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[mono-project.git] / mono / metadata / sgen-tarjan-bridge.c

/*
 * sgen-tarjan-bridge.c: Tarjan-based bridge implementation.
 *
 * Copyright 2011 Novell, Inc (http://www.novell.com)
 * Copyright 2014 Xamarin Inc (http://www.xamarin.com)
 *
 *
 * Copyright 2001-2003 Ximian, Inc
 * Copyright 2003-2010 Novell, Inc.
 *
 * Licensed under the MIT license. See LICENSE file in the project root for full license information.
 */

#include "config.h"

#ifdef HAVE_SGEN_GC

#include <stdlib.h>

#include "sgen/sgen-gc.h"
#include "sgen-bridge-internals.h"
#include "tabledefs.h"
#include "utils/mono-logger-internals.h"

#include "sgen-dynarray.h"

/*
 * See comments in sgen-bridge.h
 *
 * This bridge implementation is based on the tarjan algorithm for strongly
 * connected components. It has two elements:
 *
 *   - Standard tarjan SCC algorithm to convert graph to SCC forest
 *
 *   - "Colors": We reduce the SCC forest to bridged-SCCs-only by using a
 *     "color" algorithm devised by Kumpera. Consider the set of bridged SCCs
 *     which is reachable from a given object. We call each such unique set a
 *     "color". We compute the set of colors and which colors contain links to
 *     which colors. The color graph then becomes the reduced SCC graph.
 */

// Is this class bridged or not, and should its dependencies be scanned or not?
// The result of this callback will be cached for use by is_opaque_object later.
static MonoGCBridgeObjectKind
class_kind (MonoClass *klass)
{
        MonoGCBridgeObjectKind res = bridge_callbacks.bridge_class_kind (klass);

        /* If it's a bridge, nothing we can do about it. */
        if (res == GC_BRIDGE_TRANSPARENT_BRIDGE_CLASS || res == GC_BRIDGE_OPAQUE_BRIDGE_CLASS)
                return res;

        /* Non bridge classes with no pointers will never point to a bridge, so we can savely ignore them. */
        if (!klass->has_references) {
                SGEN_LOG (6, "class %s is opaque\n", klass->name);
                return GC_BRIDGE_OPAQUE_CLASS;
        }

        /* Some arrays can be ignored */
        if (klass->rank == 1) {
                MonoClass *elem_class = klass->element_class;

                /* FIXME the bridge check can be quite expensive, cache it at the class level. */
                /* An array of a sealed type that is not a bridge will never get to a bridge */
                if ((mono_class_get_flags (elem_class) & TYPE_ATTRIBUTE_SEALED) && !elem_class->has_references && !bridge_callbacks.bridge_class_kind (elem_class)) {
                        SGEN_LOG (6, "class %s is opaque\n", klass->name);
                        return GC_BRIDGE_OPAQUE_CLASS;
                }
        }

        return GC_BRIDGE_TRANSPARENT_CLASS;
}

//enable usage logging
// #define DUMP_GRAPH 1

/* Used in bridgeless_color_is_heavy:
 * The idea here is as long as the reference fanin and fanout on a node are both 2 or greater, then
 * removing that node will result in a net increase in edge count. So the question is which node
 * removals are counterproductive (i.e., how many edges saved balances out one node added).
 * The number of edges saved by preserving a node is (fanin*fanout - fanin - fanout).
 *
 * With all that in mind:
 *
 * - HEAVY_REFS_MIN is the number that *both* fanin and fanout must meet to preserve the node.
 * - HEAVY_COMBINED_REFS_MIN is the number (fanin*fanout) must meet to preserve the node.
 *
 * Note HEAVY_COMBINED_REFS_MIN must be <= 2*INCOMING_COLORS_MAX, or we won't know the true fanin.
 */

#define HEAVY_REFS_MIN 2
#define HEAVY_COMBINED_REFS_MIN 60

/* Used in ColorData:
 * The higher INCOMING_COLORS_BITS is the higher HEAVY_COMBINED_REFS_MIN can be (see above).
 * However, API_INDEX_BITS + INCOMING_COLORS_BITS must be equal to 31, and if API_INDEX_BITS is too
 * low then terrible things will happen if too many colors are generated. (The number of colors we
 * will ever attempt to generate is currently naturally limited by the JNI GREF limit.)
 */

#define API_INDEX_BITS        26
#define INCOMING_COLORS_BITS  5

#define API_INDEX_MAX         ((1<<API_INDEX_BITS)-1)
#define INCOMING_COLORS_MAX   ((1<<INCOMING_COLORS_BITS)-1)

// ScanData state
enum {
        INITIAL,
        SCANNED,
        FINISHED_ON_STACK,
        FINISHED_OFF_STACK
};

/*
Optimizations:
        We can split this data structure in two, those with bridges and those without
        (and only bridgeless need to record incoming_colors)
*/
typedef struct {
        // Colors (ColorDatas) linked to by objects with this color
        DynPtrArray other_colors;
        // Bridge objects (GCObjects) held by objects with this color
        DynPtrArray bridges;
        // Index of this color's MonoGCBridgeSCC in the array passed to the client (or -1 for none)
        signed api_index         : API_INDEX_BITS;
        // Count of colors that list this color in their other_colors
        unsigned incoming_colors : INCOMING_COLORS_BITS;
        unsigned visited : 1;
} ColorData;

