typos

[model-checker.git] / execution.cc

#include <stdio.h>
#include <algorithm>
#include <mutex>
#include <new>
#include <stdarg.h>

#include "model.h"
#include "execution.h"
#include "action.h"
#include "nodestack.h"
#include "schedule.h"
#include "common.h"
#include "clockvector.h"
#include "cyclegraph.h"
#include "promise.h"
#include "datarace.h"
#include "threads-model.h"
#include "bugmessage.h"

#define INITIAL_THREAD_ID       0

/**
 * Structure for holding small ModelChecker members that should be snapshotted
 */
struct model_snapshot_members {
        model_snapshot_members() :
                /* First thread created will have id INITIAL_THREAD_ID */
                next_thread_id(INITIAL_THREAD_ID),
                used_sequence_numbers(0),
                next_backtrack(NULL),
                bugs(),
                failed_promise(false),
                too_many_reads(false),
                no_valid_reads(false),
                bad_synchronization(false),
                asserted(false)
        { }

        ~model_snapshot_members() {
                for (unsigned int i = 0; i < bugs.size(); i++)
                        delete bugs[i];
                bugs.clear();
        }

        unsigned int next_thread_id;
        modelclock_t used_sequence_numbers;
        ModelAction *next_backtrack;
        SnapVector<bug_message *> bugs;
        bool failed_promise;
        bool too_many_reads;
        bool no_valid_reads;
        /** @brief Incorrectly-ordered synchronization was made */
        bool bad_synchronization;
        bool asserted;

        SNAPSHOTALLOC
};

/** @brief Constructor */
ModelExecution::ModelExecution(ModelChecker *m,
                const struct model_params *params,
                Scheduler *scheduler,
                NodeStack *node_stack) :
        model(m),
        params(params),
        scheduler(scheduler),
        action_trace(),
        thread_map(2), /* We'll always need at least 2 threads */
        obj_map(),
        condvar_waiters_map(),
        obj_thrd_map(),
        promises(),
        futurevalues(),
        pending_rel_seqs(),
        thrd_last_action(1),
        thrd_last_fence_release(),
        node_stack(node_stack),
        priv(new struct model_snapshot_members()),
        mo_graph(new CycleGraph())
{
        /* Initialize a model-checker thread, for special ModelActions */
        model_thread = new Thread(get_next_id());
        add_thread(model_thread);
        scheduler->register_engine(this);
        node_stack->register_engine(this);
}

/** @brief Destructor */
ModelExecution::~ModelExecution()
{
        for (unsigned int i = 0; i < get_num_threads(); i++)
                delete get_thread(int_to_id(i));

        for (unsigned int i = 0; i < promises.size(); i++)
                delete promises[i];

        delete mo_graph;
        delete priv;
}

int ModelExecution::get_execution_number() const
{
        return model->get_execution_number();
}

static action_list_t * get_safe_ptr_action(HashTable<const void *, action_list_t *, uintptr_t, 4> * hash, void * ptr)
{
        action_list_t *tmp = hash->get(ptr);
        if (tmp == NULL) {
                tmp = new action_list_t();
                hash->put(ptr, tmp);
        }
        return tmp;
}

static SnapVector<action_list_t> * get_safe_ptr_vect_action(HashTable<void *, SnapVector<action_list_t> *, uintptr_t, 4> * hash, void * ptr)
{
        SnapVector<action_list_t> *tmp = hash->get(ptr);
        if (tmp == NULL) {
                tmp = new SnapVector<action_list_t>();
                hash->put(ptr, tmp);
        }
        return tmp;
}

action_list_t * ModelExecution::get_actions_on_obj(void * obj, thread_id_t tid) const
{
        SnapVector<action_list_t> *wrv = obj_thrd_map.get(obj);
        if (wrv==NULL)
                return NULL;
        unsigned int thread=id_to_int(tid);
        if (thread < wrv->size())
                return &(*wrv)[thread];
        else
                return NULL;
}

/** @return a thread ID for a new Thread */
thread_id_t ModelExecution::get_next_id()
{
        return priv->next_thread_id++;
}

/** @return the number of user threads created during this execution */
unsigned int ModelExecution::get_num_threads() const
{
        return priv->next_thread_id;
}

/** @return a sequence number for a new ModelAction */
modelclock_t ModelExecution::get_next_seq_num()
{
        return ++priv->used_sequence_numbers;
}

/**
 * @brief Should the current action wake up a given thread?
 *
 * @param curr The current action
 * @param thread The thread that we might wake up
 * @return True, if we should wake up the sleeping thread; false otherwise
 */
bool ModelExecution::should_wake_up(const ModelAction *curr, const Thread *thread) const
{
        const ModelAction *asleep = thread->get_pending();
        /* Don't allow partial RMW to wake anyone up */
        if (curr->is_rmwr())
                return false;
        /* Synchronizing actions may have been backtracked */
        if (asleep->could_synchronize_with(curr))
                return true;
        /* All acquire/release fences and fence-acquire/store-release */
        if (asleep->is_fence() && asleep->is_acquire() && curr->is_release())
                return true;
        /* Fence-release + store can awake load-acquire on the same location */
        if (asleep->is_read() && asleep->is_acquire() && curr->same_var(asleep) && curr->is_write()) {
                ModelAction *fence_release = get_last_fence_release(curr->get_tid());
                if (fence_release && *(get_last_action(thread->get_id())) < *fence_release)
                        return true;
        }
        return false;
}

void ModelExecution::wake_up_sleeping_actions(ModelAction *curr)
{
        for (unsigned int i = 0; i < get_num_threads(); i++) {
                Thread *thr = get_thread(int_to_id(i));
                if (scheduler->is_sleep_set(thr)) {
                        if (should_wake_up(curr, thr))
                                /* Remove this thread from sleep set */
                                scheduler->remove_sleep(thr);
                }
        }
}

/** @brief Alert the model-checker that an incorrectly-ordered
 * synchronization was made */
void ModelExecution::set_bad_synchronization()
{
        priv->bad_synchronization = true;
}

bool ModelExecution::assert_bug(const char *msg)
{
        priv->bugs.push_back(new bug_message(msg));

        if (isfeasibleprefix()) {
                set_assert();
                return true;
        }
        return false;
}

/** @return True, if any bugs have been reported for this execution */
bool ModelExecution::have_bug_reports() const
{
        return priv->bugs.size() != 0;
}

SnapVector<bug_message *> * ModelExecution::get_bugs() const
{
        return &priv->bugs;
}

/**
 * Check whether the current trace has triggered an assertion which should halt
 * its execution.
 *
 * @return True, if the execution should be aborted; false otherwise
 */
bool ModelExecution::has_asserted() const
{
        return priv->asserted;
}

/**
 * Trigger a trace assertion which should cause this execution to be halted.
 * This can be due to a detected bug or due to an infeasibility that should
 * halt ASAP.
 */
void ModelExecution::set_assert()
{
        priv->asserted = true;
}

/**
 * Check if we are in a deadlock. Should only be called at the end of an
 * execution, although it should not give false positives in the middle of an
 * execution (there should be some ENABLED thread).
 *
 * @return True if program is in a deadlock; false otherwise
 */
bool ModelExecution::is_deadlocked() const
{
        bool blocking_threads = false;
        for (unsigned int i = 0; i < get_num_threads(); i++) {
                thread_id_t tid = int_to_id(i);
                if (is_enabled(tid))
                        return false;
                Thread *t = get_thread(tid);
                if (!t->is_model_thread() && t->get_pending())
                        blocking_threads = true;
        }
        return blocking_threads;
}

/**
 * @brief Check if we are yield-blocked
 *
 * A program can be "yield-blocked" if all threads are ready to execute a
 * yield.
 *
 * @return True if the program is yield-blocked; false otherwise
 */
bool ModelExecution::is_yieldblocked() const
{
        if (!params->yieldblock)
                return false;

        for (unsigned int i = 0; i < get_num_threads(); i++) {
                thread_id_t tid = int_to_id(i);
                Thread *t = get_thread(tid);
                if (t->get_pending() && t->get_pending()->is_yield())
                        return true;
        }
        return false;
}

/**
 * Check if this is a complete execution. That is, have all thread completed
 * execution (rather than exiting because sleep sets have forced a redundant
 * execution).
 *
 * @return True if the execution is complete.
 */
bool ModelExecution::is_complete_execution() const
{
        if (is_yieldblocked())
                return false;
        for (unsigned int i = 0; i < get_num_threads(); i++)
                if (is_enabled(int_to_id(i)))
                        return false;
        return true;
}

/**
 * @brief Find the last fence-related backtracking conflict for a ModelAction
 *
 * This function performs the search for the most recent conflicting action
 * against which we should perform backtracking, as affected by fence
 * operations. This includes pairs of potentially-synchronizing actions which
 * occur due to fence-acquire or fence-release, and hence should be explored in
 * the opposite execution order.
 *
 * @param act The current action
 * @return The most recent action which conflicts with act due to fences
 */
ModelAction * ModelExecution::get_last_fence_conflict(ModelAction *act) const
{
        /* Only perform release/acquire fence backtracking for stores */
        if (!act->is_write())
                return NULL;

        /* Find a fence-release (or, act is a release) */
        ModelAction *last_release;
        if (act->is_release())
                last_release = act;
        else
                last_release = get_last_fence_release(act->get_tid());
        if (!last_release)
                return NULL;

        /* Skip past the release */
        const action_list_t *list = &action_trace;
        action_list_t::const_reverse_iterator rit;
        for (rit = list->rbegin(); rit != list->rend(); rit++)
                if (*rit == last_release)
                        break;
        ASSERT(rit != list->rend());

        /* Find a prior:
         *   load-acquire
         * or
         *   load --sb-> fence-acquire */
        ModelVector<ModelAction *> acquire_fences(get_num_threads(), NULL);
        ModelVector<ModelAction *> prior_loads(get_num_threads(), NULL);
        bool found_acquire_fences = false;
        for ( ; rit != list->rend(); rit++) {
                ModelAction *prev = *rit;
                if (act->same_thread(prev))
                        continue;

                int tid = id_to_int(prev->get_tid());

                if (prev->is_read() && act->same_var(prev)) {
                        if (prev->is_acquire()) {
                                /* Found most recent load-acquire, don't need
                                 * to search for more fences */
                                if (!found_acquire_fences)
                                        return NULL;
                        } else {
                                prior_loads[tid] = prev;
                        }
                }
                if (prev->is_acquire() && prev->is_fence() && !acquire_fences[tid]) {
                        found_acquire_fences = true;
                        acquire_fences[tid] = prev;
                }
        }

        ModelAction *latest_backtrack = NULL;
        for (unsigned int i = 0; i < acquire_fences.size(); i++)
                if (acquire_fences[i] && prior_loads[i])
                        if (!latest_backtrack || *latest_backtrack < *acquire_fences[i])
                                latest_backtrack = acquire_fences[i];
        return latest_backtrack;
}

/**
 * @brief Find the last backtracking conflict for a ModelAction
 *
 * This function performs the search for the most recent conflicting action
 * against which we should perform backtracking. This primary includes pairs of
 * synchronizing actions which should be explored in the opposite execution
 * order.
 *
 * @param act The current action
 * @return The most recent action which conflicts with act
 */
ModelAction * ModelExecution::get_last_conflict(ModelAction *act) const
{
        switch (act->get_type()) {
        case ATOMIC_FENCE:
                /* Only seq-cst fences can (directly) cause backtracking */
                if (!act->is_seqcst())
                        break;
        case ATOMIC_READ:
        case ATOMIC_WRITE:
        case ATOMIC_RMW: {
                ModelAction *ret = NULL;

