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// Copyright 2014 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/graph-reducer.h"
#include <functional>
#include "src/compiler/graph-inl.h"
namespace v8 {
namespace internal {
namespace compiler {
enum class GraphReducer::State : uint8_t {
kUnvisited,
kRevisit,
kOnStack,
kVisited
};
GraphReducer::GraphReducer(Graph* graph, Zone* zone)
: graph_(graph),
state_(graph, 4),
reducers_(zone),
revisit_(zone),
stack_(zone) {}
void GraphReducer::AddReducer(Reducer* reducer) {
reducers_.push_back(reducer);
}
void GraphReducer::ReduceNode(Node* node) {
DCHECK(stack_.empty());
DCHECK(revisit_.empty());
Push(node);
for (;;) {
if (!stack_.empty()) {
// Process the node on the top of the stack, potentially pushing more or
// popping the node off the stack.
ReduceTop();
} else if (!revisit_.empty()) {
// If the stack becomes empty, revisit any nodes in the revisit queue.
Node* const node = revisit_.top();
revisit_.pop();
if (state_.Get(node) == State::kRevisit) {
// state can change while in queue.
Push(node);
}
} else {
break;
}
}
DCHECK(revisit_.empty());
DCHECK(stack_.empty());
}
void GraphReducer::ReduceGraph() { ReduceNode(graph()->end()); }
Reduction GraphReducer::Reduce(Node* const node) {
auto skip = reducers_.end();
for (auto i = reducers_.begin(); i != reducers_.end();) {
if (i != skip) {
Reduction reduction = (*i)->Reduce(node);
if (!reduction.Changed()) {
// No change from this reducer.
} else if (reduction.replacement() == node) {
// {replacement} == {node} represents an in-place reduction. Rerun
// all the other reducers for this node, as now there may be more
// opportunities for reduction.
skip = i;
i = reducers_.begin();
continue;
} else {
// {node} was replaced by another node.
return reduction;
}
}
++i;
}
if (skip == reducers_.end()) {
// No change from any reducer.
return Reducer::NoChange();
}
// At least one reducer did some in-place reduction.
return Reducer::Changed(node);
}
void GraphReducer::ReduceTop() {
NodeState& entry = stack_.top();
Node* node = entry.node;
DCHECK(state_.Get(node) == State::kOnStack);
if (node->IsDead()) return Pop(); // Node was killed while on stack.
// Recurse on an input if necessary.
int start = entry.input_index < node->InputCount() ? entry.input_index : 0;
for (int i = start; i < node->InputCount(); i++) {
Node* input = node->InputAt(i);
entry.input_index = i + 1;
if (input != node && Recurse(input)) return;
}
for (int i = 0; i < start; i++) {
Node* input = node->InputAt(i);
entry.input_index = i + 1;
if (input != node && Recurse(input)) return;
}
// Remember the node count before reduction.
const int node_count = graph()->NodeCount();
// All inputs should be visited or on stack. Apply reductions to node.
Reduction reduction = Reduce(node);
// If there was no reduction, pop {node} and continue.
if (!reduction.Changed()) return Pop();
// Check if the reduction is an in-place update of the {node}.
Node* const replacement = reduction.replacement();
if (replacement == node) {
// In-place update of {node}, may need to recurse on an input.
for (int i = 0; i < node->InputCount(); ++i) {
Node* input = node->InputAt(i);
entry.input_index = i + 1;
if (input != node && Recurse(input)) return;
}
}
// After reducing the node, pop it off the stack.
Pop();
// Revisit all uses of the node.
for (Node* const use : node->uses()) {
// Don't revisit this node if it refers to itself.
if (use != node) Revisit(use);
}
// Check if we have a new replacement.
if (replacement != node) {
if (node == graph()->start()) graph()->SetStart(replacement);
if (node == graph()->end()) graph()->SetEnd(replacement);
// If {node} was replaced by an old node, unlink {node} and assume that
// {replacement} was already reduced and finish.
if (replacement->id() < node_count) {
node->ReplaceUses(replacement);
node->Kill();
} else {
// Otherwise {node} was replaced by a new node. Replace all old uses of
// {node} with {replacement}. New nodes created by this reduction can
// use {node}.
node->ReplaceUsesIf(
[node_count](Node* const node) { return node->id() < node_count; },
replacement);
// Unlink {node} if it's no longer used.
if (node->uses().empty()) {
node->Kill();
}
// If there was a replacement, reduce it after popping {node}.
Recurse(replacement);
}
}
}
void GraphReducer::Pop() {
Node* node = stack_.top().node;
state_.Set(node, State::kVisited);
stack_.pop();
}
void GraphReducer::Push(Node* const node) {
DCHECK(state_.Get(node) != State::kOnStack);
state_.Set(node, State::kOnStack);
stack_.push({node, 0});
}
bool GraphReducer::Recurse(Node* node) {
if (state_.Get(node) > State::kRevisit) return false;
Push(node);
return true;
}
void GraphReducer::Revisit(Node* node) {
if (state_.Get(node) == State::kVisited) {
state_.Set(node, State::kRevisit);
revisit_.push(node);
}
}
} // namespace compiler
} // namespace internal
} // namespace v8
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