X-Git-Url: http://plrg.eecs.uci.edu/git/?p=model-checker.git;a=blobdiff_plain;f=nodestack.cc;h=00cc30f8d7efb22cf9e5b110d4628ac83db55e0f;hp=47f724d856652a501805241df58e939121429e43;hb=9eee54de6cd97a5a58f3185de0c4635123d398b2;hpb=061b34a5d84d00df340550d75f664be5e174276c

diff --git a/nodestack.cc b/nodestack.cc
index 47f724d..00cc30f 100644
--- a/nodestack.cc
+++ b/nodestack.cc
@@ -1,7 +1,14 @@
+#define __STDC_FORMAT_MACROS
+#include <inttypes.h>
+
+#include <string.h>
+
 #include "nodestack.h"
 #include "action.h"
 #include "common.h"
 #include "model.h"
+#include "threads-model.h"
+#include "modeltypes.h"
 
 /**
  * @brief Node constructor
@@ -17,80 +24,144 @@
  * @param nthreads The number of threads which exist at this point in the
  * execution trace.
  */
-Node::Node(ModelAction *act, Node *par, int nthreads)
-	: action(act),
+Node::Node(ModelAction *act, Node *par, int nthreads, Node *prevfairness) :
+	read_from_status(READ_FROM_PAST),
+	action(act),
 	parent(par),
 	num_threads(nthreads),
 	explored_children(num_threads),
 	backtrack(num_threads),
+	fairness(num_threads),
 	numBacktracks(0),
-	may_read_from(),
-	read_from_index(0),
+	enabled_array(NULL),
+	read_from_past(),
+	read_from_past_idx(0),
+	read_from_promises(),
+	read_from_promise_idx(-1),
 	future_values(),
-	future_index(-1)
+	future_index(-1),
+	resolve_promise(),
+	resolve_promise_idx(-1),
+	relseq_break_writes(),
+	relseq_break_index(0),
+	misc_index(0),
+	misc_max(0)
 {
-	if (act)
-		act->set_node(this);
+	ASSERT(act);
+	act->set_node(this);
+	int currtid = id_to_int(act->get_tid());
+	int prevtid = prevfairness ? id_to_int(prevfairness->action->get_tid()) : 0;
+
+	if (model->params.fairwindow != 0) {
+		for (int i = 0; i < num_threads; i++) {
+			ASSERT(i < ((int)fairness.size()));
+			struct fairness_info *fi = &fairness[i];
+			struct fairness_info *prevfi = (parent && i < parent->get_num_threads()) ? &parent->fairness[i] : NULL;
+			if (prevfi) {
+				*fi = *prevfi;
+			}
+			if (parent && parent->is_enabled(int_to_id(i))) {
+				fi->enabled_count++;
+			}
+			if (i == currtid) {
+				fi->turns++;
+				fi->priority = false;
+			}
+			/* Do window processing */
+			if (prevfairness != NULL) {
+				if (prevfairness->parent->is_enabled(int_to_id(i)))
+					fi->enabled_count--;
+				if (i == prevtid) {
+					fi->turns--;
+				}
+				/* Need full window to start evaluating
+				 * conditions
+				 * If we meet the enabled count and have no
+				 * turns, give us priority */
+				if ((fi->enabled_count >= model->params.enabledcount) &&
+						(fi->turns == 0))
+					fi->priority = true;
+			}
+		}
+	}
 }
 
 /** @brief Node desctructor */
 Node::~Node()
 {
-	if (action)
-		delete action;
+	delete action;
+	if (enabled_array)
+		model_free(enabled_array);
 }
 
 /** Prints debugging info for the ModelAction associated with this Node */
-void Node::print()
+void Node::print() const
 {
-	if (action)
-		action->print();
-	else
-		printf("******** empty action ********\n");
-}
+	action->print();
+	model_print("          backtrack: %s", backtrack_empty() ? "empty" : "non-empty ");
+	for (int i = 0; i < (int)backtrack.size(); i++)
+		if (backtrack[i] == true)
+			model_print("[%d]", i);
+	model_print("\n");
 