                /* linear search: from most recent to oldest */
                action_list_t *list = obj_map.get(act->get_location());
                action_list_t::reverse_iterator rit;
                for (rit = list->rbegin(); rit != list->rend(); rit++) {
                        ModelAction *prev = *rit;
                        if (prev == act)
                                continue;
                        if (prev->could_synchronize_with(act)) {
                                ret = prev;
                                break;
                        }
                }

                ModelAction *ret2 = get_last_fence_conflict(act);
                if (!ret2)
                        return ret;
                if (!ret)
                        return ret2;
                if (*ret < *ret2)
                        return ret2;
                return ret;
        }
        case ATOMIC_LOCK:
        case ATOMIC_TRYLOCK: {
                /* linear search: from most recent to oldest */
                action_list_t *list = obj_map.get(act->get_location());
                action_list_t::reverse_iterator rit;
                for (rit = list->rbegin(); rit != list->rend(); rit++) {
                        ModelAction *prev = *rit;
                        if (act->is_conflicting_lock(prev))
                                return prev;
                }
                break;
        }
        case ATOMIC_UNLOCK: {
                /* linear search: from most recent to oldest */
                action_list_t *list = obj_map.get(act->get_location());
                action_list_t::reverse_iterator rit;
                for (rit = list->rbegin(); rit != list->rend(); rit++) {
                        ModelAction *prev = *rit;
                        if (!act->same_thread(prev) && prev->is_failed_trylock())
                                return prev;
                }
                break;
        }
        case ATOMIC_WAIT: {
                /* linear search: from most recent to oldest */
                action_list_t *list = obj_map.get(act->get_location());
                action_list_t::reverse_iterator rit;
                for (rit = list->rbegin(); rit != list->rend(); rit++) {
                        ModelAction *prev = *rit;
                        if (!act->same_thread(prev) && prev->is_failed_trylock())
                                return prev;
                        if (!act->same_thread(prev) && prev->is_notify())
                                return prev;
                }
                break;
        }

        case ATOMIC_NOTIFY_ALL:
        case ATOMIC_NOTIFY_ONE: {
                /* linear search: from most recent to oldest */
                action_list_t *list = obj_map.get(act->get_location());
                action_list_t::reverse_iterator rit;
                for (rit = list->rbegin(); rit != list->rend(); rit++) {
                        ModelAction *prev = *rit;
                        if (!act->same_thread(prev) && prev->is_wait())
                                return prev;
                }
                break;
        }
        default:
                break;
        }
        return NULL;
}

/** This method finds backtracking points where we should try to
 * reorder the parameter ModelAction against.
 *
 * @param the ModelAction to find backtracking points for.
 */
void ModelExecution::set_backtracking(ModelAction *act)
{
        Thread *t = get_thread(act);
        ModelAction *prev = get_last_conflict(act);
        if (prev == NULL)
                return;

        Node *node = prev->get_node()->get_parent();

        /* See Dynamic Partial Order Reduction (addendum), POPL '05 */
        int low_tid, high_tid;
        if (node->enabled_status(t->get_id()) == THREAD_ENABLED) {
                low_tid = id_to_int(act->get_tid());
                high_tid = low_tid + 1;
        } else {
                low_tid = 0;
                high_tid = get_num_threads();
        }

        for (int i = low_tid; i < high_tid; i++) {
                thread_id_t tid = int_to_id(i);

                /* Make sure this thread can be enabled here. */
                if (i >= node->get_num_threads())
                        break;

                /* See Dynamic Partial Order Reduction (addendum), POPL '05 */
                /* Don't backtrack into a point where the thread is disabled or sleeping. */
                if (node->enabled_status(tid) != THREAD_ENABLED)
                        continue;

                /* Check if this has been explored already */
                if (node->has_been_explored(tid))
                        continue;

                /* See if fairness allows */
                if (params->fairwindow != 0 && !node->has_priority(tid)) {
                        bool unfair = false;
                        for (int t = 0; t < node->get_num_threads(); t++) {
                                thread_id_t tother = int_to_id(t);
                                if (node->is_enabled(tother) && node->has_priority(tother)) {
                                        unfair = true;
                                        break;
                                }
                        }
                        if (unfair)
                                continue;
                }

                /* See if CHESS-like yield fairness allows */
                if (params->yieldon) {
                        bool unfair = false;
                        for (int t = 0; t < node->get_num_threads(); t++) {
                                thread_id_t tother = int_to_id(t);
                                if (node->is_enabled(tother) && node->has_priority_over(tid, tother)) {
                                        unfair = true;
                                        break;
                                }
                        }
                        if (unfair)
                                continue;
                }

                /* Cache the latest backtracking point */
                set_latest_backtrack(prev);

                /* If this is a new backtracking point, mark the tree */
                if (!node->set_backtrack(tid))
                        continue;
                DEBUG("Setting backtrack: conflict = %d, instead tid = %d\n",
                                        id_to_int(prev->get_tid()),
                                        id_to_int(t->get_id()));
                if (DBG_ENABLED()) {
                        prev->print();
                        act->print();
                }
        }
}

/**
 * @brief Cache the a backtracking point as the "most recent", if eligible
 *
 * Note that this does not prepare the NodeStack for this backtracking
 * operation, it only caches the action on a per-execution basis
 *
 * @param act The operation at which we should explore a different next action
 * (i.e., backtracking point)
 * @return True, if this action is now the most recent backtracking point;
 * false otherwise
 */
bool ModelExecution::set_latest_backtrack(ModelAction *act)
{
        if (!priv->next_backtrack || *act > *priv->next_backtrack) {
                priv->next_backtrack = act;
                return true;
        }
        return false;
}

/**
 * Returns last backtracking point. The model checker will explore a different
 * path for this point in the next execution.
 * @return The ModelAction at which the next execution should diverge.
 */
ModelAction * ModelExecution::get_next_backtrack()
{
        ModelAction *next = priv->next_backtrack;
        priv->next_backtrack = NULL;
        return next;
}

/**
 * Processes a read model action.
 * @param curr is the read model action to process.
 * @return True if processing this read updates the mo_graph.
 */
bool ModelExecution::process_read(ModelAction *curr)
{
        Node *node = curr->get_node();
        while (true) {
                bool updated = false;
                switch (node->get_read_from_status()) {
                case READ_FROM_PAST: {
                        const ModelAction *rf = node->get_read_from_past();
                        ASSERT(rf);

                        mo_graph->startChanges();

                        ASSERT(!is_infeasible());
                        if (!check_recency(curr, rf)) {
                                if (node->increment_read_from()) {
                                        mo_graph->rollbackChanges();
                                        continue;
                                } else {
                                        priv->too_many_reads = true;
                                }
                        }

                        updated = r_modification_order(curr, rf);
                        read_from(curr, rf);
                        mo_graph->commitChanges();
                        mo_check_promises(curr, true);
                        break;
                }
                case READ_FROM_PROMISE: {
                        Promise *promise = curr->get_node()->get_read_from_promise();
                        if (promise->add_reader(curr))
                                priv->failed_promise = true;
                        curr->set_read_from_promise(promise);
                        mo_graph->startChanges();
                        if (!check_recency(curr, promise))
                                priv->too_many_reads = true;
                        updated = r_modification_order(curr, promise);
                        mo_graph->commitChanges();
                        break;
                }
                case READ_FROM_FUTURE: {
                        /* Read from future value */
                        struct future_value fv = node->get_future_value();
                        Promise *promise = new Promise(this, curr, fv);
                        curr->set_read_from_promise(promise);
                        promises.push_back(promise);
                        mo_graph->startChanges();
                        updated = r_modification_order(curr, promise);
                        mo_graph->commitChanges();
                        break;
                }
                default:
                        ASSERT(false);
                }
                get_thread(curr)->set_return_value(curr->get_return_value());
                return updated;
        }
}

/**
 * Processes a lock, trylock, or unlock model action.  @param curr is
 * the read model action to process.
 *
 * The try lock operation checks whether the lock is taken.  If not,
 * it falls to the normal lock operation case.  If so, it returns
 * fail.
 *
 * The lock operation has already been checked that it is enabled, so
 * it just grabs the lock and synchronizes with the previous unlock.
 *
 * The unlock operation has to re-enable all of the threads that are
 * waiting on the lock.
 *
 * @return True if synchronization was updated; false otherwise
 */
bool ModelExecution::process_mutex(ModelAction *curr)
{
        std::mutex *mutex = curr->get_mutex();
        struct std::mutex_state *state = NULL;

        if (mutex)
                state = mutex->get_state();

        switch (curr->get_type()) {
        case ATOMIC_TRYLOCK: {
                bool success = !state->locked;
                curr->set_try_lock(success);
                if (!success) {
                        get_thread(curr)->set_return_value(0);
                        break;
                }
                get_thread(curr)->set_return_value(1);
        }
                //otherwise fall into the lock case
        case ATOMIC_LOCK: {
                if (curr->get_cv()->getClock(state->alloc_tid) <= state->alloc_clock)
                        assert_bug("Lock access before initialization");
                state->locked = get_thread(curr);
                ModelAction *unlock = get_last_unlock(curr);
                //synchronize with the previous unlock statement
                if (unlock != NULL) {
                        synchronize(unlock, curr);
                        return true;
                }
                break;
        }
        case ATOMIC_WAIT:
        case ATOMIC_UNLOCK: {
                /* wake up the other threads */
                for (unsigned int i = 0; i < get_num_threads(); i++) {
                        Thread *t = get_thread(int_to_id(i));
                        Thread *curr_thrd = get_thread(curr);
                        if (t->waiting_on() == curr_thrd && t->get_pending()->is_lock())
                                scheduler->wake(t);
                }

                /* unlock the lock - after checking who was waiting on it */
                state->locked = NULL;

                if (!curr->is_wait())
                        break; /* The rest is only for ATOMIC_WAIT */

                /* Should we go to sleep? (simulate spurious failures) */
                if (curr->get_node()->get_misc() == 0) {
                        get_safe_ptr_action(&condvar_waiters_map, curr->get_location())->push_back(curr);
                        /* disable us */
                        scheduler->sleep(get_thread(curr));
                }
                break;
        }
        case ATOMIC_NOTIFY_ALL: {
                action_list_t *waiters = get_safe_ptr_action(&condvar_waiters_map, curr->get_location());
                //activate all the waiting threads
                for (action_list_t::iterator rit = waiters->begin(); rit != waiters->end(); rit++) {
                        scheduler->wake(get_thread(*rit));
                }
                waiters->clear();
                break;
        }
        case ATOMIC_NOTIFY_ONE: {
                action_list_t *waiters = get_safe_ptr_action(&condvar_waiters_map, curr->get_location());
                int wakeupthread = curr->get_node()->get_misc();
                action_list_t::iterator it = waiters->begin();
                advance(it, wakeupthread);
                scheduler->wake(get_thread(*it));
                waiters->erase(it);
                break;
        }

        default:
                ASSERT(0);
        }
        return false;
}

/**
 * @brief Check if the current pending promises allow a future value to be sent
 *
 * If one of the following is true:
 *  (a) there are no pending promises
 *  (b) the reader and writer do not cross any promises
 * Then, it is safe to pass a future value back now.
 *
 * Otherwise, we must save the pending future value until (a) or (b) is true
 *
 * @param writer The operation which sends the future value. Must be a write.
 * @param reader The operation which will observe the value. Must be a read.
 * @return True if the future value can be sent now; false if it must wait.
 */
bool ModelExecution::promises_may_allow(const ModelAction *writer,
                const ModelAction *reader) const
{
        if (promises.empty())
                return true;
        for (int i = promises.size() - 1; i >= 0; i--) {
                ModelAction *pr = promises[i]->get_reader(0);
                //reader is after promise...doesn't cross any promise
                if (*reader > *pr)
                        return true;
                //writer is after promise, reader before...bad...
                if (*writer > *pr)
                        return false;
        }
        return true;
}

