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gc.c

// This file is part of Oaklisp.
//
// This program is free software; you can redistribute it and/or
// modify it under the terms of the GNU General Public License as
// published by the Free Software Foundation; either version 2 of the
// License, or (at your option) any later version.
//
// This program 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 General Public License for more details.
//
// The GNU GPL is available at http://www.gnu.org/licenses/gpl.html
// or from the Free Software Foundation, 59 Temple Place - Suite 330,
// Boston, MA 02111-1307, USA


/**********************************************************************
 *  Copyright (c) by Barak A. Pearlmutter and Kevin J. Lang, 1987-99. *
 *  Copyright (c) by Alex Stuebinger, 1998-99.                        *
 *  Distributed under the GNU General Public License v2 or later      *
 **********************************************************************/

#define _REENTRANT

#include <stddef.h>
#include <stdlib.h>
#include <stdio.h>
#include "data.h"
#include "weak.h"
#include "xmalloc.h"
#include "stacks.h"
#include "gc.h"


#ifdef USE_VADVISE
#include <sys/vadvise.h>
#endif


#define FORTHREADS THREADY( for (my_index=0; my_index<next_index; my_index++) )



/* 1/RECLAIM_FACTOR is the target for how much of new space should 
   be used after a gc.  If more than this is used, the next new 
   space allocated will be bigger. */

#define RECLAIM_FACTOR 3

bool full_gc = false;

ref_t pre_gc_nil;

unsigned long transport_count;
unsigned long loc_transport_count;

#ifndef GC_EXAMINE_BUFFER_SIZE
#define GC_EXAMINE_BUFFER_SIZE 16
#endif

#ifdef THREADS
ref_t gc_examine_buffer_array[MAX_THREAD_COUNT][GC_EXAMINE_BUFFER_SIZE];
ref_t *gc_examine_ptr_array[MAX_THREAD_COUNT];
#else
ref_t gc_examine_buffer[GC_EXAMINE_BUFFER_SIZE];
ref_t *gc_examine_ptr = gc_examine_buffer;
#endif

#define GC_TOUCH(x)                 \
{                             \
  if ((x)&PTR_MASK)                 \
    {                         \
      ref_t *MACROp = ANY_TO_PTR((x));    \
                              \
      if (OLD_PTR(MACROp))          \
      (x) = gc_touch0((x));         \
    }                         \
}

#define GC_TOUCH_PTR(r,o)                             \
{                                               \
  (r) = REF_TO_PTR(gc_touch0(PTR_TO_REF((r)-(o)))) + (o);   \
}



#define LOC_TOUCH(x)                      \
{                                   \
  if (TAG_IS((x),LOC_TAG))                \
    {                               \
      ref_t *MACROp = LOC_TO_PTR((x));          \
                                    \
      if (OLD_PTR(MACROp))                \
      (x)=loc_touch0((x),0);              \
    }                               \
}

#define LOC_TOUCH_PTR(x)                        \
{                                         \
  (x) = LOC_TO_PTR(loc_touch0(PTR_TO_LOC(x),1));      \
}

void
printref(FILE * fd, ref_t refin)
{
  long i;
  char suffix = '?';

  if (refin & PTR_MASK)
    {
      ref_t *p = ANY_TO_PTR(refin);

      if (SPATIC_PTR(p))
      {
        i = p - spatic.start;
        suffix = 's';
      }
      else if (NEW_PTR(p))
      {
        i = p - new_space.start + spatic.size;
        suffix = 'n';
      }
      else if (OLD_PTR(p))
      {
        i = p - old_space.start + spatic.size;
        suffix = 'o';
      }
      else
      i = (long)p >> 2;

      fprintf(fd, "[%ld;tag:%d;%c]", i, refin & TAG_MASK, suffix);
    }
  else
    fprintf(fd, "[%ld;tag:%d]", (long)(refin >> 2), refin & TAG_MASK);
}

#define GC_NULL(r) ((r)==pre_gc_nil || (r)==e_nil)


/* This variant of get_length has to follow forwarding pointers so
   that it will work in the middle of a gc, when an object's type might
   already have been transported. */

static unsigned long
gc_get_length(ref_t x)
{
  if (TAG_IS(x, PTR_TAG))
    {
      ref_t typ = REF_SLOT(x, 0);
      ref_t vlen_p = REF_SLOT(typ, TYPE_VAR_LEN_P_OFF);
      ref_t len;

