Compact.c 23.9 KB
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/* -----------------------------------------------------------------------------
 *
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 * (c) The GHC Team 2001-2008
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 *
 * Compacting garbage collector
 *
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 * Documentation on the architecture of the Garbage Collector can be
 * found in the online commentary:
 * 
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 *   http://ghc.haskell.org/trac/ghc/wiki/Commentary/Rts/Storage/GC
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 *
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 * ---------------------------------------------------------------------------*/

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#include "PosixSource.h"
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#include "Rts.h"
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#include "GCThread.h"
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#include "Storage.h"
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#include "RtsUtils.h"
#include "BlockAlloc.h"
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#include "GC.h"
#include "Compact.h"
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#include "Schedule.h"
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#include "Apply.h"
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#include "Trace.h"
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#include "Weak.h"
#include "MarkWeak.h"
#include "Stable.h"
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// Turn off inlining when debugging - it obfuscates things
#ifdef DEBUG
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# undef  STATIC_INLINE
# define STATIC_INLINE static
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#endif

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/* ----------------------------------------------------------------------------
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   Threading / unthreading pointers.

   The basic idea here is to chain together all the fields pointing at
   a particular object, with the root of the chain in the object's
   info table field.  The original contents of the info pointer goes
   at the end of the chain.

   Adding a new field to the chain is a matter of swapping the
   contents of the field with the contents of the object's info table
   field.

   To unthread the chain, we walk down it updating all the fields on
   the chain with the new location of the object.  We stop when we
   reach the info pointer at the end.

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   The main difficulty here is that we need to be able to identify the
   info pointer at the end of the chain.  We can't use the low bits of
   the pointer for this; they are already being used for
   pointer-tagging.  What's more, we need to retain the
   pointer-tagging tag bits on each pointer during the
   threading/unthreading process.

   Our solution is as follows: 
     - an info pointer (chain length zero) is identified by having tag 0
     - in a threaded chain of length > 0:
        - the pointer-tagging tag bits are attached to the info pointer
        - the first entry in the chain has tag 1
        - second and subsequent entries in the chain have tag 2

   This exploits the fact that the tag on each pointer to a given
   closure is normally the same (if they are not the same, then
   presumably the tag is not essential and it therefore doesn't matter
   if we throw away some of the tags).
   ------------------------------------------------------------------------- */
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STATIC_INLINE void
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thread (StgClosure **p)
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{
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    StgClosure *q0;
    StgPtr q;
    StgWord iptr;
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    bdescr *bd;

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    q0  = *p;
    q   = (StgPtr)UNTAG_CLOSURE(q0);

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    // It doesn't look like a closure at the moment, because the info
    // ptr is possibly threaded:
    // ASSERT(LOOKS_LIKE_CLOSURE_PTR(q));
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    if (HEAP_ALLOCED(q)) {
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	bd = Bdescr(q); 
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	if (bd->flags & BF_MARKED)
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        {
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            iptr = *q;
            switch (GET_CLOSURE_TAG((StgClosure *)iptr))
            {
            case 0: 
                // this is the info pointer; we are creating a new chain.
                // save the original tag at the end of the chain.
                *p = (StgClosure *)((StgWord)iptr + GET_CLOSURE_TAG(q0));
                *q = (StgWord)p + 1;
                break;
            case 1:
            case 2:
                // this is a chain of length 1 or more
                *p = (StgClosure *)iptr;
                *q = (StgWord)p + 2;
                break;
            }
        }
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    }
}

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static void
thread_root (void *user STG_UNUSED, StgClosure **p)
{
    thread(p);
}

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// This version of thread() takes a (void *), used to circumvent
// warnings from gcc about pointer punning and strict aliasing.
STATIC_INLINE void thread_ (void *p) { thread((StgClosure **)p); }

