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/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*-
* vim: set ts=8 sts=2 et sw=2 tw=80:
* This Source Code Form is subject to the terms of the Mozilla Public
* License, v. 2.0. If a copy of the MPL was not distributed with this
* file, You can obtain one at http://mozilla.org/MPL/2.0/. */
#ifndef gc_FindSCCs_h
#define gc_FindSCCs_h
#include "mozilla/Assertions.h" // MOZ_ASSERT
#include <algorithm> // std::min
#include <stdint.h> // uintptr_t
#include "js/AllocPolicy.h" // js::SystemAllocPolicy
#include "js/friend/StackLimits.h" // JS_CHECK_STACK_SIZE
#include "js/HashTable.h" // js::HashSet, js::DefaultHasher
namespace js {
namespace gc {
template <typename Node>
struct GraphNodeBase {
using NodeSet =
js::HashSet<Node*, js::DefaultHasher<Node*>, js::SystemAllocPolicy>;
NodeSet gcGraphEdges;
Node* gcNextGraphNode = nullptr;
Node* gcNextGraphComponent = nullptr;
unsigned gcDiscoveryTime = 0;
unsigned gcLowLink = 0;
Node* nextNodeInGroup() const {
if (gcNextGraphNode &&
gcNextGraphNode->gcNextGraphComponent == gcNextGraphComponent) {
return gcNextGraphNode;
}
return nullptr;
}
Node* nextGroup() const { return gcNextGraphComponent; }
};
/*
* Find the strongly connected components of a graph using Tarjan's algorithm,
* and return them in topological order.
*
* Nodes derive from GraphNodeBase and add target edge pointers to
* sourceNode.gcGraphEdges to describe the graph:
*
* struct MyGraphNode : public GraphNodeBase<MyGraphNode>
* {
* ...
* }
*
* MyGraphNode node1, node2, node3;
* node1.gcGraphEdges.put(node2); // Error checking elided.
* node2.gcGraphEdges.put(node3);
* node3.gcGraphEdges.put(node2);
*
* ComponentFinder<MyGraphNode> finder;
* finder.addNode(node1);
* finder.addNode(node2);
* finder.addNode(node3);
* MyGraphNode* result = finder.getResultsList();
*/
template <typename Node>
class ComponentFinder {
public:
explicit ComponentFinder(uintptr_t sl) : stackLimit(sl) {}
~ComponentFinder() {
MOZ_ASSERT(!stack);
MOZ_ASSERT(!firstComponent);
}
/* Forces all nodes to be added to a single component. */
void useOneComponent() { stackFull = true; }
void addNode(Node* v) {
if (v->gcDiscoveryTime == Undefined) {
MOZ_ASSERT(v->gcLowLink == Undefined);
processNode(v);
}
}
Node* getResultsList() {
if (stackFull) {
/*
* All nodes after the stack overflow are in |stack|. Put them all in
* one big component of their own.
*/
Node* firstGoodComponent = firstComponent;
for (Node* v = stack; v; v = stack) {
stack = v->gcNextGraphNode;
v->gcNextGraphComponent = firstGoodComponent;
v->gcNextGraphNode = firstComponent;
firstComponent = v;
}
stackFull = false;
}
MOZ_ASSERT(!stack);
Node* result = firstComponent;
firstComponent = nullptr;
for (Node* v = result; v; v = v->gcNextGraphNode) {
v->gcDiscoveryTime = Undefined;
v->gcLowLink = Undefined;
}
return result;
}
static void mergeGroups(Node* first) {
for (Node* v = first; v; v = v->gcNextGraphNode) {
v->gcNextGraphComponent = nullptr;
}
}
private:
// Constant used to indicate an unprocessed vertex.
static const unsigned Undefined = 0;
// Constant used to indicate a processed vertex that is no longer on the
// stack.
static const unsigned Finished = (unsigned)-1;
void addEdgeTo(Node* w) {
if (w->gcDiscoveryTime == Undefined) {
processNode(w);
cur->gcLowLink = std::min(cur->gcLowLink, w->gcLowLink);
} else if (w->gcDiscoveryTime != Finished) {
cur->gcLowLink = std::min(cur->gcLowLink, w->gcDiscoveryTime);
}
}
void processNode(Node* v) {
v->gcDiscoveryTime = clock;
v->gcLowLink = clock;
++clock;
v->gcNextGraphNode = stack;
stack = v;
int stackDummy;
if (stackFull || !JS_CHECK_STACK_SIZE(stackLimit, &stackDummy)) {
stackFull = true;
return;
}
Node* old = cur;
cur = v;
for (auto r = cur->gcGraphEdges.all(); !r.empty(); r.popFront()) {
addEdgeTo(r.front());
}
cur = old;
if (stackFull) {
return;
}
if (v->gcLowLink == v->gcDiscoveryTime) {
Node* nextComponent = firstComponent;
Node* w;
do {
MOZ_ASSERT(stack);
w = stack;
stack = w->gcNextGraphNode;
/*
* Record that the element is no longer on the stack by setting the
* discovery time to a special value that's not Undefined.
*/
w->gcDiscoveryTime = Finished;
/* Figure out which group we're in. */
w->gcNextGraphComponent = nextComponent;
/*
* Prepend the component to the beginning of the output list to
* reverse the list and achieve the desired order.
*/
w->gcNextGraphNode = firstComponent;
firstComponent = w;
} while (w != v);
}
}
private:
unsigned clock = 1;
Node* stack = nullptr;
Node* firstComponent = nullptr;
Node* cur = nullptr;
uintptr_t stackLimit;
bool stackFull = false;
};
} /* namespace gc */
} /* namespace js */
#endif /* gc_FindSCCs_h */