-/** @brief Prints info about may_read_from set */
-void Node::print_may_read_from()
-{
-	for (unsigned int i = 0; i < may_read_from.size(); i++)
-		may_read_from[i]->print();
+	model_print("          read from past: %s", read_from_past_empty() ? "empty" : "non-empty ");
+	for (int i = read_from_past_idx + 1; i < (int)read_from_past.size(); i++)
+		model_print("[%d]", read_from_past[i]->get_seq_number());
+	model_print("\n");
+
+	model_print("          read-from promises: %s", read_from_promise_empty() ? "empty" : "non-empty ");
+	for (int i = read_from_promise_idx + 1; i < (int)read_from_promises.size(); i++)
+		model_print("[%d]", read_from_promises[i]->get_seq_number());
+	model_print("\n");
+
+	model_print("          future values: %s", future_value_empty() ? "empty" : "non-empty ");
+	for (int i = future_index + 1; i < (int)future_values.size(); i++)
+		model_print("[%#" PRIx64 "]", future_values[i].value);
+	model_print("\n");
+
+	model_print("          promises: %s\n", promise_empty() ? "empty" : "non-empty");
+	model_print("          misc: %s\n", misc_empty() ? "empty" : "non-empty");
+	model_print("          rel seq break: %s\n", relseq_break_empty() ? "empty" : "non-empty");
 }
 
+/*********************************** promise **********************************/
+
 /**
  * Sets a promise to explore meeting with the given node.
  * @param i is the promise index.
  */
-void Node::set_promise(unsigned int i) {
-	if (i>=promises.size())
-		promises.resize(i+1,0);
-	promises[i]=1;
+void Node::set_promise(unsigned int i)
+{
+	if (i >= resolve_promise.size())
+		resolve_promise.resize(i + 1, false);
+	resolve_promise[i] = true;
 }
 
 /**
  * Looks up whether a given promise should be satisfied by this node.
  * @param i The promise index.
- * @return true if the promise should be satisfied by the given model action.
+ * @return true if the promise should be satisfied by the given ModelAction.
  */
-bool Node::get_promise(unsigned int i) {
-	return (i<promises.size())&&(promises[i]==2);
+bool Node::get_promise(unsigned int i) const
+{
+	return (i < resolve_promise.size()) && (int)i == resolve_promise_idx;
 }
 
 /**
- * Increments to the next combination of promises.
+ * Increments to the next promise to resolve.
  * @return true if we have a valid combination.
  */
-bool Node::increment_promise() {
-	for (unsigned int i=0;i<promises.size();i++) {
-		if (promises[i]==1) {
-			promises[i]=2;
-			while (i>0) {
-				i--;
-				if (promises[i]==2)
-					promises[i]=1;
-			}
+bool Node::increment_promise()
+{
+	DBG();
+	if (resolve_promise.empty())
+		return false;
+	int prev_idx = resolve_promise_idx;
+	resolve_promise_idx++;
+	for ( ; resolve_promise_idx < (int)resolve_promise.size(); resolve_promise_idx++)
+		if (resolve_promise[resolve_promise_idx])
 			return true;
-		}
-	}
+	resolve_promise_idx = prev_idx;
 	return false;
 }
 
@@ -98,32 +169,41 @@ bool Node::increment_promise() {
  * Returns whether the promise set is empty.
  * @return true if we have explored all promise combinations.
  */
-bool Node::promise_empty() {
-	for (unsigned int i=0;i<promises.size();i++)
-		if (promises[i]==1)
+bool Node::promise_empty() const
+{
+	for (int i = resolve_promise_idx + 1; i < (int)resolve_promise.size(); i++)
+		if (i >= 0 && resolve_promise[i])
 			return false;
 	return true;
 }
 
-/**
- * Adds a value from a weakly ordered future write to backtrack to.
- * @param value is the value to backtrack to.
- */
-bool Node::add_future_value(uint64_t value) {
-	for(unsigned int i=0;i<future_values.size();i++)
-		if (future_values[i]==value)
-			return false;
+/** @brief Clear any promise-resolution information for this Node */
+void Node::clear_promise_resolutions()
+{
+	resolve_promise.clear();
+	resolve_promise_idx = -1;
+}
 
-	future_values.push_back(value);
-	return true;
+/******************************* end promise **********************************/
+
+void Node::set_misc_max(int i)
+{
+	misc_max = i;
 }
 
-/**
- * Checks whether the future_values set for this node is empty.
- * @return true if the future_values set is empty.
- */
-bool Node::future_value_empty() {
-	return ((future_index+1)>=future_values.size());
+int Node::get_misc() const
+{
+	return misc_index;
+}
+
+bool Node::increment_misc()
+{
+	return (misc_index < misc_max) && ((++misc_index) < misc_max);
+}
+
+bool Node::misc_empty() const
+{
+	return (misc_index + 1) >= misc_max;
 }
 