      /* Is vlen_p forwarded? */
      if (TAG_IS(vlen_p, LOC_TAG))
      vlen_p = *LOC_TO_PTR(vlen_p);

      /* Is this object variable length? */
      if (GC_NULL(vlen_p))
      {
        /* Not variable length. */
        len = REF_SLOT(typ, TYPE_LEN_OFF);

        /* Is length forwarded? */
        if (TAG_IS(len, LOC_TAG))
          len = *LOC_TO_PTR(len);

        return REF_TO_INT(len);
      }
      else
      return REF_TO_INT(REF_SLOT(x, 1));
    }
  else
    {
      fprintf(stderr, "; WARNING!!!  gc_get_length(");
      printref(stderr, x);
      fprintf(stderr, ") called; only a tag of %d is allowed.\n", PTR_TAG);
      return 0;
    }
}


static ref_t
gc_touch0(ref_t r)
{
  ref_t *p = ANY_TO_PTR(r);

  if (OLD_PTR(p))
    if (r & 1)
      {
      ref_t type_slot = *p;
      if (TAG_IS(type_slot, LOC_TAG))
        /* Already been transported. */
        /* Tag magic transforms this:
           return(PTR_TO_REF(LOC_TO_PTR(type_slot)));
           to this: */
        return type_slot | 1L;
      else
        {
          /* Transport it */
          long i;
          long len = gc_get_length(r);
          ref_t *new_place = free_point;
          ref_t *p0 = p;
          ref_t *q0 = new_place;

          transport_count += 1;

          /*
             fprintf(stderr, "About to transport ");
             printref(r);
             fprintf(stderr, " len = %ld.\n", len);
           */

          free_point += len;

#ifndef FAST
          if (free_point >= new_space.end)
            {
            fprintf(stderr,
                  "\n; New space exhausted while transporting ");
            printref(stderr, r);
            fprintf(stderr,
                  ".\n; This indicates a bug in the garbage collector.\n");
            exit(EXIT_FAILURE);
            }
#endif
          for (i = 0; i < len; i++, p0++, q0++)
            {
            *q0 = *p0;
            *p0 = PTR_TO_LOC(q0);
            }

          return (PTR_TO_REF(new_place));
        }
      }
    else
      {
      /* Follow the chain of locatives to oldspace until we find a
         real object or a circularity. */
      ref_t r0 = r, r1 = *p, *pp;
      /* int chain_len = 1; */

      while (TAG_IS(r1, LOC_TAG) && (pp = LOC_TO_PTR(r1), OLD_PTR(pp)))
        {
          if (r0 == r1)
            {
            /* fprintf(stderr, "Circular locative chain.\n"); */
            goto forwarded_loc;
            }
          r0 = *LOC_TO_PTR(r0);
          r1 = *pp;
          /* chain_len += 1; */

          if (r0 == r1)
            {
            /* fprintf(stderr, "Circular locative chain.\n"); */
            goto forwarded_loc;
            }
          if (!TAG_IS(r1, LOC_TAG) || (pp = LOC_TO_PTR(r1), !OLD_PTR(pp)))
            break;

          r1 = *pp;
          /* chain_len += 1; */
        }

      /* We're on an object, so touch it. */
      /*
         fprintf(stderr, "Locative chain followed to ");
         printref(r1);
         fprintf(stderr, " requiring %d dereferences.\n", chain_len);
       */
      GC_TOUCH(r1);
      /* (void)gc_touch(r1); */

      /* Now see if we're looking at a forwarding pointer. */
      forwarded_loc:
      return (r);
      }
  else
    return (r);
}

static ref_t
loc_touch0(ref_t r, bool warn_if_unmoved)
{
  ref_t *p = LOC_TO_PTR(r);

  if (OLD_PTR(p))
    {
      /* A locative into old space.  See if it's been transported yet. */
      ref_t r1 = *p;
      if (TAG_IS(r1, LOC_TAG) && NEW_PTR(LOC_TO_PTR(r1)))
      /* Already been transported. */
      return (r1);
      else
      {
        /* Better transport this lonely cell. */

        ref_t *new_place = free_point++;  /* make a new cell. */
        ref_t new_r = PTR_TO_LOC(new_place);

