0.8.3.76

[sbcl.git] / src / runtime / gencgc.c

/*
 * GENerational Conservative Garbage Collector for SBCL x86
 */

/*
 * This software is part of the SBCL system. See the README file for
 * more information.
 *
 * This software is derived from the CMU CL system, which was
 * written at Carnegie Mellon University and released into the
 * public domain. The software is in the public domain and is
 * provided with absolutely no warranty. See the COPYING and CREDITS
 * files for more information.
 */

/*
 * For a review of garbage collection techniques (e.g. generational
 * GC) and terminology (e.g. "scavenging") see Paul R. Wilson,
 * "Uniprocessor Garbage Collection Techniques". As of 20000618, this
 * had been accepted for _ACM Computing Surveys_ and was available
 * as a PostScript preprint through
 *   <http://www.cs.utexas.edu/users/oops/papers.html>
 * as
 *   <ftp://ftp.cs.utexas.edu/pub/garbage/bigsurv.ps>.
 */

#include <stdio.h>
#include <signal.h>
#include <errno.h>
#include "runtime.h"
#include "sbcl.h"
#include "os.h"
#include "interr.h"
#include "globals.h"
#include "interrupt.h"
#include "validate.h"
#include "lispregs.h"
#include "arch.h"
#include "gc.h"
#include "gc-internal.h"
#include "thread.h"
#include "genesis/vector.h"
#include "genesis/weak-pointer.h"
#include "genesis/simple-fun.h"

#ifdef LISP_FEATURE_SB_THREAD
#include <sys/ptrace.h>
#include <linux/user.h>         /* threading is presently linux-only */
#endif

/* assembly language stub that executes trap_PendingInterrupt */
void do_pending_interrupt(void);

/* forward declarations */
int gc_find_freeish_pages(int *restart_page_ptr, int nbytes, int unboxed, struct alloc_region *alloc_region);
void  gc_set_region_empty(struct alloc_region *region);
void gc_alloc_update_all_page_tables(void);
static void  gencgc_pickup_dynamic(void);
boolean interrupt_maybe_gc_int(int, siginfo_t *, void *);

\f
/*
 * GC parameters
 */

/* the number of actual generations. (The number of 'struct
 * generation' objects is one more than this, because one object
 * serves as scratch when GC'ing.) */
#define NUM_GENERATIONS 6

/* Should we use page protection to help avoid the scavenging of pages
 * that don't have pointers to younger generations? */
boolean enable_page_protection = 1;

/* Should we unmap a page and re-mmap it to have it zero filled? */
#if defined(__FreeBSD__) || defined(__OpenBSD__)
/* comment from cmucl-2.4.8: This can waste a lot of swap on FreeBSD
 * so don't unmap there.
 *
 * The CMU CL comment didn't specify a version, but was probably an
 * old version of FreeBSD (pre-4.0), so this might no longer be true.
 * OTOH, if it is true, this behavior might exist on OpenBSD too, so
 * for now we don't unmap there either. -- WHN 2001-04-07 */
boolean gencgc_unmap_zero = 0;
#else
boolean gencgc_unmap_zero = 1;
#endif

/* the minimum size (in bytes) for a large object*/
unsigned large_object_size = 4 * 4096;
\f
/*
 * debugging
 */



/* the verbosity level. All non-error messages are disabled at level 0;
 * and only a few rare messages are printed at level 1. */
unsigned gencgc_verbose = (QSHOW ? 1 : 0);

/* FIXME: At some point enable the various error-checking things below
 * and see what they say. */

/* We hunt for pointers to old-space, when GCing generations >= verify_gen.
 * Set verify_gens to NUM_GENERATIONS to disable this kind of check. */
int verify_gens = NUM_GENERATIONS;

/* Should we do a pre-scan verify of generation 0 before it's GCed? */
boolean pre_verify_gen_0 = 0;

/* Should we check for bad pointers after gc_free_heap is called
 * from Lisp PURIFY? */
boolean verify_after_free_heap = 0;

/* Should we print a note when code objects are found in the dynamic space
 * during a heap verify? */
boolean verify_dynamic_code_check = 0;

/* Should we check code objects for fixup errors after they are transported? */
boolean check_code_fixups = 0;

/* Should we check that newly allocated regions are zero filled? */
boolean gencgc_zero_check = 0;

/* Should we check that the free space is zero filled? */
boolean gencgc_enable_verify_zero_fill = 0;

/* Should we check that free pages are zero filled during gc_free_heap
 * called after Lisp PURIFY? */
boolean gencgc_zero_check_during_free_heap = 0;
\f
/*
 * GC structures and variables
 */

/* the total bytes allocated. These are seen by Lisp DYNAMIC-USAGE. */
unsigned long bytes_allocated = 0;
extern unsigned long bytes_consed_between_gcs; /* gc-common.c */
unsigned long auto_gc_trigger = 0;

/* the source and destination generations. These are set before a GC starts
 * scavenging. */
int from_space;
int new_space;


/* FIXME: It would be nice to use this symbolic constant instead of
 * bare 4096 almost everywhere. We could also use an assertion that
 * it's equal to getpagesize(). */

#define PAGE_BYTES 4096

/* An array of page structures is statically allocated.
 * This helps quickly map between an address its page structure.
 * NUM_PAGES is set from the size of the dynamic space. */
struct page page_table[NUM_PAGES];

/* To map addresses to page structures the address of the first page
 * is needed. */
static void *heap_base = NULL;


/* Calculate the start address for the given page number. */
inline void *
page_address(int page_num)
{
    return (heap_base + (page_num * 4096));
}

/* Find the page index within the page_table for the given
 * address. Return -1 on failure. */
inline int
find_page_index(void *addr)
{
    int index = addr-heap_base;

    if (index >= 0) {
        index = ((unsigned int)index)/4096;
        if (index < NUM_PAGES)
            return (index);
    }

    return (-1);
}

/* a structure to hold the state of a generation */
struct generation {

    /* the first page that gc_alloc() checks on its next call */
    int alloc_start_page;

    /* the first page that gc_alloc_unboxed() checks on its next call */
    int alloc_unboxed_start_page;

    /* the first page that gc_alloc_large (boxed) considers on its next
     * call. (Although it always allocates after the boxed_region.) */
    int alloc_large_start_page;

    /* the first page that gc_alloc_large (unboxed) considers on its
     * next call. (Although it always allocates after the
     * current_unboxed_region.) */
    int alloc_large_unboxed_start_page;

    /* the bytes allocated to this generation */
    int bytes_allocated;

    /* the number of bytes at which to trigger a GC */
    int gc_trigger;

    /* to calculate a new level for gc_trigger */
    int bytes_consed_between_gc;

    /* the number of GCs since the last raise */
    int num_gc;

    /* the average age after which a GC will raise objects to the
     * next generation */
    int trigger_age;

    /* the cumulative sum of the bytes allocated to this generation. It is
     * cleared after a GC on this generations, and update before new
     * objects are added from a GC of a younger generation. Dividing by
     * the bytes_allocated will give the average age of the memory in
     * this generation since its last GC. */
    int cum_sum_bytes_allocated;

    /* a minimum average memory age before a GC will occur helps
     * prevent a GC when a large number of new live objects have been
     * added, in which case a GC could be a waste of time */
    double min_av_mem_age;
};
/* the number of actual generations. (The number of 'struct
 * generation' objects is one more than this, because one object
 * serves as scratch when GC'ing.) */
#define NUM_GENERATIONS 6

/* an array of generation structures. There needs to be one more
 * generation structure than actual generations as the oldest
 * generation is temporarily raised then lowered. */
struct generation generations[NUM_GENERATIONS+1];

/* the oldest generation that is will currently be GCed by default.
 * Valid values are: 0, 1, ... (NUM_GENERATIONS-1)
 *
 * The default of (NUM_GENERATIONS-1) enables GC on all generations.
 *
 * Setting this to 0 effectively disables the generational nature of
 * the GC. In some applications generational GC may not be useful
 * because there are no long-lived objects.
 *
 * An intermediate value could be handy after moving long-lived data
 * into an older generation so an unnecessary GC of this long-lived
 * data can be avoided. */
unsigned int  gencgc_oldest_gen_to_gc = NUM_GENERATIONS-1;

/* The maximum free page in the heap is maintained and used to update
 * ALLOCATION_POINTER which is used by the room function to limit its
 * search of the heap. XX Gencgc obviously needs to be better
 * integrated with the Lisp code. */
static int  last_free_page;
\f
/* This lock is to prevent multiple threads from simultaneously
 * allocating new regions which overlap each other.  Note that the
 * majority of GC is single-threaded, but alloc() may be called from
 * >1 thread at a time and must be thread-safe.  This lock must be
 * seized before all accesses to generations[] or to parts of
 * page_table[] that other threads may want to see */

static lispobj free_pages_lock=0;

\f
/*
 * miscellaneous heap functions
 */

/* Count the number of pages which are write-protected within the
 * given generation. */
static int
count_write_protect_generation_pages(int generation)
{
    int i;
    int count = 0;

    for (i = 0; i < last_free_page; i++)
        if ((page_table[i].allocated != FREE_PAGE)
            && (page_table[i].gen == generation)
            && (page_table[i].write_protected == 1))
            count++;
    return count;
}

/* Count the number of pages within the given generation. */
static int
count_generation_pages(int generation)
{
    int i;
    int count = 0;

    for (i = 0; i < last_free_page; i++)
        if ((page_table[i].allocated != 0)
            && (page_table[i].gen == generation))
            count++;
    return count;
}

/* Count the number of dont_move pages. */
static int
count_dont_move_pages(void)
{
    int i;
    int count = 0;
    for (i = 0; i < last_free_page; i++) {
        if ((page_table[i].allocated != 0) && (page_table[i].dont_move != 0)) {
            ++count;
        }
    }
    return count;
}

/* Work through the pages and add up the number of bytes used for the
 * given generation. */
static int
count_generation_bytes_allocated (int gen)
{
    int i;
    int result = 0;
    for (i = 0; i < last_free_page; i++) {
        if ((page_table[i].allocated != 0) && (page_table[i].gen == gen))
            result += page_table[i].bytes_used;
    }
    return result;
}

/* Return the average age of the memory in a generation. */
static double
gen_av_mem_age(int gen)
{
    if (generations[gen].bytes_allocated == 0)
        return 0.0;

    return
        ((double)generations[gen].cum_sum_bytes_allocated)
        / ((double)generations[gen].bytes_allocated);
}

void fpu_save(int *);           /* defined in x86-assem.S */
void fpu_restore(int *);        /* defined in x86-assem.S */
/* The verbose argument controls how much to print: 0 for normal
 * level of detail; 1 for debugging. */
static void
print_generation_stats(int verbose) /* FIXME: should take FILE argument */
{
    int i, gens;
    int fpu_state[27];

    /* This code uses the FP instructions which may be set up for Lisp
     * so they need to be saved and reset for C. */
    fpu_save(fpu_state);

    /* number of generations to print */
    if (verbose)
        gens = NUM_GENERATIONS+1;
    else
        gens = NUM_GENERATIONS;

    /* Print the heap stats. */
    fprintf(stderr,
            "   Generation Boxed Unboxed LB   LUB    Alloc  Waste   Trig    WP  GCs Mem-age\n");

    for (i = 0; i < gens; i++) {
        int j;
        int boxed_cnt = 0;
        int unboxed_cnt = 0;
        int large_boxed_cnt = 0;
        int large_unboxed_cnt = 0;

        for (j = 0; j < last_free_page; j++)
            if (page_table[j].gen == i) {

                /* Count the number of boxed pages within the given
                 * generation. */
                if (page_table[j].allocated & BOXED_PAGE) {
                    if (page_table[j].large_object)
                        large_boxed_cnt++;
                    else
                        boxed_cnt++;
                }

                /* Count the number of unboxed pages within the given
                 * generation. */
                if (page_table[j].allocated & UNBOXED_PAGE) {
                    if (page_table[j].large_object)
                        large_unboxed_cnt++;
                    else
                        unboxed_cnt++;
                }
            }

        gc_assert(generations[i].bytes_allocated
                  == count_generation_bytes_allocated(i));
        fprintf(stderr,
                "   %8d: %5d %5d %5d %5d %8d %5d %8d %4d %3d %7.4f\n",
                i,
                boxed_cnt, unboxed_cnt, large_boxed_cnt, large_unboxed_cnt,
                generations[i].bytes_allocated,
                (count_generation_pages(i)*4096
                 - generations[i].bytes_allocated),
                generations[i].gc_trigger,
                count_write_protect_generation_pages(i),
                generations[i].num_gc,
                gen_av_mem_age(i));
    }
    fprintf(stderr,"   Total bytes allocated=%ld\n", bytes_allocated);

    fpu_restore(fpu_state);
}
\f
/*
 * allocation routines
 */

/*
 * To support quick and inline allocation, regions of memory can be
 * allocated and then allocated from with just a free pointer and a
 * check against an end address.
 *
 * Since objects can be allocated to spaces with different properties
 * e.g. boxed/unboxed, generation, ages; there may need to be many
 * allocation regions.
 *
 * Each allocation region may be start within a partly used page. Many
 * features of memory use are noted on a page wise basis, e.g. the
 * generation; so if a region starts within an existing allocated page
 * it must be consistent with this page.
 *
 * During the scavenging of the newspace, objects will be transported
 * into an allocation region, and pointers updated to point to this
 * allocation region. It is possible that these pointers will be
 * scavenged again before the allocation region is closed, e.g. due to
 * trans_list which jumps all over the place to cleanup the list. It
 * is important to be able to determine properties of all objects
 * pointed to when scavenging, e.g to detect pointers to the oldspace.
 * Thus it's important that the allocation regions have the correct
 * properties set when allocated, and not just set when closed. The
 * region allocation routines return regions with the specified
 * properties, and grab all the pages, setting their properties
 * appropriately, except that the amount used is not known.
 *
 * These regions are used to support quicker allocation using just a
 * free pointer. The actual space used by the region is not reflected
 * in the pages tables until it is closed. It can't be scavenged until
 * closed.
 *
 * When finished with the region it should be closed, which will
 * update the page tables for the actual space used returning unused
 * space. Further it may be noted in the new regions which is
 * necessary when scavenging the newspace.
 *
 * Large objects may be allocated directly without an allocation
 * region, the page tables are updated immediately.
 *
 * Unboxed objects don't contain pointers to other objects and so
 * don't need scavenging. Further they can't contain pointers to
 * younger generations so WP is not needed. By allocating pages to
 * unboxed objects the whole page never needs scavenging or
 * write-protecting. */

/* We are only using two regions at present. Both are for the current
 * newspace generation. */
struct alloc_region boxed_region;
struct alloc_region unboxed_region;

/* The generation currently being allocated to. */
static int gc_alloc_generation;

/* Find a new region with room for at least the given number of bytes.
 *
 * It starts looking at the current generation's alloc_start_page. So
 * may pick up from the previous region if there is enough space. This
 * keeps the allocation contiguous when scavenging the newspace.
 *
 * The alloc_region should have been closed by a call to
 * gc_alloc_update_page_tables(), and will thus be in an empty state.
 *
 * To assist the scavenging functions write-protected pages are not
 * used. Free pages should not be write-protected.
 *
 * It is critical to the conservative GC that the start of regions be
 * known. To help achieve this only small regions are allocated at a
 * time.
 *
 * During scavenging, pointers may be found to within the current
 * region and the page generation must be set so that pointers to the
 * from space can be recognized. Therefore the generation of pages in
 * the region are set to gc_alloc_generation. To prevent another
 * allocation call using the same pages, all the pages in the region
 * are allocated, although they will initially be empty.
 */
static void
gc_alloc_new_region(int nbytes, int unboxed, struct alloc_region *alloc_region)
{
    int first_page;
    int last_page;
    int bytes_found;
    int i;

    /*
    FSHOW((stderr,
           "/alloc_new_region for %d bytes from gen %d\n",
           nbytes, gc_alloc_generation));
    */

    /* Check that the region is in a reset state. */
    gc_assert((alloc_region->first_page == 0)
              && (alloc_region->last_page == -1)
              && (alloc_region->free_pointer == alloc_region->end_addr));
    get_spinlock(&free_pages_lock,(int) alloc_region);
    if (unboxed) {
        first_page =
            generations[gc_alloc_generation].alloc_unboxed_start_page;
    } else {
        first_page =
            generations[gc_alloc_generation].alloc_start_page;
    }
    last_page=gc_find_freeish_pages(&first_page,nbytes,unboxed,alloc_region);
    bytes_found=(4096 - page_table[first_page].bytes_used)
            + 4096*(last_page-first_page);

    /* Set up the alloc_region. */
    alloc_region->first_page = first_page;
    alloc_region->last_page = last_page;
    alloc_region->start_addr = page_table[first_page].bytes_used
        + page_address(first_page);
    alloc_region->free_pointer = alloc_region->start_addr;
    alloc_region->end_addr = alloc_region->start_addr + bytes_found;

    /* Set up the pages. */

    /* The first page may have already been in use. */
    if (page_table[first_page].bytes_used == 0) {
        if (unboxed)
            page_table[first_page].allocated = UNBOXED_PAGE;
        else
            page_table[first_page].allocated = BOXED_PAGE;
        page_table[first_page].gen = gc_alloc_generation;
        page_table[first_page].large_object = 0;
        page_table[first_page].first_object_offset = 0;
    }

    if (unboxed)
        gc_assert(page_table[first_page].allocated == UNBOXED_PAGE);
    else
        gc_assert(page_table[first_page].allocated == BOXED_PAGE);
    page_table[first_page].allocated |= OPEN_REGION_PAGE; 

    gc_assert(page_table[first_page].gen == gc_alloc_generation);
    gc_assert(page_table[first_page].large_object == 0);

    for (i = first_page+1; i <= last_page; i++) {
        if (unboxed)
            page_table[i].allocated = UNBOXED_PAGE;
        else
            page_table[i].allocated = BOXED_PAGE;
        page_table[i].gen = gc_alloc_generation;
        page_table[i].large_object = 0;
        /* This may not be necessary for unboxed regions (think it was
         * broken before!) */
        page_table[i].first_object_offset =
            alloc_region->start_addr - page_address(i);
        page_table[i].allocated |= OPEN_REGION_PAGE ;
    }
    /* Bump up last_free_page. */
    if (last_page+1 > last_free_page) {
        last_free_page = last_page+1;
        SetSymbolValue(ALLOCATION_POINTER,
                       (lispobj)(((char *)heap_base) + last_free_page*4096),
                       0);
    }
    release_spinlock(&free_pages_lock);
    
    /* we can do this after releasing free_pages_lock */
    if (gencgc_zero_check) {
        int *p;
        for (p = (int *)alloc_region->start_addr;
             p < (int *)alloc_region->end_addr; p++) {
            if (*p != 0) {
                /* KLUDGE: It would be nice to use %lx and explicit casts
                 * (long) in code like this, so that it is less likely to
                 * break randomly when running on a machine with different
                 * word sizes. -- WHN 19991129 */
                lose("The new region at %x is not zero.", p);
            }
    }
}

