rosella/gc.c

#include <assert.h>
#include <inttypes.h>
#include <stdbool.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <time.h>

#include "gc.h"

gc_stats_t gc_stats;

/* Helper macros to reduce duplication */
#define VECTOR_BYTES(nelem)  (sizeof(vector_t) + (sizeof(value_t) * (nelem)))
#define BYTESTR_BYTES(size)  (sizeof(byte_string_t) + (size))
#define STRUCT_BYTES(nslots) VECTOR_BYTES(nslots)

/* Alignment must ensure each object has enough room to hold a forwarding object */
#define GC_ALIGNMENT  ((size_t)(sizeof(object_t)))

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

static char   *gc_ranges[2];
static size_t  gc_min_size;
static size_t  gc_max_size;
static size_t  gc_soft_limit;
static bool    gc_enabled;

static int     gc_current_range;
static char   *gc_free_ptr;
static char   *gc_range_end;

static value_t gc_weak_box_list;
static value_t gc_will_list;
static value_t gc_will_active_list;

static gc_root_t gc_root_list = {
  .value = NIL,
  .prev  = &gc_root_list,
  .next  = &gc_root_list
};

void register_gc_root(gc_root_t *root, value_t v)
{
  root->value = v;
  root->next = &gc_root_list;
  gc_root_list.prev->next = root;
  root->prev = gc_root_list.prev;
  gc_root_list.prev = root;
}

void unregister_gc_root(gc_root_t *root)
{
  assert(root && root->prev && root->next);             /* Uninitialized */
  assert((root->prev != root) && (root->next != root)); /* Already removed */

  /* Cut the given root out of the list */
  root->prev->next = root->next;
  root->next->prev = root->prev;

  /* Remove dead references to root list; protects against double-removal */
  root->prev = root->next = root;
}

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

object_t *get_object(value_t v)
{
  if (is_object(v))
    return _get_object(v);
  else
    abort();
}

/* No one outside this module should care... */
static inline bool is_broken_heart(value_t v)
{
  return is_object(v) && (_get_object(v)->tag == BROKEN_HEART);
}

value_t cons(value_t car, value_t cdr)
{
  gc_root_t car_root, cdr_root;
  pair_t *p;

  register_gc_root(&car_root, car);
  register_gc_root(&cdr_root, cdr);

  p = (pair_t*)gc_alloc(sizeof(pair_t));
  p->car = car_root.value;
  p->cdr = cdr_root.value;

  unregister_gc_root(&car_root);
  unregister_gc_root(&cdr_root);

  return pair_value(p);
}

pair_t *get_pair(value_t v)
{
  if (is_pair(v))
    return _get_pair(v);
  else
    abort();
}

value_t make_box(value_t initial_value)
{
  gc_root_t iv_root;
  box_t *box;

  register_gc_root(&iv_root, initial_value);

  box = (box_t*)gc_alloc(sizeof(box_t));
  box->tag = TYPE_TAG_BOX;
  box->value = iv_root.value;

  unregister_gc_root(&iv_root);

  return object_value(box);
}

box_t *get_box(value_t v)
{
  if (is_box(v))
    return _get_box(v);
  else
    abort();
}

value_t make_vector(size_t nelem, value_t initial_value)
{
  gc_root_t iv_root;
  vector_t *vec;

  register_gc_root(&iv_root, initial_value);

  vec = (vector_t*)gc_alloc(VECTOR_BYTES(nelem));
  vec->tag = TYPE_TAG_VECTOR;
  vec->size = nelem;

  for (int i = 0; i < nelem; ++i)
    vec->elements[i] = iv_root.value;

  unregister_gc_root(&iv_root);

  return object_value(vec);
}

vector_t *get_vector(value_t v)
{
  if (is_vector(v))
    return _get_vector(v);
  else
    abort();
}

value_t make_byte_string(size_t size, int default_value)
{
  const size_t nbytes = BYTESTR_BYTES(size);
  byte_string_t *str;
  
  str = (byte_string_t*)gc_alloc(nbytes);
  str->tag = TYPE_TAG_BYTESTR;
  str->size = size;

  memset(str->bytes, default_value, size);

  return object_value(str);
}

byte_string_t *get_byte_string(value_t v)
{
  if (is_byte_string(v))
    return _get_byte_string(v);
  else
    abort();
}

value_t make_struct(value_t type, size_t nslots)
{
  gc_root_t type_root;
  struct_t *s;

  assert(nslots >= 1);