// Represents one managed object. Equivalent of new/old bridge "HashEntry"
typedef struct _ScanData {
        // FIXME this can be eliminated; if we have a ScanData we generally looked it up from its GCObject
        GCObject *obj;
        // We use the sgen lock_word in GCObject to store a pointer to the ScanData. Cache the original here to restore later:
        mword lock_word;

        ColorData *color;
        // Tarjan algorithm index (order visited)
        int index;
        // Tarjan index of lowest-index object known reachable from here
        signed low_index : 27;

        // See "ScanData state" enum above
        unsigned state : 2;
        unsigned is_bridge : 1;
        // Similar to lock_word, we use these bits in the GCObject as scratch space then restore them when done
        unsigned obj_state : 2;
} ScanData;

/* Should color be made visible to client even though it has no bridges?
 * True if we predict the number of reduced edges to be enough to justify the extra node.
 */
static inline gboolean
bridgeless_color_is_heavy (ColorData *data) {
        int fanin = data->incoming_colors;
        int fanout = dyn_array_ptr_size (&data->other_colors);
        return fanin > HEAVY_REFS_MIN && fanout > HEAVY_REFS_MIN
                && fanin*fanout >= HEAVY_COMBINED_REFS_MIN;
}

// Should color be made visible to client?
static inline gboolean
color_visible_to_client (ColorData *data) {
        return dyn_array_ptr_size (&data->bridges) || bridgeless_color_is_heavy (data);
}

// Stacks of ScanData objects used for tarjan algorithm.
// The Tarjan algorithm is normally defined recursively; here scan_stack simulates the call stack of a recursive algorithm,
// and loop_stack is the stack structure used by the algorithm itself.
static DynPtrArray scan_stack, loop_stack;

// GCObjects on which register_finalized_object has been called
static DynPtrArray registered_bridges;

// As we traverse the graph, which ColorData objects are accessible from our current position?
static DynPtrArray color_merge_array;
// Running hash of the contents of the color_merge_array.
static unsigned int color_merge_array_hash;

static void color_merge_array_empty ()
{
        dyn_array_ptr_empty (&color_merge_array);
        color_merge_array_hash = 0;
}

static int ignored_objects;
static int object_index;
static int num_colors_with_bridges;
static int num_sccs;
static int xref_count;

static size_t setup_time, tarjan_time, scc_setup_time, gather_xref_time, xref_setup_time, cleanup_time;
static SgenBridgeProcessor *bridge_processor;

#define BUCKET_SIZE 8184

//ScanData buckets
#define NUM_SCAN_ENTRIES ((BUCKET_SIZE - SIZEOF_VOID_P * 2) / sizeof (ScanData))

typedef struct _ObjectBucket ObjectBucket;
struct _ObjectBucket {
        ObjectBucket *next;
        ScanData *next_data;
        ScanData data [NUM_SCAN_ENTRIES];
};

static ObjectBucket *root_object_bucket;
static ObjectBucket *cur_object_bucket;
static int object_data_count;

// Arenas to allocate ScanData from
static ObjectBucket*
new_object_bucket (void)
{
        ObjectBucket *res = (ObjectBucket *)sgen_alloc_internal (INTERNAL_MEM_TARJAN_OBJ_BUCKET);
        res->next_data = &res->data [0];
        return res;
}

static void
object_alloc_init (void)
{
        root_object_bucket = cur_object_bucket = new_object_bucket ();
}

static ScanData*
alloc_object_data (void)
{
        ScanData *res;
retry:

        /* next_data points to the first free entry */
        res = cur_object_bucket->next_data;
        if (res >= &cur_object_bucket->data [NUM_SCAN_ENTRIES]) {
                ObjectBucket *b = new_object_bucket ();
                cur_object_bucket->next = b;
                cur_object_bucket = b;
                goto retry;
        }
        cur_object_bucket->next_data = res + 1;
        object_data_count++;
        return res;
}

static void
free_object_buckets (void)
{
        ObjectBucket *cur = root_object_bucket;

        object_data_count = 0;

        while (cur) {
                ObjectBucket *tmp = cur->next;
                sgen_free_internal (cur, INTERNAL_MEM_TARJAN_OBJ_BUCKET);
                cur = tmp;
        }

        root_object_bucket = cur_object_bucket = NULL;
}

//ColorData buckets
#define NUM_COLOR_ENTRIES ((BUCKET_SIZE - SIZEOF_VOID_P * 2) / sizeof (ColorData))

// Arenas for ColorDatas, same as ObjectBucket except items-per-bucket differs
typedef struct _ColorBucket ColorBucket;
struct _ColorBucket {
        ColorBucket *next;
        ColorData *next_data;
        ColorData data [NUM_COLOR_ENTRIES];
};

static ColorBucket *root_color_bucket;
static ColorBucket *cur_color_bucket;
static int color_data_count;


static ColorBucket*
new_color_bucket (void)
{
        ColorBucket *res = (ColorBucket *)sgen_alloc_internal (INTERNAL_MEM_TARJAN_OBJ_BUCKET);
        res->next_data = &res->data [0];
        return res;
}

static void
color_alloc_init (void)
{
        root_color_bucket = cur_color_bucket = new_color_bucket ();
}

static ColorData*
alloc_color_data (void)
{
        ColorData *res;
retry:

        /* next_data points to the first free entry */
        res = cur_color_bucket->next_data;
        if (res >= &cur_color_bucket->data [NUM_COLOR_ENTRIES]) {
                ColorBucket *b = new_color_bucket ();
                cur_color_bucket->next = b;
                cur_color_bucket = b;
                goto retry;
        }
        cur_color_bucket->next_data = res + 1;
        color_data_count++;
        return res;
}

static void
free_color_buckets (void)
{
        ColorBucket *cur, *tmp;

        color_data_count = 0;

        for (cur = root_color_bucket; cur; cur = tmp) {
                ColorData *cd;
                for (cd = &cur->data [0]; cd < cur->next_data; ++cd) {
                        dyn_array_ptr_uninit (&cd->other_colors);
                        dyn_array_ptr_uninit (&cd->bridges);
                }
                tmp = cur->next;
                sgen_free_internal (cur, INTERNAL_MEM_TARJAN_OBJ_BUCKET);
        }
        root_color_bucket = cur_color_bucket = NULL;
}


static ScanData*
create_data (GCObject *obj)
{
        mword *o = (mword*)obj;
        ScanData *res = alloc_object_data ();
        res->obj = obj;
        res->color = NULL;
        res->index = res->low_index = -1;
        res->state = INITIAL;
        res->is_bridge = FALSE;
        res->obj_state = o [0] & SGEN_VTABLE_BITS_MASK;
        res->lock_word = o [1];

        o [0] |= SGEN_VTABLE_BITS_MASK;
        o [1] = (mword)res;
        return res;
}

static ScanData*
find_data (GCObject *obj)
{
        ScanData *a = NULL;
        mword *o = (mword*)obj;
        if ((o [0] & SGEN_VTABLE_BITS_MASK) == SGEN_VTABLE_BITS_MASK)
                a = (ScanData*)o [1];
        return a;
}

static void
clear_after_processing (void)
{
        ObjectBucket *cur;

        for (cur = root_object_bucket; cur; cur = cur->next) {
                ScanData *sd;
                for (sd = &cur->data [0]; sd < cur->next_data; ++sd) {
                        mword *o = (mword*)sd->obj;
                        o [0] &= ~SGEN_VTABLE_BITS_MASK;
                        o [0] |= sd->obj_state;
                        o [1] = sd->lock_word;
                }
        }
}

static GCObject*
bridge_object_forward (GCObject *obj)
{
        GCObject *fwd;
        mword *o = (mword*)obj;
        if ((o [0] & SGEN_VTABLE_BITS_MASK) == SGEN_VTABLE_BITS_MASK)
                return obj;

        fwd = SGEN_OBJECT_IS_FORWARDED (obj);
        return fwd ? fwd : obj;
}

#ifdef DUMP_GRAPH
static const char*
safe_name_bridge (GCObject *obj)
{
        GCVTable vt = SGEN_LOAD_VTABLE (obj);
        return vt->klass->name;
}

static ScanData*
find_or_create_data (GCObject *obj)
{
        ScanData *entry = find_data (obj);
        if (!entry)
                entry = create_data (obj);
        return entry;
}
#endif

//----------
typedef struct {
        ColorData *color;
        unsigned int hash;
} HashEntry;

/*
The merge cache maps an ordered list of ColorDatas [the color_merge_array] to a single ColorData.

About cache bucket tuning: We tried 2/32, 2/128, 4/32, 4/128, 6/128 and 8/128.

The performance cost between 4/128 and 8/128 is so small since cache movement happens completely in the same cacheline,
making the extra space pretty much free.

The cost between 32 and 128 itens is minimal too, it's mostly a fixed, setup cost.

Memory wise, 4/32 takes 512 and 8/128 takes 8k, so it's quite reasonable.
*/

#define ELEMENTS_PER_BUCKET 8
#define COLOR_CACHE_SIZE 128
static HashEntry merge_cache [COLOR_CACHE_SIZE][ELEMENTS_PER_BUCKET];
static unsigned int hash_perturb;

// If true, disable an optimization where sometimes SCC nodes are merged without a perfect check
static gboolean scc_precise_merge;

static unsigned int
mix_hash (uintptr_t source)
{
        unsigned int hash = source;

        // The full hash determines whether two colors can be merged-- sometimes exclusively.
        // This value changes every GC, so XORing it in before performing the hash will make the
        // chance that two different colors will produce the same hash on successive GCs very low.
        hash = hash ^ hash_perturb;

        // Actual hash
        hash = (((hash * 215497) >> 16) ^ ((hash * 1823231) + hash));

        // Mix in highest bits on 64-bit systems only
        if (sizeof (source) > 4)
                hash = hash ^ (source >> 32);

        return hash;
}

static void
reset_cache (void)
{
        memset (merge_cache, 0, sizeof (merge_cache));

