RetainerProfile.c 71 KB
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/* -----------------------------------------------------------------------------
 *
 * (c) The GHC Team, 2001
 * Author: Sungwoo Park
 *
 * Retainer profiling.
 *
 * ---------------------------------------------------------------------------*/

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#if defined(PROFILING)
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// Turn off inlining when debugging - it obfuscates things
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#if defined(DEBUG)
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#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 "StablePtr.h" /* markStablePtrTable */
#include "StableName.h" /* rememberOldStableNameAddresses */
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#include "sm/Storage.h" // for END_OF_STATIC_LIST
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/* Note [What is a retainer?]
   ~~~~~~~~~~~~~~~~~~~~~~~~~~
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Retainer profiling is a profiling technique that gives information why
objects can't be freed and lists the consumers that hold pointers to
the heap objects. It does not list all the objects that keeps references
to the other, because then we would keep too much information that will
make the report unusable, for example the cons element of the list would keep
all the tail cells. As a result we are keeping only the objects of the
certain types, see 'isRetainer()' function for more discussion.

More formal definition of the retainer can be given the following way.

An object p is a retainer object of the object l, if all requirements
hold:

  1. p can be a retainer (see `isRetainer()`)
  2. l is reachable from p
  3. There are no other retainers on the path from p to l.

Exact algorithm and additional information can be found the historical
document 'docs/storage-mgt/rp.tex'. Details that are related to the
RTS implementation may be out of date, but the general
information about the retainers is still applicable.
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*/


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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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#if defined(DEBUG_RETAINER)
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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 {
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    // Object with fixed layout. Keeps an information about that
    // element was processed. (stackPos.next.step)
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    posTypeStep,
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    // Description of the pointers-first heap object. Keeps information
    // about layout. (stackPos.next.ptrs)
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    posTypePtrs,
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    // Keeps SRT bitmap (stackPos.next.srt)
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    posTypeSRT,
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    // Keeps a new object that was not inspected yet. Keeps a parent
    // element (stackPos.next.parent)
    posTypeFresh
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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;
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    } srt;
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    // parent of the current object, used
    // when posTypeFresh is set
    StgClosure *parent;
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} nextPos;

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// Tagged stack element, that keeps information how to process
// the next element in the traverse stack.
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typedef struct {
    nextPosType type;
    nextPos next;
} stackPos;

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// Element in the traverse stack, keeps the element, information
// how to continue processing the element, and it's retainer set.
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typedef struct {
    StgClosure *c;
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    retainer c_child_r;
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    stackPos info;
} stackElement;

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typedef struct {
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/*
  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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    bdescr *firstStack;
    bdescr *currentStack;
    stackElement *stackBottom, *stackTop, *stackLimit;
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/*
  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.
 */
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    stackElement *currentStackBoundary;
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#if defined(DEBUG_RETAINER)
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/*
  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
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    retainer profiling, maxStackSize is some value no greater
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    than the actual depth of the graph.
 */
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    int stackSize, maxStackSize;
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#endif
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} traverseState;

traverseState g_retainerTraverseState;


static void retainStack(traverseState *, StgClosure *, retainer, StgPtr, StgPtr);
static void retainClosure(traverseState *, StgClosure *, StgClosure *, retainer);
static void retainPushClosure(traverseState *, StgClosure *, StgClosure *, retainer);
static void retainActualPush(traverseState *, stackElement *);

#if defined(DEBUG_RETAINER)
static void belongToHeap(StgPtr p);
static uint32_t checkHeapSanityForRetainerProfiling( void );
#endif

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// 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( traverseState *ts, bdescr *bd )
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{
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    ts->currentStack = bd;
    ts->stackTop     = (stackElement *)(bd->start + BLOCK_SIZE_W * bd->blocks);
    ts->stackBottom  = (stackElement *)bd->start;
    ts->stackLimit   = (stackElement *)ts->stackTop;
    bd->free     = (StgPtr)ts->stackLimit;
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}

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

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

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    ts->firstStack = allocGroup(BLOCKS_IN_STACK);
    ts->firstStack->link = NULL;
    ts->firstStack->u.back = NULL;
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    newStackBlock(ts, ts->firstStack);
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}

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

/* -----------------------------------------------------------------------------
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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( traverseState *ts )
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{
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    return (ts->firstStack == ts->currentStack) && ts->stackTop == ts->stackLimit;
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}

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/* -----------------------------------------------------------------------------
 * Returns size of stack
 * -------------------------------------------------------------------------- */
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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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    traverseState *ts = &g_retainerTraverseState;
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    for (bd = ts->firstStack; bd != NULL; bd = bd->link)
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      res += bd->blocks;

    return res;
}

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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( traverseState *ts )
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{
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    return ts->stackTop == ts->currentStackBoundary;
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}

/* -----------------------------------------------------------------------------
 * 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, const StgFunInfoTable *infoTable )
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{
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    info->type = posTypeSRT;
    if (infoTable->i.srt) {
        info->next.srt.srt = (StgClosure*)GET_FUN_SRT(infoTable);
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    } else {
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        info->next.srt.srt = NULL;
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    }
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}

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static INLINE void
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init_srt_thunk( stackPos *info, const StgThunkInfoTable *infoTable )
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{
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    info->type = posTypeSRT;
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    if (infoTable->i.srt) {
        info->next.srt.srt = (StgClosure*)GET_SRT(infoTable);
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    } else {
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        info->next.srt.srt = NULL;
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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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    if (info->type == posTypeSRT) {
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        c = info->next.srt.srt;
        info->next.srt.srt = NULL;
        return c;
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    }
}

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/* -----------------------------------------------------------------------------
 * Pushes an element onto traverse stack
 * -------------------------------------------------------------------------- */
static void
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retainActualPush(traverseState *ts, stackElement *se) {
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    bdescr *nbd;      // Next Block Descriptor
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    if (ts->stackTop - 1 < ts->stackBottom) {
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#if defined(DEBUG_RETAINER)
        // debugBelch("push() to the next stack.\n");
#endif
        // currentStack->free is updated when the active stack is switched
        // to the next stack.
