* doc/gcov.texi: Use more logical markup.

[official-gcc.git] / boehm-gc / linux_threads.c

/* 
 * Copyright (c) 1994 by Xerox Corporation.  All rights reserved.
 * Copyright (c) 1996 by Silicon Graphics.  All rights reserved.
 * Copyright (c) 1998 by Fergus Henderson.  All rights reserved.
 * Copyright (c) 2000-2001 by Hewlett-Packard Company.  All rights reserved.
 *
 * THIS MATERIAL IS PROVIDED AS IS, WITH ABSOLUTELY NO WARRANTY EXPRESSED
 * OR IMPLIED.  ANY USE IS AT YOUR OWN RISK.
 *
 * Permission is hereby granted to use or copy this program
 * for any purpose,  provided the above notices are retained on all copies.
 * Permission to modify the code and to distribute modified code is granted,
 * provided the above notices are retained, and a notice that the code was
 * modified is included with the above copyright notice.
 */
/*
 * Support code for LinuxThreads, the clone()-based kernel
 * thread package for Linux which is included in libc6.
 *
 * This code relies on implementation details of LinuxThreads,
 * (i.e. properties not guaranteed by the Pthread standard),
 * though this version now does less of that than the other Pthreads
 * support code.
 *
 * Note that there is a lot of code duplication between linux_threads.c
 * and thread support for some of the other Posix platforms; any changes
 * made here may need to be reflected there too.
 */
/*
 * Linux_threads.c now also includes some code to support HPUX and
 * OSF1 (Compaq Tru64 Unix, really).  The OSF1 support is not yet
 * functional.  The OSF1 code is based on Eric Benson's
 * patch, though that was originally against hpux_irix_threads.  The code
 * here is completely untested.  With 0.0000001% probability, it might
 * actually work.
 *
 * Eric also suggested an alternate basis for a lock implementation in
 * his code:
 * + #elif defined(OSF1)
 * +    unsigned long GC_allocate_lock = 0;
 * +    msemaphore GC_allocate_semaphore;
 * + #  define GC_TRY_LOCK() \
 * +    ((msem_lock(&GC_allocate_semaphore, MSEM_IF_NOWAIT) == 0) \
 * +     ? (GC_allocate_lock = 1) \
 * +     : 0)
 * + #  define GC_LOCK_TAKEN GC_allocate_lock
 */

/* #define DEBUG_THREADS 1 */

/* ANSI C requires that a compilation unit contains something */

# if defined(GC_LINUX_THREADS) || defined(LINUX_THREADS) \
     || defined(GC_HPUX_THREADS) || defined(HPUX_THREADS) \
     || defined(GC_OSF1_THREADS) || defined(OSF1_THREADS) \

# include "private/gc_priv.h"
# ifdef THREAD_LOCAL_ALLOC
#   if !defined(USE_PTHREAD_SPECIFIC) && !defined(USE_HPUX_TLS)
#     include "private/specific.h"
#   endif
#   if defined(USE_PTHREAD_SPECIFIC)
#     define GC_getspecific pthread_getspecific
#     define GC_setspecific pthread_setspecific
#     define GC_key_create pthread_key_create
      typedef pthread_key_t GC_key_t;
#   endif
#   if defined(USE_HPUX_TLS)
#     define GC_getspecific(x) (x)
#     define GC_setspecific(key, v) ((key) = (v), 0)
#     define GC_key_create(key, d) 0
      typedef void * GC_key_t;
#   endif
# endif
# include <stdlib.h>
# include <pthread.h>
# include <sched.h>
# include <time.h>
# include <errno.h>
# include <unistd.h>
# include <sys/mman.h>
# include <sys/time.h>
# include <semaphore.h>
# include <signal.h>
# include <sys/types.h>
# include <sys/stat.h>
# include <fcntl.h>

#ifndef __GNUC__
#   define __inline__
#endif

#ifdef GC_USE_LD_WRAP
#   define WRAP_FUNC(f) __wrap_##f
#   define REAL_FUNC(f) __real_##f
#else
#   define WRAP_FUNC(f) GC_##f
#   define REAL_FUNC(f) f
#   undef pthread_create
#   undef pthread_sigmask
#   undef pthread_join
#   undef pthread_detach
#endif


void GC_thr_init();

#if 0
void GC_print_sig_mask()
{
    sigset_t blocked;
    int i;

    if (pthread_sigmask(SIG_BLOCK, NULL, &blocked) != 0)
        ABORT("pthread_sigmask");
    GC_printf0("Blocked: ");
    for (i = 1; i <= MAXSIG; i++) {
        if (sigismember(&blocked, i)) { GC_printf1("%ld ",(long) i); }
    }
    GC_printf0("\n");
}
#endif


/* We use the allocation lock to protect thread-related data structures. */

/* The set of all known threads.  We intercept thread creation and      */
/* joins.                                                               */
/* Protected by allocation/GC lock.                                     */
/* Some of this should be declared volatile, but that's inconsistent    */
/* with some library routine declarations.                              */
typedef struct GC_Thread_Rep {
    struct GC_Thread_Rep * next;  /* More recently allocated threads    */
                                  /* with a given pthread id come       */
                                  /* first.  (All but the first are     */
                                  /* guaranteed to be dead, but we may  */
                                  /* not yet have registered the join.) */
    pthread_t id;
    short flags;
#       define FINISHED 1       /* Thread has exited.   */
#       define DETACHED 2       /* Thread is intended to be detached.   */
#       define MAIN_THREAD 4    /* True for the original thread only.   */
    short thread_blocked;       /* Protected by GC lock.                */
                                /* Treated as a boolean value.  If set, */
                                /* thread will acquire GC lock before   */
                                /* doing any pointer manipulations, and */
                                /* has set its sp value.  Thus it does  */
                                /* not need to be sent a signal to stop */
                                /* it.                                  */
    ptr_t stack_end;            /* Cold end of the stack.               */
    ptr_t stack_ptr;            /* Valid only when stopped.             */
#   ifdef IA64
        ptr_t backing_store_end;
        ptr_t backing_store_ptr;
#   endif
    int signal;
    void * status;              /* The value returned from the thread.  */
                                /* Used only to avoid premature         */
                                /* reclamation of any data it might     */
                                /* reference.                           */
#   ifdef THREAD_LOCAL_ALLOC
#       if CPP_WORDSZ == 64 && defined(ALIGN_DOUBLE)
#           define GRANULARITY 16
#           define NFREELISTS 48
#       else
#           define GRANULARITY 8
#           define NFREELISTS 64
#       endif
        /* The ith free list corresponds to size (i+1)*GRANULARITY */
#       define INDEX_FROM_BYTES(n) (ADD_SLOP(n) - 1)/GRANULARITY
#       define BYTES_FROM_INDEX(i) (((i) + 1) * GRANULARITY - EXTRA_BYTES)
#       define SMALL_ENOUGH(bytes) (ADD_SLOP(bytes) <= NFREELISTS*GRANULARITY)
        ptr_t ptrfree_freelists[NFREELISTS];
        ptr_t normal_freelists[NFREELISTS];
#       ifdef GC_GCJ_SUPPORT
          ptr_t gcj_freelists[NFREELISTS];
#       endif
                /* Free lists contain either a pointer or a small count */
                /* reflecting the number of granules allocated at that  */
                /* size.                                                */
                /* 0 ==> thread-local allocation in use, free list      */
                /*       empty.                                         */
                /* > 0, <= DIRECT_GRANULES ==> Using global allocation, */
                /*       too few objects of this size have been         */
                /*       allocated by this thread.                      */
                /* >= HBLKSIZE  => pointer to nonempty free list.       */
                /* > DIRECT_GRANULES, < HBLKSIZE ==> transition to      */
                /*    local alloc, equivalent to 0.                     */
#       define DIRECT_GRANULES (HBLKSIZE/GRANULARITY)
                /* Don't use local free lists for up to this much       */
                /* allocation.                                          */
#   endif
} * GC_thread;

