/**************************************************************************/
/*                                                                        */
/*                                 OCaml                                  */
/*                                                                        */
/*             Xavier Leroy, projet Cristal, INRIA Rocquencourt           */
/*                                                                        */
/*   Copyright 1996 Institut National de Recherche en Informatique et     */
/*     en Automatique.                                                    */
/*                                                                        */
/*   All rights reserved.  This file is distributed under the terms of    */
/*   the GNU Lesser General Public License version 2.1, with the          */
/*   special exception on linking described in the file LICENSE.          */
/*                                                                        */
/**************************************************************************/

#define CAML_INTERNALS

/* Structured output */

/* The interface of this file is "caml/intext.h" */

#include <string.h>
#include "caml/alloc.h"
#include "caml/codefrag.h"
#include "caml/config.h"
#include "caml/custom.h"
#include "caml/fail.h"
#include "caml/gc.h"
#include "caml/intext.h"
#include "caml/io.h"
#include "caml/memory.h"
#include "caml/misc.h"
#include "caml/mlvalues.h"
#include "caml/reverse.h"
#include "caml/shared_heap.h"

/* Flags affecting marshaling */

enum {
  NO_SHARING = 1,               /* Flag to ignore sharing */
  CLOSURES = 2,                 /* Flag to allow marshaling code pointers */
  COMPAT_32 = 4                 /* Flag to ensure that output can safely
                                   be read back on a 32-bit platform */
};

/* Stack for pending values to marshal */

#define EXTERN_STACK_INIT_SIZE 256
#define EXTERN_STACK_MAX_SIZE (1024*1024*100)

struct extern_item { volatile value * v; mlsize_t count; };

/* Hash table to record already-marshaled objects and their positions */

struct object_position { value obj; uintnat pos; };

/* The hash table uses open addressing, linear probing, and a redundant
   representation:
   - a bitvector [present] records which entries of the table are occupied;
   - an array [entries] records (object, position) pairs for the entries
     that are occupied.
   The bitvector is much smaller than the array (1/128th on 64-bit
   platforms, 1/64th on 32-bit platforms), so it has better locality,
   making it faster to determine that an object is not in the table.
   Also, it makes it faster to empty or initialize a table: only the
   [present] bitvector needs to be filled with zeros, the [entries]
   array can be left uninitialized.
*/

struct position_table {
  int shift;
  mlsize_t size;                    /* size == 1 << (wordsize - shift) */
  mlsize_t mask;                    /* mask == size - 1 */
  mlsize_t threshold;               /* threshold == a fixed fraction of size */
  uintnat * present;                /* [Bitvect_size(size)] */
  struct object_position * entries; /* [size]  */
};

#define Bits_word (8 * sizeof(uintnat))
#define Bitvect_size(n) (((n) + Bits_word - 1) / Bits_word)

#define POS_TABLE_INIT_SIZE_LOG2 8
#define POS_TABLE_INIT_SIZE (1 << POS_TABLE_INIT_SIZE_LOG2)

struct output_block {
  struct output_block * next;
  char * end;
  char data[SIZE_EXTERN_OUTPUT_BLOCK];
};

struct caml_extern_state {

  int extern_flags;        /* logical or of some of the flags */

  uintnat obj_counter;    /* Number of objects emitted so far */
  uintnat size_32;        /* Size in words of 32-bit block for struct. */
  uintnat size_64;        /* Size in words of 64-bit block for struct. */

  /* Stack for pending value to marshal */
  struct extern_item extern_stack_init[EXTERN_STACK_INIT_SIZE];
  struct extern_item * extern_stack;
  struct extern_item * extern_stack_limit;

  /* Hash table to record already marshalled objects */
  uintnat pos_table_present_init[Bitvect_size(POS_TABLE_INIT_SIZE)];
  struct object_position pos_table_entries_init[POS_TABLE_INIT_SIZE];
  struct position_table pos_table;

  /* To buffer the output */

  char * extern_userprovided_output;
  char * extern_ptr;
  char * extern_limit;

  struct output_block * extern_output_first;
  struct output_block * extern_output_block;
};

static struct caml_extern_state* get_extern_state (void)
{
  Caml_check_caml_state();
  struct caml_extern_state* extern_state;

  if (Caml_state->extern_state != NULL)
    return Caml_state->extern_state;

  extern_state =
    caml_stat_alloc_noexc(sizeof(struct caml_extern_state));
  if (extern_state == NULL) {
    return NULL;
  }

  extern_state->extern_flags = 0;
  extern_state->obj_counter = 0;
  extern_state->size_32 = 0;
  extern_state->size_64 = 0;
  extern_state->extern_stack = extern_state->extern_stack_init;
  extern_state->extern_stack_limit =
    extern_state->extern_stack + EXTERN_STACK_INIT_SIZE;

  Caml_state->extern_state = extern_state;
  return extern_state;
}

void caml_free_extern_state (void)
{
  if (Caml_state->extern_state != NULL) {
    caml_stat_free(Caml_state->extern_state);
    Caml_state->extern_state = NULL;
  }
}

/* Forward declarations */

CAMLnoreturn_start
static void extern_out_of_memory(struct caml_extern_state* s)
CAMLnoreturn_end;

CAMLnoreturn_start
static void extern_invalid_argument(struct caml_extern_state* s, char *msg)
CAMLnoreturn_end;

CAMLnoreturn_start
static void extern_failwith(struct caml_extern_state* s, char *msg)
CAMLnoreturn_end;

