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// 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.go for overview.
package runtime
import (
"internal/cpu"
"runtime/internal/atomic"
"runtime/internal/sys"
"unsafe"
)
// minPhysPageSize is a lower-bound on the physical page size. The
// true physical page size may be larger than this. In contrast,
// sys.PhysPageSize is an upper-bound on the physical page size.
const minPhysPageSize = 4096
// Main malloc heap.
// The heap itself is the "free[]" and "large" arrays,
// but all the other global data is here too.
//
// mheap must not be heap-allocated because it contains mSpanLists,
// which must not be heap-allocated.
//
//go:notinheap
type mheap struct {
lock mutex
free mTreap // free treap of spans
busy mSpanList // busy list of spans
sweepgen uint32 // sweep generation, see comment in mspan
sweepdone uint32 // all spans are swept
sweepers uint32 // number of active sweepone calls
// allspans is a slice of all mspans ever created. Each mspan
// appears exactly once.
//
// The memory for allspans is manually managed and can be
// reallocated and move as the heap grows.
//
// In general, allspans is protected by mheap_.lock, which
// prevents concurrent access as well as freeing the backing
// store. Accesses during STW might not hold the lock, but
// must ensure that allocation cannot happen around the
// access (since that may free the backing store).
allspans []*mspan // all spans out there
// sweepSpans contains two mspan stacks: one of swept in-use
// spans, and one of unswept in-use spans. These two trade
// roles on each GC cycle. Since the sweepgen increases by 2
// on each cycle, this means the swept spans are in
// sweepSpans[sweepgen/2%2] and the unswept spans are in
// sweepSpans[1-sweepgen/2%2]. Sweeping pops spans from the
// unswept stack and pushes spans that are still in-use on the
// swept stack. Likewise, allocating an in-use span pushes it
// on the swept stack.
sweepSpans [2]gcSweepBuf
//_ uint32 // align uint64 fields on 32-bit for atomics
// Proportional sweep
//
// These parameters represent a linear function from heap_live
// to page sweep count. The proportional sweep system works to
// stay in the black by keeping the current page sweep count
// above this line at the current heap_live.
//
// The line has slope sweepPagesPerByte and passes through a
// basis point at (sweepHeapLiveBasis, pagesSweptBasis). At
// any given time, the system is at (memstats.heap_live,
// pagesSwept) in this space.
//
// It's important that the line pass through a point we
// control rather than simply starting at a (0,0) origin
// because that lets us adjust sweep pacing at any time while
// accounting for current progress. If we could only adjust
// the slope, it would create a discontinuity in debt if any
// progress has already been made.
pagesInUse uint64 // pages of spans in stats mSpanInUse; R/W with mheap.lock
pagesSwept uint64 // pages swept this cycle; updated atomically
pagesSweptBasis uint64 // pagesSwept to use as the origin of the sweep ratio; updated atomically
sweepHeapLiveBasis uint64 // value of heap_live to use as the origin of sweep ratio; written with lock, read without
sweepPagesPerByte float64 // proportional sweep ratio; written with lock, read without
// TODO(austin): pagesInUse should be a uintptr, but the 386
// compiler can't 8-byte align fields.
// Malloc stats.
largealloc uint64 // bytes allocated for large objects
nlargealloc uint64 // number of large object allocations
largefree uint64 // bytes freed for large objects (>maxsmallsize)
nlargefree uint64 // number of frees for large objects (>maxsmallsize)
nsmallfree [_NumSizeClasses]uint64 // number of frees for small objects (<=maxsmallsize)
// arenas is the heap arena map. It points to the metadata for
// the heap for every arena frame of the entire usable virtual
// address space.
//
// Use arenaIndex to compute indexes into this array.
//
// For regions of the address space that are not backed by the
// Go heap, the arena map contains nil.
//
// Modifications are protected by mheap_.lock. Reads can be
// performed without locking; however, a given entry can
// transition from nil to non-nil at any time when the lock
// isn't held. (Entries never transitions back to nil.)
//
// In general, this is a two-level mapping consisting of an L1
// map and possibly many L2 maps. This saves space when there
// are a huge number of arena frames. However, on many
// platforms (even 64-bit), arenaL1Bits is 0, making this
// effectively a single-level map. In this case, arenas[0]
// will never be nil.
arenas [1 << arenaL1Bits]*[1 << arenaL2Bits]*heapArena
// heapArenaAlloc is pre-reserved space for allocating heapArena
// objects. This is only used on 32-bit, where we pre-reserve
// this space to avoid interleaving it with the heap itself.
heapArenaAlloc linearAlloc
// arenaHints is a list of addresses at which to attempt to
// add more heap arenas. This is initially populated with a
// set of general hint addresses, and grown with the bounds of
// actual heap arena ranges.
arenaHints *arenaHint
// arena is a pre-reserved space for allocating heap arenas
// (the actual arenas). This is only used on 32-bit.
arena linearAlloc
//_ uint32 // ensure 64-bit alignment of central
// central free lists for small size classes.
// the padding makes sure that the MCentrals are
// spaced CacheLinePadSize bytes apart, so that each MCentral.lock
// gets its own cache line.
