guile/libguile/weak-set.c

/* Copyright 2011-2013,2018
     Free Software Foundation, Inc.

   This file is part of Guile.

   Guile is free software: you can redistribute it and/or modify it
   under the terms of the GNU Lesser General Public License as published
   by the Free Software Foundation, either version 3 of the License, or
   (at your option) any later version.

   Guile is distributed in the hope that it will be useful, but WITHOUT
   ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
   FITNESS FOR A PARTICULAR PURPOSE.  See the GNU Lesser General Public
   License for more details.

   You should have received a copy of the GNU Lesser General Public
   License along with Guile.  If not, see
   <https://www.gnu.org/licenses/>.  */




#ifdef HAVE_CONFIG_H
# include <config.h>
#endif

#include <assert.h>
#include <string.h>

#include "bdw-gc.h"
#include "eval.h"
#include "finalizers.h"
#include "hash.h"
#include "pairs.h"
#include "ports.h"
#include "threads.h"
#include "weak-set.h"

#include "weak-list.h"


/* Weak Sets

   This file implements weak sets.  One example of a weak set is the
   symbol table, where you want all instances of the `foo' symbol to map
   to one object.  So when you load a file and it wants a symbol with
   the characters "foo", you one up in the table, using custom hash and
   equality predicates.  Only if one is not found will you bother to
   cons one up and intern it.

   Another use case for weak sets is the set of open ports.  Guile needs
   to be able to flush them all when the process exits, but the set
   shouldn't prevent the GC from collecting the port (and thus closing
   it).

   Weak sets are implemented using an open-addressed hash table.
   Basically this means that there is an array of entries, and the item
   is expected to be found the slot corresponding to its hash code,
   modulo the length of the array.

   Collisions are handled using linear probing with the Robin Hood
   technique.  See Pedro Celis' paper, "Robin Hood Hashing":

     http://www.cs.uwaterloo.ca/research/tr/1986/CS-86-14.pdf

   The vector of entries is allocated as an "atomic" piece of memory, so
   that the GC doesn't trace it.  When an item is added to the set, a
   disappearing link is registered to its location.  If the item is
   collected, then that link will be zeroed out.

   An entry is not just an item, though; the hash code is also stored in
   the entry.  We munge hash codes so that they are never 0.  In this
   way we can detect removed entries (key of zero but nonzero hash
   code), and can then reshuffle elements as needed to maintain the
   robin hood ordering.

   Compared to buckets-and-chains hash tables, open addressing has the
   advantage that it is very cache-friendly.  It also uses less memory.

   Implementation-wise, there are two things to note.

     1. We assume that hash codes are evenly distributed across the
        range of unsigned longs.  The actual hash code stored in the
        entry is left-shifted by 1 bit (losing 1 bit of hash precision),
        and then or'd with 1.  In this way we ensure that the hash field
        of an occupied entry is nonzero.  To map to an index, we
        right-shift the hash by one, divide by the size, and take the
        remainder.

     2. Since the "keys" (the objects in the set) are stored in an
        atomic region with disappearing links, they need to be accessed
        with the GC alloc lock.  `copy_weak_entry' will do that for
        you.  The hash code itself can be read outside the lock,
        though.
*/


typedef struct {
  unsigned long hash;
  scm_t_bits key;
} scm_t_weak_entry;


struct weak_entry_data {
  scm_t_weak_entry *in;
  scm_t_weak_entry *out;
};
  
static void*
do_copy_weak_entry (void *data)
{
  struct weak_entry_data *e = data;

  e->out->hash = e->in->hash;
  e->out->key = e->in->key;

  return NULL;
}

static void
copy_weak_entry (scm_t_weak_entry *src, scm_t_weak_entry *dst)
{
  struct weak_entry_data data;

  data.in = src;
  data.out = dst;
      
  GC_call_with_alloc_lock (do_copy_weak_entry, &data);
}
  

typedef struct {
  scm_t_weak_entry *entries;    /* the data */
  scm_i_pthread_mutex_t lock;   /* the lock */
  unsigned long size;    	/* total number of slots. */
  unsigned long n_items;	/* number of items in set */
  unsigned long lower;		/* when to shrink */
  unsigned long upper;		/* when to grow */
  int size_index;		/* index into hashset_size */
  int min_size_index;		/* minimum size_index */
} scm_t_weak_set;


