X-Git-Url: http://plrg.eecs.uci.edu/git/?p=model-checker.git;a=blobdiff_plain;f=cyclegraph.cc;h=49ff00249fe9c49dbe9a1cda9fdad32af78c0f70;hp=2bfe76ac424c274bf8152638543ad7dd6bc1d31f;hb=0330918c2798a36363a197bf500a5646b048a687;hpb=0170878f8a8be6aa06af6591e50fffdb2ce54022

diff --git a/cyclegraph.cc b/cyclegraph.cc
index 2bfe76a..49ff002 100644
--- a/cyclegraph.cc
+++ b/cyclegraph.cc
@@ -1,99 +1,266 @@
 #include "cyclegraph.h"
 #include "action.h"
 #include "common.h"
+#include "promise.h"
+#include "model.h"
+#include "threads-model.h"
 
 /** Initializes a CycleGraph object. */
 CycleGraph::CycleGraph() :
+	discovered(new HashTable<const CycleNode *, const CycleNode *, uintptr_t, 4, model_malloc, model_calloc, model_free>(16)),
 	hasCycles(false),
-	oldCycles(false),
-	hasRMWViolation(false),
-	oldRMWViolation(false)
+	oldCycles(false)
 {
 }
 
 /** CycleGraph destructor */
-CycleGraph::~CycleGraph() {
+CycleGraph::~CycleGraph()
+{
+	delete discovered;
+}
+
+/**
+ * Add a CycleNode to the graph, corresponding to a store ModelAction
+ * @param act The write action that should be added
+ * @param node The CycleNode that corresponds to the store
+ */
+void CycleGraph::putNode(const ModelAction *act, CycleNode *node)
+{
+	actionToNode.put(act, node);
+#if SUPPORT_MOD_ORDER_DUMP
+	nodeList.push_back(node);
+#endif
+}
+
+/**
+ * Add a CycleNode to the graph, corresponding to a Promise
+ * @param promise The Promise that should be added
+ * @param node The CycleNode that corresponds to the Promise
+ */
+void CycleGraph::putNode(const Promise *promise, CycleNode *node)
+{
+	promiseToNode.put(promise, node);
+#if SUPPORT_MOD_ORDER_DUMP
+	nodeList.push_back(node);
+#endif
+}
+
+/**
+ * @brief Remove the Promise node from the graph
+ * @param promise The promise to remove from the graph
+ */
+void CycleGraph::erasePromiseNode(const Promise *promise)
+{
+	promiseToNode.put(promise, NULL);
+#if SUPPORT_MOD_ORDER_DUMP
+	/* Remove the promise node from nodeList */
+	CycleNode *node = getNode_noCreate(promise);
+	for (unsigned int i = 0; i < nodeList.size(); )
+		if (nodeList[i] == node)
+			nodeList.erase(nodeList.begin() + i);
+		else
+			i++;
+#endif
+}
+
+/** @return The corresponding CycleNode, if exists; otherwise NULL */
+CycleNode * CycleGraph::getNode_noCreate(const ModelAction *act) const
+{
+	return actionToNode.get(act);
+}
+
+/** @return The corresponding CycleNode, if exists; otherwise NULL */
+CycleNode * CycleGraph::getNode_noCreate(const Promise *promise) const
+{
+	return promiseToNode.get(promise);
 }
 
 /**
  * @brief Returns the CycleNode corresponding to a given ModelAction
+ *
+ * Gets (or creates, if none exist) a CycleNode corresponding to a ModelAction
+ *
  * @param action The ModelAction to find a node for
  * @return The CycleNode paired with this action
  */
-CycleNode * CycleGraph::getNode(const ModelAction *action) {
-	CycleNode *node=actionToNode.get(action);
-	if (node==NULL) {
-		node=new CycleNode(action);
-		actionToNode.put(action, node);
-#if SUPPORT_MOD_ORDER_DUMP
-		nodeList.push_back(node);
-#endif
+CycleNode * CycleGraph::getNode(const ModelAction *action)
+{
+	CycleNode *node = getNode_noCreate(action);
+	if (node == NULL) {
+		node = new CycleNode(action);
+		putNode(action, node);
 	}
 	return node;
 }
 
