path: root/libcilkrts/runtime/scheduler.c

/* scheduler.c                  -*-C-*-
 *
 *************************************************************************
 *
 *  Copyright (C) 2007-2016, Intel Corporation
 *  All rights reserved.
 *  
 *  Redistribution and use in source and binary forms, with or without
 *  modification, are permitted provided that the following conditions
 *  are met:
 *  
 *    * Redistributions of source code must retain the above copyright
 *      notice, this list of conditions and the following disclaimer.
 *    * Redistributions in binary form must reproduce the above copyright
 *      notice, this list of conditions and the following disclaimer in
 *      the documentation and/or other materials provided with the
 *      distribution.
 *    * Neither the name of Intel Corporation nor the names of its
 *      contributors may be used to endorse or promote products derived
 *      from this software without specific prior written permission.
 *  
 *  THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
 *  "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
 *  LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
 *  A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
 *  HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
 *  INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
 *  BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS
 *  OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED
 *  AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
 *  LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY
 *  WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
 *  POSSIBILITY OF SUCH DAMAGE.
 *  
 *  *********************************************************************
 *  
 *  PLEASE NOTE: This file is a downstream copy of a file mainitained in
 *  a repository at cilkplus.org. Changes made to this file that are not
 *  submitted through the contribution process detailed at
 *  http://www.cilkplus.org/submit-cilk-contribution will be lost the next
 *  time that a new version is released. Changes only submitted to the
 *  GNU compiler collection or posted to the git repository at
 *  https://bitbucket.org/intelcilkruntime/intel-cilk-runtime.git are
 *  not tracked.
 *  
 *  We welcome your contributions to this open source project. Thank you
 *  for your assistance in helping us improve Cilk Plus.
 *
 **************************************************************************/

/*
 * Cilk scheduler
 */

#include "scheduler.h"
#include "bug.h"
#include "os.h"
#include "os_mutex.h"
#include "local_state.h"
#include "signal_node.h"
#include "full_frame.h"
#include "sysdep.h"
#include "except.h"
#include "cilk_malloc.h"
#include "pedigrees.h"
#include "record-replay.h"

#include <limits.h>
#include <string.h> /* memcpy */
#include <stdio.h>  // sprintf
#include <stdlib.h> // malloc, free, abort

#ifdef _WIN32
#   pragma warning(disable:1786)   // disable warning: sprintf is deprecated
#   include "sysdep-win.h"
#   include "except-win32.h"
#endif  // _WIN32

// ICL: Don't complain about conversion from pointer to same-sized integral
// type in __cilkrts_put_stack.  That's why we're using ptrdiff_t
#ifdef _WIN32
#   pragma warning(disable: 1684)
#endif

#include "cilk/cilk_api.h"
#include "frame_malloc.h"
#include "metacall_impl.h"
#include "reducer_impl.h"
#include "cilk-tbb-interop.h"
#include "cilk-ittnotify.h"
#include "stats.h"

// ICL: Don't complain about loss of precision in myrand
// I tried restoring the warning after the function, but it didn't
// suppress it
#ifdef _WIN32
#   pragma warning(disable: 2259)
#endif

#ifndef _WIN32
#   include <unistd.h>
#endif

#ifdef __VXWORKS__
// redeclare longjmp() with noreturn to stop warnings
extern __attribute__((noreturn)) 
		void longjmp(jmp_buf, int);
#endif

//#define DEBUG_LOCKS 1
#ifdef DEBUG_LOCKS
// The currently executing worker must own this worker's lock
#   define ASSERT_WORKER_LOCK_OWNED(w) \
        { \
            __cilkrts_worker *tls_worker = __cilkrts_get_tls_worker(); \
            CILK_ASSERT((w)->l->lock.owner == tls_worker); \
        }
#else
#   define ASSERT_WORKER_LOCK_OWNED(w)
#endif // DEBUG_LOCKS

// Options for the scheduler.
enum schedule_t { SCHEDULE_RUN,
                  SCHEDULE_WAIT,
                  SCHEDULE_EXIT };

// Return values for provably_good_steal()
enum provably_good_steal_t
{
    ABANDON_EXECUTION,  // Not the last child to the sync - attempt to steal work
    CONTINUE_EXECUTION, // Last child to the sync - continue executing on this worker
    WAIT_FOR_CONTINUE   // The replay log indicates that this was the worker
                        // which continued.  Loop until we are the last worker
                        // to the sync.
};


// Verify that "w" is the worker we are currently executing on.
// Because this check is expensive, this method is usually a no-op.
static inline void verify_current_wkr(__cilkrts_worker *w)
{
#if ((REDPAR_DEBUG >= 3) || (FIBER_DEBUG >= 1))
    // Lookup the worker from TLS and compare to w. 
    __cilkrts_worker* tmp = __cilkrts_get_tls_worker();
    if (w != tmp) {
        fprintf(stderr, "Error.  W=%d, actual worker =%d...\n",
                w->self,
                tmp->self);
    }
    CILK_ASSERT(w == tmp);
#endif
}                                                            

static enum schedule_t worker_runnable(__cilkrts_worker *w);

// Scheduling-fiber functions:
static void do_return_from_spawn (__cilkrts_worker *w,
                                  full_frame *ff,
                                  __cilkrts_stack_frame *sf);
static void do_sync (__cilkrts_worker *w,
                     full_frame *ff,
                     __cilkrts_stack_frame *sf);

// max is defined on Windows and VxWorks
#if (! defined(_WIN32)) && (! defined(__VXWORKS__))
    // TBD: definition of max() for Linux.
#   define max(a, b) ((a) < (b) ? (b) : (a))
#endif

void __cilkrts_dump_stats_to_stderr(global_state_t *g)
{
#ifdef CILK_PROFILE
    int i;
    for (i = 0; i < g->total_workers; ++i) {
        // Print out statistics for each worker.  We collected them,
        // so why not print them out?
        fprintf(stderr, "Stats for worker %d\n", i);
        dump_stats_to_file(stderr, g->workers[i]->l->stats);
        __cilkrts_accum_stats(&g->stats, g->workers[i]->l->stats);
    }

    // Also print out aggregate statistics.
    dump_stats_to_file(stderr, &g->stats);
#endif
    fprintf(stderr,
            "CILK PLUS Thread Info: P=%d, Q=%d\n",
            g->P,
            g->Q);
    fprintf(stderr,
            "CILK PLUS RUNTIME MEMORY USAGE: %lld bytes",
            (long long)g->frame_malloc.allocated_from_os);
#ifdef CILK_PROFILE
    if (g->stats.stack_hwm)
        fprintf(stderr, ", %ld stacks", g->stats.stack_hwm);
#endif
    fputc('\n', stderr);
}

static void validate_worker(__cilkrts_worker *w)
{
    /* check the magic numbers, for debugging purposes */
    if (w->l->worker_magic_0 != WORKER_MAGIC_0 ||
        w->l->worker_magic_1 != WORKER_MAGIC_1)
        abort_because_rts_is_corrupted();
}

static void double_link(full_frame *left_ff, full_frame *right_ff)
{
    if (left_ff)
        left_ff->right_sibling = right_ff;
    if (right_ff)
        right_ff->left_sibling = left_ff;
}

/* add CHILD to the right of all children of PARENT */
static void push_child(full_frame *parent_ff, full_frame *child_ff)
{
    double_link(parent_ff->rightmost_child, child_ff);
    double_link(child_ff, 0);
    parent_ff->rightmost_child = child_ff;
}

/* unlink CHILD from the list of all children of PARENT */
static void unlink_child(full_frame *parent_ff, full_frame *child_ff)
{
    double_link(child_ff->left_sibling, child_ff->right_sibling);

    if (!child_ff->right_sibling) {
        /* this is the rightmost child -- update parent link */
        CILK_ASSERT(parent_ff->rightmost_child == child_ff);
        parent_ff->rightmost_child = child_ff->left_sibling;
    }
    child_ff->left_sibling = child_ff->right_sibling = 0; /* paranoia */
}

static void incjoin(full_frame *ff)
{
    ++ff->join_counter;
}

static int decjoin(full_frame *ff)
{
    CILK_ASSERT(ff->join_counter > 0);
    return (--ff->join_counter);
}

static int simulate_decjoin(full_frame *ff)
{
  CILK_ASSERT(ff->join_counter > 0);
  return (ff->join_counter - 1);
}

/*
 * Pseudo-random generator defined by the congruence S' = 69070 * S
 * mod (2^32 - 5).  Marsaglia (CACM July 1993) says on page 107 that
 * this is a ``good one''.  There you go.
 *
 * The literature makes a big fuss about avoiding the division, but
 * for us it is not worth the hassle.
 */
static const unsigned RNGMOD = ((1ULL << 32) - 5);
static const unsigned RNGMUL = 69070U;

static unsigned myrand(__cilkrts_worker *w)
{
    unsigned state = w->l->rand_seed;
    state = (unsigned)((RNGMUL * (unsigned long long)state) % RNGMOD);
    w->l->rand_seed = state;
    return state;
}

static void mysrand(__cilkrts_worker *w, unsigned seed)
{
    seed %= RNGMOD;
    seed += (seed == 0); /* 0 does not belong to the multiplicative
                            group.  Use 1 instead */
    w->l->rand_seed = seed;
}

/* W grabs its own lock */
void __cilkrts_worker_lock(__cilkrts_worker *w)
{
    validate_worker(w);
    CILK_ASSERT(w->l->do_not_steal == 0);

    /* tell thieves to stay out of the way */
    w->l->do_not_steal = 1;
    __cilkrts_fence(); /* probably redundant */

    __cilkrts_mutex_lock(w, &w->l->lock);
}

void __cilkrts_worker_unlock(__cilkrts_worker *w)
{
    __cilkrts_mutex_unlock(w, &w->l->lock);
    CILK_ASSERT(w->l->do_not_steal == 1);
    /* The fence is probably redundant.  Use a release
       operation when supported (gcc and compatibile);
       that is faster on x86 which serializes normal stores. */
#if defined __GNUC__ && (__GNUC__ * 10 + __GNUC_MINOR__ > 43 || __ICC >= 1110)
    __sync_lock_release(&w->l->do_not_steal);
#else
    w->l->do_not_steal = 0;
    __cilkrts_fence(); /* store-store barrier, redundant on x86 */
#endif
}

/* try to acquire the lock of some *other* worker */
static int worker_trylock_other(__cilkrts_worker *w,
                                __cilkrts_worker *other)
{
    int status = 0;

    validate_worker(other);

    /* This protocol guarantees that, after setting the DO_NOT_STEAL
       flag, worker W can enter its critical section after waiting for
       the thief currently in the critical section (if any) and at
       most one other thief.  

       This requirement is overly paranoid, but it should protect us
       against future nonsense from OS implementors.
    */

    /* compete for the right to disturb OTHER */
    if (__cilkrts_mutex_trylock(w, &other->l->steal_lock)) {
        if (other->l->do_not_steal) {
            /* leave it alone */
        } else {
            status = __cilkrts_mutex_trylock(w, &other->l->lock);
        }
        __cilkrts_mutex_unlock(w, &other->l->steal_lock);
    }


    return status;
}

static void worker_unlock_other(__cilkrts_worker *w,
                                __cilkrts_worker *other)
{
    __cilkrts_mutex_unlock(w, &other->l->lock);
}


/* Lock macro Usage:
    BEGIN_WITH_WORKER_LOCK(w) {
        statement;
        statement;
        BEGIN_WITH_FRAME_LOCK(w, ff) {
            statement;
            statement;
        } END_WITH_FRAME_LOCK(w, ff);
    } END_WITH_WORKER_LOCK(w);
 */
#define BEGIN_WITH_WORKER_LOCK(w) __cilkrts_worker_lock(w); do
#define END_WITH_WORKER_LOCK(w)   while (__cilkrts_worker_unlock(w), 0)

// TBD(jsukha): These are worker lock acquistions on
// a worker whose deque is empty.  My conjecture is that we
// do not need to hold the worker lock at these points.
// I have left them in for now, however.
//
// #define REMOVE_POSSIBLY_OPTIONAL_LOCKS
#ifdef REMOVE_POSSIBLY_OPTIONAL_LOCKS
    #define BEGIN_WITH_WORKER_LOCK_OPTIONAL(w) do
    #define END_WITH_WORKER_LOCK_OPTIONAL(w)   while (0)
#else
    #define BEGIN_WITH_WORKER_LOCK_OPTIONAL(w) __cilkrts_worker_lock(w); do
    #define END_WITH_WORKER_LOCK_OPTIONAL(w)   while (__cilkrts_worker_unlock(w), 0)
#endif


#define BEGIN_WITH_FRAME_LOCK(w, ff)                                     \
    do { full_frame *_locked_ff = ff; __cilkrts_frame_lock(w, _locked_ff); do

#define END_WITH_FRAME_LOCK(w, ff)                       \
    while (__cilkrts_frame_unlock(w, _locked_ff), 0); } while (0)

/* W becomes the owner of F and F can be stolen from W */
static void make_runnable(__cilkrts_worker *w, full_frame *ff)
{
    w->l->frame_ff = ff;

    /* CALL_STACK is invalid (the information is stored implicitly in W) */
    ff->call_stack = 0;
}

/*
 * The worker parameter is unused, except for print-debugging purposes.
 */
static void make_unrunnable(__cilkrts_worker *w,
                            full_frame *ff,
                            __cilkrts_stack_frame *sf,
                            int is_loot,
                            const char *why)
{
    /* CALL_STACK becomes valid again */
    ff->call_stack = sf;

    if (sf) {
#if CILK_LIB_DEBUG
        if (__builtin_expect(sf->flags & CILK_FRAME_EXITING, 0))
            __cilkrts_bug("W%d suspending exiting frame %p/%p\n", w->self, ff, sf);
#endif
        sf->flags |= CILK_FRAME_STOLEN | CILK_FRAME_SUSPENDED;
        sf->worker = 0;

        if (is_loot)
            __cilkrts_put_stack(ff, sf);

        /* perform any system-dependent action, such as saving the
           state of the stack */
        __cilkrts_make_unrunnable_sysdep(w, ff, sf, is_loot, why);
    }
}


/* Push the next full frame to be made active in this worker and increment its
 * join counter.  __cilkrts_push_next_frame and pop_next_frame work on a
 * one-element queue.  This queue is used to communicate across the runtime
 * from the code that wants to activate a frame to the code that can actually
 * begin execution on that frame.  They are asymetrical in that push
 * increments the join counter but pop does not decrement it.  Rather, a
 * single push/pop combination makes a frame active and increments its join
 * counter once. */
void __cilkrts_push_next_frame(__cilkrts_worker *w, full_frame *ff)
{
    CILK_ASSERT(ff);
    CILK_ASSERT(!w->l->next_frame_ff);
    incjoin(ff);
    w->l->next_frame_ff = ff;
}

/* Get the next full-frame to be made active in this worker.  The join count
 * of the full frame will have been incremented by the corresponding push
 * event.  See __cilkrts_push_next_frame, above.
 */
static full_frame *pop_next_frame(__cilkrts_worker *w)
{
    full_frame *ff;
    ff = w->l->next_frame_ff;
    // Remove the frame from the next_frame field.
    //
    // If this is a user worker, then there is a chance that another worker
    // from our team could push work into our next_frame (if it is the last
    // worker doing work for this team).  The other worker's setting of the
    // next_frame could race with our setting of next_frame to NULL.  This is
    // the only possible race condition on next_frame.  However, if next_frame
    // has a non-NULL value, then it means the team still has work to do, and
    // there is no chance of another team member populating next_frame.  Thus,
    // it is safe to set next_frame to NULL, if it was populated.  There is no
    // need for an atomic op.
    if (NULL != ff) {
        w->l->next_frame_ff = NULL;
    }
    return ff;
}

/*
 * Identify the single worker that is allowed to cross a sync in this frame.  A
 * thief should call this function when it is the first to steal work from a
 * user worker.  "First to steal work" may mean that there has been parallelism
 * in the user worker before, but the whole team sync'd, and this is the first
 * steal after that.
 *
 * This should happen while holding the worker and frame lock.
 */
static void set_sync_master(__cilkrts_worker *w, full_frame *ff)
{
    w->l->last_full_frame = ff;
    ff->sync_master = w;
}

/*
 * The sync that ends all parallelism for a particular user worker is about to
 * be crossed.  Decouple the worker and frame.
 *
 * No locks need to be held since the user worker isn't doing anything, and none
 * of the system workers can steal from it.  But unset_sync_master() should be
 * called before the user worker knows about this work (i.e., before it is
 * inserted into the w->l->next_frame_ff is set).
 */
static void unset_sync_master(__cilkrts_worker *w, full_frame *ff)
{
    CILK_ASSERT(WORKER_USER == w->l->type);
    CILK_ASSERT(ff->sync_master == w);
    ff->sync_master = NULL;
    w->l->last_full_frame = NULL;
}