/**
 * @brief Add a future value to a reader
 *
 * This function performs a few additional checks to ensure that the future
 * value can be feasibly observed by the reader
 *
 * @param writer The operation whose value is sent. Must be a write.
 * @param reader The read operation which may read the future value. Must be a read.
 */
void ModelExecution::add_future_value(const ModelAction *writer, ModelAction *reader)
{
        /* Do more ambitious checks now that mo is more complete */
        if (!mo_may_allow(writer, reader))
                return;

        Node *node = reader->get_node();

        /* Find an ancestor thread which exists at the time of the reader */
        Thread *write_thread = get_thread(writer);
        while (id_to_int(write_thread->get_id()) >= node->get_num_threads())
                write_thread = write_thread->get_parent();

        struct future_value fv = {
                writer->get_write_value(),
                writer->get_seq_number() + params->maxfuturedelay,
                write_thread->get_id(),
        };
        if (node->add_future_value(fv))
                set_latest_backtrack(reader);
}

/**
 * Process a write ModelAction
 * @param curr The ModelAction to process
 * @return True if the mo_graph was updated or promises were resolved
 */
bool ModelExecution::process_write(ModelAction *curr)
{
        /* Readers to which we may send our future value */
        ModelVector<ModelAction *> send_fv;

        const ModelAction *earliest_promise_reader;
        bool updated_promises = false;

        bool updated_mod_order = w_modification_order(curr, &send_fv);
        Promise *promise = pop_promise_to_resolve(curr);

        if (promise) {
                earliest_promise_reader = promise->get_reader(0);
                updated_promises = resolve_promise(curr, promise);
        } else
                earliest_promise_reader = NULL;

        for (unsigned int i = 0; i < send_fv.size(); i++) {
                ModelAction *read = send_fv[i];

                /* Don't send future values to reads after the Promise we resolve */
                if (!earliest_promise_reader || *read < *earliest_promise_reader) {
                        /* Check if future value can be sent immediately */
                        if (promises_may_allow(curr, read)) {
                                add_future_value(curr, read);
                        } else {
                                futurevalues.push_back(PendingFutureValue(curr, read));
                        }
                }
        }

        /* Check the pending future values */
        for (int i = (int)futurevalues.size() - 1; i >= 0; i--) {
                struct PendingFutureValue pfv = futurevalues[i];
                if (promises_may_allow(pfv.writer, pfv.reader)) {
                        add_future_value(pfv.writer, pfv.reader);
                        futurevalues.erase(futurevalues.begin() + i);
                }
        }

        mo_graph->commitChanges();
        mo_check_promises(curr, false);

        get_thread(curr)->set_return_value(VALUE_NONE);
        return updated_mod_order || updated_promises;
}

/**
 * Process a fence ModelAction
 * @param curr The ModelAction to process
 * @return True if synchronization was updated
 */
bool ModelExecution::process_fence(ModelAction *curr)
{
        /*
         * fence-relaxed: no-op
         * fence-release: only log the occurence (not in this function), for
         *   use in later synchronization
         * fence-acquire (this function): search for hypothetical release
         *   sequences
         * fence-seq-cst: MO constraints formed in {r,w}_modification_order
         */
        bool updated = false;
        if (curr->is_acquire()) {
                action_list_t *list = &action_trace;
                action_list_t::reverse_iterator rit;
                /* Find X : is_read(X) && X --sb-> curr */
                for (rit = list->rbegin(); rit != list->rend(); rit++) {
                        ModelAction *act = *rit;
                        if (act == curr)
                                continue;
                        if (act->get_tid() != curr->get_tid())
                                continue;
                        /* Stop at the beginning of the thread */
                        if (act->is_thread_start())
                                break;
                        /* Stop once we reach a prior fence-acquire */
                        if (act->is_fence() && act->is_acquire())
                                break;
                        if (!act->is_read())
                                continue;
                        /* read-acquire will find its own release sequences */
                        if (act->is_acquire())
                                continue;

                        /* Establish hypothetical release sequences */
                        rel_heads_list_t release_heads;
                        get_release_seq_heads(curr, act, &release_heads);
                        for (unsigned int i = 0; i < release_heads.size(); i++)
                                synchronize(release_heads[i], curr);
                        if (release_heads.size() != 0)
                                updated = true;
                }
        }
        return updated;
}

/**
 * @brief Process the current action for thread-related activity
 *
 * Performs current-action processing for a THREAD_* ModelAction. Proccesses
 * may include setting Thread status, completing THREAD_FINISH/THREAD_JOIN
 * synchronization, etc.  This function is a no-op for non-THREAD actions
 * (e.g., ATOMIC_{READ,WRITE,RMW,LOCK}, etc.)
 *
 * @param curr The current action
 * @return True if synchronization was updated or a thread completed
 */
bool ModelExecution::process_thread_action(ModelAction *curr)
{
        bool updated = false;

        switch (curr->get_type()) {
        case THREAD_CREATE: {
                thrd_t *thrd = (thrd_t *)curr->get_location();
                struct thread_params *params = (struct thread_params *)curr->get_value();
                Thread *th = new Thread(get_next_id(), thrd, params->func, params->arg, get_thread(curr));
                add_thread(th);
                th->set_creation(curr);
                /* Promises can be satisfied by children */
                for (unsigned int i = 0; i < promises.size(); i++) {
                        Promise *promise = promises[i];
                        if (promise->thread_is_available(curr->get_tid()))
                                promise->add_thread(th->get_id());
                }
                break;
        }
        case THREAD_JOIN: {
                Thread *blocking = curr->get_thread_operand();
                ModelAction *act = get_last_action(blocking->get_id());
                synchronize(act, curr);
                updated = true; /* trigger rel-seq checks */
                break;
        }
        case THREAD_FINISH: {
                Thread *th = get_thread(curr);
                /* Wake up any joining threads */
                for (unsigned int i = 0; i < get_num_threads(); i++) {
                        Thread *waiting = get_thread(int_to_id(i));
                        if (waiting->waiting_on() == th &&
                                        waiting->get_pending()->is_thread_join())
                                scheduler->wake(waiting);
                }
                th->complete();
                /* Completed thread can't satisfy promises */
                for (unsigned int i = 0; i < promises.size(); i++) {
                        Promise *promise = promises[i];
                        if (promise->thread_is_available(th->get_id()))
                                if (promise->eliminate_thread(th->get_id()))
                                        priv->failed_promise = true;
                }
                updated = true; /* trigger rel-seq checks */
                break;
        }
        case THREAD_START: {
                check_promises(curr->get_tid(), NULL, curr->get_cv());
                break;
        }
        default:
                break;
        }

        return updated;
}

/**
 * @brief Process the current action for release sequence fixup activity
 *
 * Performs model-checker release sequence fixups for the current action,
 * forcing a single pending release sequence to break (with a given, potential
 * "loose" write) or to complete (i.e., synchronize). If a pending release
 * sequence forms a complete release sequence, then we must perform the fixup
 * synchronization, mo_graph additions, etc.
 *
 * @param curr The current action; must be a release sequence fixup action
 * @param work_queue The work queue to which to add work items as they are
 * generated
 */
void ModelExecution::process_relseq_fixup(ModelAction *curr, work_queue_t *work_queue)
{
        const ModelAction *write = curr->get_node()->get_relseq_break();
        struct release_seq *sequence = pending_rel_seqs.back();
        pending_rel_seqs.pop_back();
        ASSERT(sequence);
        ModelAction *acquire = sequence->acquire;
        const ModelAction *rf = sequence->rf;
        const ModelAction *release = sequence->release;
        ASSERT(acquire);
        ASSERT(release);
        ASSERT(rf);
        ASSERT(release->same_thread(rf));

        if (write == NULL) {
                /**
                 * @todo Forcing a synchronization requires that we set
                 * modification order constraints. For instance, we can't allow
                 * a fixup sequence in which two separate read-acquire
                 * operations read from the same sequence, where the first one
                 * synchronizes and the other doesn't. Essentially, we can't
                 * allow any writes to insert themselves between 'release' and
                 * 'rf'
                 */

                /* Must synchronize */
                if (!synchronize(release, acquire))
                        return;
                /* Re-check all pending release sequences */
                work_queue->push_back(CheckRelSeqWorkEntry(NULL));
                /* Re-check act for mo_graph edges */
                work_queue->push_back(MOEdgeWorkEntry(acquire));

                /* propagate synchronization to later actions */
                action_list_t::reverse_iterator rit = action_trace.rbegin();
                for (; (*rit) != acquire; rit++) {
                        ModelAction *propagate = *rit;
                        if (acquire->happens_before(propagate)) {
                                synchronize(acquire, propagate);
                                /* Re-check 'propagate' for mo_graph edges */
                                work_queue->push_back(MOEdgeWorkEntry(propagate));
                        }
                }
        } else {
                /* Break release sequence with new edges:
                 *   release --mo--> write --mo--> rf */
                mo_graph->addEdge(release, write);
                mo_graph->addEdge(write, rf);
        }

        /* See if we have realized a data race */
        checkDataRaces();
}

/**
 * Initialize the current action by performing one or more of the following
 * actions, as appropriate: merging RMWR and RMWC/RMW actions, stepping forward
 * in the NodeStack, manipulating backtracking sets, allocating and
 * initializing clock vectors, and computing the promises to fulfill.
 *
 * @param curr The current action, as passed from the user context; may be
 * freed/invalidated after the execution of this function, with a different
 * action "returned" its place (pass-by-reference)
 * @return True if curr is a newly-explored action; false otherwise
 */
bool ModelExecution::initialize_curr_action(ModelAction **curr)
{
        ModelAction *newcurr;

        if ((*curr)->is_rmwc() || (*curr)->is_rmw()) {
                newcurr = process_rmw(*curr);
                delete *curr;

                if (newcurr->is_rmw())
                        compute_promises(newcurr);

                *curr = newcurr;
                return false;
        }

        (*curr)->set_seq_number(get_next_seq_num());

        newcurr = node_stack->explore_action(*curr, scheduler->get_enabled_array());
        if (newcurr) {
                /* First restore type and order in case of RMW operation */
                if ((*curr)->is_rmwr())
                        newcurr->copy_typeandorder(*curr);

                ASSERT((*curr)->get_location() == newcurr->get_location());
                newcurr->copy_from_new(*curr);

                /* Discard duplicate ModelAction; use action from NodeStack */
                delete *curr;

                /* Always compute new clock vector */
                newcurr->create_cv(get_parent_action(newcurr->get_tid()));

                *curr = newcurr;
                return false; /* Action was explored previously */
        } else {
                newcurr = *curr;

                /* Always compute new clock vector */
                newcurr->create_cv(get_parent_action(newcurr->get_tid()));

                /* Assign most recent release fence */
                newcurr->set_last_fence_release(get_last_fence_release(newcurr->get_tid()));

                /*
                 * Perform one-time actions when pushing new ModelAction onto
                 * NodeStack
                 */
                if (newcurr->is_write())
                        compute_promises(newcurr);
                else if (newcurr->is_relseq_fixup())
                        compute_relseq_breakwrites(newcurr);
                else if (newcurr->is_wait())
                        newcurr->get_node()->set_misc_max(2);
                else if (newcurr->is_notify_one()) {
                        newcurr->get_node()->set_misc_max(get_safe_ptr_action(&condvar_waiters_map, newcurr->get_location())->size());
                }
                return true; /* This was a new ModelAction */
        }
}