 /**
- * Adds an edge between two ModelActions. The ModelAction @a to is ordered
- * after the ModelAction @a from.
- * @param to The edge points to this ModelAction
- * @param from The edge comes from this ModelAction
+ * @brief Returns a CycleNode corresponding to a promise
+ *
+ * Gets (or creates, if none exist) a CycleNode corresponding to a promised
+ * value.
+ *
+ * @param promise The Promise generated by a reader
+ * @return The CycleNode corresponding to the Promise
  */
-void CycleGraph::addEdge(const ModelAction *from, const ModelAction *to) {
-	ASSERT(from);
-	ASSERT(to);
-	ASSERT(from != to);
+CycleNode * CycleGraph::getNode(const Promise *promise)
+{
+	CycleNode *node = getNode_noCreate(promise);
+	if (node == NULL) {
+		node = new CycleNode(promise);
+		putNode(promise, node);
+	}
+	return node;
+}
+
+/**
+ * @return false if the resolution results in a cycle; true otherwise
+ */
+bool CycleGraph::resolvePromise(const Promise *promise, ModelAction *writer,
+		promise_list_t *mustResolve)
+{
+	CycleNode *promise_node = promiseToNode.get(promise);
+	CycleNode *w_node = actionToNode.get(writer);
+	ASSERT(promise_node);
+
+	if (w_node)
+		return mergeNodes(w_node, promise_node, mustResolve);
+	/* No existing write-node; just convert the promise-node */
+	promise_node->resolvePromise(writer);
+	erasePromiseNode(promise_node->getPromise());
+	putNode(writer, promise_node);
+	return true;
+}
 
-	CycleNode *fromnode=getNode(from);
-	CycleNode *tonode=getNode(to);
+/**
+ * @brief Merge two CycleNodes that represent the same write
+ *
+ * Note that this operation cannot be rolled back.
+ *
+ * @param w_node The write ModelAction node with which to merge
+ * @param p_node The Promise node to merge. Will be destroyed after this
+ * function.
+ * @param mustMerge Return (pass-by-reference) any additional Promises that
+ * must also be merged with w_node
+ *
+ * @return false if the merge results in a cycle; true otherwise
+ */
+bool CycleGraph::mergeNodes(CycleNode *w_node, CycleNode *p_node,
+		promise_list_t *mustMerge)
+{
+	ASSERT(!w_node->is_promise());
+	ASSERT(p_node->is_promise());
 
-	if (!hasCycles) {
-		// Check for Cycles
-		hasCycles=checkReachable(tonode, fromnode);
+	const Promise *promise = p_node->getPromise();
+	if (!promise->is_compatible(w_node->getAction())) {
+		hasCycles = true;
+		return false;
 	}
 
-	if (fromnode->addEdge(tonode))
-		rollbackvector.push_back(fromnode);
+	/* Transfer the RMW */
+	CycleNode *promise_rmw = p_node->getRMW();
+	if (promise_rmw && promise_rmw != w_node->getRMW() && w_node->setRMW(promise_rmw)) {
+		hasCycles = true;
+		return false;
+	}
 
+	/* Transfer back edges to w_node */
+	while (p_node->getNumBackEdges() > 0) {
+		CycleNode *back = p_node->removeBackEdge();
+		if (back == w_node)
+			continue;
+		if (back->is_promise()) {
+			if (checkReachable(w_node, back)) {
+				/* Edge would create cycle; merge instead */
+				mustMerge->push_back(back->getPromise());
+				if (!mergeNodes(w_node, back, mustMerge))
+					return false;
+			} else
+				back->addEdge(w_node);
+		} else
+			addNodeEdge(back, w_node);
+	}
 
-	CycleNode * rmwnode=fromnode->getRMW();
+	/* Transfer forward edges to w_node */
+	while (p_node->getNumEdges() > 0) {
+		CycleNode *forward = p_node->removeEdge();
+		if (forward == w_node)
+			continue;
+		if (forward->is_promise()) {
+			if (checkReachable(forward, w_node)) {
+				mustMerge->push_back(forward->getPromise());
+				if (!mergeNodes(w_node, forward, mustMerge))
+					return false;
+			} else
+				w_node->addEdge(forward);
+		} else
+			addNodeEdge(w_node, forward);
+	}
 