#define SCM_WEAK_SET_P(x) (SCM_HAS_TYP7 (x, scm_tc7_weak_set))
#define SCM_VALIDATE_WEAK_SET(pos, arg) \
  SCM_MAKE_VALIDATE_MSG (pos, arg, WEAK_SET_P, "weak-set")
#define SCM_WEAK_SET(x) ((scm_t_weak_set *) SCM_CELL_WORD_1 (x))


static unsigned long
hash_to_index (unsigned long hash, unsigned long size)
{
  return (hash >> 1) % size;
}

static unsigned long
entry_distance (unsigned long hash, unsigned long k, unsigned long size)
{
  unsigned long origin = hash_to_index (hash, size);

  if (k >= origin)
    return k - origin;
  else
    /* The other key was displaced and wrapped around.  */
    return size - origin + k;
}

#ifndef HAVE_GC_MOVE_DISAPPEARING_LINK
static void
GC_move_disappearing_link (void **from, void **to)
{
  GC_unregister_disappearing_link (from);
  SCM_I_REGISTER_DISAPPEARING_LINK (to, *to);
}
#endif

static void
move_weak_entry (scm_t_weak_entry *from, scm_t_weak_entry *to)
{
  if (from->hash)
    {
      scm_t_weak_entry copy;
      
      copy_weak_entry (from, &copy);
      to->hash = copy.hash;
      to->key = copy.key;

      if (copy.key && SCM_HEAP_OBJECT_P (SCM_PACK (copy.key)))
        GC_move_disappearing_link ((void **) &from->key, (void **) &to->key);
    }
  else
    {
      to->hash = 0;
      to->key = 0;
    }
}

static void
rob_from_rich (scm_t_weak_set *set, unsigned long k)
{
  unsigned long empty, size;

  size = set->size;

  /* If we are to free up slot K in the set, we need room to do so.  */
  assert (set->n_items < size);
  
  empty = k;
  do 
    empty = (empty + 1) % size;
  /* Here we access key outside the lock.  Is this a problem?  At first
     glance, I wouldn't think so.  */
  while (set->entries[empty].key);

  do
    {
      unsigned long last = empty ? (empty - 1) : (size - 1);
      move_weak_entry (&set->entries[last], &set->entries[empty]);
      empty = last;
    }
  while (empty != k);

  /* Just for sanity.  */
  set->entries[empty].hash = 0;
  set->entries[empty].key = 0;
}

static void
give_to_poor (scm_t_weak_set *set, unsigned long k)
{
  /* Slot K was just freed up; possibly shuffle others down.  */
  unsigned long size = set->size;

  while (1)
    {
      unsigned long next = (k + 1) % size;
      unsigned long hash;
      scm_t_weak_entry copy;

      hash = set->entries[next].hash;

      if (!hash || hash_to_index (hash, size) == next)
        break;

      copy_weak_entry (&set->entries[next], &copy);

      if (!copy.key)
        /* Lost weak reference.  */
        {
          give_to_poor (set, next);
          set->n_items--;
          continue;
        }

      move_weak_entry (&set->entries[next], &set->entries[k]);

      k = next;
    }

  /* We have shuffled down any entries that should be shuffled down; now
     free the end.  */
  set->entries[k].hash = 0;
  set->entries[k].key = 0;
}




/* Growing or shrinking is triggered when the load factor
 *
 *   L = N / S    (N: number of items in set, S: bucket vector length)
 *
 * passes an upper limit of 0.9 or a lower limit of 0.2.
 *
 * The implementation stores the upper and lower number of items which
 * trigger a resize in the hashset object.
 *
 * Possible hash set sizes (primes) are stored in the array
 * hashset_size.
 */

static unsigned long hashset_size[] = {
  31, 61, 113, 223, 443, 883, 1759, 3517, 7027, 14051, 28099, 56197, 112363,
  224717, 449419, 898823, 1797641, 3595271, 7190537, 14381041, 28762081,
  57524111, 115048217, 230096423
};