CAMLnoreturn_start
static void extern_stack_overflow(struct caml_extern_state* s)
CAMLnoreturn_end;

static void free_extern_output(struct caml_extern_state* s);

/* Free the extern stack if needed */
static void extern_free_stack(struct caml_extern_state* s)
{
  if (s->extern_stack != s->extern_stack_init) {
    caml_stat_free(s->extern_stack);
    /* Reinitialize the globals for next time around */
    s->extern_stack = s->extern_stack_init;
    s->extern_stack_limit = s->extern_stack + EXTERN_STACK_INIT_SIZE;
  }
}


static struct extern_item * extern_resize_stack(struct caml_extern_state* s,
                                                struct extern_item * sp)
{
  asize_t newsize = 2 * (s->extern_stack_limit - s->extern_stack);
  asize_t sp_offset = sp - s->extern_stack;
  struct extern_item * newstack;

  if (newsize >= EXTERN_STACK_MAX_SIZE) extern_stack_overflow(s);
  newstack = caml_stat_calloc_noexc(newsize, sizeof(struct extern_item));
  if (newstack == NULL) extern_stack_overflow(s);

  /* Copy items from the old stack to the new stack */
  memcpy (newstack, s->extern_stack,
          sizeof(struct extern_item) * sp_offset);

  /* Free the old stack if it is not the initial stack */
  if (s->extern_stack != s->extern_stack_init)
    caml_stat_free(s->extern_stack);

  s->extern_stack = newstack;
  s->extern_stack_limit = newstack + newsize;
  return newstack + sp_offset;
}

/* Multiplicative Fibonacci hashing
   (Knuth, TAOCP vol 3, section 6.4, page 518).
   HASH_FACTOR is (sqrt(5) - 1) / 2 * 2^wordsize. */
#ifdef ARCH_SIXTYFOUR
#define HASH_FACTOR 11400714819323198486UL
#else
#define HASH_FACTOR 2654435769UL
#endif
#define Hash(v,shift) (((uintnat)(v) * HASH_FACTOR) >> (shift))

/* When the table becomes 2/3 full, its size is increased. */
#define Threshold(sz) (((sz) * 2) / 3)

/* Initialize the position table */

static void extern_init_position_table(struct caml_extern_state* s)
{
  if (s->extern_flags & NO_SHARING) return;
  s->pos_table.size = POS_TABLE_INIT_SIZE;
  s->pos_table.shift = 8 * sizeof(value) - POS_TABLE_INIT_SIZE_LOG2;
  s->pos_table.mask = POS_TABLE_INIT_SIZE - 1;
  s->pos_table.threshold = Threshold(POS_TABLE_INIT_SIZE);
  s->pos_table.present = s->pos_table_present_init;
  s->pos_table.entries = s->pos_table_entries_init;
  memset(s->pos_table_present_init, 0,
         Bitvect_size(POS_TABLE_INIT_SIZE) * sizeof(uintnat));
}

/* Free the position table */

static void extern_free_position_table(struct caml_extern_state* s)
{
  if (s->extern_flags & NO_SHARING) return;
  if (s->pos_table.present != s->pos_table_present_init) {
    caml_stat_free(s->pos_table.present);
    caml_stat_free(s->pos_table.entries);
    /* Protect against repeated calls to extern_free_position_table */
    s->pos_table.present = s->pos_table_present_init;
    s->pos_table.entries = s->pos_table_entries_init;
  }
}

/* Accessing bitvectors */

Caml_inline uintnat bitvect_test(uintnat * bv, uintnat i)
{
  return bv[i / Bits_word] & ((uintnat) 1 << (i & (Bits_word - 1)));
}

Caml_inline void bitvect_set(uintnat * bv, uintnat i)
{
  bv[i / Bits_word] |= ((uintnat) 1 << (i & (Bits_word - 1)));
}

/* Grow the position table */

static void extern_resize_position_table(struct caml_extern_state *s)
{
  mlsize_t new_size, new_byte_size;
  int new_shift;
  uintnat * new_present;
  struct object_position * new_entries;
  uintnat i, h;
  struct position_table old = s->pos_table;

  /* Grow the table quickly (x 8) up to 10^6 entries,
     more slowly (x 2) afterwards. */
  if (old.size < 1000000) {
    new_size = 8 * old.size;
    new_shift = old.shift - 3;
  } else {
    new_size = 2 * old.size;
    new_shift = old.shift - 1;
  }
  if (new_size == 0
      || caml_umul_overflow(new_size, sizeof(struct object_position),
                            &new_byte_size))
    extern_out_of_memory(s);
  new_entries = caml_stat_alloc_noexc(new_byte_size);
  if (new_entries == NULL) extern_out_of_memory(s);
  new_present =
    caml_stat_calloc_noexc(Bitvect_size(new_size), sizeof(uintnat));
  if (new_present == NULL) {
    caml_stat_free(new_entries);
    extern_out_of_memory(s);
  }
  s->pos_table.size = new_size;
  s->pos_table.shift = new_shift;
  s->pos_table.mask = new_size - 1;
  s->pos_table.threshold = Threshold(new_size);
  s->pos_table.present = new_present;
  s->pos_table.entries = new_entries;

  /* Insert every entry of the old table in the new table */
  for (i = 0; i < old.size; i++) {
    if (! bitvect_test(old.present, i)) continue;
    h = Hash(old.entries[i].obj, s->pos_table.shift);
    while (bitvect_test(new_present, h)) {
      h = (h + 1) & s->pos_table.mask;
    }
    bitvect_set(new_present, h);
    new_entries[h] = old.entries[i];
  }