 /**
@@ -133,7 +213,7 @@ bool Node::future_value_empty() {
  * @return true if this thread choice has been explored already, false
  * otherwise
  */
-bool Node::has_been_explored(thread_id_t tid)
+bool Node::has_been_explored(thread_id_t tid) const
 {
 	int id = id_to_int(tid);
 	return explored_children[id];
@@ -143,25 +223,26 @@ bool Node::has_been_explored(thread_id_t tid)
  * Checks if the backtracking set is empty.
  * @return true if the backtracking set is empty
  */
-bool Node::backtrack_empty()
+bool Node::backtrack_empty() const
 {
 	return (numBacktracks == 0);
 }
 
-/**
- * Checks whether the readsfrom set for this node is empty.
- * @return true if the readsfrom set is empty.
- */
-bool Node::read_from_empty() {
-	return ((read_from_index+1)>=may_read_from.size());
-}
-
 /**
  * Mark the appropriate backtracking information for exploring a thread choice.
  * @param act The ModelAction to explore
  */
-void Node::explore_child(ModelAction *act)
+void Node::explore_child(ModelAction *act, enabled_type_t *is_enabled)
 {
+	if (!enabled_array)
+		enabled_array = (enabled_type_t *)model_malloc(sizeof(enabled_type_t) * num_threads);
+	if (is_enabled != NULL)
+		memcpy(enabled_array, is_enabled, sizeof(enabled_type_t) * num_threads);
+	else {
+		for (int i = 0; i < num_threads; i++)
+			enabled_array[i] = THREAD_DISABLED;
+	}
+
 	explore(act->get_tid());
 }
 
@@ -175,6 +256,7 @@ void Node::explore_child(ModelAction *act)
 bool Node::set_backtrack(thread_id_t id)
 {
 	int i = id_to_int(id);
+	ASSERT(i < ((int)backtrack.size()));
 	if (backtrack[i])
 		return false;
 	backtrack[i] = true;
@@ -197,63 +279,350 @@ thread_id_t Node::get_next_backtrack()
 	return int_to_id(i);
 }
 
-bool Node::is_enabled(Thread *t)
+void Node::clear_backtracking()
 {
-	return id_to_int(t->get_id()) < num_threads;
+	for (unsigned int i = 0; i < backtrack.size(); i++)
+		backtrack[i] = false;
+	for (unsigned int i = 0; i < explored_children.size(); i++)
+		explored_children[i] = false;
+	numBacktracks = 0;
+}
+
+bool Node::is_enabled(Thread *t) const
+{
+	int thread_id = id_to_int(t->get_id());
+	return thread_id < num_threads && (enabled_array[thread_id] != THREAD_DISABLED);
+}
+
+enabled_type_t Node::enabled_status(thread_id_t tid) const
+{
+	int thread_id = id_to_int(tid);
+	if (thread_id < num_threads)
+		return enabled_array[thread_id];
+	else
+		return THREAD_DISABLED;
+}
+
+bool Node::is_enabled(thread_id_t tid) const
+{
+	int thread_id = id_to_int(tid);
+	return thread_id < num_threads && (enabled_array[thread_id] != THREAD_DISABLED);
+}
+
+bool Node::has_priority(thread_id_t tid) const
+{
+	return fairness[id_to_int(tid)].priority;
 }
 
+/*********************************** read from ********************************/
+
 /**
- * Add an action to the may_read_from set.
- * @param act is the action to add
+ * Get the current state of the may-read-from set iteration
+ * @return The read-from type we should currently be checking (past or future)
  */
-void Node::add_read_from(const ModelAction *act)
+read_from_type_t Node::get_read_from_status()
 {
-	may_read_from.push_back(act);
+	if (read_from_status == READ_FROM_PAST && read_from_past.empty())
+		increment_read_from();
+	return read_from_status;
 }
 
 /**
- * Gets the next 'future_value' value from this Node. Only valid for a node
- * where this->action is a 'read'.
- * @return The first element in future_values
+ * Iterate one step in the may-read-from iteration. This includes a step in
+ * reading from the either the past or the future.
+ * @return True if there is a new read-from to explore; false otherwise
  */
-uint64_t Node::get_future_value() {
-	ASSERT(future_index<future_values.size());
-	return future_values[future_index];
+bool Node::increment_read_from()
+{
+	clear_promise_resolutions();
+	if (increment_read_from_past()) {
+	       read_from_status = READ_FROM_PAST;
+	       return true;
+	} else if (increment_read_from_promise()) {
+		read_from_status = READ_FROM_PROMISE;
+		return true;
+	} else if (increment_future_value()) {
+		read_from_status = READ_FROM_FUTURE;
+		return true;
+	}
+	read_from_status = READ_FROM_NONE;
+	return false;
+}
+
+/**
+ * @return True if there are any new read-froms to explore
+ */
+bool Node::read_from_empty() const
+{
+	return read_from_past_empty() &&
+		read_from_promise_empty() &&
+		future_value_empty();
 }
 
