source: trunk/gc6.8/allchblk.c@ 239

/* 
 * Copyright 1988, 1989 Hans-J. Boehm, Alan J. Demers
 * Copyright (c) 1991-1994 by Xerox Corporation.  All rights reserved.
 * Copyright (c) 1998-1999 by Silicon Graphics.  All rights reserved.
 * Copyright (c) 1999 by Hewlett-Packard Company. All rights reserved.
 *
 * THIS MATERIAL IS PROVIDED AS IS, WITH ABSOLUTELY NO WARRANTY EXPRESSED
 * OR IMPLIED.  ANY USE IS AT YOUR OWN RISK.
 *
 * Permission is hereby granted to use or copy this program
 * for any purpose,  provided the above notices are retained on all copies.
 * Permission to modify the code and to distribute modified code is granted,
 * provided the above notices are retained, and a notice that the code was
 * modified is included with the above copyright notice.
 */

/* #define DEBUG */
#include <stdio.h>
#include "private/gc_priv.h"

GC_bool GC_use_entire_heap = 0;

/*
 * Free heap blocks are kept on one of several free lists,
 * depending on the size of the block.  Each free list is doubly linked.
 * Adjacent free blocks are coalesced.
 */

 
# define MAX_BLACK_LIST_ALLOC (2*HBLKSIZE)
                /* largest block we will allocate starting on a black   */
                /* listed block.  Must be >= HBLKSIZE.                  */


# define UNIQUE_THRESHOLD 32
        /* Sizes up to this many HBLKs each have their own free list    */
# define HUGE_THRESHOLD 256
        /* Sizes of at least this many heap blocks are mapped to a      */
        /* single free list.                                            */
# define FL_COMPRESSION 8
        /* In between sizes map this many distinct sizes to a single    */
        /* bin.                                                         */

# define N_HBLK_FLS (HUGE_THRESHOLD - UNIQUE_THRESHOLD)/FL_COMPRESSION \
                                 + UNIQUE_THRESHOLD

struct hblk * GC_hblkfreelist[N_HBLK_FLS+1] = { 0 };

#ifndef USE_MUNMAP

  word GC_free_bytes[N_HBLK_FLS+1] = { 0 };
        /* Number of free bytes on each list.   */

  /* Is bytes + the number of free bytes on lists n .. N_HBLK_FLS       */
  /* > GC_max_large_allocd_bytes?                                       */
# ifdef __GNUC__
  __inline__
# endif
  static GC_bool GC_enough_large_bytes_left(bytes,n)
  word bytes;
  int n;
  {
    int i;
    for (i = N_HBLK_FLS; i >= n; --i) {
        bytes += GC_free_bytes[i];
        if (bytes > GC_max_large_allocd_bytes) return TRUE;
    }
    return FALSE;
  }

# define INCR_FREE_BYTES(n, b) GC_free_bytes[n] += (b);

# define FREE_ASSERT(e) GC_ASSERT(e)

#else /* USE_MUNMAP */

# define INCR_FREE_BYTES(n, b)
# define FREE_ASSERT(e)

#endif /* USE_MUNMAP */

/* Map a number of blocks to the appropriate large block free list index. */
int GC_hblk_fl_from_blocks(blocks_needed)
word blocks_needed;
{
    if (blocks_needed <= UNIQUE_THRESHOLD) return blocks_needed;
    if (blocks_needed >= HUGE_THRESHOLD) return N_HBLK_FLS;
    return (blocks_needed - UNIQUE_THRESHOLD)/FL_COMPRESSION
                                        + UNIQUE_THRESHOLD;
    
}

# define PHDR(hhdr) HDR(hhdr -> hb_prev)
# define NHDR(hhdr) HDR(hhdr -> hb_next)

# ifdef USE_MUNMAP
#   define IS_MAPPED(hhdr) (((hhdr) -> hb_flags & WAS_UNMAPPED) == 0)
# else  /* !USE_MMAP */
#   define IS_MAPPED(hhdr) 1
# endif /* USE_MUNMAP */