-	//If the fromnode has a rmwnode that is not the tonode, we
-	//should add an edge between its rmwnode and the tonode
+	erasePromiseNode(promise);
+	/* Not deleting p_node, to maintain consistency if mergeNodes() fails */
 
-	//If tonode is also a rmw, don't do this check as the execution is
-	//doomed and we'll catch the problem elsewhere, but we want to allow
-	//for the possibility of sending to's write value to rmwnode
+	return !hasCycles;
+}
 
-	if (rmwnode!=NULL&&!to->is_rmw()) {
-		if (!hasCycles) {
-			// Check for Cycles
-			hasCycles=checkReachable(tonode, rmwnode);
-		}
+/**
+ * Adds an edge between two CycleNodes.
+ * @param fromnode The edge comes from this CycleNode
+ * @param tonode The edge points to this CycleNode
+ * @return True, if new edge(s) are added; otherwise false
+ */
+bool CycleGraph::addNodeEdge(CycleNode *fromnode, CycleNode *tonode)
+{
+	bool added;
+
+	if (!hasCycles)
+		hasCycles = checkReachable(tonode, fromnode);
+
+	if ((added = fromnode->addEdge(tonode)))
+		rollbackvector.push_back(fromnode);
 
-		if (rmwnode->addEdge(tonode))
+	/*
+	 * If the fromnode has a rmwnode that is not the tonode, we should add
+	 * an edge between its rmwnode and the tonode
+	 */
+	CycleNode *rmwnode = fromnode->getRMW();
+	if (rmwnode && rmwnode != tonode) {
+		if (!hasCycles)
+			hasCycles = checkReachable(tonode, rmwnode);
+
+		if (rmwnode->addEdge(tonode)) {
 			rollbackvector.push_back(rmwnode);
+			added = true;
+		}
 	}
+	return added;
 }
 
-/** Handles special case of a RMW action.  The ModelAction rmw reads
- *  from the ModelAction from.  The key differences are: (1) no write
- *  can occur in between the rmw and the from action.  Only one RMW
- *  action can read from a given write.
+/**
+ * @brief Add an edge between a write and the RMW which reads from it
+ *
+ * Handles special case of a RMW action, where the ModelAction rmw reads from
+ * the ModelAction/Promise from. The key differences are:
+ * (1) no write can occur in between the rmw and the from action.
+ * (2) Only one RMW action can read from a given write.
+ *
+ * @param from The edge comes from this ModelAction/Promise
+ * @param rmw The edge points to this ModelAction; this action must read from
+ * the ModelAction/Promise from
  */
-void CycleGraph::addRMWEdge(const ModelAction *from, const ModelAction *rmw) {
+template <typename T>
+void CycleGraph::addRMWEdge(const T *from, const ModelAction *rmw)
+{
 	ASSERT(from);
 	ASSERT(rmw);
-	ASSERT(from != rmw);
 
-	CycleNode *fromnode=getNode(from);
-	CycleNode *rmwnode=getNode(rmw);
+	CycleNode *fromnode = getNode(from);
+	CycleNode *rmwnode = getNode(rmw);
 
 	/* Two RMW actions cannot read from the same write. */
-	if (fromnode->setRMW(rmwnode)) {
-		hasRMWViolation=true;
-	} else {
+	if (fromnode->setRMW(rmwnode))
+		hasCycles = true;
+	else
 		rmwrollbackvector.push_back(fromnode);
-	}
 
 	/* Transfer all outgoing edges from the from node to the rmw node */
 	/* This process should not add a cycle because either:
@@ -102,88 +269,190 @@ void CycleGraph::addRMWEdge(const ModelAction *from, const ModelAction *rmw) {
 	 * (2) the fromnode is the new node and therefore it should not
 	 * have any outgoing edges.
 	 */
-	std::vector<CycleNode *> * edges=fromnode->getEdges();
-	for(unsigned int i=0;i<edges->size();i++) {
-		CycleNode * tonode=(*edges)[i];
-		if (tonode!=rmwnode) {
+	for (unsigned int i = 0; i < fromnode->getNumEdges(); i++) {
+		CycleNode *tonode = fromnode->getEdge(i);
+		if (tonode != rmwnode) {
 			if (rmwnode->addEdge(tonode))
 				rollbackvector.push_back(rmwnode);
 		}
 	}
 
