, the live input is at offset 0.
for (Node* const use : node->uses()) {
if (NodeProperties::IsPhi(use)) {
Replace(use, use->InputAt(0));
} else if (use->opcode() == IrOpcode::kLoopExit &&
use->InputAt(1) == node) {
RemoveLoopExit(use);
} else if (use->opcode() == IrOpcode::kTerminate) {
DCHECK_EQ(IrOpcode::kLoop, node->opcode());
Replace(use, dead());
}
}
return Replace(node->InputAt(0));
}
DCHECK_LE(2, live_input_count);
DCHECK_LE(live_input_count, inputs.count());
// Trim input count for the {Merge} or {Loop} node.
if (live_input_count < inputs.count()) {
// Trim input counts for all phi uses and revisit them.
for (Node* const use : node->uses()) {
if (NodeProperties::IsPhi(use)) {
use->ReplaceInput(live_input_count, node);
TrimMergeOrPhi(use, live_input_count);
Revisit(use);
}
}
TrimMergeOrPhi(node, live_input_count);
return Changed(node);
}
return NoChange();
}
Reduction DeadCodeElimination::RemoveLoopExit(Node* node) {
DCHECK_EQ(IrOpcode::kLoopExit, node->opcode());
for (Node* const use : node->uses()) {
if (use->opcode() == IrOpcode::kLoopExitValue ||
use->opcode() == IrOpcode::kLoopExitEffect) {
Replace(use, use->InputAt(0));
}
}
Node* control = NodeProperties::GetControlInput(node, 0);
Replace(node, control);
return Replace(control);
}
Reduction DeadCodeElimination::ReduceNode(Node* node) {
// If {node} has exactly one control input and this is {Dead},
// replace {node} with {Dead}.
int const control_input_count = node->op()->ControlInputCount();
if (control_input_count == 0) return NoChange();
DCHECK_EQ(1, control_input_count);
Node* control = NodeProperties::GetControlInput(node);
if (control->opcode() == IrOpcode::kDead) return Replace(control);
return NoChange();
}
Reduction DeadCodeElimination::ReduceLoopExit(Node* node) {
Node* control = NodeProperties::GetControlInput(node, 0);
Node* loop = NodeProperties::GetControlInput(node, 1);
if (control->opcode() == IrOpcode::kDead ||
loop->opcode() == IrOpcode::kDead) {
return RemoveLoopExit(node);
}
return NoChange();
}
void DeadCodeElimination::TrimMergeOrPhi(Node* node, int size) {
const Operator* const op = common()->ResizeMergeOrPhi(node->op(), size);
node->TrimInputCount(OperatorProperties::GetTotalInputCount(op));
NodeProperties::ChangeOp(node, op);
}
} // namespace compiler
} // namespace internal
} // namespace v8
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