2017-04-28 Hristian Kirtchev <kirtchev@adacore.com>

[official-gcc.git] / libgo / runtime / mheap.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.

// Page heap.
//
// See malloc.h for overview.
//
// When a MSpan is in the heap free list, state == MSpanFree
// and heapmap(s->start) == span, heapmap(s->start+s->npages-1) == span.
//
// When a MSpan is allocated, state == MSpanInUse
// and heapmap(i) == span for all s->start <= i < s->start+s->npages.

#include "runtime.h"
#include "arch.h"
#include "malloc.h"

static MSpan *MHeap_AllocLocked(MHeap*, uintptr, int32);
static bool MHeap_Grow(MHeap*, uintptr);
static void MHeap_FreeLocked(MHeap*, MSpan*);
static MSpan *MHeap_AllocLarge(MHeap*, uintptr);
static MSpan *BestFit(MSpan*, uintptr, MSpan*);

static void
RecordSpan(void *vh, byte *p)
{
        MHeap *h;
        MSpan *s;
        MSpan **all;
        uint32 cap;

        h = vh;
        s = (MSpan*)p;
        if(h->nspan >= h->nspancap) {
                cap = 64*1024/sizeof(all[0]);
                if(cap < h->nspancap*3/2)
                        cap = h->nspancap*3/2;
                all = (MSpan**)runtime_SysAlloc(cap*sizeof(all[0]), &mstats()->other_sys);
                if(all == nil)
                        runtime_throw("runtime: cannot allocate memory");
                if(h->allspans) {
                        runtime_memmove(all, h->allspans, h->nspancap*sizeof(all[0]));
                        // Don't free the old array if it's referenced by sweep.
                        // See the comment in mgc0.c.
                        if(h->allspans != runtime_mheap.sweepspans)
                                runtime_SysFree(h->allspans, h->nspancap*sizeof(all[0]), &mstats()->other_sys);
                }
                h->allspans = all;
                h->nspancap = cap;
        }
        h->allspans[h->nspan++] = s;
}

// Initialize the heap; fetch memory using alloc.
void
runtime_MHeap_Init(MHeap *h)
{
        MStats *pmstats;
        uint32 i;

        pmstats = mstats();
        runtime_FixAlloc_Init(&h->spanalloc, sizeof(MSpan), RecordSpan, h, &pmstats->mspan_sys);
        runtime_FixAlloc_Init(&h->cachealloc, sizeof(MCache), nil, nil, &pmstats->mcache_sys);
        runtime_FixAlloc_Init(&h->specialfinalizeralloc, sizeof(SpecialFinalizer), nil, nil, &pmstats->other_sys);
        runtime_FixAlloc_Init(&h->specialprofilealloc, sizeof(SpecialProfile), nil, nil, &pmstats->other_sys);
        // h->mapcache needs no init
        for(i=0; i<nelem(h->free); i++) {
                runtime_MSpanList_Init(&h->free[i]);
                runtime_MSpanList_Init(&h->busy[i]);
        }
        runtime_MSpanList_Init(&h->freelarge);
        runtime_MSpanList_Init(&h->busylarge);
        for(i=0; i<nelem(h->central); i++)
                runtime_MCentral_Init(&h->central[i], i);
}

void
runtime_MHeap_MapSpans(MHeap *h)
{
        uintptr pagesize;
        uintptr n;

        // Map spans array, PageSize at a time.
        n = (uintptr)h->arena_used;
        n -= (uintptr)h->arena_start;
        n = n / PageSize * sizeof(h->spans[0]);
        n = ROUND(n, PageSize);
        pagesize = getpagesize();
        n = ROUND(n, pagesize);
        if(h->spans_mapped >= n)
                return;
        runtime_SysMap((byte*)h->spans + h->spans_mapped, n - h->spans_mapped, h->arena_reserved, &mstats()->other_sys);
        h->spans_mapped = n;
}

