RetainerProfile.c 71.4 KB
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/* -----------------------------------------------------------------------------
 *
 * (c) The GHC Team, 2001
 * Author: Sungwoo Park
 *
 * Retainer profiling.
 *
 * ---------------------------------------------------------------------------*/

#ifdef PROFILING

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// Turn off inlining when debugging - it obfuscates things
#ifdef DEBUG
#define INLINE
#else
#define INLINE inline
#endif

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#include "PosixSource.h"
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#include "Rts.h"
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#include "RtsUtils.h"
#include "RetainerProfile.h"
#include "RetainerSet.h"
#include "Schedule.h"
#include "Printer.h"
#include "Weak.h"
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#include "sm/Sanity.h"
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#include "Profiling.h"
#include "Stats.h"
#include "ProfHeap.h"
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#include "Apply.h"
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#include "Stable.h" /* markStableTables */
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#include "sm/Storage.h" // for END_OF_STATIC_LIST
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/*
  Note: what to change in order to plug-in a new retainer profiling scheme?
    (1) type retainer in ../includes/StgRetainerProf.h
    (2) retainer function R(), i.e., getRetainerFrom()
    (3) the two hashing functions, hashKeySingleton() and hashKeyAddElement(),
        in RetainerSet.h, if needed.
    (4) printRetainer() and printRetainerSetShort() in RetainerSet.c.
 */

/* -----------------------------------------------------------------------------
 * Declarations...
 * -------------------------------------------------------------------------- */

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static uint32_t retainerGeneration;  // generation
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static uint32_t numObjectVisited;    // total number of objects visited
static uint32_t timesAnyObjectVisited;  // number of times any objects are
                                        // visited
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/*
  The rs field in the profile header of any object points to its retainer
  set in an indirect way: if flip is 0, it points to the retainer set;
  if flip is 1, it points to the next byte after the retainer set (even
  for NULL pointers). Therefore, with flip 1, (rs ^ 1) is the actual
  pointer. See retainerSetOf().
 */

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StgWord flip = 0;     // flip bit
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                      // must be 0 if DEBUG_RETAINER is on (for static closures)

#define setRetainerSetToNull(c)   \
  (c)->header.prof.hp.rs = (RetainerSet *)((StgWord)NULL | flip)

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static void retainStack(StgClosure *, retainer, StgPtr, StgPtr);
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static void retainClosure(StgClosure *, StgClosure *, retainer);
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#ifdef DEBUG_RETAINER
static void belongToHeap(StgPtr p);
#endif

#ifdef DEBUG_RETAINER
/*
  cStackSize records how many times retainStack() has been invoked recursively,
  that is, the number of activation records for retainStack() on the C stack.
  maxCStackSize records its max value.
  Invariants:
    cStackSize <= maxCStackSize
 */
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static uint32_t cStackSize, maxCStackSize;
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static uint32_t sumOfNewCost;        // sum of the cost of each object, computed
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                                // when the object is first visited
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static uint32_t sumOfNewCostExtra;   // for those objects not visited during
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                                // retainer profiling, e.g., MUT_VAR
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static uint32_t costArray[N_CLOSURE_TYPES];
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uint32_t sumOfCostLinear;            // sum of the costs of all object, computed
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                                // when linearly traversing the heap after
                                // retainer profiling
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uint32_t costArrayLinear[N_CLOSURE_TYPES];
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#endif

/* -----------------------------------------------------------------------------
 * Retainer stack - header
 *   Note:
 *     Although the retainer stack implementation could be separated *
 *     from the retainer profiling engine, there does not seem to be
 *     any advantage in doing that; retainer stack is an integral part
 *     of retainer profiling engine and cannot be use elsewhere at
 *     all.
 * -------------------------------------------------------------------------- */

typedef enum {
    posTypeStep,
    posTypePtrs,
    posTypeSRT,
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    posTypeLargeSRT,
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} nextPosType;

typedef union {
    // fixed layout or layout specified by a field in the closure
    StgWord step;

    // layout.payload
    struct {
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        // See StgClosureInfo in InfoTables.h
        StgHalfWord pos;
        StgHalfWord ptrs;
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        StgPtr payload;
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    } ptrs;

    // SRT
    struct {
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        StgClosure **srt;
        StgWord    srt_bitmap;
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    } srt;
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    // Large SRT
    struct {
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        StgLargeSRT *srt;
        StgWord offset;
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    } large_srt;
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} nextPos;

typedef struct {
    nextPosType type;
    nextPos next;
} stackPos;

typedef struct {
    StgClosure *c;
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    retainer c_child_r;
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    stackPos info;
} stackElement;

/*
  Invariants:
    firstStack points to the first block group.
    currentStack points to the block group currently being used.
    currentStack->free == stackLimit.
    stackTop points to the topmost byte in the stack of currentStack.
    Unless the whole stack is empty, stackTop must point to the topmost
    object (or byte) in the whole stack. Thus, it is only when the whole stack
    is empty that stackTop == stackLimit (not during the execution of push()
    and pop()).
    stackBottom == currentStack->start.
    stackLimit == currentStack->start + BLOCK_SIZE_W * currentStack->blocks.
  Note:
    When a current stack becomes empty, stackTop is set to point to
    the topmost element on the previous block group so as to satisfy
    the invariants described above.
 */
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static bdescr *firstStack = NULL;
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static bdescr *currentStack;
static stackElement *stackBottom, *stackTop, *stackLimit;

/*
  currentStackBoundary is used to mark the current stack chunk.
  If stackTop == currentStackBoundary, it means that the current stack chunk
  is empty. It is the responsibility of the user to keep currentStackBoundary
  valid all the time if it is to be employed.
 */
static stackElement *currentStackBoundary;