# if !defined(NO_DEBUGGING)
void GC_print_hblkfreelist()
{
    struct hblk * h;
    word total_free = 0;
    hdr * hhdr;
    word sz;
    int i;
    
    for (i = 0; i <= N_HBLK_FLS; ++i) {
      h = GC_hblkfreelist[i];
#     ifdef USE_MUNMAP
        if (0 != h) GC_printf1("Free list %ld:\n",
                               (unsigned long)i);
#     else
        if (0 != h) GC_printf2("Free list %ld (Total size %ld):\n",
                               (unsigned long)i,
                               (unsigned long)GC_free_bytes[i]);
#     endif
      while (h != 0) {
        hhdr = HDR(h);
        sz = hhdr -> hb_sz;
        GC_printf2("\t0x%lx size %lu ", (unsigned long)h, (unsigned long)sz);
        total_free += sz;
        if (GC_is_black_listed(h, HBLKSIZE) != 0) {
             GC_printf0("start black listed\n");
        } else if (GC_is_black_listed(h, hhdr -> hb_sz) != 0) {
             GC_printf0("partially black listed\n");
        } else {
             GC_printf0("not black listed\n");
        }
        h = hhdr -> hb_next;
      }
    }
#   ifndef USE_MUNMAP
      if (total_free != GC_large_free_bytes) {
        GC_printf1("GC_large_free_bytes = %lu (INCONSISTENT!!)\n",
                   (unsigned long) GC_large_free_bytes);
      }
#   endif
    GC_printf1("Total of %lu bytes on free list\n", (unsigned long)total_free);
}

/* Return the free list index on which the block described by the header */
/* appears, or -1 if it appears nowhere.                                 */
int free_list_index_of(wanted)
hdr * wanted;
{
    struct hblk * h;
    hdr * hhdr;
    int i;
    
    for (i = 0; i <= N_HBLK_FLS; ++i) {
      h = GC_hblkfreelist[i];
      while (h != 0) {
        hhdr = HDR(h);
        if (hhdr == wanted) return i;
        h = hhdr -> hb_next;
      }
    }
    return -1;
}

void GC_dump_regions()
{
    unsigned i;
    ptr_t start, end;
    ptr_t p;
    size_t bytes;
    hdr *hhdr;
    for (i = 0; i < GC_n_heap_sects; ++i) {
        start = GC_heap_sects[i].hs_start;
        bytes = GC_heap_sects[i].hs_bytes;
        end = start + bytes;
        /* Merge in contiguous sections.        */
          while (i+1 < GC_n_heap_sects && GC_heap_sects[i+1].hs_start == end) {
            ++i;
            end = GC_heap_sects[i].hs_start + GC_heap_sects[i].hs_bytes;
          }
        GC_printf2("***Section from 0x%lx to 0x%lx\n", start, end);
        for (p = start; p < end;) {
            hhdr = HDR(p);
            GC_printf1("\t0x%lx ", (unsigned long)p);
            if (IS_FORWARDING_ADDR_OR_NIL(hhdr)) {
                GC_printf1("Missing header!!(%ld)\n", hhdr);
                p += HBLKSIZE;
                continue;
            }
            if (HBLK_IS_FREE(hhdr)) {
                int correct_index = GC_hblk_fl_from_blocks(
                                        divHBLKSZ(hhdr -> hb_sz));
                int actual_index;
                
                GC_printf1("\tfree block of size 0x%lx bytes",
                           (unsigned long)(hhdr -> hb_sz));
                if (IS_MAPPED(hhdr)) {
                    GC_printf0("\n");
                } else {
                    GC_printf0("(unmapped)\n");
                }
                actual_index = free_list_index_of(hhdr);
                if (-1 == actual_index) {
                    GC_printf1("\t\tBlock not on free list %ld!!\n",
                                correct_index);
                } else if (correct_index != actual_index) {
                    GC_printf2("\t\tBlock on list %ld, should be on %ld!!\n",
                               actual_index, correct_index);
                }
                p += hhdr -> hb_sz;
            } else {
                GC_printf1("\tused for blocks of size 0x%lx bytes\n",
                           (unsigned long)WORDS_TO_BYTES(hhdr -> hb_sz));
                p += HBLKSIZE * OBJ_SZ_TO_BLOCKS(hhdr -> hb_sz);
            }
        }
    }
}