  /* Ensure that there is always a slot for the type */
  if (nslots < 1)
    nslots = 1;

  register_gc_root(&type_root, type);

  s = (struct_t*)gc_alloc(STRUCT_BYTES(nslots));
  s->tag = TYPE_TAG_VECTOR;
  s->nslots = nslots;
  s->slots[0] = type_root.value;

  for (int i = 1; i < nslots; ++i)
    s->slots[i] = NIL;

  unregister_gc_root(&type_root);

  return object_value(s);
}

struct_t *get_struct(value_t v)
{
  if (is_struct(v))
    return _get_struct(v);
  else
    abort();
}

value_t make_weak_box(value_t initial_value)
{
  gc_root_t iv_root;
  weak_box_t *box;

  register_gc_root(&iv_root, initial_value);

  box = (weak_box_t*)gc_alloc(sizeof(weak_box_t));
  box->tag = TYPE_TAG_WEAK_BOX;
  box->value = iv_root.value;
  box->next = gc_weak_box_list;
  gc_weak_box_list = object_value(box);

  unregister_gc_root(&iv_root);

  return object_value(box);
}

weak_box_t *get_weak_box(value_t v)
{
  if (is_weak_box(v))
    return _get_weak_box(v);
  else
    abort();
}

void register_finalizer(value_t value, value_t finalizer)
{
  /* Non-objects are never GC'd, so their finalizers will never be invoked. */
  if (is_object(value))
  {
    gc_root_t value_root, finalizer_root;
    will_t *w;

    register_gc_root(&value_root, value);
    register_gc_root(&finalizer_root, finalizer);

    w = (will_t*)gc_alloc(sizeof(will_t));
    w->tag = TYPE_TAG_WILL;
    w->value = value_root.value;
    w->finalizer = finalizer_root.value;
    w->next = gc_will_list;

    gc_will_list = object_value(w);

    unregister_gc_root(&value_root);
    unregister_gc_root(&finalizer_root);
  }
}

/* The 'will' accessors are private to the GC, unlike other similar routines. */
static inline will_t *_get_will(value_t v)
{
  return (will_t*)_get_object(v);
}

static will_t *get_will(value_t v)
{
  if (is_will(v))
    return _get_will(v);
  else
    abort();
}

intptr_t get_fixnum(value_t v)
{
  if (is_fixnum(v))
    return _get_fixnum(v);
  else
    abort();
}

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

static inline size_t gc_align(size_t nbytes) __attribute__ ((const));
static int gc_range_of(void *object) __attribute__ ((const));
static void transfer_object(value_t *value);
static size_t transfer_children(object_t *object);
static void _collect_garbage(size_t min_free);

static inline size_t gc_align(size_t nbytes)
{
  return ((nbytes + GC_ALIGNMENT - 1) & ~(GC_ALIGNMENT - 1));
}

static int gc_range_of(void *object)
{
  if (((uintptr_t)object >= (uintptr_t)gc_ranges[0]) &&
      ((uintptr_t)object <  (uintptr_t)gc_ranges[1]))
    return 0;
  if (((uintptr_t)object >= (uintptr_t)gc_ranges[1]) &&
      ((uintptr_t)object <  (uintptr_t)gc_ranges[2]))
    return 1;
  return -1;
}

static inline size_t gc_free_space(void)
{
  return gc_range_end - gc_free_ptr;
}

void gc_init(size_t min_size, size_t max_size)
{
  assert(min_size <= max_size);

  gc_ranges[0] = (char*)malloc(max_size);
  gc_ranges[1] = (char*)malloc(max_size);

  assert(gc_ranges[0] && gc_ranges[1]);

  gc_current_range = 0;
  gc_free_ptr = gc_ranges[gc_current_range];

  gc_min_size   = min_size;
  gc_max_size   = max_size;
  gc_soft_limit = gc_min_size;
  gc_range_end  = gc_free_ptr + gc_soft_limit;

  gc_stats.collections = 0;
  gc_stats.total_ticks = 0;
  gc_stats.high_water  = 0;

  gc_weak_box_list    = NIL;
  gc_will_list        = NIL;
  gc_will_active_list = NIL;

  gc_enabled = true;
}

/* Preconditions: nbytes pre-aligned a la gc_align(), and space exists. */
static inline void *_gc_alloc(size_t nbytes)
{
  void *p = gc_free_ptr;
  gc_free_ptr += nbytes;
  return p;
}

void *gc_alloc(size_t nbytes)
{
  nbytes = gc_align(nbytes);

  if (nbytes > gc_free_space())
    _collect_garbage(nbytes);