/*
  stackSize records the current size of the stack.
  maxStackSize records its high water mark.
  Invariants:
    stackSize <= maxStackSize
  Note:
    stackSize is just an estimate measure of the depth of the graph. The reason
    is that some heap objects have only a single child and may not result
    in a new element being pushed onto the stack. Therefore, at the end of
    retainer profiling, maxStackSize + maxCStackSize is some value no greater
    than the actual depth of the graph.
 */
#ifdef DEBUG_RETAINER
static int stackSize, maxStackSize;
#endif

// number of blocks allocated for one stack
#define BLOCKS_IN_STACK 1

/* -----------------------------------------------------------------------------
 * Add a new block group to the stack.
 * Invariants:
 *  currentStack->link == s.
 * -------------------------------------------------------------------------- */
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static INLINE void
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newStackBlock( bdescr *bd )
{
    currentStack = bd;
    stackTop     = (stackElement *)(bd->start + BLOCK_SIZE_W * bd->blocks);
    stackBottom  = (stackElement *)bd->start;
    stackLimit   = (stackElement *)stackTop;
    bd->free     = (StgPtr)stackLimit;
}

/* -----------------------------------------------------------------------------
 * Return to the previous block group.
 * Invariants:
 *   s->link == currentStack.
 * -------------------------------------------------------------------------- */
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static INLINE void
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returnToOldStack( bdescr *bd )
{
    currentStack = bd;
    stackTop = (stackElement *)bd->free;
    stackBottom = (stackElement *)bd->start;
    stackLimit = (stackElement *)(bd->start + BLOCK_SIZE_W * bd->blocks);
    bd->free = (StgPtr)stackLimit;
}

/* -----------------------------------------------------------------------------
 *  Initializes the traverse stack.
 * -------------------------------------------------------------------------- */
static void
initializeTraverseStack( void )
{
    if (firstStack != NULL) {
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        freeChain(firstStack);
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    }

    firstStack = allocGroup(BLOCKS_IN_STACK);
    firstStack->link = NULL;
    firstStack->u.back = NULL;

    newStackBlock(firstStack);
}

/* -----------------------------------------------------------------------------
 * Frees all the block groups in the traverse stack.
 * Invariants:
 *   firstStack != NULL
 * -------------------------------------------------------------------------- */
static void
closeTraverseStack( void )
{
    freeChain(firstStack);
    firstStack = NULL;
}

/* -----------------------------------------------------------------------------
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 * Returns true if the whole stack is empty.
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 * -------------------------------------------------------------------------- */
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static INLINE bool
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isEmptyRetainerStack( void )
{
    return (firstStack == currentStack) && stackTop == stackLimit;
}

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/* -----------------------------------------------------------------------------
 * Returns size of stack
 * -------------------------------------------------------------------------- */
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#ifdef DEBUG
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W_
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retainerStackBlocks( void )
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{
    bdescr* bd;
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    W_ res = 0;
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    for (bd = firstStack; bd != NULL; bd = bd->link)
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      res += bd->blocks;

    return res;
}
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#endif
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/* -----------------------------------------------------------------------------
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 * Returns true if stackTop is at the stack boundary of the current stack,
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 * i.e., if the current stack chunk is empty.
 * -------------------------------------------------------------------------- */
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static INLINE bool
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isOnBoundary( void )
{
    return stackTop == currentStackBoundary;
}

/* -----------------------------------------------------------------------------
 * Initializes *info from ptrs and payload.
 * Invariants:
 *   payload[] begins with ptrs pointers followed by non-pointers.
 * -------------------------------------------------------------------------- */
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static INLINE void
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init_ptrs( stackPos *info, uint32_t ptrs, StgPtr payload )
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{
    info->type              = posTypePtrs;
    info->next.ptrs.pos     = 0;
    info->next.ptrs.ptrs    = ptrs;
    info->next.ptrs.payload = payload;
}

/* -----------------------------------------------------------------------------
 * Find the next object from *info.
 * -------------------------------------------------------------------------- */
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static INLINE StgClosure *
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find_ptrs( stackPos *info )
{
    if (info->next.ptrs.pos < info->next.ptrs.ptrs) {
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        return (StgClosure *)info->next.ptrs.payload[info->next.ptrs.pos++];
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    } else {
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        return NULL;
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    }
}

/* -----------------------------------------------------------------------------
 *  Initializes *info from SRT information stored in *infoTable.
 * -------------------------------------------------------------------------- */
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static INLINE void
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init_srt_fun( stackPos *info, StgFunInfoTable *infoTable )
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{
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    if (infoTable->i.srt_bitmap == (StgHalfWord)(-1)) {
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        info->type = posTypeLargeSRT;
        info->next.large_srt.srt = (StgLargeSRT *)GET_FUN_SRT(infoTable);
        info->next.large_srt.offset = 0;
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    } else {
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        info->type = posTypeSRT;
        info->next.srt.srt = (StgClosure **)GET_FUN_SRT(infoTable);
        info->next.srt.srt_bitmap = infoTable->i.srt_bitmap;
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    }
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}

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static INLINE void
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init_srt_thunk( stackPos *info, StgThunkInfoTable *infoTable )
{
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    if (infoTable->i.srt_bitmap == (StgHalfWord)(-1)) {
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        info->type = posTypeLargeSRT;
        info->next.large_srt.srt = (StgLargeSRT *)GET_SRT(infoTable);
        info->next.large_srt.offset = 0;
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    } else {
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        info->type = posTypeSRT;
        info->next.srt.srt = (StgClosure **)GET_SRT(infoTable);
        info->next.srt.srt_bitmap = infoTable->i.srt_bitmap;
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    }
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}