}

/* If the record_new_objects flag is 2 then all new regions created
 * are recorded.
 *
 * If it's 1 then then it is only recorded if the first page of the
 * current region is <= new_areas_ignore_page. This helps avoid
 * unnecessary recording when doing full scavenge pass.
 *
 * The new_object structure holds the page, byte offset, and size of
 * new regions of objects. Each new area is placed in the array of
 * these structures pointer to by new_areas. new_areas_index holds the
 * offset into new_areas.
 *
 * If new_area overflows NUM_NEW_AREAS then it stops adding them. The
 * later code must detect this and handle it, probably by doing a full
 * scavenge of a generation. */
#define NUM_NEW_AREAS 512
static int record_new_objects = 0;
static int new_areas_ignore_page;
struct new_area {
    int  page;
    int  offset;
    int  size;
};
static struct new_area (*new_areas)[];
static int new_areas_index;
int max_new_areas;

/* Add a new area to new_areas. */
static void
add_new_area(int first_page, int offset, int size)
{
    unsigned new_area_start,c;
    int i;

    /* Ignore if full. */
    if (new_areas_index >= NUM_NEW_AREAS)
        return;

    switch (record_new_objects) {
    case 0:
        return;
    case 1:
        if (first_page > new_areas_ignore_page)
            return;
        break;
    case 2:
        break;
    default:
        gc_abort();
    }

    new_area_start = 4096*first_page + offset;

    /* Search backwards for a prior area that this follows from. If
       found this will save adding a new area. */
    for (i = new_areas_index-1, c = 0; (i >= 0) && (c < 8); i--, c++) {
        unsigned area_end =
            4096*((*new_areas)[i].page)
            + (*new_areas)[i].offset
            + (*new_areas)[i].size;
        /*FSHOW((stderr,
               "/add_new_area S1 %d %d %d %d\n",
               i, c, new_area_start, area_end));*/
        if (new_area_start == area_end) {
            /*FSHOW((stderr,
                   "/adding to [%d] %d %d %d with %d %d %d:\n",
                   i,
                   (*new_areas)[i].page,
                   (*new_areas)[i].offset,
                   (*new_areas)[i].size,
                   first_page,
                   offset,
                    size);*/
            (*new_areas)[i].size += size;
            return;
        }
    }

    (*new_areas)[new_areas_index].page = first_page;
    (*new_areas)[new_areas_index].offset = offset;
    (*new_areas)[new_areas_index].size = size;
    /*FSHOW((stderr,
           "/new_area %d page %d offset %d size %d\n",
           new_areas_index, first_page, offset, size));*/
    new_areas_index++;

    /* Note the max new_areas used. */
    if (new_areas_index > max_new_areas)
        max_new_areas = new_areas_index;
}

/* Update the tables for the alloc_region. The region maybe added to
 * the new_areas.
 *
 * When done the alloc_region is set up so that the next quick alloc
 * will fail safely and thus a new region will be allocated. Further
 * it is safe to try to re-update the page table of this reset
 * alloc_region. */
void
gc_alloc_update_page_tables(int unboxed, struct alloc_region *alloc_region)
{
    int more;
    int first_page;
    int next_page;
    int bytes_used;
    int orig_first_page_bytes_used;
    int region_size;
    int byte_cnt;

    /*
    FSHOW((stderr,
           "/gc_alloc_update_page_tables() to gen %d:\n",
           gc_alloc_generation));
    */

    first_page = alloc_region->first_page;

    /* Catch an unused alloc_region. */
    if ((first_page == 0) && (alloc_region->last_page == -1))
        return;

    next_page = first_page+1;

    get_spinlock(&free_pages_lock,(int) alloc_region);
    if (alloc_region->free_pointer != alloc_region->start_addr) {
        /* some bytes were allocated in the region */
        orig_first_page_bytes_used = page_table[first_page].bytes_used;

        gc_assert(alloc_region->start_addr == (page_address(first_page) + page_table[first_page].bytes_used));

        /* All the pages used need to be updated */

        /* Update the first page. */

        /* If the page was free then set up the gen, and
         * first_object_offset. */
        if (page_table[first_page].bytes_used == 0)
            gc_assert(page_table[first_page].first_object_offset == 0);
        page_table[first_page].allocated &= ~(OPEN_REGION_PAGE);

        if (unboxed)
            gc_assert(page_table[first_page].allocated == UNBOXED_PAGE);
        else
            gc_assert(page_table[first_page].allocated == BOXED_PAGE);
        gc_assert(page_table[first_page].gen == gc_alloc_generation);
        gc_assert(page_table[first_page].large_object == 0);

        byte_cnt = 0;

        /* Calculate the number of bytes used in this page. This is not
         * always the number of new bytes, unless it was free. */
        more = 0;
        if ((bytes_used = (alloc_region->free_pointer - page_address(first_page)))>4096) {
            bytes_used = 4096;
            more = 1;
        }
        page_table[first_page].bytes_used = bytes_used;
        byte_cnt += bytes_used;


        /* All the rest of the pages should be free. We need to set their
         * first_object_offset pointer to the start of the region, and set
         * the bytes_used. */
        while (more) {
            page_table[next_page].allocated &= ~(OPEN_REGION_PAGE);
            if (unboxed)
                gc_assert(page_table[next_page].allocated == UNBOXED_PAGE);
            else
                gc_assert(page_table[next_page].allocated == BOXED_PAGE);
            gc_assert(page_table[next_page].bytes_used == 0);
            gc_assert(page_table[next_page].gen == gc_alloc_generation);
            gc_assert(page_table[next_page].large_object == 0);

            gc_assert(page_table[next_page].first_object_offset ==
                      alloc_region->start_addr - page_address(next_page));

            /* Calculate the number of bytes used in this page. */
            more = 0;
            if ((bytes_used = (alloc_region->free_pointer
                               - page_address(next_page)))>4096) {
                bytes_used = 4096;
                more = 1;
            }
            page_table[next_page].bytes_used = bytes_used;
            byte_cnt += bytes_used;

            next_page++;
        }

        region_size = alloc_region->free_pointer - alloc_region->start_addr;
        bytes_allocated += region_size;
        generations[gc_alloc_generation].bytes_allocated += region_size;

        gc_assert((byte_cnt- orig_first_page_bytes_used) == region_size);

        /* Set the generations alloc restart page to the last page of
         * the region. */
        if (unboxed)
            generations[gc_alloc_generation].alloc_unboxed_start_page =
                next_page-1;
        else
            generations[gc_alloc_generation].alloc_start_page = next_page-1;

        /* Add the region to the new_areas if requested. */
        if (!unboxed)
            add_new_area(first_page,orig_first_page_bytes_used, region_size);

        /*
        FSHOW((stderr,
               "/gc_alloc_update_page_tables update %d bytes to gen %d\n",
               region_size,
               gc_alloc_generation));
        */
    } else {
        /* There are no bytes allocated. Unallocate the first_page if
         * there are 0 bytes_used. */
        page_table[first_page].allocated &= ~(OPEN_REGION_PAGE);
        if (page_table[first_page].bytes_used == 0)
            page_table[first_page].allocated = FREE_PAGE;
    }

    /* Unallocate any unused pages. */
    while (next_page <= alloc_region->last_page) {
        gc_assert(page_table[next_page].bytes_used == 0);
        page_table[next_page].allocated = FREE_PAGE;
        next_page++;
    }
    release_spinlock(&free_pages_lock);
    /* alloc_region is per-thread, we're ok to do this unlocked */
    gc_set_region_empty(alloc_region);
}

static inline void *gc_quick_alloc(int nbytes);

/* Allocate a possibly large object. */
void *
gc_alloc_large(int nbytes, int unboxed, struct alloc_region *alloc_region)
{
    int first_page;
    int last_page;
    int orig_first_page_bytes_used;
    int byte_cnt;
    int more;
    int bytes_used;
    int next_page;
    int large = (nbytes >= large_object_size);

    /*
    if (nbytes > 200000)
        FSHOW((stderr, "/alloc_large %d\n", nbytes));
    */

    /*
    FSHOW((stderr,
           "/gc_alloc_large() for %d bytes from gen %d\n",
           nbytes, gc_alloc_generation));
    */

    /* If the object is small, and there is room in the current region
       then allocate it in the current region. */
    if (!large
        && ((alloc_region->end_addr-alloc_region->free_pointer) >= nbytes))
        return gc_quick_alloc(nbytes);

    /* To allow the allocation of small objects without the danger of
       using a page in the current boxed region, the search starts after
       the current boxed free region. XX could probably keep a page
       index ahead of the current region and bumped up here to save a
       lot of re-scanning. */

    get_spinlock(&free_pages_lock,(int) alloc_region);

    if (unboxed) {
        first_page =
            generations[gc_alloc_generation].alloc_large_unboxed_start_page;
    } else {
        first_page = generations[gc_alloc_generation].alloc_large_start_page;
    }
    if (first_page <= alloc_region->last_page) {
        first_page = alloc_region->last_page+1;
    }

    last_page=gc_find_freeish_pages(&first_page,nbytes,unboxed,0);

    gc_assert(first_page > alloc_region->last_page);
    if (unboxed)
        generations[gc_alloc_generation].alloc_large_unboxed_start_page =
            last_page;
    else
        generations[gc_alloc_generation].alloc_large_start_page = last_page;

    /* Set up the pages. */
    orig_first_page_bytes_used = page_table[first_page].bytes_used;

    /* If the first page was free then set up the gen, and
     * first_object_offset. */
    if (page_table[first_page].bytes_used == 0) {
        if (unboxed)
            page_table[first_page].allocated = UNBOXED_PAGE;
        else
            page_table[first_page].allocated = BOXED_PAGE;
        page_table[first_page].gen = gc_alloc_generation;
        page_table[first_page].first_object_offset = 0;
        page_table[first_page].large_object = large;
    }

    if (unboxed)
        gc_assert(page_table[first_page].allocated == UNBOXED_PAGE);
    else
        gc_assert(page_table[first_page].allocated == BOXED_PAGE);
    gc_assert(page_table[first_page].gen == gc_alloc_generation);
    gc_assert(page_table[first_page].large_object == large);

    byte_cnt = 0;

    /* Calc. the number of bytes used in this page. This is not
     * always the number of new bytes, unless it was free. */
    more = 0;
    if ((bytes_used = nbytes+orig_first_page_bytes_used) > 4096) {
        bytes_used = 4096;
        more = 1;
    }
    page_table[first_page].bytes_used = bytes_used;
    byte_cnt += bytes_used;

    next_page = first_page+1;

    /* All the rest of the pages should be free. We need to set their
     * first_object_offset pointer to the start of the region, and
     * set the bytes_used. */
    while (more) {
        gc_assert(page_table[next_page].allocated == FREE_PAGE);
        gc_assert(page_table[next_page].bytes_used == 0);
        if (unboxed)
            page_table[next_page].allocated = UNBOXED_PAGE;
        else
            page_table[next_page].allocated = BOXED_PAGE;
        page_table[next_page].gen = gc_alloc_generation;
        page_table[next_page].large_object = large;

        page_table[next_page].first_object_offset =
            orig_first_page_bytes_used - 4096*(next_page-first_page);

        /* Calculate the number of bytes used in this page. */
        more = 0;
        if ((bytes_used=(nbytes+orig_first_page_bytes_used)-byte_cnt) > 4096) {
            bytes_used = 4096;
            more = 1;
        }
        page_table[next_page].bytes_used = bytes_used;
        byte_cnt += bytes_used;

        next_page++;
    }

    gc_assert((byte_cnt-orig_first_page_bytes_used) == nbytes);

    bytes_allocated += nbytes;
    generations[gc_alloc_generation].bytes_allocated += nbytes;

    /* Add the region to the new_areas if requested. */
    if (!unboxed)
        add_new_area(first_page,orig_first_page_bytes_used,nbytes);

    /* Bump up last_free_page */
    if (last_page+1 > last_free_page) {
        last_free_page = last_page+1;
        SetSymbolValue(ALLOCATION_POINTER,
                       (lispobj)(((char *)heap_base) + last_free_page*4096),0);
    }
    release_spinlock(&free_pages_lock);

    return((void *)(page_address(first_page)+orig_first_page_bytes_used));
}

int
gc_find_freeish_pages(int *restart_page_ptr, int nbytes, int unboxed, struct alloc_region *alloc_region)
{
    /* if alloc_region is 0, we assume this is for a potentially large
       object */
    int first_page;
    int last_page;
    int region_size;
    int restart_page=*restart_page_ptr;
    int bytes_found;
    int num_pages;
    int large = !alloc_region && (nbytes >= large_object_size);

    gc_assert(free_pages_lock);
    /* Search for a contiguous free space of at least nbytes. If it's a
       large object then align it on a page boundary by searching for a
       free page. */

    /* To allow the allocation of small objects without the danger of
       using a page in the current boxed region, the search starts after
       the current boxed free region. XX could probably keep a page
       index ahead of the current region and bumped up here to save a
       lot of re-scanning. */

    do {
        first_page = restart_page;
        if (large)              
            while ((first_page < NUM_PAGES)
                   && (page_table[first_page].allocated != FREE_PAGE))
                first_page++;
        else
            while (first_page < NUM_PAGES) {
                if(page_table[first_page].allocated == FREE_PAGE)
                    break;
                /* I don't know why we need the gen=0 test, but it
                 * breaks randomly if that's omitted -dan 2003.02.26
                 */
                if((page_table[first_page].allocated ==
                    (unboxed ? UNBOXED_PAGE : BOXED_PAGE)) &&
                   (page_table[first_page].large_object == 0) &&
                   (gc_alloc_generation == 0) &&
                   (page_table[first_page].gen == gc_alloc_generation) &&
                   (page_table[first_page].bytes_used < (4096-32)) &&
                   (page_table[first_page].write_protected == 0) &&
                   (page_table[first_page].dont_move == 0))
                    break;
                first_page++;
            }
        
        if (first_page >= NUM_PAGES) {
            fprintf(stderr,
                    "Argh! gc_find_free_space failed (first_page), nbytes=%d.\n",
                    nbytes);
            print_generation_stats(1);
            lose(NULL);
        }

        gc_assert(page_table[first_page].write_protected == 0);

        last_page = first_page;
        bytes_found = 4096 - page_table[first_page].bytes_used;
        num_pages = 1;
        while (((bytes_found < nbytes) 
                || (alloc_region && (num_pages < 2)))
               && (last_page < (NUM_PAGES-1))
               && (page_table[last_page+1].allocated == FREE_PAGE)) {
            last_page++;
            num_pages++;
            bytes_found += 4096;
            gc_assert(page_table[last_page].write_protected == 0);
        }

        region_size = (4096 - page_table[first_page].bytes_used)
            + 4096*(last_page-first_page);

        gc_assert(bytes_found == region_size);
        restart_page = last_page + 1;
    } while ((restart_page < NUM_PAGES) && (bytes_found < nbytes));

    /* Check for a failure */
    if ((restart_page >= NUM_PAGES) && (bytes_found < nbytes)) {
        fprintf(stderr,
                "Argh! gc_find_freeish_pages failed (restart_page), nbytes=%d.\n",
                nbytes);
        print_generation_stats(1);
        lose(NULL);
    }
    *restart_page_ptr=first_page;
    return last_page;
}

/* Allocate bytes.  All the rest of the special-purpose allocation
 * functions will eventually call this (instead of just duplicating
 * parts of its code) */

void *
gc_alloc_with_region(int nbytes,int unboxed_p, struct alloc_region *my_region,
                     int quick_p)
{
    void *new_free_pointer;

    /* FSHOW((stderr, "/gc_alloc %d\n", nbytes)); */

    /* Check whether there is room in the current alloc region. */
    new_free_pointer = my_region->free_pointer + nbytes;

    if (new_free_pointer <= my_region->end_addr) {
        /* If so then allocate from the current alloc region. */
        void *new_obj = my_region->free_pointer;
        my_region->free_pointer = new_free_pointer;

        /* Unless a `quick' alloc was requested, check whether the
           alloc region is almost empty. */
        if (!quick_p &&
            (my_region->end_addr - my_region->free_pointer) <= 32) {
            /* If so, finished with the current region. */
            gc_alloc_update_page_tables(unboxed_p, my_region);
            /* Set up a new region. */
            gc_alloc_new_region(32 /*bytes*/, unboxed_p, my_region);
        }

        return((void *)new_obj);
    }

    /* Else not enough free space in the current region. */

    /* If there some room left in the current region, enough to be worth
     * saving, then allocate a large object. */
    /* FIXME: "32" should be a named parameter. */
    if ((my_region->end_addr-my_region->free_pointer) > 32)
        return gc_alloc_large(nbytes, unboxed_p, my_region);

    /* Else find a new region. */

    /* Finished with the current region. */
    gc_alloc_update_page_tables(unboxed_p, my_region);

    /* Set up a new region. */
    gc_alloc_new_region(nbytes, unboxed_p, my_region);

    /* Should now be enough room. */

    /* Check whether there is room in the current region. */
    new_free_pointer = my_region->free_pointer + nbytes;

    if (new_free_pointer <= my_region->end_addr) {
        /* If so then allocate from the current region. */
        void *new_obj = my_region->free_pointer;
        my_region->free_pointer = new_free_pointer;
        /* Check whether the current region is almost empty. */
        if ((my_region->end_addr - my_region->free_pointer) <= 32) {
            /* If so find, finished with the current region. */
            gc_alloc_update_page_tables(unboxed_p, my_region);

            /* Set up a new region. */
            gc_alloc_new_region(32, unboxed_p, my_region);
        }

        return((void *)new_obj);
    }

    /* shouldn't happen */
    gc_assert(0);
    return((void *) NIL); /* dummy value: return something ... */
}

void *
gc_general_alloc(int nbytes,int unboxed_p,int quick_p)
{
    struct alloc_region *my_region = 
      unboxed_p ? &unboxed_region : &boxed_region;
    return gc_alloc_with_region(nbytes,unboxed_p, my_region,quick_p);
}



static void *
gc_alloc(int nbytes,int unboxed_p)
{
    /* this is the only function that the external interface to
     * allocation presently knows how to call: Lisp code will never
     * allocate large objects, or to unboxed space, or `quick'ly.
     * Any of that stuff will only ever happen inside of GC */
    return gc_general_alloc(nbytes,unboxed_p,0);
}

/* Allocate space from the boxed_region. If there is not enough free
 * space then call gc_alloc to do the job. A pointer to the start of
 * the object is returned. */
static inline void *
gc_quick_alloc(int nbytes)
{
    return gc_general_alloc(nbytes,ALLOC_BOXED,ALLOC_QUICK);
}