// Sweeps spans in list until reclaims at least npages into heap.
// Returns the actual number of pages reclaimed.
static uintptr
MHeap_ReclaimList(MHeap *h, MSpan *list, uintptr npages)
{
        MSpan *s;
        uintptr n;
        uint32 sg;

        n = 0;
        sg = runtime_mheap.sweepgen;
retry:
        for(s = list->next; s != list; s = s->next) {
                if(s->sweepgen == sg-2 && runtime_cas(&s->sweepgen, sg-2, sg-1)) {
                        runtime_MSpanList_Remove(s);
                        // swept spans are at the end of the list
                        runtime_MSpanList_InsertBack(list, s);
                        runtime_unlock(h);
                        n += runtime_MSpan_Sweep(s);
                        runtime_lock(h);
                        if(n >= npages)
                                return n;
                        // the span could have been moved elsewhere
                        goto retry;
                }
                if(s->sweepgen == sg-1) {
                        // the span is being sweept by background sweeper, skip
                        continue;
                }
                // already swept empty span,
                // all subsequent ones must also be either swept or in process of sweeping
                break;
        }
        return n;
}

// Sweeps and reclaims at least npage pages into heap.
// Called before allocating npage pages.
static void
MHeap_Reclaim(MHeap *h, uintptr npage)
{
        uintptr reclaimed, n;

        // First try to sweep busy spans with large objects of size >= npage,
        // this has good chances of reclaiming the necessary space.
        for(n=npage; n < nelem(h->busy); n++) {
                if(MHeap_ReclaimList(h, &h->busy[n], npage))
                        return;  // Bingo!
        }

        // Then -- even larger objects.
        if(MHeap_ReclaimList(h, &h->busylarge, npage))
                return;  // Bingo!

        // Now try smaller objects.
        // One such object is not enough, so we need to reclaim several of them.
        reclaimed = 0;
        for(n=0; n < npage && n < nelem(h->busy); n++) {
                reclaimed += MHeap_ReclaimList(h, &h->busy[n], npage-reclaimed);
                if(reclaimed >= npage)
                        return;
        }

        // Now sweep everything that is not yet swept.
        runtime_unlock(h);
        for(;;) {
                n = runtime_sweepone();
                if(n == (uintptr)-1)  // all spans are swept
                        break;
                reclaimed += n;
                if(reclaimed >= npage)
                        break;
        }
        runtime_lock(h);
}

// Allocate a new span of npage pages from the heap
// and record its size class in the HeapMap and HeapMapCache.
MSpan*
runtime_MHeap_Alloc(MHeap *h, uintptr npage, int32 sizeclass, bool large, bool needzero)
{
        MStats *pmstats;
        MSpan *s;

        runtime_lock(h);
        pmstats = mstats();
        pmstats->heap_alloc += (intptr)runtime_m()->mcache->local_cachealloc;
        runtime_m()->mcache->local_cachealloc = 0;
        s = MHeap_AllocLocked(h, npage, sizeclass);
        if(s != nil) {
                pmstats->heap_inuse += npage<<PageShift;
                if(large) {
                        pmstats->heap_objects++;
                        pmstats->heap_alloc += npage<<PageShift;
                        // Swept spans are at the end of lists.
                        if(s->npages < nelem(h->free))
                                runtime_MSpanList_InsertBack(&h->busy[s->npages], s);
                        else
                                runtime_MSpanList_InsertBack(&h->busylarge, s);
                }
        }
        runtime_unlock(h);
        if(s != nil) {
                if(needzero && s->needzero)
                        runtime_memclr((byte*)(s->start<<PageShift), s->npages<<PageShift);
                s->needzero = 0;
        }
        return s;
}

static MSpan*
MHeap_AllocLocked(MHeap *h, uintptr npage, int32 sizeclass)
{
        uintptr n;
        MSpan *s, *t;
        PageID p;

        // To prevent excessive heap growth, before allocating n pages
        // we need to sweep and reclaim at least n pages.
        if(!h->sweepdone)
                MHeap_Reclaim(h, npage);

        // Try in fixed-size lists up to max.
        for(n=npage; n < nelem(h->free); n++) {
                if(!runtime_MSpanList_IsEmpty(&h->free[n])) {
                        s = h->free[n].next;
                        goto HaveSpan;
                }
        }