  /* Free the old tables if they are not the initial ones */
  if (old.present != s->pos_table_present_init) {
    caml_stat_free(old.present);
    caml_stat_free(old.entries);
  }
}

/* Determine whether the given object [obj] is in the hash table.
   If so, set [*pos_out] to its position in the output and return 1.
   If not, set [*h_out] to the hash value appropriate for
   [extern_record_location] and return 0. */

Caml_inline int extern_lookup_position(struct caml_extern_state *s, value obj,
                                       uintnat * pos_out, uintnat * h_out)
{
  uintnat h = Hash(obj, s->pos_table.shift);
  while (1) {
    if (! bitvect_test(s->pos_table.present, h)) {
      *h_out = h;
      return 0;
    }
    if (s->pos_table.entries[h].obj == obj) {
      *pos_out = s->pos_table.entries[h].pos;
      return 1;
    }
    h = (h + 1) & s->pos_table.mask;
  }
}

/* Record the output position for the given object [obj]. */
/* The [h] parameter is the index in the hash table where the object
   must be inserted.  It was determined during lookup. */

static void extern_record_location(struct caml_extern_state* s,
                                   value obj, uintnat h)
{
  if (s->extern_flags & NO_SHARING) return;
  bitvect_set(s->pos_table.present, h);
  s->pos_table.entries[h].obj = obj;
  s->pos_table.entries[h].pos = s->obj_counter;
  s->obj_counter++;
  if (s->obj_counter >= s->pos_table.threshold)
    extern_resize_position_table(s);
}

/* To buffer the output */

static void init_extern_output(struct caml_extern_state* s)
{
  s->extern_userprovided_output = NULL;
  s->extern_output_first = caml_stat_alloc_noexc(sizeof(struct output_block));
  if (s->extern_output_first == NULL) caml_raise_out_of_memory();
  s->extern_output_block = s->extern_output_first;
  s->extern_output_block->next = NULL;
  s->extern_ptr = s->extern_output_block->data;
  s->extern_limit = s->extern_output_block->data + SIZE_EXTERN_OUTPUT_BLOCK;
}

static void close_extern_output(struct caml_extern_state* s)
{
  if (s->extern_userprovided_output == NULL){
    s->extern_output_block->end = s->extern_ptr;
  }
}

static void free_extern_output(struct caml_extern_state* s)
{
  struct output_block * blk, * nextblk;

  if (s->extern_userprovided_output == NULL) {
    for (blk = s->extern_output_first; blk != NULL; blk = nextblk) {
      nextblk = blk->next;
      caml_stat_free(blk);
    }
    s->extern_output_first = NULL;
  }
  extern_free_stack(s);
  extern_free_position_table(s);
}

static void grow_extern_output(struct caml_extern_state *s, intnat required)
{
  struct output_block * blk;
  intnat extra;

  if (s->extern_userprovided_output != NULL) {
    extern_failwith(s, "Marshal.to_buffer: buffer overflow");
  }
  s->extern_output_block->end = s->extern_ptr;
  if (required <= SIZE_EXTERN_OUTPUT_BLOCK / 2)
    extra = 0;
  else
    extra = required;
  blk = caml_stat_alloc_noexc(sizeof(struct output_block) + extra);
  if (blk == NULL) extern_out_of_memory(s);
  s->extern_output_block->next = blk;
  s->extern_output_block = blk;
  s->extern_output_block->next = NULL;
  s->extern_ptr = s->extern_output_block->data;
  s->extern_limit =
    s->extern_output_block->data + SIZE_EXTERN_OUTPUT_BLOCK + extra;
}

static intnat extern_output_length(struct caml_extern_state* s)
{
  struct output_block * blk;
  intnat len;

  if (s->extern_userprovided_output != NULL) {
    return s->extern_ptr - s->extern_userprovided_output;
  } else {
    for (len = 0, blk = s->extern_output_first; blk != NULL; blk = blk->next)
      len += blk->end - blk->data;
    return len;
  }
}

/* Exception raising, with cleanup */

static void extern_out_of_memory(struct caml_extern_state* s)
{
  free_extern_output(s);
  caml_raise_out_of_memory();
}

static void extern_invalid_argument(struct caml_extern_state *s, char *msg)
{
  free_extern_output(s);
  caml_invalid_argument(msg);
}

static void extern_failwith(struct caml_extern_state* s, char *msg)
{
  free_extern_output(s);
  caml_failwith(msg);
}

static void extern_stack_overflow(struct caml_extern_state* s)
{
  caml_gc_message (0x04, "Stack overflow in marshaling value\n");
  free_extern_output(s);
  caml_raise_out_of_memory();
}

/* Conversion to big-endian */

Caml_inline void store16(char * dst, int n)
{
  dst[0] = n >> 8;  dst[1] = n;
}

Caml_inline void store32(char * dst, intnat n)
{
  dst[0] = n >> 24;  dst[1] = n >> 16;  dst[2] = n >> 8;  dst[3] = n;
}

Caml_inline void store64(char * dst, int64_t n)
{
  dst[0] = n >> 56;  dst[1] = n >> 48;  dst[2] = n >> 40;  dst[3] = n >> 32;
  dst[4] = n >> 24;  dst[5] = n >> 16;  dst[6] = n >> 8;   dst[7] = n;
}