/********************************************************************
 * THE protocol:
 ********************************************************************/
/*
 * This is a protocol for work stealing that minimizes the overhead on
 * the victim.
 *
 * The protocol uses three shared pointers into the worker's deque:
 * - T - the "tail"
 * - H - the "head"
 * - E - the "exception"  NB: In this case, "exception" has nothing to do
 * with C++ throw-catch exceptions -- it refers only to a non-normal return,
 * i.e., a steal or similar scheduling exception.
 *
 * with H <= E, H <= T.  
 *
 * Stack frames SF, where H <= E < T, are available for stealing. 
 *
 * The worker operates on the T end of the stack.  The frame being
 * worked on is not on the stack.  To make a continuation available for
 * stealing the worker pushes a from onto the stack: stores *T++ = SF.
 * To return, it pops the frame off the stack: obtains SF = *--T.
 *
 * After decrementing T, the condition E > T signals to the victim that
 * it should invoke the runtime system's "THE" exception handler.  The
 * pointer E can become INFINITY, in which case the victim must invoke
 * the THE exception handler as soon as possible.
 *
 * See "The implementation of the Cilk-5 multithreaded language", PLDI 1998,
 * http://portal.acm.org/citation.cfm?doid=277652.277725, for more information
 * on the THE protocol.
 */

/* the infinity value of E */
#define EXC_INFINITY  ((__cilkrts_stack_frame **) (-1))

static void increment_E(__cilkrts_worker *victim)
{
    __cilkrts_stack_frame *volatile *tmp;

    // The currently executing worker must own the worker lock to touch
    // victim->exc
    ASSERT_WORKER_LOCK_OWNED(victim);

    tmp = victim->exc;
    if (tmp != EXC_INFINITY) {
        /* On most x86 this pair of operations would be slightly faster
           as an atomic exchange due to the implicit memory barrier in
           an atomic instruction. */
        victim->exc = tmp + 1;
        __cilkrts_fence();
    }
}

static void decrement_E(__cilkrts_worker *victim)
{
    __cilkrts_stack_frame *volatile *tmp;

    // The currently executing worker must own the worker lock to touch
    // victim->exc
    ASSERT_WORKER_LOCK_OWNED(victim);

    tmp = victim->exc;
    if (tmp != EXC_INFINITY) {
        /* On most x86 this pair of operations would be slightly faster
           as an atomic exchange due to the implicit memory barrier in
           an atomic instruction. */
        victim->exc = tmp - 1;
        __cilkrts_fence(); /* memory fence not really necessary */
    }
}

#if 0
/* for now unused, will be necessary if we implement abort */
static void signal_THE_exception(__cilkrts_worker *wparent)
{
    wparent->exc = EXC_INFINITY;
    __cilkrts_fence();
}
#endif

static void reset_THE_exception(__cilkrts_worker *w)
{
    // The currently executing worker must own the worker lock to touch
    // w->exc
    ASSERT_WORKER_LOCK_OWNED(w);

    w->exc = w->head;
    __cilkrts_fence();
}

/* conditions under which victim->head can be stolen: */
static int can_steal_from(__cilkrts_worker *victim)
{
    return ((victim->head < victim->tail) && 
            (victim->head < victim->protected_tail));
}

/* Return TRUE if the frame can be stolen, false otherwise */
static int dekker_protocol(__cilkrts_worker *victim)
{
    // increment_E and decrement_E are going to touch victim->exc.  The
    // currently executing worker must own victim's lock before they can
    // modify it
    ASSERT_WORKER_LOCK_OWNED(victim);

    /* ASSERT(E >= H); */

    increment_E(victim);

    /* ASSERT(E >= H + 1); */
    if (can_steal_from(victim)) {
        /* success, we can steal victim->head and set H <- H + 1
           in detach() */
        return 1;
    } else {
        /* failure, restore previous state */
        decrement_E(victim);
        return 0;    
    }
}


/* Link PARENT and CHILD in the spawn tree */
static full_frame *make_child(__cilkrts_worker *w, 
                              full_frame *parent_ff,
                              __cilkrts_stack_frame *child_sf,
                              cilk_fiber *fiber) 
{
    full_frame *child_ff = __cilkrts_make_full_frame(w, child_sf);

    child_ff->parent = parent_ff;
    push_child(parent_ff, child_ff);

    //DBGPRINTF("%d-          make_child - child_frame: %p, parent_frame: %p, child_sf: %p\n"
    //    "            parent - parent: %p, left_sibling: %p, right_sibling: %p, rightmost_child: %p\n"
    //    "            child  - parent: %p, left_sibling: %p, right_sibling: %p, rightmost_child: %p\n",
    //          w->self, child, parent, child_sf,
    //          parent->parent, parent->left_sibling, parent->right_sibling, parent->rightmost_child,
    //          child->parent, child->left_sibling, child->right_sibling, child->rightmost_child);
    CILK_ASSERT(parent_ff->call_stack);
    child_ff->is_call_child = (fiber == NULL);

    /* PLACEHOLDER_FIBER is used as non-null marker indicating that
       child should be treated as a spawn child even though we have not
       yet assigned a real fiber to its parent. */
    if (fiber == PLACEHOLDER_FIBER)
        fiber = NULL; /* Parent actually gets a null fiber, for now */

    /* perform any system-dependent actions, such as capturing
       parameter passing information */
    /*__cilkrts_make_child_sysdep(child, parent);*/

    /* Child gets reducer map and stack of parent.
       Parent gets a new map and new stack. */
    child_ff->fiber_self = parent_ff->fiber_self;
    child_ff->sync_master = NULL;

    if (child_ff->is_call_child) {
        /* Cause segfault on any attempted access.  The parent gets
           the child map and stack when the child completes. */
        parent_ff->fiber_self = 0;
    } else {
        parent_ff->fiber_self = fiber;
    }

    incjoin(parent_ff);
    return child_ff;
}

static inline __cilkrts_stack_frame *__cilkrts_advance_frame(__cilkrts_stack_frame *sf)
{
    __cilkrts_stack_frame *p = sf->call_parent;
    sf->call_parent = 0;
    return p;
}

/* w should be the currently executing worker.  
 * loot_sf is the youngest stack frame in the call stack being 
 *   unrolled (i.e., the most deeply nested stack frame.)
 *
 * When this method is called for a steal, loot_sf should be on a
 * victim worker which is different from w.
 * For CILK_FORCE_REDUCE, the victim worker will equal w.
 *
 * Before execution, the __cilkrts_stack_frame's have pointers from
 * older to younger, i.e., a __cilkrts_stack_frame points to parent.
 *
 * This method creates a full frame for each __cilkrts_stack_frame in
 * the call stack, with each full frame also pointing to its parent. 
 *
 * The method returns the full frame created for loot_sf, i.e., the
 * youngest full frame.
 */
static full_frame *unroll_call_stack(__cilkrts_worker *w, 
                                     full_frame *ff, 
                                     __cilkrts_stack_frame *const loot_sf)
{
    __cilkrts_stack_frame *sf = loot_sf;
    __cilkrts_stack_frame *rev_sf = 0;
    __cilkrts_stack_frame *t_sf;

    CILK_ASSERT(sf);
    /*CILK_ASSERT(sf->call_parent != sf);*/

    /* The leafmost frame is unsynched. */
    if (sf->worker != w)
        sf->flags |= CILK_FRAME_UNSYNCHED;

    /* Reverse the call stack to make a linked list ordered from parent
       to child.  sf->call_parent points to the child of SF instead of
       the parent.  */
    do {
        t_sf = (sf->flags & (CILK_FRAME_DETACHED|CILK_FRAME_STOLEN|CILK_FRAME_LAST))? 0 : sf->call_parent;
        sf->call_parent = rev_sf;
        rev_sf = sf;
        sf = t_sf;
    } while (sf);
    sf = rev_sf;

    /* Promote each stack frame to a full frame in order from parent
       to child, following the reversed list we just built. */
    make_unrunnable(w, ff, sf, sf == loot_sf, "steal 1");
    /* T is the *child* of SF, because we have reversed the list */
    for (t_sf = __cilkrts_advance_frame(sf); t_sf;
         sf = t_sf, t_sf = __cilkrts_advance_frame(sf)) {
        ff = make_child(w, ff, t_sf, NULL);
        make_unrunnable(w, ff, t_sf, t_sf == loot_sf, "steal 2");
    }

    /* XXX What if the leafmost frame does not contain a sync
       and this steal is from promote own deque? */
    /*sf->flags |= CILK_FRAME_UNSYNCHED;*/

    CILK_ASSERT(!sf->call_parent);
    return ff;
}

/* detach the top of the deque frame from the VICTIM and install a new
   CHILD frame in its place */
static void detach_for_steal(__cilkrts_worker *w,
                             __cilkrts_worker *victim,
                             cilk_fiber* fiber)
{
    /* ASSERT: we own victim->lock */

    full_frame *parent_ff, *child_ff, *loot_ff;
    __cilkrts_stack_frame *volatile *h;
    __cilkrts_stack_frame *sf;

    w->l->team = victim->l->team;

    CILK_ASSERT(w->l->frame_ff == 0 || w == victim);

    h = victim->head;

    CILK_ASSERT(*h);

    victim->head = h + 1;

    parent_ff = victim->l->frame_ff;
    BEGIN_WITH_FRAME_LOCK(w, parent_ff) {
        /* parent no longer referenced by victim */
        decjoin(parent_ff);

        /* obtain the victim call stack */
        sf = *h;

        /* perform system-dependent normalizations */
        /*__cilkrts_normalize_call_stack_on_steal(sf);*/

        /* unroll PARENT_FF with call stack SF, adopt the youngest
           frame LOOT.  If loot_ff == parent_ff, then we hold loot_ff->lock,
           otherwise, loot_ff is newly created and we can modify it without
           holding its lock. */
        loot_ff = unroll_call_stack(w, parent_ff, sf);

        #if REDPAR_DEBUG >= 3
        fprintf(stderr, "[W=%d, victim=%d, desc=detach, parent_ff=%p, loot=%p]\n",
                w->self, victim->self,
                parent_ff, loot_ff);
        #endif

        if (WORKER_USER == victim->l->type &&
            NULL == victim->l->last_full_frame) {
            // Mark this looted frame as special: only the original user worker
            // may cross the sync.
            // 
            // This call is a shared access to
            // victim->l->last_full_frame.
            set_sync_master(victim, loot_ff);
        }

        /* LOOT is the next frame that the thief W is supposed to
           run, unless the thief is stealing from itself, in which
           case the thief W == VICTIM executes CHILD and nobody
           executes LOOT. */
        if (w == victim) {
            /* Pretend that frame has been stolen */
            loot_ff->call_stack->flags |= CILK_FRAME_UNSYNCHED;
            loot_ff->simulated_stolen = 1;
        }
        else
            __cilkrts_push_next_frame(w, loot_ff);

        // After this "push_next_frame" call, w now owns loot_ff.
        child_ff = make_child(w, loot_ff, 0, fiber);

        BEGIN_WITH_FRAME_LOCK(w, child_ff) {
            /* install child in the victim's work queue, taking
               the parent_ff's place */
            /* child is referenced by victim */
            incjoin(child_ff);

            // With this call, w is bestowing ownership of the newly
            // created frame child_ff to the victim, and victim is
            // giving up ownership of parent_ff.
            //
            // Worker w will either take ownership of parent_ff
            // if parent_ff == loot_ff, or parent_ff will be
            // suspended.
            //
            // Note that this call changes the victim->frame_ff
            // while the victim may be executing.
            make_runnable(victim, child_ff);
        } END_WITH_FRAME_LOCK(w, child_ff);
    } END_WITH_FRAME_LOCK(w, parent_ff);
}

/**
 * @brief cilk_fiber_proc that resumes user code after a successful
 * random steal.

 * This function longjmps back into the user code whose state is
 * stored in cilk_fiber_get_data(fiber)->resume_sf.  The stack pointer
 * is adjusted so that the code resumes on the specified fiber stack
 * instead of its original stack.
 *
 * This method gets executed only on a fiber freshly allocated from a
 * pool.
 *
 * @param fiber   The fiber being used to resume user code.
 * @param arg     Unused.
 */
static
void fiber_proc_to_resume_user_code_for_random_steal(cilk_fiber *fiber)
{
    cilk_fiber_data *data = cilk_fiber_get_data(fiber);
    __cilkrts_stack_frame* sf = data->resume_sf;
    full_frame *ff;

    CILK_ASSERT(sf);

    // When we pull the resume_sf out of the fiber to resume it, clear
    // the old value.
    data->resume_sf = NULL;
    CILK_ASSERT(sf->worker == data->owner);
    ff = sf->worker->l->frame_ff;

    // For Win32, we need to overwrite the default exception handler
    // in this function, so that when the OS exception handling code
    // walks off the top of the current Cilk stack, it reaches our stub
    // handler.
    
    // Also, this function needs to be wrapped into a try-catch block
    // so the compiler generates the appropriate exception information
    // in this frame.
    
    // TBD: IS THIS HANDLER IN THE WRONG PLACE?  Can we longjmp out of
    // this function (and does it matter?)
#if defined(_WIN32) && !defined(_WIN64)
    install_exception_stub_handler();
    __try 
#endif
    {
        char* new_sp = sysdep_reset_jump_buffers_for_resume(fiber, ff, sf);
        
        // Notify the Intel tools that we're stealing code
        ITT_SYNC_ACQUIRED(sf->worker);
        NOTIFY_ZC_INTRINSIC("cilk_continue", sf);

        // TBD: We'd like to move TBB-interop methods into the fiber
        // eventually.
        cilk_fiber_invoke_tbb_stack_op(fiber, CILK_TBB_STACK_ADOPT);
        
        sf->flags &= ~CILK_FRAME_SUSPENDED;

        // longjmp to user code.  Don't process exceptions here,
        // because we are resuming a stolen frame.
        sysdep_longjmp_to_sf(new_sp, sf, NULL);
        /*NOTREACHED*/
        // Intel's C compiler respects the preceding lint pragma
    }
#if defined(_WIN32) && !defined(_WIN64)
    __except (CILK_ASSERT(!"should not execute the the stub filter"),
              EXCEPTION_EXECUTE_HANDLER)
    {
        // If we are here, that means something very wrong
        // has happened in our exception processing...
        CILK_ASSERT(! "should not be here!");
    }
#endif
}

static void random_steal(__cilkrts_worker *w)
{
    __cilkrts_worker *victim = NULL;
    cilk_fiber *fiber = NULL;
    int n;
    int success = 0;
    int32_t victim_id;

    // Nothing's been stolen yet. When true, this will flag
    // setup_for_execution_pedigree to increment the pedigree
    w->l->work_stolen = 0;

    /* If the user has disabled stealing (using the debugger) we fail */
    if (__builtin_expect(w->g->stealing_disabled, 0))
        return;

    CILK_ASSERT(w->l->type == WORKER_SYSTEM || w->l->team == w);

    /* If there is only one processor work can still be stolen.
       There must be only one worker to prevent stealing. */
    CILK_ASSERT(w->g->total_workers > 1);

    /* pick random *other* victim */
    n = myrand(w) % (w->g->total_workers - 1);
    if (n >= w->self)
        ++n;

    // If we're replaying a log, override the victim.  -1 indicates that
    // we've exhausted the list of things this worker stole when we recorded
    // the log so just return.  If we're not replaying a log,
    // replay_get_next_recorded_victim() just returns the victim ID passed in.
    n = replay_get_next_recorded_victim(w, n);
    if (-1 == n)
        return;

    victim = w->g->workers[n];

    START_INTERVAL(w, INTERVAL_FIBER_ALLOCATE) {
        /* Verify that we can get a stack.  If not, no need to continue. */
        fiber = cilk_fiber_allocate(&w->l->fiber_pool);
    } STOP_INTERVAL(w, INTERVAL_FIBER_ALLOCATE);


    if (NULL == fiber) {
#if FIBER_DEBUG >= 2
        fprintf(stderr, "w=%d: failed steal because we could not get a fiber\n",
                w->self);
#endif        
        return;
    }

    /* do not steal from self */
    CILK_ASSERT (victim != w);

    /* Execute a quick check before engaging in the THE protocol.
       Avoid grabbing locks if there is nothing to steal. */
    if (!can_steal_from(victim)) {
        NOTE_INTERVAL(w, INTERVAL_STEAL_FAIL_EMPTYQ);
        START_INTERVAL(w, INTERVAL_FIBER_DEALLOCATE) {
            int ref_count = cilk_fiber_remove_reference(fiber, &w->l->fiber_pool);
            // Fibers we use when trying to steal should not be active,
            // and thus should not have any other references.
            CILK_ASSERT(0 == ref_count);
        } STOP_INTERVAL(w, INTERVAL_FIBER_DEALLOCATE);
        return;
    }
    
    /* Attempt to steal work from the victim */
    if (worker_trylock_other(w, victim)) {
        if (w->l->type == WORKER_USER && victim->l->team != w) {

            // Fail to steal if this is a user worker and the victim is not
            // on this team.  If a user worker were allowed to steal work
            // descended from another user worker, the former might not be
            // done with its work by the time it was needed to resume and
            // unbind.  Therefore, user workers are not permitted to change
            // teams.