# endif /* NO_DEBUGGING */

/* Initialize hdr for a block containing the indicated size and         */
/* kind of objects.                                                     */
/* Return FALSE on failure.                                             */
static GC_bool setup_header(hhdr, sz, kind, flags)
register hdr * hhdr;
word sz;        /* object size in words */
int kind;
unsigned char flags;
{
    register word descr;
    
    /* Add description of valid object pointers */
      if (!GC_add_map_entry(sz)) return(FALSE);
      hhdr -> hb_map = GC_obj_map[sz > MAXOBJSZ? 0 : sz];
      
    /* Set size, kind and mark proc fields */
      hhdr -> hb_sz = sz;
      hhdr -> hb_obj_kind = kind;
      hhdr -> hb_flags = flags;
      descr = GC_obj_kinds[kind].ok_descriptor;
      if (GC_obj_kinds[kind].ok_relocate_descr) descr += WORDS_TO_BYTES(sz);
      hhdr -> hb_descr = descr;
      
    /* Clear mark bits */
      GC_clear_hdr_marks(hhdr);
      
    hhdr -> hb_last_reclaimed = (unsigned short)GC_gc_no;
    return(TRUE);
}

#define FL_UNKNOWN -1
/*
 * Remove hhdr from the appropriate free list.
 * We assume it is on the nth free list, or on the size
 * appropriate free list if n is FL_UNKNOWN.
 */
void GC_remove_from_fl(hhdr, n)
hdr * hhdr;
int n;
{
    int index;

    GC_ASSERT(((hhdr -> hb_sz) & (HBLKSIZE-1)) == 0);
#   ifndef USE_MUNMAP
      /* We always need index to mainatin free counts.  */
      if (FL_UNKNOWN == n) {
          index = GC_hblk_fl_from_blocks(divHBLKSZ(hhdr -> hb_sz));
      } else {
          index = n;
      }
#   endif
    if (hhdr -> hb_prev == 0) {
#       ifdef USE_MUNMAP
          if (FL_UNKNOWN == n) {
            index = GC_hblk_fl_from_blocks(divHBLKSZ(hhdr -> hb_sz));
          } else {
            index = n;
          }
#       endif
        GC_ASSERT(HDR(GC_hblkfreelist[index]) == hhdr);
        GC_hblkfreelist[index] = hhdr -> hb_next;
    } else {
        hdr *phdr;
        GET_HDR(hhdr -> hb_prev, phdr);
        phdr -> hb_next = hhdr -> hb_next;
    }
    FREE_ASSERT(GC_free_bytes[index] >= hhdr -> hb_sz);
    INCR_FREE_BYTES(index, - (signed_word)(hhdr -> hb_sz));
    if (0 != hhdr -> hb_next) {
        hdr * nhdr;
        GC_ASSERT(!IS_FORWARDING_ADDR_OR_NIL(NHDR(hhdr)));
        GET_HDR(hhdr -> hb_next, nhdr);
        nhdr -> hb_prev = hhdr -> hb_prev;
    }
}

/*
 * Return a pointer to the free block ending just before h, if any.
 */
struct hblk * GC_free_block_ending_at(h)
struct hblk *h;
{
    struct hblk * p = h - 1;
    hdr * phdr;

    GET_HDR(p, phdr);
    while (0 != phdr && IS_FORWARDING_ADDR_OR_NIL(phdr)) {
        p = FORWARDED_ADDR(p,phdr);
        phdr = HDR(p);
    }
    if (0 != phdr) {
        if(HBLK_IS_FREE(phdr)) {
            return p;
        } else {
            return 0;
        }
    }
    p = GC_prev_block(h - 1);
    if (0 != p) {
      phdr = HDR(p);
      if (HBLK_IS_FREE(phdr) && (ptr_t)p + phdr -> hb_sz == (ptr_t)h) {
        return p;
      }
    }
    return 0;
}