/* Write characters, integers, and blocks in the output buffer */

Caml_inline void write(struct caml_extern_state* s, int c)
{
  if (s->extern_ptr >= s->extern_limit) grow_extern_output(s, 1);
  *s->extern_ptr++ = c;
}

static void writeblock(struct caml_extern_state* s, const char * data,
                       intnat len)
{
  if (s->extern_ptr + len > s->extern_limit) grow_extern_output(s, len);
  memcpy(s->extern_ptr, data, len);
  s->extern_ptr += len;
}

Caml_inline void writeblock_float8(struct caml_extern_state* s,
                                   const double * data, intnat ndoubles)
{
#if ARCH_FLOAT_ENDIANNESS == 0x01234567 || ARCH_FLOAT_ENDIANNESS == 0x76543210
  writeblock(s, (const char *) data, ndoubles * 8);
#else
  caml_serialize_block_float_8(data, ndoubles);
#endif
}

static void writecode8(struct caml_extern_state* s,
                       int code, intnat val)
{
  if (s->extern_ptr + 2 > s->extern_limit) grow_extern_output(s, 2);
  s->extern_ptr[0] = code;
  s->extern_ptr[1] = val;
  s->extern_ptr += 2;
}

static void writecode16(struct caml_extern_state* s,
                        int code, intnat val)
{
  if (s->extern_ptr + 3 > s->extern_limit) grow_extern_output(s, 3);
  s->extern_ptr[0] = code;
  store16(s->extern_ptr + 1, (int) val);
  s->extern_ptr += 3;
}

static void writecode32(struct caml_extern_state* s,
                        int code, intnat val)
{
  if (s->extern_ptr + 5 > s->extern_limit) grow_extern_output(s, 5);
  s->extern_ptr[0] = code;
  store32(s->extern_ptr + 1, val);
  s->extern_ptr += 5;
}

#ifdef ARCH_SIXTYFOUR
static void writecode64(struct caml_extern_state* s,
                        int code, intnat val)
{
  if (s->extern_ptr + 9 > s->extern_limit) grow_extern_output(s, 9);
  s->extern_ptr[0] = code;
  store64(s->extern_ptr + 1, val);
  s->extern_ptr += 9;
}
#endif

/* Marshaling integers */

Caml_inline void extern_int(struct caml_extern_state* s, intnat n)
{
  if (n >= 0 && n < 0x40) {
    write(s, PREFIX_SMALL_INT + n);
  } else if (n >= -(1 << 7) && n < (1 << 7)) {
    writecode8(s, CODE_INT8, n);
  } else if (n >= -(1 << 15) && n < (1 << 15)) {
    writecode16(s, CODE_INT16, n);
#ifdef ARCH_SIXTYFOUR
  } else if (n < -((intnat)1 << 30) || n >= ((intnat)1 << 30)) {
      if (s->extern_flags & COMPAT_32)
        extern_failwith(s, "output_value: integer cannot be read back on "
                        "32-bit platform");
      writecode64(s, CODE_INT64, n);
#endif
  } else {
    writecode32(s, CODE_INT32, n);
  }
}

/* Marshaling references to previously-marshaled blocks */

Caml_inline void extern_shared_reference(struct caml_extern_state* s,
                                         uintnat d)
{
  if (d < 0x100) {
    writecode8(s, CODE_SHARED8, d);
  } else if (d < 0x10000) {
    writecode16(s, CODE_SHARED16, d);
#ifdef ARCH_SIXTYFOUR
  } else if (d >= (uintnat)1 << 32) {
    writecode64(s, CODE_SHARED64, d);
#endif
  } else {
    writecode32(s, CODE_SHARED32, d);
  }
}

/* Marshaling block headers */

Caml_inline void extern_header(struct caml_extern_state* s,
                               mlsize_t sz, tag_t tag)
{
  if (tag < 16 && sz < 8) {
    write(s, PREFIX_SMALL_BLOCK + tag + (sz << 4));
  } else {
    header_t hd = Make_header(sz, tag, NOT_MARKABLE);
#ifdef ARCH_SIXTYFOUR
    if (sz > 0x3FFFFF && (s->extern_flags & COMPAT_32))
      extern_failwith(s, "output_value: array cannot be read back on "
                      "32-bit platform");
    if (hd < (uintnat)1 << 32)
      writecode32(s, CODE_BLOCK32, hd);
    else
      writecode64(s, CODE_BLOCK64, hd);
#else
    writecode32(s, CODE_BLOCK32, hd);
#endif
  }
}

/* Marshaling strings */

Caml_inline void extern_string(struct caml_extern_state *s,
                               value v, mlsize_t len)
{
  if (len < 0x20) {
    write(s, PREFIX_SMALL_STRING + len);
  } else if (len < 0x100) {
    writecode8(s, CODE_STRING8, len);
  } else {
#ifdef ARCH_SIXTYFOUR
    if (len > 0xFFFFFB && (s->extern_flags & COMPAT_32))
      extern_failwith(s, "output_value: string cannot be read back on "
                      "32-bit platform");
    if (len < (uintnat)1 << 32)
      writecode32(s, CODE_STRING32, len);
    else
      writecode64(s, CODE_STRING64, len);
#else
    writecode32(s, CODE_STRING32, len);
#endif
  }
  writeblock(s, String_val(v), len);
}

/* Marshaling FP numbers */

Caml_inline void extern_double(struct caml_extern_state* s, value v)
{
  write(s, CODE_DOUBLE_NATIVE);
  writeblock_float8(s, (double *) v, 1);
}