+	addNodeEdge(fromnode, rmwnode);
+}
+/* Instantiate two forms of CycleGraph::addRMWEdge */
+template void CycleGraph::addRMWEdge(const ModelAction *from, const ModelAction *rmw);
+template void CycleGraph::addRMWEdge(const Promise *from, const ModelAction *rmw);
 
-	if (!hasCycles) {
-		// With promises we could be setting up a cycle here if we aren't
-		// careful...avoid it..
-		hasCycles=checkReachable(rmwnode, fromnode);
-	}
-	if(fromnode->addEdge(rmwnode))
-		rollbackvector.push_back(fromnode);
+/**
+ * @brief Adds an edge between objects
+ *
+ * This function will add an edge between any two objects which can be
+ * associated with a CycleNode. That is, if they have a CycleGraph::getNode
+ * implementation.
+ *
+ * The object to is ordered after the object from.
+ *
+ * @param to The edge points to this object, of type T
+ * @param from The edge comes from this object, of type U
+ * @return True, if new edge(s) are added; otherwise false
+ */
+template <typename T, typename U>
+bool CycleGraph::addEdge(const T *from, const U *to)
+{
+	ASSERT(from);
+	ASSERT(to);
+
+	CycleNode *fromnode = getNode(from);
+	CycleNode *tonode = getNode(to);
+
+	return addNodeEdge(fromnode, tonode);
 }
+/* Instantiate four forms of CycleGraph::addEdge */
+template bool CycleGraph::addEdge(const ModelAction *from, const ModelAction *to);
+template bool CycleGraph::addEdge(const ModelAction *from, const Promise *to);
+template bool CycleGraph::addEdge(const Promise *from, const ModelAction *to);
+template bool CycleGraph::addEdge(const Promise *from, const Promise *to);
 
 #if SUPPORT_MOD_ORDER_DUMP
-void CycleGraph::dumpGraphToFile(const char *filename) {
-	char buffer[200];
-  sprintf(buffer, "%s.dot",filename);
-  FILE *file=fopen(buffer, "w");
-  fprintf(file, "digraph %s {\n",filename);
-  for(unsigned int i=0;i<nodeList.size();i++) {
-		CycleNode *cn=nodeList[i];
-		std::vector<CycleNode *> * edges=cn->getEdges();
-		const ModelAction *action=cn->getAction();
-		fprintf(file, "N%u [label=\"%u, T%u\"];\n",action->get_seq_number(),action->get_seq_number(), action->get_tid());
-		if (cn->getRMW()!=NULL) {
-			fprintf(file, "N%u -> N%u[style=dotted];\n", action->get_seq_number(), cn->getRMW()->getAction()->get_seq_number());
+
+static void print_node(FILE *file, const CycleNode *node, int label)
+{
+	if (node->is_promise()) {
+		const Promise *promise = node->getPromise();
+		int idx = model->get_promise_number(promise);
+		fprintf(file, "P%d", idx);
+		if (label) {
+			int first = 1;
+			fprintf(file, " [label=\"P%d, T", idx);
+			for (unsigned int i = 0 ; i < model->get_num_threads(); i++)
+				if (promise->thread_is_available(int_to_id(i))) {
+					fprintf(file, "%s%u", first ? "": ",", i);
+					first = 0;
+				}
+			fprintf(file, "\"]");
 		}
-		for(unsigned int j=0;j<edges->size();j++) {
-		  CycleNode *dst=(*edges)[j];
-			const ModelAction *dstaction=dst->getAction();
-      fprintf(file, "N%u -> N%u;\n", action->get_seq_number(), dstaction->get_seq_number());
-	  }
+	} else {
+		const ModelAction *act = node->getAction();
+		modelclock_t idx = act->get_seq_number();
+		fprintf(file, "N%u", idx);
+		if (label)
+			fprintf(file, " [label=\"N%u, T%u\"]", idx, act->get_tid());
 	}
-  fprintf(file,"}\n");
-  fclose(file);	
+}
+
+static void print_edge(FILE *file, const CycleNode *from, const CycleNode *to, const char *prop)
+{
+	print_node(file, from, 0);
+	fprintf(file, " -> ");
+	print_node(file, to, 0);
+	if (prop && strlen(prop))
+		fprintf(file, " [%s]", prop);
+	fprintf(file, ";\n");
+}
+
+void CycleGraph::dumpNodes(FILE *file) const
+{
+	for (unsigned int i = 0; i < nodeList.size(); i++) {
+		CycleNode *n = nodeList[i];
+		print_node(file, n, 1);
+		fprintf(file, ";\n");
+		if (n->getRMW())
+			print_edge(file, n, n->getRMW(), "style=dotted");
+		for (unsigned int j = 0; j < n->getNumEdges(); j++)
+			print_edge(file, n, n->getEdge(j), NULL);
+	}
+}
+
+void CycleGraph::dumpGraphToFile(const char *filename) const
+{
+	char buffer[200];
+	sprintf(buffer, "%s.dot", filename);
+	FILE *file = fopen(buffer, "w");
+	fprintf(file, "digraph %s {\n", filename);
+	dumpNodes(file);
+	fprintf(file, "}\n");
+	fclose(file);
 }
 #endif
 
