Reverting merge from trunk

[official-gcc.git] / libgo / runtime / mgc0.c

// Copyright 2009 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

// Garbage collector.

#include <unistd.h>

#include "runtime.h"
#include "arch.h"
#include "malloc.h"
#include "mgc0.h"
#include "race.h"
#include "go-type.h"

// Map gccgo field names to gc field names.
// Slice aka __go_open_array.
#define array __values
#define cap __capacity
// Iface aka __go_interface
#define tab __methods
// Eface aka __go_empty_interface.
#define type __type_descriptor
// Hmap aka __go_map
typedef struct __go_map Hmap;
// Type aka __go_type_descriptor
#define kind __code
#define string __reflection
#define KindPtr GO_PTR
#define KindNoPointers GO_NO_POINTERS
// PtrType aka __go_ptr_type
#define elem __element_type

#ifdef USING_SPLIT_STACK

extern void * __splitstack_find (void *, void *, size_t *, void **, void **,
                                 void **);

extern void * __splitstack_find_context (void *context[10], size_t *, void **,
                                         void **, void **);

#endif

enum {
        Debug = 0,
        DebugMark = 0,  // run second pass to check mark
        CollectStats = 0,
        ScanStackByFrames = 0,
        IgnorePreciseGC = 0,

        // Four bits per word (see #defines below).
        wordsPerBitmapWord = sizeof(void*)*8/4,
        bitShift = sizeof(void*)*8/4,

        handoffThreshold = 4,
        IntermediateBufferCapacity = 64,

        // Bits in type information
        PRECISE = 1,
        LOOP = 2,
        PC_BITS = PRECISE | LOOP,

        // Pointer map
        BitsPerPointer = 2,
        BitsNoPointer = 0,
        BitsPointer = 1,
        BitsIface = 2,
        BitsEface = 3,
};

// Bits in per-word bitmap.
// #defines because enum might not be able to hold the values.
//
// Each word in the bitmap describes wordsPerBitmapWord words
// of heap memory.  There are 4 bitmap bits dedicated to each heap word,
// so on a 64-bit system there is one bitmap word per 16 heap words.
// The bits in the word are packed together by type first, then by
// heap location, so each 64-bit bitmap word consists of, from top to bottom,
// the 16 bitSpecial bits for the corresponding heap words, then the 16 bitMarked bits,
// then the 16 bitNoScan/bitBlockBoundary bits, then the 16 bitAllocated bits.
// This layout makes it easier to iterate over the bits of a given type.
//
// The bitmap starts at mheap.arena_start and extends *backward* from
// there.  On a 64-bit system the off'th word in the arena is tracked by
// the off/16+1'th word before mheap.arena_start.  (On a 32-bit system,
// the only difference is that the divisor is 8.)
//
// To pull out the bits corresponding to a given pointer p, we use:
//
//      off = p - (uintptr*)mheap.arena_start;  // word offset
//      b = (uintptr*)mheap.arena_start - off/wordsPerBitmapWord - 1;
//      shift = off % wordsPerBitmapWord
//      bits = *b >> shift;
//      /* then test bits & bitAllocated, bits & bitMarked, etc. */
//
#define bitAllocated            ((uintptr)1<<(bitShift*0))
#define bitNoScan               ((uintptr)1<<(bitShift*1))      /* when bitAllocated is set */
#define bitMarked               ((uintptr)1<<(bitShift*2))      /* when bitAllocated is set */
#define bitSpecial              ((uintptr)1<<(bitShift*3))      /* when bitAllocated is set - has finalizer or being profiled */
#define bitBlockBoundary        ((uintptr)1<<(bitShift*1))      /* when bitAllocated is NOT set */

#define bitMask (bitBlockBoundary | bitAllocated | bitMarked | bitSpecial)

// Holding worldsema grants an M the right to try to stop the world.
// The procedure is:
//
//      runtime_semacquire(&runtime_worldsema);
//      m->gcing = 1;
//      runtime_stoptheworld();
//
//      ... do stuff ...
//
//      m->gcing = 0;
//      runtime_semrelease(&runtime_worldsema);
//      runtime_starttheworld();
//
uint32 runtime_worldsema = 1;

// The size of Workbuf is N*PageSize.
typedef struct Workbuf Workbuf;
struct Workbuf
{
#define SIZE (2*PageSize-sizeof(LFNode)-sizeof(uintptr))
        LFNode  node; // must be first
        uintptr nobj;
        Obj     obj[SIZE/sizeof(Obj) - 1];
        uint8   _padding[SIZE%sizeof(Obj) + sizeof(Obj)];
#undef SIZE
};

typedef struct Finalizer Finalizer;
struct Finalizer
{
        FuncVal *fn;
        void *arg;
        const struct __go_func_type *ft;
        const struct __go_ptr_type *ot;
};

typedef struct FinBlock FinBlock;
struct FinBlock
{
        FinBlock *alllink;
        FinBlock *next;
        int32 cnt;
        int32 cap;
        Finalizer fin[1];
};

static G *fing;
static FinBlock *finq; // list of finalizers that are to be executed
static FinBlock *finc; // cache of free blocks
static FinBlock *allfin; // list of all blocks
static Lock finlock;
static int32 fingwait;

static void runfinq(void*);
static Workbuf* getempty(Workbuf*);
static Workbuf* getfull(Workbuf*);
static void     putempty(Workbuf*);
static Workbuf* handoff(Workbuf*);
static void     gchelperstart(void);

static struct {
        uint64  full;  // lock-free list of full blocks
        uint64  empty; // lock-free list of empty blocks
        byte    pad0[CacheLineSize]; // prevents false-sharing between full/empty and nproc/nwait
        uint32  nproc;
        volatile uint32 nwait;
        volatile uint32 ndone;
        volatile uint32 debugmarkdone;
        Note    alldone;
        ParFor  *markfor;
        ParFor  *sweepfor;

        Lock;
        byte    *chunk;
        uintptr nchunk;

        Obj     *roots;
        uint32  nroot;
        uint32  rootcap;
} work;

enum {
        GC_DEFAULT_PTR = GC_NUM_INSTR,
        GC_CHAN,

        GC_NUM_INSTR2
};

static struct {
        struct {
                uint64 sum;
                uint64 cnt;
        } ptr;
        uint64 nbytes;
        struct {
                uint64 sum;
                uint64 cnt;
                uint64 notype;
                uint64 typelookup;
        } obj;
        uint64 rescan;
        uint64 rescanbytes;
        uint64 instr[GC_NUM_INSTR2];
        uint64 putempty;
        uint64 getfull;
        struct {
                uint64 foundbit;
                uint64 foundword;
                uint64 foundspan;
        } flushptrbuf;
        struct {
                uint64 foundbit;
                uint64 foundword;
                uint64 foundspan;
        } markonly;
} gcstats;

// markonly marks an object. It returns true if the object
// has been marked by this function, false otherwise.
// This function doesn't append the object to any buffer.
static bool
markonly(void *obj)
{
        byte *p;
        uintptr *bitp, bits, shift, x, xbits, off, j;
        MSpan *s;
        PageID k;

        // Words outside the arena cannot be pointers.
        if((byte*)obj < runtime_mheap.arena_start || (byte*)obj >= runtime_mheap.arena_used)
                return false;

        // obj may be a pointer to a live object.
        // Try to find the beginning of the object.

        // Round down to word boundary.
        obj = (void*)((uintptr)obj & ~((uintptr)PtrSize-1));

        // Find bits for this word.
        off = (uintptr*)obj - (uintptr*)runtime_mheap.arena_start;
        bitp = (uintptr*)runtime_mheap.arena_start - off/wordsPerBitmapWord - 1;
        shift = off % wordsPerBitmapWord;
        xbits = *bitp;
        bits = xbits >> shift;

        // Pointing at the beginning of a block?
        if((bits & (bitAllocated|bitBlockBoundary)) != 0) {
                if(CollectStats)
                        runtime_xadd64(&gcstats.markonly.foundbit, 1);
                goto found;
        }

        // Pointing just past the beginning?
        // Scan backward a little to find a block boundary.
        for(j=shift; j-->0; ) {
                if(((xbits>>j) & (bitAllocated|bitBlockBoundary)) != 0) {
                        shift = j;
                        bits = xbits>>shift;
                        if(CollectStats)
                                runtime_xadd64(&gcstats.markonly.foundword, 1);
                        goto found;
                }
        }

        // Otherwise consult span table to find beginning.
        // (Manually inlined copy of MHeap_LookupMaybe.)
        k = (uintptr)obj>>PageShift;
        x = k;
        if(sizeof(void*) == 8)
                x -= (uintptr)runtime_mheap.arena_start>>PageShift;
        s = runtime_mheap.spans[x];
        if(s == nil || k < s->start || (byte*)obj >= s->limit || s->state != MSpanInUse)
                return false;
        p = (byte*)((uintptr)s->start<<PageShift);
        if(s->sizeclass == 0) {
                obj = p;
        } else {
                uintptr size = s->elemsize;
                int32 i = ((byte*)obj - p)/size;
                obj = p+i*size;
        }

        // Now that we know the object header, reload bits.
        off = (uintptr*)obj - (uintptr*)runtime_mheap.arena_start;
        bitp = (uintptr*)runtime_mheap.arena_start - off/wordsPerBitmapWord - 1;
        shift = off % wordsPerBitmapWord;
        xbits = *bitp;
        bits = xbits >> shift;
        if(CollectStats)
                runtime_xadd64(&gcstats.markonly.foundspan, 1);

found:
        // Now we have bits, bitp, and shift correct for
        // obj pointing at the base of the object.
        // Only care about allocated and not marked.
        if((bits & (bitAllocated|bitMarked)) != bitAllocated)
                return false;
        if(work.nproc == 1)
                *bitp |= bitMarked<<shift;
        else {
                for(;;) {
                        x = *bitp;
                        if(x & (bitMarked<<shift))
                                return false;
                        if(runtime_casp((void**)bitp, (void*)x, (void*)(x|(bitMarked<<shift))))
                                break;
                }
        }