/* Marshaling FP arrays */

Caml_inline void extern_double_array(struct caml_extern_state* s,
                                     value v, mlsize_t nfloats)
{
  if (nfloats < 0x100) {
    writecode8(s, CODE_DOUBLE_ARRAY8_NATIVE, nfloats);
  } else {
#ifdef ARCH_SIXTYFOUR
    if (nfloats > 0x1FFFFF && (s->extern_flags & COMPAT_32))
      extern_failwith(s, "output_value: float array cannot be read back on "
                      "32-bit platform");
    if (nfloats < (uintnat) 1 << 32)
      writecode32(s, CODE_DOUBLE_ARRAY32_NATIVE, nfloats);
    else
      writecode64(s, CODE_DOUBLE_ARRAY64_NATIVE, nfloats);
#else
    writecode32(s, CODE_DOUBLE_ARRAY32_NATIVE, nfloats);
#endif
  }
  writeblock_float8(s, (double *) v, nfloats);
}

/* Marshaling custom blocks */

Caml_inline void extern_custom(struct caml_extern_state* s, value v,
                               /*out*/ uintnat * sz_32,
                               /*out*/ uintnat * sz_64)
{
  char * size_header;
  char const * ident = Custom_ops_val(v)->identifier;
  void (*serialize)(value v, uintnat * bsize_32, uintnat * bsize_64)
        = Custom_ops_val(v)->serialize;
  const struct custom_fixed_length* fixed_length
        = Custom_ops_val(v)->fixed_length;
  if (serialize == NULL)
    extern_invalid_argument(s, "output_value: abstract value (Custom)");
  if (fixed_length == NULL) {
    write(s, CODE_CUSTOM_LEN);
    writeblock(s, ident, strlen(ident) + 1);
    /* Reserve 12 bytes for the lengths (sz_32 and sz_64). */
    if (s->extern_ptr + 12 >= s->extern_limit) grow_extern_output(s, 12);
    size_header = s->extern_ptr;
    s->extern_ptr += 12;
    serialize(v, sz_32, sz_64);
    /* Store length before serialized block */
    store32(size_header, *sz_32);
    store64(size_header + 4, *sz_64);
  } else {
    write(s, CODE_CUSTOM_FIXED);
    writeblock(s, ident, strlen(ident) + 1);
    serialize(v, sz_32, sz_64);
        if (*sz_32 != fixed_length->bsize_32 ||
            *sz_64 != fixed_length->bsize_64)
          caml_fatal_error(
            "output_value: incorrect fixed sizes specified by %s",
            ident);
  }
}

/* Marshaling code pointers */

static void extern_code_pointer(struct caml_extern_state* s, char * codeptr)
{
  struct code_fragment * cf;
  const char * digest;

  cf = caml_find_code_fragment_by_pc(codeptr);
  if (cf != NULL) {
    if ((s->extern_flags & CLOSURES) == 0)
      extern_invalid_argument(s, "output_value: functional value");
    digest = (const char *) caml_digest_of_code_fragment(cf);
    if (digest == NULL)
      extern_invalid_argument(s, "output_value: private function");
    CAMLassert(cf == caml_find_code_fragment_by_digest((unsigned char*)digest));
    writecode32(s, CODE_CODEPOINTER, codeptr - cf->code_start);
    writeblock(s, digest, 16);
  } else {
    extern_invalid_argument(s, "output_value: abstract value (outside heap)");
  }
}

/* Marshaling the non-environment part of closures */

Caml_inline mlsize_t extern_closure_up_to_env(struct caml_extern_state* s,
                                              value v)
{
  mlsize_t startenv, i;
  value info;

  startenv = Start_env_closinfo(Closinfo_val(v));
  i = 0;
  do {
    /* The infix header */
    if (i > 0) extern_int(s, Long_val(Field(v, i++)));
    /* The default entry point */
    extern_code_pointer(s, (char *) Field(v, i++));
    /* The closure info. */
    info = Field(v, i++);
    extern_int(s, Long_val(info));
    /* The direct entry point if arity is neither 0 nor 1 */
    if (Arity_closinfo(info) != 0 && Arity_closinfo(info) != 1) {
      extern_code_pointer(s, (char *) Field(v, i++));
    }
  } while (i < startenv);
  CAMLassert(i == startenv);
  return startenv;
}

/* Marshal the given value in the output buffer */

static void extern_rec(struct caml_extern_state* s, value v)
{
  struct extern_item * sp;
  uintnat h = 0;
  uintnat pos = 0;

  extern_init_position_table(s);
  sp = s->extern_stack;

  while(1) {
  if (Is_long(v)) {
    extern_int(s, Long_val(v));
  }
  else {
    header_t hd = Hd_val(v);
    tag_t tag = Tag_hd(hd);
    mlsize_t sz = Wosize_hd(hd);