        // Best fit in list of large spans.
        if((s = MHeap_AllocLarge(h, npage)) == nil) {
                if(!MHeap_Grow(h, npage))
                        return nil;
                if((s = MHeap_AllocLarge(h, npage)) == nil)
                        return nil;
        }

HaveSpan:
        // Mark span in use.
        if(s->state != MSpanFree)
                runtime_throw("MHeap_AllocLocked - MSpan not free");
        if(s->npages < npage)
                runtime_throw("MHeap_AllocLocked - bad npages");
        runtime_MSpanList_Remove(s);
        runtime_atomicstore(&s->sweepgen, h->sweepgen);
        s->state = MSpanInUse;
        mstats()->heap_idle -= s->npages<<PageShift;
        mstats()->heap_released -= s->npreleased<<PageShift;
        if(s->npreleased > 0)
                runtime_SysUsed((void*)(s->start<<PageShift), s->npages<<PageShift);
        s->npreleased = 0;

        if(s->npages > npage) {
                // Trim extra and put it back in the heap.
                t = runtime_FixAlloc_Alloc(&h->spanalloc);
                runtime_MSpan_Init(t, s->start + npage, s->npages - npage);
                s->npages = npage;
                p = t->start;
                p -= ((uintptr)h->arena_start>>PageShift);
                if(p > 0)
                        h->spans[p-1] = s;
                h->spans[p] = t;
                h->spans[p+t->npages-1] = t;
                t->needzero = s->needzero;
                runtime_atomicstore(&t->sweepgen, h->sweepgen);
                t->state = MSpanInUse;
                MHeap_FreeLocked(h, t);
                t->unusedsince = s->unusedsince; // preserve age
        }
        s->unusedsince = 0;

        // Record span info, because gc needs to be
        // able to map interior pointer to containing span.
        s->sizeclass = sizeclass;
        s->elemsize = (sizeclass==0 ? s->npages<<PageShift : (uintptr)runtime_class_to_size[sizeclass]);
        s->types.compression = MTypes_Empty;
        p = s->start;
        p -= ((uintptr)h->arena_start>>PageShift);
        for(n=0; n<npage; n++)
                h->spans[p+n] = s;
        return s;
}

// Allocate a span of exactly npage pages from the list of large spans.
static MSpan*
MHeap_AllocLarge(MHeap *h, uintptr npage)
{
        return BestFit(&h->freelarge, npage, nil);
}

// Search list for smallest span with >= npage pages.
// If there are multiple smallest spans, take the one
// with the earliest starting address.
static MSpan*
BestFit(MSpan *list, uintptr npage, MSpan *best)
{
        MSpan *s;

        for(s=list->next; s != list; s=s->next) {
                if(s->npages < npage)
                        continue;
                if(best == nil
                || s->npages < best->npages
                || (s->npages == best->npages && s->start < best->start))
                        best = s;
        }
        return best;
}

// Try to add at least npage pages of memory to the heap,
// returning whether it worked.
static bool
MHeap_Grow(MHeap *h, uintptr npage)
{
        uintptr ask;
        void *v;
        MSpan *s;
        PageID p;

        // Ask for a big chunk, to reduce the number of mappings
        // the operating system needs to track; also amortizes
        // the overhead of an operating system mapping.
        // Allocate a multiple of 64kB (16 pages).
        npage = (npage+15)&~15;
        ask = npage<<PageShift;
        if(ask < HeapAllocChunk)
                ask = HeapAllocChunk;

        v = runtime_MHeap_SysAlloc(h, ask);
        if(v == nil) {
                if(ask > (npage<<PageShift)) {
                        ask = npage<<PageShift;
                        v = runtime_MHeap_SysAlloc(h, ask);
                }
                if(v == nil) {
                        runtime_printf("runtime: out of memory: cannot allocate %D-byte block (%D in use)\n", (uint64)ask, mstats()->heap_sys);
                        return false;
                }
        }