        // The object is now marked
        return true;
}

// PtrTarget is a structure used by intermediate buffers.
// The intermediate buffers hold GC data before it
// is moved/flushed to the work buffer (Workbuf).
// The size of an intermediate buffer is very small,
// such as 32 or 64 elements.
typedef struct PtrTarget PtrTarget;
struct PtrTarget
{
        void *p;
        uintptr ti;
};

typedef struct BufferList BufferList;
struct BufferList
{
        PtrTarget ptrtarget[IntermediateBufferCapacity];
        Obj obj[IntermediateBufferCapacity];
        uint32 busy;
        byte pad[CacheLineSize];
};
static BufferList bufferList[MaxGcproc];

static Type *itabtype;

static void enqueue(Obj obj, Workbuf **_wbuf, Obj **_wp, uintptr *_nobj);

// flushptrbuf moves data from the PtrTarget buffer to the work buffer.
// The PtrTarget buffer contains blocks irrespective of whether the blocks have been marked or scanned,
// while the work buffer contains blocks which have been marked
// and are prepared to be scanned by the garbage collector.
//
// _wp, _wbuf, _nobj are input/output parameters and are specifying the work buffer.
//
// A simplified drawing explaining how the todo-list moves from a structure to another:
//
//     scanblock
//  (find pointers)
//    Obj ------> PtrTarget (pointer targets)
//     ↑          |
//     |          |
//     `----------'
//     flushptrbuf
//  (find block start, mark and enqueue)
static void
flushptrbuf(PtrTarget *ptrbuf, PtrTarget **ptrbufpos, Obj **_wp, Workbuf **_wbuf, uintptr *_nobj)
{
        byte *p, *arena_start, *obj;
        uintptr size, *bitp, bits, shift, j, x, xbits, off, nobj, ti, n;
        MSpan *s;
        PageID k;
        Obj *wp;
        Workbuf *wbuf;
        PtrTarget *ptrbuf_end;

        arena_start = runtime_mheap.arena_start;

        wp = *_wp;
        wbuf = *_wbuf;
        nobj = *_nobj;

        ptrbuf_end = *ptrbufpos;
        n = ptrbuf_end - ptrbuf;
        *ptrbufpos = ptrbuf;

        if(CollectStats) {
                runtime_xadd64(&gcstats.ptr.sum, n);
                runtime_xadd64(&gcstats.ptr.cnt, 1);
        }

        // If buffer is nearly full, get a new one.
        if(wbuf == nil || nobj+n >= nelem(wbuf->obj)) {
                if(wbuf != nil)
                        wbuf->nobj = nobj;
                wbuf = getempty(wbuf);
                wp = wbuf->obj;
                nobj = 0;

                if(n >= nelem(wbuf->obj))
                        runtime_throw("ptrbuf has to be smaller than WorkBuf");
        }

        // TODO(atom): This block is a branch of an if-then-else statement.
        //             The single-threaded branch may be added in a next CL.
        {
                // Multi-threaded version.

                while(ptrbuf < ptrbuf_end) {
                        obj = ptrbuf->p;
                        ti = ptrbuf->ti;
                        ptrbuf++;

                        // obj belongs to interval [mheap.arena_start, mheap.arena_used).
                        if(Debug > 1) {
                                if(obj < runtime_mheap.arena_start || obj >= runtime_mheap.arena_used)
                                        runtime_throw("object is outside of mheap");
                        }

                        // obj may be a pointer to a live object.
                        // Try to find the beginning of the object.

                        // Round down to word boundary.
                        if(((uintptr)obj & ((uintptr)PtrSize-1)) != 0) {
                                obj = (void*)((uintptr)obj & ~((uintptr)PtrSize-1));
                                ti = 0;
                        }

                        // Find bits for this word.
                        off = (uintptr*)obj - (uintptr*)arena_start;
                        bitp = (uintptr*)arena_start - off/wordsPerBitmapWord - 1;
                        shift = off % wordsPerBitmapWord;
                        xbits = *bitp;
                        bits = xbits >> shift;

                        // Pointing at the beginning of a block?
                        if((bits & (bitAllocated|bitBlockBoundary)) != 0) {
                                if(CollectStats)
                                        runtime_xadd64(&gcstats.flushptrbuf.foundbit, 1);
                                goto found;
                        }

                        ti = 0;

                        // Pointing just past the beginning?
                        // Scan backward a little to find a block boundary.
                        for(j=shift; j-->0; ) {
                                if(((xbits>>j) & (bitAllocated|bitBlockBoundary)) != 0) {
                                        obj = (byte*)obj - (shift-j)*PtrSize;
                                        shift = j;
                                        bits = xbits>>shift;
                                        if(CollectStats)
                                                runtime_xadd64(&gcstats.flushptrbuf.foundword, 1);
                                        goto found;
                                }
                        }

                        // Otherwise consult span table to find beginning.
                        // (Manually inlined copy of MHeap_LookupMaybe.)
                        k = (uintptr)obj>>PageShift;
                        x = k;
                        if(sizeof(void*) == 8)
                                x -= (uintptr)arena_start>>PageShift;
                        s = runtime_mheap.spans[x];
                        if(s == nil || k < s->start || obj >= s->limit || s->state != MSpanInUse)
                                continue;
                        p = (byte*)((uintptr)s->start<<PageShift);
                        if(s->sizeclass == 0) {
                                obj = p;
                        } else {
                                size = s->elemsize;
                                int32 i = ((byte*)obj - p)/size;
                                obj = p+i*size;
                        }

                        // Now that we know the object header, reload bits.
                        off = (uintptr*)obj - (uintptr*)arena_start;
                        bitp = (uintptr*)arena_start - off/wordsPerBitmapWord - 1;
                        shift = off % wordsPerBitmapWord;
                        xbits = *bitp;
                        bits = xbits >> shift;
                        if(CollectStats)
                                runtime_xadd64(&gcstats.flushptrbuf.foundspan, 1);

                found:
                        // Now we have bits, bitp, and shift correct for
                        // obj pointing at the base of the object.
                        // Only care about allocated and not marked.
                        if((bits & (bitAllocated|bitMarked)) != bitAllocated)
                                continue;
                        if(work.nproc == 1)
                                *bitp |= bitMarked<<shift;
                        else {
                                for(;;) {
                                        x = *bitp;
                                        if(x & (bitMarked<<shift))
                                                goto continue_obj;
                                        if(runtime_casp((void**)bitp, (void*)x, (void*)(x|(bitMarked<<shift))))
                                                break;
                                }
                        }

                        // If object has no pointers, don't need to scan further.
                        if((bits & bitNoScan) != 0)
                                continue;

                        // Ask span about size class.
                        // (Manually inlined copy of MHeap_Lookup.)
                        x = (uintptr)obj >> PageShift;
                        if(sizeof(void*) == 8)
                                x -= (uintptr)arena_start>>PageShift;
                        s = runtime_mheap.spans[x];

                        PREFETCH(obj);

                        *wp = (Obj){obj, s->elemsize, ti};
                        wp++;
                        nobj++;
                continue_obj:;
                }

                // If another proc wants a pointer, give it some.
                if(work.nwait > 0 && nobj > handoffThreshold && work.full == 0) {
                        wbuf->nobj = nobj;
                        wbuf = handoff(wbuf);
                        nobj = wbuf->nobj;
                        wp = wbuf->obj + nobj;
                }
        }

        *_wp = wp;
        *_wbuf = wbuf;
        *_nobj = nobj;
}

static void
flushobjbuf(Obj *objbuf, Obj **objbufpos, Obj **_wp, Workbuf **_wbuf, uintptr *_nobj)
{
        uintptr nobj, off;
        Obj *wp, obj;
        Workbuf *wbuf;
        Obj *objbuf_end;

        wp = *_wp;
        wbuf = *_wbuf;
        nobj = *_nobj;

        objbuf_end = *objbufpos;
        *objbufpos = objbuf;

        while(objbuf < objbuf_end) {
                obj = *objbuf++;

                // Align obj.b to a word boundary.
                off = (uintptr)obj.p & (PtrSize-1);
                if(off != 0) {
                        obj.p += PtrSize - off;
                        obj.n -= PtrSize - off;
                        obj.ti = 0;
                }

                if(obj.p == nil || obj.n == 0)
                        continue;

                // If buffer is full, get a new one.
                if(wbuf == nil || nobj >= nelem(wbuf->obj)) {
                        if(wbuf != nil)
                                wbuf->nobj = nobj;
                        wbuf = getempty(wbuf);
                        wp = wbuf->obj;
                        nobj = 0;
                }

                *wp = obj;
                wp++;
                nobj++;
        }

        // If another proc wants a pointer, give it some.
        if(work.nwait > 0 && nobj > handoffThreshold && work.full == 0) {
                wbuf->nobj = nobj;
                wbuf = handoff(wbuf);
                nobj = wbuf->nobj;
                wp = wbuf->obj + nobj;
        }

        *_wp = wp;
        *_wbuf = wbuf;
        *_nobj = nobj;
}

// Program that scans the whole block and treats every block element as a potential pointer
static uintptr defaultProg[2] = {PtrSize, GC_DEFAULT_PTR};

#if 0
// Hchan program
static uintptr chanProg[2] = {0, GC_CHAN};
#endif

// Local variables of a program fragment or loop
typedef struct Frame Frame;
struct Frame {
        uintptr count, elemsize, b;
        uintptr *loop_or_ret;
};

// Sanity check for the derived type info objti.
static void
checkptr(void *obj, uintptr objti)
{
        uintptr type, tisize, i, x;
        byte *objstart;
        Type *t;
        MSpan *s;

        if(!Debug)
                runtime_throw("checkptr is debug only");

        if((byte*)obj < runtime_mheap.arena_start || (byte*)obj >= runtime_mheap.arena_used)
                return;
        type = runtime_gettype(obj);
        t = (Type*)(type & ~(uintptr)(PtrSize-1));
        if(t == nil)
                return;
        x = (uintptr)obj >> PageShift;
        if(sizeof(void*) == 8)
                x -= (uintptr)(runtime_mheap.arena_start)>>PageShift;
        s = runtime_mheap.spans[x];
        objstart = (byte*)((uintptr)s->start<<PageShift);
        if(s->sizeclass != 0) {
                i = ((byte*)obj - objstart)/s->elemsize;
                objstart += i*s->elemsize;
        }
        tisize = *(uintptr*)objti;
        // Sanity check for object size: it should fit into the memory block.
        if((byte*)obj + tisize > objstart + s->elemsize) {
                runtime_printf("object of type '%S' at %p/%p does not fit in block %p/%p\n",
                               *t->string, obj, tisize, objstart, s->elemsize);
                runtime_throw("invalid gc type info");
        }
        if(obj != objstart)
                return;
        // If obj points to the beginning of the memory block,
        // check type info as well.
        if(t->string == nil ||
                // Gob allocates unsafe pointers for indirection.
                (runtime_strcmp((const char *)t->string->str, (const char*)"unsafe.Pointer") &&
                // Runtime and gc think differently about closures.
                 runtime_strstr((const char *)t->string->str, (const char*)"struct { F uintptr") != (const char *)t->string->str)) {
#if 0
                pc1 = (uintptr*)objti;
                pc2 = (uintptr*)t->gc;
                // A simple best-effort check until first GC_END.
                for(j = 1; pc1[j] != GC_END && pc2[j] != GC_END; j++) {
                        if(pc1[j] != pc2[j]) {
                                runtime_printf("invalid gc type info for '%s' at %p, type info %p, block info %p\n",
                                               t->string ? (const int8*)t->string->str : (const int8*)"?", j, pc1[j], pc2[j]);
                                runtime_throw("invalid gc type info");
                        }
                }
#endif
        }
}                                       