/* -----------------------------------------------------------------------------
 * Find the next object from *info.
 * -------------------------------------------------------------------------- */
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static INLINE StgClosure *
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find_srt( stackPos *info )
{
    StgClosure *c;
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    StgWord bitmap;
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    if (info->type == posTypeSRT) {
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        // Small SRT bitmap
        bitmap = info->next.srt.srt_bitmap;
        while (bitmap != 0) {
            if ((bitmap & 1) != 0) {
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#if defined(COMPILING_WINDOWS_DLL)
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                if ((unsigned long)(*(info->next.srt.srt)) & 0x1)
                    c = (* (StgClosure **)((unsigned long)*(info->next.srt.srt)) & ~0x1);
                else
                    c = *(info->next.srt.srt);
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#else
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                c = *(info->next.srt.srt);
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#endif
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                bitmap = bitmap >> 1;
                info->next.srt.srt++;
                info->next.srt.srt_bitmap = bitmap;
                return c;
            }
            bitmap = bitmap >> 1;
            info->next.srt.srt++;
        }
        // bitmap is now zero...
        return NULL;
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    }
    else {
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        // Large SRT bitmap
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        uint32_t i = info->next.large_srt.offset;
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        StgWord bitmap;

        // Follow the pattern from GC.c:scavenge_large_srt_bitmap().
        bitmap = info->next.large_srt.srt->l.bitmap[i / BITS_IN(W_)];
        bitmap = bitmap >> (i % BITS_IN(StgWord));
        while (i < info->next.large_srt.srt->l.size) {
            if ((bitmap & 1) != 0) {
                c = ((StgClosure **)info->next.large_srt.srt->srt)[i];
                i++;
                info->next.large_srt.offset = i;
                return c;
            }
            i++;
            if (i % BITS_IN(W_) == 0) {
                bitmap = info->next.large_srt.srt->l.bitmap[i / BITS_IN(W_)];
            } else {
                bitmap = bitmap >> 1;
            }
        }
        // reached the end of this bitmap.
        info->next.large_srt.offset = i;
        return NULL;
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    }
}

/* -----------------------------------------------------------------------------
 *  push() pushes a stackElement representing the next child of *c
 *  onto the traverse stack. If *c has no child, *first_child is set
 *  to NULL and nothing is pushed onto the stack. If *c has only one
 *  child, *c_chlid is set to that child and nothing is pushed onto
 *  the stack.  If *c has more than two children, *first_child is set
 *  to the first child and a stackElement representing the second
 *  child is pushed onto the stack.

 *  Invariants:
 *     *c_child_r is the most recent retainer of *c's children.
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 *     *c is not any of TSO, AP, PAP, AP_STACK, which means that
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 *        there cannot be any stack objects.
 *  Note: SRTs are considered to  be children as well.
 * -------------------------------------------------------------------------- */
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static INLINE void
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push( StgClosure *c, retainer c_child_r, StgClosure **first_child )
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{
    stackElement se;
    bdescr *nbd;      // Next Block Descriptor

#ifdef DEBUG_RETAINER
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    // debugBelch("push(): stackTop = 0x%x, currentStackBoundary = 0x%x\n", stackTop, currentStackBoundary);
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#endif

    ASSERT(get_itbl(c)->type != TSO);
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    ASSERT(get_itbl(c)->type != AP_STACK);
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    //
    // fill in se
    //

    se.c = c;
    se.c_child_r = c_child_r;

    // fill in se.info
    switch (get_itbl(c)->type) {
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        // no child, no SRT
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    case CONSTR_0_1:
    case CONSTR_0_2:
    case ARR_WORDS:
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    case COMPACT_NFDATA:
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        *first_child = NULL;
        return;
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        // one child (fixed), no SRT
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    case MUT_VAR_CLEAN:
    case MUT_VAR_DIRTY:
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        *first_child = ((StgMutVar *)c)->var;
        return;
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    case THUNK_SELECTOR:
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        *first_child = ((StgSelector *)c)->selectee;
        return;
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    case BLACKHOLE:
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        *first_child = ((StgInd *)c)->indirectee;
        return;
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    case CONSTR_1_0:
    case CONSTR_1_1:
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        *first_child = c->payload[0];
        return;
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        // For CONSTR_2_0 and MVAR, we use se.info.step to record the position
        // of the next child. We do not write a separate initialization code.
        // Also we do not have to initialize info.type;
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        // two children (fixed), no SRT
        // need to push a stackElement, but nothing to store in se.info
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    case CONSTR_2_0:
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        *first_child = c->payload[0];         // return the first pointer
        // se.info.type = posTypeStep;
        // se.info.next.step = 2;            // 2 = second
        break;
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        // three children (fixed), no SRT
        // need to push a stackElement
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    case MVAR_CLEAN:
    case MVAR_DIRTY:
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        // head must be TSO and the head of a linked list of TSOs.
        // Shoule it be a child? Seems to be yes.
        *first_child = (StgClosure *)((StgMVar *)c)->head;
        // se.info.type = posTypeStep;
        se.info.next.step = 2;            // 2 = second
        break;

        // three children (fixed), no SRT
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    case WEAK:
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        *first_child = ((StgWeak *)c)->key;
        // se.info.type = posTypeStep;
        se.info.next.step = 2;
        break;
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        // layout.payload.ptrs, no SRT
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    case TVAR:
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    case CONSTR:
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    case CONSTR_NOCAF:
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    case PRIM:
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    case MUT_PRIM:
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    case BCO:
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        init_ptrs(&se.info, get_itbl(c)->layout.payload.ptrs,
                  (StgPtr)c->payload);
        *first_child = find_ptrs(&se.info);
        if (*first_child == NULL)
            return;   // no child
        break;