            // There is no race on the victim's team because the victim cannot
            // change its team until it runs out of work to do, at which point
            // it will try to take out its own lock, and this worker already
            // holds it.
            NOTE_INTERVAL(w, INTERVAL_STEAL_FAIL_USER_WORKER);

        } else if (victim->l->frame_ff) {
            // A successful steal will change victim->frame_ff, even
            // though the victim may be executing.  Thus, the lock on
            // the victim's deque is also protecting victim->frame_ff.
            if (dekker_protocol(victim)) {
                int proceed_with_steal = 1; // optimistic

                // If we're replaying a log, verify that this the correct frame
                // to steal from the victim
                if (! replay_match_victim_pedigree(w, victim))
                {
                    // Abort the steal attempt. decrement_E(victim) to
                    // counter the increment_E(victim) done by the
                    // dekker protocol
                    decrement_E(victim);
                    proceed_with_steal = 0;
                }

                if (proceed_with_steal)
                {
                    START_INTERVAL(w, INTERVAL_STEAL_SUCCESS) {
                        success = 1;
                        detach_for_steal(w, victim, fiber);
                        victim_id = victim->self;

                        #if REDPAR_DEBUG >= 1
                        fprintf(stderr, "Wkr %d stole from victim %d, fiber = %p\n",
                                w->self, victim->self, fiber);
                        #endif

                        // The use of victim->self contradicts our
                        // classification of the "self" field as 
                        // local.  But since this code is only for
                        // debugging, it is ok.
                        DBGPRINTF ("%d-%p: Stealing work from worker %d\n"
                            "            sf: %p, call parent: %p\n",
                            w->self, GetCurrentFiber(), victim->self,
                            w->l->next_frame_ff->call_stack,
                            w->l->next_frame_ff->call_stack->call_parent);
                    } STOP_INTERVAL(w, INTERVAL_STEAL_SUCCESS);
                }  // end if(proceed_with_steal)
            } else {
                NOTE_INTERVAL(w, INTERVAL_STEAL_FAIL_DEKKER);
            }
        } else {
            NOTE_INTERVAL(w, INTERVAL_STEAL_FAIL_EMPTYQ);
        }
        worker_unlock_other(w, victim);
    } else {
        NOTE_INTERVAL(w, INTERVAL_STEAL_FAIL_LOCK);
    }

    // Record whether work was stolen.  When true, this will flag
    // setup_for_execution_pedigree to increment the pedigree
    w->l->work_stolen = success;

    if (0 == success) {
        // failed to steal work.  Return the fiber to the pool.
        START_INTERVAL(w, INTERVAL_FIBER_DEALLOCATE) {
            int ref_count = cilk_fiber_remove_reference(fiber, &w->l->fiber_pool);
            // Fibers we use when trying to steal should not be active,
            // and thus should not have any other references.
            CILK_ASSERT(0 == ref_count);
        } STOP_INTERVAL(w, INTERVAL_FIBER_DEALLOCATE);
    }
    else
    {
        // Since our steal was successful, finish initialization of
        // the fiber.
        cilk_fiber_reset_state(fiber,
                               fiber_proc_to_resume_user_code_for_random_steal);
        // Record the pedigree of the frame that w has stolen.
        // record only if CILK_RECORD_LOG is set
        replay_record_steal(w, victim_id);
    }
}



/**
 * At a provably good steal, we need to transfer the child reducer map
 * from ff->children_reducer_map into v->reducer_map, where v is the
 * worker that resumes execution of ff.
 *
 * Normally, we have v == w, where w is the currently executing
 * worker.  In the case where we are resuming a team leader on a user
 * worker, however, v might differ from w.

 * Thus, this, operation is a no-op, since we can't really move
 * ff->children_reducer_map into w here.
 *
 * Instead, this work is done in setup_for_execution_reducers().
 */
static inline void provably_good_steal_reducers(__cilkrts_worker *w,
                                                full_frame       *ff)
{
    // No-op.
}

/* at a provably good steal, incorporate the accumulated exceptions of
   children into the parent's exception */
static void provably_good_steal_exceptions(__cilkrts_worker *w, 
                                           full_frame       *ff)
{
    // ASSERT: we own ff->lock
    ff->pending_exception =
        __cilkrts_merge_pending_exceptions(w,
                                           ff->child_pending_exception,
                                           ff->pending_exception);
    ff->child_pending_exception = NULL;
}

/* At sync discard the frame's old stack and take the leftmost child's. */
static void provably_good_steal_stacks(__cilkrts_worker *w, full_frame *ff)
{
    CILK_ASSERT(NULL == ff->fiber_self);
    ff->fiber_self = ff->fiber_child;
    ff->fiber_child = NULL;
}

static void __cilkrts_mark_synched(full_frame *ff)
{
    ff->call_stack->flags &= ~CILK_FRAME_UNSYNCHED;
    ff->simulated_stolen = 0;
}

static
enum provably_good_steal_t provably_good_steal(__cilkrts_worker *w,
                                               full_frame       *ff)
{
    // ASSERT: we hold w->lock and ff->lock

    enum provably_good_steal_t result = ABANDON_EXECUTION;

    // If the current replay entry is a sync record matching the worker's
    // pedigree, AND this isn't the last child to the sync, return
    // WAIT_FOR_CONTINUE to indicate that the caller should loop until
    // we find the right frame to steal and CONTINUE_EXECUTION is returned.
    int match_found = replay_match_sync_pedigree(w);
    if (match_found && (0 != simulate_decjoin(ff)))
        return WAIT_FOR_CONTINUE;

    START_INTERVAL(w, INTERVAL_PROVABLY_GOOD_STEAL) {
        if (decjoin(ff) == 0) {
            provably_good_steal_reducers(w, ff);
            provably_good_steal_exceptions(w, ff);
            provably_good_steal_stacks(w, ff);
            __cilkrts_mark_synched(ff);

            // If the original owner wants this frame back (to resume
            // it on its original thread) pass it back now.
            if (NULL != ff->sync_master) {
                // The frame wants to go back and be executed by the original
                // user thread.  We can throw caution to the wind and push the
                // frame straight onto its queue because the only way we have
                // gotten to this point of being able to continue execution of
                // the frame is if the original user worker is spinning without
                // work.

                unset_sync_master(w->l->team, ff);
                __cilkrts_push_next_frame(w->l->team, ff);

                // If this is the team leader we're not abandoning the work
                if (w == w->l->team)
                    result = CONTINUE_EXECUTION;
            } else {
                __cilkrts_push_next_frame(w, ff);
                result = CONTINUE_EXECUTION;  // Continue working on this thread
            }

            // The __cilkrts_push_next_frame() call changes ownership
            // of ff to the specified worker.
        }
    } STOP_INTERVAL(w, INTERVAL_PROVABLY_GOOD_STEAL);

    // Only write a SYNC record if:
    // - We're recording a log *AND*
    // - We're the worker continuing from this sync
    replay_record_sync(w, result == CONTINUE_EXECUTION);

    // If we're replaying a log, and matched a sync from the log, mark the
    // sync record seen if the sync isn't going to be abandoned.
    replay_advance_from_sync (w, match_found, result == CONTINUE_EXECUTION);

    return result;
}

static void unconditional_steal(__cilkrts_worker *w,
                                full_frame *ff)
{
    // ASSERT: we hold ff->lock

    START_INTERVAL(w, INTERVAL_UNCONDITIONAL_STEAL) {
        decjoin(ff);
        __cilkrts_push_next_frame(w, ff);
    } STOP_INTERVAL(w, INTERVAL_UNCONDITIONAL_STEAL);
}


/* CHILD is about to die.  Give its exceptions to a sibling or to the
   parent.  */
static inline void splice_exceptions_for_call(__cilkrts_worker *w,
                                              full_frame *parent_ff,
                                              full_frame *child_ff)
{
    // ASSERT: We own parent_ff->lock
    CILK_ASSERT(child_ff->is_call_child);
    CILK_ASSERT(NULL == child_ff->right_pending_exception);
    CILK_ASSERT(NULL == parent_ff->pending_exception);
    
    parent_ff->pending_exception = child_ff->pending_exception;
    child_ff->pending_exception = NULL;
}

/**
 * Merge exceptions for a dying child. 
 *
 * @param w                   The currently executing worker.
 * @param ff                  The child frame that is dying.
 * @param left_exception_ptr  Pointer to the exception that is to our left.
 */ 
static inline
void splice_exceptions_for_spawn(__cilkrts_worker *w,
                                 full_frame *ff,
                                 struct pending_exception_info **left_exception_ptr)
{
    // ASSERT: parent_ff == child_ff->parent.
    // ASSERT: We own parent_ff->lock

    // Merge current exception into the slot where the left
    // exception should go.
    *left_exception_ptr =
        __cilkrts_merge_pending_exceptions(w,
                                           *left_exception_ptr,
                                           ff->pending_exception);
    ff->pending_exception = NULL;


    // Merge right exception into the slot where the left exception
    // should go.
    *left_exception_ptr =
        __cilkrts_merge_pending_exceptions(w,
                                           *left_exception_ptr,
                                           ff->right_pending_exception);
    ff->right_pending_exception = NULL;
}


static inline void splice_stacks_for_call(__cilkrts_worker *w,
                                          full_frame *parent_ff,
                                          full_frame *child_ff)
{
#if CILK_LIB_DEBUG
    if (parent_ff->call_stack)
        CILK_ASSERT(!(parent_ff->call_stack->flags & CILK_FRAME_MBZ));
#endif

    /* A synched frame does not have accumulated child reducers. */
    CILK_ASSERT(!child_ff->fiber_child);
    CILK_ASSERT(child_ff->is_call_child);

    /* An attached parent has no self fiber.  It may have
       accumulated child fibers or child owners, which should be
       ignored until sync. */
    CILK_ASSERT(!parent_ff->fiber_self);
    parent_ff->fiber_self = child_ff->fiber_self;
    child_ff->fiber_self = NULL;
}

static void finalize_child_for_call(__cilkrts_worker *w,
                                    full_frame *parent_ff,
                                    full_frame *child_ff)
{
    // ASSERT: we hold w->lock and parent_ff->lock
    
    START_INTERVAL(w, INTERVAL_FINALIZE_CHILD) {
        CILK_ASSERT(child_ff->is_call_child);
        CILK_ASSERT(child_ff->join_counter == 0);
        CILK_ASSERT(!child_ff->rightmost_child);
        CILK_ASSERT(child_ff == parent_ff->rightmost_child);

        // CHILD is about to die. 
        // Splicing out reducers is a no-op for a call since
        // w->reducer_map should already store the correct 
        // reducer map.
        
        // ASSERT there are no maps left to reduce.
        CILK_ASSERT(NULL == child_ff->children_reducer_map);
        CILK_ASSERT(NULL == child_ff->right_reducer_map);
        
        splice_exceptions_for_call(w, parent_ff, child_ff);

        splice_stacks_for_call(w, parent_ff, child_ff);

        /* remove CHILD from list of children of PARENT */
        unlink_child(parent_ff, child_ff);

        /* continue with the parent. */
        unconditional_steal(w, parent_ff);
        __cilkrts_destroy_full_frame(w, child_ff);
    } STOP_INTERVAL(w, INTERVAL_FINALIZE_CHILD);
}


/**
 * The invariant on ff->children_reducer_map is that when ff is
 * synched and when we are about to resume execution of ff, at least
 * one of ff->children_reducer_map and w->reducer_map must be NULL.
 *
 * Consider the two possibilities before resuming execution of ff:
 *
 * 1.  Suppose ff is synched and suspended.  Then either
 *
 *     (a) ff->children_reducer_map stores the reducer map that w
 *         should use, where w is the worker resuming execution of ff, 
 *         OR
 *     (b) w already has a user map, and ff->children_reducer_map is NULL. 
 *
 *     Case (a) happens when we are resuming execution of ff as a
 *     provably good steal.  In this case, w->reducer_map should be
 *     NULL and ff->children_reducer_map is valid.  To resume
 *     execution of ff on w, set w->reducer_map to
 *     ff->children_reducer_map.
 * 
 *     Case (b) occurs when we resume execution of ff because ff is a
 *     called child.  Then, ff->children_reducer_map should be NULL,
 *     and w should already have a valid reducer map when resuming
 *     execution of ff.  We resume execution of ff without changing
 *     w->reducer_map.
 *
 * 2. Suppose frame ff is not synched (i.e., it is active and might have
 *    active children).   Then ff->children_reducer_map is the slot for
 *    storing the reducer map from ff's leftmost child, as in the reducer
 *    protocol.   The runtime may resume execution of ff while it is not 
 *    synched only because of a steal.
 *    In this case, while we are resuming ff, ff->children_reducer_map
 *    may be non-NULL (because one of ff's children has completed).
 *    We resume execution of ff without changing w->reducer_map.
 */ 
static void setup_for_execution_reducers(__cilkrts_worker *w,
                                         full_frame *ff)
{
    // We only need to move ff->children_reducer_map into
    // w->reducer_map in case 1(a).
    //
    // First check whether ff is synched.
    __cilkrts_stack_frame *sf = ff->call_stack;
    if (!(sf->flags & CILK_FRAME_UNSYNCHED)) {
        // In this case, ff is synched. (Case 1).
        CILK_ASSERT(!ff->rightmost_child);

        // Test whether we are in case 1(a) and have
        // something to do.  Note that if both
        // ff->children_reducer_map and w->reducer_map are NULL, we
        // can't distinguish between cases 1(a) and 1(b) here.
        if (ff->children_reducer_map) {
            // We are in Case 1(a).
            CILK_ASSERT(!w->reducer_map);
            w->reducer_map = ff->children_reducer_map;
            ff->children_reducer_map = NULL;
        }
    }
}

static void setup_for_execution_exceptions(__cilkrts_worker *w, 
                                           full_frame *ff)
{
    CILK_ASSERT(NULL == w->l->pending_exception);
    w->l->pending_exception = ff->pending_exception;
    ff->pending_exception = NULL;
}

#if 0 /* unused */
static void setup_for_execution_stack(__cilkrts_worker *w, 
                                      full_frame *ff)
{
}
#endif

/*
 * setup_for_execution_pedigree
 *
 * Copies the pedigree information from the frame we're resuming to the
 * worker.  Increments the pedigree if this is work that has been stolen
 * to match the increment on a return from a spawn helper.
 */
static void setup_for_execution_pedigree(__cilkrts_worker *w)
{
    int pedigree_unsynched;
    __cilkrts_stack_frame *sf = w->current_stack_frame;

    CILK_ASSERT(NULL != sf);

    // If this isn't an ABI 1 or later frame, there's no pedigree information
    if (0 == CILK_FRAME_VERSION_VALUE(sf->flags))
        return;

    // Note whether the pedigree is unsynched and clear the flag before
    // we forget
    pedigree_unsynched = sf->flags & CILK_FRAME_SF_PEDIGREE_UNSYNCHED;
    sf->flags &= ~CILK_FRAME_SF_PEDIGREE_UNSYNCHED;

    // If we're just marshalling onto this worker, do not increment
    // the rank since that wouldn't happen in a sequential execution
    if (w->l->work_stolen || pedigree_unsynched)
    {
        if (w->l->work_stolen)
            w->pedigree.rank = sf->parent_pedigree.rank + 1;
        else
            w->pedigree.rank = sf->parent_pedigree.rank;
    }

    w->pedigree.parent = sf->parent_pedigree.parent;
    w->l->work_stolen = 0;
}

static void setup_for_execution(__cilkrts_worker *w, 
                                full_frame *ff,
                                int is_return_from_call)
{
    // ASSERT: We own w->lock and ff->lock || P == 1

    setup_for_execution_reducers(w, ff);
    setup_for_execution_exceptions(w, ff);
    /*setup_for_execution_stack(w, ff);*/

    ff->call_stack->worker = w;
    w->current_stack_frame = ff->call_stack;

    // If this is a return from a call, leave the pedigree alone
    if (! is_return_from_call)
        setup_for_execution_pedigree(w);

    __cilkrts_setup_for_execution_sysdep(w, ff);

    w->head = w->tail = w->l->ltq;
    reset_THE_exception(w);

    make_runnable(w, ff);
}


/*
 * Called by the scheduling fiber, right before
 * resuming a sf/ff for user code.
 *
 * This method associates the specified sf with the worker.
 *
 * It also asserts that w, ff, and sf all have the expected properties
 * for resuming user code.
 */ 
void scheduling_fiber_prepare_to_resume_user_code(__cilkrts_worker *w,
                                                  full_frame *ff,
                                                  __cilkrts_stack_frame *sf)
{
    w->current_stack_frame = sf;
    sf->worker = w;

    // Lots of debugging checks on the state of the fiber we might be
    // resuming.
#if FIBER_DEBUG >= 1
#   if FIBER_DEBUG >= 3
    {
        fprintf(stderr, "w=%d: ff=%p, sf=%p. about to resume user code\n",
                w->self, ff, sf);
    }
#   endif

    const int flags = sf->flags;
    CILK_ASSERT(flags & CILK_FRAME_SUSPENDED);
    CILK_ASSERT(!sf->call_parent);
    CILK_ASSERT(w->head == w->tail);

    /* A frame can not be resumed unless it was suspended. */
    CILK_ASSERT(ff->sync_sp != NULL);

    /* The leftmost frame has no allocated stack */
    if (ff->simulated_stolen)
        CILK_ASSERT(flags & CILK_FRAME_UNSYNCHED);
    else if (flags & CILK_FRAME_UNSYNCHED)
        /* XXX By coincidence sync_sp could be null. */
        CILK_ASSERT(ff->fiber_self != NULL);
    else
        /* XXX This frame could be resumed unsynched on the leftmost stack */
        CILK_ASSERT((ff->sync_master == 0 || ff->sync_master == w));
    CILK_ASSERT(w->l->frame_ff == ff);
#endif    
}