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        ts->currentStack->free = (StgPtr)ts->stackTop;
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        if (ts->currentStack->link == NULL) {
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            nbd = allocGroup(BLOCKS_IN_STACK);
            nbd->link = NULL;
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            nbd->u.back = ts->currentStack;
            ts->currentStack->link = nbd;
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        } else
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            nbd = ts->currentStack->link;
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        newStackBlock(ts, nbd);
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    }

    // adjust stackTop (acutal push)
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    ts->stackTop--;
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    // 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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    *ts->stackTop = *se;
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#if defined(DEBUG_RETAINER)
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    ts->stackSize++;
    if (ts->stackSize > ts->maxStackSize) ts->maxStackSize = ts->stackSize;
    ASSERT(ts->stackSize >= 0);
    debugBelch("stackSize = %d\n", ts->stackSize);
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#endif
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}

/* Push an object onto traverse stack. This method can be used anytime
 * instead of calling retainClosure(), it exists in order to use an
 * explicit stack instead of direct recursion.
 *
 *  *p - object's parent
 *  *c - closure
 *  c_child_r - closure retainer.
 */
static INLINE void
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retainPushClosure( traverseState *ts, StgClosure *c, StgClosure *p, retainer c_child_r) {
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    stackElement se;

    se.c = c;
    se.c_child_r = c_child_r;
    se.info.next.parent = p;
    se.info.type = posTypeFresh;

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    retainActualPush(ts, &se);
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};

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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
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 *  child, *c_child is set to that child and nothing is pushed onto
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 *  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( traverseState *ts, StgClosure *c, retainer c_child_r, StgClosure **first_child )
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{
    stackElement se;
    bdescr *nbd;      // Next Block Descriptor

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#if defined(DEBUG_RETAINER)
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    debugBelch("push(): stackTop = 0x%x, currentStackBoundary = 0x%x\n", ts->stackTop, ts->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
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        se.info.type = posTypeStep;
        se.info.next.step = 2;            // 2 = second
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        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;
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        se.info.type = posTypeStep;
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        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;
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        se.info.type = posTypeStep;
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        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_CLEAN:
    case MUT_ARR_PTRS_FROZEN_DIRTY:
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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:
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    case SMALL_MUT_ARR_PTRS_FROZEN_CLEAN:
    case SMALL_MUT_ARR_PTRS_FROZEN_DIRTY:
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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
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    case FUN_STATIC:
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    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_0_1:      // *c is a heap object.
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    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 != 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;
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        se.info.type = posTypeStep;
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        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(): %d", get_itbl(c)->type);
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        return;
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    }

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    retainActualPush(ts, &se);
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}

/* -----------------------------------------------------------------------------
 *  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
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popOffReal(traverseState *ts)
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{
    bdescr *pbd;    // Previous Block Descriptor

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

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    ASSERT(ts->stackTop + 1 == ts->stackLimit);
    ASSERT(ts->stackBottom == (stackElement *)ts->currentStack->start);
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    if (ts->firstStack == ts->currentStack) {
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        // The stack is completely empty.
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        ts->stackTop++;
        ASSERT(ts->stackTop == ts->stackLimit);
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#if defined(DEBUG_RETAINER)
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        ts->stackSize--;
        if (ts->stackSize > ts->maxStackSize) ts->maxStackSize = ts->stackSize;
        ASSERT(ts->stackSize >= 0);
        debugBelch("stackSize = %d\n", ts->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.