        // StgMutArrPtr.ptrs, no SRT
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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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        init_ptrs(&se.info, ((StgMutArrPtrs *)c)->ptrs,
                  (StgPtr)(((StgMutArrPtrs *)c)->payload));
        *first_child = find_ptrs(&se.info);
        if (*first_child == NULL)
            return;
        break;

        // StgMutArrPtr.ptrs, no SRT
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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:
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        init_ptrs(&se.info, ((StgSmallMutArrPtrs *)c)->ptrs,
                  (StgPtr)(((StgSmallMutArrPtrs *)c)->payload));
        *first_child = find_ptrs(&se.info);
        if (*first_child == NULL)
            return;
        break;
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    // layout.payload.ptrs, SRT
    case FUN:           // *c is a heap object.
    case FUN_2_0:
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        init_ptrs(&se.info, get_itbl(c)->layout.payload.ptrs, (StgPtr)c->payload);
        *first_child = find_ptrs(&se.info);
        if (*first_child == NULL)
            // no child from ptrs, so check SRT
            goto fun_srt_only;
        break;
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    case THUNK:
    case THUNK_2_0:
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        init_ptrs(&se.info, get_itbl(c)->layout.payload.ptrs,
                  (StgPtr)((StgThunk *)c)->payload);
        *first_child = find_ptrs(&se.info);
        if (*first_child == NULL)
            // no child from ptrs, so check SRT
            goto thunk_srt_only;
        break;

        // 1 fixed child, SRT
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    case FUN_1_0:
    case FUN_1_1:
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        *first_child = c->payload[0];
        ASSERT(*first_child != NULL);
        init_srt_fun(&se.info, get_fun_itbl(c));
        break;
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    case THUNK_1_0:
    case THUNK_1_1:
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        *first_child = ((StgThunk *)c)->payload[0];
        ASSERT(*first_child != NULL);
        init_srt_thunk(&se.info, get_thunk_itbl(c));
        break;
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    case FUN_STATIC:      // *c is a heap object.
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        ASSERT(get_itbl(c)->srt_bitmap != 0);
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    case FUN_0_1:
    case FUN_0_2:
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    fun_srt_only:
        init_srt_fun(&se.info, get_fun_itbl(c));
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        *first_child = find_srt(&se.info);
        if (*first_child == NULL)
            return;     // no child
        break;
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    // SRT only
    case THUNK_STATIC:
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        ASSERT(get_itbl(c)->srt_bitmap != 0);
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    case THUNK_0_1:
    case THUNK_0_2:
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    thunk_srt_only:
        init_srt_thunk(&se.info, get_thunk_itbl(c));
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        *first_child = find_srt(&se.info);
        if (*first_child == NULL)
            return;     // no child
        break;

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    case TREC_CHUNK:
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        *first_child = (StgClosure *)((StgTRecChunk *)c)->prev_chunk;
        se.info.next.step = 0;  // entry no.
        break;
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        // cannot appear
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    case PAP:
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    case AP:
    case AP_STACK:
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    case TSO:
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    case STACK:
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    case IND_STATIC:
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        // stack objects
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    case UPDATE_FRAME:
    case CATCH_FRAME:
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    case UNDERFLOW_FRAME:
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    case STOP_FRAME:
    case RET_BCO:
    case RET_SMALL:
    case RET_BIG:
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        // invalid objects
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    case IND:
    case INVALID_OBJECT:
    default:
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        barf("Invalid object *c in push()");
        return;
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    }

    if (stackTop - 1 < stackBottom) {
#ifdef DEBUG_RETAINER
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        // debugBelch("push() to the next stack.\n");
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#endif
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        // currentStack->free is updated when the active stack is switched
        // to the next stack.
        currentStack->free = (StgPtr)stackTop;

        if (currentStack->link == NULL) {
            nbd = allocGroup(BLOCKS_IN_STACK);
            nbd->link = NULL;
            nbd->u.back = currentStack;
            currentStack->link = nbd;
        } else
            nbd = currentStack->link;

        newStackBlock(nbd);
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    }

    // adjust stackTop (acutal push)
    stackTop--;
    // If the size of stackElement was huge, we would better replace the
    // following statement by either a memcpy() call or a switch statement
    // on the type of the element. Currently, the size of stackElement is
    // small enough (5 words) that this direct assignment seems to be enough.
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    // ToDo: The line below leads to the warning:
    //    warning: 'se.info.type' may be used uninitialized in this function
    // This is caused by the fact that there are execution paths through the
    // large switch statement above where some cases do not initialize this
    // field. Is this really harmless? Can we avoid the warning?
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    *stackTop = se;

#ifdef DEBUG_RETAINER
    stackSize++;
    if (stackSize > maxStackSize) maxStackSize = stackSize;
    // ASSERT(stackSize >= 0);
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    // debugBelch("stackSize = %d\n", stackSize);
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#endif
}

/* -----------------------------------------------------------------------------
 *  popOff() and popOffReal(): Pop a stackElement off the traverse stack.
 *  Invariants:
 *    stackTop cannot be equal to stackLimit unless the whole stack is
 *    empty, in which case popOff() is not allowed.
 *  Note:
 *    You can think of popOffReal() as a part of popOff() which is
 *    executed at the end of popOff() in necessary. Since popOff() is
 *    likely to be executed quite often while popOffReal() is not, we
 *    separate popOffReal() from popOff(), which is declared as an
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 *    INLINE function (for the sake of execution speed).  popOffReal()
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 *    is called only within popOff() and nowhere else.
 * -------------------------------------------------------------------------- */
static void
popOffReal(void)
{
    bdescr *pbd;    // Previous Block Descriptor

#ifdef DEBUG_RETAINER
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    // debugBelch("pop() to the previous stack.\n");
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#endif