/**
 * This method is the first method that should execute after we've
 * switched to a scheduling fiber from user code.
 *
 * @param fiber The scheduling fiber for the current worker.
 * @param wptr  The current worker.
 */
static void enter_runtime_transition_proc(cilk_fiber *fiber)
{
    // We can execute this method for one of three reasons:
    // 1. Undo-detach finds parent stolen.
    // 2. Sync suspends frame.
    // 3. Return from Cilk entry point.
    //
    //
    // In cases 1 and 2, the frame may be truly suspended or
    // may be immediately executed by this worker after provably_good_steal.
    //
    // 
    // There is a fourth case, which can, but does not need to execute
    // this function:
    //   4. Starting up the scheduling loop on a user or
    //      system worker.  In this case, we won't have
    //      a scheduling stack function to run.
    __cilkrts_worker* w = cilk_fiber_get_owner(fiber);
    if (w->l->post_suspend) {
        // Run the continuation function passed to longjmp_into_runtime
        run_scheduling_stack_fcn(w);

        // After we have jumped into the runtime and run the
        // scheduling function, any reducer map the worker had before entering the runtime
        // should have already been saved into the appropriate full
        // frame.
        CILK_ASSERT(NULL == w->reducer_map);

        // There shouldn't be any uncaught exceptions.
        //
        // In Windows, the OS catches any exceptions not caught by the
        // user code.  Thus, we are omitting the check on Windows.
        //
        // On Android, calling std::uncaught_exception with the stlport
        // library causes a seg fault.  Since we're not supporting
        // exceptions there at this point, just don't do the check
        //
        // TBD: Is this check also safe to do on Windows? 
        CILKBUG_ASSERT_NO_UNCAUGHT_EXCEPTION();
    }
}


/**
 * Method called to jump back to executing user code.
 *
 * A normal return from the runtime back to resuming user code calls
 * this method.  A computation executed using force_reduce also calls
 * this method to return to user code.
 *
 * This function should not contain any code that depends on a fiber.
 * In a force-reduce case, the user worker may not have a fiber.  In
 * the force-reduce case, we call this method directly instead of
 * calling @c user_code_resume_after_switch_into_runtime.
 */
static inline NORETURN
cilkrts_resume(__cilkrts_stack_frame *sf, full_frame *ff)
{
    // Save the sync stack pointer, and do the bookkeeping
    char* sync_sp = ff->sync_sp;
    __cilkrts_take_stack(ff, sync_sp);  // leaves ff->sync_sp null

    sf->flags &= ~CILK_FRAME_SUSPENDED;
    // Actually longjmp to the user code.
    // We may have exceptions to deal with, since we are resuming
    // a previous-suspended frame.
    sysdep_longjmp_to_sf(sync_sp, sf, ff);
}


/**
 * Called by the user-code fiber right before resuming a full frame
 * (sf/ff).
 *
 * This method pulls sf/ff out of the worker, and then calls
 * cilkrts_resume to jump to user code.
 */
static NORETURN
user_code_resume_after_switch_into_runtime(cilk_fiber *fiber)
{
    __cilkrts_worker *w = cilk_fiber_get_owner(fiber);
    __cilkrts_stack_frame *sf;
    full_frame *ff;
    sf = w->current_stack_frame;
    ff = sf->worker->l->frame_ff;

#if FIBER_DEBUG >= 1    
    CILK_ASSERT(ff->fiber_self == fiber);
    cilk_fiber_data *fdata = cilk_fiber_get_data(fiber);
    DBGPRINTF ("%d-%p: resume_after_switch_into_runtime, fiber=%p\n",
               w->self, w, fiber);
    CILK_ASSERT(sf == fdata->resume_sf);
#endif

    // Notify the Intel tools that we're stealing code
    ITT_SYNC_ACQUIRED(sf->worker);
    NOTIFY_ZC_INTRINSIC("cilk_continue", sf);
    cilk_fiber_invoke_tbb_stack_op(fiber, CILK_TBB_STACK_ADOPT);

    // Actually jump to user code.
    cilkrts_resume(sf, ff);
 }


/* The current stack is about to either be suspended or destroyed.  This
 * function will switch to the stack on which the scheduler is suspended and
 * resume running the scheduler within function do_work().  Upon waking up,
 * the scheduler will run the 'cont' function, using the supplied worker and
 * frame.
 */
static NORETURN
longjmp_into_runtime(__cilkrts_worker *w,
                     scheduling_stack_fcn_t fcn,
                     __cilkrts_stack_frame *sf)
{
    full_frame *ff, *ff2;

    CILK_ASSERT(!w->l->post_suspend);
    ff = w->l->frame_ff;

    // If we've got only one worker, stealing shouldn't be possible.
    // Assume that this is a steal or return from spawn in a force-reduce case.
    // We don't have a scheduling stack to switch to, so call the continuation
    // function directly.
    if (1 == w->g->P) {
        fcn(w, ff, sf);

        /* The call to function c() will have pushed ff as the next frame.  If
         * this were a normal (non-forced-reduce) execution, there would have
         * been a pop_next_frame call in a separate part of the runtime.  We
         * must call pop_next_frame here to complete the push/pop cycle. */
        ff2 = pop_next_frame(w);

        setup_for_execution(w, ff2, 0);
        scheduling_fiber_prepare_to_resume_user_code(w, ff2, w->current_stack_frame);
        cilkrts_resume(w->current_stack_frame, ff2);
        
// Suppress clang warning that the expression result is unused
#if defined(__clang__) && (! defined(__INTEL_COMPILER))
#   pragma clang diagnostic push
#   pragma clang diagnostic ignored "-Wunused-value"
#endif // __clang__
        /* no return */
        CILK_ASSERT(((void)"returned from __cilkrts_resume", 0));
#if defined(__clang__) && (! defined(__INTEL_COMPILER))
#   pragma clang diagnostic pop
#endif // __clang__
    }

    w->l->post_suspend = fcn;
    w->l->suspended_stack = sf;

    ITT_SYNC_RELEASING(w);
    ITT_SYNC_PREPARE(w);

#if FIBER_DEBUG >= 2
    fprintf(stderr, "ThreadId=%p, W=%d: about to switch into runtime... w->l->frame_ff = %p, sf=%p\n",
            cilkos_get_current_thread_id(),
            w->self, w->l->frame_ff,
            sf);
#endif

    // Current fiber is either the (1) one we are about to free,
    // or (2) it has been passed up to the parent.
    cilk_fiber *current_fiber = ( w->l->fiber_to_free ?
                                  w->l->fiber_to_free :
                                  w->l->frame_ff->parent->fiber_child );
    cilk_fiber_data* fdata = cilk_fiber_get_data(current_fiber);
    CILK_ASSERT(NULL == w->l->frame_ff->fiber_self);

    // Clear the sf in the current fiber for cleanliness, to prevent
    // us from accidentally resuming a bad sf.
    // Technically, resume_sf gets overwritten for a fiber when
    // we are about to resume it anyway.
    fdata->resume_sf = NULL;
    CILK_ASSERT(fdata->owner == w);

    // Set the function to execute immediately after switching to the
    // scheduling fiber, but before freeing any fibers.
    cilk_fiber_set_post_switch_proc(w->l->scheduling_fiber,
                                    enter_runtime_transition_proc);
    cilk_fiber_invoke_tbb_stack_op(current_fiber, CILK_TBB_STACK_ORPHAN);
    
    if (w->l->fiber_to_free) {
        // Case 1: we are freeing this fiber.  We never
        // resume this fiber again after jumping into the runtime.
        w->l->fiber_to_free = NULL;

        // Extra check. Normally, the fiber we are about to switch to
        // should have a NULL owner.
        CILK_ASSERT(NULL == cilk_fiber_get_data(w->l->scheduling_fiber)->owner);
#if FIBER_DEBUG >= 4
        fprintf(stderr, "ThreadId=%p, W=%d: about to switch into runtime.. current_fiber = %p, deallcoate, switch to fiber %p\n",
                cilkos_get_current_thread_id(),
                w->self,
                current_fiber, w->l->scheduling_fiber);
#endif
        cilk_fiber_invoke_tbb_stack_op(current_fiber, CILK_TBB_STACK_RELEASE);
        NOTE_INTERVAL(w, INTERVAL_DEALLOCATE_RESUME_OTHER);
        cilk_fiber_remove_reference_from_self_and_resume_other(current_fiber,
                                                               &w->l->fiber_pool,
                                                               w->l->scheduling_fiber);
        // We should never come back here!
        CILK_ASSERT(0);
    }
    else {        
        // Case 2: We are passing the fiber to our parent because we
        // are leftmost.  We should come back later to
        // resume execution of user code.
        //
        // If we are not freeing a fiber, there we must be
        // returning from a spawn or processing an exception.  The
        // "sync" path always frees a fiber.
        // 
        // We must be the leftmost child, and by left holder logic, we
        // have already moved the current fiber into our parent full
        // frame.
#if FIBER_DEBUG >= 2
        fprintf(stderr, "ThreadId=%p, W=%d: about to suspend self into runtime.. current_fiber = %p, deallcoate, switch to fiber %p\n",
                cilkos_get_current_thread_id(),
                w->self,
                current_fiber, w->l->scheduling_fiber);
#endif

        NOTE_INTERVAL(w, INTERVAL_SUSPEND_RESUME_OTHER);

        cilk_fiber_suspend_self_and_resume_other(current_fiber,
                                                 w->l->scheduling_fiber);
        // Resuming this fiber returns control back to
        // this function because our implementation uses OS fibers.
        //
        // On Unix, we could have the choice of passing the
        // user_code_resume_after_switch_into_runtime as an extra "resume_proc"
        // that resumes execution of user code instead of the
        // jumping back here, and then jumping back to user code.
#if FIBER_DEBUG >= 2
        CILK_ASSERT(fdata->owner == __cilkrts_get_tls_worker());
#endif
        user_code_resume_after_switch_into_runtime(current_fiber);
    }
}

/*
 * Send a message to the children of the specified worker: run or wait.
 */
static void notify_children(__cilkrts_worker *w, unsigned int msg)
{
    int child_num;
    __cilkrts_worker *child;
    int num_sys_workers = w->g->P - 1;

    // If worker is "n", then its children are 2n + 1, and 2n + 2.
    child_num = (w->self << 1) + 1;
    if (child_num < num_sys_workers) {
        child = w->g->workers[child_num];
        CILK_ASSERT(child->l->signal_node);
        signal_node_msg(child->l->signal_node, msg);
        child_num++;
        if (child_num < num_sys_workers) {
            child = w->g->workers[child_num];
            CILK_ASSERT(child->l->signal_node);
            signal_node_msg(child->l->signal_node, msg);
        }
    }
}

/*
 * Notify this worker's children that they need to wait.
 */
static void notify_children_wait(__cilkrts_worker *w)
{
    notify_children(w, 0);
}

/*
 * Notify this worker's children to run and start trying to steal.
 */
static void notify_children_run(__cilkrts_worker *w)
{
    notify_children(w, 1);
}

/**
 * A single "check" to find work, either on our queue or through a
 * steal attempt.  This method checks our local queue once, and
 * performs one steal attempt.
 */
static full_frame* check_for_work(__cilkrts_worker *w)
{
    full_frame *ff = NULL;
    ff = pop_next_frame(w);
    // If there is no work on the queue, try to steal some.
    if (NULL == ff) {
        START_INTERVAL(w, INTERVAL_STEALING) {
            if (w->l->type != WORKER_USER && w->l->team != NULL) {
                // At this point, the worker knows for certain that it has run
                // out of work.  Therefore, it loses its team affiliation.  User
                // workers never change teams, of course.
                __cilkrts_worker_lock(w);
                w->l->team = NULL;
                __cilkrts_worker_unlock(w);
            }

            // If we are about to do a random steal, we should have no
            // full frame...
            CILK_ASSERT(NULL == w->l->frame_ff);
            random_steal(w);
        } STOP_INTERVAL(w, INTERVAL_STEALING);

        // If the steal was successful, then the worker has populated its next
        // frame with the work to resume.
        ff = pop_next_frame(w);
        if (NULL == ff) {
            // Punish the worker for failing to steal.
            // No quantum for you!
            unsigned int max_fails = w->g->max_steal_failures << 1;
            if (w->l->has_stolen == 0 &&
                w->l->steal_failure_count % max_fails == max_fails - 1) {
                // Idle briefly if the worker has never stolen anything for
                // the given grace period
                __cilkrts_idle();
            } else {
                __cilkrts_yield();
            }
            w->l->steal_failure_count++;
            if (w->l->steal_failure_count > (max_fails << 8)) {
                // Reset the flag after certain amount of failures
                // - This will reduce cpu time in top-level synched regions
                // - max_fails can be controlled by user (CILK_STEAL_FAILURES)
                w->l->has_stolen = 0;
            }
        } else {
            // Reset steal_failure_count since there is obviously still work to
            // be done.
            w->l->steal_failure_count = 0;
            w->l->has_stolen = 1;
        }
    }
    return ff;
}

/**
 * Keep stealing or looking on our queue.
 *
 * Returns either when a full frame is found, or NULL if the
 * computation is done.
 */ 
static full_frame* search_until_work_found_or_done(__cilkrts_worker *w)
{
    full_frame *ff = NULL;
    // Find a full frame to execute (either through random stealing,
    // or because we pull it off w's 1-element queue).
    while (!ff) {
        // Check worker state to figure out our next action.
        switch (worker_runnable(w))    
        {
        case SCHEDULE_RUN:             // One attempt at checking for work.
            ff = check_for_work(w);
            break;
        case SCHEDULE_WAIT:            // go into wait-mode.
            START_INTERVAL(w, INTERVAL_SCHEDULE_WAIT);
            CILK_ASSERT(WORKER_SYSTEM == w->l->type);
            // If we are about to wait, then we better not have
            // a frame that we should execute...
            CILK_ASSERT(NULL == w->l->next_frame_ff);
            notify_children_wait(w);
            signal_node_wait(w->l->signal_node);
            // ...
            // Runtime is waking up.
            notify_children_run(w);
            w->l->steal_failure_count = 0;
            STOP_INTERVAL(w, INTERVAL_SCHEDULE_WAIT);
            break;
        case SCHEDULE_EXIT:            // exit the scheduler.
            CILK_ASSERT(WORKER_USER != w->l->type);
            return NULL;
        default:
            CILK_ASSERT(0);
            abort();
        }
    }
    return ff;
}

/**
 * The proc method for a scheduling fiber on a user worker.
 * 
 * When a user worker jumps into the runtime, it jumps into this
 * method by either starting it if the scheduling fiber has never run
 * before, or resuming the fiber if it was previously suspended.
 */
COMMON_PORTABLE
void scheduler_fiber_proc_for_user_worker(cilk_fiber *fiber)
{
    __cilkrts_worker* w = cilk_fiber_get_owner(fiber);
    CILK_ASSERT(w);

    // This must be a user worker
    CILK_ASSERT(WORKER_USER == w->l->type);

    // If we aren't the current worker, then something is very wrong
    // here..
    verify_current_wkr(w);

    __cilkrts_run_scheduler_with_exceptions(w);
}


/**
 * The body of the runtime scheduling loop.  This function executes in
 * 4 stages:
 *
 * 1. Transitions from the user code into the runtime by
 *    executing any scheduling-stack functions.
 * 2. Looks for a full frame enqueued from a successful provably
 *    good steal.
 * 3. If no full frame is found in step 2, steal until
 *    a frame is found or we are done.  If we are done, finish
 *    the scheduling loop. 
 * 4. When a frame is found, setup to resume user code.
 *    In particular, suspend the current fiber and resume the
 *    user fiber to execute the frame.
 *
 * Returns a fiber object that we should switch to after completing
 * the body of the loop, or NULL if we should continue executing on
 * this fiber.
 *
 * @pre @c current_fiber should equal @c wptr->l->scheduling_fiber
 * 
 * @param current_fiber   The currently executing (scheduling_ fiber
 * @param wptr            The currently executing worker.
 * @param return          The next fiber we should switch to.
 */
static cilk_fiber* worker_scheduling_loop_body(cilk_fiber* current_fiber,
                                               void* wptr)
{
    __cilkrts_worker *w = (__cilkrts_worker*) wptr;
    CILK_ASSERT(current_fiber == w->l->scheduling_fiber);

    // Stage 1: Transition from executing user code to the runtime code.
    // We don't need to do this call here any more, because 
    // every switch to the scheduling fiber should make this call
    // using a post_switch_proc on the fiber.
    //
    //  enter_runtime_transition_proc(w->l->scheduling_fiber, wptr);

    // After Stage 1 is complete, w should no longer have
    // an associated full frame.
    CILK_ASSERT(NULL == w->l->frame_ff);

    // Stage 2.  First do a quick check of our 1-element queue.
    full_frame *ff = pop_next_frame(w);

    if (!ff) {
        // Stage 3.  We didn't find anything from our 1-element
        // queue.  Now go through the steal loop to find work. 
        ff = search_until_work_found_or_done(w);
        if (!ff) {
            CILK_ASSERT(w->g->work_done);
            return NULL;
        }
    }

    // Stage 4.  Now that we have found a full frame to work on,
    // actually execute it.
    __cilkrts_stack_frame *sf;

    // There shouldn't be any uncaught exceptions.
    //
    // In Windows, the OS catches any exceptions not caught by the
    // user code.  Thus, we are omitting the check on Windows.
    //
    // On Android, calling std::uncaught_exception with the stlport
    // library causes a seg fault.  Since we're not supporting
    // exceptions there at this point, just don't do the check
    CILKBUG_ASSERT_NO_UNCAUGHT_EXCEPTION();

    BEGIN_WITH_WORKER_LOCK(w) {
        CILK_ASSERT(!w->l->frame_ff);
        BEGIN_WITH_FRAME_LOCK(w, ff) {
            sf = ff->call_stack;
            CILK_ASSERT(sf && !sf->call_parent);
            setup_for_execution(w, ff, 0);
        } END_WITH_FRAME_LOCK(w, ff);
    } END_WITH_WORKER_LOCK(w);

    /* run it */
    //
    // Prepare to run the full frame.  To do so, we need to:
    //   (a) Execute some code on this fiber (the scheduling
    //       fiber) to set up data structures, and
    //   (b) Suspend the scheduling fiber, and resume the
    //       user-code fiber.