// central is indexed by spanClass.
central [numSpanClasses]struct {
mcentral mcentral
pad [cpu.CacheLinePadSize - unsafe.Sizeof(mcentral{})%cpu.CacheLinePadSize]byte
}
spanalloc fixalloc // allocator for span*
cachealloc fixalloc // allocator for mcache*
treapalloc fixalloc // allocator for treapNodes* used by large objects
specialfinalizeralloc fixalloc // allocator for specialfinalizer*
specialprofilealloc fixalloc // allocator for specialprofile*
speciallock mutex // lock for special record allocators.
arenaHintAlloc fixalloc // allocator for arenaHints
unused *specialfinalizer // never set, just here to force the specialfinalizer type into DWARF
}
var mheap_ mheap
// A heapArena stores metadata for a heap arena. heapArenas are stored
// outside of the Go heap and accessed via the mheap_.arenas index.
//
// This gets allocated directly from the OS, so ideally it should be a
// multiple of the system page size. For example, avoid adding small
// fields.
//
//go:notinheap
type heapArena struct {
// bitmap stores the pointer/scalar bitmap for the words in
// this arena. See mbitmap.go for a description. Use the
// heapBits type to access this.
bitmap [heapArenaBitmapBytes]byte
// spans maps from virtual address page ID within this arena to *mspan.
// For allocated spans, their pages map to the span itself.
// For free spans, only the lowest and highest pages map to the span itself.
// Internal pages map to an arbitrary span.
// For pages that have never been allocated, spans entries are nil.
//
// Modifications are protected by mheap.lock. Reads can be
// performed without locking, but ONLY from indexes that are
// known to contain in-use or stack spans. This means there
// must not be a safe-point between establishing that an
// address is live and looking it up in the spans array.
spans [pagesPerArena]*mspan
}
// arenaHint is a hint for where to grow the heap arenas. See
// mheap_.arenaHints.
//
//go:notinheap
type arenaHint struct {
addr uintptr
down bool
next *arenaHint
}
// An MSpan is a run of pages.
//
// 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 or mSpanManual
// and heapmap(i) == span for all s->start <= i < s->start+s->npages.
// Every MSpan is in one doubly-linked list,
// either one of the MHeap's free lists or one of the
// MCentral's span lists.
// An MSpan representing actual memory has state mSpanInUse,
// mSpanManual, or mSpanFree. Transitions between these states are
// constrained as follows:
//
// * A span may transition from free to in-use or manual during any GC
// phase.
//
// * During sweeping (gcphase == _GCoff), a span may transition from
// in-use to free (as a result of sweeping) or manual to free (as a
// result of stacks being freed).
//
// * During GC (gcphase != _GCoff), a span *must not* transition from
// manual or in-use to free. Because concurrent GC may read a pointer
// and then look up its span, the span state must be monotonic.
type mSpanState uint8
const (
mSpanDead mSpanState = iota
mSpanInUse // allocated for garbage collected heap
mSpanManual // allocated for manual management (e.g., stack allocator)
mSpanFree
)
// mSpanStateNames are the names of the span states, indexed by
// mSpanState.
var mSpanStateNames = []string{
"mSpanDead",
"mSpanInUse",
"mSpanManual",
"mSpanFree",
}
// mSpanList heads a linked list of spans.
//
//go:notinheap
type mSpanList struct {
first *mspan // first span in list, or nil if none
last *mspan // last span in list, or nil if none
}
//go:notinheap
type mspan struct {
next *mspan // next span in list, or nil if none
prev *mspan // previous span in list, or nil if none
list *mSpanList // For debugging. TODO: Remove.
startAddr uintptr // address of first byte of span aka s.base()
npages uintptr // number of pages in span
manualFreeList gclinkptr // list of free objects in mSpanManual spans
// freeindex is the slot index between 0 and nelems at which to begin scanning
// for the next free object in this span.
// Each allocation scans allocBits starting at freeindex until it encounters a 0
// indicating a free object. freeindex is then adjusted so that subsequent scans begin
// just past the newly discovered free object.
//
// If freeindex == nelem, this span has no free objects.
//
// allocBits is a bitmap of objects in this span.
// If n >= freeindex and allocBits[n/8] & (1<<(n%8)) is 0
// then object n is free;
// otherwise, object n is allocated. Bits starting at nelem are
// undefined and should never be referenced.
//
// Object n starts at address n*elemsize + (start << pageShift).
freeindex uintptr
// TODO: Look up nelems from sizeclass and remove this field if it
// helps performance.
nelems uintptr // number of object in the span.
// Cache of the allocBits at freeindex. allocCache is shifted
// such that the lowest bit corresponds to the bit freeindex.
// allocCache holds the complement of allocBits, thus allowing
// ctz (count trailing zero) to use it directly.
// allocCache may contain bits beyond s.nelems; the caller must ignore
// these.
allocCache uint64
// allocBits and gcmarkBits hold pointers to a span's mark and
// allocation bits. The pointers are 8 byte aligned.
// There are three arenas where this data is held.
// free: Dirty arenas that are no longer accessed
// and can be reused.
// next: Holds information to be used in the next GC cycle.
// current: Information being used during this GC cycle.
// previous: Information being used during the last GC cycle.
// A new GC cycle starts with the call to finishsweep_m.