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STATIC_INLINE void
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unthread( StgPtr p, StgWord free )
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{
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    StgWord q, r;
    StgPtr q0;

    q = *p;
loop:
    switch (GET_CLOSURE_TAG((StgClosure *)q))
    {
    case 0:
        // nothing to do; the chain is length zero
        return;
    case 1:
        q0 = (StgPtr)(q-1);
        r = *q0;  // r is the info ptr, tagged with the pointer-tag
        *q0 = free;
        *p = (StgWord)UNTAG_CLOSURE((StgClosure *)r);
        return;
    case 2:
        q0 = (StgPtr)(q-2);
        r = *q0;
        *q0 = free;
        q = r;
        goto loop;
    default:
        barf("unthread");
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    }
}

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// Traverse a threaded chain and pull out the info pointer at the end.
// The info pointer is also tagged with the appropriate pointer tag
// for this closure, which should be attached to the pointer
// subsequently passed to unthread().
STATIC_INLINE StgWord
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get_threaded_info( StgPtr p )
{
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    StgWord q;
    
    q = (W_)GET_INFO(UNTAG_CLOSURE((StgClosure *)p));
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loop:
    switch (GET_CLOSURE_TAG((StgClosure *)q)) 
    {
    case 0:
        ASSERT(LOOKS_LIKE_INFO_PTR(q));
        return q;
    case 1:
    {
        StgWord r = *(StgPtr)(q-1);
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        ASSERT(LOOKS_LIKE_INFO_PTR((StgWord)UNTAG_CLOSURE((StgClosure *)r)));
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        return r;
    }
    case 2:
        q = *(StgPtr)(q-2);
        goto loop;
    default:
        barf("get_threaded_info");
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    }
}

// A word-aligned memmove will be faster for small objects than libc's or gcc's.
// Remember, the two regions *might* overlap, but: to <= from.
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STATIC_INLINE void
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move(StgPtr to, StgPtr from, StgWord size)
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{
    for(; size > 0; --size) {
	*to++ = *from++;
    }
}

static void
thread_static( StgClosure* p )
{
  const StgInfoTable *info;

  // keep going until we've threaded all the objects on the linked
  // list... 
  while (p != END_OF_STATIC_LIST) {

    info = get_itbl(p);
    switch (info->type) {
      
    case IND_STATIC:
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	thread(&((StgInd *)p)->indirectee);
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	p = *IND_STATIC_LINK(p);
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	continue;
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    case THUNK_STATIC:
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	p = *THUNK_STATIC_LINK(p);
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	continue;
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    case FUN_STATIC:
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	p = *FUN_STATIC_LINK(p);
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	continue;
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    case CONSTR_STATIC:
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	p = *STATIC_LINK(info,p);
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	continue;
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    default:
	barf("thread_static: strange closure %d", (int)(info->type));
    }

  }
}

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STATIC_INLINE void
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thread_large_bitmap( StgPtr p, StgLargeBitmap *large_bitmap, StgWord size )
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{
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    W_ i, b;
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    StgWord bitmap;

    b = 0;
    bitmap = large_bitmap->bitmap[b];
    for (i = 0; i < size; ) {
	if ((bitmap & 1) == 0) {
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	    thread((StgClosure **)p);
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	}
	i++;
	p++;
	if (i % BITS_IN(W_) == 0) {
	    b++;
	    bitmap = large_bitmap->bitmap[b];
	} else {
	    bitmap = bitmap >> 1;
	}
    }
}

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STATIC_INLINE StgPtr
thread_small_bitmap (StgPtr p, StgWord size, StgWord bitmap)
{
    while (size > 0) {
        if ((bitmap & 1) == 0) {
            thread((StgClosure **)p);
        }
        p++;
        bitmap = bitmap >> 1;
        size--;
    }
    return p;
}