 /**
- * Checks whether one ModelAction can reach another.
- * @param from The ModelAction from which to begin exploration
- * @param to The ModelAction to reach
+ * Checks whether one CycleNode can reach another.
+ * @param from The CycleNode from which to begin exploration
+ * @param to The CycleNode to reach
  * @return True, @a from can reach @a to; otherwise, false
  */
-bool CycleGraph::checkReachable(const ModelAction *from, const ModelAction *to) {
-	CycleNode *fromnode = actionToNode.get(from);
-	CycleNode *tonode = actionToNode.get(to);
+bool CycleGraph::checkReachable(const CycleNode *from, const CycleNode *to) const
+{
+	std::vector< const CycleNode *, ModelAlloc<const CycleNode *> > queue;
+	discovered->reset();
 
-	if (!fromnode || !tonode)
-		return false;
+	queue.push_back(from);
+	discovered->put(from, from);
+	while (!queue.empty()) {
+		const CycleNode *node = queue.back();
+		queue.pop_back();
+		if (node == to)
+			return true;
 
-	return checkReachable(fromnode, tonode);
+		for (unsigned int i = 0; i < node->getNumEdges(); i++) {
+			CycleNode *next = node->getEdge(i);
+			if (!discovered->contains(next)) {
+				discovered->put(next, next);
+				queue.push_back(next);
+			}
+		}
+	}
+	return false;
 }
 
 /**
- * Checks whether one CycleNode can reach another.
- * @param from The CycleNode from which to begin exploration
- * @param to The CycleNode to reach
+ * Checks whether one ModelAction/Promise can reach another ModelAction/Promise
+ * @param from The ModelAction or Promise from which to begin exploration
+ * @param to The ModelAction or Promise to reach
  * @return True, @a from can reach @a to; otherwise, false
  */
-bool CycleGraph::checkReachable(CycleNode *from, CycleNode *to) {
-	std::vector<CycleNode *, MyAlloc<CycleNode *> > queue;
-	HashTable<CycleNode *, CycleNode *, uintptr_t, 4, MYMALLOC, MYCALLOC, MYFREE> discovered;
+template <typename T, typename U>
+bool CycleGraph::checkReachable(const T *from, const U *to) const
+{
+	CycleNode *fromnode = getNode_noCreate(from);
+	CycleNode *tonode = getNode_noCreate(to);
+
+	if (!fromnode || !tonode)
+		return false;
+
+	return checkReachable(fromnode, tonode);
+}
+/* Instantiate three forms of CycleGraph::checkReachable */
+template bool CycleGraph::checkReachable(const ModelAction *from,
+		const ModelAction *to) const;
+template bool CycleGraph::checkReachable(const ModelAction *from,
+		const Promise *to) const;
+template bool CycleGraph::checkReachable(const Promise *from,
+		const ModelAction *to) const;
+
+/** @return True, if the promise has failed; false otherwise */
+bool CycleGraph::checkPromise(const ModelAction *fromact, Promise *promise) const
+{
+	std::vector< CycleNode *, ModelAlloc<CycleNode *> > queue;
+	discovered->reset();
+	CycleNode *from = actionToNode.get(fromact);
 