        // Create a fake "in use" span and free it, so that the
        // right coalescing happens.
        s = runtime_FixAlloc_Alloc(&h->spanalloc);
        runtime_MSpan_Init(s, (uintptr)v>>PageShift, ask>>PageShift);
        p = s->start;
        p -= ((uintptr)h->arena_start>>PageShift);
        h->spans[p] = s;
        h->spans[p + s->npages - 1] = s;
        runtime_atomicstore(&s->sweepgen, h->sweepgen);
        s->state = MSpanInUse;
        MHeap_FreeLocked(h, s);
        return true;
}

// Look up the span at the given address.
// Address is guaranteed to be in map
// and is guaranteed to be start or end of span.
MSpan*
runtime_MHeap_Lookup(MHeap *h, void *v)
{
        uintptr p;
        
        p = (uintptr)v;
        p -= (uintptr)h->arena_start;
        return h->spans[p >> PageShift];
}

// Look up the span at the given address.
// Address is *not* guaranteed to be in map
// and may be anywhere in the span.
// Map entries for the middle of a span are only
// valid for allocated spans.  Free spans may have
// other garbage in their middles, so we have to
// check for that.
MSpan*
runtime_MHeap_LookupMaybe(MHeap *h, void *v)
{
        MSpan *s;
        PageID p, q;

        if((byte*)v < h->arena_start || (byte*)v >= h->arena_used)
                return nil;
        p = (uintptr)v>>PageShift;
        q = p;
        q -= (uintptr)h->arena_start >> PageShift;
        s = h->spans[q];
        if(s == nil || p < s->start || (uintptr)v >= s->limit || s->state != MSpanInUse)
                return nil;
        return s;
}

// Free the span back into the heap.
void
runtime_MHeap_Free(MHeap *h, MSpan *s, int32 acct)
{
        MStats *pmstats;

        runtime_lock(h);
        pmstats = mstats();
        pmstats->heap_alloc += (intptr)runtime_m()->mcache->local_cachealloc;
        runtime_m()->mcache->local_cachealloc = 0;
        pmstats->heap_inuse -= s->npages<<PageShift;
        if(acct) {
                pmstats->heap_alloc -= s->npages<<PageShift;
                pmstats->heap_objects--;
        }
        MHeap_FreeLocked(h, s);
        runtime_unlock(h);
}

static void
MHeap_FreeLocked(MHeap *h, MSpan *s)
{
        MSpan *t;
        PageID p;

        s->types.compression = MTypes_Empty;

        if(s->state != MSpanInUse || s->ref != 0 || s->sweepgen != h->sweepgen) {
                runtime_printf("MHeap_FreeLocked - span %p ptr %p state %d ref %d sweepgen %d/%d\n",
                        s, s->start<<PageShift, s->state, s->ref, s->sweepgen, h->sweepgen);
                runtime_throw("MHeap_FreeLocked - invalid free");
        }
        mstats()->heap_idle += s->npages<<PageShift;
        s->state = MSpanFree;
        runtime_MSpanList_Remove(s);
        // Stamp newly unused spans. The scavenger will use that
        // info to potentially give back some pages to the OS.
        s->unusedsince = runtime_nanotime();
        s->npreleased = 0;

        // Coalesce with earlier, later spans.
        p = s->start;
        p -= (uintptr)h->arena_start >> PageShift;
        if(p > 0 && (t = h->spans[p-1]) != nil && t->state != MSpanInUse) {
                s->start = t->start;
                s->npages += t->npages;
                s->npreleased = t->npreleased; // absorb released pages
                s->needzero |= t->needzero;
                p -= t->npages;
                h->spans[p] = s;
                runtime_MSpanList_Remove(t);
                t->state = MSpanDead;
                runtime_FixAlloc_Free(&h->spanalloc, t);
        }
        if((p+s->npages)*sizeof(h->spans[0]) < h->spans_mapped && (t = h->spans[p+s->npages]) != nil && t->state != MSpanInUse) {
                s->npages += t->npages;
                s->npreleased += t->npreleased;
                s->needzero |= t->needzero;
                h->spans[p + s->npages - 1] = s;
                runtime_MSpanList_Remove(t);
                t->state = MSpanDead;
                runtime_FixAlloc_Free(&h->spanalloc, t);
        }