// finishsweep_m moves the previous arena to the free arena,
// the current arena to the previous arena, and
// the next arena to the current arena.
// The next arena is populated as the spans request
// memory to hold gcmarkBits for the next GC cycle as well
// as allocBits for newly allocated spans.
//
// The pointer arithmetic is done "by hand" instead of using
// arrays to avoid bounds checks along critical performance
// paths.
// The sweep will free the old allocBits and set allocBits to the
// gcmarkBits. The gcmarkBits are replaced with a fresh zeroed
// out memory.
allocBits *gcBits
gcmarkBits *gcBits
// sweep generation:
// if sweepgen == h->sweepgen - 2, the span needs sweeping
// if sweepgen == h->sweepgen - 1, the span is currently being swept
// if sweepgen == h->sweepgen, the span is swept and ready to use
// if sweepgen == h->sweepgen + 1, the span was cached before sweep began and is still cached, and needs sweeping
// if sweepgen == h->sweepgen + 3, the span was swept and then cached and is still cached
// h->sweepgen is incremented by 2 after every GC
sweepgen uint32
divMul uint16 // for divide by elemsize - divMagic.mul
baseMask uint16 // if non-0, elemsize is a power of 2, & this will get object allocation base
allocCount uint16 // number of allocated objects
spanclass spanClass // size class and noscan (uint8)
state mSpanState // mspaninuse etc
needzero uint8 // needs to be zeroed before allocation
divShift uint8 // for divide by elemsize - divMagic.shift
divShift2 uint8 // for divide by elemsize - divMagic.shift2
elemsize uintptr // computed from sizeclass or from npages
unusedsince int64 // first time spotted by gc in mspanfree state
npreleased uintptr // number of pages released to the os
limit uintptr // end of data in span
speciallock mutex // guards specials list
specials *special // linked list of special records sorted by offset.
}
func (s *mspan) base() uintptr {
return s.startAddr
}
func (s *mspan) layout() (size, n, total uintptr) {
total = s.npages << _PageShift
size = s.elemsize
if size > 0 {
n = total / size
}
return
}
func (s *mspan) scavenge() uintptr {
start := s.base()
end := start + s.npages<<_PageShift
if physPageSize > _PageSize {
// We can only release pages in
// physPageSize blocks, so round start
// and end in. (Otherwise, madvise
// will round them *out* and release
// more memory than we want.)
start = (start + physPageSize - 1) &^ (physPageSize - 1)
end &^= physPageSize - 1
if end <= start {
// start and end don't span a
// whole physical page.
return 0
}
}
len := end - start
released := len - (s.npreleased << _PageShift)
if physPageSize > _PageSize && released == 0 {
return 0
}
memstats.heap_released += uint64(released)
s.npreleased = len >> _PageShift
sysUnused(unsafe.Pointer(start), len)
return released
}
// recordspan adds a newly allocated span to h.allspans.
//
// This only happens the first time a span is allocated from
// mheap.spanalloc (it is not called when a span is reused).
//
// Write barriers are disallowed here because it can be called from
// gcWork when allocating new workbufs. However, because it's an
// indirect call from the fixalloc initializer, the compiler can't see
// this.
//
//go:nowritebarrierrec
func recordspan(vh unsafe.Pointer, p unsafe.Pointer) {
h := (*mheap)(vh)
s := (*mspan)(p)
if len(h.allspans) >= cap(h.allspans) {
n := 64 * 1024 / sys.PtrSize
if n < cap(h.allspans)*3/2 {
n = cap(h.allspans) * 3 / 2
}
var new []*mspan
sp := (*slice)(unsafe.Pointer(&new))
sp.array = sysAlloc(uintptr(n)*sys.PtrSize, &memstats.other_sys)
if sp.array == nil {
throw("runtime: cannot allocate memory")
}
sp.len = len(h.allspans)
sp.cap = n
if len(h.allspans) > 0 {
copy(new, h.allspans)
}
oldAllspans := h.allspans
*(*notInHeapSlice)(unsafe.Pointer(&h.allspans)) = *(*notInHeapSlice)(unsafe.Pointer(&new))
if len(oldAllspans) != 0 {
sysFree(unsafe.Pointer(&oldAllspans[0]), uintptr(cap(oldAllspans))*unsafe.Sizeof(oldAllspans[0]), &memstats.other_sys)
}
}
h.allspans = h.allspans[:len(h.allspans)+1]
h.allspans[len(h.allspans)-1] = s
}
// A spanClass represents the size class and noscan-ness of a span.
//
// Each size class has a noscan spanClass and a scan spanClass. The
// noscan spanClass contains only noscan objects, which do not contain
// pointers and thus do not need to be scanned by the garbage
// collector.
type spanClass uint8
const (
numSpanClasses = _NumSizeClasses << 1
tinySpanClass = spanClass(tinySizeClass<<1 | 1)
)
func makeSpanClass(sizeclass uint8, noscan bool) spanClass {
return spanClass(sizeclass<<1) | spanClass(bool2int(noscan))
}
func (sc spanClass) sizeclass() int8 {
return int8(sc >> 1)
}
func (sc spanClass) noscan() bool {
return sc&1 != 0
}
// arenaIndex returns the index into mheap_.arenas of the arena
// containing metadata for p. This index combines of an index into the
// L1 map and an index into the L2 map and should be used as
// mheap_.arenas[ai.l1()][ai.l2()].