GC_thread GC_lookup_thread(pthread_t id);

static GC_bool fully_initialized = FALSE;

# if defined(__GNUC__)
    void GC_full_init() __attribute__ ((constructor));
# else
    void GC_full_init();
# endif

# if defined(THREAD_LOCAL_ALLOC) && !defined(DBG_HDRS_ALL)

/* We don't really support thread-local allocation with DBG_HDRS_ALL */

#ifdef USE_HPUX_TLS
  __thread
#endif
GC_key_t GC_thread_key;

static GC_bool keys_initialized;

/* Recover the contents of the freelist array fl into the global one gfl.*/
/* Note that the indexing scheme differs, in that gfl has finer size    */
/* resolution, even if not all entries are used.                        */
/* We hold the allocator lock.                                          */
static void return_freelists(ptr_t *fl, ptr_t *gfl)
{
    int i;
    ptr_t q, *qptr;
    size_t nwords;

    for (i = 0; i < NFREELISTS; ++i) {
        nwords = (i + 1) * (GRANULARITY/sizeof(word));
        qptr = fl + i;  
        q = *qptr;
        if ((word)q < HBLKSIZE) continue;
        if (gfl[nwords] == 0) {
            gfl[nwords] = q;
        } else {
            /* Concatenate: */
            for (; (word)q >= HBLKSIZE; qptr = &(obj_link(q)), q = *qptr);
            GC_ASSERT(0 == q);
            *qptr = gfl[nwords];
            gfl[nwords] = fl[i];
        }
        /* Clear fl[i], since the thread structure may hang around.     */
        /* Do it in a way that is likely to trap if we access it.       */
        fl[i] = (ptr_t)HBLKSIZE;
    }
}

/* Each thread structure must be initialized.   */
/* This call must be made from the new thread.  */
/* Caller holds allocation lock.                */
void GC_init_thread_local(GC_thread p)
{
    int i;

    if (!keys_initialized) {
        if (0 != GC_key_create(&GC_thread_key, 0)) {
            ABORT("Failed to create key for local allocator");
        }
        keys_initialized = TRUE;
    }
    if (0 != GC_setspecific(GC_thread_key, p)) {
        ABORT("Failed to set thread specific allocation pointers");
    }
    for (i = 0; i < NFREELISTS; ++i) {
        p -> ptrfree_freelists[i] = (ptr_t)1;
        p -> normal_freelists[i] = (ptr_t)1;
#       ifdef GC_GCJ_SUPPORT
          p -> gcj_freelists[i] = (ptr_t)1;
#       endif
    }   
}

#ifdef GC_GCJ_SUPPORT
  extern ptr_t * GC_gcjobjfreelist;
#endif

/* We hold the allocator lock.  */
void GC_destroy_thread_local(GC_thread p)
{
    /* We currently only do this from the thread itself.        */
        GC_ASSERT(GC_getspecific(GC_thread_key) == (void *)p);
    return_freelists(p -> ptrfree_freelists, GC_aobjfreelist);
    return_freelists(p -> normal_freelists, GC_objfreelist);
#   ifdef GC_GCJ_SUPPORT
        return_freelists(p -> gcj_freelists, GC_gcjobjfreelist);
#   endif
}

extern GC_PTR GC_generic_malloc_many();

GC_PTR GC_local_malloc(size_t bytes)
{
    if (EXPECT(!SMALL_ENOUGH(bytes),0)) {
        return(GC_malloc(bytes));
    } else {
        int index = INDEX_FROM_BYTES(bytes);
        ptr_t * my_fl;
        ptr_t my_entry;
        GC_key_t k = GC_thread_key;
        void * tsd;

#       if defined(REDIRECT_MALLOC) && !defined(USE_PTHREAD_SPECIFIC) \
           || !defined(__GNUC__)
            if (EXPECT(0 == k, 0)) {
                /* This can happen if we get called when the world is   */
                /* being initialized.  Whether we can actually complete */
                /* the initialization then is unclear.                  */
                GC_full_init();
                k = GC_thread_key;
            }
#       endif
        tsd = GC_getspecific(GC_thread_key);
#       ifdef GC_ASSERTIONS
          LOCK();
          GC_ASSERT(tsd == (void *)GC_lookup_thread(pthread_self()));
          UNLOCK();
#       endif
        my_fl = ((GC_thread)tsd) -> normal_freelists + index;
        my_entry = *my_fl;
        if (EXPECT((word)my_entry >= HBLKSIZE, 1)) {
            ptr_t next = obj_link(my_entry);
            GC_PTR result = (GC_PTR)my_entry;
            *my_fl = next;
            obj_link(my_entry) = 0;
            PREFETCH_FOR_WRITE(next);
            return result;
        } else if ((word)my_entry - 1 < DIRECT_GRANULES) {
            *my_fl = my_entry + index + 1;
            return GC_malloc(bytes);
        } else {
            my_entry = GC_generic_malloc_many(BYTES_FROM_INDEX(index),
                                              NORMAL);
            *my_fl = my_entry;
            if (my_entry == 0) return GC_oom_fn(bytes);
            return GC_local_malloc(bytes);
        }
    }
}

GC_PTR GC_local_malloc_atomic(size_t bytes)
{
    if (EXPECT(!SMALL_ENOUGH(bytes), 0)) {
        return(GC_malloc_atomic(bytes));
    } else {
        int index = INDEX_FROM_BYTES(bytes);
        ptr_t * my_fl = ((GC_thread)GC_getspecific(GC_thread_key))
                        -> ptrfree_freelists + index;
        ptr_t my_entry = *my_fl;
        if (EXPECT((word)my_entry >= HBLKSIZE, 1)) {
            GC_PTR result = (GC_PTR)my_entry;
            *my_fl = obj_link(my_entry);
            return result;
        } else if ((word)my_entry - 1 < DIRECT_GRANULES) {
            *my_fl = my_entry + index + 1;
            return GC_malloc_atomic(bytes);
        } else {
            my_entry = GC_generic_malloc_many(BYTES_FROM_INDEX(index),
                                              PTRFREE);
            *my_fl = my_entry;
            if (my_entry == 0) return GC_oom_fn(bytes);
            return GC_local_malloc_atomic(bytes);
        }
    }
}

#ifdef GC_GCJ_SUPPORT

#include "include/gc_gcj.h"

#ifdef GC_ASSERTIONS
  extern GC_bool GC_gcj_malloc_initialized;
#endif

extern int GC_gcj_kind;