#define HASHSET_SIZE_N (sizeof(hashset_size)/sizeof(unsigned long))

static int
compute_size_index (scm_t_weak_set *set)
{
  int i = set->size_index;

  if (set->n_items < set->lower)
    {
      /* rehashing is not triggered when i <= min_size */
      do
	--i;
      while (i > set->min_size_index
	     && set->n_items < hashset_size[i] / 5);
    }
  else if (set->n_items > set->upper)
    {
      ++i;
      if (i >= HASHSET_SIZE_N)
        /* The biggest size currently is 230096423, which for a 32-bit
           machine will occupy 1.5GB of memory at a load of 80%.  There
           is probably something better to do here, but if you have a
           weak map of that size, you are hosed in any case.  */
        abort ();
    }

  return i;
}

static int
is_acceptable_size_index (scm_t_weak_set *set, int size_index)
{
  int computed = compute_size_index (set);

  if (size_index == computed)
    /* We were going to grow or shrink, and allocating the new vector
       didn't change the target size.  */
    return 1;

  if (size_index == computed + 1)
    {
      /* We were going to enlarge the set, but allocating the new
         vector finalized some objects, making an enlargement
         unnecessary.  It might still be a good idea to use the larger
         set, though.  (This branch also gets hit if, while allocating
         the vector, some other thread was actively removing items from
         the set.  That is less likely, though.)  */
      unsigned long new_lower = hashset_size[size_index] / 5;

      return set->size > new_lower;
    }

  if (size_index == computed - 1)
    {
      /* We were going to shrink the set, but when we dropped the lock
         to allocate the new vector, some other thread added elements to
         the set.  */
      return 0;
    }

  /* The computed size differs from our newly allocated size by more
     than one size index -- recalculate.  */
  return 0;
}

static void
resize_set (scm_t_weak_set *set)
{
  scm_t_weak_entry *old_entries, *new_entries;
  int new_size_index;
  unsigned long old_size, new_size, old_k;

  do 
    {
      new_size_index = compute_size_index (set);
      if (new_size_index == set->size_index)
        return;
      new_size = hashset_size[new_size_index];
    }
  while (!is_acceptable_size_index (set, new_size_index));

  new_entries = scm_gc_malloc_pointerless (new_size * sizeof (scm_t_weak_entry),
                                           "weak set");
  old_entries = set->entries;
  old_size = set->size;

  memset (new_entries, 0, new_size * sizeof(scm_t_weak_entry));

  set->size_index = new_size_index;
  set->size = new_size;
  if (new_size_index <= set->min_size_index)
    set->lower = 0;
  else
    set->lower = new_size / 5;
  set->upper = 9 * new_size / 10;
  set->n_items = 0;
  set->entries = new_entries;

  for (old_k = 0; old_k < old_size; old_k++)
    {
      scm_t_weak_entry copy;
      unsigned long new_k, distance;

      if (!old_entries[old_k].hash)
        continue;
      
      copy_weak_entry (&old_entries[old_k], &copy);
      
      if (!copy.key)
        continue;
      
      new_k = hash_to_index (copy.hash, new_size);

      for (distance = 0; ; distance++, new_k = (new_k + 1) % new_size)
        {
          unsigned long other_hash = new_entries[new_k].hash;

          if (!other_hash)
            /* Found an empty entry. */
            break;

          /* Displace the entry if our distance is less, otherwise keep
             looking. */
          if (entry_distance (other_hash, new_k, new_size) < distance)
            {
              rob_from_rich (set, new_k);
              break;
            }
        }
          
      set->n_items++;
      new_entries[new_k].hash = copy.hash;
      new_entries[new_k].key = copy.key;

      if (SCM_HEAP_OBJECT_P (SCM_PACK (copy.key)))
        SCM_I_REGISTER_DISAPPEARING_LINK ((void **) &new_entries[new_k].key,
                                          (void *) new_entries[new_k].key);
    }
}