/* Allocate space for the possibly large boxed object. If it is a
 * large object then do a large alloc else use gc_quick_alloc.  Note
 * that gc_quick_alloc will eventually fall through to
 * gc_general_alloc which may allocate the object in a large way
 * anyway, but based on decisions about the free space in the current
 * region, not the object size itself */

static inline void *
gc_quick_alloc_large(int nbytes)
{
    if (nbytes >= large_object_size)
        return gc_alloc_large(nbytes, ALLOC_BOXED, &boxed_region);
    else
        return gc_general_alloc(nbytes,ALLOC_BOXED,ALLOC_QUICK);
}

static inline void *
gc_alloc_unboxed(int nbytes)
{
    return gc_general_alloc(nbytes,ALLOC_UNBOXED,0);
}

static inline void *
gc_quick_alloc_unboxed(int nbytes)
{
    return gc_general_alloc(nbytes,ALLOC_UNBOXED,ALLOC_QUICK);
}

/* Allocate space for the object. If it is a large object then do a
 * large alloc else allocate from the current region. If there is not
 * enough free space then call general gc_alloc_unboxed() to do the job.
 *
 * A pointer to the start of the object is returned. */
static inline void *
gc_quick_alloc_large_unboxed(int nbytes)
{
    if (nbytes >= large_object_size)
        return gc_alloc_large(nbytes,ALLOC_UNBOXED,&unboxed_region);
    else
        return gc_quick_alloc_unboxed(nbytes);
}
\f
/*
 * scavenging/transporting routines derived from gc.c in CMU CL ca. 18b
 */

extern int (*scavtab[256])(lispobj *where, lispobj object);
extern lispobj (*transother[256])(lispobj object);
extern int (*sizetab[256])(lispobj *where);

/* Copy a large boxed object. If the object is in a large object
 * region then it is simply promoted, else it is copied. If it's large
 * enough then it's copied to a large object region.
 *
 * Vectors may have shrunk. If the object is not copied the space
 * needs to be reclaimed, and the page_tables corrected. */
lispobj
copy_large_object(lispobj object, int nwords)
{
    int tag;
    lispobj *new;
    lispobj *source, *dest;
    int first_page;

    gc_assert(is_lisp_pointer(object));
    gc_assert(from_space_p(object));
    gc_assert((nwords & 0x01) == 0);


    /* Check whether it's a large object. */
    first_page = find_page_index((void *)object);
    gc_assert(first_page >= 0);

    if (page_table[first_page].large_object) {

        /* Promote the object. */

        int remaining_bytes;
        int next_page;
        int bytes_freed;
        int old_bytes_used;

        /* Note: Any page write-protection must be removed, else a
         * later scavenge_newspace may incorrectly not scavenge these
         * pages. This would not be necessary if they are added to the
         * new areas, but let's do it for them all (they'll probably
         * be written anyway?). */

        gc_assert(page_table[first_page].first_object_offset == 0);

        next_page = first_page;
        remaining_bytes = nwords*4;
        while (remaining_bytes > 4096) {
            gc_assert(page_table[next_page].gen == from_space);
            gc_assert(page_table[next_page].allocated == BOXED_PAGE);
            gc_assert(page_table[next_page].large_object);
            gc_assert(page_table[next_page].first_object_offset==
                      -4096*(next_page-first_page));
            gc_assert(page_table[next_page].bytes_used == 4096);

            page_table[next_page].gen = new_space;

            /* Remove any write-protection. We should be able to rely
             * on the write-protect flag to avoid redundant calls. */
            if (page_table[next_page].write_protected) {
                os_protect(page_address(next_page), 4096, OS_VM_PROT_ALL);
                page_table[next_page].write_protected = 0;
            }
            remaining_bytes -= 4096;
            next_page++;
        }

        /* Now only one page remains, but the object may have shrunk
         * so there may be more unused pages which will be freed. */

        /* The object may have shrunk but shouldn't have grown. */
        gc_assert(page_table[next_page].bytes_used >= remaining_bytes);

        page_table[next_page].gen = new_space;
        gc_assert(page_table[next_page].allocated == BOXED_PAGE);

        /* Adjust the bytes_used. */
        old_bytes_used = page_table[next_page].bytes_used;
        page_table[next_page].bytes_used = remaining_bytes;

        bytes_freed = old_bytes_used - remaining_bytes;

        /* Free any remaining pages; needs care. */
        next_page++;
        while ((old_bytes_used == 4096) &&
               (page_table[next_page].gen == from_space) &&
               (page_table[next_page].allocated == BOXED_PAGE) &&
               page_table[next_page].large_object &&
               (page_table[next_page].first_object_offset ==
                -(next_page - first_page)*4096)) {
            /* Checks out OK, free the page. Don't need to bother zeroing
             * pages as this should have been done before shrinking the
             * object. These pages shouldn't be write-protected as they
             * should be zero filled. */
            gc_assert(page_table[next_page].write_protected == 0);

            old_bytes_used = page_table[next_page].bytes_used;
            page_table[next_page].allocated = FREE_PAGE;
            page_table[next_page].bytes_used = 0;
            bytes_freed += old_bytes_used;
            next_page++;
        }

        generations[from_space].bytes_allocated -= 4*nwords + bytes_freed;
        generations[new_space].bytes_allocated += 4*nwords;
        bytes_allocated -= bytes_freed;

        /* Add the region to the new_areas if requested. */
        add_new_area(first_page,0,nwords*4);

        return(object);
    } else {
        /* Get tag of object. */
        tag = lowtag_of(object);

        /* Allocate space. */
        new = gc_quick_alloc_large(nwords*4);

        dest = new;
        source = (lispobj *) native_pointer(object);

        /* Copy the object. */
        while (nwords > 0) {
            dest[0] = source[0];
            dest[1] = source[1];
            dest += 2;
            source += 2;
            nwords -= 2;
        }

        /* Return Lisp pointer of new object. */
        return ((lispobj) new) | tag;
    }
}

/* to copy unboxed objects */
lispobj
copy_unboxed_object(lispobj object, int nwords)
{
    int tag;
    lispobj *new;
    lispobj *source, *dest;

    gc_assert(is_lisp_pointer(object));
    gc_assert(from_space_p(object));
    gc_assert((nwords & 0x01) == 0);

    /* Get tag of object. */
    tag = lowtag_of(object);

    /* Allocate space. */
    new = gc_quick_alloc_unboxed(nwords*4);

    dest = new;
    source = (lispobj *) native_pointer(object);

    /* Copy the object. */
    while (nwords > 0) {
        dest[0] = source[0];
        dest[1] = source[1];
        dest += 2;
        source += 2;
        nwords -= 2;
    }

    /* Return Lisp pointer of new object. */
    return ((lispobj) new) | tag;
}

/* to copy large unboxed objects
 *
 * If the object is in a large object region then it is simply
 * promoted, else it is copied. If it's large enough then it's copied
 * to a large object region.
 *
 * Bignums and vectors may have shrunk. If the object is not copied
 * the space needs to be reclaimed, and the page_tables corrected.
 *
 * KLUDGE: There's a lot of cut-and-paste duplication between this
 * function and copy_large_object(..). -- WHN 20000619 */
lispobj
copy_large_unboxed_object(lispobj object, int nwords)
{
    int tag;
    lispobj *new;
    lispobj *source, *dest;
    int first_page;

    gc_assert(is_lisp_pointer(object));
    gc_assert(from_space_p(object));
    gc_assert((nwords & 0x01) == 0);

    if ((nwords > 1024*1024) && gencgc_verbose)
        FSHOW((stderr, "/copy_large_unboxed_object: %d bytes\n", nwords*4));

    /* Check whether it's a large object. */
    first_page = find_page_index((void *)object);
    gc_assert(first_page >= 0);

    if (page_table[first_page].large_object) {
        /* Promote the object. Note: Unboxed objects may have been
         * allocated to a BOXED region so it may be necessary to
         * change the region to UNBOXED. */
        int remaining_bytes;
        int next_page;
        int bytes_freed;
        int old_bytes_used;

        gc_assert(page_table[first_page].first_object_offset == 0);

        next_page = first_page;
        remaining_bytes = nwords*4;
        while (remaining_bytes > 4096) {
            gc_assert(page_table[next_page].gen == from_space);
            gc_assert((page_table[next_page].allocated == UNBOXED_PAGE)
                      || (page_table[next_page].allocated == BOXED_PAGE));
            gc_assert(page_table[next_page].large_object);
            gc_assert(page_table[next_page].first_object_offset==
                      -4096*(next_page-first_page));
            gc_assert(page_table[next_page].bytes_used == 4096);

            page_table[next_page].gen = new_space;
            page_table[next_page].allocated = UNBOXED_PAGE;
            remaining_bytes -= 4096;
            next_page++;
        }

        /* Now only one page remains, but the object may have shrunk so
         * there may be more unused pages which will be freed. */

        /* Object may have shrunk but shouldn't have grown - check. */
        gc_assert(page_table[next_page].bytes_used >= remaining_bytes);

        page_table[next_page].gen = new_space;
        page_table[next_page].allocated = UNBOXED_PAGE;

        /* Adjust the bytes_used. */
        old_bytes_used = page_table[next_page].bytes_used;
        page_table[next_page].bytes_used = remaining_bytes;

        bytes_freed = old_bytes_used - remaining_bytes;

        /* Free any remaining pages; needs care. */
        next_page++;
        while ((old_bytes_used == 4096) &&
               (page_table[next_page].gen == from_space) &&
               ((page_table[next_page].allocated == UNBOXED_PAGE)
                || (page_table[next_page].allocated == BOXED_PAGE)) &&
               page_table[next_page].large_object &&
               (page_table[next_page].first_object_offset ==
                -(next_page - first_page)*4096)) {
            /* Checks out OK, free the page. Don't need to both zeroing
             * pages as this should have been done before shrinking the
             * object. These pages shouldn't be write-protected, even if
             * boxed they should be zero filled. */
            gc_assert(page_table[next_page].write_protected == 0);

            old_bytes_used = page_table[next_page].bytes_used;
            page_table[next_page].allocated = FREE_PAGE;
            page_table[next_page].bytes_used = 0;
            bytes_freed += old_bytes_used;
            next_page++;
        }

        if ((bytes_freed > 0) && gencgc_verbose)
            FSHOW((stderr,
                   "/copy_large_unboxed bytes_freed=%d\n",
                   bytes_freed));

        generations[from_space].bytes_allocated -= 4*nwords + bytes_freed;
        generations[new_space].bytes_allocated += 4*nwords;
        bytes_allocated -= bytes_freed;

        return(object);
    }
    else {
        /* Get tag of object. */
        tag = lowtag_of(object);

        /* Allocate space. */
        new = gc_quick_alloc_large_unboxed(nwords*4);

        dest = new;
        source = (lispobj *) native_pointer(object);

        /* Copy the object. */
        while (nwords > 0) {
            dest[0] = source[0];
            dest[1] = source[1];
            dest += 2;
            source += 2;
            nwords -= 2;
        }

        /* Return Lisp pointer of new object. */
        return ((lispobj) new) | tag;
    }
}



\f

/*
 * code and code-related objects
 */
/*
static lispobj trans_fun_header(lispobj object);
static lispobj trans_boxed(lispobj object);
*/

/* Scan a x86 compiled code object, looking for possible fixups that
 * have been missed after a move.
 *
 * Two types of fixups are needed:
 * 1. Absolute fixups to within the code object.
 * 2. Relative fixups to outside the code object.
 *
 * Currently only absolute fixups to the constant vector, or to the
 * code area are checked. */
void
sniff_code_object(struct code *code, unsigned displacement)
{
    int nheader_words, ncode_words, nwords;
    void *p;
    void *constants_start_addr, *constants_end_addr;
    void *code_start_addr, *code_end_addr;
    int fixup_found = 0;

    if (!check_code_fixups)
        return;

    ncode_words = fixnum_value(code->code_size);
    nheader_words = HeaderValue(*(lispobj *)code);
    nwords = ncode_words + nheader_words;

    constants_start_addr = (void *)code + 5*4;
    constants_end_addr = (void *)code + nheader_words*4;
    code_start_addr = (void *)code + nheader_words*4;
    code_end_addr = (void *)code + nwords*4;

    /* Work through the unboxed code. */
    for (p = code_start_addr; p < code_end_addr; p++) {
        void *data = *(void **)p;
        unsigned d1 = *((unsigned char *)p - 1);
        unsigned d2 = *((unsigned char *)p - 2);
        unsigned d3 = *((unsigned char *)p - 3);
        unsigned d4 = *((unsigned char *)p - 4);
#if QSHOW
        unsigned d5 = *((unsigned char *)p - 5);
        unsigned d6 = *((unsigned char *)p - 6);
#endif

        /* Check for code references. */
        /* Check for a 32 bit word that looks like an absolute
           reference to within the code adea of the code object. */
        if ((data >= (code_start_addr-displacement))
            && (data < (code_end_addr-displacement))) {
            /* function header */
            if ((d4 == 0x5e)
                && (((unsigned)p - 4 - 4*HeaderValue(*((unsigned *)p-1))) == (unsigned)code)) {
                /* Skip the function header */
                p += 6*4 - 4 - 1;
                continue;
            }
            /* the case of PUSH imm32 */
            if (d1 == 0x68) {
                fixup_found = 1;
                FSHOW((stderr,
                       "/code ref @%x: %.2x %.2x %.2x %.2x %.2x %.2x (%.8x)\n",
                       p, d6, d5, d4, d3, d2, d1, data));
                FSHOW((stderr, "/PUSH $0x%.8x\n", data));
            }
            /* the case of MOV [reg-8],imm32 */
            if ((d3 == 0xc7)
                && (d2==0x40 || d2==0x41 || d2==0x42 || d2==0x43
                    || d2==0x45 || d2==0x46 || d2==0x47)
                && (d1 == 0xf8)) {
                fixup_found = 1;
                FSHOW((stderr,
                       "/code ref @%x: %.2x %.2x %.2x %.2x %.2x %.2x (%.8x)\n",
                       p, d6, d5, d4, d3, d2, d1, data));
                FSHOW((stderr, "/MOV [reg-8],$0x%.8x\n", data));
            }
            /* the case of LEA reg,[disp32] */
            if ((d2 == 0x8d) && ((d1 & 0xc7) == 5)) {
                fixup_found = 1;
                FSHOW((stderr,
                       "/code ref @%x: %.2x %.2x %.2x %.2x %.2x %.2x (%.8x)\n",
                       p, d6, d5, d4, d3, d2, d1, data));
                FSHOW((stderr,"/LEA reg,[$0x%.8x]\n", data));
            }
        }

        /* Check for constant references. */
        /* Check for a 32 bit word that looks like an absolute
           reference to within the constant vector. Constant references
           will be aligned. */
        if ((data >= (constants_start_addr-displacement))
            && (data < (constants_end_addr-displacement))
            && (((unsigned)data & 0x3) == 0)) {
            /*  Mov eax,m32 */
            if (d1 == 0xa1) {
                fixup_found = 1;
                FSHOW((stderr,
                       "/abs const ref @%x: %.2x %.2x %.2x %.2x %.2x %.2x (%.8x)\n",
                       p, d6, d5, d4, d3, d2, d1, data));
                FSHOW((stderr,"/MOV eax,0x%.8x\n", data));
            }

            /*  the case of MOV m32,EAX */
            if (d1 == 0xa3) {
                fixup_found = 1;
                FSHOW((stderr,
                       "/abs const ref @%x: %.2x %.2x %.2x %.2x %.2x %.2x (%.8x)\n",
                       p, d6, d5, d4, d3, d2, d1, data));
                FSHOW((stderr, "/MOV 0x%.8x,eax\n", data));
            }

            /* the case of CMP m32,imm32 */             
            if ((d1 == 0x3d) && (d2 == 0x81)) {
                fixup_found = 1;
                FSHOW((stderr,
                       "/abs const ref @%x: %.2x %.2x %.2x %.2x %.2x %.2x (%.8x)\n",
                       p, d6, d5, d4, d3, d2, d1, data));
                /* XX Check this */
                FSHOW((stderr, "/CMP 0x%.8x,immed32\n", data));
            }

            /* Check for a mod=00, r/m=101 byte. */
            if ((d1 & 0xc7) == 5) {
                /* Cmp m32,reg */
                if (d2 == 0x39) {
                    fixup_found = 1;
                    FSHOW((stderr,
                           "/abs const ref @%x: %.2x %.2x %.2x %.2x %.2x %.2x (%.8x)\n",
                           p, d6, d5, d4, d3, d2, d1, data));
                    FSHOW((stderr,"/CMP 0x%.8x,reg\n", data));
                }
                /* the case of CMP reg32,m32 */
                if (d2 == 0x3b) {
                    fixup_found = 1;
                    FSHOW((stderr,
                           "/abs const ref @%x: %.2x %.2x %.2x %.2x %.2x %.2x (%.8x)\n",
                           p, d6, d5, d4, d3, d2, d1, data));
                    FSHOW((stderr, "/CMP reg32,0x%.8x\n", data));
                }
                /* the case of MOV m32,reg32 */
                if (d2 == 0x89) {
                    fixup_found = 1;
                    FSHOW((stderr,
                           "/abs const ref @%x: %.2x %.2x %.2x %.2x %.2x %.2x (%.8x)\n",
                           p, d6, d5, d4, d3, d2, d1, data));
                    FSHOW((stderr, "/MOV 0x%.8x,reg32\n", data));
                }
                /* the case of MOV reg32,m32 */
                if (d2 == 0x8b) {
                    fixup_found = 1;
                    FSHOW((stderr,
                           "/abs const ref @%x: %.2x %.2x %.2x %.2x %.2x %.2x (%.8x)\n",
                           p, d6, d5, d4, d3, d2, d1, data));
                    FSHOW((stderr, "/MOV reg32,0x%.8x\n", data));
                }
                /* the case of LEA reg32,m32 */
                if (d2 == 0x8d) {
                    fixup_found = 1;
                    FSHOW((stderr,
                           "abs const ref @%x: %.2x %.2x %.2x %.2x %.2x %.2x (%.8x)\n",
                           p, d6, d5, d4, d3, d2, d1, data));
                    FSHOW((stderr, "/LEA reg32,0x%.8x\n", data));
                }
            }
        }
    }

    /* If anything was found, print some information on the code
     * object. */
    if (fixup_found) {
        FSHOW((stderr,
               "/compiled code object at %x: header words = %d, code words = %d\n",
               code, nheader_words, ncode_words));
        FSHOW((stderr,
               "/const start = %x, end = %x\n",
               constants_start_addr, constants_end_addr));
        FSHOW((stderr,
               "/code start = %x, end = %x\n",
               code_start_addr, code_end_addr));
    }
}

void
gencgc_apply_code_fixups(struct code *old_code, struct code *new_code)
{
    int nheader_words, ncode_words, nwords;
    void *constants_start_addr, *constants_end_addr;
    void *code_start_addr, *code_end_addr;
    lispobj fixups = NIL;
    unsigned displacement = (unsigned)new_code - (unsigned)old_code;
    struct vector *fixups_vector;

    ncode_words = fixnum_value(new_code->code_size);
    nheader_words = HeaderValue(*(lispobj *)new_code);
    nwords = ncode_words + nheader_words;
    /* FSHOW((stderr,
             "/compiled code object at %x: header words = %d, code words = %d\n",
             new_code, nheader_words, ncode_words)); */
    constants_start_addr = (void *)new_code + 5*4;
    constants_end_addr = (void *)new_code + nheader_words*4;
    code_start_addr = (void *)new_code + nheader_words*4;
    code_end_addr = (void *)new_code + nwords*4;
    /*
    FSHOW((stderr,
           "/const start = %x, end = %x\n",
           constants_start_addr,constants_end_addr));
    FSHOW((stderr,
           "/code start = %x; end = %x\n",
           code_start_addr,code_end_addr));
    */