// scanblock scans a block of n bytes starting at pointer b for references
// to other objects, scanning any it finds recursively until there are no
// unscanned objects left.  Instead of using an explicit recursion, it keeps
// a work list in the Workbuf* structures and loops in the main function
// body.  Keeping an explicit work list is easier on the stack allocator and
// more efficient.
//
// wbuf: current work buffer
// wp:   storage for next queued pointer (write pointer)
// nobj: number of queued objects
static void
scanblock(Workbuf *wbuf, Obj *wp, uintptr nobj, bool keepworking)
{
        byte *b, *arena_start, *arena_used;
        uintptr n, i, end_b, elemsize, size, ti, objti, count /* , type */;
        uintptr *pc, precise_type, nominal_size;
#if 0
        uintptr *chan_ret, chancap;
#endif
        void *obj;
        const Type *t;
        Slice *sliceptr;
        Frame *stack_ptr, stack_top, stack[GC_STACK_CAPACITY+4];
        BufferList *scanbuffers;
        PtrTarget *ptrbuf, *ptrbuf_end, *ptrbufpos;
        Obj *objbuf, *objbuf_end, *objbufpos;
        Eface *eface;
        Iface *iface;
#if 0
        Hchan *chan;
        ChanType *chantype;
#endif

        if(sizeof(Workbuf) % PageSize != 0)
                runtime_throw("scanblock: size of Workbuf is suboptimal");

        // Memory arena parameters.
        arena_start = runtime_mheap.arena_start;
        arena_used = runtime_mheap.arena_used;

        stack_ptr = stack+nelem(stack)-1;
        
        precise_type = false;
        nominal_size = 0;

        // Allocate ptrbuf
        {
                scanbuffers = &bufferList[runtime_m()->helpgc];
                ptrbuf = &scanbuffers->ptrtarget[0];
                ptrbuf_end = &scanbuffers->ptrtarget[0] + nelem(scanbuffers->ptrtarget);
                objbuf = &scanbuffers->obj[0];
                objbuf_end = &scanbuffers->obj[0] + nelem(scanbuffers->obj);
        }

        ptrbufpos = ptrbuf;
        objbufpos = objbuf;

        // (Silence the compiler)
#if 0
        chan = nil;
        chantype = nil;
        chan_ret = nil;
#endif

        goto next_block;

        for(;;) {
                // Each iteration scans the block b of length n, queueing pointers in
                // the work buffer.
                if(Debug > 1) {
                        runtime_printf("scanblock %p %D\n", b, (int64)n);
                }

                if(CollectStats) {
                        runtime_xadd64(&gcstats.nbytes, n);
                        runtime_xadd64(&gcstats.obj.sum, nobj);
                        runtime_xadd64(&gcstats.obj.cnt, 1);
                }

                if(ti != 0 && false) {
                        pc = (uintptr*)(ti & ~(uintptr)PC_BITS);
                        precise_type = (ti & PRECISE);
                        stack_top.elemsize = pc[0];
                        if(!precise_type)
                                nominal_size = pc[0];
                        if(ti & LOOP) {
                                stack_top.count = 0;    // 0 means an infinite number of iterations
                                stack_top.loop_or_ret = pc+1;
                        } else {
                                stack_top.count = 1;
                        }
                        if(Debug) {
                                // Simple sanity check for provided type info ti:
                                // The declared size of the object must be not larger than the actual size
                                // (it can be smaller due to inferior pointers).
                                // It's difficult to make a comprehensive check due to inferior pointers,
                                // reflection, gob, etc.
                                if(pc[0] > n) {
                                        runtime_printf("invalid gc type info: type info size %p, block size %p\n", pc[0], n);
                                        runtime_throw("invalid gc type info");
                                }
                        }
                } else if(UseSpanType && false) {
                        if(CollectStats)
                                runtime_xadd64(&gcstats.obj.notype, 1);

#if 0
                        type = runtime_gettype(b);
                        if(type != 0) {
                                if(CollectStats)
                                        runtime_xadd64(&gcstats.obj.typelookup, 1);

                                t = (Type*)(type & ~(uintptr)(PtrSize-1));
                                switch(type & (PtrSize-1)) {
                                case TypeInfo_SingleObject:
                                        pc = (uintptr*)t->gc;
                                        precise_type = true;  // type information about 'b' is precise
                                        stack_top.count = 1;
                                        stack_top.elemsize = pc[0];
                                        break;
                                case TypeInfo_Array:
                                        pc = (uintptr*)t->gc;
                                        if(pc[0] == 0)
                                                goto next_block;
                                        precise_type = true;  // type information about 'b' is precise
                                        stack_top.count = 0;  // 0 means an infinite number of iterations
                                        stack_top.elemsize = pc[0];
                                        stack_top.loop_or_ret = pc+1;
                                        break;
                                case TypeInfo_Chan:
                                        chan = (Hchan*)b;
                                        chantype = (ChanType*)t;
                                        chan_ret = nil;
                                        pc = chanProg;
                                        break;
                                default:
                                        runtime_throw("scanblock: invalid type");
                                        return;
                                }
                        } else {
                                pc = defaultProg;
                        }
#endif
                } else {
                        pc = defaultProg;
                }

                if(IgnorePreciseGC)
                        pc = defaultProg;

                pc++;
                stack_top.b = (uintptr)b;

                end_b = (uintptr)b + n - PtrSize;

        for(;;) {
                if(CollectStats)
                        runtime_xadd64(&gcstats.instr[pc[0]], 1);

                obj = nil;
                objti = 0;
                switch(pc[0]) {
                case GC_PTR:
                        obj = *(void**)(stack_top.b + pc[1]);
                        objti = pc[2];
                        pc += 3;
                        if(Debug)
                                checkptr(obj, objti);
                        break;

                case GC_SLICE:
                        sliceptr = (Slice*)(stack_top.b + pc[1]);
                        if(sliceptr->cap != 0) {
                                obj = sliceptr->array;
                                // Can't use slice element type for scanning,
                                // because if it points to an array embedded
                                // in the beginning of a struct,
                                // we will scan the whole struct as the slice.
                                // So just obtain type info from heap.
                        }
                        pc += 3;
                        break;

                case GC_APTR:
                        obj = *(void**)(stack_top.b + pc[1]);
                        pc += 2;
                        break;

                case GC_STRING:
                        obj = *(void**)(stack_top.b + pc[1]);
                        markonly(obj);
                        pc += 2;
                        continue;

                case GC_EFACE:
                        eface = (Eface*)(stack_top.b + pc[1]);
                        pc += 2;
                        if(eface->type == nil)
                                continue;

                        // eface->type
                        t = eface->type;
                        if((const byte*)t >= arena_start && (const byte*)t < arena_used) {
                                union { const Type *tc; Type *tr; } u;
                                u.tc = t;
                                *ptrbufpos++ = (struct PtrTarget){(void*)u.tr, 0};
                                if(ptrbufpos == ptrbuf_end)
                                        flushptrbuf(ptrbuf, &ptrbufpos, &wp, &wbuf, &nobj);
                        }

                        // eface->__object
                        if((byte*)eface->__object >= arena_start && (byte*)eface->__object < arena_used) {
                                if(t->__size <= sizeof(void*)) {
                                        if((t->kind & KindNoPointers))
                                                continue;

                                        obj = eface->__object;
                                        if((t->kind & ~KindNoPointers) == KindPtr)
                                                // objti = (uintptr)((PtrType*)t)->elem->gc;
                                                objti = 0;
                                } else {
                                        obj = eface->__object;
                                        // objti = (uintptr)t->gc;
                                        objti = 0;
                                }
                        }
                        break;

                case GC_IFACE:
                        iface = (Iface*)(stack_top.b + pc[1]);
                        pc += 2;
                        if(iface->tab == nil)
                                continue;
                        
                        // iface->tab
                        if((byte*)iface->tab >= arena_start && (byte*)iface->tab < arena_used) {
                                // *ptrbufpos++ = (struct PtrTarget){iface->tab, (uintptr)itabtype->gc};
                                *ptrbufpos++ = (struct PtrTarget){iface->tab, 0};
                                if(ptrbufpos == ptrbuf_end)
                                        flushptrbuf(ptrbuf, &ptrbufpos, &wp, &wbuf, &nobj);
                        }

                        // iface->data
                        if((byte*)iface->__object >= arena_start && (byte*)iface->__object < arena_used) {
                                // t = iface->tab->type;
                                t = nil;
                                if(t->__size <= sizeof(void*)) {
                                        if((t->kind & KindNoPointers))
                                                continue;