 	queue.push_back(from);
-	discovered.put(from, from);
-	while(!queue.empty()) {
-		CycleNode * node=queue.back();
+	discovered->put(from, from);
+	while (!queue.empty()) {
+		CycleNode *node = queue.back();
 		queue.pop_back();
-		if (node==to)
+
+		if (node->getPromise() == promise)
+			return true;
+		if (!node->is_promise() &&
+				promise->eliminate_thread(node->getAction()->get_tid()))
 			return true;
 
-		for(unsigned int i=0;i<node->getEdges()->size();i++) {
-			CycleNode *next=(*node->getEdges())[i];
-			if (!discovered.contains(next)) {
-				discovered.put(next,next);
+		for (unsigned int i = 0; i < node->getNumEdges(); i++) {
+			CycleNode *next = node->getEdge(i);
+			if (!discovered->contains(next)) {
+				discovered->put(next, next);
 				queue.push_back(next);
 			}
 		}
@@ -191,75 +460,154 @@ bool CycleGraph::checkReachable(CycleNode *from, CycleNode *to) {
 	return false;
 }
 
-void CycleGraph::startChanges() {
-	ASSERT(rollbackvector.size()==0);
-	ASSERT(rmwrollbackvector.size()==0);
-	ASSERT(oldCycles==hasCycles);
-	ASSERT(oldRMWViolation==hasRMWViolation);
+void CycleGraph::startChanges()
+{
+	ASSERT(rollbackvector.empty());
+	ASSERT(rmwrollbackvector.empty());
+	ASSERT(oldCycles == hasCycles);
 }
 
 /** Commit changes to the cyclegraph. */
-void CycleGraph::commitChanges() {
-	rollbackvector.resize(0);
-	rmwrollbackvector.resize(0);
-	oldCycles=hasCycles;
-	oldRMWViolation=hasRMWViolation;
+void CycleGraph::commitChanges()
+{
+	rollbackvector.clear();
+	rmwrollbackvector.clear();
+	oldCycles = hasCycles;
 }
 
 /** Rollback changes to the previous commit. */
-void CycleGraph::rollbackChanges() {
-	for (unsigned int i = 0; i < rollbackvector.size(); i++) {
-		rollbackvector[i]->popEdge();
-	}
+void CycleGraph::rollbackChanges()
+{
+	for (unsigned int i = 0; i < rollbackvector.size(); i++)
+		rollbackvector[i]->removeEdge();
 
-	for (unsigned int i = 0; i < rmwrollbackvector.size(); i++) {
+	for (unsigned int i = 0; i < rmwrollbackvector.size(); i++)
 		rmwrollbackvector[i]->clearRMW();
-	}
 
 	hasCycles = oldCycles;
-	hasRMWViolation = oldRMWViolation;
-	rollbackvector.resize(0);
-	rmwrollbackvector.resize(0);
+	rollbackvector.clear();
+	rmwrollbackvector.clear();
 }
 
 /** @returns whether a CycleGraph contains cycles. */
-bool CycleGraph::checkForCycles() {
+bool CycleGraph::checkForCycles() const
+{
 	return hasCycles;
 }
 
-bool CycleGraph::checkForRMWViolation() {
-	return hasRMWViolation;
+/**
+ * @brief Constructor for a CycleNode
+ * @param act The ModelAction for this node
+ */
+CycleNode::CycleNode(const ModelAction *act) :
+	action(act),
+	promise(NULL),
+	hasRMW(NULL)
+{
 }
 
 /**
- * Constructor for a CycleNode.
- * @param modelaction The ModelAction for this node
+ * @brief Constructor for a Promise CycleNode
+ * @param promise The Promise which was generated
  */
-CycleNode::CycleNode(const ModelAction *modelaction) :
-	action(modelaction),
+CycleNode::CycleNode(const Promise *promise) :
+	action(NULL),
+	promise(promise),
 	hasRMW(NULL)
 {
 }
 