        // When using the precise merge debug option, we do not want the inconsistency caused by hash_perturb.
        if (!scc_precise_merge)
                ++hash_perturb;
}


static gboolean
dyn_array_ptr_contains (DynPtrArray *da, void *x)
{
        int i;
        for (i = 0; i < dyn_array_ptr_size (da); ++i)
                if (dyn_array_ptr_get (da, i) == x)
                        return TRUE;
        return FALSE;
}

static gboolean
match_colors_estimate (DynPtrArray *a, DynPtrArray *b)
{
        return dyn_array_ptr_size (a) == dyn_array_ptr_size (b);
}


static gboolean
match_colors (DynPtrArray *a, DynPtrArray *b)
{
        int i;
        if (dyn_array_ptr_size (a) != dyn_array_ptr_size (b))
                return FALSE;

        for (i = 0; i < dyn_array_ptr_size (a); ++i) {
                if (!dyn_array_ptr_contains (b, dyn_array_ptr_get (a, i)))
                        return FALSE;
        }
        return TRUE;
}

// If scc_precise_merge, "semihits" refers to find_in_cache calls aborted because the merge array was too large.
// Otherwise "semihits" refers to cache hits where the match was only estimated.
static int cache_hits, cache_semihits, cache_misses;

// The cache contains only non-bridged colors.
static ColorData*
find_in_cache (int *insert_index)
{
        HashEntry *bucket;
        int i, size, index;

        size = dyn_array_ptr_size (&color_merge_array);

        /* Color equality checking is very expensive with a lot of elements, so when there are many
         * elements we switch to a cheap comparison method which allows false positives. When false
         * positives occur the worst that can happen is two items will be inappropriately merged
         * and memory will be retained longer than it should be. We assume that will correct itself
         * on the next GC (the hash_perturb mechanism increases the probability of this).
         *
         * Because this has *some* potential to create problems, if the user set the debug option
         * 'enable-tarjan-precise-merge' we bail out early (and never merge SCCs with >3 colors).
         */
        gboolean color_merge_array_large = size > 3;
        if (scc_precise_merge && color_merge_array_large) {
                ++cache_semihits;
                return NULL;
        }

        unsigned int hash = color_merge_array_hash;
        if (!hash) // 0 is used to indicate an empty bucket entry
                hash = 1;

        index = hash & (COLOR_CACHE_SIZE - 1);
        bucket = merge_cache [index];
        for (i = 0; i < ELEMENTS_PER_BUCKET; ++i) {
                if (bucket [i].hash != hash)
                        continue;

                if (color_merge_array_large) {
                        if (match_colors_estimate (&bucket [i].color->other_colors, &color_merge_array)) {
                                ++cache_semihits;
                                return bucket [i].color;
                        }
                } else {
                        if (match_colors (&bucket [i].color->other_colors, &color_merge_array)) {
                                ++cache_hits;
                                return bucket [i].color;
                        }
                }
        }

        //move elements to the back
        for (i = ELEMENTS_PER_BUCKET - 1; i > 0; --i)
                bucket [i] = bucket [i - 1];
        ++cache_misses;
        *insert_index = index;
        bucket [0].hash = hash;
        return NULL;
}

// A color is needed for an SCC. If the SCC has bridges, the color MUST be newly allocated.
// If the SCC lacks bridges, the allocator MAY use the cache to merge it with an existing one.
static ColorData*
new_color (gboolean has_bridges)
{
        int cacheSlot = -1;
        ColorData *cd;
        /* XXX Try to find an equal one and return it */
        if (!has_bridges) {
                cd = find_in_cache (&cacheSlot);
                if (cd)
                        return cd;
        }

        cd = alloc_color_data ();
        cd->api_index = -1;
        dyn_array_ptr_set_all (&cd->other_colors, &color_merge_array);

        // Inform targets
        for (int i = 0; i < dyn_array_ptr_size (&color_merge_array); ++i) {
                ColorData *points_to = (ColorData *)dyn_array_ptr_get (&color_merge_array, i);
                points_to->incoming_colors = MIN (points_to->incoming_colors + 1, INCOMING_COLORS_MAX);
        }

        /* if cacheSlot >= 0, it means we prepared a given slot to receive the new color */
        if (cacheSlot >= 0)
                merge_cache [cacheSlot][0].color = cd;

        return cd;
}


static void
register_bridge_object (GCObject *obj)
{
        create_data (obj)->is_bridge = TRUE;
}

static gboolean
is_opaque_object (GCObject *obj)
{
        MonoVTable *vt = SGEN_LOAD_VTABLE (obj);
        if ((vt->gc_bits & SGEN_GC_BIT_BRIDGE_OPAQUE_OBJECT) == SGEN_GC_BIT_BRIDGE_OPAQUE_OBJECT) {
                SGEN_LOG (6, "ignoring %s\n", vt->klass->name);
                ++ignored_objects;
                return TRUE;
        }
        return FALSE;
}

// Called during DFS; visits one child. If it is a candidate to be scanned, pushes it to the stacks.
static void
push_object (GCObject *obj)
{
        ScanData *data;
        obj = bridge_object_forward (obj);

#if DUMP_GRAPH
        printf ("\t= pushing %p %s -> ", obj, safe_name_bridge (obj));
#endif

        /* Object types we can ignore */
        if (is_opaque_object (obj)) {
#if DUMP_GRAPH
                printf ("opaque\n");
#endif
                return;
        }

        data = find_data (obj);