    if (tag == Forward_tag) {
      value f = Forward_val (v);
      if (Is_block (f)
          && (   Tag_val (f) == Forward_tag
              || Tag_val (f) == Lazy_tag
              || Tag_val (f) == Forcing_tag
#ifdef FLAT_FLOAT_ARRAY
              || Tag_val (f) == Double_tag
#endif
              )){
        /* Do not short-circuit the pointer. */
      }else{
        v = f;
        continue;
      }
    }
    /* Atoms are treated specially for two reasons: they are not allocated
       in the externed block, and they are automatically shared. */
    if (sz == 0) {
      extern_header(s, 0, tag);
      goto next_item;
    }
    /* Check if object already seen */
    if (! (s->extern_flags & NO_SHARING)) {
      if (extern_lookup_position(s, v, &pos, &h)) {
        extern_shared_reference(s, s->obj_counter - pos);
        goto next_item;
      }
    }
    /* Output the contents of the object */
    switch(tag) {
    case String_tag: {
      mlsize_t len = caml_string_length(v);
      extern_string(s, v, len);
      s->size_32 += 1 + (len + 4) / 4;
      s->size_64 += 1 + (len + 8) / 8;
      extern_record_location(s, v, h);
      break;
    }
    case Double_tag: {
      CAMLassert(sizeof(double) == 8);
      extern_double(s, v);
      s->size_32 += 1 + 2;
      s->size_64 += 1 + 1;
      extern_record_location(s, v, h);
      break;
    }
    case Double_array_tag: {
      mlsize_t nfloats;
      CAMLassert(sizeof(double) == 8);
      nfloats = Wosize_val(v) / Double_wosize;
      extern_double_array(s, v, nfloats);
      s->size_32 += 1 + nfloats * 2;
      s->size_64 += 1 + nfloats;
      extern_record_location(s, v, h);
      break;
    }
    case Abstract_tag:
      extern_invalid_argument(s, "output_value: abstract value (Abstract)");
      break;
    case Infix_tag:
      writecode32(s, CODE_INFIXPOINTER, Infix_offset_hd(hd));
      v = v - Infix_offset_hd(hd); /* PR#5772 */
      continue;
    case Custom_tag: {
      uintnat sz_32, sz_64;
      extern_custom(s, v, &sz_32, &sz_64);
      s->size_32 += 2 + ((sz_32 + 3) >> 2);  /* header + ops + data */
      s->size_64 += 2 + ((sz_64 + 7) >> 3);
      extern_record_location(s, v, h);
      break;
    }
    case Closure_tag: {
      mlsize_t i;
      extern_header(s, sz, tag);
      s->size_32 += 1 + sz;
      s->size_64 += 1 + sz;
      extern_record_location(s, v, h);
      i = extern_closure_up_to_env(s, v);
      if (i >= sz) goto next_item;
      /* Remember that we still have to serialize fields i + 1 ... sz - 1 */
      if (i < sz - 1) {
        sp++;
        if (sp >= s->extern_stack_limit)
          sp = extern_resize_stack(s, sp);
        sp->v = &Field(v, i + 1);
        sp->count = sz - i - 1;
      }
      /* Continue serialization with the first environment field */
      v = Field(v, i);
      continue;
    }
    default: {
      extern_header(s, sz, tag);
      s->size_32 += 1 + sz;
      s->size_64 += 1 + sz;
      extern_record_location(s, v, h);
      /* Remember that we still have to serialize fields 1 ... sz - 1 */
      if (sz > 1) {
        sp++;
        if (sp >= s->extern_stack_limit)
          sp = extern_resize_stack(s, sp);
        sp->v = &Field(v, 1);
        sp->count = sz - 1;
      }
      /* Continue serialization with the first field */
      v = Field(v, 0);
      continue;
    }
    }
  }
  next_item:
    /* Pop one more item to marshal, if any */
    if (sp == s->extern_stack) {
        /* We are done.   Cleanup the stack and leave the function */
        extern_free_stack(s);
        extern_free_position_table(s);
        return;
    }
    v = *((sp->v)++);
    if (--(sp->count) == 0) sp--;
  }
  /* Never reached as function leaves with return */
}

static int extern_flag_values[] = { NO_SHARING, CLOSURES, COMPAT_32 };

static intnat extern_value(struct caml_extern_state* s, value v, value flags,
                           /*out*/ char header[32],
                           /*out*/ int * header_len)
{
  intnat res_len;
  /* Parse flag list */
  s->extern_flags = caml_convert_flag_list(flags, extern_flag_values);
  /* Initializations */
  s->obj_counter = 0;
  s->size_32 = 0;
  s->size_64 = 0;
  /* Marshal the object */
  extern_rec(s, v);
  /* Record end of output */
  close_extern_output(s);
  /* Write the header */
  res_len = extern_output_length(s);
#ifdef ARCH_SIXTYFOUR
  if (res_len >= ((intnat)1 << 32) ||
      s->size_32 >= ((intnat)1 << 32) || s->size_64 >= ((intnat)1 << 32)) {
    /* The object is too big for the small header format.
       Fail if we are in compat32 mode, or use big header. */
    if (s->extern_flags & COMPAT_32) {
      free_extern_output(s);
      caml_failwith("output_value: object too big to be read back on "
                    "32-bit platform");
    }
    store32(header, Intext_magic_number_big);
    store32(header + 4, 0);
    store64(header + 8, res_len);
    store64(header + 16, s->obj_counter);
    store64(header + 24, s->size_64);
    *header_len = 32;
    return res_len;
  }
#endif
  /* Use the small header format */
  store32(header, Intext_magic_number_small);
  store32(header + 4, res_len);
  store32(header + 8, s->obj_counter);
  store32(header + 12, s->size_32);
  store32(header + 16, s->size_64);
  *header_len = 20;
  return res_len;
}

void caml_output_val(struct channel *chan, value v, value flags)
{
  char header[32];
  int header_len;
  struct output_block * blk, * nextblk;
  struct caml_extern_state* s = get_extern_state ();

  if (! caml_channel_binary_mode(chan))
    caml_failwith("output_value: not a binary channel");
  init_extern_output(s);
  extern_value(s, v, flags, header, &header_len);
  /* During [caml_really_putblock], concurrent [caml_output_val] operations
     can take place (via signal handlers or context switching in systhreads),
     and [extern_output_first] may change. So, save it in a local variable. */
  blk = s->extern_output_first;
  caml_really_putblock(chan, header, header_len);
  while (blk != NULL) {
    caml_really_putblock(chan, blk->data, blk->end - blk->data);
    nextblk = blk->next;
    caml_stat_free(blk);
    blk = nextblk;
  }
  Flush_if_unbuffered(chan);
}