    ASSERT(stackTop + 1 == stackLimit);
    ASSERT(stackBottom == (stackElement *)currentStack->start);

    if (firstStack == currentStack) {
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        // The stack is completely empty.
        stackTop++;
        ASSERT(stackTop == stackLimit);
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#ifdef DEBUG_RETAINER
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        stackSize--;
        if (stackSize > maxStackSize) maxStackSize = stackSize;
        /*
          ASSERT(stackSize >= 0);
          debugBelch("stackSize = %d\n", stackSize);
        */
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#endif
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        return;
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    }

    // currentStack->free is updated when the active stack is switched back
    // to the previous stack.
    currentStack->free = (StgPtr)stackLimit;

    // find the previous block descriptor
    pbd = currentStack->u.back;
    ASSERT(pbd != NULL);

    returnToOldStack(pbd);

#ifdef DEBUG_RETAINER
    stackSize--;
    if (stackSize > maxStackSize) maxStackSize = stackSize;
    /*
      ASSERT(stackSize >= 0);
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      debugBelch("stackSize = %d\n", stackSize);
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    */
#endif
}

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static INLINE void
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popOff(void) {
#ifdef DEBUG_RETAINER
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    // debugBelch("\tpopOff(): stackTop = 0x%x, currentStackBoundary = 0x%x\n", stackTop, currentStackBoundary);
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#endif

    ASSERT(stackTop != stackLimit);
    ASSERT(!isEmptyRetainerStack());

    // <= (instead of <) is wrong!
    if (stackTop + 1 < stackLimit) {
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        stackTop++;
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#ifdef DEBUG_RETAINER
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        stackSize--;
        if (stackSize > maxStackSize) maxStackSize = stackSize;
        /*
          ASSERT(stackSize >= 0);
          debugBelch("stackSize = %d\n", stackSize);
        */
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#endif
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        return;
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    }

    popOffReal();
}

/* -----------------------------------------------------------------------------
 *  Finds the next object to be considered for retainer profiling and store
 *  its pointer to *c.
 *  Test if the topmost stack element indicates that more objects are left,
 *  and if so, retrieve the first object and store its pointer to *c. Also,
 *  set *cp and *r appropriately, both of which are stored in the stack element.
 *  The topmost stack element then is overwritten so as for it to now denote
 *  the next object.
 *  If the topmost stack element indicates no more objects are left, pop
 *  off the stack element until either an object can be retrieved or
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 *  the current stack chunk becomes empty, indicated by true returned by
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 *  isOnBoundary(), in which case *c is set to NULL.
 *  Note:
 *    It is okay to call this function even when the current stack chunk
 *    is empty.
 * -------------------------------------------------------------------------- */
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static INLINE void
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pop( StgClosure **c, StgClosure **cp, retainer *r )
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{
    stackElement *se;

#ifdef DEBUG_RETAINER
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    // debugBelch("pop(): stackTop = 0x%x, currentStackBoundary = 0x%x\n", stackTop, currentStackBoundary);
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#endif

    do {
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        if (isOnBoundary()) {     // if the current stack chunk is depleted
            *c = NULL;
            return;
        }

        se = stackTop;

        switch (get_itbl(se->c)->type) {
            // two children (fixed), no SRT
            // nothing in se.info
        case CONSTR_2_0:
            *c = se->c->payload[1];
            *cp = se->c;
            *r = se->c_child_r;
            popOff();
            return;

            // three children (fixed), no SRT
            // need to push a stackElement
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        case MVAR_CLEAN:
        case MVAR_DIRTY:
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            if (se->info.next.step == 2) {
                *c = (StgClosure *)((StgMVar *)se->c)->tail;
                se->info.next.step++;             // move to the next step
                // no popOff
            } else {
                *c = ((StgMVar *)se->c)->value;
                popOff();
            }
            *cp = se->c;
            *r = se->c_child_r;
            return;

            // three children (fixed), no SRT
        case WEAK:
            if (se->info.next.step == 2) {
                *c = ((StgWeak *)se->c)->value;
                se->info.next.step++;
                // no popOff
            } else {
                *c = ((StgWeak *)se->c)->finalizer;
                popOff();
            }
            *cp = se->c;
            *r = se->c_child_r;
            return;

        case TREC_CHUNK: {
            // These are pretty complicated: we have N entries, each
            // of which contains 3 fields that we want to follow.  So
            // we divide the step counter: the 2 low bits indicate
            // which field, and the rest of the bits indicate the
            // entry number (starting from zero).
            TRecEntry *entry;
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            uint32_t entry_no = se->info.next.step >> 2;
            uint32_t field_no = se->info.next.step & 3;
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            if (entry_no == ((StgTRecChunk *)se->c)->next_entry_idx) {
                *c = NULL;
                popOff();
                return;
            }
            entry = &((StgTRecChunk *)se->c)->entries[entry_no];
            if (field_no == 0) {
                *c = (StgClosure *)entry->tvar;
            } else if (field_no == 1) {
                *c = entry->expected_value;
            } else {
                *c = entry->new_value;
            }
            *cp = se->c;
            *r = se->c_child_r;
            se->info.next.step++;
            return;
        }
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        case TVAR:
        case CONSTR:
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        case PRIM:
        case MUT_PRIM:
        case BCO:
            // StgMutArrPtr.ptrs, no SRT
        case MUT_ARR_PTRS_CLEAN:
        case MUT_ARR_PTRS_DIRTY:
        case MUT_ARR_PTRS_FROZEN:
        case MUT_ARR_PTRS_FROZEN0:
            *c = find_ptrs(&se->info);
            if (*c == NULL) {
                popOff();
                break;
            }
            *cp = se->c;
            *r = se->c_child_r;
            return;