        /* Already marked - XXX must be done this way as the bridge themselves are alive. */
        if (data && data->state != INITIAL) {
#if DUMP_GRAPH
                printf ("already marked\n");
#endif
                return;
        }

        /* We only care about dead objects */
        if (!data && sgen_object_is_live (obj)) {
#if DUMP_GRAPH
                printf ("alive\n");
#endif
                return;
        }

#if DUMP_GRAPH
        printf ("pushed!\n");
#endif

        if (!data)
                data = create_data (obj);
        g_assert (data->state == INITIAL);
        g_assert (data->index == -1);
        dyn_array_ptr_push (&scan_stack, data);
}

#undef HANDLE_PTR
#define HANDLE_PTR(ptr,obj)     do {                                    \
                GCObject *dst = (GCObject*)*(ptr);                      \
                if (dst) push_object (dst);                     \
        } while (0)

// dfs () function's queue-children-of-object operation.
static void
push_all (ScanData *data)
{
        GCObject *obj = data->obj;
        char *start = (char*)obj;
        mword desc = sgen_obj_get_descriptor_safe (obj);

#if DUMP_GRAPH
        printf ("+scanning %s (%p) index %d color %p\n", safe_name_bridge (data->obj), data->obj, data->index, data->color);
#endif

        #include "sgen/sgen-scan-object.h"
}


static void
compute_low_index (ScanData *data, GCObject *obj)
{
        ScanData *other;
        ColorData *cd;

        obj = bridge_object_forward (obj);
        other = find_data (obj);

#if DUMP_GRAPH
        printf ("\tcompute low %p ->%p (%s) %p (%d / %d)\n", data->obj, obj, safe_name_bridge (obj), other, other ? other->index : -2, other ? other->low_index : -2);
#endif
        if (!other)
                return;

        g_assert (other->state != INITIAL);

        if ((other->state == SCANNED || other->state == FINISHED_ON_STACK) && data->low_index > other->low_index)
                data->low_index = other->low_index;

        /* Compute the low color */
        if (other->color == NULL)
                return;

        cd = other->color;
        if (!cd->visited) {
                color_merge_array_hash += mix_hash ((uintptr_t) other->color);
                dyn_array_ptr_add (&color_merge_array, other->color);
                cd->visited = TRUE;
        }
}

#undef HANDLE_PTR
#define HANDLE_PTR(ptr,obj)     do {                                    \
                GCObject *dst = (GCObject*)*(ptr);                      \
                if (dst) compute_low_index (data, dst);                         \
        } while (0)

static void
compute_low (ScanData *data)
{
        GCObject *obj = data->obj;
        char *start = (char*)obj;
        mword desc = sgen_obj_get_descriptor_safe (obj);

        #include "sgen/sgen-scan-object.h"
}

// A non-bridged object needs a single color describing the current merge array.
static ColorData*
reduce_color (void)
{
        ColorData *color = NULL;
        int size = dyn_array_ptr_size (&color_merge_array);

        // Merge array is empty-- this SCC points to no bridged colors.
        // This SCC can be ignored completely.
        if (size == 0)
                color = NULL;

        // Merge array has one item-- this SCC points to a single bridged color.
        // This SCC can be forwarded to the pointed-to color.
        else if (size == 1) {
                color = (ColorData *)dyn_array_ptr_get (&color_merge_array, 0);

        // This SCC gets to talk to the color allocator.
        } else
                color = new_color (FALSE);

        return color;
}

static void
create_scc (ScanData *data)
{
        int i;
        gboolean found = FALSE;
        gboolean found_bridge = FALSE;
        ColorData *color_data = NULL;

        for (i = dyn_array_ptr_size (&loop_stack) - 1; i >= 0; --i) {
                ScanData *other = (ScanData *)dyn_array_ptr_get (&loop_stack, i);
                found_bridge |= other->is_bridge;
                if (found_bridge || other == data)
                        break;
        }

#if DUMP_GRAPH
        printf ("|SCC rooted in %s (%p) has bridge %d\n", safe_name_bridge (data->obj), data->obj, found_bridge);
        printf ("\tpoints-to-colors: ");
        for (int i = 0; i < dyn_array_ptr_size (&color_merge_array); ++i)
                printf ("%p ", dyn_array_ptr_get (&color_merge_array, i));
        printf ("\n");

        printf ("loop stack: ");
        for (int i = 0; i < dyn_array_ptr_size (&loop_stack); ++i) {
                ScanData *other = dyn_array_ptr_get (&loop_stack, i);
                printf ("(%d/%d)", other->index, other->low_index);
        }
        printf ("\n");
#endif

        if (found_bridge) {
                color_data = new_color (TRUE);
                ++num_colors_with_bridges;
        } else {
                color_data = reduce_color ();
        }

        while (dyn_array_ptr_size (&loop_stack) > 0) {
                ScanData *other = (ScanData *)dyn_array_ptr_pop (&loop_stack);