    /* The first constant should be a pointer to the fixups for this
       code objects. Check. */
    fixups = new_code->constants[0];

    /* It will be 0 or the unbound-marker if there are no fixups, and
     * will be an other pointer if it is valid. */
    if ((fixups == 0) || (fixups == UNBOUND_MARKER_WIDETAG) ||
        !is_lisp_pointer(fixups)) {
        /* Check for possible errors. */
        if (check_code_fixups)
            sniff_code_object(new_code, displacement);

        /*fprintf(stderr,"Fixups for code object not found!?\n");
          fprintf(stderr,"*** Compiled code object at %x: header_words=%d code_words=%d .\n",
          new_code, nheader_words, ncode_words);
          fprintf(stderr,"*** Const. start = %x; end= %x; Code start = %x; end = %x\n",
          constants_start_addr,constants_end_addr,
          code_start_addr,code_end_addr);*/
        return;
    }

    fixups_vector = (struct vector *)native_pointer(fixups);

    /* Could be pointing to a forwarding pointer. */
    if (is_lisp_pointer(fixups) &&
        (find_page_index((void*)fixups_vector) != -1) &&
        (fixups_vector->header == 0x01)) {
        /* If so, then follow it. */
        /*SHOW("following pointer to a forwarding pointer");*/
        fixups_vector = (struct vector *)native_pointer((lispobj)fixups_vector->length);
    }

    /*SHOW("got fixups");*/

    if (widetag_of(fixups_vector->header) ==
        SIMPLE_ARRAY_UNSIGNED_BYTE_32_WIDETAG) {
        /* Got the fixups for the code block. Now work through the vector,
           and apply a fixup at each address. */
        int length = fixnum_value(fixups_vector->length);
        int i;
        for (i = 0; i < length; i++) {
            unsigned offset = fixups_vector->data[i];
            /* Now check the current value of offset. */
            unsigned old_value =
                *(unsigned *)((unsigned)code_start_addr + offset);

            /* If it's within the old_code object then it must be an
             * absolute fixup (relative ones are not saved) */
            if ((old_value >= (unsigned)old_code)
                && (old_value < ((unsigned)old_code + nwords*4)))
                /* So add the dispacement. */
                *(unsigned *)((unsigned)code_start_addr + offset) =
                    old_value + displacement;
            else
                /* It is outside the old code object so it must be a
                 * relative fixup (absolute fixups are not saved). So
                 * subtract the displacement. */
                *(unsigned *)((unsigned)code_start_addr + offset) =
                    old_value - displacement;
        }
    }

    /* Check for possible errors. */
    if (check_code_fixups) {
        sniff_code_object(new_code,displacement);
    }
}


static lispobj
trans_boxed_large(lispobj object)
{
    lispobj header;
    unsigned long length;

    gc_assert(is_lisp_pointer(object));

    header = *((lispobj *) native_pointer(object));
    length = HeaderValue(header) + 1;
    length = CEILING(length, 2);

    return copy_large_object(object, length);
}


static lispobj
trans_unboxed_large(lispobj object)
{
    lispobj header;
    unsigned long length;


    gc_assert(is_lisp_pointer(object));

    header = *((lispobj *) native_pointer(object));
    length = HeaderValue(header) + 1;
    length = CEILING(length, 2);

    return copy_large_unboxed_object(object, length);
}

\f
/*
 * vector-like objects
 */


/* FIXME: What does this mean? */
int gencgc_hash = 1;

static int
scav_vector(lispobj *where, lispobj object)
{
    unsigned int kv_length;
    lispobj *kv_vector;
    unsigned int length = 0; /* (0 = dummy to stop GCC warning) */
    lispobj *hash_table;
    lispobj empty_symbol;
    unsigned int *index_vector = NULL; /* (NULL = dummy to stop GCC warning) */
    unsigned int *next_vector = NULL; /* (NULL = dummy to stop GCC warning) */
    unsigned int *hash_vector = NULL; /* (NULL = dummy to stop GCC warning) */
    lispobj weak_p_obj;
    unsigned next_vector_length = 0;

    /* FIXME: A comment explaining this would be nice. It looks as
     * though SB-VM:VECTOR-VALID-HASHING-SUBTYPE is set for EQ-based
     * hash tables in the Lisp HASH-TABLE code, and nowhere else. */
    if (HeaderValue(object) != subtype_VectorValidHashing)
        return 1;

    if (!gencgc_hash) {
        /* This is set for backward compatibility. FIXME: Do we need
         * this any more? */
        *where =
            (subtype_VectorMustRehash<<N_WIDETAG_BITS) | SIMPLE_VECTOR_WIDETAG;
        return 1;
    }

    kv_length = fixnum_value(where[1]);
    kv_vector = where + 2;  /* Skip the header and length. */
    /*FSHOW((stderr,"/kv_length = %d\n", kv_length));*/

    /* Scavenge element 0, which may be a hash-table structure. */
    scavenge(where+2, 1);
    if (!is_lisp_pointer(where[2])) {
        lose("no pointer at %x in hash table", where[2]);
    }
    hash_table = (lispobj *)native_pointer(where[2]);
    /*FSHOW((stderr,"/hash_table = %x\n", hash_table));*/
    if (widetag_of(hash_table[0]) != INSTANCE_HEADER_WIDETAG) {
        lose("hash table not instance (%x at %x)", hash_table[0], hash_table);
    }

    /* Scavenge element 1, which should be some internal symbol that
     * the hash table code reserves for marking empty slots. */
    scavenge(where+3, 1);
    if (!is_lisp_pointer(where[3])) {
        lose("not empty-hash-table-slot symbol pointer: %x", where[3]);
    }
    empty_symbol = where[3];
    /* fprintf(stderr,"* empty_symbol = %x\n", empty_symbol);*/
    if (widetag_of(*(lispobj *)native_pointer(empty_symbol)) !=
        SYMBOL_HEADER_WIDETAG) {
        lose("not a symbol where empty-hash-table-slot symbol expected: %x",
             *(lispobj *)native_pointer(empty_symbol));
    }

    /* Scavenge hash table, which will fix the positions of the other
     * needed objects. */
    scavenge(hash_table, 16);

    /* Cross-check the kv_vector. */
    if (where != (lispobj *)native_pointer(hash_table[9])) {
        lose("hash_table table!=this table %x", hash_table[9]);
    }

    /* WEAK-P */
    weak_p_obj = hash_table[10];

    /* index vector */
    {
        lispobj index_vector_obj = hash_table[13];

        if (is_lisp_pointer(index_vector_obj) &&
            (widetag_of(*(lispobj *)native_pointer(index_vector_obj)) ==
             SIMPLE_ARRAY_UNSIGNED_BYTE_32_WIDETAG)) {
            index_vector = ((unsigned int *)native_pointer(index_vector_obj)) + 2;
            /*FSHOW((stderr, "/index_vector = %x\n",index_vector));*/
            length = fixnum_value(((unsigned int *)native_pointer(index_vector_obj))[1]);
            /*FSHOW((stderr, "/length = %d\n", length));*/
        } else {
            lose("invalid index_vector %x", index_vector_obj);
        }
    }

    /* next vector */
    {
        lispobj next_vector_obj = hash_table[14];

        if (is_lisp_pointer(next_vector_obj) &&
            (widetag_of(*(lispobj *)native_pointer(next_vector_obj)) ==
             SIMPLE_ARRAY_UNSIGNED_BYTE_32_WIDETAG)) {
            next_vector = ((unsigned int *)native_pointer(next_vector_obj)) + 2;
            /*FSHOW((stderr, "/next_vector = %x\n", next_vector));*/
            next_vector_length = fixnum_value(((unsigned int *)native_pointer(next_vector_obj))[1]);
            /*FSHOW((stderr, "/next_vector_length = %d\n", next_vector_length));*/
        } else {
            lose("invalid next_vector %x", next_vector_obj);
        }
    }

    /* maybe hash vector */
    {
        /* FIXME: This bare "15" offset should become a symbolic
         * expression of some sort. And all the other bare offsets
         * too. And the bare "16" in scavenge(hash_table, 16). And
         * probably other stuff too. Ugh.. */
        lispobj hash_vector_obj = hash_table[15];

        if (is_lisp_pointer(hash_vector_obj) &&
            (widetag_of(*(lispobj *)native_pointer(hash_vector_obj))
             == SIMPLE_ARRAY_UNSIGNED_BYTE_32_WIDETAG)) {
            hash_vector = ((unsigned int *)native_pointer(hash_vector_obj)) + 2;
            /*FSHOW((stderr, "/hash_vector = %x\n", hash_vector));*/
            gc_assert(fixnum_value(((unsigned int *)native_pointer(hash_vector_obj))[1])
                      == next_vector_length);
        } else {
            hash_vector = NULL;
            /*FSHOW((stderr, "/no hash_vector: %x\n", hash_vector_obj));*/
        }
    }

    /* These lengths could be different as the index_vector can be a
     * different length from the others, a larger index_vector could help
     * reduce collisions. */
    gc_assert(next_vector_length*2 == kv_length);

    /* now all set up.. */

    /* Work through the KV vector. */
    {
        int i;
        for (i = 1; i < next_vector_length; i++) {
            lispobj old_key = kv_vector[2*i];
            unsigned int  old_index = (old_key & 0x1fffffff)%length;

            /* Scavenge the key and value. */
            scavenge(&kv_vector[2*i],2);

            /* Check whether the key has moved and is EQ based. */
            {
                lispobj new_key = kv_vector[2*i];
                unsigned int new_index = (new_key & 0x1fffffff)%length;

                if ((old_index != new_index) &&
                    ((!hash_vector) || (hash_vector[i] == 0x80000000)) &&
                    ((new_key != empty_symbol) ||
                     (kv_vector[2*i] != empty_symbol))) {

                    /*FSHOW((stderr,
                           "* EQ key %d moved from %x to %x; index %d to %d\n",
                           i, old_key, new_key, old_index, new_index));*/

                    if (index_vector[old_index] != 0) {
                        /*FSHOW((stderr, "/P1 %d\n", index_vector[old_index]));*/

                        /* Unlink the key from the old_index chain. */
                        if (index_vector[old_index] == i) {
                            /*FSHOW((stderr, "/P2a %d\n", next_vector[i]));*/
                            index_vector[old_index] = next_vector[i];
                            /* Link it into the needing rehash chain. */
                            next_vector[i] = fixnum_value(hash_table[11]);
                            hash_table[11] = make_fixnum(i);
                            /*SHOW("P2");*/
                        } else {
                            unsigned prior = index_vector[old_index];
                            unsigned next = next_vector[prior];

                            /*FSHOW((stderr, "/P3a %d %d\n", prior, next));*/

                            while (next != 0) {
                                /*FSHOW((stderr, "/P3b %d %d\n", prior, next));*/
                                if (next == i) {
                                    /* Unlink it. */
                                    next_vector[prior] = next_vector[next];
                                    /* Link it into the needing rehash
                                     * chain. */
                                    next_vector[next] =
                                        fixnum_value(hash_table[11]);
                                    hash_table[11] = make_fixnum(next);
                                    /*SHOW("/P3");*/
                                    break;
                                }
                                prior = next;
                                next = next_vector[next];
                            }
                        }
                    }
                }
            }
        }
    }
    return (CEILING(kv_length + 2, 2));
}


\f
/*
 * weak pointers
 */

/* XX This is a hack adapted from cgc.c. These don't work too
 * efficiently with the gencgc as a list of the weak pointers is
 * maintained within the objects which causes writes to the pages. A
 * limited attempt is made to avoid unnecessary writes, but this needs
 * a re-think. */
#define WEAK_POINTER_NWORDS \
    CEILING((sizeof(struct weak_pointer) / sizeof(lispobj)), 2)

static int
scav_weak_pointer(lispobj *where, lispobj object)
{
    struct weak_pointer *wp = weak_pointers;
    /* Push the weak pointer onto the list of weak pointers.
     * Do I have to watch for duplicates? Originally this was
     * part of trans_weak_pointer but that didn't work in the
     * case where the WP was in a promoted region.
     */

    /* Check whether it's already in the list. */
    while (wp != NULL) {
        if (wp == (struct weak_pointer*)where) {
            break;
        }
        wp = wp->next;
    }
    if (wp == NULL) {
        /* Add it to the start of the list. */
        wp = (struct weak_pointer*)where;
        if (wp->next != weak_pointers) {
            wp->next = weak_pointers;
        } else {
            /*SHOW("avoided write to weak pointer");*/
        }
        weak_pointers = wp;
    }

    /* Do not let GC scavenge the value slot of the weak pointer.
     * (That is why it is a weak pointer.) */

    return WEAK_POINTER_NWORDS;
}

\f
/* Scan an area looking for an object which encloses the given pointer.
 * Return the object start on success or NULL on failure. */
static lispobj *
search_space(lispobj *start, size_t words, lispobj *pointer)
{
    while (words > 0) {
        size_t count = 1;
        lispobj thing = *start;

        /* If thing is an immediate then this is a cons. */
        if (is_lisp_pointer(thing)
            || ((thing & 3) == 0) /* fixnum */
            || (widetag_of(thing) == BASE_CHAR_WIDETAG)
            || (widetag_of(thing) == UNBOUND_MARKER_WIDETAG))
            count = 2;
        else
            count = (sizetab[widetag_of(thing)])(start);

        /* Check whether the pointer is within this object. */
        if ((pointer >= start) && (pointer < (start+count))) {
            /* found it! */
            /*FSHOW((stderr,"/found %x in %x %x\n", pointer, start, thing));*/
            return(start);
        }

        /* Round up the count. */
        count = CEILING(count,2);

        start += count;
        words -= count;
    }
    return (NULL);
}

static lispobj*
search_read_only_space(lispobj *pointer)
{
    lispobj* start = (lispobj*)READ_ONLY_SPACE_START;
    lispobj* end = (lispobj*)SymbolValue(READ_ONLY_SPACE_FREE_POINTER,0);
    if ((pointer < start) || (pointer >= end))
        return NULL;
    return (search_space(start, (pointer+2)-start, pointer));
}

static lispobj *
search_static_space(lispobj *pointer)
{
    lispobj* start = (lispobj*)STATIC_SPACE_START;
    lispobj* end = (lispobj*)SymbolValue(STATIC_SPACE_FREE_POINTER,0);
    if ((pointer < start) || (pointer >= end))
        return NULL;
    return (search_space(start, (pointer+2)-start, pointer));
}

/* a faster version for searching the dynamic space. This will work even
 * if the object is in a current allocation region. */
lispobj *
search_dynamic_space(lispobj *pointer)
{
    int  page_index = find_page_index(pointer);
    lispobj *start;

    /* The address may be invalid, so do some checks. */
    if ((page_index == -1) || (page_table[page_index].allocated == FREE_PAGE))
        return NULL;
    start = (lispobj *)((void *)page_address(page_index)
                        + page_table[page_index].first_object_offset);
    return (search_space(start, (pointer+2)-start, pointer));
}

/* Is there any possibility that pointer is a valid Lisp object
 * reference, and/or something else (e.g. subroutine call return
 * address) which should prevent us from moving the referred-to thing?
 * This is called from preserve_pointers() */
static int
possibly_valid_dynamic_space_pointer(lispobj *pointer)
{
    lispobj *start_addr;

    /* Find the object start address. */
    if ((start_addr = search_dynamic_space(pointer)) == NULL) {
        return 0;
    }

    /* We need to allow raw pointers into Code objects for return
     * addresses. This will also pick up pointers to functions in code
     * objects. */
    if (widetag_of(*start_addr) == CODE_HEADER_WIDETAG) {
        /* XXX could do some further checks here */
        return 1;
    }

    /* If it's not a return address then it needs to be a valid Lisp
     * pointer. */
    if (!is_lisp_pointer((lispobj)pointer)) {
        return 0;
    }

    /* Check that the object pointed to is consistent with the pointer
     * low tag.
     */
    switch (lowtag_of((lispobj)pointer)) {
    case FUN_POINTER_LOWTAG:
        /* Start_addr should be the enclosing code object, or a closure
         * header. */
        switch (widetag_of(*start_addr)) {
        case CODE_HEADER_WIDETAG:
            /* This case is probably caught above. */
            break;
        case CLOSURE_HEADER_WIDETAG:
        case FUNCALLABLE_INSTANCE_HEADER_WIDETAG:
            if ((unsigned)pointer !=
                ((unsigned)start_addr+FUN_POINTER_LOWTAG)) {
                if (gencgc_verbose)
                    FSHOW((stderr,
                           "/Wf2: %x %x %x\n",
                           pointer, start_addr, *start_addr));
                return 0;
            }
            break;
        default:
            if (gencgc_verbose)
                FSHOW((stderr,
                       "/Wf3: %x %x %x\n",
                       pointer, start_addr, *start_addr));
            return 0;
        }
        break;
    case LIST_POINTER_LOWTAG:
        if ((unsigned)pointer !=
            ((unsigned)start_addr+LIST_POINTER_LOWTAG)) {
            if (gencgc_verbose)
                FSHOW((stderr,
                       "/Wl1: %x %x %x\n",
                       pointer, start_addr, *start_addr));
            return 0;
        }
        /* Is it plausible cons? */
        if ((is_lisp_pointer(start_addr[0])
            || ((start_addr[0] & 3) == 0) /* fixnum */
            || (widetag_of(start_addr[0]) == BASE_CHAR_WIDETAG)
            || (widetag_of(start_addr[0]) == UNBOUND_MARKER_WIDETAG))
           && (is_lisp_pointer(start_addr[1])
               || ((start_addr[1] & 3) == 0) /* fixnum */
               || (widetag_of(start_addr[1]) == BASE_CHAR_WIDETAG)
               || (widetag_of(start_addr[1]) == UNBOUND_MARKER_WIDETAG)))
            break;
        else {
            if (gencgc_verbose)
                FSHOW((stderr,
                       "/Wl2: %x %x %x\n",
                       pointer, start_addr, *start_addr));
            return 0;
        }
    case INSTANCE_POINTER_LOWTAG:
        if ((unsigned)pointer !=
            ((unsigned)start_addr+INSTANCE_POINTER_LOWTAG)) {
            if (gencgc_verbose)
                FSHOW((stderr,
                       "/Wi1: %x %x %x\n",
                       pointer, start_addr, *start_addr));
            return 0;
        }
        if (widetag_of(start_addr[0]) != INSTANCE_HEADER_WIDETAG) {
            if (gencgc_verbose)
                FSHOW((stderr,
                       "/Wi2: %x %x %x\n",
                       pointer, start_addr, *start_addr));
            return 0;
        }
        break;
    case OTHER_POINTER_LOWTAG:
        if ((unsigned)pointer !=
            ((int)start_addr+OTHER_POINTER_LOWTAG)) {
            if (gencgc_verbose)
                FSHOW((stderr,
                       "/Wo1: %x %x %x\n",
                       pointer, start_addr, *start_addr));
            return 0;
        }
        /* Is it plausible?  Not a cons. XXX should check the headers. */
        if (is_lisp_pointer(start_addr[0]) || ((start_addr[0] & 3) == 0)) {
            if (gencgc_verbose)
                FSHOW((stderr,
                       "/Wo2: %x %x %x\n",
                       pointer, start_addr, *start_addr));
            return 0;
        }
        switch (widetag_of(start_addr[0])) {
        case UNBOUND_MARKER_WIDETAG:
        case BASE_CHAR_WIDETAG:
            if (gencgc_verbose)
                FSHOW((stderr,
                       "*Wo3: %x %x %x\n",
                       pointer, start_addr, *start_addr));
            return 0;