#ifndef FAST
        if (free_point >= new_space.end)
          {
            fprintf(stderr,
                  "\n; New space exhausted while transporting the cell ");
            printref(stderr, r);
            fprintf(stderr,
                  ".\n; This indicates a bug in the garbage collector.\n");
            exit(EXIT_FAILURE);
          }
#endif
        *p = new_r;           /* Record the transportation. */

        /* Put the right value in the new cell. */

        *new_place =
          TAG_IS(r1, PTR_TAG) && (p = REF_TO_PTR(r1), OLD_PTR(p))
          ? *p | 1 : r1;
        /* ? PTR_TO_REF(REF_TO_PTR(*p)) : r1; */

        loc_transport_count += 1;

        if (warn_if_unmoved)
          {
            fprintf(stderr, "\nWarning: the locative ");
            printref(stderr, r);
            fprintf(stderr, " has just had its raw cell moved.\n");
          }
        return (new_r);
      }
    }
  else
    return (r);               /* Not a locative into old space. */
}


static void
scavenge(void)
{
  ref_t *scavenge_p;

  for (scavenge_p = new_space.start; scavenge_p < free_point; scavenge_p += 1)
    GC_TOUCH(*scavenge_p);
}

static void
loc_scavenge(void)
{
  ref_t *scavenge_p;

  for (scavenge_p = new_space.start; scavenge_p < free_point; scavenge_p += 1)
    LOC_TOUCH(*scavenge_p);
}

#ifndef FAST
/* This set of routines are for consistency checks */

#define GGC_CHECK(r) GC_CHECK(r,"r")

/* True if r seems like a messed up reference. */
static bool
gc_check_(ref_t r)
{
  return (r & PTR_MASK) && !NEW_PTR(ANY_TO_PTR(r))
    && (full_gc || !SPATIC_PTR(ANY_TO_PTR(r)));
}

static void
GC_CHECK(ref_t x, char *st)
{
  if (gc_check_(x))
    {
      fprintf(stderr, "%s = ", st);
      printref(stderr, x);
      if (OLD_PTR(ANY_TO_PTR(x)))
      {
        fprintf(stderr, ",  cell contains ");
        printref(stderr, *ANY_TO_PTR(x));
      }
      fprintf(stderr, "\n");
    }
}

static void
GC_CHECK1(ref_t x, char *st, long i)
{
  if (gc_check_((x)))
    {
      fprintf(stderr, (st), (i));
      printref(stderr, x);
      if (OLD_PTR(ANY_TO_PTR(x)))
      {
        fprintf(stderr, ",  cell contains ");
        printref(stderr, *ANY_TO_PTR(x));
      }
      fprintf(stderr, "\n");
    }
}
#endif


static u_int16_t *
pc_touch(u_int16_t * o_pc)
{
  ref_t *pcell = (ref_t *) ((unsigned long)o_pc & ~TAG_MASKL);

  /*
    It is possible that the gc was called while a vm was executing the last
    instruction in a code block (hopefully a branch or funcall) in
    a multithreaded enviornment.  So let's back up the pc one before gc'ing
    it.  However, this means the gc generally should not be called until the
    loop has read at least one instruction in the code block.
  */
  /* pcell--;  Changed my mind.  Moved POLL_GC_SIGNALS to top of loop. */
  LOC_TOUCH_PTR(pcell);
  /* pcell++; */

  return
    (u_int16_t *) ((u_int32_t) pcell
               | ((u_int32_t) o_pc & TAG_MASK));
}

static void
set_external_full_gc(bool full)
{
  full_gc = full;
}

void
gc(bool pre_dump, bool full_gc, char *reason, size_t amount)
/*
 *     pre_dump        About to dump world?  (discards stacks)
 *     full_gc         Reclaim garbage from spatic space too?
 *     reason          The reason for this GC, human readable.
 *     amount          The amount of space that is needed.
 */
{
  long old_taken;
  long old_spatic_taken;
  ref_t *p;
#ifdef THREADS
  bool ready=false;
  int my_index;
  int i;
  int *my_index_p;
  my_index_p = pthread_getspecific (index_key);
  my_index = *my_index_p;
  gc_ready[my_index] = 1;
  set_gc_flag (true);
#endif