GC_PTR GC_local_gcj_malloc(size_t bytes,
                           void * ptr_to_struct_containing_descr)
{
    GC_ASSERT(GC_gcj_malloc_initialized);
    if (EXPECT(!SMALL_ENOUGH(bytes), 0)) {
        return GC_gcj_malloc(bytes, ptr_to_struct_containing_descr);
    } else {
        int index = INDEX_FROM_BYTES(bytes);
        ptr_t * my_fl = ((GC_thread)GC_getspecific(GC_thread_key))
                        -> gcj_freelists + index;
        ptr_t my_entry = *my_fl;
        if (EXPECT((word)my_entry >= HBLKSIZE, 1)) {
            GC_PTR result = (GC_PTR)my_entry;
            GC_ASSERT(!GC_incremental);
            /* We assert that any concurrent marker will stop us.       */
            /* Thus it is impossible for a mark procedure to see the    */
            /* allocation of the next object, but to see this object    */
            /* still containing a free list pointer.  Otherwise the     */
            /* marker might find a random "mark descriptor".            */
            *my_fl = obj_link(my_entry);
            *(void **)result = ptr_to_struct_containing_descr; 
            return result;
        } else if ((word)my_entry - 1 < DIRECT_GRANULES) {
            *my_fl = my_entry + index + 1;
            return GC_gcj_malloc(bytes, ptr_to_struct_containing_descr);
        } else {
            my_entry = GC_generic_malloc_many(BYTES_FROM_INDEX(index),
                                              GC_gcj_kind);
            *my_fl = my_entry;
            if (my_entry == 0) return GC_oom_fn(bytes);
            return GC_gcj_malloc(bytes, ptr_to_struct_containing_descr);
        }
    }
}

#endif /* GC_GCJ_SUPPORT */

# else  /* !THREAD_LOCAL_ALLOC  && !DBG_HDRS_ALL */

#   define GC_destroy_thread_local(t)

# endif /* !THREAD_LOCAL_ALLOC */

/*
 * The only way to suspend threads given the pthread interface is to send
 * signals.  We can't use SIGSTOP directly, because we need to get the
 * thread to save its stack pointer in the GC thread table before
 * suspending.  So we have to reserve a signal of our own for this.
 * This means we have to intercept client calls to change the signal mask.
 * The linuxthreads package already uses SIGUSR1 and SIGUSR2,
 * so we need to reuse something else.  I chose SIGPWR.
 * (Perhaps SIGUNUSED would be a better choice.)
 */
#ifndef SIG_SUSPEND
#  if defined(HPUX_THREADS) || defined(GC_OSF1_THREADS)
#   define SIG_SUSPEND _SIGRTMIN + 6
#  else
#   define SIG_SUSPEND SIGPWR
#  endif
#endif

#ifndef SIG_THR_RESTART
#  if defined(HPUX_THREADS) || defined(GC_OSF1_THREADS)
#   define SIG_THR_RESTART _SIGRTMIN + 5
#  else
#   define SIG_THR_RESTART SIGXCPU
#  endif
#endif

/* SPARC/Linux doesn't properly define SIGPWR in <signal.h>.
 * It is aliased to SIGLOST in asm/signal.h, though.            */
#if defined(SPARC) && !defined(SIGPWR)
#   define SIGPWR SIGLOST
#endif

sem_t GC_suspend_ack_sem;

#if !defined(HPUX_THREADS) && !defined(GC_OSF1_THREADS)
/*
To make sure that we're using LinuxThreads and not some other thread
package, we generate a dummy reference to `pthread_kill_other_threads_np'
(was `__pthread_initial_thread_bos' but that disappeared),
which is a symbol defined in LinuxThreads, but (hopefully) not in other
thread packages.
*/
void (*dummy_var_to_force_linux_threads)() = pthread_kill_other_threads_np;
#endif /* !HPUX_THREADS */

#if defined(SPARC) || defined(IA64)
  extern word GC_save_regs_in_stack();
#endif

long GC_nprocs = 1;     /* Number of processors.  We may not have       */
                        /* access to all of them, but this is as good   */
                        /* a guess as any ...                           */

#ifdef PARALLEL_MARK

# ifndef MAX_MARKERS
#   define MAX_MARKERS 16
# endif

static ptr_t marker_sp[MAX_MARKERS] = {0};

void * GC_mark_thread(void * id)
{
  word my_mark_no = 0;

  marker_sp[(word)id] = GC_approx_sp();
  for (;; ++my_mark_no) {
    /* GC_mark_no is passed only to allow GC_help_marker to terminate   */
    /* promptly.  This is important if it were called from the signal   */
    /* handler or from the GC lock acquisition code.  Under Linux, it's */
    /* not safe to call it from a signal handler, since it uses mutexes */
    /* and condition variables.  Since it is called only here, the      */
    /* argument is unnecessary.                                         */
    if (my_mark_no < GC_mark_no || my_mark_no > GC_mark_no + 2) {
        /* resynchronize if we get far off, e.g. because GC_mark_no     */
        /* wrapped.                                                     */
        my_mark_no = GC_mark_no;
    }
#   ifdef DEBUG_THREADS
        GC_printf1("Starting mark helper for mark number %ld\n", my_mark_no);
#   endif
    GC_help_marker(my_mark_no);
  }
}

extern long GC_markers;         /* Number of mark threads we would      */
                                /* like to have.  Includes the          */
                                /* initiating thread.                   */

pthread_t GC_mark_threads[MAX_MARKERS];

#define PTHREAD_CREATE REAL_FUNC(pthread_create)

static void start_mark_threads()
{
    unsigned i;
    pthread_attr_t attr;

    if (GC_markers > MAX_MARKERS) {
        WARN("Limiting number of mark threads\n", 0);
        GC_markers = MAX_MARKERS;
    }
    if (0 != pthread_attr_init(&attr)) ABORT("pthread_attr_init failed");
        
    if (0 != pthread_attr_setdetachstate(&attr, PTHREAD_CREATE_DETACHED))
        ABORT("pthread_attr_setdetachstate failed");
#   ifdef CONDPRINT
      if (GC_print_stats) {
        GC_printf1("Starting %ld marker threads\n", GC_markers - 1);
      }
#   endif
    for (i = 0; i < GC_markers - 1; ++i) {
      if (0 != PTHREAD_CREATE(GC_mark_threads + i, &attr,
                              GC_mark_thread, (void *)(word)i)) {
        WARN("Marker thread creation failed, errno = %ld.\n", errno);
      }
    }
}

#else  /* !PARALLEL_MARK */

static __inline__ void start_mark_threads()
{
}

#endif /* !PARALLEL_MARK */

void GC_suspend_handler(int sig)
{
    int dummy;
    pthread_t my_thread = pthread_self();
    GC_thread me;
    sigset_t all_sigs;
    sigset_t old_sigs;
    int i;
    sigset_t mask;
#   ifdef PARALLEL_MARK
        word my_mark_no = GC_mark_no;
        /* Marker can't proceed until we acknowledge.  Thus this is     */
        /* guaranteed to be the mark_no correspending to our            */
        /* suspension, i.e. the marker can't have incremented it yet.   */
#   endif

    if (sig != SIG_SUSPEND) ABORT("Bad signal in suspend_handler");

#if DEBUG_THREADS
    GC_printf1("Suspending 0x%x\n", my_thread);
#endif

    me = GC_lookup_thread(my_thread);
    /* The lookup here is safe, since I'm doing this on behalf  */
    /* of a thread which holds the allocation lock in order     */
    /* to stop the world.  Thus concurrent modification of the  */
    /* data structure is impossible.                            */
#   ifdef SPARC
        me -> stack_ptr = (ptr_t)GC_save_regs_in_stack();
#   else
        me -> stack_ptr = (ptr_t)(&dummy);
#   endif
#   ifdef IA64
        me -> backing_store_ptr = (ptr_t)GC_save_regs_in_stack();
#   endif