/**
 * @brief Establish reads-from relation between two actions
 *
 * Perform basic operations involved with establishing a concrete rf relation,
 * including setting the ModelAction data and checking for release sequences.
 *
 * @param act The action that is reading (must be a read)
 * @param rf The action from which we are reading (must be a write)
 *
 * @return True if this read established synchronization
 */
bool ModelExecution::read_from(ModelAction *act, const ModelAction *rf)
{
        ASSERT(rf);
        ASSERT(rf->is_write());

        act->set_read_from(rf);
        if (act->is_acquire()) {
                rel_heads_list_t release_heads;
                get_release_seq_heads(act, act, &release_heads);
                int num_heads = release_heads.size();
                for (unsigned int i = 0; i < release_heads.size(); i++)
                        if (!synchronize(release_heads[i], act))
                                num_heads--;
                return num_heads > 0;
        }
        return false;
}

/**
 * @brief Synchronizes two actions
 *
 * When A synchronizes with B (or A --sw-> B), B inherits A's clock vector.
 * This function performs the synchronization as well as providing other hooks
 * for other checks along with synchronization.
 *
 * @param first The left-hand side of the synchronizes-with relation
 * @param second The right-hand side of the synchronizes-with relation
 * @return True if the synchronization was successful (i.e., was consistent
 * with the execution order); false otherwise
 */
bool ModelExecution::synchronize(const ModelAction *first, ModelAction *second)
{
        if (*second < *first) {
                set_bad_synchronization();
                return false;
        }
        check_promises(first->get_tid(), second->get_cv(), first->get_cv());
        return second->synchronize_with(first);
}

/**
 * Check promises and eliminate potentially-satisfying threads when a thread is
 * blocked (e.g., join, lock). A thread which is waiting on another thread can
 * no longer satisfy a promise generated from that thread.
 *
 * @param blocker The thread on which a thread is waiting
 * @param waiting The waiting thread
 */
void ModelExecution::thread_blocking_check_promises(Thread *blocker, Thread *waiting)
{
        for (unsigned int i = 0; i < promises.size(); i++) {
                Promise *promise = promises[i];
                if (!promise->thread_is_available(waiting->get_id()))
                        continue;
                for (unsigned int j = 0; j < promise->get_num_readers(); j++) {
                        ModelAction *reader = promise->get_reader(j);
                        if (reader->get_tid() != blocker->get_id())
                                continue;
                        if (promise->eliminate_thread(waiting->get_id())) {
                                /* Promise has failed */
                                priv->failed_promise = true;
                        } else {
                                /* Only eliminate the 'waiting' thread once */
                                return;
                        }
                }
        }
}

/**
 * @brief Check whether a model action is enabled.
 *
 * Checks whether an operation would be successful (i.e., is a lock already
 * locked, or is the joined thread already complete).
 *
 * For yield-blocking, yields are never enabled.
 *
 * @param curr is the ModelAction to check whether it is enabled.
 * @return a bool that indicates whether the action is enabled.
 */
bool ModelExecution::check_action_enabled(ModelAction *curr) {
        if (curr->is_lock()) {
                std::mutex *lock = curr->get_mutex();
                struct std::mutex_state *state = lock->get_state();
                if (state->locked)
                        return false;
        } else if (curr->is_thread_join()) {
                Thread *blocking = curr->get_thread_operand();
                if (!blocking->is_complete()) {
                        thread_blocking_check_promises(blocking, get_thread(curr));
                        return false;
                }
        } else if (params->yieldblock && curr->is_yield()) {
                return false;
        }

        return true;
}

/**
 * This is the heart of the model checker routine. It performs model-checking
 * actions corresponding to a given "current action." Among other processes, it
 * calculates reads-from relationships, updates synchronization clock vectors,
 * forms a memory_order constraints graph, and handles replay/backtrack
 * execution when running permutations of previously-observed executions.
 *
 * @param curr The current action to process
 * @return The ModelAction that is actually executed; may be different than
 * curr
 */
ModelAction * ModelExecution::check_current_action(ModelAction *curr)
{
        ASSERT(curr);
        bool second_part_of_rmw = curr->is_rmwc() || curr->is_rmw();
        bool newly_explored = initialize_curr_action(&curr);

        DBG();

        wake_up_sleeping_actions(curr);

        /* Compute fairness information for CHESS yield algorithm */
        if (params->yieldon) {
                curr->get_node()->update_yield(scheduler);
        }

        /* Add the action to lists before any other model-checking tasks */
        if (!second_part_of_rmw)
                add_action_to_lists(curr);

        /* Build may_read_from set for newly-created actions */
        if (newly_explored && curr->is_read())
                build_may_read_from(curr);

        /* Initialize work_queue with the "current action" work */
        work_queue_t work_queue(1, CheckCurrWorkEntry(curr));
        while (!work_queue.empty() && !has_asserted()) {
                WorkQueueEntry work = work_queue.front();
                work_queue.pop_front();

                switch (work.type) {
                case WORK_CHECK_CURR_ACTION: {
                        ModelAction *act = work.action;
                        bool update = false; /* update this location's release seq's */
                        bool update_all = false; /* update all release seq's */

                        if (process_thread_action(curr))
                                update_all = true;

                        if (act->is_read() && !second_part_of_rmw && process_read(act))
                                update = true;

                        if (act->is_write() && process_write(act))
                                update = true;

                        if (act->is_fence() && process_fence(act))
                                update_all = true;

                        if (act->is_mutex_op() && process_mutex(act))
                                update_all = true;

                        if (act->is_relseq_fixup())
                                process_relseq_fixup(curr, &work_queue);

                        if (update_all)
                                work_queue.push_back(CheckRelSeqWorkEntry(NULL));
                        else if (update)
                                work_queue.push_back(CheckRelSeqWorkEntry(act->get_location()));
                        break;
                }
                case WORK_CHECK_RELEASE_SEQ:
                        resolve_release_sequences(work.location, &work_queue);
                        break;
                case WORK_CHECK_MO_EDGES: {
                        /** @todo Complete verification of work_queue */
                        ModelAction *act = work.action;
                        bool updated = false;

                        if (act->is_read()) {
                                const ModelAction *rf = act->get_reads_from();
                                const Promise *promise = act->get_reads_from_promise();
                                if (rf) {
                                        if (r_modification_order(act, rf))
                                                updated = true;
                                } else if (promise) {
                                        if (r_modification_order(act, promise))
                                                updated = true;
                                }
                        }
                        if (act->is_write()) {
                                if (w_modification_order(act, NULL))
                                        updated = true;
                        }
                        mo_graph->commitChanges();

                        if (updated)
                                work_queue.push_back(CheckRelSeqWorkEntry(act->get_location()));
                        break;
                }
                default:
                        ASSERT(false);
                        break;
                }
        }

        check_curr_backtracking(curr);
        set_backtracking(curr);
        return curr;
}

void ModelExecution::check_curr_backtracking(ModelAction *curr)
{
        Node *currnode = curr->get_node();
        Node *parnode = currnode->get_parent();

        if ((parnode && !parnode->backtrack_empty()) ||
                         !currnode->misc_empty() ||
                         !currnode->read_from_empty() ||
                         !currnode->promise_empty() ||
                         !currnode->relseq_break_empty()) {
                set_latest_backtrack(curr);
        }
}

bool ModelExecution::promises_expired() const
{
        for (unsigned int i = 0; i < promises.size(); i++) {
                Promise *promise = promises[i];
                if (promise->get_expiration() < priv->used_sequence_numbers)
                        return true;
        }
        return false;
}

/**
 * This is the strongest feasibility check available.
 * @return whether the current trace (partial or complete) must be a prefix of
 * a feasible trace.
 */
bool ModelExecution::isfeasibleprefix() const
{
        return pending_rel_seqs.size() == 0 && is_feasible_prefix_ignore_relseq();
}

/**
 * Print disagnostic information about an infeasible execution
 * @param prefix A string to prefix the output with; if NULL, then a default
 * message prefix will be provided
 */
void ModelExecution::print_infeasibility(const char *prefix) const
{
        char buf[100];
        char *ptr = buf;
        if (mo_graph->checkForCycles())
                ptr += sprintf(ptr, "[mo cycle]");
        if (priv->failed_promise)
                ptr += sprintf(ptr, "[failed promise]");
        if (priv->too_many_reads)
                ptr += sprintf(ptr, "[too many reads]");
        if (priv->no_valid_reads)
                ptr += sprintf(ptr, "[no valid reads-from]");
        if (priv->bad_synchronization)
                ptr += sprintf(ptr, "[bad sw ordering]");
        if (promises_expired())
                ptr += sprintf(ptr, "[promise expired]");
        if (promises.size() != 0)
                ptr += sprintf(ptr, "[unresolved promise]");
        if (ptr != buf)
                model_print("%s: %s\n", prefix ? prefix : "Infeasible", buf);
}

/**
 * Returns whether the current completed trace is feasible, except for pending
 * release sequences.
 */
bool ModelExecution::is_feasible_prefix_ignore_relseq() const
{
        return !is_infeasible() && promises.size() == 0;
}

/**
 * Check if the current partial trace is infeasible. Does not check any
 * end-of-execution flags, which might rule out the execution. Thus, this is
 * useful only for ruling an execution as infeasible.
 * @return whether the current partial trace is infeasible.
 */
bool ModelExecution::is_infeasible() const
{
        return mo_graph->checkForCycles() ||
                priv->no_valid_reads ||
                priv->failed_promise ||
                priv->too_many_reads ||
                priv->bad_synchronization ||
                promises_expired();
}

/** Close out a RMWR by converting previous RMWR into a RMW or READ. */
ModelAction * ModelExecution::process_rmw(ModelAction *act) {
        ModelAction *lastread = get_last_action(act->get_tid());
        lastread->process_rmw(act);
        if (act->is_rmw()) {
                if (lastread->get_reads_from())
                        mo_graph->addRMWEdge(lastread->get_reads_from(), lastread);
                else
                        mo_graph->addRMWEdge(lastread->get_reads_from_promise(), lastread);
                mo_graph->commitChanges();
        }
        return lastread;
}

/**
 * A helper function for ModelExecution::check_recency, to check if the current
 * thread is able to read from a different write/promise for 'params.maxreads'
 * number of steps and if that write/promise should become visible (i.e., is
 * ordered later in the modification order). This helps model memory liveness.
 *
 * @param curr The current action. Must be a read.
 * @param rf The write/promise from which we plan to read
 * @param other_rf The write/promise from which we may read
 * @return True if we were able to read from other_rf for params.maxreads steps
 */
template <typename T, typename U>
bool ModelExecution::should_read_instead(const ModelAction *curr, const T *rf, const U *other_rf) const
{
        /* Need a different write/promise */
        if (other_rf->equals(rf))
                return false;

        /* Only look for "newer" writes/promises */
        if (!mo_graph->checkReachable(rf, other_rf))
                return false;

        SnapVector<action_list_t> *thrd_lists = obj_thrd_map.get(curr->get_location());
        action_list_t *list = &(*thrd_lists)[id_to_int(curr->get_tid())];
        action_list_t::reverse_iterator rit = list->rbegin();
        ASSERT((*rit) == curr);
        /* Skip past curr */
        rit++;

        /* Does this write/promise work for everyone? */
        for (int i = 0; i < params->maxreads; i++, rit++) {
                ModelAction *act = *rit;
                if (!act->may_read_from(other_rf))
                        return false;
        }
        return true;
}