 /**
- * Gets the next 'may_read_from' action from this Node. Only valid for a node
+ * Get the total size of the may-read-from set, including both past and future
+ * values
+ * @return The size of may-read-from
+ */
+unsigned int Node::read_from_size() const
+{
+	return read_from_past.size() +
+		read_from_promises.size() +
+		future_values.size();
+}
+
+/******************************* end read from ********************************/
+
+/****************************** read from past ********************************/
+
+/** @brief Prints info about read_from_past set */
+void Node::print_read_from_past()
+{
+	for (unsigned int i = 0; i < read_from_past.size(); i++)
+		read_from_past[i]->print();
+}
+
+/**
+ * Add an action to the read_from_past set.
+ * @param act is the action to add
+ */
+void Node::add_read_from_past(const ModelAction *act)
+{
+	read_from_past.push_back(act);
+}
+
+/**
+ * Gets the next 'read_from_past' action from this Node. Only valid for a node
  * where this->action is a 'read'.
- * @return The first element in may_read_from
+ * @return The first element in read_from_past
  */
-const ModelAction * Node::get_read_from() {
-	if (read_from_index<may_read_from.size())
-		return may_read_from[read_from_index];
+const ModelAction * Node::get_read_from_past() const
+{
+	if (read_from_past_idx < read_from_past.size())
+		return read_from_past[read_from_past_idx];
 	else
 		return NULL;
 }
 
+const ModelAction * Node::get_read_from_past(int i) const
+{
+	return read_from_past[i];
+}
+
+int Node::get_read_from_past_size() const
+{
+	return read_from_past.size();
+}
+
+/**
+ * Checks whether the readsfrom set for this node is empty.
+ * @return true if the readsfrom set is empty.
+ */
+bool Node::read_from_past_empty() const
+{
+	return ((read_from_past_idx + 1) >= read_from_past.size());
+}
+
 /**
  * Increments the index into the readsfrom set to explore the next item.
  * @return Returns false if we have explored all items.
  */
-bool Node::increment_read_from() {
-	read_from_index++;
-	return (read_from_index<may_read_from.size());
+bool Node::increment_read_from_past()
+{
+	DBG();
+	if (read_from_past_idx < read_from_past.size()) {
+		read_from_past_idx++;
+		return read_from_past_idx < read_from_past.size();
+	}
+	return false;
+}
+
+/************************** end read from past ********************************/
+
+/***************************** read_from_promises *****************************/
+
+/**
+ * Add an action to the read_from_promises set.
+ * @param reader The read which generated the Promise; we use the ModelAction
+ * instead of the Promise because the Promise does not last across executions
+ */
+void Node::add_read_from_promise(const ModelAction *reader)
+{
+	read_from_promises.push_back(reader);
+}
+
+/**
+ * Gets the next 'read-from-promise' from this Node. Only valid for a node
+ * where this->action is a 'read'.
+ * @return The current element in read_from_promises
+ */
+Promise * Node::get_read_from_promise() const
+{
+	if (read_from_promise_idx < 0 || read_from_promise_idx >= ((int)read_from_promises.size()))
+		return NULL;
+	return read_from_promises[read_from_promise_idx]->get_reads_from_promise();
+}
+
+/**
+ * Checks whether the read_from_promises set for this node is empty.
+ * @return true if the read_from_promises set is empty.
+ */
+bool Node::read_from_promise_empty() const
+{
+	return ((read_from_promise_idx + 1) >= ((int)read_from_promises.size()));
+}
+
+/**
+ * Increments the index into the read_from_promises set to explore the next item.
+ * @return Returns false if we have explored all promises.
+ */
+bool Node::increment_read_from_promise()
+{
+	DBG();
+	if (read_from_promise_idx < ((int)read_from_promises.size())) {
+		read_from_promise_idx++;
+		return (read_from_promise_idx < ((int)read_from_promises.size()));
+	}
+	return false;
+}
+
+/************************* end read_from_promises *****************************/
+
+/****************************** future values *********************************/
+
+/**
+ * Adds a value from a weakly ordered future write to backtrack to. This
+ * operation may "fail" if the future value has already been run (within some
+ * sloppiness window of this expiration), or if the futurevalues set has
+ * reached its maximum.
+ * @see model_params.maxfuturevalues
+ *
+ * @param value is the value to backtrack to.
+ * @return True if the future value was successully added; false otherwise
+ */
+bool Node::add_future_value(struct future_value fv)
+{
+	uint64_t value = fv.value;
+	modelclock_t expiration = fv.expiration;
+	thread_id_t tid = fv.tid;
+	int idx = -1; /* Highest index where value is found */
+	for (unsigned int i = 0; i < future_values.size(); i++) {
+		if (future_values[i].value == value && future_values[i].tid == tid) {
+			if (expiration <= future_values[i].expiration)
+				return false;
+			idx = i;
+		}
+	}
+	if (idx > future_index) {
+		/* Future value hasn't been explored; update expiration */
+		future_values[idx].expiration = expiration;
+		return true;
+	} else if (idx >= 0 && expiration <= future_values[idx].expiration + model->params.expireslop) {
+		/* Future value has been explored and is within the "sloppy" window */
+		return false;
+	}
+
+	/* Limit the size of the future-values set */
+	if (model->params.maxfuturevalues > 0 &&
+			(int)future_values.size() >= model->params.maxfuturevalues)
+		return false;
+
+	future_values.push_back(fv);
+	return true;
+}
+
+/**
+ * Gets the next 'future_value' from this Node. Only valid for a node where
+ * this->action is a 'read'.
+ * @return The first element in future_values
+ */
+struct future_value Node::get_future_value() const
+{
+	ASSERT(future_index >= 0 && future_index < ((int)future_values.size()));
+	return future_values[future_index];
+}
+
+/**
+ * Checks whether the future_values set for this node is empty.
+ * @return true if the future_values set is empty.
+ */
+bool Node::future_value_empty() const
+{
+	return ((future_index + 1) >= ((int)future_values.size()));
 }
 