    /* Tell the thread that wants to stop the world that this   */
    /* thread has been stopped.  Note that sem_post() is        */
    /* the only async-signal-safe primitive in LinuxThreads.    */
    sem_post(&GC_suspend_ack_sem);

    /* Wait until that thread tells us to restart by sending    */
    /* this thread a SIG_THR_RESTART signal.                    */
    /* SIG_THR_RESTART should be masked at this point.  Thus there      */
    /* is no race.                                              */
    if (sigfillset(&mask) != 0) ABORT("sigfillset() failed");
    if (sigdelset(&mask, SIG_THR_RESTART) != 0) ABORT("sigdelset() failed");
#   ifdef NO_SIGNALS
      if (sigdelset(&mask, SIGINT) != 0) ABORT("sigdelset() failed");
      if (sigdelset(&mask, SIGQUIT) != 0) ABORT("sigdelset() failed");
      if (sigdelset(&mask, SIGTERM) != 0) ABORT("sigdelset() failed");
      if (sigdelset(&mask, SIGABRT) != 0) ABORT("sigdelset() failed");
#   endif
    do {
            me->signal = 0;
            sigsuspend(&mask);             /* Wait for signal */
    } while (me->signal != SIG_THR_RESTART);

#if DEBUG_THREADS
    GC_printf1("Continuing 0x%x\n", my_thread);
#endif
}

void GC_restart_handler(int sig)
{
    GC_thread me;

    if (sig != SIG_THR_RESTART) ABORT("Bad signal in suspend_handler");

    /* Let the GC_suspend_handler() know that we got a SIG_THR_RESTART. */
    /* The lookup here is safe, since I'm doing this on behalf  */
    /* of a thread which holds the allocation lock in order     */
    /* to stop the world.  Thus concurrent modification of the  */
    /* data structure is impossible.                            */
    me = GC_lookup_thread(pthread_self());
    me->signal = SIG_THR_RESTART;

    /*
    ** Note: even if we didn't do anything useful here,
    ** it would still be necessary to have a signal handler,
    ** rather than ignoring the signals, otherwise
    ** the signals will not be delivered at all, and
    ** will thus not interrupt the sigsuspend() above.
    */

#if DEBUG_THREADS
    GC_printf1("In GC_restart_handler for 0x%x\n", pthread_self());
#endif
}

/* Defining INSTALL_LOOPING_SEGV_HANDLER causes SIGSEGV and SIGBUS to   */
/* result in an infinite loop in a signal handler.  This can be very    */
/* useful for debugging, since (as of RH7) gdb still seems to have      */
/* serious problems with threads.                                       */
#ifdef INSTALL_LOOPING_SEGV_HANDLER
void GC_looping_handler(int sig)
{
    GC_printf3("Signal %ld in thread %lx, pid %ld\n",
               sig, pthread_self(), getpid());
    for (;;);
}
#endif

GC_bool GC_thr_initialized = FALSE;

# define THREAD_TABLE_SZ 128    /* Must be power of 2   */
volatile GC_thread GC_threads[THREAD_TABLE_SZ];

void GC_push_thread_structures GC_PROTO((void))
{
    GC_push_all((ptr_t)(GC_threads), (ptr_t)(GC_threads)+sizeof(GC_threads));
}

/* Add a thread to GC_threads.  We assume it wasn't already there.      */
/* Caller holds allocation lock.                                        */
GC_thread GC_new_thread(pthread_t id)
{
    int hv = ((word)id) % THREAD_TABLE_SZ;
    GC_thread result;
    static struct GC_Thread_Rep first_thread;
    static GC_bool first_thread_used = FALSE;
    
    if (!first_thread_used) {
        result = &first_thread;
        first_thread_used = TRUE;
    } else {
        result = (struct GC_Thread_Rep *)
                 GC_INTERNAL_MALLOC(sizeof(struct GC_Thread_Rep), NORMAL);
    }
    if (result == 0) return(0);
    result -> id = id;
    result -> next = GC_threads[hv];
    GC_threads[hv] = result;
    GC_ASSERT(result -> flags == 0 && result -> thread_blocked == 0);
    return(result);
}

/* Delete a thread from GC_threads.  We assume it is there.     */
/* (The code intentionally traps if it wasn't.)                 */
/* Caller holds allocation lock.                                */
void GC_delete_thread(pthread_t id)
{
    int hv = ((word)id) % THREAD_TABLE_SZ;
    register GC_thread p = GC_threads[hv];
    register GC_thread prev = 0;
    
    while (!pthread_equal(p -> id, id)) {
        prev = p;
        p = p -> next;
    }
    if (prev == 0) {
        GC_threads[hv] = p -> next;
    } else {
        prev -> next = p -> next;
    }
    GC_INTERNAL_FREE(p);
}

/* If a thread has been joined, but we have not yet             */
/* been notified, then there may be more than one thread        */
/* in the table with the same pthread id.                       */
/* This is OK, but we need a way to delete a specific one.      */
void GC_delete_gc_thread(pthread_t id, GC_thread gc_id)
{
    int hv = ((word)id) % THREAD_TABLE_SZ;
    register GC_thread p = GC_threads[hv];
    register GC_thread prev = 0;

    while (p != gc_id) {
        prev = p;
        p = p -> next;
    }
    if (prev == 0) {
        GC_threads[hv] = p -> next;
    } else {
        prev -> next = p -> next;
    }
    GC_INTERNAL_FREE(p);
}

/* Return a GC_thread corresponding to a given thread_t.        */
/* Returns 0 if it's not there.                                 */
/* Caller holds  allocation lock or otherwise inhibits          */
/* updates.                                                     */
/* If there is more than one thread with the given id we        */
/* return the most recent one.                                  */
GC_thread GC_lookup_thread(pthread_t id)
{
    int hv = ((word)id) % THREAD_TABLE_SZ;
    register GC_thread p = GC_threads[hv];
    
    while (p != 0 && !pthread_equal(p -> id, id)) p = p -> next;
    return(p);
}

/* There seems to be a very rare thread stopping problem.  To help us  */
/* debug that, we save the ids of the stopping thread. */
pthread_t GC_stopping_thread;
int GC_stopping_pid;

/* Caller holds allocation lock.        */
void GC_stop_world()
{
    pthread_t my_thread = pthread_self();
    register int i;
    register GC_thread p;
    register int n_live_threads = 0;
    register int result;

    GC_stopping_thread = my_thread;    /* debugging only.      */
    GC_stopping_pid = getpid();                /* debugging only.      */

    /* Make sure all free list construction has stopped before we start. */
    /* No new construction can start, since free list construction is   */
    /* required to acquire and release the GC lock before it starts,    */
    /* and we have the lock.                                            */
#   ifdef PARALLEL_MARK
      GC_acquire_mark_lock();
      GC_ASSERT(GC_fl_builder_count == 0);
      /* We should have previously waited for it to become zero. */
#   endif /* PARALLEL_MARK */
    for (i = 0; i < THREAD_TABLE_SZ; i++) {
      for (p = GC_threads[i]; p != 0; p = p -> next) {
        if (p -> id != my_thread) {
            if (p -> flags & FINISHED) continue;
            if (p -> thread_blocked) /* Will wait */ continue;
            n_live_threads++;
            #if DEBUG_THREADS
              GC_printf1("Sending suspend signal to 0x%x\n", p -> id);
            #endif
            result = pthread_kill(p -> id, SIG_SUSPEND);
            switch(result) {
                case ESRCH:
                    /* Not really there anymore.  Possible? */
                    n_live_threads--;
                    break;
                case 0:
                    break;
                default:
                    ABORT("pthread_kill failed");
            }
        }
      }
    }
    for (i = 0; i < n_live_threads; i++) {
        if (0 != sem_wait(&GC_suspend_ack_sem))
            ABORT("sem_wait in handler failed");
    }
#   ifdef PARALLEL_MARK
      GC_release_mark_lock();
#   endif
    #if DEBUG_THREADS
      GC_printf1("World stopped 0x%x\n", pthread_self());
    #endif
}