/* Run from a finalizer via do_vacuum_weak_set, this function runs over
   the whole table, removing lost weak references, reshuffling the set
   as it goes.  It might resize the set if it reaps enough entries.  */
static void
vacuum_weak_set (scm_t_weak_set *set)
{
  scm_t_weak_entry *entries = set->entries;
  unsigned long size = set->size;
  unsigned long k;

  for (k = 0; k < size; k++)
    {
      unsigned long hash = entries[k].hash;
      
      if (hash)
        {
          scm_t_weak_entry copy;

          copy_weak_entry (&entries[k], &copy);

          if (!copy.key)
            /* Lost weak reference; reshuffle.  */
            {
              give_to_poor (set, k);
              set->n_items--;
            }
        }
    }

  if (set->n_items < set->lower)
    resize_set (set);
}




static SCM
weak_set_lookup (scm_t_weak_set *set, unsigned long hash,
                 scm_t_set_predicate_fn pred, void *closure,
                 SCM dflt)
{
  unsigned long k, distance, size;
  scm_t_weak_entry *entries;
  
  size = set->size;
  entries = set->entries;

  hash = (hash << 1) | 0x1;
  k = hash_to_index (hash, size);
  
  for (distance = 0; distance < size; distance++, k = (k + 1) % size)
    {
      unsigned long other_hash;

    retry:
      other_hash = entries[k].hash;

      if (!other_hash)
        /* Not found. */
        return dflt;

      if (hash == other_hash)
        {
          scm_t_weak_entry copy;
          
          copy_weak_entry (&entries[k], &copy);

          if (!copy.key)
            /* Lost weak reference; reshuffle.  */
            {
              give_to_poor (set, k);
              set->n_items--;
              goto retry;
            }

          if (pred (SCM_PACK (copy.key), closure))
            /* Found. */
            return SCM_PACK (copy.key);
        }

      /* If the entry's distance is less, our key is not in the set.  */
      if (entry_distance (other_hash, k, size) < distance)
        return dflt;
    }

  /* If we got here, then we were unfortunate enough to loop through the
     whole set.  Shouldn't happen, but hey.  */
  return dflt;
}


static SCM
weak_set_add_x (scm_t_weak_set *set, unsigned long hash,
                scm_t_set_predicate_fn pred, void *closure,
                SCM obj)
{
  unsigned long k, distance, size;
  scm_t_weak_entry *entries;
  
  size = set->size;
  entries = set->entries;

  hash = (hash << 1) | 0x1;
  k = hash_to_index (hash, size);

  for (distance = 0; ; distance++, k = (k + 1) % size)
    {
      unsigned long other_hash;

    retry:
      other_hash = entries[k].hash;

      if (!other_hash)
        /* Found an empty entry. */
        break;

      if (other_hash == hash)
        {
          scm_t_weak_entry copy;

          copy_weak_entry (&entries[k], &copy);
          
          if (!copy.key)
            /* Lost weak reference; reshuffle.  */
            {
              give_to_poor (set, k);
              set->n_items--;
              goto retry;
            }

          if (pred (SCM_PACK (copy.key), closure))
            /* Found an entry with this key. */
            return SCM_PACK (copy.key);
        }

      if (set->n_items > set->upper)
        /* Full set, time to resize.  */
        {
          vacuum_weak_set (set);
          resize_set (set);
          return weak_set_add_x (set, hash >> 1, pred, closure, obj);
        }

      /* Displace the entry if our distance is less, otherwise keep
         looking. */
      if (entry_distance (other_hash, k, size) < distance)
        {
          rob_from_rich (set, k);
          break;
        }
    }
          
  set->n_items++;
  entries[k].hash = hash;
  entries[k].key = SCM_UNPACK (obj);

  if (SCM_HEAP_OBJECT_P (obj))
    SCM_I_REGISTER_DISAPPEARING_LINK ((void **) &entries[k].key,
                                      (void *) SCM2PTR (obj));

  return obj;
}


static void
weak_set_remove_x (scm_t_weak_set *set, unsigned long hash,
                   scm_t_set_predicate_fn pred, void *closure)
{
  unsigned long k, distance, size;
  scm_t_weak_entry *entries;
  
  size = set->size;
  entries = set->entries;

  hash = (hash << 1) | 0x1;
  k = hash_to_index (hash, size);

  for (distance = 0; distance < size; distance++, k = (k + 1) % size)
    {
      unsigned long other_hash;

    retry:
      other_hash = entries[k].hash;

      if (!other_hash)
        /* Not found. */
        return;