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    ts->currentStack->free = (StgPtr)ts->stackLimit;
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    // find the previous block descriptor
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    pbd = ts->currentStack->u.back;
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    ASSERT(pbd != NULL);

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    returnToOldStack(ts, pbd);
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#if defined(DEBUG_RETAINER)
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    ts->stackSize--;
    if (ts->stackSize > ts->maxStackSize) ts->maxStackSize = ts->stackSize;
    ASSERT(ts->stackSize >= 0);
    debugBelch("stackSize = %d\n", ts->stackSize);
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#endif
}

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static INLINE void
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popOff(traverseState *ts) {
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#if defined(DEBUG_RETAINER)
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    debugBelch("\tpopOff(): stackTop = 0x%x, currentStackBoundary = 0x%x\n", ts->stackTop, ts->currentStackBoundary);
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#endif

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

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    popOffReal(ts);
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}

/* -----------------------------------------------------------------------------
 *  Finds the next object to be considered for retainer profiling and store
 *  its pointer to *c.
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 *  If the unprocessed object was stored in the stack (posTypeFresh), the
 *  this object is returned as-is. Otherwise Test if the topmost stack
 *  element indicates that more objects are left,
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 *  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( traverseState *ts, StgClosure **c, StgClosure **cp, retainer *r )
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{
    stackElement *se;

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#if defined(DEBUG_RETAINER)
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    debugBelch("pop(): stackTop = 0x%x, currentStackBoundary = 0x%x\n", ts->stackTop, ts->currentStackBoundary);
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#endif

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

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        se = ts->stackTop;
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        // If this is a top-level element, you should pop that out.
        if (se->info.type == posTypeFresh) {
            *cp = se->info.next.parent;
            *c = se->c;
            *r = se->c_child_r;
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            popOff(ts);
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            return;
        }

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        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;
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            popOff(ts);
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            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;
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                popOff(ts);
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            }
            *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;
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                popOff(ts);
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            }
            *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;
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                popOff(ts);
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                break;
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            }
            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:
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        case MUT_ARR_PTRS_FROZEN_CLEAN:
        case MUT_ARR_PTRS_FROZEN_DIRTY:
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        case SMALL_MUT_ARR_PTRS_CLEAN:
        case SMALL_MUT_ARR_PTRS_DIRTY:
        case SMALL_MUT_ARR_PTRS_FROZEN_CLEAN:
        case SMALL_MUT_ARR_PTRS_FROZEN_DIRTY:
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            *c = find_ptrs(&se->info);
            if (*c == NULL) {
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                popOff(ts);
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                break;
            }
            *cp = se->c;
            *r = se->c_child_r;
            return;

            // layout.payload.ptrs, SRT
        case FUN:         // always a heap object
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        case FUN_STATIC:
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        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_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;
            }
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            popOff(ts);
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            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:
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            barf("Invalid object *c in pop(): %d", get_itbl(se->c)->type);
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            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 )
{
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#if defined(SECOND_APPROACH)
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    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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 * In general the retainers are the objects that may be the roots of the
 * collection. Basically this roots represents programmers threads
 * (TSO) with their stack and thunks.
 *
 * In addition we mark all mutable objects as a retainers, the reason for
 * that decision is lost in time.
1024
 * -------------------------------------------------------------------------- */
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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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    case SMALL_MUT_ARR_PTRS_CLEAN:
    case SMALL_MUT_ARR_PTRS_DIRTY:
    case BLOCKING_QUEUE:
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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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1060
        // 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
1088
    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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    case WHITEHOLE:
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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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    case COMPACT_NFDATA:
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        // STM
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    case TREC_CHUNK:
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        // immutable arrays
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    case MUT_ARR_PTRS_FROZEN_CLEAN:
    case MUT_ARR_PTRS_FROZEN_DIRTY:
    case SMALL_MUT_ARR_PTRS_FROZEN_CLEAN:
    case SMALL_MUT_ARR_PTRS_FROZEN_DIRTY:
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        return false;
1111

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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 CATCH_RETRY_FRAME:
    case CATCH_STM_FRAME:
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    case UNDERFLOW_FRAME:
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    case ATOMICALLY_FRAME:
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    case STOP_FRAME:
    case RET_BCO:
    case RET_SMALL:
    case RET_BIG:
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    case RET_FUN:
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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));

    return c->header.prof.ccs;
}

/* -----------------------------------------------------------------------------
 *  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);
}

1174
/* -----------------------------------------------------------------------------
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   Call retainPushClosure for each of the closures covered by a large bitmap.
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   -------------------------------------------------------------------------- */

static void
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retain_large_bitmap (traverseState *ts, StgPtr p, StgLargeBitmap *large_bitmap,
                     uint32_t size, StgClosure *c, retainer c_child_r)
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{
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    uint32_t i, b;