            /* only pointed to by function pointers? */
        case CLOSURE_HEADER_WIDETAG:
        case FUNCALLABLE_INSTANCE_HEADER_WIDETAG:
            if (gencgc_verbose)
                FSHOW((stderr,
                       "*Wo4: %x %x %x\n",
                       pointer, start_addr, *start_addr));
            return 0;

        case INSTANCE_HEADER_WIDETAG:
            if (gencgc_verbose)
                FSHOW((stderr,
                       "*Wo5: %x %x %x\n",
                       pointer, start_addr, *start_addr));
            return 0;

            /* the valid other immediate pointer objects */
        case SIMPLE_VECTOR_WIDETAG:
        case RATIO_WIDETAG:
        case COMPLEX_WIDETAG:
#ifdef COMPLEX_SINGLE_FLOAT_WIDETAG
        case COMPLEX_SINGLE_FLOAT_WIDETAG:
#endif
#ifdef COMPLEX_DOUBLE_FLOAT_WIDETAG
        case COMPLEX_DOUBLE_FLOAT_WIDETAG:
#endif
#ifdef COMPLEX_LONG_FLOAT_WIDETAG
        case COMPLEX_LONG_FLOAT_WIDETAG:
#endif
        case SIMPLE_ARRAY_WIDETAG:
        case COMPLEX_BASE_STRING_WIDETAG:
        case COMPLEX_VECTOR_NIL_WIDETAG:
        case COMPLEX_BIT_VECTOR_WIDETAG:
        case COMPLEX_VECTOR_WIDETAG:
        case COMPLEX_ARRAY_WIDETAG:
        case VALUE_CELL_HEADER_WIDETAG:
        case SYMBOL_HEADER_WIDETAG:
        case FDEFN_WIDETAG:
        case CODE_HEADER_WIDETAG:
        case BIGNUM_WIDETAG:
        case SINGLE_FLOAT_WIDETAG:
        case DOUBLE_FLOAT_WIDETAG:
#ifdef LONG_FLOAT_WIDETAG
        case LONG_FLOAT_WIDETAG:
#endif
        case SIMPLE_BASE_STRING_WIDETAG:
        case SIMPLE_BIT_VECTOR_WIDETAG:
        case SIMPLE_ARRAY_NIL_WIDETAG:
        case SIMPLE_ARRAY_UNSIGNED_BYTE_2_WIDETAG:
        case SIMPLE_ARRAY_UNSIGNED_BYTE_4_WIDETAG:
        case SIMPLE_ARRAY_UNSIGNED_BYTE_7_WIDETAG:
        case SIMPLE_ARRAY_UNSIGNED_BYTE_8_WIDETAG:
        case SIMPLE_ARRAY_UNSIGNED_BYTE_15_WIDETAG:
        case SIMPLE_ARRAY_UNSIGNED_BYTE_16_WIDETAG:
        case SIMPLE_ARRAY_UNSIGNED_BYTE_29_WIDETAG:
        case SIMPLE_ARRAY_UNSIGNED_BYTE_31_WIDETAG:
        case SIMPLE_ARRAY_UNSIGNED_BYTE_32_WIDETAG:
#ifdef SIMPLE_ARRAY_SIGNED_BYTE_8_WIDETAG
        case SIMPLE_ARRAY_SIGNED_BYTE_8_WIDETAG:
#endif
#ifdef SIMPLE_ARRAY_SIGNED_BYTE_16_WIDETAG
        case SIMPLE_ARRAY_SIGNED_BYTE_16_WIDETAG:
#endif
#ifdef SIMPLE_ARRAY_SIGNED_BYTE_30_WIDETAG
        case SIMPLE_ARRAY_SIGNED_BYTE_30_WIDETAG:
#endif
#ifdef SIMPLE_ARRAY_SIGNED_BYTE_32_WIDETAG
        case SIMPLE_ARRAY_SIGNED_BYTE_32_WIDETAG:
#endif
        case SIMPLE_ARRAY_SINGLE_FLOAT_WIDETAG:
        case SIMPLE_ARRAY_DOUBLE_FLOAT_WIDETAG:
#ifdef SIMPLE_ARRAY_LONG_FLOAT_WIDETAG
        case SIMPLE_ARRAY_LONG_FLOAT_WIDETAG:
#endif
#ifdef SIMPLE_ARRAY_COMPLEX_SINGLE_FLOAT_WIDETAG
        case SIMPLE_ARRAY_COMPLEX_SINGLE_FLOAT_WIDETAG:
#endif
#ifdef SIMPLE_ARRAY_COMPLEX_DOUBLE_FLOAT_WIDETAG
        case SIMPLE_ARRAY_COMPLEX_DOUBLE_FLOAT_WIDETAG:
#endif
#ifdef SIMPLE_ARRAY_COMPLEX_LONG_FLOAT_WIDETAG
        case SIMPLE_ARRAY_COMPLEX_LONG_FLOAT_WIDETAG:
#endif
        case SAP_WIDETAG:
        case WEAK_POINTER_WIDETAG:
            break;

        default:
            if (gencgc_verbose)
                FSHOW((stderr,
                       "/Wo6: %x %x %x\n",
                       pointer, start_addr, *start_addr));
            return 0;
        }
        break;
    default:
        if (gencgc_verbose)
            FSHOW((stderr,
                   "*W?: %x %x %x\n",
                   pointer, start_addr, *start_addr));
        return 0;
    }

    /* looks good */
    return 1;
}

/* Adjust large bignum and vector objects. This will adjust the
 * allocated region if the size has shrunk, and move unboxed objects
 * into unboxed pages. The pages are not promoted here, and the
 * promoted region is not added to the new_regions; this is really
 * only designed to be called from preserve_pointer(). Shouldn't fail
 * if this is missed, just may delay the moving of objects to unboxed
 * pages, and the freeing of pages. */
static void
maybe_adjust_large_object(lispobj *where)
{
    int first_page;
    int nwords;

    int remaining_bytes;
    int next_page;
    int bytes_freed;
    int old_bytes_used;

    int boxed;

    /* Check whether it's a vector or bignum object. */
    switch (widetag_of(where[0])) {
    case SIMPLE_VECTOR_WIDETAG:
        boxed = BOXED_PAGE;
        break;
    case BIGNUM_WIDETAG:
    case SIMPLE_BASE_STRING_WIDETAG:
    case SIMPLE_BIT_VECTOR_WIDETAG:
    case SIMPLE_ARRAY_NIL_WIDETAG:
    case SIMPLE_ARRAY_UNSIGNED_BYTE_2_WIDETAG:
    case SIMPLE_ARRAY_UNSIGNED_BYTE_4_WIDETAG:
    case SIMPLE_ARRAY_UNSIGNED_BYTE_7_WIDETAG:
    case SIMPLE_ARRAY_UNSIGNED_BYTE_8_WIDETAG:
    case SIMPLE_ARRAY_UNSIGNED_BYTE_15_WIDETAG:
    case SIMPLE_ARRAY_UNSIGNED_BYTE_16_WIDETAG:
    case SIMPLE_ARRAY_UNSIGNED_BYTE_29_WIDETAG:
    case SIMPLE_ARRAY_UNSIGNED_BYTE_31_WIDETAG:
    case SIMPLE_ARRAY_UNSIGNED_BYTE_32_WIDETAG:
#ifdef SIMPLE_ARRAY_SIGNED_BYTE_8_WIDETAG
    case SIMPLE_ARRAY_SIGNED_BYTE_8_WIDETAG:
#endif
#ifdef SIMPLE_ARRAY_SIGNED_BYTE_16_WIDETAG
    case SIMPLE_ARRAY_SIGNED_BYTE_16_WIDETAG:
#endif
#ifdef SIMPLE_ARRAY_SIGNED_BYTE_30_WIDETAG
    case SIMPLE_ARRAY_SIGNED_BYTE_30_WIDETAG:
#endif
#ifdef SIMPLE_ARRAY_SIGNED_BYTE_32_WIDETAG
    case SIMPLE_ARRAY_SIGNED_BYTE_32_WIDETAG:
#endif
    case SIMPLE_ARRAY_SINGLE_FLOAT_WIDETAG:
    case SIMPLE_ARRAY_DOUBLE_FLOAT_WIDETAG:
#ifdef SIMPLE_ARRAY_LONG_FLOAT_WIDETAG
    case SIMPLE_ARRAY_LONG_FLOAT_WIDETAG:
#endif
#ifdef SIMPLE_ARRAY_COMPLEX_SINGLE_FLOAT_WIDETAG
    case SIMPLE_ARRAY_COMPLEX_SINGLE_FLOAT_WIDETAG:
#endif
#ifdef SIMPLE_ARRAY_COMPLEX_DOUBLE_FLOAT_WIDETAG
    case SIMPLE_ARRAY_COMPLEX_DOUBLE_FLOAT_WIDETAG:
#endif
#ifdef SIMPLE_ARRAY_COMPLEX_LONG_FLOAT_WIDETAG
    case SIMPLE_ARRAY_COMPLEX_LONG_FLOAT_WIDETAG:
#endif
        boxed = UNBOXED_PAGE;
        break;
    default:
        return;
    }

    /* Find its current size. */
    nwords = (sizetab[widetag_of(where[0])])(where);

    first_page = find_page_index((void *)where);
    gc_assert(first_page >= 0);

    /* Note: Any page write-protection must be removed, else a later
     * scavenge_newspace may incorrectly not scavenge these pages.
     * This would not be necessary if they are added to the new areas,
     * but lets do it for them all (they'll probably be written
     * anyway?). */

    gc_assert(page_table[first_page].first_object_offset == 0);

    next_page = first_page;
    remaining_bytes = nwords*4;
    while (remaining_bytes > 4096) {
        gc_assert(page_table[next_page].gen == from_space);
        gc_assert((page_table[next_page].allocated == BOXED_PAGE)
                  || (page_table[next_page].allocated == UNBOXED_PAGE));
        gc_assert(page_table[next_page].large_object);
        gc_assert(page_table[next_page].first_object_offset ==
                  -4096*(next_page-first_page));
        gc_assert(page_table[next_page].bytes_used == 4096);

        page_table[next_page].allocated = boxed;

        /* Shouldn't be write-protected at this stage. Essential that the
         * pages aren't. */
        gc_assert(!page_table[next_page].write_protected);
        remaining_bytes -= 4096;
        next_page++;
    }

    /* Now only one page remains, but the object may have shrunk so
     * there may be more unused pages which will be freed. */

    /* Object may have shrunk but shouldn't have grown - check. */
    gc_assert(page_table[next_page].bytes_used >= remaining_bytes);

    page_table[next_page].allocated = boxed;
    gc_assert(page_table[next_page].allocated ==
              page_table[first_page].allocated);

    /* Adjust the bytes_used. */
    old_bytes_used = page_table[next_page].bytes_used;
    page_table[next_page].bytes_used = remaining_bytes;

    bytes_freed = old_bytes_used - remaining_bytes;

    /* Free any remaining pages; needs care. */
    next_page++;
    while ((old_bytes_used == 4096) &&
           (page_table[next_page].gen == from_space) &&
           ((page_table[next_page].allocated == UNBOXED_PAGE)
            || (page_table[next_page].allocated == BOXED_PAGE)) &&
           page_table[next_page].large_object &&
           (page_table[next_page].first_object_offset ==
            -(next_page - first_page)*4096)) {
        /* It checks out OK, free the page. We don't need to both zeroing
         * pages as this should have been done before shrinking the
         * object. These pages shouldn't be write protected as they
         * should be zero filled. */
        gc_assert(page_table[next_page].write_protected == 0);

        old_bytes_used = page_table[next_page].bytes_used;
        page_table[next_page].allocated = FREE_PAGE;
        page_table[next_page].bytes_used = 0;
        bytes_freed += old_bytes_used;
        next_page++;
    }

    if ((bytes_freed > 0) && gencgc_verbose) {
        FSHOW((stderr,
               "/maybe_adjust_large_object() freed %d\n",
               bytes_freed));
    }

    generations[from_space].bytes_allocated -= bytes_freed;
    bytes_allocated -= bytes_freed;

    return;
}

/* Take a possible pointer to a Lisp object and mark its page in the
 * page_table so that it will not be relocated during a GC.
 *
 * This involves locating the page it points to, then backing up to
 * the first page that has its first object start at offset 0, and
 * then marking all pages dont_move from the first until a page that
 * ends by being full, or having free gen.
 *
 * This ensures that objects spanning pages are not broken.
 *
 * It is assumed that all the page static flags have been cleared at
 * the start of a GC.
 *
 * It is also assumed that the current gc_alloc() region has been
 * flushed and the tables updated. */
static void
preserve_pointer(void *addr)
{
    int addr_page_index = find_page_index(addr);
    int first_page;
    int i;
    unsigned region_allocation;

    /* quick check 1: Address is quite likely to have been invalid. */
    if ((addr_page_index == -1)
        || (page_table[addr_page_index].allocated == FREE_PAGE)
        || (page_table[addr_page_index].bytes_used == 0)
        || (page_table[addr_page_index].gen != from_space)
        /* Skip if already marked dont_move. */
        || (page_table[addr_page_index].dont_move != 0))
        return;
    gc_assert(!(page_table[addr_page_index].allocated & OPEN_REGION_PAGE));
    /* (Now that we know that addr_page_index is in range, it's
     * safe to index into page_table[] with it.) */
    region_allocation = page_table[addr_page_index].allocated;

    /* quick check 2: Check the offset within the page.
     *
     * FIXME: The mask should have a symbolic name, and ideally should
     * be derived from page size instead of hardwired to 0xfff.
     * (Also fix other uses of 0xfff, elsewhere.) */
    if (((unsigned)addr & 0xfff) > page_table[addr_page_index].bytes_used)
        return;

    /* Filter out anything which can't be a pointer to a Lisp object
     * (or, as a special case which also requires dont_move, a return
     * address referring to something in a CodeObject). This is
     * expensive but important, since it vastly reduces the
     * probability that random garbage will be bogusly interpreted as
     * a pointer which prevents a page from moving. */
    if (!(possibly_valid_dynamic_space_pointer(addr)))
        return;
    first_page = addr_page_index;

    /* Work backwards to find a page with a first_object_offset of 0.
     * The pages should be contiguous with all bytes used in the same
     * gen. Assumes the first_object_offset is negative or zero. */

    /* this is probably needlessly conservative.  The first object in
     * the page may not even be the one we were passed a pointer to:
     * if this is the case, we will write-protect all the previous
     * object's pages too.
     */

    while (page_table[first_page].first_object_offset != 0) {
        --first_page;
        /* Do some checks. */
        gc_assert(page_table[first_page].bytes_used == 4096);
        gc_assert(page_table[first_page].gen == from_space);
        gc_assert(page_table[first_page].allocated == region_allocation);
    }

    /* Adjust any large objects before promotion as they won't be
     * copied after promotion. */
    if (page_table[first_page].large_object) {
        maybe_adjust_large_object(page_address(first_page));
        /* If a large object has shrunk then addr may now point to a
         * free area in which case it's ignored here. Note it gets
         * through the valid pointer test above because the tail looks
         * like conses. */
        if ((page_table[addr_page_index].allocated == FREE_PAGE)
            || (page_table[addr_page_index].bytes_used == 0)
            /* Check the offset within the page. */
            || (((unsigned)addr & 0xfff)
                > page_table[addr_page_index].bytes_used)) {
            FSHOW((stderr,
                   "weird? ignore ptr 0x%x to freed area of large object\n",
                   addr));
            return;
        }
        /* It may have moved to unboxed pages. */
        region_allocation = page_table[first_page].allocated;
    }

    /* Now work forward until the end of this contiguous area is found,
     * marking all pages as dont_move. */
    for (i = first_page; ;i++) {
        gc_assert(page_table[i].allocated == region_allocation);

        /* Mark the page static. */
        page_table[i].dont_move = 1;

        /* Move the page to the new_space. XX I'd rather not do this
         * but the GC logic is not quite able to copy with the static
         * pages remaining in the from space. This also requires the
         * generation bytes_allocated counters be updated. */
        page_table[i].gen = new_space;
        generations[new_space].bytes_allocated += page_table[i].bytes_used;
        generations[from_space].bytes_allocated -= page_table[i].bytes_used;

        /* It is essential that the pages are not write protected as
         * they may have pointers into the old-space which need
         * scavenging. They shouldn't be write protected at this
         * stage. */
        gc_assert(!page_table[i].write_protected);

        /* Check whether this is the last page in this contiguous block.. */
        if ((page_table[i].bytes_used < 4096)
            /* ..or it is 4096 and is the last in the block */
            || (page_table[i+1].allocated == FREE_PAGE)
            || (page_table[i+1].bytes_used == 0) /* next page free */
            || (page_table[i+1].gen != from_space) /* diff. gen */
            || (page_table[i+1].first_object_offset == 0))
            break;
    }

    /* Check that the page is now static. */
    gc_assert(page_table[addr_page_index].dont_move != 0);
}
\f
/* If the given page is not write-protected, then scan it for pointers
 * to younger generations or the top temp. generation, if no
 * suspicious pointers are found then the page is write-protected.
 *
 * Care is taken to check for pointers to the current gc_alloc()
 * region if it is a younger generation or the temp. generation. This
 * frees the caller from doing a gc_alloc_update_page_tables(). Actually
 * the gc_alloc_generation does not need to be checked as this is only
 * called from scavenge_generation() when the gc_alloc generation is
 * younger, so it just checks if there is a pointer to the current
 * region.
 *
 * We return 1 if the page was write-protected, else 0. */
static int
update_page_write_prot(int page)
{
    int gen = page_table[page].gen;
    int j;
    int wp_it = 1;
    void **page_addr = (void **)page_address(page);
    int num_words = page_table[page].bytes_used / 4;