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STATIC_INLINE StgPtr
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thread_arg_block (StgFunInfoTable *fun_info, StgClosure **args)
{
    StgPtr p;
    StgWord bitmap;
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    StgWord size;
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    p = (StgPtr)args;
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    switch (fun_info->f.fun_type) {
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    case ARG_GEN:
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	bitmap = BITMAP_BITS(fun_info->f.b.bitmap);
	size = BITMAP_SIZE(fun_info->f.b.bitmap);
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	goto small_bitmap;
    case ARG_GEN_BIG:
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	size = GET_FUN_LARGE_BITMAP(fun_info)->size;
	thread_large_bitmap(p, GET_FUN_LARGE_BITMAP(fun_info), size);
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	p += size;
	break;
    default:
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	bitmap = BITMAP_BITS(stg_arg_bitmaps[fun_info->f.fun_type]);
	size = BITMAP_SIZE(stg_arg_bitmaps[fun_info->f.fun_type]);
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    small_bitmap:
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        p = thread_small_bitmap(p, size, bitmap);
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	break;
    }
    return p;
}

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static void
thread_stack(StgPtr p, StgPtr stack_end)
{
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    const StgRetInfoTable* info;
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    StgWord bitmap;
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    StgWord size;
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    // highly similar to scavenge_stack, but we do pointer threading here.
    
    while (p < stack_end) {

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	// *p must be the info pointer of an activation
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	// record.  All activation records have 'bitmap' style layout
	// info.
	//
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	info  = get_ret_itbl((StgClosure *)p);
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	switch (info->i.type) {
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            // small bitmap (<= 32 entries, or 64 on a 64-bit machine)
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        case CATCH_RETRY_FRAME:
        case CATCH_STM_FRAME:
        case ATOMICALLY_FRAME:
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	case UPDATE_FRAME:
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        case UNDERFLOW_FRAME:
        case STOP_FRAME:
        case CATCH_FRAME:
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	case RET_SMALL:
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	    bitmap = BITMAP_BITS(info->i.layout.bitmap);
	    size   = BITMAP_SIZE(info->i.layout.bitmap);
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	    p++;
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	    // NOTE: the payload starts immediately after the info-ptr, we
	    // don't have an StgHeader in the same sense as a heap closure.
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            p = thread_small_bitmap(p, size, bitmap);
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	    continue;

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	case RET_BCO: {
	    StgBCO *bco;
	    
	    p++;
	    bco = (StgBCO *)*p;
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	    thread((StgClosure **)p);
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	    p++;
	    size = BCO_BITMAP_SIZE(bco);
	    thread_large_bitmap(p, BCO_BITMAP(bco), size);
	    p += size;
	    continue;
	}

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	    // large bitmap (> 32 entries, or 64 on a 64-bit machine) 
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	case RET_BIG:
	    p++;
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	    size = GET_LARGE_BITMAP(&info->i)->size;
	    thread_large_bitmap(p, GET_LARGE_BITMAP(&info->i), size);
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	    p += size;
	    continue;
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	case RET_FUN:
	{
	    StgRetFun *ret_fun = (StgRetFun *)p;
	    StgFunInfoTable *fun_info;
	    
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	    fun_info = FUN_INFO_PTR_TO_STRUCT((StgInfoTable *)UNTAG_CLOSURE((StgClosure *)
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                           get_threaded_info((StgPtr)ret_fun->fun)));
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	         // *before* threading it!
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	    thread(&ret_fun->fun);
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	    p = thread_arg_block(fun_info, ret_fun->payload);
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	    continue;
	}

	default:
	    barf("thread_stack: weird activation record found on stack: %d", 
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		 (int)(info->i.type));
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	}
    }
}

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STATIC_INLINE StgPtr
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thread_PAP_payload (StgClosure *fun, StgClosure **payload, StgWord size)
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{
    StgPtr p;
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    StgWord bitmap;
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    StgFunInfoTable *fun_info;
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    fun_info = FUN_INFO_PTR_TO_STRUCT((StgInfoTable *)UNTAG_CLOSURE((StgClosure *)
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                        get_threaded_info((StgPtr)fun)));
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    ASSERT(fun_info->i.type != PAP);