    // Part (a). Set up data structures.
    scheduling_fiber_prepare_to_resume_user_code(w, ff, sf);

    cilk_fiber *other = w->l->frame_ff->fiber_self;
    cilk_fiber_data* other_data = cilk_fiber_get_data(other);
    cilk_fiber_data* current_fiber_data = cilk_fiber_get_data(current_fiber);

    // I believe two cases are possible here, both of which
    // should have other_data->resume_sf as NULL.
    //
    // 1. Resuming a fiber that was previously executing
    //    user code (i.e., a provably-good-steal).
    //    In this case, resume_sf should have been
    //    set to NULL when it was suspended.
    //
    // 2. Resuming code on a steal.  In this case, since we
    //    grabbed a new fiber, resume_sf should be NULL.
    CILK_ASSERT(NULL == other_data->resume_sf);
        
#if FIBER_DEBUG >= 2
    fprintf(stderr, "W=%d: other fiber=%p, setting resume_sf to %p\n",
            w->self, other, other_data->resume_sf);
#endif
    // Update our own fiber's data.
    current_fiber_data->resume_sf = NULL;
    // The scheduling fiber should have the right owner from before.
    CILK_ASSERT(current_fiber_data->owner == w);
    other_data->resume_sf = sf;
        

#if FIBER_DEBUG >= 3
    fprintf(stderr, "ThreadId=%p (about to suspend self resume other), W=%d: current_fiber=%p, other=%p, current_fiber->resume_sf = %p, other->resume_sf = %p\n",
            cilkos_get_current_thread_id(),
            w->self,
            current_fiber, other,
            current_fiber_data->resume_sf,
            other_data->resume_sf);
#endif
    return other;
}


/**
 * This function is executed once by each worker, to initialize its
 * scheduling loop.
 */
static void worker_scheduler_init_function(__cilkrts_worker *w)
{
    // First, execute the startup tasks that must happen for all
    // worker types.
    ITT_SYNC_PREPARE(w);
    /* Notify tools about the new worker. Inspector needs this, but we
       don't want to confuse Cilkscreen with system threads.  User threads
       do this notification in bind_thread */
    if (! w->g->under_ptool)
        __cilkrts_cilkscreen_establish_worker(w);

    // Seed the initial random number generator.
    // If we forget to do this, then the worker always steals from 0.
    // Programs will still execute correctly, but
    // you may see a subtle performance bug...
    mysrand(w, (w->self + 1));

    // The startup work varies, depending on the worker type.
    switch (w->l->type) {
    case WORKER_USER:
        break;

    case WORKER_SYSTEM:
        // If a system worker is starting, we must also be starting
        // the runtime.

        // Runtime begins in a wait-state and is woken up by the first user
        // worker when the runtime is ready.
        signal_node_wait(w->l->signal_node);
        // ...
        // Runtime is waking up.
        notify_children_run(w);
        w->l->steal_failure_count = 0;
        break;
    default:
        __cilkrts_bug("Unknown worker %p of type %d entering scheduling loop\n",
                      w, w->l->type);
    }
}

/**
 * This function is executed once by each worker, to finish its
 * scheduling loop.
 *
 * @note Currently, only system workers finish their loops.  User
 * workers will jump away to user code without exiting their
 * scheduling loop.
 */ 
static void worker_scheduler_terminate_function(__cilkrts_worker *w)
{
    // A user worker should never finish by falling through the
    // scheduling loop.
    CILK_ASSERT(WORKER_USER != w->l->type);
}

/**
 * The main scheduler function executed by a worker's scheduling
 * fiber.
 * 
 * This method is started by either a new system worker, or a user
 * worker that has stalled and just been imported into the runtime.
 */
static void worker_scheduler_function(__cilkrts_worker *w)
{
    START_INTERVAL(w, INTERVAL_INIT_WORKER);
    worker_scheduler_init_function(w);
    STOP_INTERVAL(w, INTERVAL_INIT_WORKER);
    
    // The main scheduling loop body.

    while (!w->g->work_done) {    
        // Execute the "body" of the scheduling loop, and figure
        // out the fiber to jump to next.
        START_INTERVAL(w, INTERVAL_SCHED_LOOP);
        cilk_fiber* fiber_to_resume
            = worker_scheduling_loop_body(w->l->scheduling_fiber, w);
        STOP_INTERVAL(w, INTERVAL_SCHED_LOOP);
        
        if (fiber_to_resume) {
            // Suspend the current fiber and resume next one.
            NOTE_INTERVAL(w, INTERVAL_SUSPEND_RESUME_OTHER);

            // Whenever we jump to resume user code, we stop being in
            // the runtime, and start working.
            STOP_INTERVAL(w, INTERVAL_IN_RUNTIME);
            START_INTERVAL(w, INTERVAL_WORKING);
            cilk_fiber_suspend_self_and_resume_other(w->l->scheduling_fiber,
                                                     fiber_to_resume);
            // Return here only when this (scheduling) fiber is
            // resumed (i.e., this worker wants to reenter the runtime).

            // We've already switched from WORKING to IN_RUNTIME in
            // the runtime code that handles the fiber switch.  Thus, at
            // this point we are IN_RUNTIME already.
        }
    }

    // Finish the scheduling loop.
    worker_scheduler_terminate_function(w);
}


/*************************************************************
  Forward declarations for reduction protocol.
*************************************************************/

static __cilkrts_worker*
execute_reductions_for_sync(__cilkrts_worker *w,
                            full_frame *ff,
                            __cilkrts_stack_frame *sf_at_sync);

static __cilkrts_worker*
execute_reductions_for_spawn_return(__cilkrts_worker *w,
                                    full_frame *ff,
                                    __cilkrts_stack_frame *returning_sf);

                                                             

/*************************************************************
  Scheduler functions that are callable by client code
*************************************************************/
static full_frame *disown(__cilkrts_worker *w,
                          full_frame *ff,
                          __cilkrts_stack_frame *sf,
                          const char *why)
{
    CILK_ASSERT(ff);
    make_unrunnable(w, ff, sf, sf != 0, why);
    w->l->frame_ff = 0;
    return ff->parent;
}

/**
 * Called when ff is returning from a spawn, and we need to execute a
 * reduction.
 *
 * @param w             The currently executing worker.
 * @param ff            The full frame for w.
 * @param returning_sf  The stack frame for the spawn helper that is returning.
 *
 * Normally, by the time we gain control in the runtime, the worker
 * has already popped off the __cilkrts_stack_frame "returning_sf"
 * from its call chain.
 * 
 * When we have only serial reductions, w->current_stack_frame is not
 * needed any more, because w is about to enter the runtime scheduling
 * loop anyway.  Similarly, the frame "ff" is slated to be destroyed
 * after the runtime finishes the return from spawn and splices ff out
 * of the tree of full frames.
 *
 * To execute a parallel reduction, however, we still want
 * w->current_stack_frame == returning_sf, and we are going to use the
 * frame ff for a little bit longer.
 *
 * This method:
 *
 *   1. Puts returning_sf back as w's current stack frame.
 *   2. Makes "ff" runnable again on w.
 */ 
static inline
void restore_frame_for_spawn_return_reduction(__cilkrts_worker *w,
                                              full_frame *ff,
                                              __cilkrts_stack_frame *returning_sf) {
#if REDPAR_DEBUG >= 2
    CILK_ASSERT(returning_sf);
    CILK_ASSERT(returning_sf->worker == w);
#endif
    // Change w's current stack frame back to "returning_sf".
    //
    // Intuitively, w->current_stack_frame should be
    // returning_sf->call_parent at this point.
    //
    // We can not assert this, however, because the pop of
    // returning_sf from the call chain has already cleared
    // returning_sf->call_parent.  We don't want to restore the call
    // parent of returning_sf, because its parent has been stolen, and
    // the runtime assumes that steals break this link.

    // We cannot assert call_parent is NULL either, since that's not true for
    // Win64 exception handling
//    CILK_ASSERT(returning_sf->call_parent == NULL);
    w->current_stack_frame = returning_sf;

    // Make the full frame "ff" runnable again, in preparation for
    // executing the reduction.
    make_runnable(w, ff);
}


NORETURN __cilkrts_c_sync(__cilkrts_worker *w,
                          __cilkrts_stack_frame *sf_at_sync)
{
    full_frame *ff; 
    STOP_INTERVAL(w, INTERVAL_WORKING);
    START_INTERVAL(w, INTERVAL_IN_RUNTIME);

    // Claim: This read of w->l->frame_ff can occur without
    // holding the worker lock because when w has reached a sync
    // and entered the runtime (because it stalls), w's deque is empty
    // and no one else can steal and change w->l->frame_ff.

    ff = w->l->frame_ff;
#ifdef _WIN32
    __cilkrts_save_exception_state(w, ff);
#else
    // Move any pending exceptions into the full frame
    CILK_ASSERT(NULL == ff->pending_exception);
    ff->pending_exception = w->l->pending_exception;
    w->l->pending_exception = NULL;
#endif
    
    w = execute_reductions_for_sync(w, ff, sf_at_sync);

#if FIBER_DEBUG >= 3
    fprintf(stderr, "ThreadId=%p, w->self = %d. about to longjmp_into_runtim[c_sync] with ff=%p\n",
            cilkos_get_current_thread_id(), w->self, ff);
#endif    

    longjmp_into_runtime(w, do_sync, sf_at_sync);
}

static void do_sync(__cilkrts_worker *w, full_frame *ff,
                    __cilkrts_stack_frame *sf)
{
    //int abandoned = 1;
    enum provably_good_steal_t steal_result = ABANDON_EXECUTION;

    START_INTERVAL(w, INTERVAL_SYNC_CHECK) {
        BEGIN_WITH_WORKER_LOCK_OPTIONAL(w) {

            CILK_ASSERT(ff);
            BEGIN_WITH_FRAME_LOCK(w, ff) {
                CILK_ASSERT(sf->call_parent == 0);
                CILK_ASSERT(sf->flags & CILK_FRAME_UNSYNCHED);

                // Before switching into the scheduling fiber, we should have
                // already taken care of deallocating the current
                // fiber. 
                CILK_ASSERT(NULL == ff->fiber_self);

                // Update the frame's pedigree information if this is an ABI 1
                // or later frame
                if (CILK_FRAME_VERSION_VALUE(sf->flags) >= 1)
                {
                    sf->parent_pedigree.rank = w->pedigree.rank;
                    sf->parent_pedigree.parent = w->pedigree.parent;

                    // Note that the pedigree rank needs to be updated
                    // when setup_for_execution_pedigree runs
                    sf->flags |= CILK_FRAME_SF_PEDIGREE_UNSYNCHED;
                }

                /* the decjoin() occurs in provably_good_steal() */
                steal_result = provably_good_steal(w, ff);

            } END_WITH_FRAME_LOCK(w, ff);
            // set w->l->frame_ff = NULL after checking abandoned
            if (WAIT_FOR_CONTINUE != steal_result) {
                w->l->frame_ff = NULL;
            }
        } END_WITH_WORKER_LOCK_OPTIONAL(w);
    } STOP_INTERVAL(w, INTERVAL_SYNC_CHECK);

    // Now, if we are in a replay situation and provably_good_steal() returned
    // WAIT_FOR_CONTINUE, we should sleep, reacquire locks, call
    // provably_good_steal(), and release locks until we get a value other
    // than WAIT_FOR_CONTINUE from the function.
#ifdef CILK_RECORD_REPLAY
    // We don't have to explicitly check for REPLAY_LOG below because
    // steal_result can only be set to WAIT_FOR_CONTINUE during replay
    while(WAIT_FOR_CONTINUE == steal_result)
    {
        __cilkrts_sleep();
        BEGIN_WITH_WORKER_LOCK_OPTIONAL(w)
        {
            ff = w->l->frame_ff;
            BEGIN_WITH_FRAME_LOCK(w, ff)
            {
                steal_result = provably_good_steal(w, ff);
            } END_WITH_FRAME_LOCK(w, ff);
            if (WAIT_FOR_CONTINUE != steal_result)
                w->l->frame_ff = NULL;
        } END_WITH_WORKER_LOCK_OPTIONAL(w);
    }
#endif  // CILK_RECORD_REPLAY

#ifdef ENABLE_NOTIFY_ZC_INTRINSIC
    // If we can't make any further progress on this thread, tell Inspector
    // that we're abandoning the work and will go find something else to do.
    if (ABANDON_EXECUTION == steal_result)
    {
        NOTIFY_ZC_INTRINSIC("cilk_sync_abandon", 0);
    }
#endif // defined ENABLE_NOTIFY_ZC_INTRINSIC

    return; /* back to scheduler loop */
}

/* worker W completely promotes its own deque, simulating the case
   where the whole deque is stolen.  We use this mechanism to force
   the allocation of new storage for reducers for race-detection
   purposes. */
void __cilkrts_promote_own_deque(__cilkrts_worker *w)
{
    // Remember the fiber we start this method on.
    CILK_ASSERT(w->l->frame_ff);
    cilk_fiber* starting_fiber = w->l->frame_ff->fiber_self;
    
    BEGIN_WITH_WORKER_LOCK(w) {
        while (dekker_protocol(w)) {
            /* PLACEHOLDER_FIBER is used as non-null marker to tell detach()
               and make_child() that this frame should be treated as a spawn
               parent, even though we have not assigned it a stack. */
            detach_for_steal(w, w, PLACEHOLDER_FIBER);
        }
    } END_WITH_WORKER_LOCK(w);


    // TBD: The management of full frames and fibers is a bit
    // sketchy here.  We are promoting stack frames into full frames,
    // and pretending they are stolen away, but no other worker is
    // actually working on them.  Some runtime invariants
    // may be broken here.
    //
    // Technically, if we are simulating a steal from w
    // w should get a new full frame, but
    // keep the same fiber.  A real thief would be taking the
    // loot frame away, get a new fiber, and starting executing the
    // loot frame.
    //
    // What should a fake thief do?  Where does the frame go? 

    // In any case, we should be finishing the promotion process with
    // the same fiber with.
    CILK_ASSERT(w->l->frame_ff);
    CILK_ASSERT(w->l->frame_ff->fiber_self == starting_fiber);
}



/* the client code calls this function after a spawn when the dekker
   protocol fails.  The function may either return or longjmp
   into the rts

   This function takes in a "returning_sf" argument which corresponds
   to the __cilkrts_stack_frame that we are finishing (i.e., the
   argument to __cilkrts_leave_frame).
   */
void __cilkrts_c_THE_exception_check(__cilkrts_worker *w, 
                                     __cilkrts_stack_frame *returning_sf)
{
    full_frame *ff;
    int stolen_p;
    __cilkrts_stack_frame *saved_sf = NULL;

    // For the exception check, stop working and count as time in
    // runtime.
    STOP_INTERVAL(w, INTERVAL_WORKING);
    START_INTERVAL(w, INTERVAL_IN_RUNTIME);

    START_INTERVAL(w, INTERVAL_THE_EXCEPTION_CHECK);

    BEGIN_WITH_WORKER_LOCK(w) {
        ff = w->l->frame_ff;
        CILK_ASSERT(ff);
        /* This code is called only upon a normal return and never
           upon an exceptional return.  Assert that this is the
           case. */
        CILK_ASSERT(!w->l->pending_exception);

        reset_THE_exception(w);
        stolen_p = !(w->head < (w->tail + 1)); /* +1 because tail was
                                                  speculatively
                                                  decremented by the
                                                  compiled code */

        if (stolen_p) {
            /* XXX This will be charged to THE for accounting purposes */
            __cilkrts_save_exception_state(w, ff);

            // Save the value of the current stack frame.
            saved_sf = w->current_stack_frame;

            // Reverse the decrement from undo_detach.
            // This update effectively resets the deque to be
            // empty (i.e., changes w->tail back to equal w->head). 
            // We need to reset the deque to execute parallel
            // reductions.  When we have only serial reductions, it
            // does not matter, since serial reductions do not
            // change the deque.
            w->tail++;
#if REDPAR_DEBUG > 1            
            // ASSERT our deque is empty.
            CILK_ASSERT(w->head == w->tail);
#endif
        }
    } END_WITH_WORKER_LOCK(w);

    STOP_INTERVAL(w, INTERVAL_THE_EXCEPTION_CHECK);

    if (stolen_p)
    {
        w = execute_reductions_for_spawn_return(w, ff, returning_sf);

        // "Mr. Policeman?  My parent always told me that if I was in trouble
        // I should ask a nice policeman for help.  I can't find my parent
        // anywhere..."
        //
        // Write a record to the replay log for an attempt to return to a stolen parent
        replay_record_orphaned(w);

        // Update the pedigree only after we've finished the
        // reductions.
        update_pedigree_on_leave_frame(w, returning_sf);

        // Notify Inspector that the parent has been stolen and we're
        // going to abandon this work and go do something else.  This
        // will match the cilk_leave_begin in the compiled code
        NOTIFY_ZC_INTRINSIC("cilk_leave_stolen", saved_sf);