            // layout.payload.ptrs, SRT
        case FUN:         // always a heap object
        case FUN_2_0:
            if (se->info.type == posTypePtrs) {
                *c = find_ptrs(&se->info);
                if (*c != NULL) {
                    *cp = se->c;
                    *r = se->c_child_r;
                    return;
                }
                init_srt_fun(&se->info, get_fun_itbl(se->c));
            }
            goto do_srt;

        case THUNK:
        case THUNK_2_0:
            if (se->info.type == posTypePtrs) {
                *c = find_ptrs(&se->info);
                if (*c != NULL) {
                    *cp = se->c;
                    *r = se->c_child_r;
                    return;
                }
                init_srt_thunk(&se->info, get_thunk_itbl(se->c));
            }
            goto do_srt;

            // SRT
        do_srt:
        case THUNK_STATIC:
        case FUN_STATIC:
        case FUN_0_1:
        case FUN_0_2:
        case THUNK_0_1:
        case THUNK_0_2:
        case FUN_1_0:
        case FUN_1_1:
        case THUNK_1_0:
        case THUNK_1_1:
            *c = find_srt(&se->info);
            if (*c != NULL) {
                *cp = se->c;
                *r = se->c_child_r;
                return;
            }
            popOff();
            break;

            // no child (fixed), no SRT
        case CONSTR_0_1:
        case CONSTR_0_2:
        case ARR_WORDS:
            // one child (fixed), no SRT
        case MUT_VAR_CLEAN:
        case MUT_VAR_DIRTY:
        case THUNK_SELECTOR:
        case CONSTR_1_1:
            // cannot appear
        case PAP:
        case AP:
        case AP_STACK:
        case TSO:
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        case STACK:
        case IND_STATIC:
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        case CONSTR_NOCAF:
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            // stack objects
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        case UPDATE_FRAME:
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        case CATCH_FRAME:
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        case UNDERFLOW_FRAME:
        case STOP_FRAME:
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        case RET_BCO:
        case RET_SMALL:
        case RET_BIG:
            // invalid objects
        case IND:
        case INVALID_OBJECT:
        default:
            barf("Invalid object *c in pop()");
            return;
        }
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    } while (true);
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}

/* -----------------------------------------------------------------------------
 * RETAINER PROFILING ENGINE
 * -------------------------------------------------------------------------- */

void
initRetainerProfiling( void )
{
    initializeAllRetainerSet();
    retainerGeneration = 0;
}

/* -----------------------------------------------------------------------------
 *  This function must be called before f-closing prof_file.
 * -------------------------------------------------------------------------- */
void
endRetainerProfiling( void )
{
#ifdef SECOND_APPROACH
    outputAllRetainerSet(prof_file);
#endif
}

/* -----------------------------------------------------------------------------
 *  Returns the actual pointer to the retainer set of the closure *c.
 *  It may adjust RSET(c) subject to flip.
 *  Side effects:
 *    RSET(c) is initialized to NULL if its current value does not
 *    conform to flip.
 *  Note:
 *    Even though this function has side effects, they CAN be ignored because
 *    subsequent calls to retainerSetOf() always result in the same return value
 *    and retainerSetOf() is the only way to retrieve retainerSet of a given
 *    closure.
 *    We have to perform an XOR (^) operation each time a closure is examined.
 *    The reason is that we do not know when a closure is visited last.
 * -------------------------------------------------------------------------- */
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static INLINE void
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maybeInitRetainerSet( StgClosure *c )
{
    if (!isRetainerSetFieldValid(c)) {
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        setRetainerSetToNull(c);
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    }
}

/* -----------------------------------------------------------------------------
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 * Returns true if *c is a retainer.
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 * -------------------------------------------------------------------------- */
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static INLINE bool
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isRetainer( StgClosure *c )
{
    switch (get_itbl(c)->type) {
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        //
        //  True case
        //
        // TSOs MUST be retainers: they constitute the set of roots.
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    case TSO:
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    case STACK:
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        // mutable objects
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    case MUT_PRIM:
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    case MVAR_CLEAN:
    case MVAR_DIRTY:
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    case TVAR:
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    case MUT_VAR_CLEAN:
    case MUT_VAR_DIRTY:
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    case MUT_ARR_PTRS_CLEAN:
    case MUT_ARR_PTRS_DIRTY:
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        // thunks are retainers.
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    case THUNK:
    case THUNK_1_0:
    case THUNK_0_1:
    case THUNK_2_0:
    case THUNK_1_1:
    case THUNK_0_2:
    case THUNK_SELECTOR:
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    case AP:
    case AP_STACK:
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        // Static thunks, or CAFS, are obviously retainers.
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    case THUNK_STATIC:

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        // WEAK objects are roots; there is separate code in which traversing
        // begins from WEAK objects.
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    case WEAK:
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        return true;
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        //
        // False case
        //
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        // constructors
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    case CONSTR:
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    case CONSTR_NOCAF:
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    case CONSTR_1_0:
    case CONSTR_0_1:
    case CONSTR_2_0:
    case CONSTR_1_1:
    case CONSTR_0_2:
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        // functions
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    case FUN:
    case FUN_1_0:
    case FUN_0_1:
    case FUN_2_0:
    case FUN_1_1:
    case FUN_0_2:
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        // partial applications
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    case PAP:
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        // indirection
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    // IND_STATIC used to be an error, but at the moment it can happen
    // as isAlive doesn't look through IND_STATIC as it ignores static
    // closures. See trac #3956 for a program that hit this error.
    case IND_STATIC:
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    case BLACKHOLE:
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        // static objects
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    case FUN_STATIC:
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        // misc
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    case PRIM:
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    case BCO:
    case ARR_WORDS:
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        // STM
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    case TREC_CHUNK:
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        // immutable arrays
    case MUT_ARR_PTRS_FROZEN:
    case MUT_ARR_PTRS_FROZEN0:
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        return false;
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        //
        // Error case
        //
        // Stack objects are invalid because they are never treated as
        // legal objects during retainer profiling.
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    case UPDATE_FRAME:
    case CATCH_FRAME:
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    case UNDERFLOW_FRAME:
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    case STOP_FRAME:
    case RET_BCO:
    case RET_SMALL:
    case RET_BIG:
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        // other cases
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    case IND:
    case INVALID_OBJECT:
    default:
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        barf("Invalid object in isRetainer(): %d", get_itbl(c)->type);
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        return false;
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    }
}