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    p = (StgPtr)payload;
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    switch (fun_info->f.fun_type) {
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    case ARG_GEN:
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	bitmap = BITMAP_BITS(fun_info->f.b.bitmap);
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	goto small_bitmap;
    case ARG_GEN_BIG:
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	thread_large_bitmap(p, GET_FUN_LARGE_BITMAP(fun_info), size);
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	p += size;
	break;
    case ARG_BCO:
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	thread_large_bitmap((StgPtr)payload, BCO_BITMAP(fun), size);
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	p += size;
	break;
    default:
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	bitmap = BITMAP_BITS(stg_arg_bitmaps[fun_info->f.fun_type]);
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    small_bitmap:
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        p = thread_small_bitmap(p, size, bitmap);
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	break;
    }
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    return p;
}

STATIC_INLINE StgPtr
thread_PAP (StgPAP *pap)
{
    StgPtr p;
    p = thread_PAP_payload(pap->fun, pap->payload, pap->n_args);
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    thread(&pap->fun);
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    return p;
}
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STATIC_INLINE StgPtr
thread_AP (StgAP *ap)
{
    StgPtr p;
    p = thread_PAP_payload(ap->fun, ap->payload, ap->n_args);
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    thread(&ap->fun);
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    return p;
}    
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STATIC_INLINE StgPtr
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thread_AP_STACK (StgAP_STACK *ap)
{
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    thread(&ap->fun);
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    thread_stack((P_)ap->payload, (P_)ap->payload + ap->size);
    return (P_)ap + sizeofW(StgAP_STACK) + ap->size;
}

static StgPtr
thread_TSO (StgTSO *tso)
{
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    thread_(&tso->_link);
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    thread_(&tso->global_link);
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    if (   tso->why_blocked == BlockedOnMVar
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        || tso->why_blocked == BlockedOnMVarRead
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	|| tso->why_blocked == BlockedOnBlackHole
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	|| tso->why_blocked == BlockedOnMsgThrowTo
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        || tso->why_blocked == NotBlocked
        ) {
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	thread_(&tso->block_info.closure);
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    }
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    thread_(&tso->blocked_exceptions);
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    thread_(&tso->bq);
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    thread_(&tso->trec);
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    thread_(&tso->stackobj);
    return (StgPtr)tso + sizeofW(StgTSO);
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}


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static void
update_fwd_large( bdescr *bd )
{
  StgPtr p;
  const StgInfoTable* info;

  for (; bd != NULL; bd = bd->link) {

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    // nothing to do in a pinned block; it might not even have an object
    // at the beginning.
    if (bd->flags & BF_PINNED) continue;

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    p = bd->start;
    info  = get_itbl((StgClosure *)p);

    switch (info->type) {

    case ARR_WORDS:
      // nothing to follow 
      continue;

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    case MUT_ARR_PTRS_CLEAN:
    case MUT_ARR_PTRS_DIRTY:
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    case MUT_ARR_PTRS_FROZEN:
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    case MUT_ARR_PTRS_FROZEN0:
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      // follow everything 
      {
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          StgMutArrPtrs *a;
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          a = (StgMutArrPtrs*)p;
          for (p = (P_)a->payload; p < (P_)&a->payload[a->ptrs]; p++) {
              thread((StgClosure **)p);
          }
          continue;
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      }

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    case SMALL_MUT_ARR_PTRS_CLEAN:
    case SMALL_MUT_ARR_PTRS_DIRTY:
    case SMALL_MUT_ARR_PTRS_FROZEN:
    case SMALL_MUT_ARR_PTRS_FROZEN0:
      // follow everything 
      {
          StgSmallMutArrPtrs *a;

          a = (StgSmallMutArrPtrs*)p;
          for (p = (P_)a->payload; p < (P_)&a->payload[a->ptrs]; p++) {
              thread((StgClosure **)p);
          }
          continue;
      }