                                        obj = iface->__object;
                                        if((t->kind & ~KindNoPointers) == KindPtr)
                                                // objti = (uintptr)((const PtrType*)t)->elem->gc;
                                                objti = 0;
                                } else {
                                        obj = iface->__object;
                                        // objti = (uintptr)t->gc;
                                        objti = 0;
                                }
                        }
                        break;

                case GC_DEFAULT_PTR:
                        while(stack_top.b <= end_b) {
                                obj = *(byte**)stack_top.b;
                                stack_top.b += PtrSize;
                                if((byte*)obj >= arena_start && (byte*)obj < arena_used) {
                                        *ptrbufpos++ = (struct PtrTarget){obj, 0};
                                        if(ptrbufpos == ptrbuf_end)
                                                flushptrbuf(ptrbuf, &ptrbufpos, &wp, &wbuf, &nobj);
                                }
                        }
                        goto next_block;

                case GC_END:
                        if(--stack_top.count != 0) {
                                // Next iteration of a loop if possible.
                                stack_top.b += stack_top.elemsize;
                                if(stack_top.b + stack_top.elemsize <= end_b+PtrSize) {
                                        pc = stack_top.loop_or_ret;
                                        continue;
                                }
                                i = stack_top.b;
                        } else {
                                // Stack pop if possible.
                                if(stack_ptr+1 < stack+nelem(stack)) {
                                        pc = stack_top.loop_or_ret;
                                        stack_top = *(++stack_ptr);
                                        continue;
                                }
                                i = (uintptr)b + nominal_size;
                        }
                        if(!precise_type) {
                                // Quickly scan [b+i,b+n) for possible pointers.
                                for(; i<=end_b; i+=PtrSize) {
                                        if(*(byte**)i != nil) {
                                                // Found a value that may be a pointer.
                                                // Do a rescan of the entire block.
                                                enqueue((Obj){b, n, 0}, &wbuf, &wp, &nobj);
                                                if(CollectStats) {
                                                        runtime_xadd64(&gcstats.rescan, 1);
                                                        runtime_xadd64(&gcstats.rescanbytes, n);
                                                }
                                                break;
                                        }
                                }
                        }
                        goto next_block;

                case GC_ARRAY_START:
                        i = stack_top.b + pc[1];
                        count = pc[2];
                        elemsize = pc[3];
                        pc += 4;

                        // Stack push.
                        *stack_ptr-- = stack_top;
                        stack_top = (Frame){count, elemsize, i, pc};
                        continue;

                case GC_ARRAY_NEXT:
                        if(--stack_top.count != 0) {
                                stack_top.b += stack_top.elemsize;
                                pc = stack_top.loop_or_ret;
                        } else {
                                // Stack pop.
                                stack_top = *(++stack_ptr);
                                pc += 1;
                        }
                        continue;

                case GC_CALL:
                        // Stack push.
                        *stack_ptr-- = stack_top;
                        stack_top = (Frame){1, 0, stack_top.b + pc[1], pc+3 /*return address*/};
                        pc = (uintptr*)((byte*)pc + *(int32*)(pc+2));  // target of the CALL instruction
                        continue;

                case GC_REGION:
                        obj = (void*)(stack_top.b + pc[1]);
                        size = pc[2];
                        objti = pc[3];
                        pc += 4;

                        *objbufpos++ = (Obj){obj, size, objti};
                        if(objbufpos == objbuf_end)
                                flushobjbuf(objbuf, &objbufpos, &wp, &wbuf, &nobj);
                        continue;

#if 0
                case GC_CHAN_PTR:
                        chan = *(Hchan**)(stack_top.b + pc[1]);
                        if(chan == nil) {
                                pc += 3;
                                continue;
                        }
                        if(markonly(chan)) {
                                chantype = (ChanType*)pc[2];
                                if(!(chantype->elem->kind & KindNoPointers)) {
                                        // Start chanProg.
                                        chan_ret = pc+3;
                                        pc = chanProg+1;
                                        continue;
                                }
                        }
                        pc += 3;
                        continue;

                case GC_CHAN:
                        // There are no heap pointers in struct Hchan,
                        // so we can ignore the leading sizeof(Hchan) bytes.
                        if(!(chantype->elem->kind & KindNoPointers)) {
                                // Channel's buffer follows Hchan immediately in memory.
                                // Size of buffer (cap(c)) is second int in the chan struct.
                                chancap = ((uintgo*)chan)[1];
                                if(chancap > 0) {
                                        // TODO(atom): split into two chunks so that only the
                                        // in-use part of the circular buffer is scanned.
                                        // (Channel routines zero the unused part, so the current
                                        // code does not lead to leaks, it's just a little inefficient.)
                                        *objbufpos++ = (Obj){(byte*)chan+runtime_Hchansize, chancap*chantype->elem->size,
                                                (uintptr)chantype->elem->gc | PRECISE | LOOP};
                                        if(objbufpos == objbuf_end)
                                                flushobjbuf(objbuf, &objbufpos, &wp, &wbuf, &nobj);
                                }
                        }
                        if(chan_ret == nil)
                                goto next_block;
                        pc = chan_ret;
                        continue;
#endif

                default:
                        runtime_throw("scanblock: invalid GC instruction");
                        return;
                }

                if((byte*)obj >= arena_start && (byte*)obj < arena_used) {
                        *ptrbufpos++ = (struct PtrTarget){obj, objti};
                        if(ptrbufpos == ptrbuf_end)
                                flushptrbuf(ptrbuf, &ptrbufpos, &wp, &wbuf, &nobj);
                }
        }

        next_block:
                // Done scanning [b, b+n).  Prepare for the next iteration of
                // the loop by setting b, n, ti to the parameters for the next block.

                if(nobj == 0) {
                        flushptrbuf(ptrbuf, &ptrbufpos, &wp, &wbuf, &nobj);
                        flushobjbuf(objbuf, &objbufpos, &wp, &wbuf, &nobj);

                        if(nobj == 0) {
                                if(!keepworking) {
                                        if(wbuf)
                                                putempty(wbuf);
                                        goto endscan;
                                }
                                // Emptied our buffer: refill.
                                wbuf = getfull(wbuf);
                                if(wbuf == nil)
                                        goto endscan;
                                nobj = wbuf->nobj;
                                wp = wbuf->obj + wbuf->nobj;
                        }
                }

                // Fetch b from the work buffer.
                --wp;
                b = wp->p;
                n = wp->n;
                ti = wp->ti;
                nobj--;
        }

endscan:;
}

// debug_scanblock is the debug copy of scanblock.
// it is simpler, slower, single-threaded, recursive,
// and uses bitSpecial as the mark bit.
static void
debug_scanblock(byte *b, uintptr n)
{
        byte *obj, *p;
        void **vp;
        uintptr size, *bitp, bits, shift, i, xbits, off;
        MSpan *s;

        if(!DebugMark)
                runtime_throw("debug_scanblock without DebugMark");

        if((intptr)n < 0) {
                runtime_printf("debug_scanblock %p %D\n", b, (int64)n);
                runtime_throw("debug_scanblock");
        }

        // Align b to a word boundary.
        off = (uintptr)b & (PtrSize-1);
        if(off != 0) {
                b += PtrSize - off;
                n -= PtrSize - off;
        }

        vp = (void**)b;
        n /= PtrSize;
        for(i=0; i<(uintptr)n; i++) {
                obj = (byte*)vp[i];

                // Words outside the arena cannot be pointers.
                if((byte*)obj < runtime_mheap.arena_start || (byte*)obj >= runtime_mheap.arena_used)
                        continue;

                // Round down to word boundary.
                obj = (void*)((uintptr)obj & ~((uintptr)PtrSize-1));

                // Consult span table to find beginning.
                s = runtime_MHeap_LookupMaybe(&runtime_mheap, obj);
                if(s == nil)
                        continue;

                p =  (byte*)((uintptr)s->start<<PageShift);
                size = s->elemsize;
                if(s->sizeclass == 0) {
                        obj = p;
                } else {
                        int32 i = ((byte*)obj - p)/size;
                        obj = p+i*size;
                }

                // Now that we know the object header, reload bits.
                off = (uintptr*)obj - (uintptr*)runtime_mheap.arena_start;
                bitp = (uintptr*)runtime_mheap.arena_start - off/wordsPerBitmapWord - 1;
                shift = off % wordsPerBitmapWord;
                xbits = *bitp;
                bits = xbits >> shift;

                // Now we have bits, bitp, and shift correct for
                // obj pointing at the base of the object.
                // If not allocated or already marked, done.
                if((bits & bitAllocated) == 0 || (bits & bitSpecial) != 0)  // NOTE: bitSpecial not bitMarked
                        continue;
                *bitp |= bitSpecial<<shift;
                if(!(bits & bitMarked))
                        runtime_printf("found unmarked block %p in %p\n", obj, vp+i);

                // If object has no pointers, don't need to scan further.
                if((bits & bitNoScan) != 0)
                        continue;

                debug_scanblock(obj, size);
        }
}

// Append obj to the work buffer.
// _wbuf, _wp, _nobj are input/output parameters and are specifying the work buffer.
static void
enqueue(Obj obj, Workbuf **_wbuf, Obj **_wp, uintptr *_nobj)
{
        uintptr nobj, off;
        Obj *wp;
        Workbuf *wbuf;

        if(Debug > 1)
                runtime_printf("append obj(%p %D %p)\n", obj.p, (int64)obj.n, obj.ti);

        // Align obj.b to a word boundary.
        off = (uintptr)obj.p & (PtrSize-1);
        if(off != 0) {
                obj.p += PtrSize - off;
                obj.n -= PtrSize - off;
                obj.ti = 0;
        }

        if(obj.p == nil || obj.n == 0)
                return;

        // Load work buffer state
        wp = *_wp;
        wbuf = *_wbuf;
        nobj = *_nobj;

        // If another proc wants a pointer, give it some.
        if(work.nwait > 0 && nobj > handoffThreshold && work.full == 0) {
                wbuf->nobj = nobj;
                wbuf = handoff(wbuf);
                nobj = wbuf->nobj;
                wp = wbuf->obj + nobj;
        }

        // If buffer is full, get a new one.
        if(wbuf == nil || nobj >= nelem(wbuf->obj)) {
                if(wbuf != nil)
                        wbuf->nobj = nobj;
                wbuf = getempty(wbuf);
                wp = wbuf->obj;
                nobj = 0;
        }

        *wp = obj;
        wp++;
        nobj++;

        // Save work buffer state
        *_wp = wp;
        *_wbuf = wbuf;
        *_nobj = nobj;
}

static void
markroot(ParFor *desc, uint32 i)
{
        Obj *wp;
        Workbuf *wbuf;
        uintptr nobj;

        USED(&desc);
        wp = nil;
        wbuf = nil;
        nobj = 0;
        enqueue(work.roots[i], &wbuf, &wp, &nobj);
        scanblock(wbuf, wp, nobj, false);
}