/*
 * Add hhdr to the appropriate free list.
 * We maintain individual free lists sorted by address.
 */
void GC_add_to_fl(h, hhdr)
struct hblk *h;
hdr * hhdr;
{
    int index = GC_hblk_fl_from_blocks(divHBLKSZ(hhdr -> hb_sz));
    struct hblk *second = GC_hblkfreelist[index];
    hdr * second_hdr;
#   ifdef GC_ASSERTIONS
      struct hblk *next = (struct hblk *)((word)h + hhdr -> hb_sz);
      hdr * nexthdr = HDR(next);
      struct hblk *prev = GC_free_block_ending_at(h);
      hdr * prevhdr = HDR(prev);
      GC_ASSERT(nexthdr == 0 || !HBLK_IS_FREE(nexthdr) || !IS_MAPPED(nexthdr));
      GC_ASSERT(prev == 0 || !HBLK_IS_FREE(prevhdr) || !IS_MAPPED(prevhdr));
#   endif
    GC_ASSERT(((hhdr -> hb_sz) & (HBLKSIZE-1)) == 0);
    GC_hblkfreelist[index] = h;
    INCR_FREE_BYTES(index, hhdr -> hb_sz);
    FREE_ASSERT(GC_free_bytes[index] <= GC_large_free_bytes)
    hhdr -> hb_next = second;
    hhdr -> hb_prev = 0;
    if (0 != second) {
      GET_HDR(second, second_hdr);
      second_hdr -> hb_prev = h;
    }
    GC_invalidate_map(hhdr);
}

#ifdef USE_MUNMAP

/* Unmap blocks that haven't been recently touched.  This is the only way */
/* way blocks are ever unmapped.                                          */
void GC_unmap_old(void)
{
    struct hblk * h;
    hdr * hhdr;
    word sz;
    unsigned short last_rec, threshold;
    int i;
#   define UNMAP_THRESHOLD 6
    
    for (i = 0; i <= N_HBLK_FLS; ++i) {
      for (h = GC_hblkfreelist[i]; 0 != h; h = hhdr -> hb_next) {
        hhdr = HDR(h);
        if (!IS_MAPPED(hhdr)) continue;
        threshold = (unsigned short)(GC_gc_no - UNMAP_THRESHOLD);
        last_rec = hhdr -> hb_last_reclaimed;
        if ((last_rec > GC_gc_no || last_rec < threshold)
            && threshold < GC_gc_no /* not recently wrapped */) {
          sz = hhdr -> hb_sz;
          GC_unmap((ptr_t)h, sz);
          hhdr -> hb_flags |= WAS_UNMAPPED;
        }
      }
    }  
}

/* Merge all unmapped blocks that are adjacent to other free            */
/* blocks.  This may involve remapping, since all blocks are either     */
/* fully mapped or fully unmapped.                                      */
void GC_merge_unmapped(void)
{
    struct hblk * h, *next;
    hdr * hhdr, *nexthdr;
    word size, nextsize;
    int i;
    