/**
 * Checks whether a thread has read from the same write or Promise for too many
 * times without seeing the effects of a later write/Promise.
 *
 * Basic idea:
 * 1) there must a different write/promise that we could read from,
 * 2) we must have read from the same write/promise in excess of maxreads times,
 * 3) that other write/promise must have been in the reads_from set for maxreads times, and
 * 4) that other write/promise must be mod-ordered after the write/promise we are reading.
 *
 * If so, we decide that the execution is no longer feasible.
 *
 * @param curr The current action. Must be a read.
 * @param rf The ModelAction/Promise from which we might read.
 * @return True if the read should succeed; false otherwise
 */
template <typename T>
bool ModelExecution::check_recency(ModelAction *curr, const T *rf) const
{
        if (!params->maxreads)
                return true;

        //NOTE: Next check is just optimization, not really necessary....
        if (curr->get_node()->get_read_from_past_size() +
                        curr->get_node()->get_read_from_promise_size() <= 1)
                return true;

        SnapVector<action_list_t> *thrd_lists = obj_thrd_map.get(curr->get_location());
        int tid = id_to_int(curr->get_tid());
        ASSERT(tid < (int)thrd_lists->size());
        action_list_t *list = &(*thrd_lists)[tid];
        action_list_t::reverse_iterator rit = list->rbegin();
        ASSERT((*rit) == curr);
        /* Skip past curr */
        rit++;

        action_list_t::reverse_iterator ritcopy = rit;
        /* See if we have enough reads from the same value */
        for (int count = 0; count < params->maxreads; ritcopy++, count++) {
                if (ritcopy == list->rend())
                        return true;
                ModelAction *act = *ritcopy;
                if (!act->is_read())
                        return true;
                if (act->get_reads_from_promise() && !act->get_reads_from_promise()->equals(rf))
                        return true;
                if (act->get_reads_from() && !act->get_reads_from()->equals(rf))
                        return true;
                if (act->get_node()->get_read_from_past_size() +
                                act->get_node()->get_read_from_promise_size() <= 1)
                        return true;
        }
        for (int i = 0; i < curr->get_node()->get_read_from_past_size(); i++) {
                const ModelAction *write = curr->get_node()->get_read_from_past(i);
                if (should_read_instead(curr, rf, write))
                        return false; /* liveness failure */
        }
        for (int i = 0; i < curr->get_node()->get_read_from_promise_size(); i++) {
                const Promise *promise = curr->get_node()->get_read_from_promise(i);
                if (should_read_instead(curr, rf, promise))
                        return false; /* liveness failure */
        }
        return true;
}

/**
 * @brief Updates the mo_graph with the constraints imposed from the current
 * read.
 *
 * Basic idea is the following: Go through each other thread and find
 * the last action that happened before our read.  Two cases:
 *
 * -# The action is a write: that write must either occur before
 * the write we read from or be the write we read from.
 * -# The action is a read: the write that that action read from
 * must occur before the write we read from or be the same write.
 *
 * @param curr The current action. Must be a read.
 * @param rf The ModelAction or Promise that curr reads from. Must be a write.
 * @return True if modification order edges were added; false otherwise
 */
template <typename rf_type>
bool ModelExecution::r_modification_order(ModelAction *curr, const rf_type *rf)
{
        SnapVector<action_list_t> *thrd_lists = obj_thrd_map.get(curr->get_location());
        unsigned int i;
        bool added = false;
        ASSERT(curr->is_read());

        /* Last SC fence in the current thread */
        ModelAction *last_sc_fence_local = get_last_seq_cst_fence(curr->get_tid(), NULL);
        ModelAction *last_sc_write = NULL;
        if (curr->is_seqcst())
                last_sc_write = get_last_seq_cst_write(curr);

        /* Iterate over all threads */
        for (i = 0; i < thrd_lists->size(); i++) {
                /* Last SC fence in thread i */
                ModelAction *last_sc_fence_thread_local = NULL;
                if (int_to_id((int)i) != curr->get_tid())
                        last_sc_fence_thread_local = get_last_seq_cst_fence(int_to_id(i), NULL);

                /* Last SC fence in thread i, before last SC fence in current thread */
                ModelAction *last_sc_fence_thread_before = NULL;
                if (last_sc_fence_local)
                        last_sc_fence_thread_before = get_last_seq_cst_fence(int_to_id(i), last_sc_fence_local);

                /* Iterate over actions in thread, starting from most recent */
                action_list_t *list = &(*thrd_lists)[i];
                action_list_t::reverse_iterator rit;
                for (rit = list->rbegin(); rit != list->rend(); rit++) {
                        ModelAction *act = *rit;

                        /* Skip curr */
                        if (act == curr)
                                continue;
                        /* Don't want to add reflexive edges on 'rf' */
                        if (act->equals(rf)) {
                                if (act->happens_before(curr))
                                        break;
                                else
                                        continue;
                        }

                        if (act->is_write()) {
                                /* C++, Section 29.3 statement 5 */
                                if (curr->is_seqcst() && last_sc_fence_thread_local &&
                                                *act < *last_sc_fence_thread_local) {
                                        added = mo_graph->addEdge(act, rf) || added;
                                        break;
                                }
                                /* C++, Section 29.3 statement 4 */
                                else if (act->is_seqcst() && last_sc_fence_local &&
                                                *act < *last_sc_fence_local) {
                                        added = mo_graph->addEdge(act, rf) || added;
                                        break;
                                }
                                /* C++, Section 29.3 statement 6 */
                                else if (last_sc_fence_thread_before &&
                                                *act < *last_sc_fence_thread_before) {
                                        added = mo_graph->addEdge(act, rf) || added;
                                        break;
                                }
                        }

                        /* C++, Section 29.3 statement 3 (second subpoint) */
                        if (curr->is_seqcst() && last_sc_write && act == last_sc_write) {
                                added = mo_graph->addEdge(act, rf) || added;
                                break;
                        }

                        /*
                         * Include at most one act per-thread that "happens
                         * before" curr
                         */
                        if (act->happens_before(curr)) {
                                if (act->is_write()) {
                                        added = mo_graph->addEdge(act, rf) || added;
                                } else {
                                        const ModelAction *prevrf = act->get_reads_from();
                                        const Promise *prevrf_promise = act->get_reads_from_promise();
                                        if (prevrf) {
                                                if (!prevrf->equals(rf))
                                                        added = mo_graph->addEdge(prevrf, rf) || added;
                                        } else if (!prevrf_promise->equals(rf)) {
                                                added = mo_graph->addEdge(prevrf_promise, rf) || added;
                                        }
                                }
                                break;
                        }
                }
        }

        /*
         * All compatible, thread-exclusive promises must be ordered after any
         * concrete loads from the same thread
         */
        for (unsigned int i = 0; i < promises.size(); i++)
                if (promises[i]->is_compatible_exclusive(curr))
                        added = mo_graph->addEdge(rf, promises[i]) || added;

        return added;
}

/**
 * Updates the mo_graph with the constraints imposed from the current write.
 *
 * Basic idea is the following: Go through each other thread and find
 * the lastest action that happened before our write.  Two cases:
 *
 * (1) The action is a write => that write must occur before
 * the current write
 *
 * (2) The action is a read => the write that that action read from
 * must occur before the current write.
 *
 * This method also handles two other issues:
 *
 * (I) Sequential Consistency: Making sure that if the current write is
 * seq_cst, that it occurs after the previous seq_cst write.
 *
 * (II) Sending the write back to non-synchronizing reads.
 *
 * @param curr The current action. Must be a write.
 * @param send_fv A vector for stashing reads to which we may pass our future
 * value. If NULL, then don't record any future values.
 * @return True if modification order edges were added; false otherwise
 */
bool ModelExecution::w_modification_order(ModelAction *curr, ModelVector<ModelAction *> *send_fv)
{
        SnapVector<action_list_t> *thrd_lists = obj_thrd_map.get(curr->get_location());
        unsigned int i;
        bool added = false;
        ASSERT(curr->is_write());

        if (curr->is_seqcst()) {
                /* We have to at least see the last sequentially consistent write,
                         so we are initialized. */
                ModelAction *last_seq_cst = get_last_seq_cst_write(curr);
                if (last_seq_cst != NULL) {
                        added = mo_graph->addEdge(last_seq_cst, curr) || added;
                }
        }

        /* Last SC fence in the current thread */
        ModelAction *last_sc_fence_local = get_last_seq_cst_fence(curr->get_tid(), NULL);

        /* Iterate over all threads */
        for (i = 0; i < thrd_lists->size(); i++) {
                /* Last SC fence in thread i, before last SC fence in current thread */
                ModelAction *last_sc_fence_thread_before = NULL;
                if (last_sc_fence_local && int_to_id((int)i) != curr->get_tid())
                        last_sc_fence_thread_before = get_last_seq_cst_fence(int_to_id(i), last_sc_fence_local);

                /* Iterate over actions in thread, starting from most recent */
                action_list_t *list = &(*thrd_lists)[i];
                action_list_t::reverse_iterator rit;
                for (rit = list->rbegin(); rit != list->rend(); rit++) {
                        ModelAction *act = *rit;
                        if (act == curr) {
                                /*
                                 * 1) If RMW and it actually read from something, then we
                                 * already have all relevant edges, so just skip to next
                                 * thread.
                                 *
                                 * 2) If RMW and it didn't read from anything, we should
                                 * whatever edge we can get to speed up convergence.
                                 *
                                 * 3) If normal write, we need to look at earlier actions, so
                                 * continue processing list.
                                 */
                                if (curr->is_rmw()) {
                                        if (curr->get_reads_from() != NULL)
                                                break;
                                        else
                                                continue;
                                } else
                                        continue;
                        }

                        /* C++, Section 29.3 statement 7 */
                        if (last_sc_fence_thread_before && act->is_write() &&
                                        *act < *last_sc_fence_thread_before) {
                                added = mo_graph->addEdge(act, curr) || added;
                                break;
                        }

                        /*
                         * Include at most one act per-thread that "happens
                         * before" curr
                         */
                        if (act->happens_before(curr)) {
                                /*
                                 * Note: if act is RMW, just add edge:
                                 *   act --mo--> curr
                                 * The following edge should be handled elsewhere:
                                 *   readfrom(act) --mo--> act
                                 */
                                if (act->is_write())
                                        added = mo_graph->addEdge(act, curr) || added;
                                else if (act->is_read()) {
                                        //if previous read accessed a null, just keep going
                                        if (act->get_reads_from() == NULL)
                                                continue;
                                        added = mo_graph->addEdge(act->get_reads_from(), curr) || added;
                                }
                                break;
                        } else if (act->is_read() && !act->could_synchronize_with(curr) &&
                                                     !act->same_thread(curr)) {
                                /* We have an action that:
                                   (1) did not happen before us
                                   (2) is a read and we are a write
                                   (3) cannot synchronize with us
                                   (4) is in a different thread
                                   =>
                                   that read could potentially read from our write.  Note that
                                   these checks are overly conservative at this point, we'll
                                   do more checks before actually removing the
                                   pendingfuturevalue.