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    case STACK:
    {
        StgStack *stack = (StgStack*)p;
        thread_stack(stack->sp, stack->stack + stack->stack_size);
        continue;
    }
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    case AP_STACK:
	thread_AP_STACK((StgAP_STACK *)p);
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	continue;

    case PAP:
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	thread_PAP((StgPAP *)p);
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	continue;

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    case TREC_CHUNK:
    {
        StgWord i;
        StgTRecChunk *tc = (StgTRecChunk *)p;
	TRecEntry *e = &(tc -> entries[0]);
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	thread_(&tc->prev_chunk);
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	for (i = 0; i < tc -> next_entry_idx; i ++, e++ ) {
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	  thread_(&e->tvar);
	  thread(&e->expected_value);
	  thread(&e->new_value);
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	}
	continue;
    }

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    default:
      barf("update_fwd_large: unknown/strange object  %d", (int)(info->type));
    }
  }
}

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// ToDo: too big to inline
static /* STATIC_INLINE */ StgPtr
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thread_obj (StgInfoTable *info, StgPtr p)
{
    switch (info->type) {
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    case THUNK_0_1:
	return p + sizeofW(StgThunk) + 1;

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    case FUN_0_1:
    case CONSTR_0_1:
	return p + sizeofW(StgHeader) + 1;
	
    case FUN_1_0:
    case CONSTR_1_0:
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	thread(&((StgClosure *)p)->payload[0]);
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	return p + sizeofW(StgHeader) + 1;
	
    case THUNK_1_0:
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	thread(&((StgThunk *)p)->payload[0]);
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	return p + sizeofW(StgThunk) + 1;
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    case THUNK_0_2:
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	return p + sizeofW(StgThunk) + 2;

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    case FUN_0_2:
    case CONSTR_0_2:
	return p + sizeofW(StgHeader) + 2;
	
    case THUNK_1_1:
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	thread(&((StgThunk *)p)->payload[0]);
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	return p + sizeofW(StgThunk) + 2;

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    case FUN_1_1:
    case CONSTR_1_1:
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	thread(&((StgClosure *)p)->payload[0]);
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	return p + sizeofW(StgHeader) + 2;
	
    case THUNK_2_0:
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	thread(&((StgThunk *)p)->payload[0]);
	thread(&((StgThunk *)p)->payload[1]);
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	return p + sizeofW(StgThunk) + 2;

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    case FUN_2_0:
    case CONSTR_2_0:
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	thread(&((StgClosure *)p)->payload[0]);
	thread(&((StgClosure *)p)->payload[1]);
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	return p + sizeofW(StgHeader) + 2;
	
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    case BCO: {
	StgBCO *bco = (StgBCO *)p;
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	thread_(&bco->instrs);
	thread_(&bco->literals);
	thread_(&bco->ptrs);
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	return p + bco_sizeW(bco);
    }

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    case THUNK:
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    {
	StgPtr end;
	
	end = (P_)((StgThunk *)p)->payload + 
	    info->layout.payload.ptrs;
	for (p = (P_)((StgThunk *)p)->payload; p < end; p++) {
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	    thread((StgClosure **)p);
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	}
	return p + info->layout.payload.nptrs;
    }

    case FUN:
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    case CONSTR:
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    case PRIM:
    case MUT_PRIM:
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    case MUT_VAR_CLEAN:
    case MUT_VAR_DIRTY:
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    case TVAR:
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    case BLACKHOLE:
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    case BLOCKING_QUEUE:
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    {
	StgPtr end;
	
	end = (P_)((StgClosure *)p)->payload + 
	    info->layout.payload.ptrs;
	for (p = (P_)((StgClosure *)p)->payload; p < end; p++) {
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	    thread((StgClosure **)p);
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	}
	return p + info->layout.payload.nptrs;
    }
    
    case WEAK:
    {
	StgWeak *w = (StgWeak *)p;
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	thread(&w->cfinalizers);
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	thread(&w->key);
	thread(&w->value);
	thread(&w->finalizer);
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	if (w->link != NULL) {
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	    thread_(&w->link);
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	}
	return p + sizeofW(StgWeak);
    }
    