    /* Shouldn't be a free page. */
    gc_assert(page_table[page].allocated != FREE_PAGE);
    gc_assert(page_table[page].bytes_used != 0);

    /* Skip if it's already write-protected, pinned, or unboxed */
    if (page_table[page].write_protected
        || page_table[page].dont_move
        || (page_table[page].allocated & UNBOXED_PAGE))
        return (0);

    /* Scan the page for pointers to younger generations or the
     * top temp. generation. */

    for (j = 0; j < num_words; j++) {
        void *ptr = *(page_addr+j);
        int index = find_page_index(ptr);

        /* Check that it's in the dynamic space */
        if (index != -1)
            if (/* Does it point to a younger or the temp. generation? */
                ((page_table[index].allocated != FREE_PAGE)
                 && (page_table[index].bytes_used != 0)
                 && ((page_table[index].gen < gen)
                     || (page_table[index].gen == NUM_GENERATIONS)))

                /* Or does it point within a current gc_alloc() region? */
                || ((boxed_region.start_addr <= ptr)
                    && (ptr <= boxed_region.free_pointer))
                || ((unboxed_region.start_addr <= ptr)
                    && (ptr <= unboxed_region.free_pointer))) {
                wp_it = 0;
                break;
            }
    }

    if (wp_it == 1) {
        /* Write-protect the page. */
        /*FSHOW((stderr, "/write-protecting page %d gen %d\n", page, gen));*/

        os_protect((void *)page_addr,
                   4096,
                   OS_VM_PROT_READ|OS_VM_PROT_EXECUTE);

        /* Note the page as protected in the page tables. */
        page_table[page].write_protected = 1;
    }

    return (wp_it);
}

/* Scavenge a generation.
 *
 * This will not resolve all pointers when generation is the new
 * space, as new objects may be added which are not check here - use
 * scavenge_newspace generation.
 *
 * Write-protected pages should not have any pointers to the
 * from_space so do need scavenging; thus write-protected pages are
 * not always scavenged. There is some code to check that these pages
 * are not written; but to check fully the write-protected pages need
 * to be scavenged by disabling the code to skip them.
 *
 * Under the current scheme when a generation is GCed the younger
 * generations will be empty. So, when a generation is being GCed it
 * is only necessary to scavenge the older generations for pointers
 * not the younger. So a page that does not have pointers to younger
 * generations does not need to be scavenged.
 *
 * The write-protection can be used to note pages that don't have
 * pointers to younger pages. But pages can be written without having
 * pointers to younger generations. After the pages are scavenged here
 * they can be scanned for pointers to younger generations and if
 * there are none the page can be write-protected.
 *
 * One complication is when the newspace is the top temp. generation.
 *
 * Enabling SC_GEN_CK scavenges the write-protected pages and checks
 * that none were written, which they shouldn't be as they should have
 * no pointers to younger generations. This breaks down for weak
 * pointers as the objects contain a link to the next and are written
 * if a weak pointer is scavenged. Still it's a useful check. */
static void
scavenge_generation(int generation)
{
    int i;
    int num_wp = 0;

#define SC_GEN_CK 0
#if SC_GEN_CK
    /* Clear the write_protected_cleared flags on all pages. */
    for (i = 0; i < NUM_PAGES; i++)
        page_table[i].write_protected_cleared = 0;
#endif

    for (i = 0; i < last_free_page; i++) {
        if ((page_table[i].allocated & BOXED_PAGE)
            && (page_table[i].bytes_used != 0)
            && (page_table[i].gen == generation)) {
            int last_page;

            /* This should be the start of a contiguous block. */
            gc_assert(page_table[i].first_object_offset == 0);

            /* We need to find the full extent of this contiguous
             * block in case objects span pages. */

            /* Now work forward until the end of this contiguous area
             * is found. A small area is preferred as there is a
             * better chance of its pages being write-protected. */
            for (last_page = i; ; last_page++)
                /* Check whether this is the last page in this contiguous
                 * block. */
                if ((page_table[last_page].bytes_used < 4096)
                    /* Or it is 4096 and is the last in the block */
                    || (!(page_table[last_page+1].allocated & BOXED_PAGE))
                    || (page_table[last_page+1].bytes_used == 0)
                    || (page_table[last_page+1].gen != generation)
                    || (page_table[last_page+1].first_object_offset == 0))
                    break;

            /* Do a limited check for write_protected pages. If all pages
             * are write_protected then there is no need to scavenge. */
            {
                int j, all_wp = 1;
                for (j = i; j <= last_page; j++)
                    if (page_table[j].write_protected == 0) {
                        all_wp = 0;
                        break;
                    }
#if !SC_GEN_CK
                if (all_wp == 0)
#endif
                    {
                        scavenge(page_address(i), (page_table[last_page].bytes_used
                                                   + (last_page-i)*4096)/4);

                        /* Now scan the pages and write protect those
                         * that don't have pointers to younger
                         * generations. */
                        if (enable_page_protection) {
                            for (j = i; j <= last_page; j++) {
                                num_wp += update_page_write_prot(j);
                            }
                        }
                    }
            }
            i = last_page;
        }
    }

    if ((gencgc_verbose > 1) && (num_wp != 0)) {
        FSHOW((stderr,
               "/write protected %d pages within generation %d\n",
               num_wp, generation));
    }

#if SC_GEN_CK
    /* Check that none of the write_protected pages in this generation
     * have been written to. */
    for (i = 0; i < NUM_PAGES; i++) {
        if ((page_table[i].allocation ! =FREE_PAGE)
            && (page_table[i].bytes_used != 0)
            && (page_table[i].gen == generation)
            && (page_table[i].write_protected_cleared != 0)) {
            FSHOW((stderr, "/scavenge_generation() %d\n", generation));
            FSHOW((stderr,
                   "/page bytes_used=%d first_object_offset=%d dont_move=%d\n",
                    page_table[i].bytes_used,
                    page_table[i].first_object_offset,
                    page_table[i].dont_move));
            lose("write to protected page %d in scavenge_generation()", i);
        }
    }
#endif
}

\f
/* Scavenge a newspace generation. As it is scavenged new objects may
 * be allocated to it; these will also need to be scavenged. This
 * repeats until there are no more objects unscavenged in the
 * newspace generation.
 *
 * To help improve the efficiency, areas written are recorded by
 * gc_alloc() and only these scavenged. Sometimes a little more will be
 * scavenged, but this causes no harm. An easy check is done that the
 * scavenged bytes equals the number allocated in the previous
 * scavenge.
 *
 * Write-protected pages are not scanned except if they are marked
 * dont_move in which case they may have been promoted and still have
 * pointers to the from space.
 *
 * Write-protected pages could potentially be written by alloc however
 * to avoid having to handle re-scavenging of write-protected pages
 * gc_alloc() does not write to write-protected pages.
 *
 * New areas of objects allocated are recorded alternatively in the two
 * new_areas arrays below. */
static struct new_area new_areas_1[NUM_NEW_AREAS];
static struct new_area new_areas_2[NUM_NEW_AREAS];

/* Do one full scan of the new space generation. This is not enough to
 * complete the job as new objects may be added to the generation in
 * the process which are not scavenged. */
static void
scavenge_newspace_generation_one_scan(int generation)
{
    int i;

    FSHOW((stderr,
           "/starting one full scan of newspace generation %d\n",
           generation));
    for (i = 0; i < last_free_page; i++) {
        /* note that this skips over open regions when it encounters them */
        if ((page_table[i].allocated == BOXED_PAGE)
            && (page_table[i].bytes_used != 0)
            && (page_table[i].gen == generation)
            && ((page_table[i].write_protected == 0)
                /* (This may be redundant as write_protected is now
                 * cleared before promotion.) */
                || (page_table[i].dont_move == 1))) {
            int last_page;

            /* The scavenge will start at the first_object_offset of page i.
             *
             * We need to find the full extent of this contiguous
             * block in case objects span pages.
             *
             * Now work forward until the end of this contiguous area
             * is found. A small area is preferred as there is a
             * better chance of its pages being write-protected. */
            for (last_page = i; ;last_page++) {
                /* Check whether this is the last page in this
                 * contiguous block */
                if ((page_table[last_page].bytes_used < 4096)
                    /* Or it is 4096 and is the last in the block */
                    || (!(page_table[last_page+1].allocated & BOXED_PAGE))
                    || (page_table[last_page+1].bytes_used == 0)
                    || (page_table[last_page+1].gen != generation)
                    || (page_table[last_page+1].first_object_offset == 0))
                    break;
            }

            /* Do a limited check for write-protected pages. If all
             * pages are write-protected then no need to scavenge,
             * except if the pages are marked dont_move. */
            {
                int j, all_wp = 1;
                for (j = i; j <= last_page; j++)
                    if ((page_table[j].write_protected == 0)
                        || (page_table[j].dont_move != 0)) {
                        all_wp = 0;
                        break;
                    }

                if (!all_wp) {
                    int size;

                    /* Calculate the size. */
                    if (last_page == i)
                        size = (page_table[last_page].bytes_used
                                - page_table[i].first_object_offset)/4;
                    else
                        size = (page_table[last_page].bytes_used
                                + (last_page-i)*4096
                                - page_table[i].first_object_offset)/4;
                    
                    {
                        new_areas_ignore_page = last_page;
                        
                        scavenge(page_address(i) +
                                 page_table[i].first_object_offset,
                                 size);

                    }
                }
            }

            i = last_page;
        }
    }
    FSHOW((stderr,
           "/done with one full scan of newspace generation %d\n",
           generation));
}

/* Do a complete scavenge of the newspace generation. */
static void
scavenge_newspace_generation(int generation)
{
    int i;

    /* the new_areas array currently being written to by gc_alloc() */
    struct new_area (*current_new_areas)[] = &new_areas_1;
    int current_new_areas_index;

    /* the new_areas created but the previous scavenge cycle */
    struct new_area (*previous_new_areas)[] = NULL;
    int previous_new_areas_index;

    /* Flush the current regions updating the tables. */
    gc_alloc_update_all_page_tables();

    /* Turn on the recording of new areas by gc_alloc(). */
    new_areas = current_new_areas;
    new_areas_index = 0;

    /* Don't need to record new areas that get scavenged anyway during
     * scavenge_newspace_generation_one_scan. */
    record_new_objects = 1;

    /* Start with a full scavenge. */
    scavenge_newspace_generation_one_scan(generation);

    /* Record all new areas now. */
    record_new_objects = 2;

    /* Flush the current regions updating the tables. */
    gc_alloc_update_all_page_tables();

    /* Grab new_areas_index. */
    current_new_areas_index = new_areas_index;

    /*FSHOW((stderr,
             "The first scan is finished; current_new_areas_index=%d.\n",
             current_new_areas_index));*/

    while (current_new_areas_index > 0) {
        /* Move the current to the previous new areas */
        previous_new_areas = current_new_areas;
        previous_new_areas_index = current_new_areas_index;

        /* Scavenge all the areas in previous new areas. Any new areas
         * allocated are saved in current_new_areas. */

        /* Allocate an array for current_new_areas; alternating between
         * new_areas_1 and 2 */
        if (previous_new_areas == &new_areas_1)
            current_new_areas = &new_areas_2;
        else
            current_new_areas = &new_areas_1;

        /* Set up for gc_alloc(). */
        new_areas = current_new_areas;
        new_areas_index = 0;

        /* Check whether previous_new_areas had overflowed. */
        if (previous_new_areas_index >= NUM_NEW_AREAS) {

            /* New areas of objects allocated have been lost so need to do a
             * full scan to be sure! If this becomes a problem try
             * increasing NUM_NEW_AREAS. */
            if (gencgc_verbose)
                SHOW("new_areas overflow, doing full scavenge");

            /* Don't need to record new areas that get scavenge anyway
             * during scavenge_newspace_generation_one_scan. */
            record_new_objects = 1;

            scavenge_newspace_generation_one_scan(generation);

            /* Record all new areas now. */
            record_new_objects = 2;

            /* Flush the current regions updating the tables. */
            gc_alloc_update_all_page_tables();

        } else {

            /* Work through previous_new_areas. */
            for (i = 0; i < previous_new_areas_index; i++) {
                /* FIXME: All these bare *4 and /4 should be something
                 * like BYTES_PER_WORD or WBYTES. */
                int page = (*previous_new_areas)[i].page;
                int offset = (*previous_new_areas)[i].offset;
                int size = (*previous_new_areas)[i].size / 4;
                gc_assert((*previous_new_areas)[i].size % 4 == 0);
                scavenge(page_address(page)+offset, size);
            }

            /* Flush the current regions updating the tables. */
            gc_alloc_update_all_page_tables();
        }

        current_new_areas_index = new_areas_index;

        /*FSHOW((stderr,
                 "The re-scan has finished; current_new_areas_index=%d.\n",
                 current_new_areas_index));*/
    }

    /* Turn off recording of areas allocated by gc_alloc(). */
    record_new_objects = 0;

#if SC_NS_GEN_CK
    /* Check that none of the write_protected pages in this generation
     * have been written to. */
    for (i = 0; i < NUM_PAGES; i++) {
        if ((page_table[i].allocation != FREE_PAGE)
            && (page_table[i].bytes_used != 0)
            && (page_table[i].gen == generation)
            && (page_table[i].write_protected_cleared != 0)
            && (page_table[i].dont_move == 0)) {
            lose("write protected page %d written to in scavenge_newspace_generation\ngeneration=%d dont_move=%d",
                 i, generation, page_table[i].dont_move);
        }
    }
#endif
}
\f
/* Un-write-protect all the pages in from_space. This is done at the
 * start of a GC else there may be many page faults while scavenging
 * the newspace (I've seen drive the system time to 99%). These pages
 * would need to be unprotected anyway before unmapping in
 * free_oldspace; not sure what effect this has on paging.. */
static void
unprotect_oldspace(void)
{
    int i;

    for (i = 0; i < last_free_page; i++) {
        if ((page_table[i].allocated != FREE_PAGE)
            && (page_table[i].bytes_used != 0)
            && (page_table[i].gen == from_space)) {
            void *page_start;

            page_start = (void *)page_address(i);

            /* Remove any write-protection. We should be able to rely
             * on the write-protect flag to avoid redundant calls. */
            if (page_table[i].write_protected) {
                os_protect(page_start, 4096, OS_VM_PROT_ALL);
                page_table[i].write_protected = 0;
            }
        }
    }
}

/* Work through all the pages and free any in from_space. This
 * assumes that all objects have been copied or promoted to an older
 * generation. Bytes_allocated and the generation bytes_allocated
 * counter are updated. The number of bytes freed is returned. */
extern void i586_bzero(void *addr, int nbytes);
static int
free_oldspace(void)
{
    int bytes_freed = 0;
    int first_page, last_page;

    first_page = 0;

    do {
        /* Find a first page for the next region of pages. */
        while ((first_page < last_free_page)
               && ((page_table[first_page].allocated == FREE_PAGE)
                   || (page_table[first_page].bytes_used == 0)
                   || (page_table[first_page].gen != from_space)))
            first_page++;

        if (first_page >= last_free_page)
            break;

        /* Find the last page of this region. */
        last_page = first_page;

        do {
            /* Free the page. */
            bytes_freed += page_table[last_page].bytes_used;
            generations[page_table[last_page].gen].bytes_allocated -=
                page_table[last_page].bytes_used;
            page_table[last_page].allocated = FREE_PAGE;
            page_table[last_page].bytes_used = 0;

            /* Remove any write-protection. We should be able to rely
             * on the write-protect flag to avoid redundant calls. */
            {
                void  *page_start = (void *)page_address(last_page);
        
                if (page_table[last_page].write_protected) {
                    os_protect(page_start, 4096, OS_VM_PROT_ALL);
                    page_table[last_page].write_protected = 0;
                }
            }
            last_page++;
        }
        while ((last_page < last_free_page)
               && (page_table[last_page].allocated != FREE_PAGE)
               && (page_table[last_page].bytes_used != 0)
               && (page_table[last_page].gen == from_space));

        /* Zero pages from first_page to (last_page-1).
         *
         * FIXME: Why not use os_zero(..) function instead of
         * hand-coding this again? (Check other gencgc_unmap_zero
         * stuff too. */
        if (gencgc_unmap_zero) {
            void *page_start, *addr;

            page_start = (void *)page_address(first_page);

            os_invalidate(page_start, 4096*(last_page-first_page));
            addr = os_validate(page_start, 4096*(last_page-first_page));
            if (addr == NULL || addr != page_start) {
                /* Is this an error condition? I couldn't really tell from
                 * the old CMU CL code, which fprintf'ed a message with
                 * an exclamation point at the end. But I've never seen the
                 * message, so it must at least be unusual..
                 *
                 * (The same condition is also tested for in gc_free_heap.)
                 *
                 * -- WHN 19991129 */
                lose("i586_bzero: page moved, 0x%08x ==> 0x%08x",
                     page_start,
                     addr);
            }
        } else {
            int *page_start;

            page_start = (int *)page_address(first_page);
            i586_bzero(page_start, 4096*(last_page-first_page));
        }

        first_page = last_page;

    } while (first_page < last_free_page);

    bytes_allocated -= bytes_freed;
    return bytes_freed;
}
\f
#if 0
/* Print some information about a pointer at the given address. */
static void
print_ptr(lispobj *addr)
{
    /* If addr is in the dynamic space then out the page information. */
    int pi1 = find_page_index((void*)addr);

    if (pi1 != -1)
        fprintf(stderr,"  %x: page %d  alloc %d  gen %d  bytes_used %d  offset %d  dont_move %d\n",
                (unsigned int) addr,
                pi1,
                page_table[pi1].allocated,
                page_table[pi1].gen,
                page_table[pi1].bytes_used,
                page_table[pi1].first_object_offset,
                page_table[pi1].dont_move);
    fprintf(stderr,"  %x %x %x %x (%x) %x %x %x %x\n",
            *(addr-4),
            *(addr-3),
            *(addr-2),
            *(addr-1),
            *(addr-0),
            *(addr+1),
            *(addr+2),
            *(addr+3),
            *(addr+4));
}
#endif

extern int undefined_tramp;

static void
verify_space(lispobj *start, size_t words)
{
    int is_in_dynamic_space = (find_page_index((void*)start) != -1);
    int is_in_readonly_space =
        (READ_ONLY_SPACE_START <= (unsigned)start &&
         (unsigned)start < SymbolValue(READ_ONLY_SPACE_FREE_POINTER,0));

    while (words > 0) {
        size_t count = 1;
        lispobj thing = *(lispobj*)start;

        if (is_lisp_pointer(thing)) {
            int page_index = find_page_index((void*)thing);
            int to_readonly_space =
                (READ_ONLY_SPACE_START <= thing &&
                 thing < SymbolValue(READ_ONLY_SPACE_FREE_POINTER,0));
            int to_static_space =
                (STATIC_SPACE_START <= thing &&
                 thing < SymbolValue(STATIC_SPACE_FREE_POINTER,0));