  return _gc_alloc(nbytes);
}

/* Precondition: *value refers to an object (or pair). */
static void transfer_object(value_t *value)
{
  if (is_object(*value))
  {
    object_t *obj;
    size_t nbytes;
    void *newobj;
    value_t new_value;

    assert(gc_range_of(obj) != gc_current_range);
    assert(is_object(*value));

    obj = _get_object(*value);

    if (obj->tag == BROKEN_HEART)
    {
      /* Object has already been moved; just update the reference */
      *value = obj->forward;
      return;
    }

    switch (obj->tag)
    {
    case TYPE_TAG_VECTOR:
    case TYPE_TAG_STRUCT:
      nbytes = VECTOR_BYTES(((const vector_t*)obj)->size);
      break;
    case TYPE_TAG_BYTESTR:
      nbytes = BYTESTR_BYTES(((const byte_string_t*)obj)->size);
      break;
    case TYPE_TAG_WEAK_BOX:
      nbytes = sizeof(weak_box_t);
      break;
    case TYPE_TAG_WILL:
      nbytes = sizeof(will_t);
      break;
    case TYPE_TAG_BOX:
    default: /* pair */
      nbytes = sizeof(pair_t);
      break;
    }

    newobj = _gc_alloc(gc_align(nbytes));

    memcpy(newobj, obj, nbytes);

    /* Keep the original tag bits (pair or object) */
    new_value = object_value(newobj) | (*value & 2);

    obj->tag = BROKEN_HEART;
    obj->forward = new_value;

    *value = new_value;
  }
}

/* Also works on structs, which share the same layout */
static size_t transfer_vector(vector_t *vec)
{
  for (size_t i = 0; i < vec->size; ++i)
    transfer_object(&vec->elements[i]);

  return VECTOR_BYTES(vec->size);
}

static size_t transfer_pair(pair_t *p)
{
  transfer_object(&p->car);
  transfer_object(&p->cdr);
  return sizeof(pair_t);
}

static size_t transfer_will(will_t *w)
{
  transfer_object(&w->finalizer);

  /* Weak boxes are discarded when there are no other references,
   * but wills need to remain until their finalizers are invoked. */
  transfer_object(&w->next);

  return sizeof(will_t);
}

static size_t transfer_children(object_t *obj)
{
  switch (obj->tag)
  {
  case TYPE_TAG_VECTOR:
  case TYPE_TAG_STRUCT:
    return transfer_vector((vector_t*)obj);
  case TYPE_TAG_BYTESTR:
    return BYTESTR_BYTES(((const byte_string_t*)obj)->size);
  case TYPE_TAG_WEAK_BOX:
    return sizeof(weak_box_t);
  case TYPE_TAG_WILL:
    return transfer_will((will_t*)obj);
  case TYPE_TAG_BOX:
  default: /* pair */
    return transfer_pair((pair_t*)obj);
  }
}

static void swap_gc_ranges(void)
{
  gc_current_range = 1 - gc_current_range;
  gc_free_ptr      = gc_ranges[gc_current_range];
  gc_range_end     = gc_free_ptr + gc_soft_limit;
}

static void transfer_roots(void)
{
  /* Transfer registered GC roots */
  for (gc_root_t *root = gc_root_list.next; root != &gc_root_list; root = root->next)
    transfer_object(&root->value);

  /* Ensure pending will list is transferred */
  transfer_object(&gc_will_list);

  /* The values associated with active wills are also roots */
  for (value_t *will = &gc_will_active_list; !is_nil(*will); will = &_get_will(*will)->next)
    transfer_object(&get_will(*will)->value);

  /* Ensure active list itself is transferred */
  transfer_object(&gc_will_active_list);
}

static void process_weak_boxes(void)
{
  value_t *wb = &gc_weak_box_list;

  while (!is_nil(*wb))
  {
    if (is_broken_heart(*wb))
    {
      /* The box itself is reachable; need to update 'next' pointer to new location */
      weak_box_t *box;

      *wb = _get_object(*wb)->forward;
      assert(is_weak_box(*wb));
      box = _get_weak_box(*wb);

      if (is_broken_heart(box->value))
      {
        /* The value in the box is also reachable; update w/ new location. */
        box->value = _get_object(box->value)->forward;
      }
      else if (is_object(box->value))
      {
        /* The value in the box is an unreachable object; change to #f */
        box->value = FALSE_VALUE;
      }