/* -----------------------------------------------------------------------------
 *  Returns the retainer function value for the closure *c, i.e., R(*c).
 *  This function does NOT return the retainer(s) of *c.
 *  Invariants:
 *    *c must be a retainer.
 *  Note:
 *    Depending on the definition of this function, the maintenance of retainer
 *    sets can be made easier. If most retainer sets are likely to be created
 *    again across garbage collections, refreshAllRetainerSet() in
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 *    RetainerSet.c can simply do nothing.
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 *    If this is not the case, we can free all the retainer sets and
 *    re-initialize the hash table.
 *    See refreshAllRetainerSet() in RetainerSet.c.
 * -------------------------------------------------------------------------- */
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static INLINE retainer
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getRetainerFrom( StgClosure *c )
{
    ASSERT(isRetainer(c));

#if defined(RETAINER_SCHEME_INFO)
    // Retainer scheme 1: retainer = info table
    return get_itbl(c);
#elif defined(RETAINER_SCHEME_CCS)
    // Retainer scheme 2: retainer = cost centre stack
    return c->header.prof.ccs;
#elif defined(RETAINER_SCHEME_CC)
    // Retainer scheme 3: retainer = cost centre
    return c->header.prof.ccs->cc;
#endif
}

/* -----------------------------------------------------------------------------
 *  Associates the retainer set *s with the closure *c, that is, *s becomes
 *  the retainer set of *c.
 *  Invariants:
 *    c != NULL
 *    s != NULL
 * -------------------------------------------------------------------------- */
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static INLINE void
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associate( StgClosure *c, RetainerSet *s )
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{
    // StgWord has the same size as pointers, so the following type
    // casting is okay.
    RSET(c) = (RetainerSet *)((StgWord)s | flip);
}

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/* -----------------------------------------------------------------------------
   Call retainClosure for each of the closures covered by a large bitmap.
   -------------------------------------------------------------------------- */

static void
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retain_large_bitmap (StgPtr p, StgLargeBitmap *large_bitmap, uint32_t size,
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                     StgClosure *c, retainer c_child_r)
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{
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    uint32_t i, b;
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    StgWord bitmap;
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    b = 0;
    bitmap = large_bitmap->bitmap[b];
    for (i = 0; i < size; ) {
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        if ((bitmap & 1) == 0) {
            retainClosure((StgClosure *)*p, c, c_child_r);
        }
        i++;
        p++;
        if (i % BITS_IN(W_) == 0) {
            b++;
            bitmap = large_bitmap->bitmap[b];
        } else {
            bitmap = bitmap >> 1;
        }
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    }
}

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static INLINE StgPtr
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retain_small_bitmap (StgPtr p, uint32_t size, StgWord bitmap,
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                     StgClosure *c, retainer c_child_r)
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{
    while (size > 0) {
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        if ((bitmap & 1) == 0) {
            retainClosure((StgClosure *)*p, c, c_child_r);
        }
        p++;
        bitmap = bitmap >> 1;
        size--;
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    }
    return p;
}

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/* -----------------------------------------------------------------------------
 * Call retainClosure for each of the closures in an SRT.
 * ------------------------------------------------------------------------- */

static void
retain_large_srt_bitmap (StgLargeSRT *srt, StgClosure *c, retainer c_child_r)
{
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    uint32_t i, b, size;
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    StgWord bitmap;
    StgClosure **p;
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    b = 0;
    p = (StgClosure **)srt->srt;
    size   = srt->l.size;
    bitmap = srt->l.bitmap[b];
    for (i = 0; i < size; ) {
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        if ((bitmap & 1) != 0) {
            retainClosure((StgClosure *)*p, c, c_child_r);
        }
        i++;
        p++;
        if (i % BITS_IN(W_) == 0) {
            b++;
            bitmap = srt->l.bitmap[b];
        } else {
            bitmap = bitmap >> 1;
        }
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    }
}

static INLINE void
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retainSRT (StgClosure **srt, uint32_t srt_bitmap, StgClosure *c,
            retainer c_child_r)
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{
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  uint32_t bitmap;
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  StgClosure **p;

  bitmap = srt_bitmap;
  p = srt;

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  if (bitmap == (StgHalfWord)(-1)) {
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      retain_large_srt_bitmap( (StgLargeSRT *)srt, c, c_child_r );
      return;
  }

  while (bitmap != 0) {
      if ((bitmap & 1) != 0) {
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#if defined(COMPILING_WINDOWS_DLL)
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          if ( (unsigned long)(*srt) & 0x1 ) {
              retainClosure(* (StgClosure**) ((unsigned long) (*srt) & ~0x1),
                            c, c_child_r);
          } else {
              retainClosure(*srt,c,c_child_r);
          }
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#else
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          retainClosure(*srt,c,c_child_r);
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#endif
      }
      p++;
      bitmap = bitmap >> 1;
  }
}