            /* Does it point to the dynamic space? */
            if (page_index != -1) {
                /* If it's within the dynamic space it should point to a used
                 * page. XX Could check the offset too. */
                if ((page_table[page_index].allocated != FREE_PAGE)
                    && (page_table[page_index].bytes_used == 0))
                    lose ("Ptr %x @ %x sees free page.", thing, start);
                /* Check that it doesn't point to a forwarding pointer! */
                if (*((lispobj *)native_pointer(thing)) == 0x01) {
                    lose("Ptr %x @ %x sees forwarding ptr.", thing, start);
                }
                /* Check that its not in the RO space as it would then be a
                 * pointer from the RO to the dynamic space. */
                if (is_in_readonly_space) {
                    lose("ptr to dynamic space %x from RO space %x",
                         thing, start);
                }
                /* Does it point to a plausible object? This check slows
                 * it down a lot (so it's commented out).
                 *
                 * "a lot" is serious: it ate 50 minutes cpu time on
                 * my duron 950 before I came back from lunch and
                 * killed it.
                 *
                 *   FIXME: Add a variable to enable this
                 * dynamically. */
                /*
                if (!possibly_valid_dynamic_space_pointer((lispobj *)thing)) {
                    lose("ptr %x to invalid object %x", thing, start); 
                }
                */
            } else {
                /* Verify that it points to another valid space. */
                if (!to_readonly_space && !to_static_space
                    && (thing != (unsigned)&undefined_tramp)) {
                    lose("Ptr %x @ %x sees junk.", thing, start);
                }
            }
        } else {
            if (thing & 0x3) { /* Skip fixnums. FIXME: There should be an
                                * is_fixnum for this. */

                switch(widetag_of(*start)) {

                    /* boxed objects */
                case SIMPLE_VECTOR_WIDETAG:
                case RATIO_WIDETAG:
                case COMPLEX_WIDETAG:
                case SIMPLE_ARRAY_WIDETAG:
                case COMPLEX_BASE_STRING_WIDETAG:
                case COMPLEX_VECTOR_NIL_WIDETAG:
                case COMPLEX_BIT_VECTOR_WIDETAG:
                case COMPLEX_VECTOR_WIDETAG:
                case COMPLEX_ARRAY_WIDETAG:
                case CLOSURE_HEADER_WIDETAG:
                case FUNCALLABLE_INSTANCE_HEADER_WIDETAG:
                case VALUE_CELL_HEADER_WIDETAG:
                case SYMBOL_HEADER_WIDETAG:
                case BASE_CHAR_WIDETAG:
                case UNBOUND_MARKER_WIDETAG:
                case INSTANCE_HEADER_WIDETAG:
                case FDEFN_WIDETAG:
                    count = 1;
                    break;

                case CODE_HEADER_WIDETAG:
                    {
                        lispobj object = *start;
                        struct code *code;
                        int nheader_words, ncode_words, nwords;
                        lispobj fheaderl;
                        struct simple_fun *fheaderp;

                        code = (struct code *) start;

                        /* Check that it's not in the dynamic space.
                         * FIXME: Isn't is supposed to be OK for code
                         * objects to be in the dynamic space these days? */
                        if (is_in_dynamic_space
                            /* It's ok if it's byte compiled code. The trace
                             * table offset will be a fixnum if it's x86
                             * compiled code - check.
                             *
                             * FIXME: #^#@@! lack of abstraction here..
                             * This line can probably go away now that
                             * there's no byte compiler, but I've got
                             * too much to worry about right now to try
                             * to make sure. -- WHN 2001-10-06 */
                            && !(code->trace_table_offset & 0x3)
                            /* Only when enabled */
                            && verify_dynamic_code_check) {
                            FSHOW((stderr,
                                   "/code object at %x in the dynamic space\n",
                                   start));
                        }

                        ncode_words = fixnum_value(code->code_size);
                        nheader_words = HeaderValue(object);
                        nwords = ncode_words + nheader_words;
                        nwords = CEILING(nwords, 2);
                        /* Scavenge the boxed section of the code data block */
                        verify_space(start + 1, nheader_words - 1);

                        /* Scavenge the boxed section of each function
                         * object in the code data block. */
                        fheaderl = code->entry_points;
                        while (fheaderl != NIL) {
                            fheaderp =
                                (struct simple_fun *) native_pointer(fheaderl);
                            gc_assert(widetag_of(fheaderp->header) == SIMPLE_FUN_HEADER_WIDETAG);
                            verify_space(&fheaderp->name, 1);
                            verify_space(&fheaderp->arglist, 1);
                            verify_space(&fheaderp->type, 1);
                            fheaderl = fheaderp->next;
                        }
                        count = nwords;
                        break;
                    }
        
                    /* unboxed objects */
                case BIGNUM_WIDETAG:
                case SINGLE_FLOAT_WIDETAG:
                case DOUBLE_FLOAT_WIDETAG:
#ifdef COMPLEX_LONG_FLOAT_WIDETAG
                case LONG_FLOAT_WIDETAG:
#endif
#ifdef COMPLEX_SINGLE_FLOAT_WIDETAG
                case COMPLEX_SINGLE_FLOAT_WIDETAG:
#endif
#ifdef COMPLEX_DOUBLE_FLOAT_WIDETAG
                case COMPLEX_DOUBLE_FLOAT_WIDETAG:
#endif
#ifdef COMPLEX_LONG_FLOAT_WIDETAG
                case COMPLEX_LONG_FLOAT_WIDETAG:
#endif
                case SIMPLE_BASE_STRING_WIDETAG:
                case SIMPLE_BIT_VECTOR_WIDETAG:
                case SIMPLE_ARRAY_NIL_WIDETAG:
                case SIMPLE_ARRAY_UNSIGNED_BYTE_2_WIDETAG:
                case SIMPLE_ARRAY_UNSIGNED_BYTE_4_WIDETAG:
                case SIMPLE_ARRAY_UNSIGNED_BYTE_7_WIDETAG:
                case SIMPLE_ARRAY_UNSIGNED_BYTE_8_WIDETAG:
                case SIMPLE_ARRAY_UNSIGNED_BYTE_15_WIDETAG:
                case SIMPLE_ARRAY_UNSIGNED_BYTE_16_WIDETAG:
                case SIMPLE_ARRAY_UNSIGNED_BYTE_29_WIDETAG:
                case SIMPLE_ARRAY_UNSIGNED_BYTE_31_WIDETAG:
                case SIMPLE_ARRAY_UNSIGNED_BYTE_32_WIDETAG:
#ifdef SIMPLE_ARRAY_SIGNED_BYTE_8_WIDETAG
                case SIMPLE_ARRAY_SIGNED_BYTE_8_WIDETAG:
#endif
#ifdef SIMPLE_ARRAY_SIGNED_BYTE_16_WIDETAG
                case SIMPLE_ARRAY_SIGNED_BYTE_16_WIDETAG:
#endif
#ifdef SIMPLE_ARRAY_SIGNED_BYTE_30_WIDETAG
                case SIMPLE_ARRAY_SIGNED_BYTE_30_WIDETAG:
#endif
#ifdef SIMPLE_ARRAY_SIGNED_BYTE_32_WIDETAG
                case SIMPLE_ARRAY_SIGNED_BYTE_32_WIDETAG:
#endif
                case SIMPLE_ARRAY_SINGLE_FLOAT_WIDETAG:
                case SIMPLE_ARRAY_DOUBLE_FLOAT_WIDETAG:
#ifdef SIMPLE_ARRAY_COMPLEX_LONG_FLOAT_WIDETAG
                case SIMPLE_ARRAY_LONG_FLOAT_WIDETAG:
#endif
#ifdef SIMPLE_ARRAY_COMPLEX_SINGLE_FLOAT_WIDETAG
                case SIMPLE_ARRAY_COMPLEX_SINGLE_FLOAT_WIDETAG:
#endif
#ifdef SIMPLE_ARRAY_COMPLEX_DOUBLE_FLOAT_WIDETAG
                case SIMPLE_ARRAY_COMPLEX_DOUBLE_FLOAT_WIDETAG:
#endif
#ifdef SIMPLE_ARRAY_COMPLEX_LONG_FLOAT_WIDETAG
                case SIMPLE_ARRAY_COMPLEX_LONG_FLOAT_WIDETAG:
#endif
                case SAP_WIDETAG:
                case WEAK_POINTER_WIDETAG:
                    count = (sizetab[widetag_of(*start)])(start);
                    break;

                default:
                    gc_abort();
                }
            }
        }
        start += count;
        words -= count;
    }
}

static void
verify_gc(void)
{
    /* FIXME: It would be nice to make names consistent so that
     * foo_size meant size *in* *bytes* instead of size in some
     * arbitrary units. (Yes, this caused a bug, how did you guess?:-)
     * Some counts of lispobjs are called foo_count; it might be good
     * to grep for all foo_size and rename the appropriate ones to
     * foo_count. */
    int read_only_space_size =
        (lispobj*)SymbolValue(READ_ONLY_SPACE_FREE_POINTER,0)
        - (lispobj*)READ_ONLY_SPACE_START;
    int static_space_size =
        (lispobj*)SymbolValue(STATIC_SPACE_FREE_POINTER,0)
        - (lispobj*)STATIC_SPACE_START;
    struct thread *th;
    for_each_thread(th) {
    int binding_stack_size =
            (lispobj*)SymbolValue(BINDING_STACK_POINTER,th)
            - (lispobj*)th->binding_stack_start;
        verify_space(th->binding_stack_start, binding_stack_size);
    }
    verify_space((lispobj*)READ_ONLY_SPACE_START, read_only_space_size);
    verify_space((lispobj*)STATIC_SPACE_START   , static_space_size);
}

static void
verify_generation(int  generation)
{
    int i;

    for (i = 0; i < last_free_page; i++) {
        if ((page_table[i].allocated != FREE_PAGE)
            && (page_table[i].bytes_used != 0)
            && (page_table[i].gen == generation)) {
            int last_page;
            int region_allocation = page_table[i].allocated;

            /* This should be the start of a contiguous block */
            gc_assert(page_table[i].first_object_offset == 0);

            /* Need to find the full extent of this contiguous block in case
               objects span pages. */

            /* Now work forward until the end of this contiguous area is
               found. */
            for (last_page = i; ;last_page++)
                /* Check whether this is the last page in this contiguous
                 * block. */
                if ((page_table[last_page].bytes_used < 4096)
                    /* Or it is 4096 and is the last in the block */
                    || (page_table[last_page+1].allocated != region_allocation)
                    || (page_table[last_page+1].bytes_used == 0)
                    || (page_table[last_page+1].gen != generation)
                    || (page_table[last_page+1].first_object_offset == 0))
                    break;

            verify_space(page_address(i), (page_table[last_page].bytes_used
                                           + (last_page-i)*4096)/4);
            i = last_page;
        }
    }
}

/* Check that all the free space is zero filled. */
static void
verify_zero_fill(void)
{
    int page;

    for (page = 0; page < last_free_page; page++) {
        if (page_table[page].allocated == FREE_PAGE) {
            /* The whole page should be zero filled. */
            int *start_addr = (int *)page_address(page);
            int size = 1024;
            int i;
            for (i = 0; i < size; i++) {
                if (start_addr[i] != 0) {
                    lose("free page not zero at %x", start_addr + i);
                }
            }
        } else {
            int free_bytes = 4096 - page_table[page].bytes_used;
            if (free_bytes > 0) {
                int *start_addr = (int *)((unsigned)page_address(page)
                                          + page_table[page].bytes_used);
                int size = free_bytes / 4;
                int i;
                for (i = 0; i < size; i++) {
                    if (start_addr[i] != 0) {
                        lose("free region not zero at %x", start_addr + i);
                    }
                }
            }
        }
    }
}

/* External entry point for verify_zero_fill */
void
gencgc_verify_zero_fill(void)
{
    /* Flush the alloc regions updating the tables. */
    gc_alloc_update_all_page_tables();
    SHOW("verifying zero fill");
    verify_zero_fill();
}

static void
verify_dynamic_space(void)
{
    int i;

    for (i = 0; i < NUM_GENERATIONS; i++)
        verify_generation(i);

    if (gencgc_enable_verify_zero_fill)
        verify_zero_fill();
}
\f
/* Write-protect all the dynamic boxed pages in the given generation. */
static void
write_protect_generation_pages(int generation)
{
    int i;

    gc_assert(generation < NUM_GENERATIONS);

    for (i = 0; i < last_free_page; i++)
        if ((page_table[i].allocated == BOXED_PAGE)
            && (page_table[i].bytes_used != 0)
            && !page_table[i].dont_move
            && (page_table[i].gen == generation))  {
            void *page_start;

            page_start = (void *)page_address(i);

            os_protect(page_start,
                       4096,
                       OS_VM_PROT_READ | OS_VM_PROT_EXECUTE);

            /* Note the page as protected in the page tables. */
            page_table[i].write_protected = 1;
        }

    if (gencgc_verbose > 1) {
        FSHOW((stderr,
               "/write protected %d of %d pages in generation %d\n",
               count_write_protect_generation_pages(generation),
               count_generation_pages(generation),
               generation));
    }
}

/* Garbage collect a generation. If raise is 0 then the remains of the
 * generation are not raised to the next generation. */
static void
garbage_collect_generation(int generation, int raise)
{
    unsigned long bytes_freed;
    unsigned long i;
    unsigned long static_space_size;
    struct thread *th;
    gc_assert(generation <= (NUM_GENERATIONS-1));

    /* The oldest generation can't be raised. */
    gc_assert((generation != (NUM_GENERATIONS-1)) || (raise == 0));

    /* Initialize the weak pointer list. */
    weak_pointers = NULL;

    /* When a generation is not being raised it is transported to a
     * temporary generation (NUM_GENERATIONS), and lowered when
     * done. Set up this new generation. There should be no pages
     * allocated to it yet. */
    if (!raise)
        gc_assert(generations[NUM_GENERATIONS].bytes_allocated == 0);

    /* Set the global src and dest. generations */
    from_space = generation;
    if (raise)
        new_space = generation+1;
    else
        new_space = NUM_GENERATIONS;

    /* Change to a new space for allocation, resetting the alloc_start_page */
    gc_alloc_generation = new_space;
    generations[new_space].alloc_start_page = 0;
    generations[new_space].alloc_unboxed_start_page = 0;
    generations[new_space].alloc_large_start_page = 0;
    generations[new_space].alloc_large_unboxed_start_page = 0;

    /* Before any pointers are preserved, the dont_move flags on the
     * pages need to be cleared. */
    for (i = 0; i < last_free_page; i++)
        if(page_table[i].gen==from_space)
            page_table[i].dont_move = 0;

    /* Un-write-protect the old-space pages. This is essential for the
     * promoted pages as they may contain pointers into the old-space
     * which need to be scavenged. It also helps avoid unnecessary page
     * faults as forwarding pointers are written into them. They need to
     * be un-protected anyway before unmapping later. */
    unprotect_oldspace();

    /* Scavenge the stacks' conservative roots. */
    for_each_thread(th) {
        void **ptr;
        void **esp= (void **) &raise;
        int i=0,free;
#ifdef LISP_FEATURE_SB_THREAD
        if(th!=arch_os_get_current_thread()) {
            os_context_t *last_context=get_interrupt_context_for_thread(th);
            esp = (void **)*os_context_register_addr(last_context,reg_ESP);
        }
#endif
        for (ptr = (void **)th->control_stack_end; ptr > esp;  ptr--) {
            preserve_pointer(*ptr);
        }
        /* also need to check registers in any interrupt contexts on
         * an alternate signal stack */
        free=fixnum_value(SymbolValue(FREE_INTERRUPT_CONTEXT_INDEX,th));
        for(i=0;i<free;i++){
            os_context_t *c=th->interrupt_contexts[i];
            if(c>=th->control_stack_end && c<esp) continue;
            for(ptr = (void **)(c+1); ptr>=(void **)c; ptr--) {
                preserve_pointer(*ptr);
            }
        }
    }

#if QSHOW
    if (gencgc_verbose > 1) {
        int num_dont_move_pages = count_dont_move_pages();
        fprintf(stderr,
                "/non-movable pages due to conservative pointers = %d (%d bytes)\n",
                num_dont_move_pages,
                /* FIXME: 4096 should be symbolic constant here and
                 * prob'ly elsewhere too. */
                num_dont_move_pages * 4096);
    }
#endif

    /* Scavenge all the rest of the roots. */

    /* Scavenge the Lisp functions of the interrupt handlers, taking
     * care to avoid SIG_DFL and SIG_IGN. */
    for_each_thread(th) {
        struct interrupt_data *data=th->interrupt_data;
    for (i = 0; i < NSIG; i++) {
            union interrupt_handler handler = data->interrupt_handlers[i];
        if (!ARE_SAME_HANDLER(handler.c, SIG_IGN) &&
            !ARE_SAME_HANDLER(handler.c, SIG_DFL)) {
                scavenge((lispobj *)(data->interrupt_handlers + i), 1);
            }
        }
    }
    /* Scavenge the binding stacks. */
 {
     struct thread *th;
     for_each_thread(th) {
         long len= (lispobj *)SymbolValue(BINDING_STACK_POINTER,th) -
             th->binding_stack_start;
         scavenge((lispobj *) th->binding_stack_start,len);
#ifdef LISP_FEATURE_SB_THREAD
         /* do the tls as well */
         len=fixnum_value(SymbolValue(FREE_TLS_INDEX,0)) -
             (sizeof (struct thread))/(sizeof (lispobj));
         scavenge((lispobj *) (th+1),len);
#endif
        }
    }

    /* The original CMU CL code had scavenge-read-only-space code
     * controlled by the Lisp-level variable
     * *SCAVENGE-READ-ONLY-SPACE*. It was disabled by default, and it
     * wasn't documented under what circumstances it was useful or
     * safe to turn it on, so it's been turned off in SBCL. If you
     * want/need this functionality, and can test and document it,
     * please submit a patch. */
#if 0
    if (SymbolValue(SCAVENGE_READ_ONLY_SPACE) != NIL) {
        unsigned long read_only_space_size =
            (lispobj*)SymbolValue(READ_ONLY_SPACE_FREE_POINTER) -
            (lispobj*)READ_ONLY_SPACE_START;
        FSHOW((stderr,
               "/scavenge read only space: %d bytes\n",
               read_only_space_size * sizeof(lispobj)));
        scavenge( (lispobj *) READ_ONLY_SPACE_START, read_only_space_size);
    }
#endif

    /* Scavenge static space. */
    static_space_size =
        (lispobj *)SymbolValue(STATIC_SPACE_FREE_POINTER,0) -
        (lispobj *)STATIC_SPACE_START;
    if (gencgc_verbose > 1) {
        FSHOW((stderr,
               "/scavenge static space: %d bytes\n",
               static_space_size * sizeof(lispobj)));
    }
    scavenge( (lispobj *) STATIC_SPACE_START, static_space_size);