        DBGPRINTF ("%d: longjmp_into_runtime from __cilkrts_c_THE_exception_check\n", w->self);
        longjmp_into_runtime(w, do_return_from_spawn, 0);
        DBGPRINTF ("%d: returned from longjmp_into_runtime from __cilkrts_c_THE_exception_check?!\n", w->self);
    }
    else
    {
        NOTE_INTERVAL(w, INTERVAL_THE_EXCEPTION_CHECK_USELESS);

        // If we fail the exception check and return, then switch back
        // to working.
        STOP_INTERVAL(w, INTERVAL_IN_RUNTIME);
        START_INTERVAL(w, INTERVAL_WORKING);
        return;
    }
}

/* Return an exception to a stolen parent. */
NORETURN __cilkrts_exception_from_spawn(__cilkrts_worker *w,
                                        __cilkrts_stack_frame *returning_sf) 
{
    full_frame *ff = w->l->frame_ff;
    STOP_INTERVAL(w, INTERVAL_WORKING);
    START_INTERVAL(w, INTERVAL_IN_RUNTIME);

    // This is almost the same as THE_exception_check, except
    // the detach didn't happen, we don't need to undo the tail
    // update.
    CILK_ASSERT(w->head == w->tail);
    w = execute_reductions_for_spawn_return(w, ff, returning_sf);

    longjmp_into_runtime(w, do_return_from_spawn, 0);
    CILK_ASSERT(0);
}

static void do_return_from_spawn(__cilkrts_worker *w,
                                 full_frame *ff,
                                 __cilkrts_stack_frame *sf)
{
    full_frame *parent_ff;
    enum provably_good_steal_t steal_result = ABANDON_EXECUTION;

    BEGIN_WITH_WORKER_LOCK_OPTIONAL(w) {
        CILK_ASSERT(ff);
        CILK_ASSERT(!ff->is_call_child);
        CILK_ASSERT(sf == NULL);
        parent_ff = ff->parent;
    
        BEGIN_WITH_FRAME_LOCK(w, ff) {
            decjoin(ff);
        } END_WITH_FRAME_LOCK(w, ff);

        BEGIN_WITH_FRAME_LOCK(w, parent_ff) {
            if (parent_ff->simulated_stolen)
                unconditional_steal(w, parent_ff);
            else
                steal_result = provably_good_steal(w, parent_ff);
        } END_WITH_FRAME_LOCK(w, parent_ff);

    } END_WITH_WORKER_LOCK_OPTIONAL(w);

    // Loop here in replay mode
#ifdef CILK_RECORD_REPLAY
    // We don't have to explicitly check for REPLAY_LOG below because
    // steal_result can only get set to WAIT_FOR_CONTINUE during replay.
    // We also don't have to worry about the simulated_stolen flag
    // because steal_result can only be set to WAIT_FOR_CONTINUE by
    // provably_good_steal().
    while(WAIT_FOR_CONTINUE == steal_result)
    {
        __cilkrts_sleep();
        BEGIN_WITH_WORKER_LOCK_OPTIONAL(w)
        {
            BEGIN_WITH_FRAME_LOCK(w, parent_ff)
            {
                steal_result = provably_good_steal(w, parent_ff);
            } END_WITH_FRAME_LOCK(w, parent_ff);
        } END_WITH_WORKER_LOCK_OPTIONAL(w);
    }
#endif  // CILK_RECORD_REPLAY

    // Cleanup the child frame.
    __cilkrts_destroy_full_frame(w, ff);
    return;
}

#ifdef _WIN32
/* migrate an exception across fibers.  Call this function when an exception has
 * been thrown and has to traverse across a steal.  The exception has already
 * been wrapped up, so all that remains is to longjmp() into the continuation,
 * sync, and re-raise it.
 */
void __cilkrts_migrate_exception(__cilkrts_stack_frame *sf) {

    __cilkrts_worker *w = sf->worker;
    full_frame *ff;

    BEGIN_WITH_WORKER_LOCK(w) {
        ff = w->l->frame_ff;
        reset_THE_exception(w);
        /* there is no need to check for a steal because we wouldn't be here if
           there weren't a steal. */
        __cilkrts_save_exception_state(w, ff);

        CILK_ASSERT(w->head == w->tail);
    } END_WITH_WORKER_LOCK(w);

    {
        // TBD(jsukha): This function emulates the
        // the "do_return_from_spawn" path.
        w = execute_reductions_for_spawn_return(w, ff, sf);
    }

    longjmp_into_runtime(w, do_return_from_spawn, 0); /* does not return. */
    CILK_ASSERT(! "Shouldn't be here...");
}
#endif


/* Pop a call stack from TAIL.  Return the call stack, or NULL if the
   queue is empty */
__cilkrts_stack_frame *__cilkrts_pop_tail(__cilkrts_worker *w)
{
    __cilkrts_stack_frame *sf;
    BEGIN_WITH_WORKER_LOCK(w) {
        __cilkrts_stack_frame *volatile *tail = w->tail;
        if (w->head < tail) {
            --tail;
            sf = *tail;
            w->tail = tail;
        } else {
            sf = 0;
        }
    } END_WITH_WORKER_LOCK(w);
    return sf;
}

#ifdef CILK_RECORD_REPLAY
__cilkrts_stack_frame *simulate_pop_tail(__cilkrts_worker *w)
{
    __cilkrts_stack_frame *sf;
    BEGIN_WITH_WORKER_LOCK(w) {
        if (w->head < w->tail) {
            sf = *(w->tail-1);
        } else {
            sf = 0;
        }
    } END_WITH_WORKER_LOCK(w);
    return sf;
}
#endif


/* Return from a call, not a spawn. */
void __cilkrts_return(__cilkrts_worker *w)
{
    full_frame *ff, *parent_ff;

    // Count time during the return as in the runtime.
    STOP_INTERVAL(w, INTERVAL_WORKING);
    START_INTERVAL(w, INTERVAL_IN_RUNTIME);
    START_INTERVAL(w, INTERVAL_RETURNING);

    BEGIN_WITH_WORKER_LOCK_OPTIONAL(w) {
        ff = w->l->frame_ff;
        CILK_ASSERT(ff);
        CILK_ASSERT(ff->join_counter == 1);
        /* This path is not used to return from spawn. */
        CILK_ASSERT(ff->is_call_child);

        BEGIN_WITH_FRAME_LOCK(w, ff) {
            // After this call, w->l->frame_ff != ff.
            // Technically, w will "own" ff until ff is freed,
            // however, because ff is a dying leaf full frame.
            parent_ff = disown(w, ff, 0, "return");
            decjoin(ff);

#ifdef _WIN32
            __cilkrts_save_exception_state(w, ff);
#else
            // Move the pending exceptions into the full frame
            // This should always be NULL if this isn't a
            // return with an exception
            CILK_ASSERT(NULL == ff->pending_exception);
            ff->pending_exception = w->l->pending_exception;
            w->l->pending_exception = NULL;
#endif  // _WIN32

        } END_WITH_FRAME_LOCK(w, ff);

        __cilkrts_fence(); /* redundant */

        CILK_ASSERT(parent_ff);

        BEGIN_WITH_FRAME_LOCK(w, parent_ff) {
            finalize_child_for_call(w, parent_ff, ff);
        } END_WITH_FRAME_LOCK(w, parent_ff);

        ff = pop_next_frame(w);
        /* ff will be non-null except when the parent frame is owned
           by another worker.
           CILK_ASSERT(ff)
        */
        CILK_ASSERT(!w->l->frame_ff);
        if (ff) {
            BEGIN_WITH_FRAME_LOCK(w, ff) {
                __cilkrts_stack_frame *sf = ff->call_stack;
                CILK_ASSERT(sf && !sf->call_parent);
                setup_for_execution(w, ff, 1);
            } END_WITH_FRAME_LOCK(w, ff);
        }
    } END_WITH_WORKER_LOCK_OPTIONAL(w);

    STOP_INTERVAL(w, INTERVAL_RETURNING);
    STOP_INTERVAL(w, INTERVAL_IN_RUNTIME);
    START_INTERVAL(w, INTERVAL_WORKING);
}

static void __cilkrts_unbind_thread()
{
    int stop_cilkscreen = 0;
    global_state_t *g;

    // Take out the global OS mutex to protect accesses to the table of workers
    global_os_mutex_lock();

    if (cilkg_is_published()) {
        __cilkrts_worker *w = __cilkrts_get_tls_worker();
        if (w) {
            g = w->g;


            // Matches the START in bind_thread in cilk-abi.c.
            STOP_INTERVAL(w, INTERVAL_IN_RUNTIME);
            STOP_INTERVAL(w, INTERVAL_IN_SCHEDULER);

            __cilkrts_set_tls_worker(0);

            if (w->self == -1) {
                // This worker is an overflow worker.  I.e., it was created on-
                // demand when the global pool ran out of workers.
                destroy_worker(w);
                __cilkrts_free(w);
            } else {
                // This is a normal user worker and needs to be counted by the
                // global state for the purposes of throttling system workers.
                w->l->type = WORKER_FREE;
                __cilkrts_leave_cilk(g);
            }

            stop_cilkscreen = (0 == g->Q);
        }
    }
    global_os_mutex_unlock();

    /* Turn off Cilkscreen.  This needs to be done when we are NOT holding the
     * os mutex. */
    if (stop_cilkscreen)
        __cilkrts_cilkscreen_disable_instrumentation();
}

/* special return from the initial frame */

void __cilkrts_c_return_from_initial(__cilkrts_worker *w)
{
    struct cilkred_map *rm;

    // When we are returning from the initial frame, switch from
    // INTERVAL_WORKING into INTERVAL_IN_RUNTIME. 
    STOP_INTERVAL(w, INTERVAL_WORKING);
    START_INTERVAL(w, INTERVAL_IN_RUNTIME);

    /* This is only called on a user thread worker. */
    CILK_ASSERT(w->l->type == WORKER_USER);

    #if REDPAR_DEBUG >= 3
    fprintf(stderr, "[W=%d, desc=cilkrts_c_return_from_initial, ff=%p]\n",
            w->self, w->l->frame_ff);
    #endif
    
    BEGIN_WITH_WORKER_LOCK_OPTIONAL(w) {
        full_frame *ff = w->l->frame_ff;
        CILK_ASSERT(ff);
        CILK_ASSERT(ff->join_counter == 1);
        w->l->frame_ff = 0;

        CILK_ASSERT(ff->fiber_self);
        // Save any TBB interop data for the next time this thread enters Cilk
        cilk_fiber_tbb_interop_save_info_from_stack(ff->fiber_self);

        // Deallocate cilk_fiber that mapped to the user stack.  The stack
        // itself does not get deallocated (of course) but our data
        // structure becomes divorced from it.

#if FIBER_DEBUG >= 1
        fprintf(stderr, "ThreadId=%p: w=%d: We are about to deallocate ff->fiber_self  = %p here. w->l->scheduling_fiber = %p. w->l->type = %d\n",
                cilkos_get_current_thread_id(),
                w->self,
                ff->fiber_self,
                w->l->scheduling_fiber,
                w->l->type);
#endif
        // The fiber in ff is a user-code fiber.  The fiber in
        // w->l->scheduling_fiber is a scheduling fiber.  These fibers should
        // never be equal.  When a user worker returns (and will unbind), we
        // should destroy only the fiber in ff.  The scheduling fiber will be
        // re-used.

        CILK_ASSERT(ff->fiber_self != w->l->scheduling_fiber);

        START_INTERVAL(w, INTERVAL_FIBER_DEALLOCATE) {
            // This fiber might not be deallocated here if there
            // is a pending exception on Windows that refers
            // to this fiber.
            //
            // First "suspend" the fiber, and then try to delete it.
            cilk_fiber_deallocate_from_thread(ff->fiber_self);
        } STOP_INTERVAL(w, INTERVAL_FIBER_DEALLOCATE);
        ff->fiber_self = NULL;

        /* Save reducer map into global_state object */
        rm = w->reducer_map;
        w->reducer_map = NULL;

#if REDPAR_DEBUG >= 3
        fprintf(stderr, "W=%d, reducer_map_to_delete=%p, was in ff=%p\n",
                w->self,
                rm,
                ff);
#endif
        __cilkrts_destroy_full_frame(w, ff);


        /* Work is never done. w->g->work_done = 1; __cilkrts_fence(); */
    } END_WITH_WORKER_LOCK_OPTIONAL(w);


    save_pedigree_leaf_from_user_worker(w);

    // Workers can have NULL reducer maps now.
    if (rm) {
        __cilkrts_destroy_reducer_map(w, rm);
    }


#if FIBER_DEBUG >= 1
    __cilkrts_worker* tmp = w;
    int tmp_id = w->self;
    fprintf(stderr, "w=%d: We are about unbind thread (w= %p)\n",
            w->self,
            w);
#endif

    w = NULL;
    
    __cilkrts_unbind_thread();

#if FIBER_DEBUG >= 1
    
    fprintf(stderr, "w=%p, %d: Finished unbind\n",
            tmp, tmp_id);
#endif

    /* Other workers will stop trying to steal if this was the last worker. */

    return;
}


/*
 * __cilkrts_restore_stealing
 *
 * Restore the protected_tail to a previous state, possibly allowing frames
 * to be stolen.  The dekker_protocol has been extended to steal only if
 * head+1 is < protected_tail.
 */

void __cilkrts_restore_stealing(
    __cilkrts_worker *w,
    __cilkrts_stack_frame *volatile *saved_protected_tail)
{
    /* On most x86 this pair of operations would be slightly faster
       as an atomic exchange due to the implicit memory barrier in
       an atomic instruction. */
    w->protected_tail = saved_protected_tail;
    __cilkrts_fence();
}

/*
 * __cilkrts_disallow_stealing
 *
 * Move the protected_tail to NEW_PROTECTED_TAIL, preventing any
 * frames from being stolen.  If NEW_PROTECTED_TAIL is NULL, prevent
 * stealing from the whole queue.  The dekker_protocol has been
 * extended to only steal if head+1 is also < protected_tail.
 */

__cilkrts_stack_frame *volatile *__cilkrts_disallow_stealing(
    __cilkrts_worker *w,
    __cilkrts_stack_frame *volatile *new_protected_tail)
{
    __cilkrts_stack_frame *volatile *saved_protected_tail = w->protected_tail;

    if (!new_protected_tail)
        new_protected_tail = w->l->ltq;

    if (w->protected_tail > new_protected_tail) {
        w->protected_tail = new_protected_tail;
        /* Issue a store-store barrier.  The update to protected_tail
           here must precede the update to tail in the next spawn.
           On x86 this is probably not needed. */
#if defined __GNUC__ && __ICC >= 1200 && !(__MIC__ ||__MIC2__)
        _mm_sfence();
#else
        __cilkrts_fence();
#endif
    }

    return saved_protected_tail;
}

/*************************************************************
  Initialization and startup 
*************************************************************/

__cilkrts_worker *make_worker(global_state_t *g,
                              int self, __cilkrts_worker *w)
{
    w->self = self;
    w->g = g;

    w->pedigree.rank = 0;    // Initial rank is 0
    w->pedigree.parent = NULL;

    w->l = (local_state *)__cilkrts_malloc(sizeof(*w->l));

    __cilkrts_frame_malloc_per_worker_init(w);

    w->reducer_map = NULL;
    w->current_stack_frame = NULL;
    w->reserved = NULL;
    
    w->l->worker_magic_0 = WORKER_MAGIC_0;
    w->l->team = NULL;
    w->l->type = WORKER_FREE;
    
    __cilkrts_mutex_init(&w->l->lock);
    __cilkrts_mutex_init(&w->l->steal_lock);
    w->l->do_not_steal = 0;
    w->l->frame_ff = 0;
    w->l->next_frame_ff = 0;
    w->l->last_full_frame = NULL;

    w->l->ltq = (__cilkrts_stack_frame **)
        __cilkrts_malloc(g->ltqsize * sizeof(*w->l->ltq));
    w->ltq_limit = w->l->ltq + g->ltqsize;
    w->head = w->tail = w->l->ltq;
    
    cilk_fiber_pool_init(&w->l->fiber_pool,
                         &g->fiber_pool,
                         g->stack_size,
                         g->fiber_pool_size,
                         0,   // alloc_max is 0.  We don't allocate from the heap directly without checking the parent pool.
                         0);
#if FIBER_DEBUG >= 2
    fprintf(stderr, "ThreadId=%p: Making w=%d (%p), pool = %p\n",
            cilkos_get_current_thread_id(),
            w->self, w, 
            &w->l->fiber_pool);
#endif
    w->l->scheduling_fiber = NULL;
    w->l->original_pedigree_leaf = NULL;
    w->l->rand_seed = 0; /* the scheduler will overwrite this field */

    w->l->post_suspend = 0;
    w->l->suspended_stack = 0;
    w->l->fiber_to_free = NULL;
    w->l->pending_exception = NULL;

#if CILK_PROFILE
    w->l->stats = __cilkrts_malloc(sizeof(statistics));
    __cilkrts_init_stats(w->l->stats);
#else
    w->l->stats = NULL;
#endif    
    w->l->steal_failure_count = 0;
    w->l->has_stolen = 0;

    w->l->work_stolen = 0;

    // Initialize record/replay assuming we're doing neither
    w->l->record_replay_fptr = NULL;
    w->l->replay_list_root = NULL;
    w->l->replay_list_entry = NULL;
    w->l->signal_node = NULL;
    // Nothing's been stolen yet
    w->l->worker_magic_1 = WORKER_MAGIC_1;

    /*w->parallelism_disabled = 0;*/

    // Allow stealing all frames. Sets w->saved_protected_tail
    __cilkrts_restore_stealing(w, w->ltq_limit);
    