                                 */
                                if (send_fv && thin_air_constraint_may_allow(curr, act)) {
                                        if (!is_infeasible())
                                                send_fv->push_back(act);
                                        else if (curr->is_rmw() && act->is_rmw() && curr->get_reads_from() && curr->get_reads_from() == act->get_reads_from())
                                                add_future_value(curr, act);
                                }
                        }
                }
        }

        /*
         * All compatible, thread-exclusive promises must be ordered after any
         * concrete stores to the same thread, or else they can be merged with
         * this store later
         */
        for (unsigned int i = 0; i < promises.size(); i++)
                if (promises[i]->is_compatible_exclusive(curr))
                        added = mo_graph->addEdge(curr, promises[i]) || added;

        return added;
}

/** Arbitrary reads from the future are not allowed.  Section 29.3
 * part 9 places some constraints.  This method checks one result of constraint
 * constraint.  Others require compiler support. */
bool ModelExecution::thin_air_constraint_may_allow(const ModelAction *writer, const ModelAction *reader) const
{
        if (!writer->is_rmw())
                return true;

        if (!reader->is_rmw())
                return true;

        for (const ModelAction *search = writer->get_reads_from(); search != NULL; search = search->get_reads_from()) {
                if (search == reader)
                        return false;
                if (search->get_tid() == reader->get_tid() &&
                                search->happens_before(reader))
                        break;
        }

        return true;
}

/**
 * Arbitrary reads from the future are not allowed. Section 29.3 part 9 places
 * some constraints. This method checks one the following constraint (others
 * require compiler support):
 *
 *   If X --hb-> Y --mo-> Z, then X should not read from Z.
 */
bool ModelExecution::mo_may_allow(const ModelAction *writer, const ModelAction *reader)
{
        SnapVector<action_list_t> *thrd_lists = obj_thrd_map.get(reader->get_location());
        unsigned int i;
        /* Iterate over all threads */
        for (i = 0; i < thrd_lists->size(); i++) {
                const ModelAction *write_after_read = NULL;

                /* Iterate over actions in thread, starting from most recent */
                action_list_t *list = &(*thrd_lists)[i];
                action_list_t::reverse_iterator rit;
                for (rit = list->rbegin(); rit != list->rend(); rit++) {
                        ModelAction *act = *rit;

                        /* Don't disallow due to act == reader */
                        if (!reader->happens_before(act) || reader == act)
                                break;
                        else if (act->is_write())
                                write_after_read = act;
                        else if (act->is_read() && act->get_reads_from() != NULL)
                                write_after_read = act->get_reads_from();
                }

                if (write_after_read && write_after_read != writer && mo_graph->checkReachable(write_after_read, writer))
                        return false;
        }
        return true;
}

/**
 * Finds the head(s) of the release sequence(s) containing a given ModelAction.
 * The ModelAction under consideration is expected to be taking part in
 * release/acquire synchronization as an object of the "reads from" relation.
 * Note that this can only provide release sequence support for RMW chains
 * which do not read from the future, as those actions cannot be traced until
 * their "promise" is fulfilled. Similarly, we may not even establish the
 * presence of a release sequence with certainty, as some modification order
 * constraints may be decided further in the future. Thus, this function
 * "returns" two pieces of data: a pass-by-reference vector of @a release_heads
 * and a boolean representing certainty.
 *
 * @param rf The action that might be part of a release sequence. Must be a
 * write.
 * @param release_heads A pass-by-reference style return parameter. After
 * execution of this function, release_heads will contain the heads of all the
 * relevant release sequences, if any exists with certainty
 * @param pending A pass-by-reference style return parameter which is only used
 * when returning false (i.e., uncertain). Returns most information regarding
 * an uncertain release sequence, including any write operations that might
 * break the sequence.
 * @return true, if the ModelExecution is certain that release_heads is complete;
 * false otherwise
 */
bool ModelExecution::release_seq_heads(const ModelAction *rf,
                rel_heads_list_t *release_heads,
                struct release_seq *pending) const
{
        /* Only check for release sequences if there are no cycles */
        if (mo_graph->checkForCycles())
                return false;

        for ( ; rf != NULL; rf = rf->get_reads_from()) {
                ASSERT(rf->is_write());

                if (rf->is_release())
                        release_heads->push_back(rf);
                else if (rf->get_last_fence_release())
                        release_heads->push_back(rf->get_last_fence_release());
                if (!rf->is_rmw())
                        break; /* End of RMW chain */

                /** @todo Need to be smarter here...  In the linux lock
                 * example, this will run to the beginning of the program for
                 * every acquire. */
                /** @todo The way to be smarter here is to keep going until 1
                 * thread has a release preceded by an acquire and you've seen
                 *       both. */

                /* acq_rel RMW is a sufficient stopping condition */
                if (rf->is_acquire() && rf->is_release())
                        return true; /* complete */
        };
        if (!rf) {
                /* read from future: need to settle this later */
                pending->rf = NULL;
                return false; /* incomplete */
        }

        if (rf->is_release())
                return true; /* complete */

        /* else relaxed write
         * - check for fence-release in the same thread (29.8, stmt. 3)
         * - check modification order for contiguous subsequence
         *   -> rf must be same thread as release */

        const ModelAction *fence_release = rf->get_last_fence_release();
        /* Synchronize with a fence-release unconditionally; we don't need to
         * find any more "contiguous subsequence..." for it */
        if (fence_release)
                release_heads->push_back(fence_release);

        int tid = id_to_int(rf->get_tid());
        SnapVector<action_list_t> *thrd_lists = obj_thrd_map.get(rf->get_location());
        action_list_t *list = &(*thrd_lists)[tid];
        action_list_t::const_reverse_iterator rit;

        /* Find rf in the thread list */
        rit = std::find(list->rbegin(), list->rend(), rf);
        ASSERT(rit != list->rend());

        /* Find the last {write,fence}-release */
        for (; rit != list->rend(); rit++) {
                if (fence_release && *(*rit) < *fence_release)
                        break;
                if ((*rit)->is_release())
                        break;
        }
        if (rit == list->rend()) {
                /* No write-release in this thread */
                return true; /* complete */
        } else if (fence_release && *(*rit) < *fence_release) {
                /* The fence-release is more recent (and so, "stronger") than
                 * the most recent write-release */
                return true; /* complete */
        } /* else, need to establish contiguous release sequence */
        ModelAction *release = *rit;

        ASSERT(rf->same_thread(release));

        pending->writes.clear();

        bool certain = true;
        for (unsigned int i = 0; i < thrd_lists->size(); i++) {
                if (id_to_int(rf->get_tid()) == (int)i)
                        continue;
                list = &(*thrd_lists)[i];

                /* Can we ensure no future writes from this thread may break
                 * the release seq? */
                bool future_ordered = false;

                ModelAction *last = get_last_action(int_to_id(i));
                Thread *th = get_thread(int_to_id(i));
                if ((last && rf->happens_before(last)) ||
                                !is_enabled(th) ||
                                th->is_complete())
                        future_ordered = true;

                ASSERT(!th->is_model_thread() || future_ordered);

                for (rit = list->rbegin(); rit != list->rend(); rit++) {
                        const ModelAction *act = *rit;
                        /* Reach synchronization -> this thread is complete */
                        if (act->happens_before(release))
                                break;
                        if (rf->happens_before(act)) {
                                future_ordered = true;
                                continue;
                        }

                        /* Only non-RMW writes can break release sequences */
                        if (!act->is_write() || act->is_rmw())
                                continue;

                        /* Check modification order */
                        if (mo_graph->checkReachable(rf, act)) {
                                /* rf --mo--> act */
                                future_ordered = true;
                                continue;
                        }
                        if (mo_graph->checkReachable(act, release))
                                /* act --mo--> release */
                                break;
                        if (mo_graph->checkReachable(release, act) &&
                                      mo_graph->checkReachable(act, rf)) {
                                /* release --mo-> act --mo--> rf */
                                return true; /* complete */
                        }
                        /* act may break release sequence */
                        pending->writes.push_back(act);
                        certain = false;
                }
                if (!future_ordered)
                        certain = false; /* This thread is uncertain */
        }

        if (certain) {
                release_heads->push_back(release);
                pending->writes.clear();
        } else {
                pending->release = release;
                pending->rf = rf;
        }
        return certain;
}

/**
 * An interface for getting the release sequence head(s) with which a
 * given ModelAction must synchronize. This function only returns a non-empty
 * result when it can locate a release sequence head with certainty. Otherwise,
 * it may mark the internal state of the ModelExecution so that it will handle
 * the release sequence at a later time, causing @a acquire to update its
 * synchronization at some later point in execution.
 *
 * @param acquire The 'acquire' action that may synchronize with a release
 * sequence
 * @param read The read action that may read from a release sequence; this may
 * be the same as acquire, or else an earlier action in the same thread (i.e.,
 * when 'acquire' is a fence-acquire)
 * @param release_heads A pass-by-reference return parameter. Will be filled
 * with the head(s) of the release sequence(s), if they exists with certainty.
 * @see ModelExecution::release_seq_heads
 */
void ModelExecution::get_release_seq_heads(ModelAction *acquire,
                ModelAction *read, rel_heads_list_t *release_heads)
{
        const ModelAction *rf = read->get_reads_from();
        struct release_seq *sequence = (struct release_seq *)snapshot_calloc(1, sizeof(struct release_seq));
        sequence->acquire = acquire;
        sequence->read = read;

        if (!release_seq_heads(rf, release_heads, sequence)) {
                /* add act to 'lazy checking' list */
                pending_rel_seqs.push_back(sequence);
        } else {
                snapshot_free(sequence);
        }
}

/**
 * Attempt to resolve all stashed operations that might synchronize with a
 * release sequence for a given location. This implements the "lazy" portion of
 * determining whether or not a release sequence was contiguous, since not all
 * modification order information is present at the time an action occurs.
 *
 * @param location The location/object that should be checked for release
 * sequence resolutions. A NULL value means to check all locations.
 * @param work_queue The work queue to which to add work items as they are
 * generated
 * @return True if any updates occurred (new synchronization, new mo_graph
 * edges)
 */
bool ModelExecution::resolve_release_sequences(void *location, work_queue_t *work_queue)
{
        bool updated = false;
        SnapVector<struct release_seq *>::iterator it = pending_rel_seqs.begin();
        while (it != pending_rel_seqs.end()) {
                struct release_seq *pending = *it;
                ModelAction *acquire = pending->acquire;
                const ModelAction *read = pending->read;

                /* Only resolve sequences on the given location, if provided */
                if (location && read->get_location() != location) {
                        it++;
                        continue;
                }

                const ModelAction *rf = read->get_reads_from();
                rel_heads_list_t release_heads;
                bool complete;
                complete = release_seq_heads(rf, &release_heads, pending);
                for (unsigned int i = 0; i < release_heads.size(); i++)
                        if (!acquire->has_synchronized_with(release_heads[i]))
                                if (synchronize(release_heads[i], acquire))
                                        updated = true;

                if (updated) {
                        /* Re-check all pending release sequences */
                        work_queue->push_back(CheckRelSeqWorkEntry(NULL));
                        /* Re-check read-acquire for mo_graph edges */
                        if (acquire->is_read())
                                work_queue->push_back(MOEdgeWorkEntry(acquire));

                        /* propagate synchronization to later actions */
                        action_list_t::reverse_iterator rit = action_trace.rbegin();
                        for (; (*rit) != acquire; rit++) {
                                ModelAction *propagate = *rit;
                                if (acquire->happens_before(propagate)) {
                                        synchronize(acquire, propagate);
                                        /* Re-check 'propagate' for mo_graph edges */
                                        work_queue->push_back(MOEdgeWorkEntry(propagate));
                                }
                        }
                }
                if (complete) {
                        it = pending_rel_seqs.erase(it);
                        snapshot_free(pending);
                } else {
                        it++;
                }
        }