// Get an empty work buffer off the work.empty list,
// allocating new buffers as needed.
static Workbuf*
getempty(Workbuf *b)
{
        if(b != nil)
                runtime_lfstackpush(&work.full, &b->node);
        b = (Workbuf*)runtime_lfstackpop(&work.empty);
        if(b == nil) {
                // Need to allocate.
                runtime_lock(&work);
                if(work.nchunk < sizeof *b) {
                        work.nchunk = 1<<20;
                        work.chunk = runtime_SysAlloc(work.nchunk, &mstats.gc_sys);
                        if(work.chunk == nil)
                                runtime_throw("runtime: cannot allocate memory");
                }
                b = (Workbuf*)work.chunk;
                work.chunk += sizeof *b;
                work.nchunk -= sizeof *b;
                runtime_unlock(&work);
        }
        b->nobj = 0;
        return b;
}

static void
putempty(Workbuf *b)
{
        if(CollectStats)
                runtime_xadd64(&gcstats.putempty, 1);

        runtime_lfstackpush(&work.empty, &b->node);
}

// Get a full work buffer off the work.full list, or return nil.
static Workbuf*
getfull(Workbuf *b)
{
        M *m;
        int32 i;

        if(CollectStats)
                runtime_xadd64(&gcstats.getfull, 1);

        if(b != nil)
                runtime_lfstackpush(&work.empty, &b->node);
        b = (Workbuf*)runtime_lfstackpop(&work.full);
        if(b != nil || work.nproc == 1)
                return b;

        m = runtime_m();
        runtime_xadd(&work.nwait, +1);
        for(i=0;; i++) {
                if(work.full != 0) {
                        runtime_xadd(&work.nwait, -1);
                        b = (Workbuf*)runtime_lfstackpop(&work.full);
                        if(b != nil)
                                return b;
                        runtime_xadd(&work.nwait, +1);
                }
                if(work.nwait == work.nproc)
                        return nil;
                if(i < 10) {
                        m->gcstats.nprocyield++;
                        runtime_procyield(20);
                } else if(i < 20) {
                        m->gcstats.nosyield++;
                        runtime_osyield();
                } else {
                        m->gcstats.nsleep++;
                        runtime_usleep(100);
                }
        }
}

static Workbuf*
handoff(Workbuf *b)
{
        M *m;
        int32 n;
        Workbuf *b1;

        m = runtime_m();

        // Make new buffer with half of b's pointers.
        b1 = getempty(nil);
        n = b->nobj/2;
        b->nobj -= n;
        b1->nobj = n;
        runtime_memmove(b1->obj, b->obj+b->nobj, n*sizeof b1->obj[0]);
        m->gcstats.nhandoff++;
        m->gcstats.nhandoffcnt += n;

        // Put b on full list - let first half of b get stolen.
        runtime_lfstackpush(&work.full, &b->node);
        return b1;
}

static void
addroot(Obj obj)
{
        uint32 cap;
        Obj *new;

        if(work.nroot >= work.rootcap) {
                cap = PageSize/sizeof(Obj);
                if(cap < 2*work.rootcap)
                        cap = 2*work.rootcap;
                new = (Obj*)runtime_SysAlloc(cap*sizeof(Obj), &mstats.gc_sys);
                if(new == nil)
                        runtime_throw("runtime: cannot allocate memory");
                if(work.roots != nil) {
                        runtime_memmove(new, work.roots, work.rootcap*sizeof(Obj));
                        runtime_SysFree(work.roots, work.rootcap*sizeof(Obj), &mstats.gc_sys);
                }
                work.roots = new;
                work.rootcap = cap;
        }
        work.roots[work.nroot] = obj;
        work.nroot++;
}

static void
addstackroots(G *gp)
{
#ifdef USING_SPLIT_STACK
        M *mp;
        void* sp;
        size_t spsize;
        void* next_segment;
        void* next_sp;
        void* initial_sp;

        if(gp == runtime_g()) {
                // Scanning our own stack.
                sp = __splitstack_find(nil, nil, &spsize, &next_segment,
                                       &next_sp, &initial_sp);
        } else if((mp = gp->m) != nil && mp->helpgc) {
                // gchelper's stack is in active use and has no interesting pointers.
                return;
        } else {
                // Scanning another goroutine's stack.
                // The goroutine is usually asleep (the world is stopped).

                // The exception is that if the goroutine is about to enter or might
                // have just exited a system call, it may be executing code such
                // as schedlock and may have needed to start a new stack segment.
                // Use the stack segment and stack pointer at the time of
                // the system call instead, since that won't change underfoot.
                if(gp->gcstack != nil) {
                        sp = gp->gcstack;
                        spsize = gp->gcstack_size;
                        next_segment = gp->gcnext_segment;
                        next_sp = gp->gcnext_sp;
                        initial_sp = gp->gcinitial_sp;
                } else {
                        sp = __splitstack_find_context(&gp->stack_context[0],
                                                       &spsize, &next_segment,
                                                       &next_sp, &initial_sp);
                }
        }
        if(sp != nil) {
                addroot((Obj){sp, spsize, 0});
                while((sp = __splitstack_find(next_segment, next_sp,
                                              &spsize, &next_segment,
                                              &next_sp, &initial_sp)) != nil)
                        addroot((Obj){sp, spsize, 0});
        }
#else
        M *mp;
        byte* bottom;
        byte* top;

        if(gp == runtime_g()) {
                // Scanning our own stack.
                bottom = (byte*)&gp;
        } else if((mp = gp->m) != nil && mp->helpgc) {
                // gchelper's stack is in active use and has no interesting pointers.
                return;
        } else {
                // Scanning another goroutine's stack.
                // The goroutine is usually asleep (the world is stopped).
                bottom = (byte*)gp->gcnext_sp;
                if(bottom == nil)
                        return;
        }
        top = (byte*)gp->gcinitial_sp + gp->gcstack_size;
        if(top > bottom)
                addroot((Obj){bottom, top - bottom, 0});
        else
                addroot((Obj){top, bottom - top, 0});
#endif
}

static void
addfinroots(void *v)
{
        uintptr size;
        void *base;

        size = 0;
        if(!runtime_mlookup(v, (byte**)&base, &size, nil) || !runtime_blockspecial(base))
                runtime_throw("mark - finalizer inconsistency");

        // do not mark the finalizer block itself.  just mark the things it points at.
        addroot((Obj){base, size, 0});
}

static struct root_list* roots;

void
__go_register_gc_roots (struct root_list* r)
{
        // FIXME: This needs locking if multiple goroutines can call
        // dlopen simultaneously.
        r->next = roots;
        roots = r;
}

static void
addroots(void)
{
        struct root_list *pl;
        G *gp;
        FinBlock *fb;
        MSpan *s, **allspans;
        uint32 spanidx;

        work.nroot = 0;

        // mark data+bss.
        for(pl = roots; pl != nil; pl = pl->next) {
                struct root* pr = &pl->roots[0];
                while(1) {
                        void *decl = pr->decl;
                        if(decl == nil)
                                break;
                        addroot((Obj){decl, pr->size, 0});
                        pr++;
                }
        }

        addroot((Obj){(byte*)&runtime_m0, sizeof runtime_m0, 0});
        addroot((Obj){(byte*)&runtime_g0, sizeof runtime_g0, 0});
        addroot((Obj){(byte*)&runtime_allg, sizeof runtime_allg, 0});
        addroot((Obj){(byte*)&runtime_allm, sizeof runtime_allm, 0});
        addroot((Obj){(byte*)&runtime_allp, sizeof runtime_allp, 0});
        runtime_proc_scan(addroot);
        runtime_MProf_Mark(addroot);
        runtime_time_scan(addroot);
        runtime_netpoll_scan(addroot);

        // MSpan.types
        allspans = runtime_mheap.allspans;
        for(spanidx=0; spanidx<runtime_mheap.nspan; spanidx++) {
                s = allspans[spanidx];
                if(s->state == MSpanInUse) {
                        // The garbage collector ignores type pointers stored in MSpan.types:
                        //  - Compiler-generated types are stored outside of heap.
                        //  - The reflect package has runtime-generated types cached in its data structures.
                        //    The garbage collector relies on finding the references via that cache.
                        switch(s->types.compression) {
                        case MTypes_Empty:
                        case MTypes_Single:
                                break;
                        case MTypes_Words:
                        case MTypes_Bytes:
                                markonly((byte*)s->types.data);
                                break;
                        }
                }
        }

        // stacks
        for(gp=runtime_allg; gp!=nil; gp=gp->alllink) {
                switch(gp->status){
                default:
                        runtime_printf("unexpected G.status %d\n", gp->status);
                        runtime_throw("mark - bad status");
                case Gdead:
                        break;
                case Grunning:
                        runtime_throw("mark - world not stopped");
                case Grunnable:
                case Gsyscall:
                case Gwaiting:
                        addstackroots(gp);
                        break;
                }
        }

        runtime_walkfintab(addfinroots, addroot);

        for(fb=allfin; fb; fb=fb->alllink)
                addroot((Obj){(byte*)fb->fin, fb->cnt*sizeof(fb->fin[0]), 0});

        addroot((Obj){(byte*)&work, sizeof work, 0});
}

static bool
handlespecial(byte *p, uintptr size)
{
        FuncVal *fn;
        const struct __go_func_type *ft;
        const struct __go_ptr_type *ot;
        FinBlock *block;
        Finalizer *f;
        
        if(!runtime_getfinalizer(p, true, &fn, &ft, &ot)) {
                runtime_setblockspecial(p, false);
                runtime_MProf_Free(p, size);
                return false;
        }

        runtime_lock(&finlock);
        if(finq == nil || finq->cnt == finq->cap) {
                if(finc == nil) {
                        finc = runtime_persistentalloc(PageSize, 0, &mstats.gc_sys);
                        finc->cap = (PageSize - sizeof(FinBlock)) / sizeof(Finalizer) + 1;
                        finc->alllink = allfin;
                        allfin = finc;
                }
                block = finc;
                finc = block->next;
                block->next = finq;
                finq = block;
        }
        f = &finq->fin[finq->cnt];
        finq->cnt++;
        f->fn = fn;
        f->ft = ft;
        f->ot = ot;
        f->arg = p;
        runtime_unlock(&finlock);
        return true;
}

// Sweep frees or collects finalizers for blocks not marked in the mark phase.
// It clears the mark bits in preparation for the next GC round.
static void
sweepspan(ParFor *desc, uint32 idx)
{
        M *m;
        int32 cl, n, npages;
        uintptr size;
        byte *p;
        MCache *c;
        byte *arena_start;
        MLink head, *end;
        int32 nfree;
        byte *type_data;
        byte compression;
        uintptr type_data_inc;
        MSpan *s;

        m = runtime_m();