        // Insert s into appropriate list.
        if(s->npages < nelem(h->free))
                runtime_MSpanList_Insert(&h->free[s->npages], s);
        else
                runtime_MSpanList_Insert(&h->freelarge, s);
}

static void
forcegchelper(void *vnote)
{
        Note *note = (Note*)vnote;

        runtime_gc(1);
        runtime_notewakeup(note);
}

static uintptr
scavengelist(MSpan *list, uint64 now, uint64 limit)
{
        uintptr released, sumreleased, start, end, pagesize;
        MSpan *s;

        if(runtime_MSpanList_IsEmpty(list))
                return 0;

        sumreleased = 0;
        for(s=list->next; s != list; s=s->next) {
                if((now - s->unusedsince) > limit && s->npreleased != s->npages) {
                        released = (s->npages - s->npreleased) << PageShift;
                        mstats()->heap_released += released;
                        sumreleased += released;
                        s->npreleased = s->npages;

                        start = s->start << PageShift;
                        end = start + (s->npages << PageShift);

                        // Round start up and end down to ensure we
                        // are acting on entire pages.
                        pagesize = getpagesize();
                        start = ROUND(start, pagesize);
                        end &= ~(pagesize - 1);
                        if(end > start)
                                runtime_SysUnused((void*)start, end - start);
                }
        }
        return sumreleased;
}

static void
scavenge(int32 k, uint64 now, uint64 limit)
{
        uint32 i;
        uintptr sumreleased;
        MHeap *h;
        
        h = &runtime_mheap;
        sumreleased = 0;
        for(i=0; i < nelem(h->free); i++)
                sumreleased += scavengelist(&h->free[i], now, limit);
        sumreleased += scavengelist(&h->freelarge, now, limit);

        if(runtime_debug.gctrace > 0) {
                if(sumreleased > 0)
                        runtime_printf("scvg%d: %D MB released\n", k, (uint64)sumreleased>>20);
                runtime_printf("scvg%d: inuse: %D, idle: %D, sys: %D, released: %D, consumed: %D (MB)\n",
                        k, mstats()->heap_inuse>>20, mstats()->heap_idle>>20, mstats()->heap_sys>>20,
                        mstats()->heap_released>>20, (mstats()->heap_sys - mstats()->heap_released)>>20);
        }
}

// Release (part of) unused memory to OS.
// Goroutine created at startup.
// Loop forever.
void
runtime_MHeap_Scavenger(void* dummy)
{
        G *g;
        MHeap *h;
        uint64 tick, now, forcegc, limit;
        int64 unixnow;
        uint32 k;
        Note note, *notep;

        USED(dummy);

        g = runtime_g();
        g->issystem = true;
        g->isbackground = true;

        // If we go two minutes without a garbage collection, force one to run.
        forcegc = 2*60*1e9;
        // If a span goes unused for 5 minutes after a garbage collection,
        // we hand it back to the operating system.
        limit = 5*60*1e9;
        // Make wake-up period small enough for the sampling to be correct.
        if(forcegc < limit)
                tick = forcegc/2;
        else
                tick = limit/2;

        h = &runtime_mheap;
        for(k=0;; k++) {
                runtime_noteclear(&note);
                runtime_notetsleepg(&note, tick);

                runtime_lock(h);
                unixnow = runtime_unixnanotime();
                if(unixnow - mstats()->last_gc > forcegc) {
                        runtime_unlock(h);
                        // The scavenger can not block other goroutines,
                        // otherwise deadlock detector can fire spuriously.
                        // GC blocks other goroutines via the runtime_worldsema.
                        runtime_noteclear(&note);
                        notep = &note;
                        __go_go(forcegchelper, (void*)notep);
                        runtime_notetsleepg(&note, -1);
                        if(runtime_debug.gctrace > 0)
                                runtime_printf("scvg%d: GC forced\n", k);
                        runtime_lock(h);
                }
                now = runtime_nanotime();
                scavenge(k, now, limit);
                runtime_unlock(h);
        }
}

void runtime_debug_freeOSMemory(void) __asm__("runtime_debug.freeOSMemory");

void
runtime_debug_freeOSMemory(void)
{
        runtime_gc(2);  // force GC and do eager sweep
        runtime_lock(&runtime_mheap);
        scavenge(-1, ~(uintptr)0, 0);
        runtime_unlock(&runtime_mheap);
}