    __cilkrts_init_worker_sysdep(w);

    reset_THE_exception(w); 

    return w;
}

void destroy_worker(__cilkrts_worker *w)
{
    CILK_ASSERT (NULL == w->l->pending_exception);

    // Deallocate the scheduling fiber
    if (NULL != w->l->scheduling_fiber)
    {
        // The scheduling fiber is the main fiber for system workers and must
        // be deallocated by the thread that created it.  Thus, we can
        // deallocate only free workers' (formerly user workers) scheduling
        // fibers here. 
        CILK_ASSERT(WORKER_FREE == w->l->type);

#if FIBER_DEBUG >=1
        fprintf(stderr, "ThreadId=%p, w=%p, %d, deallocating scheduling fiber = %p, \n",
                cilkos_get_current_thread_id(),
                w,
                w->self,
                w->l->scheduling_fiber);
#endif
        int ref_count = cilk_fiber_remove_reference(w->l->scheduling_fiber, NULL);
        // Scheduling fiber should never have extra references because of exceptions.
        CILK_ASSERT(0 == ref_count);
        w->l->scheduling_fiber = NULL;
    }

#if CILK_PROFILE
    if (w->l->stats) {
        __cilkrts_free(w->l->stats);
    }
#else
    CILK_ASSERT(NULL == w->l->stats);
#endif
    
    /* Free any cached fibers. */
    cilk_fiber_pool_destroy(&w->l->fiber_pool);

    __cilkrts_destroy_worker_sysdep(w);

    if (w->l->signal_node) {
        CILK_ASSERT(WORKER_SYSTEM == w->l->type);
        signal_node_destroy(w->l->signal_node);
    }

    __cilkrts_free(w->l->ltq);
    __cilkrts_mutex_destroy(0, &w->l->lock);
    __cilkrts_mutex_destroy(0, &w->l->steal_lock);
    __cilkrts_frame_malloc_per_worker_cleanup(w);

    __cilkrts_free(w->l);

    // The caller is responsible for freeing the worker memory
}

/*
 * Make a worker into a system worker.
 */
static void make_worker_system(__cilkrts_worker *w) {
    CILK_ASSERT(WORKER_FREE == w->l->type);
    w->l->type = WORKER_SYSTEM;
    w->l->signal_node = signal_node_create();
}

void __cilkrts_deinit_internal(global_state_t *g)
{
    int i;
    __cilkrts_worker *w;

    // If there's no global state then we're done
    if (NULL == g)
        return;

#ifdef CILK_PROFILE
    __cilkrts_dump_stats_to_stderr(g);
#endif

    w = g->workers[0];
    if (w->l->frame_ff) {
        __cilkrts_destroy_full_frame(w, w->l->frame_ff);
        w->l->frame_ff = 0;
    }

    // Release any resources used for record/replay
    replay_term(g);

    // Destroy any system dependent global state
    __cilkrts_destroy_global_sysdep(g);

    for (i = 0; i < g->total_workers; ++i)
        destroy_worker(g->workers[i]);

    // Free memory for all worker blocks which were allocated contiguously
    __cilkrts_free(g->workers[0]);

    __cilkrts_free(g->workers);

    cilk_fiber_pool_destroy(&g->fiber_pool);
    __cilkrts_frame_malloc_global_cleanup(g);

    cilkg_deinit_global_state();
}

/*
 * Wake the runtime by notifying the system workers that they can steal.  The
 * first user worker into the runtime should call this.
 */
static void wake_runtime(global_state_t *g)
{
    __cilkrts_worker *root;
    if (g->P > 1) {
        // Send a message to the root node.  The message will propagate.
        root = g->workers[0];
        CILK_ASSERT(root->l->signal_node);
        signal_node_msg(root->l->signal_node, 1);
    }
}

/*
 * Put the runtime to sleep.  The last user worker out of the runtime should
 * call this.  Like Dad always said, turn out the lights when nobody's in the
 * room.
 */
static void sleep_runtime(global_state_t *g)
{
    __cilkrts_worker *root;
    if (g->P > 1) {
        // Send a message to the root node.  The message will propagate.
        root = g->workers[0];
        CILK_ASSERT(root->l->signal_node);
        signal_node_msg(root->l->signal_node, 0);
    }
}

/* Called when a user thread joins Cilk.
   Global lock must be held. */
void __cilkrts_enter_cilk(global_state_t *g)
{
    if (g->Q++ == 0) {
        // If this is the first user thread to enter Cilk wake
        // up all the workers.
        wake_runtime(g);
    }
}

/* Called when a user thread leaves Cilk.
   Global lock must be held. */
void __cilkrts_leave_cilk(global_state_t *g)
{
    if (--g->Q == 0) {
        // Put the runtime to sleep.
        sleep_runtime(g);
    }
}

/*
 * worker_runnable
 *
 * Return true if the worker should continue to try to steal.  False, otherwise.
 */

NOINLINE
static enum schedule_t worker_runnable(__cilkrts_worker *w)
{
    global_state_t *g = w->g;

    /* If this worker has something to do, do it.
       Otherwise the work would be lost. */
    if (w->l->next_frame_ff)
        return SCHEDULE_RUN;

    // If Cilk has explicitly (by the user) been told to exit (i.e., by
    // __cilkrts_end_cilk() -> __cilkrts_stop_workers(g)), then return 0.
    if (g->work_done)
        return SCHEDULE_EXIT;

    if (0 == w->self) {
        // This worker is the root node and is the only one that may query the
        // global state to see if there are still any user workers in Cilk.
        if (w->l->steal_failure_count > g->max_steal_failures) {
            if (signal_node_should_wait(w->l->signal_node)) {
                return SCHEDULE_WAIT;
            } else {
                // Reset the steal_failure_count since we have verified that
                // user workers are still in Cilk.
                w->l->steal_failure_count = 0;
            }
        }
    } else if (WORKER_SYSTEM == w->l->type &&
               signal_node_should_wait(w->l->signal_node)) {
        // This worker has been notified by its parent that it should stop
        // trying to steal.
        return SCHEDULE_WAIT;
    }

    return SCHEDULE_RUN;
}



// Initialize the worker structs, but don't start the workers themselves.
static void init_workers(global_state_t *g)
{
    int total_workers = g->total_workers;
    int i;
    struct CILK_ALIGNAS(256) buffered_worker {
        __cilkrts_worker w;
        char buf[64];
    } *workers_memory;

    /* not needed if only one worker */
    cilk_fiber_pool_init(&g->fiber_pool,
                         NULL,
                         g->stack_size,
                         g->global_fiber_pool_size,           // buffer_size
                         g->max_stacks,                       // maximum # to allocate
                         1);

    cilk_fiber_pool_set_fiber_limit(&g->fiber_pool,
                                    (g->max_stacks ? g->max_stacks : INT_MAX));

    g->workers = (__cilkrts_worker **)
        __cilkrts_malloc(total_workers * sizeof(*g->workers));

    // Allocate 1 block of memory for workers to make life easier for tools
    // like Inspector which run multithreaded and need to know the memory
    // range for all the workers that will be accessed in a user's program
    workers_memory = (struct buffered_worker*)
        __cilkrts_malloc(sizeof(*workers_memory) * total_workers);    
    
    // Notify any tools that care (Cilkscreen and Inspector) that they should
    // ignore memory allocated for the workers
    __cilkrts_cilkscreen_ignore_block(&workers_memory[0],
                                      &workers_memory[total_workers]);

    // Initialize worker structs, including unused worker slots.
    for (i = 0; i < total_workers; ++i) {
        g->workers[i] = make_worker(g, i, &workers_memory[i].w);
    }

    // Set the workers in the first P - 1 slots to be system workers.
    // Remaining worker structs already have type == 0.
    for (i = 0; i < g->system_workers; ++i) {
        make_worker_system(g->workers[i]);
    }
}

void __cilkrts_init_internal(int start)
{
    global_state_t *g = NULL;

    if (cilkg_is_published()) {
        g = cilkg_init_global_state();
    }
    else {

        // We think the state has not been published yet.
        // Grab the lock and try to initialize/publish.
        global_os_mutex_lock();

        if (cilkg_is_published()) {
            // Some other thread must have snuck in and published.
            g = cilkg_init_global_state();
        }
        else {
            // Initialize and retrieve global state
            g = cilkg_init_global_state();

            // Set the scheduler pointer
            g->scheduler = worker_scheduler_function;

            // If we're running under a sequential P-Tool (Cilkscreen or
            // Cilkview) then there's only one worker and we need to tell
            // the tool about the extent of the stack
            if (g->under_ptool)
                __cilkrts_establish_c_stack();     
            init_workers(g);

            // Initialize per-work record/replay logging
            replay_init_workers(g);

            // Initialize any system dependent global state
            __cilkrts_init_global_sysdep(g);


            cilkg_publish_global_state(g);
        }

        global_os_mutex_unlock();
    }

    CILK_ASSERT(g);

    if (start && !g->workers_running)
    {
        // Acquire the global OS mutex while we're starting the workers
        global_os_mutex_lock();
        if (!g->workers_running)
            // Start P - 1 system workers since P includes the first user
            // worker.
            __cilkrts_start_workers(g, g->P - 1);
        global_os_mutex_unlock();
    }
}


/************************************************************************
  Methods for reducer protocol.

  Reductions occur in two places:
    A. A full frame "ff" is returning from a spawn with a stolen parent.
    B. A full frame "ff" is stalling at a sync.

  To support parallel reductions, reduction functions need to be
  executed while control is on a user stack, before jumping into the
  runtime.  These reductions can not occur while holding a worker or
  frame lock.

  Before a worker w executes a reduction in either Case A or B, w's
  deque is empty.

  Since parallel reductions push work onto the deque, we must do extra
  work to set up runtime data structures properly before reductions
  begin to allow stealing.  ( Normally, when we have only serial
  reductions, once a worker w starts a reduction, its deque remains
  empty until w either steals another frame or resumes a suspended
  frame.  Thus, we don't care about the state of the deque, since w
  will reset its deque when setting up execution of a frame. )

  To allow for parallel reductions, we coerce the runtime data
  structures so that, from their perspective, it looks as though we
  have spliced in an "execute_reductions()" function.  Consider the
  two cases for reductions:

    Case A: Return from a spawn with a stolen parent.
      Consider a spawned function g is returning on a worker w.
      Assume:
          -   g was spawned from a parent function f.  
          -   ff is the full frame for g's spawn helper
          -   sf be the __cilkrts_stack_frame for g's spawn helper.

      We are conceptually splicing "execute_reductions()" so that it
      occurs immediately before the spawn helper of g returns to f.

      We do so by creating two different world views --- one for the
      runtime data structures, and one for the actual control flow.

        - Before reductions begin, the runtime data structures should
          look as though the spawn helper of g is calling
          "execute_reductions()", in terms of both the user stack and
          worker deque.  More precisely, w should satisfy the
          following properties:

              (a) w has ff as its full frame,
              (b) w has sf as its __cilkrts_stack_frame, and
              (c) w has an empty deque. 

          If the runtime satisfies these properties, then if w
          encounters a spawn in a parallel reduction, it can push onto
          a valid deque.  Also, when a steal from w occurs, it will
          build the correct tree of full frames when w is stolen from.

        - In actual control flow, however, once the
          "execute_reductions()" function returns, it is actually
          returning to runtime code instead of g's spawn helper. 

          At the point a worker w began executing reductions, the
          control flow / compiled code had already finished g's spawn
          helper, and w was about to enter the runtime.  With parallel
          reductions, some worker v (which might be different from w)
          is the one returning to the runtime.


      The reduction logic consists of 4 steps:

       A1. Restore runtime data structures to make it look as though
           the spawn helper of g() is still the currently executing
           frame for w.

       A2. Execute reductions on the user stack.  Reductions also
           includes the logic for exceptions and stacks.  Note that
           reductions start on w, but may finish on a different
           worker if there is parallelism in the reduce.

       A3. Splice out ff from the tree of full frames.

       A4. Jump into the runtime/scheduling stack and execute
           "do_return_from_spawn".  This method

           (a) Frees the user stack we were just on if it is no longer needed.
           (b) Decrement the join counter on ff->parent, and tries to do a
               provably good steal.
           (c) Clean up the full frame ff. 


   Case B: Stalling at a sync.

     Consider a function g(), with full frame ff and
     __cilkrts_stack_frame sf.  Suppose g() stalls at a sync, and we
     are executing reductions.

     Conceptually, we are splicing in an "execute_reductions()"
     function into g() as the last action that g() takes immediately
     before it executes the cilk_sync.

     The reduction logic for this case is similar to Case A.

       B1. Restore the runtime data structures. 

           The main difference from Case A is that ff/sf is still a
           frame that needs to be executed later (since it is stalling
           at a cilk_sync).  Thus, we also need to save the current
           stack information into "ff" so that we can correctly resume
           execution of "ff" after the sync.

       B2. Execute reductions on the user stack.

       B3. No frame to splice out of the tree.

       B4. Jump into the runtime/scheduling stack and execute "do_sync".
           This method:
           (a) Frees the user stack we were just on if it is no longer needed.
           (b) Tries to execute a provably good steal.

  Finally, for the reducer protocol, we consider two reduction paths,
  namely a "fast" and "slow" path.  On a fast path, only trivial
  merges of reducer maps happen (i.e., one or both of the maps are
  NULL).  Otherwise, on the slow path, a reduction actually needs to
  happen.

*****************************************************************/

/**
 * @brief Locations to store the result of a reduction.
 *
 * Struct storing pointers to the fields in our "left" sibling that we
 * should update when splicing out a full frame or stalling at a sync.
 */
typedef struct {
    /** A pointer to the location of our left reducer map. */
    struct cilkred_map **map_ptr;

    /** A pointer to the location of our left exception. */
    struct pending_exception_info **exception_ptr;
} splice_left_ptrs;

/**
 * For a full frame returning from a spawn, calculate the pointers to
 * the maps and exceptions to my left.
 *
 * @param w   The currently executing worker.
 * @param ff  Full frame that is dying
 * @return    Pointers to our "left" for reducers and exceptions.
 */
static inline
splice_left_ptrs compute_left_ptrs_for_spawn_return(__cilkrts_worker *w,
                                                    full_frame *ff)
{
    // ASSERT: we hold the lock on ff->parent

    splice_left_ptrs left_ptrs;
    if (ff->left_sibling) {
        left_ptrs.map_ptr = &ff->left_sibling->right_reducer_map;
        left_ptrs.exception_ptr = &ff->left_sibling->right_pending_exception;
    }
    else {
        full_frame *parent_ff = ff->parent;
        left_ptrs.map_ptr = &parent_ff->children_reducer_map;
        left_ptrs.exception_ptr = &parent_ff->child_pending_exception;
    }
    return left_ptrs;
}

/**
 * For a full frame at a sync, calculate the pointers to the maps and
 * exceptions to my left.
 *
 * @param w   The currently executing worker.
 * @param ff  Full frame that is stalling at a sync.
 * @return    Pointers to our "left" for reducers and exceptions.
 */
static inline
splice_left_ptrs compute_left_ptrs_for_sync(__cilkrts_worker *w,
                                            full_frame *ff)
{
    // ASSERT: we hold the lock on ff
    splice_left_ptrs left_ptrs;

    // Figure out which map to the left we should merge into.
    if (ff->rightmost_child) {
        CILK_ASSERT(ff->rightmost_child->parent == ff);
        left_ptrs.map_ptr = &(ff->rightmost_child->right_reducer_map);
        left_ptrs.exception_ptr = &(ff->rightmost_child->right_pending_exception);
    }
    else {
        // We have no children.  Then, we should be the last
        // worker at the sync... "left" is our child map.
        left_ptrs.map_ptr = &(ff->children_reducer_map);
        left_ptrs.exception_ptr = &(ff->child_pending_exception);
    }
    return left_ptrs;
}

/**
 * After we have completed all reductions on a spawn return, call this
 * method to finish up before jumping into the runtime.
 *
 *   1. Perform the "reduction" on stacks, i.e., execute the left
 *      holder logic to pass the leftmost stack up.
 *
 *      w->l->fiber_to_free holds any stack that needs to be freed
 *      when control switches into the runtime fiber.
 * 
 *   2. Unlink and remove child_ff from the tree of full frames.
 *
 * @param   w          The currently executing worker.
 * @param   parent_ff  The parent of child_ff.
 * @param   child_ff   The full frame returning from a spawn.
 */
static inline
void finish_spawn_return_on_user_stack(__cilkrts_worker *w,
                                       full_frame *parent_ff,
                                       full_frame *child_ff)
{
    CILK_ASSERT(w->l->fiber_to_free == NULL);

    // Execute left-holder logic for stacks.
    if (child_ff->left_sibling || parent_ff->fiber_child) {
        // Case where we are not the leftmost stack.
        CILK_ASSERT(parent_ff->fiber_child != child_ff->fiber_self);