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    case MVAR_CLEAN:
    case MVAR_DIRTY:
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    { 
	StgMVar *mvar = (StgMVar *)p;
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	thread_(&mvar->head);
	thread_(&mvar->tail);
	thread(&mvar->value);
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	return p + sizeofW(StgMVar);
    }
    
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    case IND:
    case IND_PERM:
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	thread(&((StgInd *)p)->indirectee);
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	return p + sizeofW(StgInd);
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    case THUNK_SELECTOR:
    { 
	StgSelector *s = (StgSelector *)p;
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	thread(&s->selectee);
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	return p + THUNK_SELECTOR_sizeW();
    }
    
    case AP_STACK:
	return thread_AP_STACK((StgAP_STACK *)p);
	
    case PAP:
	return thread_PAP((StgPAP *)p);
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    case AP:
	return thread_AP((StgAP *)p);
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    case ARR_WORDS:
	return p + arr_words_sizeW((StgArrWords *)p);
	
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    case MUT_ARR_PTRS_CLEAN:
    case MUT_ARR_PTRS_DIRTY:
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    case MUT_ARR_PTRS_FROZEN:
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    case MUT_ARR_PTRS_FROZEN0:
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	// follow everything 
    {
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        StgMutArrPtrs *a;

        a = (StgMutArrPtrs *)p;
	for (p = (P_)a->payload; p < (P_)&a->payload[a->ptrs]; p++) {
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	    thread((StgClosure **)p);
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	}
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	return (StgPtr)a + mut_arr_ptrs_sizeW(a);
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    }
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    case SMALL_MUT_ARR_PTRS_CLEAN:
    case SMALL_MUT_ARR_PTRS_DIRTY:
    case SMALL_MUT_ARR_PTRS_FROZEN:
    case SMALL_MUT_ARR_PTRS_FROZEN0:
	// follow everything 
    {
        StgSmallMutArrPtrs *a;

        a = (StgSmallMutArrPtrs *)p;
	for (p = (P_)a->payload; p < (P_)&a->payload[a->ptrs]; p++) {
	    thread((StgClosure **)p);
	}

	return (StgPtr)a + small_mut_arr_ptrs_sizeW(a);
    }
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    case TSO:
	return thread_TSO((StgTSO *)p);
    
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    case STACK:
    {
        StgStack *stack = (StgStack*)p;
        thread_stack(stack->sp, stack->stack + stack->stack_size);
        return p + stack_sizeW(stack);
    }

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    case TREC_CHUNK:
    {
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        StgWord i;
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        StgTRecChunk *tc = (StgTRecChunk *)p;
	TRecEntry *e = &(tc -> entries[0]);
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	thread_(&tc->prev_chunk);
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	for (i = 0; i < tc -> next_entry_idx; i ++, e++ ) {
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	  thread_(&e->tvar);
	  thread(&e->expected_value);
	  thread(&e->new_value);
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	}
	return p + sizeofW(StgTRecChunk);
    }

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    default:
	barf("update_fwd: unknown/strange object  %d", (int)(info->type));
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	return NULL;
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    }
}

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static void
update_fwd( bdescr *blocks )
{
    StgPtr p;
    bdescr *bd;
    StgInfoTable *info;

    bd = blocks;

    // cycle through all the blocks in the step
    for (; bd != NULL; bd = bd->link) {
	p = bd->start;

	// linearly scan the objects in this block
	while (p < bd->free) {
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	    ASSERT(LOOKS_LIKE_CLOSURE_PTR(p));
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	    info = get_itbl((StgClosure *)p);
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	    p = thread_obj(info, p);
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	}
    }
} 

static void
update_fwd_compact( bdescr *blocks )
{
    StgPtr p, q, free;
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#if 0
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    StgWord m;
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#endif
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    bdescr *bd, *free_bd;
    StgInfoTable *info;
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    StgWord size;
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    StgWord iptr;
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    bd = blocks;
    free_bd = blocks;
    free = free_bd->start;