 /**
  * Increments the index into the future_values set to explore the next item.
  * @return Returns false if we have explored all values.
  */
-bool Node::increment_future_value() {
-	future_index++;
-	return (future_index<future_values.size());
+bool Node::increment_future_value()
+{
+	DBG();
+	if (future_index < ((int)future_values.size())) {
+		future_index++;
+		return (future_index < ((int)future_values.size()));
+	}
+	return false;
+}
+
+/************************** end future values *********************************/
+
+/**
+ * Add a write ModelAction to the set of writes that may break the release
+ * sequence. This is used during replay exploration of pending release
+ * sequences. This Node must correspond to a release sequence fixup action.
+ *
+ * @param write The write that may break the release sequence. NULL means we
+ * allow the release sequence to synchronize.
+ */
+void Node::add_relseq_break(const ModelAction *write)
+{
+	relseq_break_writes.push_back(write);
+}
+
+/**
+ * Get the write that may break the current pending release sequence,
+ * according to the replay / divergence pattern.
+ *
+ * @return A write that may break the release sequence. If NULL, that means
+ * the release sequence should not be broken.
+ */
+const ModelAction * Node::get_relseq_break() const
+{
+	if (relseq_break_index < (int)relseq_break_writes.size())
+		return relseq_break_writes[relseq_break_index];
+	else
+		return NULL;
+}
+
+/**
+ * Increments the index into the relseq_break_writes set to explore the next
+ * item.
+ * @return Returns false if we have explored all values.
+ */
+bool Node::increment_relseq_break()
+{
+	DBG();
+	if (relseq_break_index < ((int)relseq_break_writes.size())) {
+		relseq_break_index++;
+		return (relseq_break_index < ((int)relseq_break_writes.size()));
+	}
+	return false;
+}
+
+/**
+ * @return True if all writes that may break the release sequence have been
+ * explored
+ */
+bool Node::relseq_break_empty() const
+{
+	return ((relseq_break_index + 1) >= ((int)relseq_break_writes.size()));
 }
 
 void Node::explore(thread_id_t tid)
 {
 	int i = id_to_int(tid);
+	ASSERT(i < ((int)backtrack.size()));
 	if (backtrack[i]) {
 		backtrack[i] = false;
 		numBacktracks--;
@@ -261,60 +630,58 @@ void Node::explore(thread_id_t tid)
 	explored_children[i] = true;
 }
 