#if DUMP_GRAPH
                printf ("\tmember %s (%p) index %d low-index %d color %p state %d\n", safe_name_bridge (other->obj), other->obj, other->index, other->low_index, other->color, other->state);
#endif

                other->color = color_data;
                switch (other->state) {
                case FINISHED_ON_STACK:
                        other->state = FINISHED_OFF_STACK;
                        break;
                case FINISHED_OFF_STACK:
                        break;
                default:
                        g_error ("Invalid state when building SCC %d", other->state);
                }

                if (other->is_bridge)
                        dyn_array_ptr_add (&color_data->bridges, other->obj);

                if (other == data) {
                        found = TRUE;
                        break;
                }
        }
        g_assert (found);

        for (i = 0; i < dyn_array_ptr_size (&color_merge_array); ++i) {
                ColorData *cd  = (ColorData *)dyn_array_ptr_get (&color_merge_array, i);
                g_assert (cd->visited);
                cd->visited = FALSE;
        }
        color_merge_array_empty ();
        found_bridge = FALSE;
}

static void
dfs (void)
{
        g_assert (dyn_array_ptr_size (&scan_stack) == 1);
        g_assert (dyn_array_ptr_size (&loop_stack) == 0);

        color_merge_array_empty ();

        while (dyn_array_ptr_size (&scan_stack) > 0) {
                ScanData *data = (ScanData *)dyn_array_ptr_pop (&scan_stack);

                /**
                 * Ignore finished objects on stack, they happen due to loops. For example:
                 * A -> C
                 * A -> B
                 * B -> C
                 * C -> A
                 *
                 * We start scanning from A and push C before B. So, after the first iteration, the scan stack will have: A C B.
                 * We then visit B, which will find C in its initial state and push again.
                 * Finally after finish with C and B, the stack will be left with "A C" and at this point C should be ignored.
         *
         * The above explains FINISHED_ON_STACK, to explain FINISHED_OFF_STACK, consider if the root was D, which pointed
                 * to A and C. A is processed first, leaving C on stack after that in the mentioned state.
                 */
                if (data->state == FINISHED_ON_STACK || data->state == FINISHED_OFF_STACK)
                        continue;

                if (data->state == INITIAL) {
                        g_assert (data->index == -1);
                        g_assert (data->low_index == -1);

                        data->state = SCANNED;
                        data->low_index = data->index = object_index++;
                        dyn_array_ptr_push (&scan_stack, data);
                        dyn_array_ptr_push (&loop_stack, data);

#if DUMP_GRAPH
                        printf ("+scanning %s (%p) index %d color %p\n", safe_name_bridge (data->obj), data->obj, data->index, data->color);
#endif
                        /*push all refs */
                        push_all (data);
                } else {
                        g_assert (data->state == SCANNED);
                        data->state = FINISHED_ON_STACK;

#if DUMP_GRAPH
                        printf ("-finishing %s (%p) index %d low-index %d color %p\n", safe_name_bridge (data->obj), data->obj, data->index, data->low_index, data->color);
#endif

                        /* Compute low index */
                        compute_low (data);

#if DUMP_GRAPH
                        printf ("-finished %s (%p) index %d low-index %d color %p\n", safe_name_bridge (data->obj), data->obj, data->index, data->low_index, data->color);
#endif
                        //SCC root
                        if (data->index == data->low_index)
                                create_scc (data);
                }
        }
}

static void
register_finalized_object (GCObject *obj)
{
        g_assert (sgen_need_bridge_processing ());
        dyn_array_ptr_push (&registered_bridges, obj);
}

static void
reset_data (void)
{
        dyn_array_ptr_empty (&registered_bridges);
}

static void
cleanup (void)
{
        dyn_array_ptr_empty (&scan_stack);
        dyn_array_ptr_empty (&loop_stack);
        dyn_array_ptr_empty (&registered_bridges);
        free_object_buckets ();
        free_color_buckets ();
        reset_cache ();
        object_index = 0;
        num_colors_with_bridges = 0;
}

#ifdef DUMP_GRAPH
static void
dump_color_table (const char *why, gboolean do_index)
{
        ColorBucket *cur;
        int i = 0, j;
        printf ("colors%s:\n", why);

        for (cur = root_color_bucket; cur; cur = cur->next, ++i) {
                ColorData *cd;
                for (cd = &cur->data [0]; cd < cur->next_data; ++cd) {
                        if (do_index)
                                printf ("\t%d(%d):", i, cd->api_index);
                        else
                                printf ("\t%d: ", i);
                        for (j = 0; j < dyn_array_ptr_size (&cd->other_colors); ++j) {
                                printf ("%p ", dyn_array_ptr_get (&cd->other_colors, j));
                        }
                        if (dyn_array_ptr_size (&cd->bridges)) {
                                printf (" bridges: ");
                                for (j = 0; j < dyn_array_ptr_size (&cd->bridges); ++j) {
                                        GCObject *obj = dyn_array_ptr_get (&cd->bridges, j);
                                        ScanData *data = find_or_create_data (obj);
                                        printf ("%d ", data->index);
                                }
                        }
                        printf ("\n");
                }
        }

}
#endif

static gint64
step_timer (gint64 *timer)
{
        gint64 curtime, diff;

        SGEN_TV_GETTIME (curtime); 
        diff = SGEN_TV_ELAPSED (*timer, curtime);
        *timer = curtime;
        return diff;
}
static void
processing_stw_step (void)
{
        int i;
        int bridge_count;
        gint64 curtime;

        if (!dyn_array_ptr_size (&registered_bridges))
                return;