    // cycle through all the blocks in the step
    for (; bd != NULL; bd = bd->link) {
	p = bd->start;

	while (p < bd->free ) {

	    while ( p < bd->free && !is_marked(p,bd) ) {
		p++;
	    }
	    if (p >= bd->free) {
		break;
	    }

#if 0
    next:
	m = * ((StgPtr)bd->u.bitmap + ((p - bd->start) / (BITS_IN(StgWord))));
	m >>= ((p - bd->start) & (BITS_IN(StgWord) - 1));

	while ( p < bd->free ) {

	    if ((m & 1) == 0) {
		m >>= 1;
		p++;
		if (((StgWord)p & (sizeof(W_) * BITS_IN(StgWord))) == 0) {
		    goto next;
		} else {
		    continue;
		}
	    }
#endif

	    // Problem: we need to know the destination for this cell
	    // in order to unthread its info pointer.  But we can't
	    // know the destination without the size, because we may
	    // spill into the next block.  So we have to run down the 
	    // threaded list and get the info ptr first.
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            //
            // ToDo: one possible avenue of attack is to use the fact
            // that if (p&BLOCK_MASK) >= (free&BLOCK_MASK), then we
            // definitely have enough room.  Also see bug #1147.
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            iptr = get_threaded_info(p);
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	    info = INFO_PTR_TO_STRUCT((StgInfoTable *)UNTAG_CLOSURE((StgClosure *)iptr));
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	    q = p;

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	    p = thread_obj(info, p);
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	    size = p - q;
	    if (free + size > free_bd->start + BLOCK_SIZE_W) {
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		// set the next bit in the bitmap to indicate that
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		// this object needs to be pushed into the next
		// block.  This saves us having to run down the
		// threaded info pointer list twice during the next pass.
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		mark(q+1,bd);
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		free_bd = free_bd->link;
		free = free_bd->start;
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	    } else {
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		ASSERT(!is_marked(q+1,bd));
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	    }

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	    unthread(q,(StgWord)free + GET_CLOSURE_TAG((StgClosure *)iptr));
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	    free += size;
#if 0
	    goto next;
#endif
	}
    }
}

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static W_
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update_bkwd_compact( generation *gen )
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{
    StgPtr p, free;
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#if 0
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    StgWord m;
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#endif
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    bdescr *bd, *free_bd;
    StgInfoTable *info;
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    StgWord size;
    W_ free_blocks;
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    StgWord iptr;
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    bd = free_bd = gen->old_blocks;
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    free = free_bd->start;
    free_blocks = 1;

    // cycle through all the blocks in the step
    for (; bd != NULL; bd = bd->link) {
	p = bd->start;

	while (p < bd->free ) {

	    while ( p < bd->free && !is_marked(p,bd) ) {
		p++;
	    }
	    if (p >= bd->free) {
		break;
	    }

#if 0
    next:
	m = * ((StgPtr)bd->u.bitmap + ((p - bd->start) / (BITS_IN(StgWord))));
	m >>= ((p - bd->start) & (BITS_IN(StgWord) - 1));

	while ( p < bd->free ) {

	    if ((m & 1) == 0) {
		m >>= 1;
		p++;
		if (((StgWord)p & (sizeof(W_) * BITS_IN(StgWord))) == 0) {
		    goto next;
		} else {
		    continue;
		}
	    }
#endif

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	    if (is_marked(p+1,bd)) {
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		// don't forget to update the free ptr in the block desc.
		free_bd->free = free;
		free_bd = free_bd->link;
		free = free_bd->start;
		free_blocks++;
	    }