// Initialize a new span with the given start and npages.
void
runtime_MSpan_Init(MSpan *span, PageID start, uintptr npages)
{
        span->next = nil;
        span->prev = nil;
        span->start = start;
        span->npages = npages;
        span->freelist = nil;
        span->ref = 0;
        span->sizeclass = 0;
        span->incache = false;
        span->elemsize = 0;
        span->state = MSpanDead;
        span->unusedsince = 0;
        span->npreleased = 0;
        span->types.compression = MTypes_Empty;
        span->speciallock.key = 0;
        span->specials = nil;
        span->needzero = 0;
        span->freebuf = nil;
}

// Initialize an empty doubly-linked list.
void
runtime_MSpanList_Init(MSpan *list)
{
        list->state = MSpanListHead;
        list->next = list;
        list->prev = list;
}

void
runtime_MSpanList_Remove(MSpan *span)
{
        if(span->prev == nil && span->next == nil)
                return;
        span->prev->next = span->next;
        span->next->prev = span->prev;
        span->prev = nil;
        span->next = nil;
}

bool
runtime_MSpanList_IsEmpty(MSpan *list)
{
        return list->next == list;
}

void
runtime_MSpanList_Insert(MSpan *list, MSpan *span)
{
        if(span->next != nil || span->prev != nil) {
                runtime_printf("failed MSpanList_Insert %p %p %p\n", span, span->next, span->prev);
                runtime_throw("MSpanList_Insert");
        }
        span->next = list->next;
        span->prev = list;
        span->next->prev = span;
        span->prev->next = span;
}

void
runtime_MSpanList_InsertBack(MSpan *list, MSpan *span)
{
        if(span->next != nil || span->prev != nil) {
                runtime_printf("failed MSpanList_Insert %p %p %p\n", span, span->next, span->prev);
                runtime_throw("MSpanList_Insert");
        }
        span->next = list;
        span->prev = list->prev;
        span->next->prev = span;
        span->prev->next = span;
}

// Adds the special record s to the list of special records for
// the object p.  All fields of s should be filled in except for
// offset & next, which this routine will fill in.
// Returns true if the special was successfully added, false otherwise.
// (The add will fail only if a record with the same p and s->kind
//  already exists.)
static bool
addspecial(void *p, Special *s)
{
        MSpan *span;
        Special **t, *x;
        uintptr offset;
        byte kind;

        span = runtime_MHeap_LookupMaybe(&runtime_mheap, p);
        if(span == nil)
                runtime_throw("addspecial on invalid pointer");

        // Ensure that the span is swept.
        // GC accesses specials list w/o locks. And it's just much safer.
        runtime_m()->locks++;
        runtime_MSpan_EnsureSwept(span);

        offset = (uintptr)p - (span->start << PageShift);
        kind = s->kind;

        runtime_lock(&span->speciallock);

        // Find splice point, check for existing record.
        t = &span->specials;
        while((x = *t) != nil) {
                if(offset == x->offset && kind == x->kind) {
                        runtime_unlock(&span->speciallock);
                        runtime_m()->locks--;
                        return false; // already exists
                }
                if(offset < x->offset || (offset == x->offset && kind < x->kind))
                        break;
                t = &x->next;
        }
        // Splice in record, fill in offset.
        s->offset = offset;
        s->next = x;
        *t = s;
        runtime_unlock(&span->speciallock);
        runtime_m()->locks--;
        return true;
}

// Removes the Special record of the given kind for the object p.
// Returns the record if the record existed, nil otherwise.
// The caller must FixAlloc_Free the result.
static Special*
removespecial(void *p, byte kind)
{
        MSpan *span;
        Special *s, **t;
        uintptr offset;

        span = runtime_MHeap_LookupMaybe(&runtime_mheap, p);
        if(span == nil)
                runtime_throw("removespecial on invalid pointer");