//
// If p is outside the range of valid heap addresses, either l1() or
// l2() will be out of bounds.
//
// It is nosplit because it's called by spanOf and several other
// nosplit functions.
//
//go:nosplit
func arenaIndex(p uintptr) arenaIdx {
return arenaIdx((p + arenaBaseOffset) / heapArenaBytes)
}
// arenaBase returns the low address of the region covered by heap
// arena i.
func arenaBase(i arenaIdx) uintptr {
return uintptr(i)*heapArenaBytes - arenaBaseOffset
}
type arenaIdx uint
func (i arenaIdx) l1() uint {
if arenaL1Bits == 0 {
// Let the compiler optimize this away if there's no
// L1 map.
return 0
} else {
return uint(i) >> arenaL1Shift
}
}
func (i arenaIdx) l2() uint {
if arenaL1Bits == 0 {
return uint(i)
} else {
return uint(i) & (1<<arenaL2Bits - 1)
}
}
// inheap reports whether b is a pointer into a (potentially dead) heap object.
// It returns false for pointers into mSpanManual spans.
// Non-preemptible because it is used by write barriers.
//go:nowritebarrier
//go:nosplit
func inheap(b uintptr) bool {
return spanOfHeap(b) != nil
}
// inHeapOrStack is a variant of inheap that returns true for pointers
// into any allocated heap span.
//
//go:nowritebarrier
//go:nosplit
func inHeapOrStack(b uintptr) bool {
s := spanOf(b)
if s == nil || b < s.base() {
return false
}
switch s.state {
case mSpanInUse, mSpanManual:
return b < s.limit
default:
return false
}
}
// spanOf returns the span of p. If p does not point into the heap
// arena or no span has ever contained p, spanOf returns nil.
//
// If p does not point to allocated memory, this may return a non-nil
// span that does *not* contain p. If this is a possibility, the
// caller should either call spanOfHeap or check the span bounds
// explicitly.
//
// Must be nosplit because it has callers that are nosplit.
//
//go:nosplit
func spanOf(p uintptr) *mspan {
// This function looks big, but we use a lot of constant
// folding around arenaL1Bits to get it under the inlining
// budget. Also, many of the checks here are safety checks
// that Go needs to do anyway, so the generated code is quite
// short.
ri := arenaIndex(p)
if arenaL1Bits == 0 {
// If there's no L1, then ri.l1() can't be out of bounds but ri.l2() can.
if ri.l2() >= uint(len(mheap_.arenas[0])) {
return nil
}
} else {
// If there's an L1, then ri.l1() can be out of bounds but ri.l2() can't.
if ri.l1() >= uint(len(mheap_.arenas)) {
return nil
}
}
l2 := mheap_.arenas[ri.l1()]
if arenaL1Bits != 0 && l2 == nil { // Should never happen if there's no L1.
return nil
}
ha := l2[ri.l2()]
if ha == nil {
return nil
}
return ha.spans[(p/pageSize)%pagesPerArena]
}
// spanOfUnchecked is equivalent to spanOf, but the caller must ensure
// that p points into an allocated heap arena.
//
// Must be nosplit because it has callers that are nosplit.
//
//go:nosplit
func spanOfUnchecked(p uintptr) *mspan {
ai := arenaIndex(p)
return mheap_.arenas[ai.l1()][ai.l2()].spans[(p/pageSize)%pagesPerArena]
}
// spanOfHeap is like spanOf, but returns nil if p does not point to a
// heap object.
//
// Must be nosplit because it has callers that are nosplit.
//
//go:nosplit
func spanOfHeap(p uintptr) *mspan {
s := spanOf(p)
// If p is not allocated, it may point to a stale span, so we
// have to check the span's bounds and state.
if s == nil || p < s.base() || p >= s.limit || s.state != mSpanInUse {
return nil
}
return s
}
// Initialize the heap.
func (h *mheap) init() {
h.treapalloc.init(unsafe.Sizeof(treapNode{}), nil, nil, &memstats.other_sys)
h.spanalloc.init(unsafe.Sizeof(mspan{}), recordspan, unsafe.Pointer(h), &memstats.mspan_sys)
h.cachealloc.init(unsafe.Sizeof(mcache{}), nil, nil, &memstats.mcache_sys)
h.specialfinalizeralloc.init(unsafe.Sizeof(specialfinalizer{}), nil, nil, &memstats.other_sys)
h.specialprofilealloc.init(unsafe.Sizeof(specialprofile{}), nil, nil, &memstats.other_sys)
h.arenaHintAlloc.init(unsafe.Sizeof(arenaHint{}), nil, nil, &memstats.other_sys)
// Don't zero mspan allocations. Background sweeping can
// inspect a span concurrently with allocating it, so it's
// important that the span's sweepgen survive across freeing
// and re-allocating a span to prevent background sweeping
// from improperly cas'ing it from 0.
//
// This is safe because mspan contains no heap pointers.
h.spanalloc.zero = false
// h->mapcache needs no init
h.busy.init()
for i := range h.central {
h.central[i].mcentral.init(spanClass(i))
}
}
// Sweeps spans in list until reclaims at least npages into heap.