    for (i = 0; i <= N_HBLK_FLS; ++i) {
      h = GC_hblkfreelist[i];
      while (h != 0) {
        GET_HDR(h, hhdr);
        size = hhdr->hb_sz;
        next = (struct hblk *)((word)h + size);
        GET_HDR(next, nexthdr);
        /* Coalesce with successor, if possible */
          if (0 != nexthdr && HBLK_IS_FREE(nexthdr)) {
            nextsize = nexthdr -> hb_sz;
            if (IS_MAPPED(hhdr)) {
              GC_ASSERT(!IS_MAPPED(nexthdr));
              /* make both consistent, so that we can merge */
                if (size > nextsize) {
                  GC_remap((ptr_t)next, nextsize);
                } else {
                  GC_unmap((ptr_t)h, size);
                  hhdr -> hb_flags |= WAS_UNMAPPED;
                }
            } else if (IS_MAPPED(nexthdr)) {
              GC_ASSERT(!IS_MAPPED(hhdr));
              if (size > nextsize) {
                GC_unmap((ptr_t)next, nextsize);
              } else {
                GC_remap((ptr_t)h, size);
                hhdr -> hb_flags &= ~WAS_UNMAPPED;
                hhdr -> hb_last_reclaimed = nexthdr -> hb_last_reclaimed;
              }
            } else {
              /* Unmap any gap in the middle */
                GC_unmap_gap((ptr_t)h, size, (ptr_t)next, nexthdr -> hb_sz);
            }
            /* If they are both unmapped, we merge, but leave unmapped. */
            GC_remove_from_fl(hhdr, i);
            GC_remove_from_fl(nexthdr, FL_UNKNOWN);
            hhdr -> hb_sz += nexthdr -> hb_sz; 
            GC_remove_header(next);
            GC_add_to_fl(h, hhdr); 
            /* Start over at beginning of list */
            h = GC_hblkfreelist[i];
          } else /* not mergable with successor */ {
            h = hhdr -> hb_next;
          }
      } /* while (h != 0) ... */
    } /* for ... */
}

#endif /* USE_MUNMAP */

/*
 * Return a pointer to a block starting at h of length bytes.
 * Memory for the block is mapped.
 * Remove the block from its free list, and return the remainder (if any)
 * to its appropriate free list.
 * May fail by returning 0.
 * The header for the returned block must be set up by the caller.
 * If the return value is not 0, then hhdr is the header for it.
 */
struct hblk * GC_get_first_part(h, hhdr, bytes, index)
struct hblk *h;
hdr * hhdr;
word bytes;
int index;
{
    word total_size = hhdr -> hb_sz;
    struct hblk * rest;
    hdr * rest_hdr;

    GC_ASSERT((total_size & (HBLKSIZE-1)) == 0);
    GC_remove_from_fl(hhdr, index);
    if (total_size == bytes) return h;
    rest = (struct hblk *)((word)h + bytes);
    rest_hdr = GC_install_header(rest);
    if (0 == rest_hdr) {
        /* This may be very bad news ... */
        WARN("Header allocation failed: Dropping block.\n", 0);
        return(0);
    }
    rest_hdr -> hb_sz = total_size - bytes;
    rest_hdr -> hb_flags = 0;
#   ifdef GC_ASSERTIONS
      /* Mark h not free, to avoid assertion about adjacent free blocks. */
        hhdr -> hb_map = 0;
#   endif
    GC_add_to_fl(rest, rest_hdr);
    return h;
}

/*
 * H is a free block.  N points at an address inside it.
 * A new header for n has already been set up.  Fix up h's header
 * to reflect the fact that it is being split, move it to the
 * appropriate free list.
 * N replaces h in the original free list.
 *
 * Nhdr is not completely filled in, since it is about to allocated.
 * It may in fact end up on the wrong free list for its size.
 * (Hence adding it to a free list is silly.  But this path is hopefully
 * rare enough that it doesn't matter.  The code is cleaner this way.)
 */
void GC_split_block(h, hhdr, n, nhdr, index)
struct hblk *h;
hdr * hhdr;
struct hblk *n;
hdr * nhdr;
int index;      /* Index of free list */
{
    word total_size = hhdr -> hb_sz;
    word h_size = (word)n - (word)h;
    struct hblk *prev = hhdr -> hb_prev;
    struct hblk *next = hhdr -> hb_next;