        // Ensure that the span is swept.
        // GC accesses specials list w/o locks. And it's just much safer.
        runtime_m()->locks++;
        runtime_MSpan_EnsureSwept(span);

        offset = (uintptr)p - (span->start << PageShift);

        runtime_lock(&span->speciallock);
        t = &span->specials;
        while((s = *t) != nil) {
                // This function is used for finalizers only, so we don't check for
                // "interior" specials (p must be exactly equal to s->offset).
                if(offset == s->offset && kind == s->kind) {
                        *t = s->next;
                        runtime_unlock(&span->speciallock);
                        runtime_m()->locks--;
                        return s;
                }
                t = &s->next;
        }
        runtime_unlock(&span->speciallock);
        runtime_m()->locks--;
        return nil;
}

// Adds a finalizer to the object p.  Returns true if it succeeded.
bool
runtime_addfinalizer(void *p, FuncVal *f, const FuncType *ft, const PtrType *ot)
{
        SpecialFinalizer *s;

        runtime_lock(&runtime_mheap.speciallock);
        s = runtime_FixAlloc_Alloc(&runtime_mheap.specialfinalizeralloc);
        runtime_unlock(&runtime_mheap.speciallock);
        s->kind = KindSpecialFinalizer;
        s->fn = f;
        s->ft = ft;
        s->ot = ot;
        if(addspecial(p, s))
                return true;

        // There was an old finalizer
        runtime_lock(&runtime_mheap.speciallock);
        runtime_FixAlloc_Free(&runtime_mheap.specialfinalizeralloc, s);
        runtime_unlock(&runtime_mheap.speciallock);
        return false;
}

// Removes the finalizer (if any) from the object p.
void
runtime_removefinalizer(void *p)
{
        SpecialFinalizer *s;

        s = (SpecialFinalizer*)removespecial(p, KindSpecialFinalizer);
        if(s == nil)
                return; // there wasn't a finalizer to remove
        runtime_lock(&runtime_mheap.speciallock);
        runtime_FixAlloc_Free(&runtime_mheap.specialfinalizeralloc, s);
        runtime_unlock(&runtime_mheap.speciallock);
}

// Set the heap profile bucket associated with addr to b.
void
runtime_setprofilebucket(void *p, Bucket *b)
{
        SpecialProfile *s;

        runtime_lock(&runtime_mheap.speciallock);
        s = runtime_FixAlloc_Alloc(&runtime_mheap.specialprofilealloc);
        runtime_unlock(&runtime_mheap.speciallock);
        s->kind = KindSpecialProfile;
        s->b = b;
        if(!addspecial(p, s))
                runtime_throw("setprofilebucket: profile already set");
}

// Do whatever cleanup needs to be done to deallocate s.  It has
// already been unlinked from the MSpan specials list.
// Returns true if we should keep working on deallocating p.
bool
runtime_freespecial(Special *s, void *p, uintptr size, bool freed)
{
        SpecialFinalizer *sf;
        SpecialProfile *sp;

        switch(s->kind) {
        case KindSpecialFinalizer:
                sf = (SpecialFinalizer*)s;
                runtime_queuefinalizer(p, sf->fn, sf->ft, sf->ot);
                runtime_lock(&runtime_mheap.speciallock);
                runtime_FixAlloc_Free(&runtime_mheap.specialfinalizeralloc, sf);
                runtime_unlock(&runtime_mheap.speciallock);
                return false; // don't free p until finalizer is done
        case KindSpecialProfile:
                sp = (SpecialProfile*)s;
                runtime_MProf_Free(sp->b, size, freed);
                runtime_lock(&runtime_mheap.speciallock);
                runtime_FixAlloc_Free(&runtime_mheap.specialprofilealloc, sp);
                runtime_unlock(&runtime_mheap.speciallock);
                return true;
        default:
                runtime_throw("bad special kind");
                return true;
        }
}