// Returns the actual number of pages reclaimed.
func (h *mheap) reclaimList(list *mSpanList, npages uintptr) uintptr {
n := uintptr(0)
sg := mheap_.sweepgen
retry:
for s := list.first; s != nil; s = s.next {
if s.sweepgen == sg-2 && atomic.Cas(&s.sweepgen, sg-2, sg-1) {
list.remove(s)
// swept spans are at the end of the list
list.insertBack(s) // Puts it back on a busy list. s is not in the treap at this point.
unlock(&h.lock)
snpages := s.npages
if s.sweep(false) {
n += snpages
}
lock(&h.lock)
if n >= npages {
return n
}
// the span could have been moved elsewhere
goto retry
}
if s.sweepgen == sg-1 {
// the span is being swept 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.
func (h *mheap) reclaim(npage uintptr) {
if h.reclaimList(&h.busy, npage) != 0 {
return // Bingo!
}
// Now sweep everything that is not yet swept.
var reclaimed uintptr
unlock(&h.lock)
for {
n := sweepone()
if n == ^uintptr(0) { // all spans are swept
break
}
reclaimed += n
if reclaimed >= npage {
break
}
}
lock(&h.lock)
}
// alloc_m is the internal implementation of mheap.alloc.
//
// alloc_m must run on the system stack because it locks the heap, so
// any stack growth during alloc_m would self-deadlock.
//
//go:systemstack
func (h *mheap) alloc_m(npage uintptr, spanclass spanClass, large bool) *mspan {
_g_ := getg()
lock(&h.lock)
// To prevent excessive heap growth, before allocating n pages
// we need to sweep and reclaim at least n pages.
if h.sweepdone == 0 {
// TODO(austin): This tends to sweep a large number of
// spans in order to find a few completely free spans
// (for example, in the garbage benchmark, this sweeps
// ~30x the number of pages it's trying to allocate).
// If GC kept a bit for whether there were any marks
// in a span, we could release these free spans
// at the end of GC and eliminate this entirely.
if trace.enabled {
traceGCSweepStart()
}
h.reclaim(npage)
if trace.enabled {
traceGCSweepDone()
}
}
// transfer stats from cache to global
memstats.heap_scan += uint64(_g_.m.mcache.local_scan)
_g_.m.mcache.local_scan = 0
memstats.tinyallocs += uint64(_g_.m.mcache.local_tinyallocs)
_g_.m.mcache.local_tinyallocs = 0
s := h.allocSpanLocked(npage, &memstats.heap_inuse)
if s != nil {
// Record span info, because gc needs to be
// able to map interior pointer to containing span.
atomic.Store(&s.sweepgen, h.sweepgen)
h.sweepSpans[h.sweepgen/2%2].push(s) // Add to swept in-use list.
s.state = mSpanInUse
s.allocCount = 0
s.spanclass = spanclass
if sizeclass := spanclass.sizeclass(); sizeclass == 0 {
s.elemsize = s.npages << _PageShift
s.divShift = 0
s.divMul = 0
s.divShift2 = 0
s.baseMask = 0
} else {
s.elemsize = uintptr(class_to_size[sizeclass])
m := &class_to_divmagic[sizeclass]
s.divShift = m.shift
s.divMul = m.mul
s.divShift2 = m.shift2
s.baseMask = m.baseMask
}
// update stats, sweep lists
h.pagesInUse += uint64(npage)
if large {
memstats.heap_objects++
mheap_.largealloc += uint64(s.elemsize)
mheap_.nlargealloc++
atomic.Xadd64(&memstats.heap_live, int64(npage<<_PageShift))
// Swept spans are at the end of lists.
h.busy.insertBack(s)
}
}
// heap_scan and heap_live were updated.
if gcBlackenEnabled != 0 {
gcController.revise()
}
if trace.enabled {
traceHeapAlloc()
}
// h.spans is accessed concurrently without synchronization
// from other threads. Hence, there must be a store/store
// barrier here to ensure the writes to h.spans above happen
// before the caller can publish a pointer p to an object
// allocated from s. As soon as this happens, the garbage
// collector running on another processor could read p and
// look up s in h.spans. The unlock acts as the barrier to
// order these writes. On the read side, the data dependency
// between p and the index in h.spans orders the reads.
unlock(&h.lock)
return s
}
// alloc allocates a new span of npage pages from the GC'd heap.
//
// Either large must be true or spanclass must indicates the span's
// size class and scannability.
//
// If needzero is true, the memory for the returned span will be zeroed.
func (h *mheap) alloc(npage uintptr, spanclass spanClass, large bool, needzero bool) *mspan {
// Don't do any operations that lock the heap on the G stack.
// It might trigger stack growth, and the stack growth code needs
// to be able to allocate heap.
var s *mspan
systemstack(func() {
s = h.alloc_m(npage, spanclass, large)
})
if s != nil {
if needzero && s.needzero != 0 {
memclrNoHeapPointers(unsafe.Pointer(s.base()), s.npages<<_PageShift)
}
s.needzero = 0
}
return s
}
// allocManual allocates a manually-managed span of npage pages.
// allocManual returns nil if allocation fails.