        // If we resolved promises or data races, see if we have realized a data race.
        checkDataRaces();

        return updated;
}

/**
 * Performs various bookkeeping operations for the current ModelAction. For
 * instance, adds action to the per-object, per-thread action vector and to the
 * action trace list of all thread actions.
 *
 * @param act is the ModelAction to add.
 */
void ModelExecution::add_action_to_lists(ModelAction *act)
{
        int tid = id_to_int(act->get_tid());
        ModelAction *uninit = NULL;
        int uninit_id = -1;
        action_list_t *list = get_safe_ptr_action(&obj_map, act->get_location());
        if (list->empty() && act->is_atomic_var()) {
                uninit = get_uninitialized_action(act);
                uninit_id = id_to_int(uninit->get_tid());
                list->push_front(uninit);
        }
        list->push_back(act);

        action_trace.push_back(act);
        if (uninit)
                action_trace.push_front(uninit);

        SnapVector<action_list_t> *vec = get_safe_ptr_vect_action(&obj_thrd_map, act->get_location());
        if (tid >= (int)vec->size())
                vec->resize(priv->next_thread_id);
        (*vec)[tid].push_back(act);
        if (uninit)
                (*vec)[uninit_id].push_front(uninit);

        if ((int)thrd_last_action.size() <= tid)
                thrd_last_action.resize(get_num_threads());
        thrd_last_action[tid] = act;
        if (uninit)
                thrd_last_action[uninit_id] = uninit;

        if (act->is_fence() && act->is_release()) {
                if ((int)thrd_last_fence_release.size() <= tid)
                        thrd_last_fence_release.resize(get_num_threads());
                thrd_last_fence_release[tid] = act;
        }

        if (act->is_wait()) {
                void *mutex_loc = (void *) act->get_value();
                get_safe_ptr_action(&obj_map, mutex_loc)->push_back(act);

                SnapVector<action_list_t> *vec = get_safe_ptr_vect_action(&obj_thrd_map, mutex_loc);
                if (tid >= (int)vec->size())
                        vec->resize(priv->next_thread_id);
                (*vec)[tid].push_back(act);
        }
}

/**
 * @brief Get the last action performed by a particular Thread
 * @param tid The thread ID of the Thread in question
 * @return The last action in the thread
 */
ModelAction * ModelExecution::get_last_action(thread_id_t tid) const
{
        int threadid = id_to_int(tid);
        if (threadid < (int)thrd_last_action.size())
                return thrd_last_action[id_to_int(tid)];
        else
                return NULL;
}

/**
 * @brief Get the last fence release performed by a particular Thread
 * @param tid The thread ID of the Thread in question
 * @return The last fence release in the thread, if one exists; NULL otherwise
 */
ModelAction * ModelExecution::get_last_fence_release(thread_id_t tid) const
{
        int threadid = id_to_int(tid);
        if (threadid < (int)thrd_last_fence_release.size())
                return thrd_last_fence_release[id_to_int(tid)];
        else
                return NULL;
}

/**
 * Gets the last memory_order_seq_cst write (in the total global sequence)
 * performed on a particular object (i.e., memory location), not including the
 * current action.
 * @param curr The current ModelAction; also denotes the object location to
 * check
 * @return The last seq_cst write
 */
ModelAction * ModelExecution::get_last_seq_cst_write(ModelAction *curr) const
{
        void *location = curr->get_location();
        action_list_t *list = obj_map.get(location);
        /* Find: max({i in dom(S) | seq_cst(t_i) && isWrite(t_i) && samevar(t_i, t)}) */
        action_list_t::reverse_iterator rit;
        for (rit = list->rbegin(); (*rit) != curr; rit++)
                ;
        rit++; /* Skip past curr */
        for ( ; rit != list->rend(); rit++)
                if ((*rit)->is_write() && (*rit)->is_seqcst())
                        return *rit;
        return NULL;
}

/**
 * Gets the last memory_order_seq_cst fence (in the total global sequence)
 * performed in a particular thread, prior to a particular fence.
 * @param tid The ID of the thread to check
 * @param before_fence The fence from which to begin the search; if NULL, then
 * search for the most recent fence in the thread.
 * @return The last prior seq_cst fence in the thread, if exists; otherwise, NULL
 */
ModelAction * ModelExecution::get_last_seq_cst_fence(thread_id_t tid, const ModelAction *before_fence) const
{
        /* All fences should have location FENCE_LOCATION */
        action_list_t *list = obj_map.get(FENCE_LOCATION);

        if (!list)
                return NULL;

        action_list_t::reverse_iterator rit = list->rbegin();

        if (before_fence) {
                for (; rit != list->rend(); rit++)
                        if (*rit == before_fence)
                                break;

                ASSERT(*rit == before_fence);
                rit++;
        }

        for (; rit != list->rend(); rit++)
                if ((*rit)->is_fence() && (tid == (*rit)->get_tid()) && (*rit)->is_seqcst())
                        return *rit;
        return NULL;
}

/**
 * Gets the last unlock operation performed on a particular mutex (i.e., memory
 * location). This function identifies the mutex according to the current
 * action, which is presumed to perform on the same mutex.
 * @param curr The current ModelAction; also denotes the object location to
 * check
 * @return The last unlock operation
 */
ModelAction * ModelExecution::get_last_unlock(ModelAction *curr) const
{
        void *location = curr->get_location();
        action_list_t *list = obj_map.get(location);
        /* Find: max({i in dom(S) | isUnlock(t_i) && samevar(t_i, t)}) */
        action_list_t::reverse_iterator rit;
        for (rit = list->rbegin(); rit != list->rend(); rit++)
                if ((*rit)->is_unlock() || (*rit)->is_wait())
                        return *rit;
        return NULL;
}

ModelAction * ModelExecution::get_parent_action(thread_id_t tid) const
{
        ModelAction *parent = get_last_action(tid);
        if (!parent)
                parent = get_thread(tid)->get_creation();
        return parent;
}

/**
 * Returns the clock vector for a given thread.
 * @param tid The thread whose clock vector we want
 * @return Desired clock vector
 */
ClockVector * ModelExecution::get_cv(thread_id_t tid) const
{
        return get_parent_action(tid)->get_cv();
}

/**
 * @brief Find the promise (if any) to resolve for the current action and
 * remove it from the pending promise vector
 * @param curr The current ModelAction. Should be a write.
 * @return The Promise to resolve, if any; otherwise NULL
 */
Promise * ModelExecution::pop_promise_to_resolve(const ModelAction *curr)
{
        for (unsigned int i = 0; i < promises.size(); i++)
                if (curr->get_node()->get_promise(i)) {
                        Promise *ret = promises[i];
                        promises.erase(promises.begin() + i);
                        return ret;
                }
        return NULL;
}

/**
 * Resolve a Promise with a current write.
 * @param write The ModelAction that is fulfilling Promises
 * @param promise The Promise to resolve
 * @return True if the Promise was successfully resolved; false otherwise
 */
bool ModelExecution::resolve_promise(ModelAction *write, Promise *promise)
{
        ModelVector<ModelAction *> actions_to_check;

        for (unsigned int i = 0; i < promise->get_num_readers(); i++) {
                ModelAction *read = promise->get_reader(i);
                read_from(read, write);
                actions_to_check.push_back(read);
        }
        /* Make sure the promise's value matches the write's value */
        ASSERT(promise->is_compatible(write) && promise->same_value(write));
        if (!mo_graph->resolvePromise(promise, write))
                priv->failed_promise = true;

        /**
         * @todo  It is possible to end up in an inconsistent state, where a
         * "resolved" promise may still be referenced if
         * CycleGraph::resolvePromise() failed, so don't delete 'promise'.
         *
         * Note that the inconsistency only matters when dumping mo_graph to
         * file.
         *
         * delete promise;
         */

        //Check whether reading these writes has made threads unable to
        //resolve promises
        for (unsigned int i = 0; i < actions_to_check.size(); i++) {
                ModelAction *read = actions_to_check[i];
                mo_check_promises(read, true);
        }

        return true;
}

/**
 * Compute the set of promises that could potentially be satisfied by this
 * action. Note that the set computation actually appears in the Node, not in
 * ModelExecution.
 * @param curr The ModelAction that may satisfy promises
 */
void ModelExecution::compute_promises(ModelAction *curr)
{
        for (unsigned int i = 0; i < promises.size(); i++) {
                Promise *promise = promises[i];
                if (!promise->is_compatible(curr) || !promise->same_value(curr))
                        continue;

                bool satisfy = true;
                for (unsigned int j = 0; j < promise->get_num_readers(); j++) {
                        const ModelAction *act = promise->get_reader(j);
                        if (act->happens_before(curr) ||
                                        act->could_synchronize_with(curr)) {
                                satisfy = false;
                                break;
                        }
                }
                if (satisfy)
                        curr->get_node()->set_promise(i);
        }
}

/** Checks promises in response to change in ClockVector Threads. */
void ModelExecution::check_promises(thread_id_t tid, ClockVector *old_cv, ClockVector *merge_cv)
{
        for (unsigned int i = 0; i < promises.size(); i++) {
                Promise *promise = promises[i];
                if (!promise->thread_is_available(tid))
                        continue;
                for (unsigned int j = 0; j < promise->get_num_readers(); j++) {
                        const ModelAction *act = promise->get_reader(j);
                        if ((!old_cv || !old_cv->synchronized_since(act)) &&
                                        merge_cv->synchronized_since(act)) {
                                if (promise->eliminate_thread(tid)) {
                                        /* Promise has failed */
                                        priv->failed_promise = true;
                                        return;
                                }
                        }
                }
        }
}

void ModelExecution::check_promises_thread_disabled()
{
        for (unsigned int i = 0; i < promises.size(); i++) {
                Promise *promise = promises[i];
                if (promise->has_failed()) {
                        priv->failed_promise = true;
                        return;
                }
        }
}

/**
 * @brief Checks promises in response to addition to modification order for
 * threads.
 *
 * We test whether threads are still available for satisfying promises after an
 * addition to our modification order constraints. Those that are unavailable
 * are "eliminated". Once all threads are eliminated from satisfying a promise,
 * that promise has failed.
 *
 * @param act The ModelAction which updated the modification order
 * @param is_read_check Should be true if act is a read and we must check for
 * updates to the store from which it read (there is a distinction here for
 * RMW's, which are both a load and a store)
 */
void ModelExecution::mo_check_promises(const ModelAction *act, bool is_read_check)
{
        const ModelAction *write = is_read_check ? act->get_reads_from() : act;

        for (unsigned int i = 0; i < promises.size(); i++) {
                Promise *promise = promises[i];

                // Is this promise on the same location?
                if (!promise->same_location(write))
                        continue;

                for (unsigned int j = 0; j < promise->get_num_readers(); j++) {
                        const ModelAction *pread = promise->get_reader(j);
                        if (!pread->happens_before(act))
                               continue;
                        if (mo_graph->checkPromise(write, promise)) {
                                priv->failed_promise = true;
                                return;
                        }
                        break;
                }

                // Don't do any lookups twice for the same thread
                if (!promise->thread_is_available(act->get_tid()))
                        continue;

                if (mo_graph->checkReachable(promise, write)) {
                        if (mo_graph->checkPromise(write, promise)) {
                                priv->failed_promise = true;
                                return;
                        }
                }
        }
}

/**
 * Compute the set of writes that may break the current pending release
 * sequence. This information is extracted from previou release sequence
 * calculations.
 *
 * @param curr The current ModelAction. Must be a release sequence fixup
 * action.
 */
void ModelExecution::compute_relseq_breakwrites(ModelAction *curr)
{
        if (pending_rel_seqs.empty())
                return;

        struct release_seq *pending = pending_rel_seqs.back();
        for (unsigned int i = 0; i < pending->writes.size(); i++) {
                const ModelAction *write = pending->writes[i];
                curr->get_node()->add_relseq_break(write);
        }