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/* -----------------------------------------------------------------------------
 *  Process all the objects in the stack chunk from stackStart to stackEnd
 *  with *c and *c_child_r being their parent and their most recent retainer,
 *  respectively. Treat stackOptionalFun as another child of *c if it is
 *  not NULL.
 *  Invariants:
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 *    *c is one of the following: TSO, AP_STACK.
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 *    If *c is TSO, c == c_child_r.
 *    stackStart < stackEnd.
 *    RSET(c) and RSET(c_child_r) are valid, i.e., their
 *    interpretation conforms to the current value of flip (even when they
 *    are interpreted to be NULL).
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 *    If *c is TSO, its state is not ThreadComplete,or ThreadKilled,
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 *    which means that its stack is ready to process.
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 *  Note:
 *    This code was almost plagiarzied from GC.c! For each pointer,
 *    retainClosure() is invoked instead of evacuate().
 * -------------------------------------------------------------------------- */
static void
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retainStack( StgClosure *c, retainer c_child_r,
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             StgPtr stackStart, StgPtr stackEnd )
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{
    stackElement *oldStackBoundary;
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    StgPtr p;
    StgRetInfoTable *info;
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    StgWord bitmap;
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    uint32_t size;
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#ifdef DEBUG_RETAINER
    cStackSize++;
    if (cStackSize > maxCStackSize) maxCStackSize = cStackSize;
#endif

    /*
      Each invocation of retainStack() creates a new virtual
      stack. Since all such stacks share a single common stack, we
      record the current currentStackBoundary, which will be restored
      at the exit.
    */
    oldStackBoundary = currentStackBoundary;
    currentStackBoundary = stackTop;

#ifdef DEBUG_RETAINER
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    // debugBelch("retainStack() called: oldStackBoundary = 0x%x, currentStackBoundary = 0x%x\n", oldStackBoundary, currentStackBoundary);
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#endif

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    ASSERT(get_itbl(c)->type == STACK);

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    p = stackStart;
    while (p < stackEnd) {
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        info = get_ret_itbl((StgClosure *)p);
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        switch(info->i.type) {
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        case UPDATE_FRAME:
            retainClosure(((StgUpdateFrame *)p)->updatee, c, c_child_r);
            p += sizeofW(StgUpdateFrame);
            continue;
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        case UNDERFLOW_FRAME:
        case STOP_FRAME:
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        case CATCH_FRAME:
        case CATCH_STM_FRAME:
        case CATCH_RETRY_FRAME:
        case ATOMICALLY_FRAME:
        case RET_SMALL:
            bitmap = BITMAP_BITS(info->i.layout.bitmap);
            size   = BITMAP_SIZE(info->i.layout.bitmap);
            p++;
            p = retain_small_bitmap(p, size, bitmap, c, c_child_r);

        follow_srt:
            retainSRT((StgClosure **)GET_SRT(info), info->i.srt_bitmap, c, c_child_r);
            continue;

        case RET_BCO: {
            StgBCO *bco;

            p++;
            retainClosure((StgClosure *)*p, c, c_child_r);
            bco = (StgBCO *)*p;
            p++;
            size = BCO_BITMAP_SIZE(bco);
            retain_large_bitmap(p, BCO_BITMAP(bco), size, c, c_child_r);
            p += size;
            continue;
        }

            // large bitmap (> 32 entries, or > 64 on a 64-bit machine)
        case RET_BIG:
            size = GET_LARGE_BITMAP(&info->i)->size;
            p++;
            retain_large_bitmap(p, GET_LARGE_BITMAP(&info->i),
                                size, c, c_child_r);
            p += size;
            // and don't forget to follow the SRT
            goto follow_srt;
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        case RET_FUN: {
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            StgRetFun *ret_fun = (StgRetFun *)p;
            StgFunInfoTable *fun_info;

            retainClosure(ret_fun->fun, c, c_child_r);
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            fun_info = get_fun_itbl(UNTAG_CONST_CLOSURE(ret_fun->fun));
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            p = (P_)&ret_fun->payload;
            switch (fun_info->f.fun_type) {
            case ARG_GEN:
                bitmap = BITMAP_BITS(fun_info->f.b.bitmap);
                size = BITMAP_SIZE(fun_info->f.b.bitmap);
                p = retain_small_bitmap(p, size, bitmap, c, c_child_r);
                break;
            case ARG_GEN_BIG:
                size = GET_FUN_LARGE_BITMAP(fun_info)->size;
                retain_large_bitmap(p, GET_FUN_LARGE_BITMAP(fun_info),
                                    size, c, c_child_r);
                p += size;
                break;
            default:
                bitmap = BITMAP_BITS(stg_arg_bitmaps[fun_info->f.fun_type]);
                size = BITMAP_SIZE(stg_arg_bitmaps[fun_info->f.fun_type]);
                p = retain_small_bitmap(p, size, bitmap, c, c_child_r);
                break;
            }
            goto follow_srt;
        }

        default:
            barf("Invalid object found in retainStack(): %d",
                 (int)(info->i.type));
        }
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    }

    // restore currentStackBoundary
    currentStackBoundary = oldStackBoundary;
#ifdef DEBUG_RETAINER
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    // debugBelch("retainStack() finished: currentStackBoundary = 0x%x\n", currentStackBoundary);
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#endif

#ifdef DEBUG_RETAINER
    cStackSize--;
#endif
}

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/* ----------------------------------------------------------------------------
 * Call retainClosure for each of the children of a PAP/AP
 * ------------------------------------------------------------------------- */

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static INLINE StgPtr
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retain_PAP_payload (StgClosure *pap,    /* NOT tagged */
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                    retainer c_child_r, /* NOT tagged */
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                    StgClosure *fun,    /* tagged */
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                    StgClosure** payload, StgWord n_args)
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{
    StgPtr p;
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    StgWord bitmap;
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    StgFunInfoTable *fun_info;

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    retainClosure(fun, pap, c_child_r);
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    fun = UNTAG_CLOSURE(fun);
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    fun_info = get_fun_itbl(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);
        p = retain_small_bitmap(p, n_args, bitmap,
                                pap,