/* Caller holds allocation lock, and has held it continuously since     */
/* the world stopped.                                                   */
void GC_start_world()
{
    pthread_t my_thread = pthread_self();
    register int i;
    register GC_thread p;
    register int n_live_threads = 0;
    register int result;
    
#   if DEBUG_THREADS
      GC_printf0("World starting\n");
#   endif

    for (i = 0; i < THREAD_TABLE_SZ; i++) {
      for (p = GC_threads[i]; p != 0; p = p -> next) {
        if (p -> id != my_thread) {
            if (p -> flags & FINISHED) continue;
            if (p -> thread_blocked) continue;
            n_live_threads++;
            #if DEBUG_THREADS
              GC_printf1("Sending restart signal to 0x%x\n", p -> id);
            #endif
            result = pthread_kill(p -> id, SIG_THR_RESTART);
            switch(result) {
                case ESRCH:
                    /* Not really there anymore.  Possible? */
                    n_live_threads--;
                    break;
                case 0:
                    break;
                default:
                    ABORT("pthread_kill failed");
            }
        }
      }
    }
    #if DEBUG_THREADS
      GC_printf0("World started\n");
    #endif
    GC_stopping_thread = 0;  /* debugging only */
}

# ifdef IA64
#   define IF_IA64(x) x
# else
#   define IF_IA64(x)
# endif
/* We hold allocation lock.  Should do exactly the right thing if the   */
/* world is stopped.  Should not fail if it isn't.                      */
void GC_push_all_stacks()
{
    int i;
    GC_thread p;
    ptr_t sp = GC_approx_sp();
    ptr_t lo, hi;
    /* On IA64, we also need to scan the register backing store. */
    IF_IA64(ptr_t bs_lo; ptr_t bs_hi;)
    pthread_t me = pthread_self();
    
    if (!GC_thr_initialized) GC_thr_init();
    #if DEBUG_THREADS
        GC_printf1("Pushing stacks from thread 0x%lx\n", (unsigned long) me);
    #endif
    for (i = 0; i < THREAD_TABLE_SZ; i++) {
      for (p = GC_threads[i]; p != 0; p = p -> next) {
        if (p -> flags & FINISHED) continue;
        if (pthread_equal(p -> id, me)) {
#           ifdef SPARC
                lo = (ptr_t)GC_save_regs_in_stack();
#           else
                lo = GC_approx_sp();
#           endif
            IF_IA64(bs_hi = (ptr_t)GC_save_regs_in_stack();)
        } else {
            lo = p -> stack_ptr;
            IF_IA64(bs_hi = p -> backing_store_ptr;)
        }
        if ((p -> flags & MAIN_THREAD) == 0) {
            hi = p -> stack_end;
            IF_IA64(bs_lo = p -> backing_store_end);
        } else {
            /* The original stack. */
            hi = GC_stackbottom;
            IF_IA64(bs_lo = BACKING_STORE_BASE;)
        }
        #if DEBUG_THREADS
            GC_printf3("Stack for thread 0x%lx = [%lx,%lx)\n",
                (unsigned long) p -> id,
                (unsigned long) lo, (unsigned long) hi);
        #endif
        if (0 == lo) ABORT("GC_push_all_stacks: sp not set!\n");
#       ifdef STACK_GROWS_UP
          /* We got them backwards! */
          GC_push_all_stack(hi, lo);
#       else
          GC_push_all_stack(lo, hi);
#       endif
#       ifdef IA64
          if (pthread_equal(p -> id, me)) {
            GC_push_all_eager(bs_lo, bs_hi);
          } else {
            GC_push_all_stack(bs_lo, bs_hi);
          }
#       endif
      }
    }
}

#ifdef USE_PROC_FOR_LIBRARIES
int GC_segment_is_thread_stack(ptr_t lo, ptr_t hi)
{
    int i;
    GC_thread p;
    
#   ifdef PARALLEL_MARK
      for (i = 0; i < GC_markers; ++i) {
        if (marker_sp[i] > lo & marker_sp[i] < hi) return 1;
      }
#   endif
    for (i = 0; i < THREAD_TABLE_SZ; i++) {
      for (p = GC_threads[i]; p != 0; p = p -> next) {
        if (0 != p -> stack_end) {
#         ifdef STACK_GROWS_UP
            if (p -> stack_end >= lo && p -> stack_end < hi) return 1;
#         else /* STACK_GROWS_DOWN */
            if (p -> stack_end > lo && p -> stack_end <= hi) return 1;
#         endif
        }
      }
    }
    return 0;
}
#endif /* USE_PROC_FOR_LIBRARIES */

#ifdef LINUX_THREADS
/* Return the number of processors, or i<= 0 if it can't be determined. */
int GC_get_nprocs()
{
    /* Should be "return sysconf(_SC_NPROCESSORS_ONLN);" but that       */
    /* appears to be buggy in many cases.                               */
    /* We look for lines "cpu<n>" in /proc/stat.                        */
#   define STAT_BUF_SIZE 4096
#   if defined(GC_USE_LD_WRAP)
#       define STAT_READ __real_read
#   else
#       define STAT_READ read
#   endif    
    char stat_buf[STAT_BUF_SIZE];
    int f;
    char c;
    word result = 1;
        /* Some old kernels only have a single "cpu nnnn ..."   */
        /* entry in /proc/stat.  We identify those as           */
        /* uniprocessors.                                       */
    size_t i, len = 0;

    f = open("/proc/stat", O_RDONLY);
    if (f < 0 || (len = STAT_READ(f, stat_buf, STAT_BUF_SIZE)) < 100) {
        WARN("Couldn't read /proc/stat\n", 0);
        return -1;
    }
    for (i = 0; i < len - 100; ++i) {
        if (stat_buf[i] == '\n' && stat_buf[i+1] == 'c'
            && stat_buf[i+2] == 'p' && stat_buf[i+3] == 'u') {
            int cpu_no = atoi(stat_buf + i + 4);
            if (cpu_no >= result) result = cpu_no + 1;
        }
    }
    return result;
}
#endif /* LINUX_THREADS */

/* We hold the allocation lock. */
void GC_thr_init()
{
    int dummy;
    GC_thread t;
    struct sigaction act;

    if (GC_thr_initialized) return;
    GC_thr_initialized = TRUE;

    if (sem_init(&GC_suspend_ack_sem, 0, 0) != 0)
        ABORT("sem_init failed");

    act.sa_flags = SA_RESTART;
    if (sigfillset(&act.sa_mask) != 0) {
        ABORT("sigfillset() failed");
    }
#   ifdef NO_SIGNALS
      if (sigdelset(&act.sa_mask, SIGINT) != 0
          || sigdelset(&act.sa_mask, SIGQUIT != 0)
          || sigdelset(&act.sa_mask, SIGABRT != 0)
          || sigdelset(&act.sa_mask, SIGTERM != 0)) {
        ABORT("sigdelset() failed");
      }
#   endif