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            iptr = get_threaded_info(p);
	    unthread(p, (StgWord)free + GET_CLOSURE_TAG((StgClosure *)iptr));
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	    ASSERT(LOOKS_LIKE_INFO_PTR((StgWord)((StgClosure *)p)->header.info));
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	    info = get_itbl((StgClosure *)p);
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	    size = closure_sizeW_((StgClosure *)p,info);
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	    if (free != p) {
		move(free,p,size);
	    }
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	    // relocate TSOs
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            if (info->type == STACK) {
                move_STACK((StgStack *)p, (StgStack *)free);
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	    }

	    free += size;
	    p += size;
#if 0
	    goto next;
#endif
	}
    }

    // free the remaining blocks and count what's left.
    free_bd->free = free;
    if (free_bd->link != NULL) {
	freeChain(free_bd->link);
	free_bd->link = NULL;
    }

    return free_blocks;
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}

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void
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compact(StgClosure *static_objects)
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{
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    W_ n, g, blocks;
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    generation *gen;
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    // 1. thread the roots
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    markCapabilities((evac_fn)thread_root, NULL);
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    markScheduler((evac_fn)thread_root, NULL);

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    // the weak pointer lists...
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    for (g = 0; g < RtsFlags.GcFlags.generations; g++) {
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        if (generations[g].weak_ptr_list != NULL) {
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            thread((void *)&generations[g].weak_ptr_list);
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        }
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    }
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    if (dead_weak_ptr_list != NULL) {
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        thread((void *)&dead_weak_ptr_list); // tmp
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    }

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    // mutable lists
    for (g = 1; g < RtsFlags.GcFlags.generations; g++) {
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        bdescr *bd;
        StgPtr p;
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        for (n = 0; n < n_capabilities; n++) {
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            for (bd = capabilities[n]->mut_lists[g];
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                 bd != NULL; bd = bd->link) {
                for (p = bd->start; p < bd->free; p++) {
                    thread((StgClosure **)p);
                }
            }
        }
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    }
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    // the global thread list
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    for (g = 0; g < RtsFlags.GcFlags.generations; g++) {
        thread((void *)&generations[g].threads);
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    }
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    // any threads resurrected during this GC
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    thread((void *)&resurrected_threads);
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    // the task list
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    {
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	Task *task;
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        InCall *incall;
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        for (task = all_tasks; task != NULL; task = task->all_next) {
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            for (incall = task->incall; incall != NULL; 
                 incall = incall->prev_stack) {
                if (incall->tso) {
                    thread_(&incall->tso);
                }
            }
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	}
    }

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    // the static objects
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    thread_static(static_objects /* ToDo: ok? */);
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    // the stable pointer table
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    threadStableTables((evac_fn)thread_root, NULL);
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    // the CAF list (used by GHCi)
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    markCAFs((evac_fn)thread_root, NULL);
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    // 2. update forward ptrs
    for (g = 0; g < RtsFlags.GcFlags.generations; g++) {
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        gen = &generations[g];
        debugTrace(DEBUG_gc, "update_fwd:  %d", g);

        update_fwd(gen->blocks);
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        for (n = 0; n < n_capabilities; n++) {
            update_fwd(gc_threads[n]->gens[g].todo_bd);
            update_fwd(gc_threads[n]->gens[g].part_list);
        }
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        update_fwd_large(gen->scavenged_large_objects);
        if (g == RtsFlags.GcFlags.generations-1 && gen->old_blocks != NULL) {
            debugTrace(DEBUG_gc, "update_fwd:  %d (compact)", g);
            update_fwd_compact(gen->old_blocks);
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	}
    }

    // 3. update backward ptrs
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    gen = oldest_gen;
    if (gen->old_blocks != NULL) {
	blocks = update_bkwd_compact(gen);
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	debugTrace(DEBUG_gc, 
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		   "update_bkwd: %d (compact, old: %d blocks, now %d blocks)",
		   gen->no, gen->n_old_blocks, blocks);
	gen->n_old_blocks = blocks;
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    }
}