        USED(&desc);
        s = runtime_mheap.allspans[idx];
        if(s->state != MSpanInUse)
                return;
        arena_start = runtime_mheap.arena_start;
        p = (byte*)(s->start << PageShift);
        cl = s->sizeclass;
        size = s->elemsize;
        if(cl == 0) {
                n = 1;
        } else {
                // Chunk full of small blocks.
                npages = runtime_class_to_allocnpages[cl];
                n = (npages << PageShift) / size;
        }
        nfree = 0;
        end = &head;
        c = m->mcache;
        
        type_data = (byte*)s->types.data;
        type_data_inc = sizeof(uintptr);
        compression = s->types.compression;
        switch(compression) {
        case MTypes_Bytes:
                type_data += 8*sizeof(uintptr);
                type_data_inc = 1;
                break;
        }

        // Sweep through n objects of given size starting at p.
        // This thread owns the span now, so it can manipulate
        // the block bitmap without atomic operations.
        for(; n > 0; n--, p += size, type_data+=type_data_inc) {
                uintptr off, *bitp, shift, bits;

                off = (uintptr*)p - (uintptr*)arena_start;
                bitp = (uintptr*)arena_start - off/wordsPerBitmapWord - 1;
                shift = off % wordsPerBitmapWord;
                bits = *bitp>>shift;

                if((bits & bitAllocated) == 0)
                        continue;

                if((bits & bitMarked) != 0) {
                        if(DebugMark) {
                                if(!(bits & bitSpecial))
                                        runtime_printf("found spurious mark on %p\n", p);
                                *bitp &= ~(bitSpecial<<shift);
                        }
                        *bitp &= ~(bitMarked<<shift);
                        continue;
                }

                // Special means it has a finalizer or is being profiled.
                // In DebugMark mode, the bit has been coopted so
                // we have to assume all blocks are special.
                if(DebugMark || (bits & bitSpecial) != 0) {
                        if(handlespecial(p, size))
                                continue;
                }

                // Mark freed; restore block boundary bit.
                *bitp = (*bitp & ~(bitMask<<shift)) | (bitBlockBoundary<<shift);

                if(cl == 0) {
                        // Free large span.
                        runtime_unmarkspan(p, 1<<PageShift);
                        *(uintptr*)p = (uintptr)0xdeaddeaddeaddeadll;   // needs zeroing
                        runtime_MHeap_Free(&runtime_mheap, s, 1);
                        c->local_nlargefree++;
                        c->local_largefree += size;
                } else {
                        // Free small object.
                        switch(compression) {
                        case MTypes_Words:
                                *(uintptr*)type_data = 0;
                                break;
                        case MTypes_Bytes:
                                *(byte*)type_data = 0;
                                break;
                        }
                        if(size > sizeof(uintptr))
                                ((uintptr*)p)[1] = (uintptr)0xdeaddeaddeaddeadll;       // mark as "needs to be zeroed"
                        
                        end->next = (MLink*)p;
                        end = (MLink*)p;
                        nfree++;
                }
        }

        if(nfree) {
                c->local_nsmallfree[cl] += nfree;
                c->local_cachealloc -= nfree * size;
                runtime_MCentral_FreeSpan(&runtime_mheap.central[cl], s, nfree, head.next, end);
        }
}

static void
dumpspan(uint32 idx)
{
        int32 sizeclass, n, npages, i, column;
        uintptr size;
        byte *p;
        byte *arena_start;
        MSpan *s;
        bool allocated, special;

        s = runtime_mheap.allspans[idx];
        if(s->state != MSpanInUse)
                return;
        arena_start = runtime_mheap.arena_start;
        p = (byte*)(s->start << PageShift);
        sizeclass = s->sizeclass;
        size = s->elemsize;
        if(sizeclass == 0) {
                n = 1;
        } else {
                npages = runtime_class_to_allocnpages[sizeclass];
                n = (npages << PageShift) / size;
        }
        
        runtime_printf("%p .. %p:\n", p, p+n*size);
        column = 0;
        for(; n>0; n--, p+=size) {
                uintptr off, *bitp, shift, bits;

                off = (uintptr*)p - (uintptr*)arena_start;
                bitp = (uintptr*)arena_start - off/wordsPerBitmapWord - 1;
                shift = off % wordsPerBitmapWord;
                bits = *bitp>>shift;

                allocated = ((bits & bitAllocated) != 0);
                special = ((bits & bitSpecial) != 0);

                for(i=0; (uint32)i<size; i+=sizeof(void*)) {
                        if(column == 0) {
                                runtime_printf("\t");
                        }
                        if(i == 0) {
                                runtime_printf(allocated ? "(" : "[");
                                runtime_printf(special ? "@" : "");
                                runtime_printf("%p: ", p+i);
                        } else {
                                runtime_printf(" ");
                        }

                        runtime_printf("%p", *(void**)(p+i));

                        if(i+sizeof(void*) >= size) {
                                runtime_printf(allocated ? ") " : "] ");
                        }

                        column++;
                        if(column == 8) {
                                runtime_printf("\n");
                                column = 0;
                        }
                }
        }
        runtime_printf("\n");
}

// A debugging function to dump the contents of memory
void
runtime_memorydump(void)
{
        uint32 spanidx;

        for(spanidx=0; spanidx<runtime_mheap.nspan; spanidx++) {
                dumpspan(spanidx);
        }
}

void
runtime_gchelper(void)
{
        gchelperstart();

        // parallel mark for over gc roots
        runtime_parfordo(work.markfor);

        // help other threads scan secondary blocks
        scanblock(nil, nil, 0, true);

        if(DebugMark) {
                // wait while the main thread executes mark(debug_scanblock)
                while(runtime_atomicload(&work.debugmarkdone) == 0)
                        runtime_usleep(10);
        }

        runtime_parfordo(work.sweepfor);
        bufferList[runtime_m()->helpgc].busy = 0;
        if(runtime_xadd(&work.ndone, +1) == work.nproc-1)
                runtime_notewakeup(&work.alldone);
}

#define GcpercentUnknown (-2)

// Initialized from $GOGC.  GOGC=off means no gc.
//
// Next gc is after we've allocated an extra amount of
// memory proportional to the amount already in use.
// If gcpercent=100 and we're using 4M, we'll gc again
// when we get to 8M.  This keeps the gc cost in linear
// proportion to the allocation cost.  Adjusting gcpercent
// just changes the linear constant (and also the amount of
// extra memory used).
static int32 gcpercent = GcpercentUnknown;

static void
cachestats(void)
{
        MCache *c;
        P *p, **pp;

        for(pp=runtime_allp; (p=*pp) != nil; pp++) {
                c = p->mcache;
                if(c==nil)
                        continue;
                runtime_purgecachedstats(c);
        }
}

static void
updatememstats(GCStats *stats)
{
        M *mp;
        MSpan *s;
        MCache *c;
        P *p, **pp;
        uint32 i;
        uint64 stacks_inuse, smallfree;
        uint64 *src, *dst;

        if(stats)
                runtime_memclr((byte*)stats, sizeof(*stats));
        stacks_inuse = 0;
        for(mp=runtime_allm; mp; mp=mp->alllink) {
                //stacks_inuse += mp->stackinuse*FixedStack;
                if(stats) {
                        src = (uint64*)&mp->gcstats;
                        dst = (uint64*)stats;
                        for(i=0; i<sizeof(*stats)/sizeof(uint64); i++)
                                dst[i] += src[i];
                        runtime_memclr((byte*)&mp->gcstats, sizeof(mp->gcstats));
                }
        }
        mstats.stacks_inuse = stacks_inuse;
        mstats.mcache_inuse = runtime_mheap.cachealloc.inuse;
        mstats.mspan_inuse = runtime_mheap.spanalloc.inuse;
        mstats.sys = mstats.heap_sys + mstats.stacks_sys + mstats.mspan_sys +
                mstats.mcache_sys + mstats.buckhash_sys + mstats.gc_sys + mstats.other_sys;
        
        // Calculate memory allocator stats.
        // During program execution we only count number of frees and amount of freed memory.
        // Current number of alive object in the heap and amount of alive heap memory
        // are calculated by scanning all spans.
        // Total number of mallocs is calculated as number of frees plus number of alive objects.
        // Similarly, total amount of allocated memory is calculated as amount of freed memory
        // plus amount of alive heap memory.
        mstats.alloc = 0;
        mstats.total_alloc = 0;
        mstats.nmalloc = 0;
        mstats.nfree = 0;
        for(i = 0; i < nelem(mstats.by_size); i++) {
                mstats.by_size[i].nmalloc = 0;
                mstats.by_size[i].nfree = 0;
        }

        // Flush MCache's to MCentral.
        for(pp=runtime_allp; (p=*pp) != nil; pp++) {
                c = p->mcache;
                if(c==nil)
                        continue;
                runtime_MCache_ReleaseAll(c);
        }

        // Aggregate local stats.
        cachestats();

        // Scan all spans and count number of alive objects.
        for(i = 0; i < runtime_mheap.nspan; i++) {
                s = runtime_mheap.allspans[i];
                if(s->state != MSpanInUse)
                        continue;
                if(s->sizeclass == 0) {
                        mstats.nmalloc++;
                        mstats.alloc += s->elemsize;
                } else {
                        mstats.nmalloc += s->ref;
                        mstats.by_size[s->sizeclass].nmalloc += s->ref;
                        mstats.alloc += s->ref*s->elemsize;
                }
        }

        // Aggregate by size class.
        smallfree = 0;
        mstats.nfree = runtime_mheap.nlargefree;
        for(i = 0; i < nelem(mstats.by_size); i++) {
                mstats.nfree += runtime_mheap.nsmallfree[i];
                mstats.by_size[i].nfree = runtime_mheap.nsmallfree[i];
                mstats.by_size[i].nmalloc += runtime_mheap.nsmallfree[i];
                smallfree += runtime_mheap.nsmallfree[i] * runtime_class_to_size[i];
        }
        mstats.nmalloc += mstats.nfree;

        // Calculate derived stats.
        mstats.total_alloc = mstats.alloc + runtime_mheap.largefree + smallfree;
        mstats.heap_alloc = mstats.alloc;
        mstats.heap_objects = mstats.nmalloc - mstats.nfree;
}