    /* Replace h with n on its freelist */
      nhdr -> hb_prev = prev;
      nhdr -> hb_next = next;
      nhdr -> hb_sz = total_size - h_size;
      nhdr -> hb_flags = 0;
      if (0 != prev) {
        HDR(prev) -> hb_next = n;
      } else {
        GC_hblkfreelist[index] = n;
      }
      if (0 != next) {
        HDR(next) -> hb_prev = n;
      }
      INCR_FREE_BYTES(index, -(signed_word)h_size);
      FREE_ASSERT(GC_free_bytes[index] > 0);
#     ifdef GC_ASSERTIONS
        nhdr -> hb_map = 0;     /* Don't fail test for consecutive      */
                                /* free blocks in GC_add_to_fl.         */
#     endif
#   ifdef USE_MUNMAP
      hhdr -> hb_last_reclaimed = (unsigned short)GC_gc_no;
#   endif
    hhdr -> hb_sz = h_size;
    GC_add_to_fl(h, hhdr);
    GC_invalidate_map(nhdr);
}
        
struct hblk * GC_allochblk_nth();

/*
 * Allocate (and return pointer to) a heap block
 *   for objects of size sz words, searching the nth free list.
 *
 * NOTE: We set obj_map field in header correctly.
 *       Caller is responsible for building an object freelist in block.
 *
 * Unlike older versions of the collectors, the client is responsible
 * for clearing the block, if necessary.
 */
struct hblk *
GC_allochblk(sz, kind, flags)
word sz;
int kind;
unsigned flags;  /* IGNORE_OFF_PAGE or 0 */
{
    word blocks = OBJ_SZ_TO_BLOCKS(sz);
    int start_list = GC_hblk_fl_from_blocks(blocks);
    int i;
    for (i = start_list; i <= N_HBLK_FLS; ++i) {
        struct hblk * result = GC_allochblk_nth(sz, kind, flags, i);
        if (0 != result) {
            return result;
        }
    }
    return 0;
}
/*
 * The same, but with search restricted to nth free list.
 */
struct hblk *
GC_allochblk_nth(sz, kind, flags, n)
word sz;
int kind;
unsigned char flags;  /* IGNORE_OFF_PAGE or 0 */
int n;
{
    register struct hblk *hbp;
    register hdr * hhdr;                /* Header corr. to hbp */
    register struct hblk *thishbp;
    register hdr * thishdr;             /* Header corr. to hbp */
    signed_word size_needed;    /* number of bytes in requested objects */
    signed_word size_avail;     /* bytes available in this block        */

    size_needed = HBLKSIZE * OBJ_SZ_TO_BLOCKS(sz);

    /* search for a big enough block in free list */
        hbp = GC_hblkfreelist[n];
        for(; 0 != hbp; hbp = hhdr -> hb_next) {
            GET_HDR(hbp, hhdr);
            size_avail = hhdr->hb_sz;
            if (size_avail < size_needed) continue;
            if (size_avail != size_needed
                && !GC_use_entire_heap
                && !GC_dont_gc
                && USED_HEAP_SIZE >= GC_requested_heapsize
                && !TRUE_INCREMENTAL && GC_should_collect()) {
#               ifdef USE_MUNMAP
                    continue;
#               else
                    /* If we have enough large blocks left to cover any */
                    /* previous request for large blocks, we go ahead   */
                    /* and split.  Assuming a steady state, that should */
                    /* be safe.  It means that we can use the full      */
                    /* heap if we allocate only small objects.          */
                    if (!GC_enough_large_bytes_left(GC_large_allocd_bytes, n)) {
                      continue;
                    } 
                    /* If we are deallocating lots of memory from       */
                    /* finalizers, fail and collect sooner rather       */
                    /* than later.                                      */
                    if (WORDS_TO_BYTES(GC_finalizer_mem_freed)
                        > (GC_heapsize >> 4))  {
                      continue;
                    }
#               endif /* !USE_MUNMAP */
            }
            /* If the next heap block is obviously better, go on.       */
            /* This prevents us from disassembling a single large block */
            /* to get tiny blocks.                                      */
            {
              signed_word next_size;
              
              thishbp = hhdr -> hb_next;
              if (thishbp != 0) {
                GET_HDR(thishbp, thishdr);
                next_size = (signed_word)(thishdr -> hb_sz);
                if (next_size < size_avail
                  && next_size >= size_needed
                  && !GC_is_black_listed(thishbp, (word)size_needed)) {
                  continue;
                }
              }
            }
            if ( !IS_UNCOLLECTABLE(kind) &&
                 (kind != PTRFREE || size_needed > MAX_BLACK_LIST_ALLOC)) {
              struct hblk * lasthbp = hbp;
              ptr_t search_end = (ptr_t)hbp + size_avail - size_needed;
              signed_word orig_avail = size_avail;
              signed_word eff_size_needed = ((flags & IGNORE_OFF_PAGE)?
                                                HBLKSIZE
                                                : size_needed);
              