      if (other_hash == hash)
        {
          scm_t_weak_entry copy;
      
          copy_weak_entry (&entries[k], &copy);
          
          if (!copy.key)
            /* Lost weak reference; reshuffle.  */
            {
              give_to_poor (set, k);
              set->n_items--;
              goto retry;
            }

          if (pred (SCM_PACK (copy.key), closure))
            /* Found an entry with this key. */
            {
              entries[k].hash = 0;
              entries[k].key = 0;

              if (SCM_HEAP_OBJECT_P (SCM_PACK (copy.key)))
                GC_unregister_disappearing_link ((void **) &entries[k].key);

              if (--set->n_items < set->lower)
                resize_set (set);
              else
                give_to_poor (set, k);

              return;
            }
        }

      /* If the entry's distance is less, our key is not in the set.  */
      if (entry_distance (other_hash, k, size) < distance)
        return;
    }
}



static SCM
make_weak_set (unsigned long k)
{
  scm_t_weak_set *set;

  int i = 0, n = k ? k : 31;
  while (i + 1 < HASHSET_SIZE_N && n > hashset_size[i])
    ++i;
  n = hashset_size[i];

  set = scm_gc_malloc (sizeof (*set), "weak-set");
  set->entries = scm_gc_malloc_pointerless (n * sizeof(scm_t_weak_entry),
                                            "weak-set");
  memset (set->entries, 0, n * sizeof(scm_t_weak_entry));
  set->n_items = 0;
  set->size = n;
  set->lower = 0;
  set->upper = 9 * n / 10;
  set->size_index = i;
  set->min_size_index = i;
  scm_i_pthread_mutex_init (&set->lock, NULL);

  return scm_cell (scm_tc7_weak_set, (scm_t_bits)set);
}

void
scm_i_weak_set_print (SCM exp, SCM port, scm_print_state *pstate)
{
  scm_puts ("#<", port);
  scm_puts ("weak-set ", port);
  scm_uintprint (SCM_WEAK_SET (exp)->n_items, 10, port);
  scm_putc ('/', port);
  scm_uintprint (SCM_WEAK_SET (exp)->size, 10, port);
  scm_puts (">", port);
}

static void
do_vacuum_weak_set (SCM set)
{
  scm_t_weak_set *s;

  s = SCM_WEAK_SET (set);

  /* We should always be able to grab this lock, because we are run from
     a finalizer, which runs in another thread (or an async, which is
     mostly equivalent).  */
  scm_i_pthread_mutex_lock (&s->lock);
  vacuum_weak_set (s);
  scm_i_pthread_mutex_unlock (&s->lock);
}

static scm_i_pthread_mutex_t all_weak_sets_lock = SCM_I_PTHREAD_MUTEX_INITIALIZER;
static SCM all_weak_sets = SCM_EOL;

static void
vacuum_all_weak_sets (void)
{
  scm_i_pthread_mutex_lock (&all_weak_sets_lock);
  scm_i_visit_weak_list (&all_weak_sets, do_vacuum_weak_set);
  scm_i_pthread_mutex_unlock (&all_weak_sets_lock);
}

SCM
scm_c_make_weak_set (unsigned long k)
{
  SCM ret;

  ret = make_weak_set (k);

  scm_i_pthread_mutex_lock (&all_weak_sets_lock);
  all_weak_sets = scm_i_weak_cons (ret, all_weak_sets);
  scm_i_pthread_mutex_unlock (&all_weak_sets_lock);

  return ret;
}

SCM
scm_weak_set_p (SCM obj)
{
  return scm_from_bool (SCM_WEAK_SET_P (obj));
}

SCM
scm_weak_set_clear_x (SCM set)
{
  scm_t_weak_set *s = SCM_WEAK_SET (set);

  scm_i_pthread_mutex_lock (&s->lock);

  memset (s->entries, 0, sizeof (scm_t_weak_entry) * s->size);
  s->n_items = 0;

  scm_i_pthread_mutex_unlock (&s->lock);

  return SCM_UNSPECIFIED;
}

SCM
scm_c_weak_set_lookup (SCM set, unsigned long raw_hash,
                       scm_t_set_predicate_fn pred,
                       void *closure, SCM dflt)
{
  SCM ret;
  scm_t_weak_set *s = SCM_WEAK_SET (set);