#ifdef THREADS
  /*Problem here is next_index could change if someone creates a thread
    while someone else is gc'ing*/
   while (ready == false) {
    ready = true;
    for (i = 0; i < next_index; i++) {
      if (gc_ready[i] == 0) {
          ready = false;
          break;
      }
    }
  }
#endif

  /* The full_gc flag is also a global to avoid ugly parameter passing. */
  set_external_full_gc(full_gc);

gc_top:
  if (trace_gc == 1)
    fprintf(stderr, "\n;GC");
  if (trace_gc > 1)
    fprintf(stderr, "\n; %sGC due to %s.\n", full_gc ? "Full " : "", reason);

  if (trace_gc > 2 && !pre_dump)
    {
      FORTHREADS {
      fprintf (stderr, "value ");
      dump_stack (value_stack_address);
      fprintf (stderr, "context ");
      dump_stack (context_stack_address);
      }
    }
  if (trace_gc > 1)
    fprintf(stderr, "; Flipping...");

  old_taken = free_point - new_space.start;
  old_spatic_taken = spatic.size;
  old_space = new_space;

  if (trace_gc > 2)
    fprintf(stderr, "old taken: %ld", old_taken);

  if (full_gc)
    new_space.size += spatic.size;
  else
    new_space.size = e_next_newspace_size;

  alloc_space(&new_space, new_space.size);
  free_point = new_space.start;


  transport_count = 0;

  if (trace_gc > 1)
    fprintf(stderr, " rooting...");

  {
    /* Hit the registers: */

    pre_gc_nil = e_nil;
    GC_TOUCH(e_nil);
    GC_TOUCH(e_boot_code);

    if (!pre_dump)
      {
      GC_TOUCH(e_t);
      GC_TOUCH(e_fixnum_type);
      GC_TOUCH(e_loc_type);
      GC_TOUCH(e_cons_type);
      GC_TOUCH_PTR(e_subtype_table, 2);
      /* e_nargs is a fixnum.  Nor is it global... */
      GC_TOUCH (e_env_type);
      GC_TOUCH_PTR (e_argless_tag_trap_table, 2);
      GC_TOUCH_PTR (e_arged_tag_trap_table, 2);
      GC_TOUCH (e_object_type);
      GC_TOUCH (e_segment_type);
      FORTHREADS {
        /* e_bp is a locative, but a pointer to the object should exist, so we
           need only touch it in the locative pass. */
        GC_TOUCH_PTR(e_env, 0);
        GC_TOUCH (e_code_segment);
        GC_TOUCH (e_current_method);
        GC_TOUCH (e_process);
      }
      GC_TOUCH (e_uninitialized);
      GC_TOUCH (e_method_type);
      GC_TOUCH (e_operation_type);

      FORTHREADS {
        for (p = gc_examine_buffer; p < gc_examine_ptr; p++)
          GC_TOUCH(*p);
      }

      /* Scan the stacks. */
      FORTHREADS {
        for (p = value_stack.bp; p <= value_stack.sp; p++)
          GC_TOUCH(*p);

        for (p = context_stack.bp; p <= context_stack.sp; p++)
          GC_TOUCH(*p);

        /* Scan the stack segments. */
        GC_TOUCH(value_stack.segment);
        GC_TOUCH(context_stack.segment);
      }

      /* Scan static space. */
      if (!full_gc)
        for (p = spatic.start; p < spatic.end; p++)
          GC_TOUCH(*p);
      }
    /* Scavenge. */
    if (trace_gc > 1)
      fprintf(stderr, " scavenging...");
    scavenge();

    if (trace_gc > 1)
      fprintf(stderr, " %ld object%s transported.\n",
            transport_count, transport_count != 1 ? "s" : "");



    /* Clean up the locatives. */
    if (trace_gc > 1)
      fprintf(stderr, "; Scanning locatives...");
    loc_transport_count = 0;

    if (!pre_dump)
      {
      FORTHREADS {
        LOC_TOUCH_PTR (e_bp);
        e_pc = pc_touch (e_pc);

        LOC_TOUCH(e_uninitialized);

        for (p = gc_examine_buffer; p < gc_examine_ptr; p++)
          LOC_TOUCH(*p);

        for (p = value_stack.bp; p <= value_stack.sp; p++)
          LOC_TOUCH(*p);

        for (p = context_stack.bp; p <= context_stack.sp; p++)