              
              while ((ptr_t)lasthbp <= search_end
                     && (thishbp = GC_is_black_listed(lasthbp,
                                                      (word)eff_size_needed))
                        != 0) {
                lasthbp = thishbp;
              }
              size_avail -= (ptr_t)lasthbp - (ptr_t)hbp;
              thishbp = lasthbp;
              if (size_avail >= size_needed) {
                if (thishbp != hbp &&
                    0 != (thishdr = GC_install_header(thishbp))) {
                  /* Make sure it's mapped before we mangle it. */
#                   ifdef USE_MUNMAP
                      if (!IS_MAPPED(hhdr)) {
                        GC_remap((ptr_t)hbp, hhdr -> hb_sz);
                        hhdr -> hb_flags &= ~WAS_UNMAPPED;
                      }
#                   endif
                  /* Split the block at thishbp */
                      GC_split_block(hbp, hhdr, thishbp, thishdr, n);
                  /* Advance to thishbp */
                      hbp = thishbp;
                      hhdr = thishdr;
                      /* We must now allocate thishbp, since it may     */
                      /* be on the wrong free list.                     */
                }
              } else if (size_needed > (signed_word)BL_LIMIT
                         && orig_avail - size_needed
                            > (signed_word)BL_LIMIT) {
                /* Punt, since anything else risks unreasonable heap growth. */
                if (++GC_large_alloc_warn_suppressed
                    >= GC_large_alloc_warn_interval) {
                  WARN("Repeated allocation of very large block "
                       "(appr. size %ld):\n"
                       "\tMay lead to memory leak and poor performance.\n",
                       size_needed);
                  GC_large_alloc_warn_suppressed = 0;
                }
                size_avail = orig_avail;
              } else if (size_avail == 0 && size_needed == HBLKSIZE
                         && IS_MAPPED(hhdr)) {
                if (!GC_find_leak) {
                  static unsigned count = 0;
                  
                  /* The block is completely blacklisted.  We need      */
                  /* to drop some such blocks, since otherwise we spend */
                  /* all our time traversing them if pointerfree        */
                  /* blocks are unpopular.                              */
                  /* A dropped block will be reconsidered at next GC.   */
                  if ((++count & 3) == 0) {
                    /* Allocate and drop the block in small chunks, to  */
                    /* maximize the chance that we will recover some    */
                    /* later.                                           */
                      word total_size = hhdr -> hb_sz;
                      struct hblk * limit = hbp + divHBLKSZ(total_size);
                      struct hblk * h;
                      struct hblk * prev = hhdr -> hb_prev;
                      
                      GC_words_wasted += BYTES_TO_WORDS(total_size);
                      GC_large_free_bytes -= total_size;
                      GC_remove_from_fl(hhdr, n);
                      for (h = hbp; h < limit; h++) {
                        if (h == hbp || 0 != (hhdr = GC_install_header(h))) {
                          (void) setup_header(
                                  hhdr,
                                  BYTES_TO_WORDS(HBLKSIZE),
                                  PTRFREE, 0); /* Cant fail */
                          if (GC_debugging_started) {
                            BZERO(h, HBLKSIZE);
                          }
                        }
                      }
                    /* Restore hbp to point at free block */
                      hbp = prev;
                      if (0 == hbp) {
                        return GC_allochblk_nth(sz, kind, flags, n);
                      }
                      hhdr = HDR(hbp);
                  }
                }
              }
            }
            if( size_avail >= size_needed ) {
#               ifdef USE_MUNMAP
                  if (!IS_MAPPED(hhdr)) {
                    GC_remap((ptr_t)hbp, hhdr -> hb_sz);
                    hhdr -> hb_flags &= ~WAS_UNMAPPED;
                  }
#               endif
                /* hbp may be on the wrong freelist; the parameter n    */
                /* is important.                                        */
                hbp = GC_get_first_part(hbp, hhdr, size_needed, n);
                break;
            }
        }

    if (0 == hbp) return 0;
        