//
// allocManual adds the bytes used to *stat, which should be a
// memstats in-use field. Unlike allocations in the GC'd heap, the
// allocation does *not* count toward heap_inuse or heap_sys.
//
// The memory backing the returned span may not be zeroed if
// span.needzero is set.
//
// allocManual must be called on the system stack to prevent stack
// growth. Since this is used by the stack allocator, stack growth
// during allocManual would self-deadlock.
//
//go:systemstack
func (h *mheap) allocManual(npage uintptr, stat *uint64) *mspan {
lock(&h.lock)
s := h.allocSpanLocked(npage, stat)
if s != nil {
s.state = mSpanManual
s.manualFreeList = 0
s.allocCount = 0
s.spanclass = 0
s.nelems = 0
s.elemsize = 0
s.limit = s.base() + s.npages<<_PageShift
// Manually managed memory doesn't count toward heap_sys.
memstats.heap_sys -= uint64(s.npages << _PageShift)
}
// This unlock acts as a release barrier. See mheap.alloc_m.
unlock(&h.lock)
return s
}
// setSpan modifies the span map so spanOf(base) is s.
func (h *mheap) setSpan(base uintptr, s *mspan) {
ai := arenaIndex(base)
h.arenas[ai.l1()][ai.l2()].spans[(base/pageSize)%pagesPerArena] = s
}
// setSpans modifies the span map so [spanOf(base), spanOf(base+npage*pageSize))
// is s.
func (h *mheap) setSpans(base, npage uintptr, s *mspan) {
p := base / pageSize
ai := arenaIndex(base)
ha := h.arenas[ai.l1()][ai.l2()]
for n := uintptr(0); n < npage; n++ {
i := (p + n) % pagesPerArena
if i == 0 {
ai = arenaIndex(base + n*pageSize)
ha = h.arenas[ai.l1()][ai.l2()]
}
ha.spans[i] = s
}
}
// Allocates a span of the given size. h must be locked.
// The returned span has been removed from the
// free list, but its state is still mSpanFree.
func (h *mheap) allocSpanLocked(npage uintptr, stat *uint64) *mspan {
var s *mspan
// Best fit in the treap of spans.
s = h.free.remove(npage)
if s == nil {
if !h.grow(npage) {
return nil
}
s = h.free.remove(npage)
if s == nil {
return nil
}
}
// Mark span in use.
if s.state != mSpanFree {
throw("MHeap_AllocLocked - MSpan not free")
}
if s.npages < npage {
throw("MHeap_AllocLocked - bad npages")
}
if s.npreleased > 0 {
sysUsed(unsafe.Pointer(s.base()), s.npages<<_PageShift)
memstats.heap_released -= uint64(s.npreleased << _PageShift)
s.npreleased = 0
}
if s.npages > npage {
// Trim extra and put it back in the heap.
t := (*mspan)(h.spanalloc.alloc())
t.init(s.base()+npage<<_PageShift, s.npages-npage)
s.npages = npage
h.setSpan(t.base()-1, s)
h.setSpan(t.base(), t)
h.setSpan(t.base()+t.npages*pageSize-1, t)
t.needzero = s.needzero
s.state = mSpanManual // prevent coalescing with s
t.state = mSpanManual
h.freeSpanLocked(t, false, false, s.unusedsince)
s.state = mSpanFree
}
s.unusedsince = 0
h.setSpans(s.base(), npage, s)
*stat += uint64(npage << _PageShift)
memstats.heap_idle -= uint64(npage << _PageShift)
//println("spanalloc", hex(s.start<<_PageShift))
if s.inList() {
throw("still in list")
}
return s
}
// Try to add at least npage pages of memory to the heap,
// returning whether it worked.
//
// h must be locked.
func (h *mheap) grow(npage uintptr) bool {
ask := npage << _PageShift
v, size := h.sysAlloc(ask)
if v == nil {
print("runtime: out of memory: cannot allocate ", ask, "-byte block (", memstats.heap_sys, " in use)\n")
return false
}
// Create a fake "in use" span and free it, so that the
// right coalescing happens.
s := (*mspan)(h.spanalloc.alloc())
s.init(uintptr(v), size/pageSize)
h.setSpans(s.base(), s.npages, s)
atomic.Store(&s.sweepgen, h.sweepgen)
s.state = mSpanInUse
h.pagesInUse += uint64(s.npages)
h.freeSpanLocked(s, false, true, 0)
return true
}
// Free the span back into the heap.
//
// large must match the value of large passed to mheap.alloc. This is
// used for accounting.
func (h *mheap) freeSpan(s *mspan, large bool) {
systemstack(func() {
mp := getg().m
lock(&h.lock)
memstats.heap_scan += uint64(mp.mcache.local_scan)
mp.mcache.local_scan = 0
memstats.tinyallocs += uint64(mp.mcache.local_tinyallocs)
mp.mcache.local_tinyallocs = 0
if msanenabled {
// Tell msan that this entire span is no longer in use.
base := unsafe.Pointer(s.base())
bytes := s.npages << _PageShift
msanfree(base, bytes)
}
if large {
// Match accounting done in mheap.alloc.
memstats.heap_objects--
}
if gcBlackenEnabled != 0 {
// heap_scan changed.
gcController.revise()
}
h.freeSpanLocked(s, true, true, 0)
unlock(&h.lock)
})
}
// freeManual frees a manually-managed span returned by allocManual.