    /* SIG_THR_RESTART is unmasked by the handler when necessary.       */
    act.sa_handler = GC_suspend_handler;
    if (sigaction(SIG_SUSPEND, &act, NULL) != 0) {
        ABORT("Cannot set SIG_SUSPEND handler");
    }

    act.sa_handler = GC_restart_handler;
    if (sigaction(SIG_THR_RESTART, &act, NULL) != 0) {
        ABORT("Cannot set SIG_THR_RESTART handler");
    }
#   ifdef INSTALL_LOOPING_SEGV_HANDLER
        act.sa_handler = GC_looping_handler;
        if (sigaction(SIGSEGV, &act, NULL) != 0
            || sigaction(SIGBUS, &act, NULL) != 0) {
            ABORT("Cannot set SIGSEGV or SIGBUS looping handler");
        }
#   endif  /* INSTALL_LOOPING_SEGV_HANDLER */

    /* Add the initial thread, so we can stop it.       */
      t = GC_new_thread(pthread_self());
      t -> stack_ptr = (ptr_t)(&dummy);
      t -> flags = DETACHED | MAIN_THREAD;

    /* Set GC_nprocs.  */
      {
        char * nprocs_string = GETENV("GC_NPROCS");
        GC_nprocs = -1;
        if (nprocs_string != NULL) GC_nprocs = atoi(nprocs_string);
      }
      if (GC_nprocs <= 0) {
#       if defined(HPUX_THREADS)
          GC_nprocs = pthread_num_processors_np();
#       endif
#       if defined(OSF1_THREADS)
          GC_nprocs = 1;
#       endif
#       ifdef LINUX_THREADS
          GC_nprocs = GC_get_nprocs();
#       endif
      }
      if (GC_nprocs <= 0) {
        WARN("GC_get_nprocs() returned %ld\n", GC_nprocs);
        GC_nprocs = 2;
#       ifdef PARALLEL_MARK
          GC_markers = 1;
#       endif
      } else {
#       ifdef PARALLEL_MARK
          GC_markers = GC_nprocs;
#       endif
      }
#   ifdef PARALLEL_MARK
#     ifdef CONDPRINT
        if (GC_print_stats) {
          GC_printf2("Number of processors = %ld, "
                 "number of marker threads = %ld\n", GC_nprocs, GC_markers);
        }
#     endif
      if (GC_markers == 1) {
        GC_parallel = FALSE;
#       ifdef CONDPRINT
          if (GC_print_stats) {
            GC_printf0("Single marker thread, turning off parallel marking\n");
          }
#       endif
      } else {
        GC_parallel = TRUE;
      }
#   endif
}


/* Perform all initializations, including those that    */
/* may require allocation.                              */
/* Called as constructor without allocation lock.       */
/* Must be called before a second thread is created.    */
void GC_full_init()
{
    if (fully_initialized) return;
    if (!GC_is_initialized) GC_init();
    /* If we are using a parallel marker, start the helper threads.  */
#     ifdef PARALLEL_MARK
        if (GC_parallel) start_mark_threads();
#     endif
    /* Initialize thread local free lists if used.      */
#   if defined(THREAD_LOCAL_ALLOC) && !defined(DBG_HDRS_ALL)
      LOCK();
      GC_init_thread_local(GC_lookup_thread(pthread_self()));
      UNLOCK();
#   endif
    fully_initialized = TRUE;
}


int WRAP_FUNC(pthread_sigmask)(int how, const sigset_t *set, sigset_t *oset)
{
    sigset_t fudged_set;
    
    if (set != NULL && (how == SIG_BLOCK || how == SIG_SETMASK)) {
        fudged_set = *set;
        sigdelset(&fudged_set, SIG_SUSPEND);
        set = &fudged_set;
    }
    return(REAL_FUNC(pthread_sigmask)(how, set, oset));
}

/* Wrappers for functions that are likely to block for an appreciable   */
/* length of time.  Must be called in pairs, if at all.                 */
/* Nothing much beyond the system call itself should be executed        */
/* between these.                                                       */

void GC_start_blocking(void) {
#   define SP_SLOP 128
    GC_thread me;
    LOCK();
    me = GC_lookup_thread(pthread_self());
    GC_ASSERT(!(me -> thread_blocked));
#   ifdef SPARC
        me -> stack_ptr = (ptr_t)GC_save_regs_in_stack();
#   else
        me -> stack_ptr = (ptr_t)GC_approx_sp();
#   endif
#   ifdef IA64
        me -> backing_store_ptr = (ptr_t)GC_save_regs_in_stack() + SP_SLOP;
#   endif
    /* Add some slop to the stack pointer, since the wrapped call may   */
    /* end up pushing more callee-save registers.                       */
#   ifdef STACK_GROWS_UP
        me -> stack_ptr += SP_SLOP;
#   else
        me -> stack_ptr -= SP_SLOP;
#   endif
    me -> thread_blocked = TRUE;
    UNLOCK();
}

GC_end_blocking(void) {
    GC_thread me;
    LOCK();   /* This will block if the world is stopped.       */
    me = GC_lookup_thread(pthread_self());
    GC_ASSERT(me -> thread_blocked);
    me -> thread_blocked = FALSE;
    UNLOCK();
}
    
/* A wrapper for the standard C sleep function  */
int WRAP_FUNC(sleep) (unsigned int seconds)
{
    int result;

    GC_start_blocking();
    result = REAL_FUNC(sleep)(seconds);
    GC_end_blocking();
    return result;
}

struct start_info {
    void *(*start_routine)(void *);
    void *arg;
    word flags;
    sem_t registered;           /* 1 ==> in our thread table, but       */
                                /* parent hasn't yet noticed.           */
};

/* Called at thread exit.                               */
/* Never called for main thread.  That's OK, since it   */
/* results in at most a tiny one-time leak.  And        */
/* linuxthreads doesn't reclaim the main threads        */
/* resources or id anyway.                              */
void GC_thread_exit_proc(void *arg)
{
    GC_thread me;

    LOCK();
    me = GC_lookup_thread(pthread_self());
    GC_destroy_thread_local(me);
    if (me -> flags & DETACHED) {
        GC_delete_thread(pthread_self());
    } else {
        me -> flags |= FINISHED;
    }
#   if defined(THREAD_LOCAL_ALLOC) && !defined(USE_PTHREAD_SPECIFIC) \
       && !defined(USE_HPUX_TLS) && !defined(DBG_HDRS_ALL)
      GC_remove_specific(GC_thread_key);
#   endif
    if (GC_incremental && GC_collection_in_progress()) {
        int old_gc_no = GC_gc_no;

        /* Make sure that no part of our stack is still on the mark stack, */
        /* since it's about to be unmapped.                                */
        while (GC_incremental && GC_collection_in_progress()
               && old_gc_no == GC_gc_no) {
            ENTER_GC();
            GC_collect_a_little_inner(1);
            EXIT_GC();
            UNLOCK();
            sched_yield();
            LOCK();
        }
    }
    UNLOCK();
}

int WRAP_FUNC(pthread_join)(pthread_t thread, void **retval)
{
    int result;
    GC_thread thread_gc_id;
    
    LOCK();
    thread_gc_id = GC_lookup_thread(thread);
    /* This is guaranteed to be the intended one, since the thread id   */
    /* cant have been recycled by pthreads.                             */
    UNLOCK();
    result = REAL_FUNC(pthread_join)(thread, retval);
    if (result == 0) {
        LOCK();
        /* Here the pthread thread id may have been recycled. */
        GC_delete_gc_thread(thread, thread_gc_id);
        UNLOCK();
    }
    return result;
}

int
WRAP_FUNC(pthread_detach)(pthread_t thread)
{
    int result;
    GC_thread thread_gc_id;
    