    /* Add it to map of valid blocks */
        if (!GC_install_counts(hbp, (word)size_needed)) return(0);
        /* This leaks memory under very rare conditions. */
                
    /* Set up header */
        if (!setup_header(hhdr, sz, kind, flags)) {
            GC_remove_counts(hbp, (word)size_needed);
            return(0); /* ditto */
        }

    /* Notify virtual dirty bit implementation that we are about to write.  */
    /* Ensure that pointerfree objects are not protected if it's avoidable. */
        GC_remove_protection(hbp, divHBLKSZ(size_needed),
                             (hhdr -> hb_descr == 0) /* pointer-free */);
        
    /* We just successfully allocated a block.  Restart count of        */
    /* consecutive failures.                                            */
    {
        extern unsigned GC_fail_count;
        
        GC_fail_count = 0;
    }

    GC_large_free_bytes -= size_needed;
    
    GC_ASSERT(IS_MAPPED(hhdr));
    return( hbp );
}
 
struct hblk * GC_freehblk_ptr = 0;  /* Search position hint for GC_freehblk */

/*
 * Free a heap block.
 *
 * Coalesce the block with its neighbors if possible.
 *
 * All mark words are assumed to be cleared.
 */
void
GC_freehblk(hbp)
struct hblk *hbp;
{
struct hblk *next, *prev;
hdr *hhdr, *prevhdr, *nexthdr;
signed_word size;


    GET_HDR(hbp, hhdr);
    size = hhdr->hb_sz;
    size = HBLKSIZE * OBJ_SZ_TO_BLOCKS(size);
    GC_remove_counts(hbp, (word)size);
    hhdr->hb_sz = size;
#   ifdef USE_MUNMAP
      hhdr -> hb_last_reclaimed = (unsigned short)GC_gc_no;
#   endif
    
    /* Check for duplicate deallocation in the easy case */
      if (HBLK_IS_FREE(hhdr)) {
        GC_printf1("Duplicate large block deallocation of 0x%lx\n",
                   (unsigned long) hbp);
        ABORT("Duplicate large block deallocation");
      }

    GC_ASSERT(IS_MAPPED(hhdr));
    GC_invalidate_map(hhdr);
    next = (struct hblk *)((word)hbp + size);
    GET_HDR(next, nexthdr);
    prev = GC_free_block_ending_at(hbp);
    /* Coalesce with successor, if possible */
      if(0 != nexthdr && HBLK_IS_FREE(nexthdr) && IS_MAPPED(nexthdr)) {
        GC_remove_from_fl(nexthdr, FL_UNKNOWN);
        hhdr -> hb_sz += nexthdr -> hb_sz; 
        GC_remove_header(next);
      }
    /* Coalesce with predecessor, if possible. */
      if (0 != prev) {
        prevhdr = HDR(prev);
        if (IS_MAPPED(prevhdr)) {
          GC_remove_from_fl(prevhdr, FL_UNKNOWN);
          prevhdr -> hb_sz += hhdr -> hb_sz;
#         ifdef USE_MUNMAP
            prevhdr -> hb_last_reclaimed = (unsigned short)GC_gc_no;
#         endif
          GC_remove_header(hbp);
          hbp = prev;
          hhdr = prevhdr;
        }
      }
    /* FIXME: It is not clear we really always want to do these merges  */
    /* with -DUSE_MUNMAP, since it updates ages and hence prevents      */
    /* unmapping.                                                       */

    GC_large_free_bytes += size;
    GC_add_to_fl(hbp, hhdr);    
}