-/** @returns a vector of the edges from a CycleNode. */
-std::vector<CycleNode *> * CycleNode::getEdges() {
-	return &edges;
+/**
+ * @param i The index of the edge to return
+ * @returns The a CycleNode edge indexed by i
+ */
+CycleNode * CycleNode::getEdge(unsigned int i) const
+{
+	return edges[i];
+}
+
+/** @returns The number of edges leaving this CycleNode */
+unsigned int CycleNode::getNumEdges() const
+{
+	return edges.size();
+}
+
+CycleNode * CycleNode::getBackEdge(unsigned int i) const
+{
+	return back_edges[i];
+}
+
+unsigned int CycleNode::getNumBackEdges() const
+{
+	return back_edges.size();
+}
+
+/**
+ * @brief Remove an element from a vector
+ * @param v The vector
+ * @param n The element to remove
+ * @return True if the element was found; false otherwise
+ */
+template <typename T>
+static bool vector_remove_node(std::vector<T, SnapshotAlloc<T> >& v, const T n)
+{
+	for (unsigned int i = 0; i < v.size(); i++) {
+		if (v[i] == n) {
+			v.erase(v.begin() + i);
+			return true;
+		}
+	}
+	return false;
+}
+
+/**
+ * @brief Remove a (forward) edge from this CycleNode
+ * @return The CycleNode which was popped, if one exists; otherwise NULL
+ */
+CycleNode * CycleNode::removeEdge()
+{
+	if (edges.empty())
+		return NULL;
+
+	CycleNode *ret = edges.back();
+	edges.pop_back();
+	vector_remove_node(ret->back_edges, this);
+	return ret;
+}
+
+/**
+ * @brief Remove a (back) edge from this CycleNode
+ * @return The CycleNode which was popped, if one exists; otherwise NULL
+ */
+CycleNode * CycleNode::removeBackEdge()
+{
+	if (back_edges.empty())
+		return NULL;
+
+	CycleNode *ret = back_edges.back();
+	back_edges.pop_back();
+	vector_remove_node(ret->edges, this);
+	return ret;
 }
 
 /**
  * Adds an edge from this CycleNode to another CycleNode.
  * @param node The node to which we add a directed edge
+ * @return True if this edge is a new edge; false otherwise
  */
-bool CycleNode::addEdge(CycleNode *node) {
-	for(unsigned int i=0;i<edges.size();i++)
-		if (edges[i]==node)
+bool CycleNode::addEdge(CycleNode *node)
+{
+	for (unsigned int i = 0; i < edges.size(); i++)
+		if (edges[i] == node)
 			return false;
 	edges.push_back(node);
+	node->back_edges.push_back(this);
 	return true;
 }
 
 /** @returns the RMW CycleNode that reads from the current CycleNode */
-CycleNode * CycleNode::getRMW() {
+CycleNode * CycleNode::getRMW() const
+{
 	return hasRMW;
 }
 
@@ -269,9 +617,26 @@ CycleNode * CycleNode::getRMW() {
  * @return True, if this node already was read by another RMW; false otherwise
  * @see CycleGraph::addRMWEdge
  */
-bool CycleNode::setRMW(CycleNode *node) {
-	if (hasRMW!=NULL)
+bool CycleNode::setRMW(CycleNode *node)
+{
+	if (hasRMW != NULL)
 		return true;
-	hasRMW=node;
+	hasRMW = node;
 	return false;
 }
+
+/**
+ * Convert a Promise CycleNode into a concrete-valued CycleNode. Should only be
+ * used when there's no existing ModelAction CycleNode for this write.
+ *
+ * @param writer The ModelAction which wrote the future value represented by
+ * this CycleNode
+ */
+void CycleNode::resolvePromise(const ModelAction *writer)
+{
+	ASSERT(is_promise());
+	ASSERT(promise->is_compatible(writer));
+	action = writer;
+	promise = NULL;
+	ASSERT(!is_promise());
+}