    LOCK();
    thread_gc_id = GC_lookup_thread(thread);
    UNLOCK();
    result = REAL_FUNC(pthread_detach)(thread);
    if (result == 0) {
      LOCK();
      thread_gc_id -> flags |= DETACHED;
      /* Here the pthread thread id may have been recycled. */
      if (thread_gc_id -> flags & FINISHED) {
        GC_delete_gc_thread(thread, thread_gc_id);
      }
      UNLOCK();
    }
    return result;
}

void * GC_start_routine(void * arg)
{
    int dummy;
    struct start_info * si = arg;
    void * result;
    GC_thread me;
    pthread_t my_pthread;
    void *(*start)(void *);
    void *start_arg;

    my_pthread = pthread_self();
#   ifdef DEBUG_THREADS
        GC_printf1("Starting thread 0x%lx\n", my_pthread);
        GC_printf1("pid = %ld\n", (long) getpid());
        GC_printf1("sp = 0x%lx\n", (long) &arg);
#   endif
    LOCK();
    me = GC_new_thread(my_pthread);
    me -> flags = si -> flags;
    me -> stack_ptr = 0;
    /* me -> stack_end = GC_linux_stack_base(); -- currently (11/99)    */
    /* doesn't work because the stack base in /proc/self/stat is the    */
    /* one for the main thread.  There is a strong argument that that's */
    /* a kernel bug, but a pervasive one.                               */
#   ifdef STACK_GROWS_DOWN
      me -> stack_end = (ptr_t)(((word)(&dummy) + (GC_page_size - 1))
                                & ~(GC_page_size - 1));
      me -> stack_ptr = me -> stack_end - 0x10;
        /* Needs to be plausible, since an asynchronous stack mark      */
        /* should not crash.                                            */
#   else
      me -> stack_end = (ptr_t)((word)(&dummy) & ~(GC_page_size - 1));
      me -> stack_ptr = me -> stack_end + 0x10;
#   endif
    /* This is dubious, since we may be more than a page into the stack, */
    /* and hence skip some of it, though it's not clear that matters.    */
#   ifdef IA64
      me -> backing_store_end = (ptr_t)
                        (GC_save_regs_in_stack() & ~(GC_page_size - 1));
      /* This is also < 100% convincing.  We should also read this      */
      /* from /proc, but the hook to do so isn't there yet.             */
#   endif /* IA64 */
    UNLOCK();
    start = si -> start_routine;
#   ifdef DEBUG_THREADS
        GC_printf1("start_routine = 0x%lx\n", start);
#   endif
    start_arg = si -> arg;
    sem_post(&(si -> registered));      /* Last action on si.   */
                                        /* OK to deallocate.    */
    pthread_cleanup_push(GC_thread_exit_proc, 0);
#   if defined(THREAD_LOCAL_ALLOC) && !defined(DBG_HDRS_ALL)
        LOCK();
        GC_init_thread_local(me);
        UNLOCK();
#   endif
    result = (*start)(start_arg);
#if DEBUG_THREADS
        GC_printf1("Finishing thread 0x%x\n", pthread_self());
#endif
    me -> status = result;
    me -> flags |= FINISHED;
    pthread_cleanup_pop(1);
    /* Cleanup acquires lock, ensuring that we can't exit               */
    /* while a collection that thinks we're alive is trying to stop     */
    /* us.                                                              */
    return(result);
}

int
WRAP_FUNC(pthread_create)(pthread_t *new_thread,
                  const pthread_attr_t *attr,
                  void *(*start_routine)(void *), void *arg)
{
    int result;
    GC_thread t;
    pthread_t my_new_thread;
    int detachstate;
    word my_flags = 0;
    struct start_info * si; 
        /* This is otherwise saved only in an area mmapped by the thread */
        /* library, which isn't visible to the collector.                */
 
    LOCK();
    si = (struct start_info *)GC_INTERNAL_MALLOC(sizeof(struct start_info), NORMAL);
    UNLOCK();
    if (!fully_initialized) GC_full_init();
    if (0 == si) return(ENOMEM);
    sem_init(&(si -> registered), 0, 0);
    si -> start_routine = start_routine;
    si -> arg = arg;
    LOCK();
    if (!GC_thr_initialized) GC_thr_init();
    if (NULL == attr) {
        detachstate = PTHREAD_CREATE_JOINABLE;
    } else {
        pthread_attr_getdetachstate(attr, &detachstate);
    }
    if (PTHREAD_CREATE_DETACHED == detachstate) my_flags |= DETACHED;
    si -> flags = my_flags;
    UNLOCK();
#   ifdef DEBUG_THREADS
        GC_printf1("About to start new thread from thread 0x%X\n",
                   pthread_self());
#   endif
    result = REAL_FUNC(pthread_create)(new_thread, attr, GC_start_routine, si);
#   ifdef DEBUG_THREADS
        GC_printf1("Started thread 0x%X\n", *new_thread);
#   endif
    /* Wait until child has been added to the thread table.             */
    /* This also ensures that we hold onto si until the child is done   */
    /* with it.  Thus it doesn't matter whether it is otherwise         */
    /* visible to the collector.                                        */
        while (0 != sem_wait(&(si -> registered))) {
            if (EINTR != errno) ABORT("sem_wait failed");
        }
        sem_destroy(&(si -> registered));
        LOCK();
        GC_INTERNAL_FREE(si);
        UNLOCK();
    return(result);
}

#ifdef GENERIC_COMPARE_AND_SWAP
  pthread_mutex_t GC_compare_and_swap_lock = PTHREAD_MUTEX_INITIALIZER;

  GC_bool GC_compare_and_exchange(volatile GC_word *addr,
                                  GC_word old, GC_word new_val)
  {
    GC_bool result;
    pthread_mutex_lock(&GC_compare_and_swap_lock);
    if (*addr == old) {
      *addr = new_val;
      result = TRUE;
    } else {
      result = FALSE;
    }
    pthread_mutex_unlock(&GC_compare_and_swap_lock);
    return result;
  }
  
  GC_word GC_atomic_add(volatile GC_word *addr, GC_word how_much)
  {
    GC_word old;
    pthread_mutex_lock(&GC_compare_and_swap_lock);
    old = *addr;
    *addr = old + how_much;
    pthread_mutex_unlock(&GC_compare_and_swap_lock);
    return old;
  }

#endif /* GENERIC_COMPARE_AND_SWAP */
/* Spend a few cycles in a way that can't introduce contention with     */
/* othre threads.                                                       */
void GC_pause()
{
    int i;
    volatile word dummy = 0;

    for (i = 0; i < 10; ++i) { 
#     ifdef __GNUC__
        __asm__ __volatile__ (" " : : : "memory");
#     else
        /* Something that's unlikely to be optimized away. */
        GC_noop(++dummy);
#     endif
    }
}
    
#define SPIN_MAX 1024   /* Maximum number of calls to GC_pause before   */
                        /* give up.                                     */

VOLATILE GC_bool GC_collecting = 0;
                        /* A hint that we're in the collector and       */
                        /* holding the allocation lock for an           */
                        /* extended period.                             */