// Structure of arguments passed to function gc().
// This allows the arguments to be passed via runtime_mcall.
struct gc_args
{
        int64 start_time; // start time of GC in ns (just before stoptheworld)
};

static void gc(struct gc_args *args);
static void mgc(G *gp);

static int32
readgogc(void)
{
        const byte *p;

        p = runtime_getenv("GOGC");
        if(p == nil || p[0] == '\0')
                return 100;
        if(runtime_strcmp((const char *)p, "off") == 0)
                return -1;
        return runtime_atoi(p);
}

void
runtime_gc(int32 force)
{
        M *m;
        G *g;
        struct gc_args a;
        int32 i;

        // The atomic operations are not atomic if the uint64s
        // are not aligned on uint64 boundaries. This has been
        // a problem in the past.
        if((((uintptr)&work.empty) & 7) != 0)
                runtime_throw("runtime: gc work buffer is misaligned");
        if((((uintptr)&work.full) & 7) != 0)
                runtime_throw("runtime: gc work buffer is misaligned");

        // Make sure all registers are saved on stack so that
        // scanstack sees them.
        __builtin_unwind_init();

        // The gc is turned off (via enablegc) until
        // the bootstrap has completed.
        // Also, malloc gets called in the guts
        // of a number of libraries that might be
        // holding locks.  To avoid priority inversion
        // problems, don't bother trying to run gc
        // while holding a lock.  The next mallocgc
        // without a lock will do the gc instead.
        m = runtime_m();
        if(!mstats.enablegc || runtime_g() == m->g0 || m->locks > 0 || runtime_panicking)
                return;

        if(gcpercent == GcpercentUnknown) {     // first time through
                runtime_lock(&runtime_mheap);
                if(gcpercent == GcpercentUnknown)
                        gcpercent = readgogc();
                runtime_unlock(&runtime_mheap);
        }
        if(gcpercent < 0)
                return;

        runtime_semacquire(&runtime_worldsema, false);
        if(!force && mstats.heap_alloc < mstats.next_gc) {
                // typically threads which lost the race to grab
                // worldsema exit here when gc is done.
                runtime_semrelease(&runtime_worldsema);
                return;
        }

        // Ok, we're doing it!  Stop everybody else
        a.start_time = runtime_nanotime();
        m->gcing = 1;
        runtime_stoptheworld();
        
        // Run gc on the g0 stack.  We do this so that the g stack
        // we're currently running on will no longer change.  Cuts
        // the root set down a bit (g0 stacks are not scanned, and
        // we don't need to scan gc's internal state).  Also an
        // enabler for copyable stacks.
        for(i = 0; i < (runtime_debug.gctrace > 1 ? 2 : 1); i++) {
                // switch to g0, call gc(&a), then switch back
                g = runtime_g();
                g->param = &a;
                g->status = Gwaiting;
                g->waitreason = "garbage collection";
                runtime_mcall(mgc);
                // record a new start time in case we're going around again
                a.start_time = runtime_nanotime();
        }

        // all done
        m->gcing = 0;
        m->locks++;
        runtime_semrelease(&runtime_worldsema);
        runtime_starttheworld();
        m->locks--;

        // now that gc is done, kick off finalizer thread if needed
        if(finq != nil) {
                runtime_lock(&finlock);
                // kick off or wake up goroutine to run queued finalizers
                if(fing == nil)
                        fing = __go_go(runfinq, nil);
                else if(fingwait) {
                        fingwait = 0;
                        runtime_ready(fing);
                }
                runtime_unlock(&finlock);
        }
        // give the queued finalizers, if any, a chance to run
        runtime_gosched();
}

static void
mgc(G *gp)
{
        gc(gp->param);
        gp->param = nil;
        gp->status = Grunning;
        runtime_gogo(gp);
}

static void
gc(struct gc_args *args)
{
        M *m;
        int64 t0, t1, t2, t3, t4;
        uint64 heap0, heap1, obj0, obj1, ninstr;
        GCStats stats;
        M *mp;
        uint32 i;
        // Eface eface;

        m = runtime_m();

        t0 = args->start_time;

        if(CollectStats)
                runtime_memclr((byte*)&gcstats, sizeof(gcstats));

        for(mp=runtime_allm; mp; mp=mp->alllink)
                runtime_settype_flush(mp);

        heap0 = 0;
        obj0 = 0;
        if(runtime_debug.gctrace) {
                updatememstats(nil);
                heap0 = mstats.heap_alloc;
                obj0 = mstats.nmalloc - mstats.nfree;
        }

        m->locks++;     // disable gc during mallocs in parforalloc
        if(work.markfor == nil)
                work.markfor = runtime_parforalloc(MaxGcproc);
        if(work.sweepfor == nil)
                work.sweepfor = runtime_parforalloc(MaxGcproc);
        m->locks--;

        if(itabtype == nil) {
                // get C pointer to the Go type "itab"
                // runtime_gc_itab_ptr(&eface);
                // itabtype = ((PtrType*)eface.type)->elem;
        }

        work.nwait = 0;
        work.ndone = 0;
        work.debugmarkdone = 0;
        work.nproc = runtime_gcprocs();
        addroots();
        runtime_parforsetup(work.markfor, work.nproc, work.nroot, nil, false, markroot);
        runtime_parforsetup(work.sweepfor, work.nproc, runtime_mheap.nspan, nil, true, sweepspan);
        if(work.nproc > 1) {
                runtime_noteclear(&work.alldone);
                runtime_helpgc(work.nproc);
        }

        t1 = runtime_nanotime();

        gchelperstart();
        runtime_parfordo(work.markfor);
        scanblock(nil, nil, 0, true);

        if(DebugMark) {
                for(i=0; i<work.nroot; i++)
                        debug_scanblock(work.roots[i].p, work.roots[i].n);
                runtime_atomicstore(&work.debugmarkdone, 1);
        }
        t2 = runtime_nanotime();

        runtime_parfordo(work.sweepfor);
        bufferList[m->helpgc].busy = 0;
        t3 = runtime_nanotime();

        if(work.nproc > 1)
                runtime_notesleep(&work.alldone);

        cachestats();
        mstats.next_gc = mstats.heap_alloc+mstats.heap_alloc*gcpercent/100;

        t4 = runtime_nanotime();
        mstats.last_gc = t4;
        mstats.pause_ns[mstats.numgc%nelem(mstats.pause_ns)] = t4 - t0;
        mstats.pause_total_ns += t4 - t0;
        mstats.numgc++;
        if(mstats.debuggc)
                runtime_printf("pause %D\n", t4-t0);

        if(runtime_debug.gctrace) {
                updatememstats(&stats);
                heap1 = mstats.heap_alloc;
                obj1 = mstats.nmalloc - mstats.nfree;

                stats.nprocyield += work.sweepfor->nprocyield;
                stats.nosyield += work.sweepfor->nosyield;
                stats.nsleep += work.sweepfor->nsleep;

                runtime_printf("gc%d(%d): %D+%D+%D ms, %D -> %D MB %D -> %D (%D-%D) objects,"
                                " %D(%D) handoff, %D(%D) steal, %D/%D/%D yields\n",
                        mstats.numgc, work.nproc, (t2-t1)/1000000, (t3-t2)/1000000, (t1-t0+t4-t3)/1000000,
                        heap0>>20, heap1>>20, obj0, obj1,
                        mstats.nmalloc, mstats.nfree,
                        stats.nhandoff, stats.nhandoffcnt,
                        work.sweepfor->nsteal, work.sweepfor->nstealcnt,
                        stats.nprocyield, stats.nosyield, stats.nsleep);
                if(CollectStats) {
                        runtime_printf("scan: %D bytes, %D objects, %D untyped, %D types from MSpan\n",
                                gcstats.nbytes, gcstats.obj.cnt, gcstats.obj.notype, gcstats.obj.typelookup);
                        if(gcstats.ptr.cnt != 0)
                                runtime_printf("avg ptrbufsize: %D (%D/%D)\n",
                                        gcstats.ptr.sum/gcstats.ptr.cnt, gcstats.ptr.sum, gcstats.ptr.cnt);
                        if(gcstats.obj.cnt != 0)
                                runtime_printf("avg nobj: %D (%D/%D)\n",
                                        gcstats.obj.sum/gcstats.obj.cnt, gcstats.obj.sum, gcstats.obj.cnt);
                        runtime_printf("rescans: %D, %D bytes\n", gcstats.rescan, gcstats.rescanbytes);

                        runtime_printf("instruction counts:\n");
                        ninstr = 0;
                        for(i=0; i<nelem(gcstats.instr); i++) {
                                runtime_printf("\t%d:\t%D\n", i, gcstats.instr[i]);
                                ninstr += gcstats.instr[i];
                        }
                        runtime_printf("\ttotal:\t%D\n", ninstr);

                        runtime_printf("putempty: %D, getfull: %D\n", gcstats.putempty, gcstats.getfull);

                        runtime_printf("markonly base lookup: bit %D word %D span %D\n", gcstats.markonly.foundbit, gcstats.markonly.foundword, gcstats.markonly.foundspan);
                        runtime_printf("flushptrbuf base lookup: bit %D word %D span %D\n", gcstats.flushptrbuf.foundbit, gcstats.flushptrbuf.foundword, gcstats.flushptrbuf.foundspan);
                }
        }

        runtime_MProf_GC();
}

void runtime_ReadMemStats(MStats *)
  __asm__ (GOSYM_PREFIX "runtime.ReadMemStats");

void
runtime_ReadMemStats(MStats *stats)
{
        M *m;

        // Have to acquire worldsema to stop the world,
        // because stoptheworld can only be used by
        // one goroutine at a time, and there might be
        // a pending garbage collection already calling it.
        runtime_semacquire(&runtime_worldsema, false);
        m = runtime_m();
        m->gcing = 1;
        runtime_stoptheworld();
        updatememstats(nil);
        *stats = mstats;
        m->gcing = 0;
        m->locks++;
        runtime_semrelease(&runtime_worldsema);
        runtime_starttheworld();
        m->locks--;
}

void runtime_debug_readGCStats(Slice*)
  __asm__("runtime_debug.readGCStats");

void
runtime_debug_readGCStats(Slice *pauses)
{
        uint64 *p;
        uint32 i, n;

        // Calling code in runtime/debug should make the slice large enough.
        if((size_t)pauses->cap < nelem(mstats.pause_ns)+3)
                runtime_throw("runtime: short slice passed to readGCStats");

        // Pass back: pauses, last gc (absolute time), number of gc, total pause ns.
        p = (uint64*)pauses->array;
        runtime_lock(&runtime_mheap);
        n = mstats.numgc;
        if(n > nelem(mstats.pause_ns))
                n = nelem(mstats.pause_ns);
        