        // Remember any fiber we need to free in the worker.
        // After we jump into the runtime, we will actually do the
        // free.
        w->l->fiber_to_free = child_ff->fiber_self;
    }
    else {
        // We are leftmost, pass stack/fiber up to parent.
        // Thus, no stack/fiber to free.
        parent_ff->fiber_child = child_ff->fiber_self;
        w->l->fiber_to_free = NULL;
    }

    child_ff->fiber_self = NULL;

    unlink_child(parent_ff, child_ff);
}


/**
 * Executes any fast reductions necessary to splice ff out of the tree
 * of full frames.
 *
 * This "fast" path performs only trivial merges of reducer maps,
 * i.e,. when one of them is NULL.
 * (See slow_path_reductions_for_spawn_return() for slow path.)
 *
 * Returns: 1 if we finished all reductions.
 * Returns: 0 if there are still reductions to execute, and
 *            we should execute the slow path.
 *
 * This method assumes w holds the frame lock on parent_ff.
 * After this method completes:
 *    1. We have spliced ff out of the tree of full frames.
 *    2. The reducer maps of child_ff have been deposited
 *       "left" according to the reducer protocol.
 *    3. w->l->stack_to_free stores the stack
 *       that needs to be freed once we jump into the runtime.
 *
 * We have not, however, decremented the join counter on ff->parent.
 * This prevents any other workers from resuming execution of the parent.
 *
 * @param   w    The currently executing worker.
 * @param   ff   The full frame returning from a spawn.
 * @return  NULL if we finished all reductions.
 * @return  The address where the left map is stored (which should be passed to 
 *          slow_path_reductions_for_spawn_return()) if there are
 *          still reductions to execute. 
 */
struct cilkred_map**
fast_path_reductions_for_spawn_return(__cilkrts_worker *w,
                                      full_frame *ff)
{
    // ASSERT: we hold ff->parent->lock.
    splice_left_ptrs left_ptrs;

    CILK_ASSERT(NULL == w->l->pending_exception);

    // Figure out the pointers to the left where I want
    // to put reducers and exceptions.
    left_ptrs = compute_left_ptrs_for_spawn_return(w, ff);
    
    // Go ahead and merge exceptions while holding the lock.
    splice_exceptions_for_spawn(w, ff, left_ptrs.exception_ptr);

    // Now check if we have any reductions to perform.
    //
    // Consider all the cases of left, middle and right maps.
    //  0. (-, -, -)  :  finish and return 1
    //  1. (L, -, -)  :  finish and return 1
    //  2. (-, M, -)  :  slide over to left, finish, and return 1.
    //  3. (L, M, -)  :  return 0
    //  4. (-, -, R)  :  slide over to left, finish, and return 1.
    //  5. (L, -, R)  :  return 0
    //  6. (-, M, R)  :  return 0
    //  7. (L, M, R)  :  return 0
    //
    // In terms of code:
    //  L == *left_ptrs.map_ptr
    //  M == w->reducer_map
    //  R == f->right_reducer_map.
    //
    // The goal of the code below is to execute the fast path with
    // as few branches and writes as possible.
    
    int case_value = (*(left_ptrs.map_ptr) != NULL);
    case_value += ((w->reducer_map != NULL) << 1);
    case_value += ((ff->right_reducer_map != NULL) << 2);

    // Fastest path is case_value == 0 or 1.
    if (case_value >=2) {
        switch (case_value) {
        case 2:
            *(left_ptrs.map_ptr) = w->reducer_map;
            w->reducer_map = NULL;
            return NULL;
            break;
        case 4:
            *(left_ptrs.map_ptr) = ff->right_reducer_map;
            ff->right_reducer_map = NULL;
            return NULL;
        default:
            // If we have to execute the slow path, then
            // return the pointer to the place to deposit the left
            // map.
            return left_ptrs.map_ptr;
        }
    }

    // Do nothing
    return NULL;
}


/**
 * Executes any reductions necessary to splice "ff" frame out of
 * the steal tree.
 *
 * This method executes the "slow" path for reductions on a spawn
 * return, i.e., there are non-NULL maps that need to be merged
 * together.
 *
 * This method should execute only if
 * fast_path_reductions_for_spawn_return() returns a non-NULL
 * left_map_ptr.
 *
 * Upon entry, left_map_ptr should be the location of the left map
 * at the start of the reduction, as calculated by
 * fast_path_reductions_for_spawn_return().
 *
 * After this method completes:
 *    1. We have spliced ff out of the tree of full frames.
 *    2. The reducer maps of child_ff have been deposited
 *       "left" according to the reducer protocol.
 *    3. w->l->stack_to_free stores the stack
 *       that needs to be freed once we jump into the runtime.
 * We have not, however, decremented the join counter on ff->parent,
 * so no one can resume execution of the parent yet.
 *
 * WARNING: 
 *   This method assumes the lock on ff->parent is held upon entry, and
 *   Upon exit, the worker that returns still holds a lock on ff->parent
 *   This method can, however, release and reacquire the lock on ff->parent.
 *
 * @param w             The currently executing worker.
 * @param ff            The full frame returning from a spawn.
 * @param left_map_ptr  Pointer to our initial left map.
 * @return              The worker that this method returns on. 
 */ 
static __cilkrts_worker*
slow_path_reductions_for_spawn_return(__cilkrts_worker *w,
                                      full_frame *ff,
                                      struct cilkred_map **left_map_ptr)
{

    // CILK_ASSERT: w is holding frame lock on parent_ff.
#if REDPAR_DEBUG > 0
    CILK_ASSERT(!ff->rightmost_child);
    CILK_ASSERT(!ff->is_call_child);
#endif

    // Loop invariant:
    // When beginning this loop, we should
    //   1. Be holding the lock on ff->parent.
    //   2. left_map_ptr should be the address of the pointer to the left map.
    //   3. All maps should be slid over left by one, if possible.
    //   4. All exceptions should be merged so far.
    while (1) {
        
        // Slide middle map left if possible.
        if (!(*left_map_ptr)) {
            *left_map_ptr = w->reducer_map;
            w->reducer_map = NULL;
        }
        // Slide right map to middle if possible.
        if (!w->reducer_map) {
            w->reducer_map = ff->right_reducer_map;
            ff->right_reducer_map = NULL;
        }

        // Since we slid everything left by one,
        // we are finished if there is no middle map.
        if (!w->reducer_map) {
            verify_current_wkr(w);
            return w;
        }
        else {
            struct cilkred_map* left_map;
            struct cilkred_map* middle_map;
            struct cilkred_map* right_map;

            // Take all the maps from their respective locations.
            // We can't leave them in place and execute a reduction because these fields
            // might change once we release the lock.
            left_map = *left_map_ptr;
            *left_map_ptr = NULL;
            middle_map = w->reducer_map;
            w->reducer_map = NULL;
            right_map = ff->right_reducer_map;
            ff->right_reducer_map = NULL;
        
            // WARNING!!! Lock release here.
            // We have reductions to execute (and we can't hold locks).
            __cilkrts_frame_unlock(w, ff->parent);

            // After we've released the lock, start counting time as
            // WORKING again.
            STOP_INTERVAL(w, INTERVAL_IN_RUNTIME);
            START_INTERVAL(w, INTERVAL_WORKING);

            // Merge all reducers into the left map.
            left_map = repeated_merge_reducer_maps(&w,
                                                   left_map,
                                                   middle_map);
            verify_current_wkr(w);
            left_map = repeated_merge_reducer_maps(&w,
                                                   left_map,
                                                   right_map);
            verify_current_wkr(w);
            CILK_ASSERT(NULL == w->reducer_map);
            // Put the final answer back into w->reducer_map.
            w->reducer_map = left_map;
            
            // Save any exceptions generated because of the reduction
            // process from the returning worker.  These get merged
            // the next time around the loop.
            CILK_ASSERT(NULL == ff->pending_exception);
            ff->pending_exception = w->l->pending_exception;
            w->l->pending_exception = NULL;

            STOP_INTERVAL(w, INTERVAL_WORKING);
            START_INTERVAL(w, INTERVAL_IN_RUNTIME);

            // Lock ff->parent for the next loop around.
            __cilkrts_frame_lock(w, ff->parent);

            // Once we have the lock again, recompute who is to our
            // left.
            splice_left_ptrs left_ptrs;
            left_ptrs = compute_left_ptrs_for_spawn_return(w, ff);

            // Update the pointer for the left map.
            left_map_ptr = left_ptrs.map_ptr;
            // Splice the exceptions for spawn.
            splice_exceptions_for_spawn(w, ff, left_ptrs.exception_ptr);
        }
    }
    // We should never break out of this loop.
    
    CILK_ASSERT(0);
    return NULL;
}



/**
 * Execute reductions when returning from a spawn whose parent has
 * been stolen.
 *
 * Execution may start on w, but may finish on a different worker.
 * This method acquires/releases the lock on ff->parent. 
 *
 * @param w            The currently executing worker.
 * @param ff           The full frame of the spawned function that is returning.
 * @param returning_sf The __cilkrts_stack_frame for this returning function.
 * @return             The worker returning from this method. 
 */ 
static __cilkrts_worker*
execute_reductions_for_spawn_return(__cilkrts_worker *w,
                                    full_frame *ff,
                                    __cilkrts_stack_frame *returning_sf)
{ 
    // Step A1 from reducer protocol described above.
    //
    // Coerce the runtime into thinking that 
    // ff/returning_sf are still on the bottom of
    // w's deque.
    restore_frame_for_spawn_return_reduction(w, ff, returning_sf);

    // Step A2 and A3: Execute reductions on user stack.
    BEGIN_WITH_FRAME_LOCK(w, ff->parent) {
        struct cilkred_map **left_map_ptr;
        left_map_ptr = fast_path_reductions_for_spawn_return(w, ff);

        // Pointer will be non-NULL if there are
        // still reductions to execute.
        if (left_map_ptr) {
            // WARNING: This method call may release the lock
            // on ff->parent and re-acquire it (possibly on a
            // different worker).
            // We can't hold locks while actually executing
            // reduce functions.
            w = slow_path_reductions_for_spawn_return(w,
                                                      ff,
                                                      left_map_ptr);
            verify_current_wkr(w);
        }

        finish_spawn_return_on_user_stack(w, ff->parent, ff);      
        // WARNING: the use of this lock macro is deceptive.
        // The worker may have changed here.
    } END_WITH_FRAME_LOCK(w, ff->parent);
    return w;
}



/**
 * Execute fast "reductions" when ff stalls at a sync.
 *
 * @param   w  The currently executing worker.
 * @param   ff The full frame stalling at a sync.
 * @return  1 if we are finished with all reductions after calling this method.
 * @return  0 if we still need to execute the slow path reductions.
 */ 
static inline
int fast_path_reductions_for_sync(__cilkrts_worker *w,
                                  full_frame *ff) {
    // Return 0 if there is some reduction that needs to happen.
    return !(w->reducer_map  || ff->pending_exception);
}

/**
 * Executes slow reductions when ff stalls at a sync.
 * This method should execute only if
 *   fast_path_reductions_for_sync(w, ff) returned 0.
 *
 * After this method completes:
 *   1. ff's current reducer map has been deposited into
 *       right_reducer_map of ff's rightmost child, or
 *       ff->children_reducer_map if ff has no children.
 *   2. Similarly for ff's current exception.
 *   3. Nothing to calculate for stacks --- if we are stalling
 *      we will always free a stack.
 *
 * This method may repeatedly acquire/release the lock on ff.
 *
 * @param   w  The currently executing worker.
 * @param   ff The full frame stalling at a sync.
 * @return  The worker returning from this method.
 */
static __cilkrts_worker*
slow_path_reductions_for_sync(__cilkrts_worker *w,
                              full_frame *ff)
{
    struct cilkred_map *left_map;
    struct cilkred_map *middle_map;
    
#if (REDPAR_DEBUG > 0)
    CILK_ASSERT(ff);
    CILK_ASSERT(w->head == w->tail);
#endif

    middle_map = w->reducer_map;
    w->reducer_map = NULL;

    // Loop invariant: middle_map should be valid (the current map to reduce). 
    //                 left_map is junk.
    //                 w->reducer_map == NULL.
    while (1) {
        BEGIN_WITH_FRAME_LOCK(w, ff) {
            splice_left_ptrs left_ptrs = compute_left_ptrs_for_sync(w, ff);
            
            // Grab the "left" map and store pointers to those locations.
            left_map = *(left_ptrs.map_ptr);
            *(left_ptrs.map_ptr) = NULL;
            
            // Slide the maps in our struct left as far as possible.
            if (!left_map) {
                left_map = middle_map;
                middle_map = NULL;
            }

            *(left_ptrs.exception_ptr) =
                __cilkrts_merge_pending_exceptions(w,
                                                   *left_ptrs.exception_ptr,
                                                   ff->pending_exception);
            ff->pending_exception = NULL;

            // If there is no middle map, then we are done.
            // Deposit left and return.
            if (!middle_map) {
                *(left_ptrs).map_ptr = left_map;
                #if (REDPAR_DEBUG > 0)
                CILK_ASSERT(NULL == w->reducer_map);
                #endif
                // Sanity check upon leaving the loop.
                verify_current_wkr(w);
                // Make sure to unlock before we return!
                __cilkrts_frame_unlock(w, ff);
                return w;
            }
        } END_WITH_FRAME_LOCK(w, ff);

        // After we've released the lock, start counting time as
        // WORKING again.
        STOP_INTERVAL(w, INTERVAL_IN_RUNTIME);
        START_INTERVAL(w, INTERVAL_WORKING);
        
        // If we get here, we have a nontrivial reduction to execute.
        middle_map = repeated_merge_reducer_maps(&w,
                                                 left_map,
                                                 middle_map);
        verify_current_wkr(w);

        STOP_INTERVAL(w, INTERVAL_WORKING);
        START_INTERVAL(w, INTERVAL_IN_RUNTIME);

        // Save any exceptions generated because of the reduction
        // process.  These get merged the next time around the
        // loop.
        CILK_ASSERT(NULL == ff->pending_exception);
        ff->pending_exception = w->l->pending_exception;
        w->l->pending_exception = NULL;
    }
    
    // We should never break out of the loop above.
    CILK_ASSERT(0);
    return NULL;
}


/**
 * Execute reductions when ff stalls at a sync.
 *
 * Execution starts on w, but may finish on a different worker.
 * This method may acquire/release the lock on ff.
 *
 * @param w          The currently executing worker.
 * @param ff         The full frame of the spawned function at the sync
 * @param sf_at_sync The __cilkrts_stack_frame stalling at a sync
 * @return           The worker returning from this method.
 */ 
static __cilkrts_worker*
execute_reductions_for_sync(__cilkrts_worker *w,
                            full_frame *ff,
                            __cilkrts_stack_frame *sf_at_sync)
{
    int finished_reductions;
    // Step B1 from reducer protocol above:
    // Restore runtime invariants.
    //
    // The following code for this step is almost equivalent to
    // the following sequence:
    //   1. disown(w, ff, sf_at_sync, "sync") (which itself
    //        calls make_unrunnable(w, ff, sf_at_sync))
    //   2. make_runnable(w, ff, sf_at_sync).
    //
    // The "disown" will mark the frame "sf_at_sync"
    // as stolen and suspended, and save its place on the stack,
    // so it can be resumed after the sync. 
    //
    // The difference is, that we don't want the disown to 
    // break the following connections yet, since we are
    // about to immediately make sf/ff runnable again anyway.
    //   sf_at_sync->worker == w
    //   w->l->frame_ff == ff.
    //
    // These connections are needed for parallel reductions, since
    // we will use sf / ff as the stack frame / full frame for
    // executing any potential reductions.
    //
    // TBD: Can we refactor the disown / make_unrunnable code
    // to avoid the code duplication here?

    ff->call_stack = NULL;

    // Normally, "make_unrunnable" would add CILK_FRAME_STOLEN and
    // CILK_FRAME_SUSPENDED to sf_at_sync->flags and save the state of
    // the stack so that a worker can resume the frame in the correct
    // place.
    //
    // But on this path, CILK_FRAME_STOLEN should already be set.
    // Also, we technically don't want to suspend the frame until
    // the reduction finishes.
    // We do, however, need to save the stack before
    // we start any reductions, since the reductions might push more
    // data onto the stack.
    CILK_ASSERT(sf_at_sync->flags | CILK_FRAME_STOLEN);

    __cilkrts_put_stack(ff, sf_at_sync);
    __cilkrts_make_unrunnable_sysdep(w, ff, sf_at_sync, 1,
                                     "execute_reductions_for_sync");
    CILK_ASSERT(w->l->frame_ff == ff);

    // Step B2: Execute reductions on user stack.
    // Check if we have any "real" reductions to do.
    finished_reductions = fast_path_reductions_for_sync(w, ff);
    
    if (!finished_reductions) {
        // Still have some real reductions to execute.
        // Run them here.

        // This method may acquire/release the lock on ff.
        w = slow_path_reductions_for_sync(w, ff);

        // The previous call may return on a different worker.
        // than what we started on.
        verify_current_wkr(w);
    }

#if REDPAR_DEBUG >= 0
    CILK_ASSERT(w->l->frame_ff == ff);
    CILK_ASSERT(ff->call_stack == NULL);
#endif

    // Now we suspend the frame ff (since we've
    // finished the reductions).  Roughly, we've split apart the 
    // "make_unrunnable" call here --- we've already saved the
    // stack info earlier before the reductions execute.
    // All that remains is to restore the call stack back into the
    // full frame, and mark the frame as suspended.
    ff->call_stack = sf_at_sync;
    sf_at_sync->flags |= CILK_FRAME_SUSPENDED;

    // At a nontrivial sync, we should always free the current fiber,
    // because it can not be leftmost.
    w->l->fiber_to_free = ff->fiber_self;
    ff->fiber_self = NULL;
    return w;
}


/*
  Local Variables: **
  c-file-style:"bsd" **
  c-basic-offset:4 **
  indent-tabs-mode:nil **
  End: **
*/