    /* All generations but the generation being GCed need to be
     * scavenged. The new_space generation needs special handling as
     * objects may be moved in - it is handled separately below. */
    for (i = 0; i < NUM_GENERATIONS; i++) {
        if ((i != generation) && (i != new_space)) {
            scavenge_generation(i);
        }
    }

    /* Finally scavenge the new_space generation. Keep going until no
     * more objects are moved into the new generation */
    scavenge_newspace_generation(new_space);

    /* FIXME: I tried reenabling this check when debugging unrelated
     * GC weirdness ca. sbcl-0.6.12.45, and it failed immediately.
     * Since the current GC code seems to work well, I'm guessing that
     * this debugging code is just stale, but I haven't tried to
     * figure it out. It should be figured out and then either made to
     * work or just deleted. */
#define RESCAN_CHECK 0
#if RESCAN_CHECK
    /* As a check re-scavenge the newspace once; no new objects should
     * be found. */
    {
        int old_bytes_allocated = bytes_allocated;
        int bytes_allocated;

        /* Start with a full scavenge. */
        scavenge_newspace_generation_one_scan(new_space);

        /* Flush the current regions, updating the tables. */
        gc_alloc_update_all_page_tables();

        bytes_allocated = bytes_allocated - old_bytes_allocated;

        if (bytes_allocated != 0) {
            lose("Rescan of new_space allocated %d more bytes.",
                 bytes_allocated);
        }
    }
#endif

    scan_weak_pointers();

    /* Flush the current regions, updating the tables. */
    gc_alloc_update_all_page_tables();

    /* Free the pages in oldspace, but not those marked dont_move. */
    bytes_freed = free_oldspace();

    /* If the GC is not raising the age then lower the generation back
     * to its normal generation number */
    if (!raise) {
        for (i = 0; i < last_free_page; i++)
            if ((page_table[i].bytes_used != 0)
                && (page_table[i].gen == NUM_GENERATIONS))
                page_table[i].gen = generation;
        gc_assert(generations[generation].bytes_allocated == 0);
        generations[generation].bytes_allocated =
            generations[NUM_GENERATIONS].bytes_allocated;
        generations[NUM_GENERATIONS].bytes_allocated = 0;
    }

    /* Reset the alloc_start_page for generation. */
    generations[generation].alloc_start_page = 0;
    generations[generation].alloc_unboxed_start_page = 0;
    generations[generation].alloc_large_start_page = 0;
    generations[generation].alloc_large_unboxed_start_page = 0;

    if (generation >= verify_gens) {
        if (gencgc_verbose)
            SHOW("verifying");
        verify_gc();
        verify_dynamic_space();
    }

    /* Set the new gc trigger for the GCed generation. */
    generations[generation].gc_trigger =
        generations[generation].bytes_allocated
        + generations[generation].bytes_consed_between_gc;

    if (raise)
        generations[generation].num_gc = 0;
    else
        ++generations[generation].num_gc;
}

/* Update last_free_page, then SymbolValue(ALLOCATION_POINTER). */
int
update_x86_dynamic_space_free_pointer(void)
{
    int last_page = -1;
    int i;

    for (i = 0; i < NUM_PAGES; i++)
        if ((page_table[i].allocated != FREE_PAGE)
            && (page_table[i].bytes_used != 0))
            last_page = i;

    last_free_page = last_page+1;

    SetSymbolValue(ALLOCATION_POINTER,
                   (lispobj)(((char *)heap_base) + last_free_page*4096),0);
    return 0; /* dummy value: return something ... */
}

/* GC all generations newer than last_gen, raising the objects in each
 * to the next older generation - we finish when all generations below
 * last_gen are empty.  Then if last_gen is due for a GC, or if
 * last_gen==NUM_GENERATIONS (the scratch generation?  eh?) we GC that
 * too.  The valid range for last_gen is: 0,1,...,NUM_GENERATIONS.
 *
 * We stop collecting at gencgc_oldest_gen_to_gc, even if this is less than
 * last_gen (oh, and note that by default it is NUM_GENERATIONS-1) */
 
void
collect_garbage(unsigned last_gen)
{
    int gen = 0;
    int raise;
    int gen_to_wp;
    int i;

    FSHOW((stderr, "/entering collect_garbage(%d)\n", last_gen));

    if (last_gen > NUM_GENERATIONS) {
        FSHOW((stderr,
               "/collect_garbage: last_gen = %d, doing a level 0 GC\n",
               last_gen));
        last_gen = 0;
    }

    /* Flush the alloc regions updating the tables. */
    gc_alloc_update_all_page_tables();

    /* Verify the new objects created by Lisp code. */
    if (pre_verify_gen_0) {
        FSHOW((stderr, "pre-checking generation 0\n"));
        verify_generation(0);
    }

    if (gencgc_verbose > 1)
        print_generation_stats(0);

    do {
        /* Collect the generation. */

        if (gen >= gencgc_oldest_gen_to_gc) {
            /* Never raise the oldest generation. */
            raise = 0;
        } else {
            raise =
                (gen < last_gen)
                || (generations[gen].num_gc >= generations[gen].trigger_age);
        }

        if (gencgc_verbose > 1) {
            FSHOW((stderr,
                   "starting GC of generation %d with raise=%d alloc=%d trig=%d GCs=%d\n",
                   gen,
                   raise,
                   generations[gen].bytes_allocated,
                   generations[gen].gc_trigger,
                   generations[gen].num_gc));
        }

        /* If an older generation is being filled, then update its
         * memory age. */
        if (raise == 1) {
            generations[gen+1].cum_sum_bytes_allocated +=
                generations[gen+1].bytes_allocated;
        }

        garbage_collect_generation(gen, raise);

        /* Reset the memory age cum_sum. */
        generations[gen].cum_sum_bytes_allocated = 0;

        if (gencgc_verbose > 1) {
            FSHOW((stderr, "GC of generation %d finished:\n", gen));
            print_generation_stats(0);
        }

        gen++;
    } while ((gen <= gencgc_oldest_gen_to_gc)
             && ((gen < last_gen)
                 || ((gen <= gencgc_oldest_gen_to_gc)
                     && raise
                     && (generations[gen].bytes_allocated
                         > generations[gen].gc_trigger)
                     && (gen_av_mem_age(gen)
                         > generations[gen].min_av_mem_age))));

    /* Now if gen-1 was raised all generations before gen are empty.
     * If it wasn't raised then all generations before gen-1 are empty.
     *
     * Now objects within this gen's pages cannot point to younger
     * generations unless they are written to. This can be exploited
     * by write-protecting the pages of gen; then when younger
     * generations are GCed only the pages which have been written
     * need scanning. */
    if (raise)
        gen_to_wp = gen;
    else
        gen_to_wp = gen - 1;

    /* There's not much point in WPing pages in generation 0 as it is
     * never scavenged (except promoted pages). */
    if ((gen_to_wp > 0) && enable_page_protection) {
        /* Check that they are all empty. */
        for (i = 0; i < gen_to_wp; i++) {
            if (generations[i].bytes_allocated)
                lose("trying to write-protect gen. %d when gen. %d nonempty",
                     gen_to_wp, i);
        }
        write_protect_generation_pages(gen_to_wp);
    }

    /* Set gc_alloc() back to generation 0. The current regions should
     * be flushed after the above GCs. */
    gc_assert((boxed_region.free_pointer - boxed_region.start_addr) == 0);
    gc_alloc_generation = 0;

    update_x86_dynamic_space_free_pointer();
    auto_gc_trigger = bytes_allocated + bytes_consed_between_gcs;
    if(gencgc_verbose)
        fprintf(stderr,"Next gc when %ld bytes have been consed\n",
                auto_gc_trigger);
    SHOW("returning from collect_garbage");
}

/* This is called by Lisp PURIFY when it is finished. All live objects
 * will have been moved to the RO and Static heaps. The dynamic space
 * will need a full re-initialization. We don't bother having Lisp
 * PURIFY flush the current gc_alloc() region, as the page_tables are
 * re-initialized, and every page is zeroed to be sure. */
void
gc_free_heap(void)
{
    int page;

    if (gencgc_verbose > 1)
        SHOW("entering gc_free_heap");

    for (page = 0; page < NUM_PAGES; page++) {
        /* Skip free pages which should already be zero filled. */
        if (page_table[page].allocated != FREE_PAGE) {
            void *page_start, *addr;

            /* Mark the page free. The other slots are assumed invalid
             * when it is a FREE_PAGE and bytes_used is 0 and it
             * should not be write-protected -- except that the
             * generation is used for the current region but it sets
             * that up. */
            page_table[page].allocated = FREE_PAGE;
            page_table[page].bytes_used = 0;

            /* Zero the page. */
            page_start = (void *)page_address(page);

            /* First, remove any write-protection. */
            os_protect(page_start, 4096, OS_VM_PROT_ALL);
            page_table[page].write_protected = 0;

            os_invalidate(page_start,4096);
            addr = os_validate(page_start,4096);
            if (addr == NULL || addr != page_start) {
                lose("gc_free_heap: page moved, 0x%08x ==> 0x%08x",
                     page_start,
                     addr);
            }
        } else if (gencgc_zero_check_during_free_heap) {
            /* Double-check that the page is zero filled. */
            int *page_start, i;
            gc_assert(page_table[page].allocated == FREE_PAGE);
            gc_assert(page_table[page].bytes_used == 0);
            page_start = (int *)page_address(page);
            for (i=0; i<1024; i++) {
                if (page_start[i] != 0) {
                    lose("free region not zero at %x", page_start + i);
                }
            }
        }
    }

    bytes_allocated = 0;

    /* Initialize the generations. */
    for (page = 0; page < NUM_GENERATIONS; page++) {
        generations[page].alloc_start_page = 0;
        generations[page].alloc_unboxed_start_page = 0;
        generations[page].alloc_large_start_page = 0;
        generations[page].alloc_large_unboxed_start_page = 0;
        generations[page].bytes_allocated = 0;
        generations[page].gc_trigger = 2000000;
        generations[page].num_gc = 0;
        generations[page].cum_sum_bytes_allocated = 0;
    }

    if (gencgc_verbose > 1)
        print_generation_stats(0);

    /* Initialize gc_alloc(). */
    gc_alloc_generation = 0;

    gc_set_region_empty(&boxed_region);
    gc_set_region_empty(&unboxed_region);

    last_free_page = 0;
    SetSymbolValue(ALLOCATION_POINTER, (lispobj)((char *)heap_base),0);

    if (verify_after_free_heap) {
        /* Check whether purify has left any bad pointers. */
        if (gencgc_verbose)
            SHOW("checking after free_heap\n");
        verify_gc();
    }
}
\f
void
gc_init(void)
{
    int i;

    gc_init_tables();
    scavtab[SIMPLE_VECTOR_WIDETAG] = scav_vector;
    scavtab[WEAK_POINTER_WIDETAG] = scav_weak_pointer;
    transother[SIMPLE_ARRAY_WIDETAG] = trans_boxed_large;

    heap_base = (void*)DYNAMIC_SPACE_START;

    /* Initialize each page structure. */
    for (i = 0; i < NUM_PAGES; i++) {
        /* Initialize all pages as free. */
        page_table[i].allocated = FREE_PAGE;
        page_table[i].bytes_used = 0;

        /* Pages are not write-protected at startup. */
        page_table[i].write_protected = 0;
    }

    bytes_allocated = 0;

    /* Initialize the generations.
     *
     * FIXME: very similar to code in gc_free_heap(), should be shared */
    for (i = 0; i < NUM_GENERATIONS; i++) {
        generations[i].alloc_start_page = 0;
        generations[i].alloc_unboxed_start_page = 0;
        generations[i].alloc_large_start_page = 0;
        generations[i].alloc_large_unboxed_start_page = 0;
        generations[i].bytes_allocated = 0;
        generations[i].gc_trigger = 2000000;
        generations[i].num_gc = 0;
        generations[i].cum_sum_bytes_allocated = 0;
        /* the tune-able parameters */
        generations[i].bytes_consed_between_gc = 2000000;
        generations[i].trigger_age = 1;
        generations[i].min_av_mem_age = 0.75;
    }

    /* Initialize gc_alloc. */
    gc_alloc_generation = 0;
    gc_set_region_empty(&boxed_region);
    gc_set_region_empty(&unboxed_region);

    last_free_page = 0;

}

/*  Pick up the dynamic space from after a core load.
 *
 *  The ALLOCATION_POINTER points to the end of the dynamic space.
 *
 *  XX A scan is needed to identify the closest first objects for pages. */
static void
gencgc_pickup_dynamic(void)
{
    int page = 0;
    int addr = DYNAMIC_SPACE_START;
    int alloc_ptr = SymbolValue(ALLOCATION_POINTER,0);

    /* Initialize the first region. */
    do {
        page_table[page].allocated = BOXED_PAGE;
        page_table[page].gen = 0;
        page_table[page].bytes_used = 4096;
        page_table[page].large_object = 0;
        page_table[page].first_object_offset =
            (void *)DYNAMIC_SPACE_START - page_address(page);
        addr += 4096;
        page++;
    } while (addr < alloc_ptr);

    generations[0].bytes_allocated = 4096*page;
    bytes_allocated = 4096*page;

}

void
gc_initialize_pointers(void)
{
    gencgc_pickup_dynamic();
}


\f

/* alloc(..) is the external interface for memory allocation. It
 * allocates to generation 0. It is not called from within the garbage
 * collector as it is only external uses that need the check for heap
 * size (GC trigger) and to disable the interrupts (interrupts are
 * always disabled during a GC).
 *
 * The vops that call alloc(..) assume that the returned space is zero-filled.
 * (E.g. the most significant word of a 2-word bignum in MOVE-FROM-UNSIGNED.)
 *
 * The check for a GC trigger is only performed when the current
 * region is full, so in most cases it's not needed. */

char *
alloc(int nbytes)
{
    struct thread *th=arch_os_get_current_thread();
    struct alloc_region *region= 
        th ? &(th->alloc_region) : &boxed_region; 
    void *new_obj;
    void *new_free_pointer;

    /* Check for alignment allocation problems. */
    gc_assert((((unsigned)region->free_pointer & 0x7) == 0)
              && ((nbytes & 0x7) == 0));
    if(all_threads)
        /* there are a few places in the C code that allocate data in the
         * heap before Lisp starts.  This is before interrupts are enabled,
         * so we don't need to check for pseudo-atomic */
#ifdef LISP_FEATURE_SB_THREAD
        if(!SymbolValue(PSEUDO_ATOMIC_ATOMIC,th)) {
            register u32 fs;
            fprintf(stderr, "fatal error in thread 0x%x, pid=%d\n",
                    th,getpid());
            __asm__("movl %fs,%0" : "=r" (fs)  : );
            fprintf(stderr, "fs is %x, th->tls_cookie=%x (should be identical)\n",
                    debug_get_fs(),th->tls_cookie);
            lose("If you see this message before 2003.12.01, mail details to sbcl-devel\n");
        }
#else
    gc_assert(SymbolValue(PSEUDO_ATOMIC_ATOMIC,th));
#endif
    
    /* maybe we can do this quickly ... */
    new_free_pointer = region->free_pointer + nbytes;
    if (new_free_pointer <= region->end_addr) {
        new_obj = (void*)(region->free_pointer);
        region->free_pointer = new_free_pointer;
        return(new_obj);        /* yup */
    }
    
    /* we have to go the long way around, it seems.  Check whether 
     * we should GC in the near future
     */
    if (auto_gc_trigger && bytes_allocated > auto_gc_trigger) {
        /* set things up so that GC happens when we finish the PA
         * section.  */
        struct interrupt_data *data=th->interrupt_data;
        maybe_defer_handler(interrupt_maybe_gc_int,data,0,0,0);
    }
    new_obj = gc_alloc_with_region(nbytes,0,region,0);
    return (new_obj);
}

\f
/* Find the code object for the given pc, or return NULL on failure.
 *
 * FIXME: PC shouldn't be lispobj*, should it? Maybe void*? */
lispobj *
component_ptr_from_pc(lispobj *pc)
{
    lispobj *object = NULL;

    if ( (object = search_read_only_space(pc)) )
        ;
    else if ( (object = search_static_space(pc)) )
        ;
    else
        object = search_dynamic_space(pc);

    if (object) /* if we found something */
        if (widetag_of(*object) == CODE_HEADER_WIDETAG) /* if it's a code object */
            return(object);

    return (NULL);
}
\f
/*
 * shared support for the OS-dependent signal handlers which
 * catch GENCGC-related write-protect violations
 */

void unhandled_sigmemoryfault(void);

/* Depending on which OS we're running under, different signals might
 * be raised for a violation of write protection in the heap. This
 * function factors out the common generational GC magic which needs
 * to invoked in this case, and should be called from whatever signal
 * handler is appropriate for the OS we're running under.
 *
 * Return true if this signal is a normal generational GC thing that
 * we were able to handle, or false if it was abnormal and control
 * should fall through to the general SIGSEGV/SIGBUS/whatever logic. */

int
gencgc_handle_wp_violation(void* fault_addr)
{
    int  page_index = find_page_index(fault_addr);

#if defined QSHOW_SIGNALS
    FSHOW((stderr, "heap WP violation? fault_addr=%x, page_index=%d\n",
           fault_addr, page_index));
#endif

    /* Check whether the fault is within the dynamic space. */
    if (page_index == (-1)) {

        /* It can be helpful to be able to put a breakpoint on this
         * case to help diagnose low-level problems. */
        unhandled_sigmemoryfault();

        /* not within the dynamic space -- not our responsibility */
        return 0;

    } else {
        if (page_table[page_index].write_protected) {
            /* Unprotect the page. */
            os_protect(page_address(page_index), PAGE_BYTES, OS_VM_PROT_ALL);
            page_table[page_index].write_protected_cleared = 1;
            page_table[page_index].write_protected = 0;
        } else {  
            /* The only acceptable reason for this signal on a heap
             * access is that GENCGC write-protected the page.
             * However, if two CPUs hit a wp page near-simultaneously,
             * we had better not have the second one lose here if it
             * does this test after the first one has already set wp=0
             */
            if(page_table[page_index].write_protected_cleared != 1) 
                lose("fault in heap page not marked as write-protected");
        }
        /* Don't worry, we can handle it. */
        return 1;
    }
}
/* This is to be called when we catch a SIGSEGV/SIGBUS, determine that
 * it's not just a case of the program hitting the write barrier, and
 * are about to let Lisp deal with it. It's basically just a
 * convenient place to set a gdb breakpoint. */
void
unhandled_sigmemoryfault()
{}

void gc_alloc_update_all_page_tables(void)
{
    /* Flush the alloc regions updating the tables. */
    struct thread *th;
    for_each_thread(th) 
        gc_alloc_update_page_tables(0, &th->alloc_region);
    gc_alloc_update_page_tables(1, &unboxed_region);
    gc_alloc_update_page_tables(0, &boxed_region);
}
void 
gc_set_region_empty(struct alloc_region *region)
{
    region->first_page = 0;
    region->last_page = -1;
    region->start_addr = page_address(0);
    region->free_pointer = page_address(0);
    region->end_addr = page_address(0);
}