          LOC_TOUCH(*p);
      }

      /* Scan spatic space. */
      if (!full_gc)
        for (p = spatic.start; p < spatic.end; p++)
          LOC_TOUCH(*p);
      }
    if (trace_gc > 1)
      fprintf(stderr, " scavenging...");
    loc_scavenge();

    if (trace_gc > 1)
      fprintf(stderr, " %ld naked cell%s transported.\n",
            loc_transport_count, loc_transport_count != 1 ? "s" : "");


    /* Discard weak pointers whose targets have not been transported. */
    if (trace_gc > 1)
      fprintf(stderr, "; Scanning weak pointer table...");
    {
      long count = post_gc_wp();

      if (trace_gc > 1)
      fprintf(stderr, " %ld entr%s discarded.\n",
            count, count != 1 ? "ies" : "y");
    }
  }

#ifndef FAST
  {
    /* Check GC consistency. */

    if (trace_gc > 1)
      fprintf(stderr, "; Checking consistency...\n");

    GGC_CHECK(e_nil);
    GGC_CHECK(e_boot_code);

    if (!pre_dump)
      {
      GGC_CHECK (e_t);
      GGC_CHECK (e_fixnum_type);
      GGC_CHECK (e_loc_type);
      GGC_CHECK (e_cons_type);
      GC_CHECK (PTR_TO_REF (e_subtype_table - 2), "e_subtype_table");
      FORTHREADS {
        GC_CHECK (PTR_TO_LOC (e_bp), "PTR_TO_LOC(E_BP)");
        GC_CHECK (PTR_TO_REF (e_env), "e_env");
        }
      /* e_nargs is a fixnum.  Nor is it global... */
      GGC_CHECK (e_env_type);
      GC_CHECK (PTR_TO_REF (e_argless_tag_trap_table - 2), "e_argless_tag_trap_table");
      GC_CHECK (PTR_TO_REF (e_arged_tag_trap_table - 2), "e_arged_tag_trap_table");
      GGC_CHECK (e_object_type);
      GGC_CHECK (e_segment_type);
      FORTHREADS {
        GGC_CHECK (e_code_segment);
        GGC_CHECK (e_current_method);
        GGC_CHECK (e_process);
      }
      GGC_CHECK (e_uninitialized);
      GGC_CHECK (e_method_type);
      GGC_CHECK (e_operation_type);

      /* Scan the stacks. */
      FORTHREADS {
        for (p = value_stack.bp; p <= value_stack.sp; p++)
          GC_CHECK1(*p, "value_stack.bp[%d] = ",
                  (long)(p - value_stack.bp));

        for (p = context_stack.bp; p <= context_stack.sp; p++)
          GC_CHECK1(*p, "context_stack.bp[%d] = ",
                  (long)(p - context_stack.bp));

        GGC_CHECK(value_stack.segment);
        GGC_CHECK(context_stack.segment);

        /* Make sure the program counter is okay. */
        GC_CHECK ((ref_t) ((ref_t) e_pc | LOC_TAG), "e_pc");
      }
      }
    /* Scan the heap. */

    if (!full_gc)
      for (p = spatic.start; p < spatic.end; p++)
      GC_CHECK1(*p, "static_space[%ld] = ", (long)(p - spatic.start));

    for (p = new_space.start; p < free_point; p++)
      GC_CHECK1(*p, "new_space[%ld] = ", (long)(p - new_space.start));
  }
#endif /* not defined(FAST) */

  /* Hopefully there are no more references into old space. */
  if (!pre_dump)
    free_space(&old_space);

  if (!pre_dump && full_gc)
    free_space(&spatic);