// Free all special records for p.
void
runtime_freeallspecials(MSpan *span, void *p, uintptr size)
{
        Special *s, **t, *list;
        uintptr offset;

        if(span->sweepgen != runtime_mheap.sweepgen)
                runtime_throw("runtime: freeallspecials: unswept span");
        // first, collect all specials into the list; then, free them
        // this is required to not cause deadlock between span->specialLock and proflock
        list = nil;
        offset = (uintptr)p - (span->start << PageShift);
        runtime_lock(&span->speciallock);
        t = &span->specials;
        while((s = *t) != nil) {
                if(offset + size <= s->offset)
                        break;
                if(offset <= s->offset) {
                        *t = s->next;
                        s->next = list;
                        list = s;
                } else
                        t = &s->next;
        }
        runtime_unlock(&span->speciallock);

        while(list != nil) {
                s = list;
                list = s->next;
                if(!runtime_freespecial(s, p, size, true))
                        runtime_throw("can't explicitly free an object with a finalizer");
        }
}

// Split an allocated span into two equal parts.
void
runtime_MHeap_SplitSpan(MHeap *h, MSpan *s)
{
        MSpan *t;
        MCentral *c;
        uintptr i;
        uintptr npages;
        PageID p;

        if(s->state != MSpanInUse)
                runtime_throw("MHeap_SplitSpan on a free span");
        if(s->sizeclass != 0 && s->ref != 1)
                runtime_throw("MHeap_SplitSpan doesn't have an allocated object");
        npages = s->npages;

        // remove the span from whatever list it is in now
        if(s->sizeclass > 0) {
                // must be in h->central[x].mempty
                c = &h->central[s->sizeclass];
                runtime_lock(c);
                runtime_MSpanList_Remove(s);
                runtime_unlock(c);
                runtime_lock(h);
        } else {
                // must be in h->busy/busylarge
                runtime_lock(h);
                runtime_MSpanList_Remove(s);
        }
        // heap is locked now

        if(npages == 1) {
                // convert span of 1 PageSize object to a span of 2 PageSize/2 objects.
                s->ref = 2;
                s->sizeclass = runtime_SizeToClass(PageSize/2);
                s->elemsize = PageSize/2;
        } else {
                // convert span of n>1 pages into two spans of n/2 pages each.
                if((s->npages & 1) != 0)
                        runtime_throw("MHeap_SplitSpan on an odd size span");

                // compute position in h->spans
                p = s->start;
                p -= (uintptr)h->arena_start >> PageShift;

                // Allocate a new span for the first half.
                t = runtime_FixAlloc_Alloc(&h->spanalloc);
                runtime_MSpan_Init(t, s->start, npages/2);
                t->limit = (uintptr)((t->start + npages/2) << PageShift);
                t->state = MSpanInUse;
                t->elemsize = npages << (PageShift - 1);
                t->sweepgen = s->sweepgen;
                if(t->elemsize <= MaxSmallSize) {
                        t->sizeclass = runtime_SizeToClass(t->elemsize);
                        t->ref = 1;
                }

                // the old span holds the second half.
                s->start += npages/2;
                s->npages = npages/2;
                s->elemsize = npages << (PageShift - 1);
                if(s->elemsize <= MaxSmallSize) {
                        s->sizeclass = runtime_SizeToClass(s->elemsize);
                        s->ref = 1;
                }

                // update span lookup table
                for(i = p; i < p + npages/2; i++)
                        h->spans[i] = t;
        }

        // place the span into a new list
        if(s->sizeclass > 0) {
                runtime_unlock(h);
                c = &h->central[s->sizeclass];
                runtime_lock(c);
                // swept spans are at the end of the list
                runtime_MSpanList_InsertBack(&c->mempty, s);
                runtime_unlock(c);
        } else {
                // Swept spans are at the end of lists.
                if(s->npages < nelem(h->free))
                        runtime_MSpanList_InsertBack(&h->busy[s->npages], s);
                else
                        runtime_MSpanList_InsertBack(&h->busylarge, s);
                runtime_unlock(h);
        }
}