      /* Move on to this box's 'next' pointer */
      wb = &_get_weak_box(*wb)->next;
    }
    else
    {
      /* Box is no longer reachable; remove it from the list by updating 'next' pointer. */
      assert(is_weak_box(*wb));
      *wb = _get_weak_box(*wb)->next;
    }
  }
}

/* Precondition: The wills themselves, and their finalizers,
 * have already been transferred (recursively). */
static void process_wills(void)
{
  /*
   * Was value transferred (broken heart), and thus reachable?
   *   Yes ==> update value.
   *   No ==> transfer value and move will to active list.
   */
  value_t *will = &gc_will_list;

  while (!is_nil(*will))
  {
    will_t *w = get_will(*will);

    if (is_broken_heart(w->value))
    {
      /* The value associated with the will is still reachable; update w/ new location. */
      w->value = _get_object(w->value)->forward;

      /* Move on to this will's 'next' pointer */
      will = &w->next;
    }
    else
    {
      assert(is_object(w->value));

      /*
       * The will is associated with an unreachable object; activate it.
       */

      /* First, ensure that the value remains reachable for the finalizer. */
      transfer_object(&w->value);

      /* Remove the will from the 'pending' list. */
      *will = w->next;

      /* Insert the will into the 'active' list. */
      w->next = gc_will_active_list;
      gc_will_active_list = object_value(w);
    }
  }
}

static void update_soft_limit(size_t min_free)
{
  size_t bytes_used = gc_free_ptr - gc_ranges[gc_current_range];
  size_t min_limit = bytes_used + min_free;
  size_t new_limit = (5 * min_limit) / 3;

  if (new_limit > gc_max_size)
    new_limit = gc_max_size;
#if 0
  else if (new_limit < gc_min_size)
    new_limit = gc_min_size;

  gc_soft_limit = new_limit;
#else
  if (new_limit > gc_soft_limit)
    gc_soft_limit = new_limit;
#endif

  /* Update end of range to reflect new limit */
  gc_range_end = gc_ranges[gc_current_range] + gc_soft_limit;

  if (gc_soft_limit > gc_stats.high_water)
  {
    gc_stats.high_water = gc_soft_limit;
  }
}

static void _collect_garbage(size_t min_free)
{
  if (gc_enabled)
  {
    char *object_ptr;

    gc_stats.total_ticks -= clock();
    ++gc_stats.collections;

    //debug(("Collecting garbage...\n"));

    /* Swap ranges; new "current" range is initially empty, old one is full */
    swap_gc_ranges();
    object_ptr = gc_free_ptr;

    transfer_roots();

    /* Keep transferring until no more objects in the new range refer to the old one,
     * other than pending wills and weak boxes. */
    while (object_ptr < gc_free_ptr)
    {
      object_ptr += gc_align(transfer_children((object_t*)object_ptr));
    }

    process_weak_boxes();
    process_wills();

    /* Keep transferring until no more objects in the new range refer to the old one.
     * This is so that values which are otherwise unreachable, but have finalizers which
     * may be able to reach them, are not collected prematurely. Note that these values may
     * be finalized in any order, and that any weak references have already been cleared. */
    while (object_ptr < gc_free_ptr)
    {
      object_ptr += gc_align(transfer_children((object_t*)object_ptr));
    }
    
    //debug(("Finished collection with %d bytes to spare (out of %d bytes).\n", gc_free_space(), gc_soft_limit));

    gc_stats.total_ticks += clock();
  }

  update_soft_limit(min_free);

  if (gc_free_space() < min_free)
  {
    out_of_memory();
  }
}

void collect_garbage(size_t min_free)
{
  bool was_enabled = gc_enabled;
  gc_enabled = true;
  _collect_garbage(min_free);
  gc_enabled = was_enabled;
}

bool set_gc_enabled(bool enable)
{
  bool was_enabled = gc_enabled;
  gc_enabled = enable;
  return was_enabled;
}

bool are_finalizers_pending(void)
{
  return !is_nil(gc_will_active_list);
}

/* Finalizer can be registered as #f, but value must be an object.
 * Returning with value == #f means there are no more finalizers. */
void get_next_finalizer(value_t *value, value_t *finalizer)
{
  assert(value && finalizer);

  if (is_nil(gc_will_active_list))
  {
    *value = *finalizer = FALSE_VALUE;
  }
  else
  {
    will_t *w = get_will(gc_will_active_list);

    *value = w->value;
    *finalizer = w->finalizer;

    /* Remove this finalizer from the list -- up to caller to keep values reachable. */
    gc_will_active_list = w->next;
  }
}

/* vim:set sw=2 expandtab: */