// stat must be the same as the stat passed to the allocManual that
// allocated s.
//
// This must only be called when gcphase == _GCoff. See mSpanState for
// an explanation.
//
// freeManual must be called on the system stack to prevent stack
// growth, just like allocManual.
//
//go:systemstack
func (h *mheap) freeManual(s *mspan, stat *uint64) {
s.needzero = 1
lock(&h.lock)
*stat -= uint64(s.npages << _PageShift)
memstats.heap_sys += uint64(s.npages << _PageShift)
h.freeSpanLocked(s, false, true, 0)
unlock(&h.lock)
}
// s must be on the busy list or unlinked.
func (h *mheap) freeSpanLocked(s *mspan, acctinuse, acctidle bool, unusedsince int64) {
switch s.state {
case mSpanManual:
if s.allocCount != 0 {
throw("MHeap_FreeSpanLocked - invalid stack free")
}
case mSpanInUse:
if s.allocCount != 0 || s.sweepgen != h.sweepgen {
print("MHeap_FreeSpanLocked - span ", s, " ptr ", hex(s.base()), " allocCount ", s.allocCount, " sweepgen ", s.sweepgen, "/", h.sweepgen, "\n")
throw("MHeap_FreeSpanLocked - invalid free")
}
h.pagesInUse -= uint64(s.npages)
default:
throw("MHeap_FreeSpanLocked - invalid span state")
}
if acctinuse {
memstats.heap_inuse -= uint64(s.npages << _PageShift)
}
if acctidle {
memstats.heap_idle += uint64(s.npages << _PageShift)
}
s.state = mSpanFree
if s.inList() {
h.busy.remove(s)
}
// Stamp newly unused spans. The scavenger will use that
// info to potentially give back some pages to the OS.
s.unusedsince = unusedsince
if unusedsince == 0 {
s.unusedsince = nanotime()
}
s.npreleased = 0
// Coalesce with earlier, later spans.
if before := spanOf(s.base() - 1); before != nil && before.state == mSpanFree {
// Now adjust s.
s.startAddr = before.startAddr
s.npages += before.npages
s.npreleased = before.npreleased // absorb released pages
s.needzero |= before.needzero
h.setSpan(before.base(), s)
// The size is potentially changing so the treap needs to delete adjacent nodes and
// insert back as a combined node.
h.free.removeSpan(before)
before.state = mSpanDead
h.spanalloc.free(unsafe.Pointer(before))
}
// Now check to see if next (greater addresses) span is free and can be coalesced.
if after := spanOf(s.base() + s.npages*pageSize); after != nil && after.state == mSpanFree {
s.npages += after.npages
s.npreleased += after.npreleased
s.needzero |= after.needzero
h.setSpan(s.base()+s.npages*pageSize-1, s)
h.free.removeSpan(after)
after.state = mSpanDead
h.spanalloc.free(unsafe.Pointer(after))
}
// Insert s into the free treap.
h.free.insert(s)
}
func scavengeTreapNode(t *treapNode, now, limit uint64) uintptr {
s := t.spanKey
if (now-uint64(s.unusedsince)) > limit && s.npreleased != s.npages {
if released := s.scavenge(); released != 0 {
return released
}
}
return 0
}
func (h *mheap) scavenge(k int32, now, limit uint64) {
// Disallow malloc or panic while holding the heap lock. We do
// this here because this is an non-mallocgc entry-point to
// the mheap API.
gp := getg()
gp.m.mallocing++
lock(&h.lock)
sumreleased := scavengetreap(h.free.treap, now, limit)
unlock(&h.lock)
gp.m.mallocing--
if debug.gctrace > 0 {
if sumreleased > 0 {
print("scvg", k, ": ", sumreleased>>20, " MB released\n")
}
print("scvg", k, ": inuse: ", memstats.heap_inuse>>20, ", idle: ", memstats.heap_idle>>20, ", sys: ", memstats.heap_sys>>20, ", released: ", memstats.heap_released>>20, ", consumed: ", (memstats.heap_sys-memstats.heap_released)>>20, " (MB)\n")
}
}
//go:linkname runtime_debug_freeOSMemory runtime/debug.freeOSMemory
func runtime_debug_freeOSMemory() {
GC()
systemstack(func() { mheap_.scavenge(-1, ^uint64(0), 0) })
}
// Initialize a new span with the given start and npages.
func (span *mspan) init(base uintptr, npages uintptr) {
// span is *not* zeroed.
span.next = nil
span.prev = nil
span.list = nil
span.startAddr = base
span.npages = npages
span.allocCount = 0
span.spanclass = 0
span.elemsize = 0
span.state = mSpanDead
span.unusedsince = 0
span.npreleased = 0
span.speciallock.key = 0
span.specials = nil
span.needzero = 0
span.freeindex = 0
span.allocBits = nil
span.gcmarkBits = nil
}
func (span *mspan) inList() bool {
return span.list != nil
}
// Initialize an empty doubly-linked list.