        // The pause buffer is circular. The most recent pause is at
        // pause_ns[(numgc-1)%nelem(pause_ns)], and then backward
        // from there to go back farther in time. We deliver the times
        // most recent first (in p[0]).
        for(i=0; i<n; i++)
                p[i] = mstats.pause_ns[(mstats.numgc-1-i)%nelem(mstats.pause_ns)];

        p[n] = mstats.last_gc;
        p[n+1] = mstats.numgc;
        p[n+2] = mstats.pause_total_ns; 
        runtime_unlock(&runtime_mheap);
        pauses->__count = n+3;
}

intgo runtime_debug_setGCPercent(intgo)
  __asm__("runtime_debug.setGCPercent");

intgo
runtime_debug_setGCPercent(intgo in)
{
        intgo out;

        runtime_lock(&runtime_mheap);
        if(gcpercent == GcpercentUnknown)
                gcpercent = readgogc();
        out = gcpercent;
        if(in < 0)
                in = -1;
        gcpercent = in;
        runtime_unlock(&runtime_mheap);
        return out;
}

static void
gchelperstart(void)
{
        M *m;

        m = runtime_m();
        if(m->helpgc < 0 || m->helpgc >= MaxGcproc)
                runtime_throw("gchelperstart: bad m->helpgc");
        if(runtime_xchg(&bufferList[m->helpgc].busy, 1))
                runtime_throw("gchelperstart: already busy");
        if(runtime_g() != m->g0)
                runtime_throw("gchelper not running on g0 stack");
}

static void
runfinq(void* dummy __attribute__ ((unused)))
{
        Finalizer *f;
        FinBlock *fb, *next;
        uint32 i;
        Eface ef;
        Iface iface;

        for(;;) {
                runtime_lock(&finlock);
                fb = finq;
                finq = nil;
                if(fb == nil) {
                        fingwait = 1;
                        runtime_park(runtime_unlock, &finlock, "finalizer wait");
                        continue;
                }
                runtime_unlock(&finlock);
                if(raceenabled)
                        runtime_racefingo();
                for(; fb; fb=next) {
                        next = fb->next;
                        for(i=0; i<(uint32)fb->cnt; i++) {
                                const Type *fint;
                                void *param;

                                f = &fb->fin[i];
                                fint = ((const Type**)f->ft->__in.array)[0];
                                if(fint->kind == KindPtr) {
                                        // direct use of pointer
                                        param = &f->arg;
                                } else if(((const InterfaceType*)fint)->__methods.__count == 0) {
                                        // convert to empty interface
                                        ef.type = (const Type*)f->ot;
                                        ef.__object = f->arg;
                                        param = &ef;
                                } else {
                                        // convert to interface with methods
                                        iface.__methods = __go_convert_interface_2((const Type*)fint,
                                                                                   (const Type*)f->ot,
                                                                                   1);
                                        iface.__object = f->arg;
                                        if(iface.__methods == nil)
                                                runtime_throw("invalid type conversion in runfinq");
                                        param = &iface;
                                }
                                reflect_call(f->ft, f->fn, 0, 0, &param, nil);
                                f->fn = nil;
                                f->arg = nil;
                                f->ot = nil;
                        }
                        fb->cnt = 0;
                        fb->next = finc;
                        finc = fb;
                }
                runtime_gc(1);  // trigger another gc to clean up the finalized objects, if possible
        }
}

// mark the block at v of size n as allocated.
// If noscan is true, mark it as not needing scanning.
void
runtime_markallocated(void *v, uintptr n, bool noscan)
{
        uintptr *b, obits, bits, off, shift;

        if(0)
                runtime_printf("markallocated %p+%p\n", v, n);

        if((byte*)v+n > (byte*)runtime_mheap.arena_used || (byte*)v < runtime_mheap.arena_start)
                runtime_throw("markallocated: bad pointer");

        off = (uintptr*)v - (uintptr*)runtime_mheap.arena_start;  // word offset
        b = (uintptr*)runtime_mheap.arena_start - off/wordsPerBitmapWord - 1;
        shift = off % wordsPerBitmapWord;

        for(;;) {
                obits = *b;
                bits = (obits & ~(bitMask<<shift)) | (bitAllocated<<shift);
                if(noscan)
                        bits |= bitNoScan<<shift;
                if(runtime_gomaxprocs == 1) {
                        *b = bits;
                        break;
                } else {
                        // more than one goroutine is potentially running: use atomic op
                        if(runtime_casp((void**)b, (void*)obits, (void*)bits))
                                break;
                }
        }
}

// mark the block at v of size n as freed.
void
runtime_markfreed(void *v, uintptr n)
{
        uintptr *b, obits, bits, off, shift;

        if(0)
                runtime_printf("markfreed %p+%p\n", v, n);

        if((byte*)v+n > (byte*)runtime_mheap.arena_used || (byte*)v < runtime_mheap.arena_start)
                runtime_throw("markfreed: bad pointer");

        off = (uintptr*)v - (uintptr*)runtime_mheap.arena_start;  // word offset
        b = (uintptr*)runtime_mheap.arena_start - off/wordsPerBitmapWord - 1;
        shift = off % wordsPerBitmapWord;

        for(;;) {
                obits = *b;
                bits = (obits & ~(bitMask<<shift)) | (bitBlockBoundary<<shift);
                if(runtime_gomaxprocs == 1) {
                        *b = bits;
                        break;
                } else {
                        // more than one goroutine is potentially running: use atomic op
                        if(runtime_casp((void**)b, (void*)obits, (void*)bits))
                                break;
                }
        }
}

// check that the block at v of size n is marked freed.
void
runtime_checkfreed(void *v, uintptr n)
{
        uintptr *b, bits, off, shift;

        if(!runtime_checking)
                return;

        if((byte*)v+n > (byte*)runtime_mheap.arena_used || (byte*)v < runtime_mheap.arena_start)
                return; // not allocated, so okay

        off = (uintptr*)v - (uintptr*)runtime_mheap.arena_start;  // word offset
        b = (uintptr*)runtime_mheap.arena_start - off/wordsPerBitmapWord - 1;
        shift = off % wordsPerBitmapWord;

        bits = *b>>shift;
        if((bits & bitAllocated) != 0) {
                runtime_printf("checkfreed %p+%p: off=%p have=%p\n",
                        v, n, off, bits & bitMask);
                runtime_throw("checkfreed: not freed");
        }
}

// mark the span of memory at v as having n blocks of the given size.
// if leftover is true, there is left over space at the end of the span.
void
runtime_markspan(void *v, uintptr size, uintptr n, bool leftover)
{
        uintptr *b, off, shift;
        byte *p;

        if((byte*)v+size*n > (byte*)runtime_mheap.arena_used || (byte*)v < runtime_mheap.arena_start)
                runtime_throw("markspan: bad pointer");

        p = v;
        if(leftover)    // mark a boundary just past end of last block too
                n++;
        for(; n-- > 0; p += size) {
                // Okay to use non-atomic ops here, because we control
                // the entire span, and each bitmap word has bits for only
                // one span, so no other goroutines are changing these
                // bitmap words.
                off = (uintptr*)p - (uintptr*)runtime_mheap.arena_start;  // word offset
                b = (uintptr*)runtime_mheap.arena_start - off/wordsPerBitmapWord - 1;
                shift = off % wordsPerBitmapWord;
                *b = (*b & ~(bitMask<<shift)) | (bitBlockBoundary<<shift);
        }
}

// unmark the span of memory at v of length n bytes.
void
runtime_unmarkspan(void *v, uintptr n)
{
        uintptr *p, *b, off;

        if((byte*)v+n > (byte*)runtime_mheap.arena_used || (byte*)v < runtime_mheap.arena_start)
                runtime_throw("markspan: bad pointer");

        p = v;
        off = p - (uintptr*)runtime_mheap.arena_start;  // word offset
        if(off % wordsPerBitmapWord != 0)
                runtime_throw("markspan: unaligned pointer");
        b = (uintptr*)runtime_mheap.arena_start - off/wordsPerBitmapWord - 1;
        n /= PtrSize;
        if(n%wordsPerBitmapWord != 0)
                runtime_throw("unmarkspan: unaligned length");
        // Okay to use non-atomic ops here, because we control
        // the entire span, and each bitmap word has bits for only
        // one span, so no other goroutines are changing these
        // bitmap words.
        n /= wordsPerBitmapWord;
        while(n-- > 0)
                *b-- = 0;
}

bool
runtime_blockspecial(void *v)
{
        uintptr *b, off, shift;

        if(DebugMark)
                return true;

        off = (uintptr*)v - (uintptr*)runtime_mheap.arena_start;
        b = (uintptr*)runtime_mheap.arena_start - off/wordsPerBitmapWord - 1;
        shift = off % wordsPerBitmapWord;

        return (*b & (bitSpecial<<shift)) != 0;
}

void
runtime_setblockspecial(void *v, bool s)
{
        uintptr *b, off, shift, bits, obits;

        if(DebugMark)
                return;

        off = (uintptr*)v - (uintptr*)runtime_mheap.arena_start;
        b = (uintptr*)runtime_mheap.arena_start - off/wordsPerBitmapWord - 1;
        shift = off % wordsPerBitmapWord;

        for(;;) {
                obits = *b;
                if(s)
                        bits = obits | (bitSpecial<<shift);
                else
                        bits = obits & ~(bitSpecial<<shift);
                if(runtime_gomaxprocs == 1) {
                        *b = bits;
                        break;
                } else {
                        // more than one goroutine is potentially running: use atomic op
                        if(runtime_casp((void**)b, (void*)obits, (void*)bits))
                                break;
                }
        }
}

void
runtime_MHeap_MapBits(MHeap *h)
{
        size_t page_size;

        // Caller has added extra mappings to the arena.
        // Add extra mappings of bitmap words as needed.
        // We allocate extra bitmap pieces in chunks of bitmapChunk.
        enum {
                bitmapChunk = 8192
        };
        uintptr n;

        n = (h->arena_used - h->arena_start) / wordsPerBitmapWord;
        n = ROUND(n, bitmapChunk);
        if(h->bitmap_mapped >= n)
                return;

        page_size = getpagesize();
        n = (n+page_size-1) & ~(page_size-1);

        runtime_SysMap(h->arena_start - n, n - h->bitmap_mapped, &mstats.gc_sys);
        h->bitmap_mapped = n;
}