#ifdef USE_VADVISE
#ifdef VA_FLUSH
  /* Tell the virtual memory system that recent statistics are useless. */
  vadvise(VA_FLUSH);
#endif
#endif

  if (trace_gc > 2 && !pre_dump)
    {
      FORTHREADS {
#ifdef THREADS
      fprintf (stderr, "Thread %d\n", my_index);
#endif
      fprintf (stderr, "value_stack ");
      dump_stack (value_stack_address);
      fprintf (stderr, "context_stack ");
      dump_stack (context_stack_address);
      }
    }
  {
    long new_taken = free_point - new_space.start;
    long old_total = old_taken + (full_gc ? old_spatic_taken : 0);
    long reclaimed = old_total - new_taken;

    if (trace_gc == 1)
      {
      fprintf(stderr, ":%ld%%", (100 * reclaimed) / old_total);
      }
    if (trace_gc > 1)
      {
      fprintf(stderr, "; GC complete.  %ld ", old_total);
      if (full_gc)
        fprintf(stderr, "(%ld+%ld) ", old_spatic_taken, old_taken);
      fprintf(stderr, "compacted to %ld; %ld (%ld%%) garbage.\n",
            new_taken, reclaimed, (100 * reclaimed) / old_total);
      }

    /* Make the next new space bigger if the current was too small. */
    if (!full_gc && !pre_dump
      && (RECLAIM_FACTOR * new_taken + amount > new_space.size))
      {
      e_next_newspace_size = RECLAIM_FACTOR * new_taken + amount;
#ifdef MAX_NEW_SPACE_SIZE
      if (e_next_newspace_size > MAX_NEW_SPACE_SIZE)
        e_next_newspace_size = MAX_NEW_SPACE_SIZE;
#endif
      switch (trace_gc)
        {
        case 0:
          break;
        case 1:
          fprintf(stderr, ",resize:%ld", (long)e_next_newspace_size);
          break;
        default:
          fprintf(stderr,
                "; Expanding next new space from %ld to %ld (%ld%%).\n",
                (long)new_space.size, (long)e_next_newspace_size,
                (long)(100 * (e_next_newspace_size - new_space.size))
                / new_space.size);
          break;
        }

      if ((size_t) (new_space.end - free_point) < amount)
        {
#ifdef MAX_NEW_SPACE_SIZE
          if (((new_space.end - free_point) + amount) < e_next_newspace_size)
            {
            fprintf(stderr,
                  "\nFatal GC error:"
                  "Essential new space size exceeds maximum allowable.\n");
            exit(EXIT_FAILURE);
            }
#endif
          reason = "immediate new space expansion necessity";
          goto gc_top;
        }
      }

    if (full_gc && !pre_dump)
      {
      /* move _new to spatic, and reallocate new. */

      spatic = new_space;
      realloc_space(&spatic, free_point - new_space.start);

      if (trace_gc > 1 && e_next_newspace_size != original_newspace_size)
        fprintf(stderr, "; Setting new space size to %ld.\n",
              (long)original_newspace_size);
      new_space.size = e_next_newspace_size = original_newspace_size;
      if (e_next_newspace_size <= amount)
        {
          e_next_newspace_size = RECLAIM_FACTOR * amount;
          switch (trace_gc)
            {
            case 0:
            break;
            case 1:
            fprintf(stderr, ",resize:%ld", (long)e_next_newspace_size);
            break;
            default:
            fprintf(stderr,
                  "; expanding next new space %ld to %ld (%d%%).\n",
                  (long)new_space.size,
                  (long)e_next_newspace_size,
                  (int)((100 * (e_next_newspace_size - new_space.size))
                        / new_space.size));
            break;
            }
          new_space.size = e_next_newspace_size;
        }
      alloc_space(&new_space, new_space.size);
      free_point = new_space.start;
      }

    if (trace_gc == 1)
      fprintf(stderr, "\n");

    if (trace_gc)
      fflush(stdout);
  }
#ifdef THREADS
    my_index_p = pthread_getspecific (index_key);
    my_index = *my_index_p;
    gc_ready[my_index] = 0;
    set_gc_flag (false);
#endif 
}



/* This routine takes a block of memory and scans through it, updating
   all pointers into the window starting at old_start to instead point
   into the corresponding location in new_start.  Typically new_start
   will be the same as start */

void
shift_targets(ref_t * start, size_t len,
            ref_t * old_start, size_t old_len,
            ref_t * new_start)
{
  size_t i;
  for (i = 0; i < len; i++)
    {
      ref_t x = start[i];
      if (PTR_MASK & x)       /* is it a pointer? */
      {
        ref_t *y = ANY_TO_PTR(x);
        size_t offset = y - old_start;
        if (y >= 0 && offset < old_len)   /* into old window? */
          start[i] = PTR_TO_TAGGED(new_start + offset, x);
      }
    }
}

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