  scm_i_pthread_mutex_lock (&s->lock);

  ret = weak_set_lookup (s, raw_hash, pred, closure, dflt);

  scm_i_pthread_mutex_unlock (&s->lock);

  return ret;
}

SCM
scm_c_weak_set_add_x (SCM set, unsigned long raw_hash,
                      scm_t_set_predicate_fn pred,
                      void *closure, SCM obj)
{
  SCM ret;
  scm_t_weak_set *s = SCM_WEAK_SET (set);

  scm_i_pthread_mutex_lock (&s->lock);

  ret = weak_set_add_x (s, raw_hash, pred, closure, obj);

  scm_i_pthread_mutex_unlock (&s->lock);

  return ret;
}

void
scm_c_weak_set_remove_x (SCM set, unsigned long raw_hash,
                         scm_t_set_predicate_fn pred,
                         void *closure)
{
  scm_t_weak_set *s = SCM_WEAK_SET (set);

  scm_i_pthread_mutex_lock (&s->lock);

  weak_set_remove_x (s, raw_hash, pred, closure);

  scm_i_pthread_mutex_unlock (&s->lock);
}

static int
eq_predicate (SCM x, void *closure)
{
  return scm_is_eq (x, SCM_PACK_POINTER (closure));
}

SCM
scm_weak_set_add_x (SCM set, SCM obj)
{
  return scm_c_weak_set_add_x (set, scm_ihashq (obj, -1),
                               eq_predicate, SCM_UNPACK_POINTER (obj), obj);
}

SCM
scm_weak_set_remove_x (SCM set, SCM obj)
{
  scm_c_weak_set_remove_x (set, scm_ihashq (obj, -1),
                           eq_predicate, SCM_UNPACK_POINTER (obj));

  return SCM_UNSPECIFIED;
}

SCM
scm_c_weak_set_fold (scm_t_set_fold_fn proc, void *closure,
                     SCM init, SCM set)
{
  scm_t_weak_set *s;
  scm_t_weak_entry *entries;
  unsigned long k, size;

  s = SCM_WEAK_SET (set);

  scm_i_pthread_mutex_lock (&s->lock);

  size = s->size;
  entries = s->entries;

  for (k = 0; k < size; k++)
    {
      if (entries[k].hash)
        {
          scm_t_weak_entry copy;
          
          copy_weak_entry (&entries[k], &copy);
      
          if (copy.key)
            {
              /* Release set lock while we call the function.  */
              scm_i_pthread_mutex_unlock (&s->lock);
              init = proc (closure, SCM_PACK (copy.key), init);
              scm_i_pthread_mutex_lock (&s->lock);
            }
        }
    }
  
  scm_i_pthread_mutex_unlock (&s->lock);
  
  return init;
}

static SCM
fold_trampoline (void *closure, SCM item, SCM init)
{
  return scm_call_2 (SCM_PACK_POINTER (closure), item, init);
}

SCM
scm_weak_set_fold (SCM proc, SCM init, SCM set)
{
  return scm_c_weak_set_fold (fold_trampoline, SCM_UNPACK_POINTER (proc), init, set);
}

static SCM
for_each_trampoline (void *closure, SCM item, SCM seed)
{
  scm_call_1 (SCM_PACK_POINTER (closure), item);
  return seed;
}

SCM
scm_weak_set_for_each (SCM proc, SCM set)
{
  scm_c_weak_set_fold (for_each_trampoline, SCM_UNPACK_POINTER (proc), SCM_BOOL_F, set);

  return SCM_UNSPECIFIED;
}

static SCM
map_trampoline (void *closure, SCM item, SCM seed)
{
  return scm_cons (scm_call_1 (SCM_PACK_POINTER (closure), item), seed);
}

SCM
scm_weak_set_map_to_list (SCM proc, SCM set)
{
  return scm_c_weak_set_fold (map_trampoline, SCM_UNPACK_POINTER (proc), SCM_EOL, set);
}


void
scm_init_weak_set ()
{
#include "weak-set.x"

  scm_i_register_async_gc_callback (vacuum_all_weak_sets);
}