#if !defined(USE_SPIN_LOCK) || defined(PARALLEL_MARK)
/* If we don't want to use the below spinlock implementation, either    */
/* because we don't have a GC_test_and_set implementation, or because   */
/* we don't want to risk sleeping, we can still try spinning on         */
/* pthread_mutex_trylock for a while.  This appears to be very          */
/* beneficial in many cases.                                            */
/* I suspect that under high contention this is nearly always better    */
/* than the spin lock.  But it's a bit slower on a uniprocessor.        */
/* Hence we still default to the spin lock.                             */
/* This is also used to acquire the mark lock for the parallel          */
/* marker.                                                              */

/* Here we use a strict exponential backoff scheme.  I don't know       */
/* whether that's better or worse than the above.  We eventually        */
/* yield by calling pthread_mutex_lock(); it never makes sense to       */
/* explicitly sleep.                                                    */

void GC_generic_lock(pthread_mutex_t * lock)
{
    unsigned pause_length = 1;
    unsigned i;
    
    if (0 == pthread_mutex_trylock(lock)) return;
    for (; pause_length <= SPIN_MAX; pause_length <<= 1) {
        for (i = 0; i < pause_length; ++i) {
            GC_pause();
        }
        switch(pthread_mutex_trylock(lock)) {
            case 0:
                return;
            case EBUSY:
                break;
            default:
                ABORT("Unexpected error from pthread_mutex_trylock");
        }
    }
    pthread_mutex_lock(lock);
}

#endif /* !USE_SPIN_LOCK || PARALLEL_MARK */

#if defined(USE_SPIN_LOCK)

/* Reasonably fast spin locks.  Basically the same implementation */
/* as STL alloc.h.  This isn't really the right way to do this.   */
/* but until the POSIX scheduling mess gets straightened out ...  */

volatile unsigned int GC_allocate_lock = 0;


void GC_lock()
{
#   define low_spin_max 30  /* spin cycles if we suspect uniprocessor */
#   define high_spin_max SPIN_MAX /* spin cycles for multiprocessor */
    static unsigned spin_max = low_spin_max;
    unsigned my_spin_max;
    static unsigned last_spins = 0;
    unsigned my_last_spins;
    int i;

    if (!GC_test_and_set(&GC_allocate_lock)) {
        return;
    }
    my_spin_max = spin_max;
    my_last_spins = last_spins;
    for (i = 0; i < my_spin_max; i++) {
        if (GC_collecting || GC_nprocs == 1) goto yield;
        if (i < my_last_spins/2 || GC_allocate_lock) {
            GC_pause();
            continue;
        }
        if (!GC_test_and_set(&GC_allocate_lock)) {
            /*
             * got it!
             * Spinning worked.  Thus we're probably not being scheduled
             * against the other process with which we were contending.
             * Thus it makes sense to spin longer the next time.
             */
            last_spins = i;
            spin_max = high_spin_max;
            return;
        }
    }
    /* We are probably being scheduled against the other process.  Sleep. */
    spin_max = low_spin_max;
yield:
    for (i = 0;; ++i) {
        if (!GC_test_and_set(&GC_allocate_lock)) {
            return;
        }
#       define SLEEP_THRESHOLD 12
                /* nanosleep(<= 2ms) just spins under Linux.  We        */
                /* want to be careful to avoid that behavior.           */
        if (i < SLEEP_THRESHOLD) {
            sched_yield();
        } else {
            struct timespec ts;
        
            if (i > 24) i = 24;
                        /* Don't wait for more than about 15msecs, even */
                        /* under extreme contention.                    */
            ts.tv_sec = 0;
            ts.tv_nsec = 1 << i;
            nanosleep(&ts, 0);
        }
    }
}

#else  /* !USE_SPINLOCK */

void GC_lock()
{
    if (1 == GC_nprocs || GC_collecting) {
        pthread_mutex_lock(&GC_allocate_ml);
    } else {
        GC_generic_lock(&GC_allocate_ml);
    }
}

#endif /* !USE_SPINLOCK */

#ifdef PARALLEL_MARK

#ifdef GC_ASSERTIONS
  pthread_t GC_mark_lock_holder = NO_THREAD;
#endif

#ifdef IA64
  /* Ugly workaround for a linux threads bug in the final versions      */
  /* of glibc2.1.  Pthread_mutex_trylock sets the mutex owner           */
  /* field even when it fails to acquire the mutex.  This causes        */
  /* pthread_cond_wait to die.  Remove for glibc2.2.                    */
  /* According to the man page, we should use                           */
  /* PTHREAD_ERRORCHECK_MUTEX_INITIALIZER_NP, but that isn't actually   */
  /* defined.                                                           */
  static pthread_mutex_t mark_mutex =
        {0, 0, 0, PTHREAD_MUTEX_ERRORCHECK_NP, {0, 0}};
#else
  static pthread_mutex_t mark_mutex = PTHREAD_MUTEX_INITIALIZER;
#endif

static pthread_cond_t mark_cv = PTHREAD_COND_INITIALIZER;

static pthread_cond_t builder_cv = PTHREAD_COND_INITIALIZER;

void GC_acquire_mark_lock()
{
/*
    if (pthread_mutex_lock(&mark_mutex) != 0) {
        ABORT("pthread_mutex_lock failed");
    }
*/
    GC_generic_lock(&mark_mutex);
#   ifdef GC_ASSERTIONS
        GC_mark_lock_holder = pthread_self();
#   endif
}

void GC_release_mark_lock()
{
    GC_ASSERT(GC_mark_lock_holder == pthread_self());
#   ifdef GC_ASSERTIONS
        GC_mark_lock_holder = NO_THREAD;
#   endif
    if (pthread_mutex_unlock(&mark_mutex) != 0) {
        ABORT("pthread_mutex_unlock failed");
    }
}

void GC_wait_marker()
{
    GC_ASSERT(GC_mark_lock_holder == pthread_self());
#   ifdef GC_ASSERTIONS
        GC_mark_lock_holder = NO_THREAD;
#   endif
    if (pthread_cond_wait(&mark_cv, &mark_mutex) != 0) {
        ABORT("pthread_cond_wait failed");
    }
    GC_ASSERT(GC_mark_lock_holder == NO_THREAD);
#   ifdef GC_ASSERTIONS
        GC_mark_lock_holder = pthread_self();
#   endif
}

void GC_wait_builder()
{
    GC_ASSERT(GC_mark_lock_holder == pthread_self());
#   ifdef GC_ASSERTIONS
        GC_mark_lock_holder = NO_THREAD;
#   endif
    if (pthread_cond_wait(&builder_cv, &mark_mutex) != 0) {
        ABORT("pthread_cond_wait failed");
    }
    GC_ASSERT(GC_mark_lock_holder == NO_THREAD);
#   ifdef GC_ASSERTIONS
        GC_mark_lock_holder = pthread_self();
#   endif
}

void GC_notify_all_marker()
{
    if (pthread_cond_broadcast(&mark_cv) != 0) {
        ABORT("pthread_cond_broadcast failed");
    }
}

void GC_notify_all_builder()
{
    GC_ASSERT(GC_mark_lock_holder == pthread_self());
    if (pthread_cond_broadcast(&builder_cv) != 0) {
        ABORT("pthread_cond_broadcast failed");
    }
}

void GC_wait_for_reclaim()
{
    GC_acquire_mark_lock();
    while (GC_fl_builder_count > 0) {
        GC_wait_builder();
    }
    GC_release_mark_lock();
}
#endif /* PARALLEL_MARK */

# endif /* LINUX_THREADS */