CAMLprim value caml_output_value(value vchan, value v, value flags)
{
  CAMLparam3 (vchan, v, flags);
  struct channel * channel = Channel(vchan);

  Lock(channel);
  caml_output_val(channel, v, flags);
  Unlock(channel);
  CAMLreturn (Val_unit);
}

CAMLprim value caml_output_value_to_bytes(value v, value flags)
{
  char header[32];
  int header_len;
  intnat data_len, ofs;
  value res;
  struct output_block * blk, * nextblk;
  struct caml_extern_state* s = get_extern_state ();

  init_extern_output(s);
  data_len = extern_value(s, v, flags, header, &header_len);
  /* PR#4030: it is prudent to save extern_output_first before allocating
     the result, as in caml_output_val */
  blk = s->extern_output_first;
  res = caml_alloc_string(header_len + data_len);
  ofs = 0;
  memcpy(&Byte(res, ofs), header, header_len);
  ofs += header_len;
  while (blk != NULL) {
    intnat n = blk->end - blk->data;
    memcpy(&Byte(res, ofs), blk->data, n);
    ofs += n;
    nextblk = blk->next;
    caml_stat_free(blk);
    blk = nextblk;
  }
  return res;
}

CAMLprim value caml_output_value_to_string(value v, value flags)
{
  return caml_output_value_to_bytes(v,flags);
}

CAMLexport intnat caml_output_value_to_block(value v, value flags,
                                             char * buf, intnat len)
{
  char header[32];
  int header_len;
  intnat data_len;
  struct caml_extern_state* s = get_extern_state ();

  /* At this point we don't know the size of the header.
     Guess that it is small, and fix up later if not. */
  s->extern_userprovided_output = buf + 20;
  s->extern_ptr = s->extern_userprovided_output;
  s->extern_limit = buf + len;
  data_len = extern_value(s, v, flags, header, &header_len);
  if (header_len != 20) {
    /* Bad guess!  Need to shift the output to make room for big header.
       Make sure there is room. */
    if (header_len + data_len > len)
      caml_failwith("Marshal.to_buffer: buffer overflow");
    memmove(buf + header_len, buf + 20, data_len);
  }
  memcpy(buf, header, header_len);
  return header_len + data_len;
}

CAMLprim value caml_output_value_to_buffer(value buf, value ofs, value len,
                                           value v, value flags)
{
  intnat l =
    caml_output_value_to_block(v, flags,
                               &Byte(buf, Long_val(ofs)), Long_val(len));
  return Val_long(l);
}

CAMLexport void caml_output_value_to_malloc(value v, value flags,
                                            /*out*/ char ** buf,
                                            /*out*/ intnat * len)
{
  char header[32];
  int header_len;
  intnat data_len;
  char * res;
  struct output_block * blk, * nextblk;
  struct caml_extern_state* s = get_extern_state ();

  init_extern_output(s);
  data_len = extern_value(s, v, flags, header, &header_len);
  res = caml_stat_alloc_noexc(header_len + data_len);
  if (res == NULL) extern_out_of_memory(s);
  *buf = res;
  *len = header_len + data_len;
  memcpy(res, header, header_len);
  res += header_len;
  for (blk = s->extern_output_first; blk != NULL; blk = nextblk) {
    intnat n = blk->end - blk->data;
    memcpy(res, blk->data, n);
    res += n;
    nextblk = blk->next;
    caml_stat_free(blk);
  }
}

/* Functions for writing user-defined marshallers */

CAMLexport void caml_serialize_int_1(int i)
{
  struct caml_extern_state* s = get_extern_state ();
  if (s->extern_ptr + 1 > s->extern_limit) grow_extern_output(s, 1);
  s->extern_ptr[0] = i;
  s->extern_ptr += 1;
}

CAMLexport void caml_serialize_int_2(int i)
{
  struct caml_extern_state* s = get_extern_state ();
  if (s->extern_ptr + 2 > s->extern_limit) grow_extern_output(s, 2);
  store16(s->extern_ptr, i);
  s->extern_ptr += 2;
}

CAMLexport void caml_serialize_int_4(int32_t i)
{
  struct caml_extern_state* s = get_extern_state ();
  if (s->extern_ptr + 4 > s->extern_limit) grow_extern_output(s, 4);
  store32(s->extern_ptr, i);
  s->extern_ptr += 4;
}

CAMLexport void caml_serialize_int_8(int64_t i)
{
  struct caml_extern_state* s = get_extern_state ();
  if (s->extern_ptr + 8 > s->extern_limit) grow_extern_output(s, 8);
  store64(s->extern_ptr, i);
  s->extern_ptr += 8;
}

CAMLexport void caml_serialize_float_4(float f)
{
  caml_serialize_block_4(&f, 1);
}