-static void clear_node_list(node_list_t *list, node_list_t::iterator start,
-                                               node_list_t::iterator end)
+NodeStack::NodeStack() :
+	node_list(),
+	head_idx(-1),
+	total_nodes(0)
 {
-	node_list_t::iterator it;
-
-	for (it = start; it != end; it++)
-		delete (*it);
-	list->erase(start, end);
-}
-
-NodeStack::NodeStack()
-	: total_nodes(0)
-{
-	node_list.push_back(new Node());
 	total_nodes++;
-	iter = node_list.begin();
 }
 
 NodeStack::~NodeStack()
 {
-	clear_node_list(&node_list, node_list.begin(), node_list.end());
+	for (unsigned int i = 0; i < node_list.size(); i++)
+		delete node_list[i];
 }
 
-void NodeStack::print()
+void NodeStack::print() const
 {
-	node_list_t::iterator it;
-	printf("............................................\n");
-	printf("NodeStack printing node_list:\n");
-	for (it = node_list.begin(); it != node_list.end(); it++) {
-		if (it == this->iter)
-			printf("vvv following action is the current iterator vvv\n");
-		(*it)->print();
+	model_print("............................................\n");
+	model_print("NodeStack printing node_list:\n");
+	for (unsigned int it = 0; it < node_list.size(); it++) {
+		if ((int)it == this->head_idx)
+			model_print("vvv following action is the current iterator vvv\n");
+		node_list[it]->print();
 	}
-	printf("............................................\n");
+	model_print("............................................\n");
 }
 
-ModelAction * NodeStack::explore_action(ModelAction *act)
+/** Note: The is_enabled set contains what actions were enabled when
+ *  act was chosen. */
+ModelAction * NodeStack::explore_action(ModelAction *act, enabled_type_t *is_enabled)
 {
 	DBG();
 
-	ASSERT(!node_list.empty());
-	node_list_t::iterator it=iter;
-	it++;
-
-	if (it != node_list.end()) {
-		iter++;
-		return (*iter)->get_action();
+	if ((head_idx + 1) < (int)node_list.size()) {
+		head_idx++;
+		return node_list[head_idx]->get_action();
 	}
 
 	/* Record action */
-	get_head()->explore_child(act);
-	node_list.push_back(new Node(act, get_head(), model->get_num_threads()));
+	Node *head = get_head();
+	Node *prevfairness = NULL;
+	if (head) {
+		head->explore_child(act, is_enabled);
+		if (model->params.fairwindow != 0 && head_idx > (int)model->params.fairwindow)
+			prevfairness = node_list[head_idx - model->params.fairwindow];
+	}
+
+	int next_threads = model->get_num_threads();
+	if (act->get_type() == THREAD_CREATE)
+		next_threads++;
+	node_list.push_back(new Node(act, head, next_threads, prevfairness));
 	total_nodes++;
-	iter++;
+	head_idx++;
 	return NULL;
 }
 
@@ -329,35 +696,35 @@ ModelAction * NodeStack::explore_action(ModelAction *act)
 void NodeStack::pop_restofstack(int numAhead)
 {
 	/* Diverging from previous execution; clear out remainder of list */
-	node_list_t::iterator it = iter;
-	while (numAhead--)
-		it++;
-	clear_node_list(&node_list, it, node_list.end());
+	unsigned int it = head_idx + numAhead;
+	for (unsigned int i = it; i < node_list.size(); i++)
+		delete node_list[i];
+	node_list.resize(it);
+	node_list.back()->clear_backtracking();
 }
 
-Node * NodeStack::get_head()
+Node * NodeStack::get_head() const
 {
-	if (node_list.empty())
+	if (node_list.empty() || head_idx < 0)
 		return NULL;
-	return *iter;
+	return node_list[head_idx];
 }
 
-Node * NodeStack::get_next()
+Node * NodeStack::get_next() const
 {
-	node_list_t::iterator it = iter;
 	if (node_list.empty()) {
 		DEBUG("Empty\n");
 		return NULL;
 	}
-	it++;
-	if (it == node_list.end()) {
+	unsigned int it = head_idx + 1;
+	if (it == node_list.size()) {
 		DEBUG("At end\n");
 		return NULL;
 	}
-	return *it;
+	return node_list[it];
 }
 
 void NodeStack::reset_execution()
 {
-	iter = node_list.begin();
+	head_idx = -1;
 }