#if defined (DUMP_GRAPH)
        printf ("-----------------\n");
#endif

        SGEN_TV_GETTIME (curtime);

        object_alloc_init ();
        color_alloc_init ();

        bridge_count = dyn_array_ptr_size (&registered_bridges);
        for (i = 0; i < bridge_count ; ++i)
                register_bridge_object ((GCObject *)dyn_array_ptr_get (&registered_bridges, i));

        setup_time = step_timer (&curtime);

        for (i = 0; i < bridge_count; ++i) {
                ScanData *sd = find_data ((GCObject *)dyn_array_ptr_get (&registered_bridges, i));
                if (sd->state == INITIAL) {
                        dyn_array_ptr_push (&scan_stack, sd);
                        dfs ();
                } else {
                        g_assert (sd->state == FINISHED_OFF_STACK);
                }
        }

        tarjan_time = step_timer (&curtime);

#if defined (DUMP_GRAPH)
        printf ("----summary----\n");
        printf ("bridges:\n");
        for (int i = 0; i < bridge_count; ++i) {
                ScanData *sd = find_data (dyn_array_ptr_get (&registered_bridges, i));
                printf ("\t%s (%p) index %d color %p\n", safe_name_bridge (sd->obj), sd->obj, sd->index, sd->color);
        }

        dump_color_table (" after tarjan", FALSE);
#endif

        clear_after_processing ();
}


static void
gather_xrefs (ColorData *color)
{
        int i;
        for (i = 0; i < dyn_array_ptr_size (&color->other_colors); ++i) {
                ColorData *src = (ColorData *)dyn_array_ptr_get (&color->other_colors, i);
                if (src->visited)
                        continue;
                src->visited = TRUE;
                if (color_visible_to_client (src))
                        dyn_array_ptr_add (&color_merge_array, src);
                else
                        gather_xrefs (src);
        }
}

static void
reset_xrefs (ColorData *color)
{
        int i;
        for (i = 0; i < dyn_array_ptr_size (&color->other_colors); ++i) {
                ColorData *src = (ColorData *)dyn_array_ptr_get (&color->other_colors, i);
                if (!src->visited)
                        continue;
                src->visited = FALSE;
                if (!color_visible_to_client (src))
                        reset_xrefs (src);
        }
}

static void
processing_build_callback_data (int generation)
{
        int j;
        gint64 curtime;
        ColorBucket *cur;

        g_assert (bridge_processor->num_sccs == 0 && bridge_processor->num_xrefs == 0);
        g_assert (!bridge_processor->api_sccs && !bridge_processor->api_xrefs);

        if (!dyn_array_ptr_size (&registered_bridges))
                return;

        SGEN_TV_GETTIME (curtime);

        /*create API objects */

#if defined (DUMP_GRAPH)
        printf ("***** API *****\n");
        printf ("number of SCCs %d\n", num_colors_with_bridges);
#endif

        // Count the number of SCCs visible to the client
        num_sccs = 0;
        for (cur = root_color_bucket; cur; cur = cur->next) {
                ColorData *cd;
                for (cd = &cur->data [0]; cd < cur->next_data; ++cd) {
                        if (color_visible_to_client (cd))
                                num_sccs++;
                }
        }

        /* This is a straightforward translation from colors to the bridge callback format. */
        MonoGCBridgeSCC **api_sccs = (MonoGCBridgeSCC **)sgen_alloc_internal_dynamic (sizeof (MonoGCBridgeSCC*) * num_sccs, INTERNAL_MEM_BRIDGE_DATA, TRUE);
        int api_index = 0;
        xref_count = 0;

        // Convert visible SCCs, along with their bridged object list, to MonoGCBridgeSCCs in the client's SCC list
        for (cur = root_color_bucket; cur; cur = cur->next) {
                ColorData *cd;
                for (cd = &cur->data [0]; cd < cur->next_data; ++cd) {
                        int bridges = dyn_array_ptr_size (&cd->bridges);
                        if (!(bridges || bridgeless_color_is_heavy (cd)))
                                continue;

                        api_sccs [api_index] = (MonoGCBridgeSCC *)sgen_alloc_internal_dynamic (sizeof (MonoGCBridgeSCC) + sizeof (MonoObject*) * bridges, INTERNAL_MEM_BRIDGE_DATA, TRUE);
                        api_sccs [api_index]->is_alive = FALSE;
                        api_sccs [api_index]->num_objs = bridges;

                        cd->api_index = api_index;

                        for (j = 0; j < bridges; ++j)
                                api_sccs [api_index]->objs [j] = (MonoObject *)dyn_array_ptr_get (&cd->bridges, j);

                        g_assert(api_index < API_INDEX_MAX);
                        api_index++;
                }
        }

        scc_setup_time = step_timer (&curtime);

        // Eliminate non-visible SCCs from the SCC list and redistribute xrefs
        for (cur = root_color_bucket; cur; cur = cur->next) {
                ColorData *cd;
                for (cd = &cur->data [0]; cd < cur->next_data; ++cd) {
                        if (!color_visible_to_client (cd))
                                continue;