CAMLexport void caml_serialize_float_8(double f)
{
  caml_serialize_block_float_8(&f, 1);
}

CAMLexport void caml_serialize_block_1(void * data, intnat len)
{
  struct caml_extern_state* s = get_extern_state ();
  if (s->extern_ptr + len > s->extern_limit) grow_extern_output(s, len);
  memcpy(s->extern_ptr, data, len);
  s->extern_ptr += len;
}

CAMLexport void caml_serialize_block_2(void * data, intnat len)
{
  struct caml_extern_state* s = get_extern_state ();
  if (s->extern_ptr + 2 * len > s->extern_limit)
    grow_extern_output(s, 2 * len);
#ifndef ARCH_BIG_ENDIAN
  {
    unsigned char * p;
    char * q;
    for (p = data, q = s->extern_ptr; len > 0; len--, p += 2, q += 2)
      Reverse_16(q, p);
    s->extern_ptr = q;
  }
#else
  memcpy(s->extern_ptr, data, len * 2);
  s->extern_ptr += len * 2;
#endif
}

CAMLexport void caml_serialize_block_4(void * data, intnat len)
{
  struct caml_extern_state* s = get_extern_state ();
  if (s->extern_ptr + 4 * len > s->extern_limit)
    grow_extern_output(s, 4 * len);
#ifndef ARCH_BIG_ENDIAN
  {
    unsigned char * p;
    char * q;
    for (p = data, q = s->extern_ptr; len > 0; len--, p += 4, q += 4)
      Reverse_32(q, p);
    s->extern_ptr = q;
  }
#else
  memcpy(s->extern_ptr, data, len * 4);
  s->extern_ptr += len * 4;
#endif
}

CAMLexport void caml_serialize_block_8(void * data, intnat len)
{
  struct caml_extern_state* s = get_extern_state ();
  if (s->extern_ptr + 8 * len > s->extern_limit)
    grow_extern_output(s, 8 * len);
#ifndef ARCH_BIG_ENDIAN
  {
    unsigned char * p;
    char * q;
    for (p = data, q = s->extern_ptr; len > 0; len--, p += 8, q += 8)
      Reverse_64(q, p);
    s->extern_ptr = q;
  }
#else
  memcpy(s->extern_ptr, data, len * 8);
  s->extern_ptr += len * 8;
#endif
}

CAMLexport void caml_serialize_block_float_8(void * data, intnat len)
{
  struct caml_extern_state* s = get_extern_state ();
  if (s->extern_ptr + 8 * len > s->extern_limit) grow_extern_output(s, 8 * len);
#if ARCH_FLOAT_ENDIANNESS == 0x01234567
  memcpy(s->extern_ptr, data, len * 8);
  s->extern_ptr += len * 8;
#elif ARCH_FLOAT_ENDIANNESS == 0x76543210
  {
    unsigned char * p;
    char * q;
    for (p = data, q = s->extern_ptr; len > 0; len--, p += 8, q += 8)
      Reverse_64(q, p);
    s->extern_ptr = q;
  }
#else
  {
    unsigned char * p;
    char * q;
    for (p = data, q = s->extern_ptr; len > 0; len--, p += 8, q += 8)
      Permute_64(q, 0x01234567, p, ARCH_FLOAT_ENDIANNESS);
    s->extern_ptr = q;
  }
#endif
}

CAMLprim value caml_obj_reachable_words(value v)
{
  intnat size;
  struct extern_item * sp;
  uintnat h = 0;
  uintnat pos = 0;
  struct caml_extern_state *s = get_extern_state ();

  s->obj_counter = 0;
  s->extern_flags = 0;
  extern_init_position_table(s);
  sp = s->extern_stack;
  size = 0;

  /* In Multicore OCaml, we don't distinguish between major heap blocks and
   * out-of-heap blocks, so we end up counting out-of-heap blocks too. */
  while (1) {
    if (Is_long(v)) {
      /* Tagged integers contribute 0 to the size, nothing to do */
    } else if (extern_lookup_position(s, v, &pos, &h)) {
      /* Already seen and counted, nothing to do */
    } else {
      header_t hd = Hd_val(v);
      tag_t tag = Tag_hd(hd);
      mlsize_t sz = Wosize_hd(hd);
      /* Infix pointer: go back to containing closure */
      if (tag == Infix_tag) {
        v = v - Infix_offset_hd(hd);
        continue;
      }
      /* Remember that we've visited this block */
      extern_record_location(s, v, h);
      /* The block contributes to the total size */
      size += 1 + sz;           /* header word included */
      if (tag < No_scan_tag) {
        /* i is the position of the first field to traverse recursively */
        uintnat i =
          tag == Closure_tag ? Start_env_closinfo(Closinfo_val(v)) : 0;
        if (i < sz) {
          if (i < sz - 1) {
            /* Remember that we need to count fields i + 1 ... sz - 1 */
            sp++;
            if (sp >= s->extern_stack_limit)
              sp = extern_resize_stack(s, sp);
            sp->v = &Field(v, i + 1);
            sp->count = sz - i - 1;
          }
          /* Continue with field i */
          v = Field(v, i);
          continue;
        }
      }
    }
    /* Pop one more item to traverse, if any */
    if (sp == s->extern_stack) break;
    v = *((sp->v)++);
    if (--(sp->count) == 0) sp--;
  }
  extern_free_stack(s);
  extern_free_position_table(s);
  return Val_long(size);
}