        /* NULL means don't break the sequence; just synchronize */
        curr->get_node()->add_relseq_break(NULL);
}

/**
 * Build up an initial set of all past writes that this 'read' action may read
 * from, as well as any previously-observed future values that must still be valid.
 *
 * @param curr is the current ModelAction that we are exploring; it must be a
 * 'read' operation.
 */
void ModelExecution::build_may_read_from(ModelAction *curr)
{
        SnapVector<action_list_t> *thrd_lists = obj_thrd_map.get(curr->get_location());
        unsigned int i;
        ASSERT(curr->is_read());

        ModelAction *last_sc_write = NULL;

        if (curr->is_seqcst())
                last_sc_write = get_last_seq_cst_write(curr);

        /* Iterate over all threads */
        for (i = 0; i < thrd_lists->size(); i++) {
                /* Iterate over actions in thread, starting from most recent */
                action_list_t *list = &(*thrd_lists)[i];
                action_list_t::reverse_iterator rit;
                for (rit = list->rbegin(); rit != list->rend(); rit++) {
                        ModelAction *act = *rit;

                        /* Only consider 'write' actions */
                        if (!act->is_write() || act == curr)
                                continue;

                        /* Don't consider more than one seq_cst write if we are a seq_cst read. */
                        bool allow_read = true;

                        if (curr->is_seqcst() && (act->is_seqcst() || (last_sc_write != NULL && act->happens_before(last_sc_write))) && act != last_sc_write)
                                allow_read = false;
                        else if (curr->get_sleep_flag() && !curr->is_seqcst() && !sleep_can_read_from(curr, act))
                                allow_read = false;

                        if (allow_read) {
                                /* Only add feasible reads */
                                mo_graph->startChanges();
                                r_modification_order(curr, act);
                                if (!is_infeasible())
                                        curr->get_node()->add_read_from_past(act);
                                mo_graph->rollbackChanges();
                        }

                        /* Include at most one act per-thread that "happens before" curr */
                        if (act->happens_before(curr))
                                break;
                }
        }

        /* Inherit existing, promised future values */
        for (i = 0; i < promises.size(); i++) {
                const Promise *promise = promises[i];
                const ModelAction *promise_read = promise->get_reader(0);
                if (promise_read->same_var(curr)) {
                        /* Only add feasible future-values */
                        mo_graph->startChanges();
                        r_modification_order(curr, promise);
                        if (!is_infeasible())
                                curr->get_node()->add_read_from_promise(promise_read);
                        mo_graph->rollbackChanges();
                }
        }

        /* We may find no valid may-read-from only if the execution is doomed */
        if (!curr->get_node()->read_from_size()) {
                priv->no_valid_reads = true;
                set_assert();
        }

        if (DBG_ENABLED()) {
                model_print("Reached read action:\n");
                curr->print();
                model_print("Printing read_from_past\n");
                curr->get_node()->print_read_from_past();
                model_print("End printing read_from_past\n");
        }
}

bool ModelExecution::sleep_can_read_from(ModelAction *curr, const ModelAction *write)
{
        for ( ; write != NULL; write = write->get_reads_from()) {
                /* UNINIT actions don't have a Node, and they never sleep */
                if (write->is_uninitialized())
                        return true;
                Node *prevnode = write->get_node()->get_parent();

                bool thread_sleep = prevnode->enabled_status(curr->get_tid()) == THREAD_SLEEP_SET;
                if (write->is_release() && thread_sleep)
                        return true;
                if (!write->is_rmw())
                        return false;
        }
        return true;
}

/**
 * @brief Get an action representing an uninitialized atomic
 *
 * This function may create a new one or try to retrieve one from the NodeStack
 *
 * @param curr The current action, which prompts the creation of an UNINIT action
 * @return A pointer to the UNINIT ModelAction
 */
ModelAction * ModelExecution::get_uninitialized_action(const ModelAction *curr) const
{
        Node *node = curr->get_node();
        ModelAction *act = node->get_uninit_action();
        if (!act) {
                act = new ModelAction(ATOMIC_UNINIT, std::memory_order_relaxed, curr->get_location(), params->uninitvalue, model_thread);
                node->set_uninit_action(act);
        }
        act->create_cv(NULL);
        return act;
}

static void print_list(const action_list_t *list)
{
        action_list_t::const_iterator it;

        model_print("---------------------------------------------------------------------\n");

        unsigned int hash = 0;

        for (it = list->begin(); it != list->end(); it++) {
                const ModelAction *act = *it;
                if (act->get_seq_number() > 0)
                        act->print();
                hash = hash^(hash<<3)^((*it)->hash());
        }
        model_print("HASH %u\n", hash);
        model_print("---------------------------------------------------------------------\n");
}

#if SUPPORT_MOD_ORDER_DUMP
void ModelExecution::dumpGraph(char *filename) const
{
        char buffer[200];
        sprintf(buffer, "%s.dot", filename);
        FILE *file = fopen(buffer, "w");
        fprintf(file, "digraph %s {\n", filename);
        mo_graph->dumpNodes(file);
        ModelAction **thread_array = (ModelAction **)model_calloc(1, sizeof(ModelAction *) * get_num_threads());

        for (action_list_t::const_iterator it = action_trace.begin(); it != action_trace.end(); it++) {
                ModelAction *act = *it;
                if (act->is_read()) {
                        mo_graph->dot_print_node(file, act);
                        if (act->get_reads_from())
                                mo_graph->dot_print_edge(file,
                                                act->get_reads_from(),
                                                act,
                                                "label=\"rf\", color=red, weight=2");
                        else
                                mo_graph->dot_print_edge(file,
                                                act->get_reads_from_promise(),
                                                act,
                                                "label=\"rf\", color=red");
                }
                if (thread_array[act->get_tid()]) {
                        mo_graph->dot_print_edge(file,
                                        thread_array[id_to_int(act->get_tid())],
                                        act,
                                        "label=\"sb\", color=blue, weight=400");
                }

                thread_array[act->get_tid()] = act;
        }
        fprintf(file, "}\n");
        model_free(thread_array);
        fclose(file);
}
#endif

/** @brief Prints an execution trace summary. */
void ModelExecution::print_summary() const
{
#if SUPPORT_MOD_ORDER_DUMP
        char buffername[100];
        sprintf(buffername, "exec%04u", get_execution_number());
        mo_graph->dumpGraphToFile(buffername);
        sprintf(buffername, "graph%04u", get_execution_number());
        dumpGraph(buffername);
#endif

        model_print("Execution %d:", get_execution_number());
        if (isfeasibleprefix()) {
                if (is_yieldblocked())
                        model_print(" YIELD BLOCKED");
                if (scheduler->all_threads_sleeping())
                        model_print(" SLEEP-SET REDUNDANT");
                model_print("\n");
        } else
                print_infeasibility(" INFEASIBLE");
        print_list(&action_trace);
        model_print("\n");
        if (!promises.empty()) {
                model_print("Pending promises:\n");
                for (unsigned int i = 0; i < promises.size(); i++) {
                        model_print(" [P%u] ", i);
                        promises[i]->print();
                }
                model_print("\n");
        }
}

/**
 * Add a Thread to the system for the first time. Should only be called once
 * per thread.
 * @param t The Thread to add
 */
void ModelExecution::add_thread(Thread *t)
{
        unsigned int i = id_to_int(t->get_id());
        if (i >= thread_map.size())
                thread_map.resize(i + 1);
        thread_map[i] = t;
        if (!t->is_model_thread())
                scheduler->add_thread(t);
}

/**
 * @brief Get a Thread reference by its ID
 * @param tid The Thread's ID
 * @return A Thread reference
 */
Thread * ModelExecution::get_thread(thread_id_t tid) const
{
        unsigned int i = id_to_int(tid);
        if (i < thread_map.size())
                return thread_map[i];
        return NULL;
}

/**
 * @brief Get a reference to the Thread in which a ModelAction was executed
 * @param act The ModelAction
 * @return A Thread reference
 */
Thread * ModelExecution::get_thread(const ModelAction *act) const
{
        return get_thread(act->get_tid());
}

/**
 * @brief Get a Promise's "promise number"
 *
 * A "promise number" is an index number that is unique to a promise, valid
 * only for a specific snapshot of an execution trace. Promises may come and go
 * as they are generated an resolved, so an index only retains meaning for the
 * current snapshot.
 *
 * @param promise The Promise to check
 * @return The promise index, if the promise still is valid; otherwise -1
 */
int ModelExecution::get_promise_number(const Promise *promise) const
{
        for (unsigned int i = 0; i < promises.size(); i++)
                if (promises[i] == promise)
                        return i;
        /* Not found */
        return -1;
}

/**
 * @brief Check if a Thread is currently enabled
 * @param t The Thread to check
 * @return True if the Thread is currently enabled
 */
bool ModelExecution::is_enabled(Thread *t) const
{
        return scheduler->is_enabled(t);
}

/**
 * @brief Check if a Thread is currently enabled
 * @param tid The ID of the Thread to check
 * @return True if the Thread is currently enabled
 */
bool ModelExecution::is_enabled(thread_id_t tid) const
{
        return scheduler->is_enabled(tid);
}

/**
 * @brief Select the next thread to execute based on the curren action
 *
 * RMW actions occur in two parts, and we cannot split them. And THREAD_CREATE
 * actions should be followed by the execution of their child thread. In either
 * case, the current action should determine the next thread schedule.
 *
 * @param curr The current action
 * @return The next thread to run, if the current action will determine this
 * selection; otherwise NULL
 */
Thread * ModelExecution::action_select_next_thread(const ModelAction *curr) const
{
        /* Do not split atomic RMW */
        if (curr->is_rmwr())
                return get_thread(curr);
        /* Follow CREATE with the created thread */
        if (curr->get_type() == THREAD_CREATE)
                return curr->get_thread_operand();
        return NULL;
}

/** @return True if the execution has taken too many steps */
bool ModelExecution::too_many_steps() const
{
        return params->bound != 0 && priv->used_sequence_numbers > params->bound;
}

/**
 * Takes the next step in the execution, if possible.
 * @param curr The current step to take
 * @return Returns the next Thread to run, if any; NULL if this execution
 * should terminate
 */
Thread * ModelExecution::take_step(ModelAction *curr)
{
        Thread *curr_thrd = get_thread(curr);
        ASSERT(curr_thrd->get_state() == THREAD_READY);

        ASSERT(check_action_enabled(curr)); /* May have side effects? */
        curr = check_current_action(curr);
        ASSERT(curr);

        if (curr_thrd->is_blocked() || curr_thrd->is_complete())
                scheduler->remove_thread(curr_thrd);

        return action_select_next_thread(curr);
}

/**
 * Launch end-of-execution release sequence fixups only when
 * the execution is otherwise feasible AND there are:
 *
 * (1) pending release sequences
 * (2) pending assertions that could be invalidated by a change
 * in clock vectors (i.e., data races)
 * (3) no pending promises
 */
void ModelExecution::fixup_release_sequences()
{
        while (!pending_rel_seqs.empty() &&
                        is_feasible_prefix_ignore_relseq() &&
                        haveUnrealizedRaces()) {
                model_print("*** WARNING: release sequence fixup action "
                                "(%zu pending release seuqence(s)) ***\n",
                                pending_rel_seqs.size());
                ModelAction *fixup = new ModelAction(MODEL_FIXUP_RELSEQ,
                                std::memory_order_seq_cst, NULL, VALUE_NONE,
                                model_thread);
                take_step(fixup);
        };
}