                        color_merge_array_empty ();
                        gather_xrefs (cd);
                        reset_xrefs (cd);
                        dyn_array_ptr_set_all (&cd->other_colors, &color_merge_array);
                        xref_count += dyn_array_ptr_size (&cd->other_colors);
                }
        }

        gather_xref_time = step_timer (&curtime);

#if defined (DUMP_GRAPH)
        printf ("TOTAL XREFS %d\n", xref_count);
        dump_color_table (" after xref pass", TRUE);
#endif

        // Write out xrefs array
        MonoGCBridgeXRef *api_xrefs = (MonoGCBridgeXRef *)sgen_alloc_internal_dynamic (sizeof (MonoGCBridgeXRef) * xref_count, INTERNAL_MEM_BRIDGE_DATA, TRUE);
        int xref_index = 0;
        for (cur = root_color_bucket; cur; cur = cur->next) {
                ColorData *src;
                for (src = &cur->data [0]; src < cur->next_data; ++src) {
                        if (!color_visible_to_client (src))
                                continue;

                        for (j = 0; j < dyn_array_ptr_size (&src->other_colors); ++j) {
                                ColorData *dest = (ColorData *)dyn_array_ptr_get (&src->other_colors, j);
                                g_assert (color_visible_to_client (dest)); /* Supposedly we already eliminated all xrefs to non-visible objects. */

                                api_xrefs [xref_index].src_scc_index = src->api_index;
                                api_xrefs [xref_index].dst_scc_index = dest->api_index;

                                ++xref_index;
                        }
                }
        }

        g_assert (xref_count == xref_index);
        xref_setup_time = step_timer (&curtime);

#if defined (DUMP_GRAPH)
        printf ("---xrefs:\n");
        for (int i = 0; i < xref_count; ++i)
                printf ("\t%d -> %d\n", api_xrefs [i].src_scc_index, api_xrefs [i].dst_scc_index);
#endif

        //FIXME move half of the cleanup to before the bridge callback?
        bridge_processor->num_sccs = num_sccs;
        bridge_processor->api_sccs = api_sccs;
        bridge_processor->num_xrefs = xref_count;
        bridge_processor->api_xrefs = api_xrefs;
}

static void
processing_after_callback (int generation)
{
        gint64 curtime;
        int bridge_count = dyn_array_ptr_size (&registered_bridges);
        int object_count = object_data_count;
        int color_count = color_data_count;
        int colors_with_bridges_count = num_colors_with_bridges;

        SGEN_TV_GETTIME (curtime);

        /* cleanup */
        cleanup ();

        cleanup_time = step_timer (&curtime);

        mono_trace (G_LOG_LEVEL_INFO, MONO_TRACE_GC, "GC_TAR_BRIDGE bridges %d objects %d opaque %d colors %d colors-bridged %d colors-visible %d xref %d cache-hit %d cache-%s %d cache-miss %d setup %.2fms tarjan %.2fms scc-setup %.2fms gather-xref %.2fms xref-setup %.2fms cleanup %.2fms",
                bridge_count, object_count, ignored_objects,
                color_count, colors_with_bridges_count, num_sccs, xref_count,
                cache_hits, (scc_precise_merge ? "abstain" : "semihit"), cache_semihits, cache_misses,
                setup_time / 10000.0f,
                tarjan_time / 10000.0f,
                scc_setup_time / 10000.0f,
                gather_xref_time / 10000.0f,
                xref_setup_time / 10000.0f,
                cleanup_time / 10000.0f);

        cache_hits = cache_semihits = cache_misses = 0;
        ignored_objects = 0;
}

static void
describe_pointer (GCObject *obj)
{
        // HashEntry *entry;
        int i;

        for (i = 0; i < dyn_array_ptr_size (&registered_bridges); ++i) {
                if (obj == dyn_array_ptr_get (&registered_bridges, i)) {
                        printf ("Pointer is a registered bridge object.\n");
                        break;
                }
        }

        // entry = sgen_hash_table_lookup (&hash_table, obj);
        // if (!entry)
        //      return;
        //
        // printf ("Bridge hash table entry %p:\n", entry);
        // printf ("  is bridge: %d\n", (int)entry->is_bridge);
        // printf ("  is visited: %d\n", (int)entry->is_visited);
}

static void
set_config (const SgenBridgeProcessorConfig *config)
{
        if (config->scc_precise_merge) {
                hash_perturb = 0;
                scc_precise_merge = TRUE;
        }
}

void
sgen_tarjan_bridge_init (SgenBridgeProcessor *collector)
{
        collector->reset_data = reset_data;
        collector->processing_stw_step = processing_stw_step;
        collector->processing_build_callback_data = processing_build_callback_data;
        collector->processing_after_callback = processing_after_callback;
        collector->class_kind = class_kind;
        collector->register_finalized_object = register_finalized_object;
        collector->describe_pointer = describe_pointer;
        collector->set_config = set_config;

        sgen_register_fixed_internal_mem_type (INTERNAL_MEM_TARJAN_OBJ_BUCKET, BUCKET_SIZE);
        g_assert (sizeof (ObjectBucket) <= BUCKET_SIZE);
        g_assert (sizeof (ColorBucket) <= BUCKET_SIZE);
        g_assert (API_INDEX_BITS + INCOMING_COLORS_BITS <= 31);
        bridge_processor = collector;
}

#endif