func (list *mSpanList) init() {
list.first = nil
list.last = nil
}
func (list *mSpanList) remove(span *mspan) {
if span.list != list {
print("runtime: failed MSpanList_Remove span.npages=", span.npages,
" span=", span, " prev=", span.prev, " span.list=", span.list, " list=", list, "\n")
throw("MSpanList_Remove")
}
if list.first == span {
list.first = span.next
} else {
span.prev.next = span.next
}
if list.last == span {
list.last = span.prev
} else {
span.next.prev = span.prev
}
span.next = nil
span.prev = nil
span.list = nil
}
func (list *mSpanList) isEmpty() bool {
return list.first == nil
}
func (list *mSpanList) insert(span *mspan) {
if span.next != nil || span.prev != nil || span.list != nil {
println("runtime: failed MSpanList_Insert", span, span.next, span.prev, span.list)
throw("MSpanList_Insert")
}
span.next = list.first
if list.first != nil {
// The list contains at least one span; link it in.
// The last span in the list doesn't change.
list.first.prev = span
} else {
// The list contains no spans, so this is also the last span.
list.last = span
}
list.first = span
span.list = list
}
func (list *mSpanList) insertBack(span *mspan) {
if span.next != nil || span.prev != nil || span.list != nil {
println("runtime: failed MSpanList_InsertBack", span, span.next, span.prev, span.list)
throw("MSpanList_InsertBack")
}
span.prev = list.last
if list.last != nil {
// The list contains at least one span.
list.last.next = span
} else {
// The list contains no spans, so this is also the first span.
list.first = span
}
list.last = span
span.list = list
}
// takeAll removes all spans from other and inserts them at the front
// of list.
func (list *mSpanList) takeAll(other *mSpanList) {
if other.isEmpty() {
return
}
// Reparent everything in other to list.
for s := other.first; s != nil; s = s.next {
s.list = list
}
// Concatenate the lists.
if list.isEmpty() {
*list = *other
} else {
// Neither list is empty. Put other before list.
other.last.next = list.first
list.first.prev = other.last
list.first = other.first
}
other.first, other.last = nil, nil
}
const (
_KindSpecialFinalizer = 1
_KindSpecialProfile = 2
// Note: The finalizer special must be first because if we're freeing
// an object, a finalizer special will cause the freeing operation
// to abort, and we want to keep the other special records around
// if that happens.
)
//go:notinheap
type special struct {
next *special // linked list in span
offset uint16 // span offset of object
kind byte // kind of special
}
// 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.)
func addspecial(p unsafe.Pointer, s *special) bool {
span := spanOfHeap(uintptr(p))
if span == nil {
throw("addspecial on invalid pointer")
}
// Ensure that the span is swept.
// Sweeping accesses the specials list w/o locks, so we have
// to synchronize with it. And it's just much safer.
mp := acquirem()
span.ensureSwept()
offset := uintptr(p) - span.base()
kind := s.kind
lock(&span.speciallock)
// Find splice point, check for existing record.
t := &span.specials
for {
x := *t
if x == nil {
break
}
if offset == uintptr(x.offset) && kind == x.kind {
unlock(&span.speciallock)
releasem(mp)
return false // already exists
}
if offset < uintptr(x.offset) || (offset == uintptr(x.offset) && kind < x.kind) {
break
}
t = &x.next
}
// Splice in record, fill in offset.
s.offset = uint16(offset)
s.next = *t
*t = s
unlock(&span.speciallock)
releasem(mp)
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.
func removespecial(p unsafe.Pointer, kind uint8) *special {
span := spanOfHeap(uintptr(p))
if span == nil {
throw("removespecial on invalid pointer")
}
// Ensure that the span is swept.
// Sweeping accesses the specials list w/o locks, so we have
// to synchronize with it. And it's just much safer.
mp := acquirem()
span.ensureSwept()
offset := uintptr(p) - span.base()
lock(&span.speciallock)
t := &span.specials
for {
s := *t
if s == nil {
break
}
// 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 == uintptr(s.offset) && kind == s.kind {
*t = s.next
unlock(&span.speciallock)
releasem(mp)
return s
}
t = &s.next
}
unlock(&span.speciallock)
releasem(mp)
return nil
}
// The described object has a finalizer set for it.
//
// specialfinalizer is allocated from non-GC'd memory, so any heap
// pointers must be specially handled.
//
//go:notinheap
type specialfinalizer struct {
special special
fn *funcval // May be a heap pointer.
nret uintptr
fint *_type // May be a heap pointer, but always live.
ot *ptrtype // May be a heap pointer, but always live.
}
// Adds a finalizer to the object p. Returns true if it succeeded.
func addfinalizer(p unsafe.Pointer, f *funcval, nret uintptr, fint *_type, ot *ptrtype) bool {
lock(&mheap_.speciallock)
s := (*specialfinalizer)(mheap_.specialfinalizeralloc.alloc())
unlock(&mheap_.speciallock)
s.special.kind = _KindSpecialFinalizer
s.fn = f
s.nret = nret
s.fint = fint
s.ot = ot
if addspecial(p, &s.special) {
// This is responsible for maintaining the same
// GC-related invariants as markrootSpans in any
// situation where it's possible that markrootSpans
// has already run but mark termination hasn't yet.
if gcphase != _GCoff {
base, _, _ := findObject(uintptr(p), 0, 0)
mp := acquirem()
gcw := &mp.p.ptr().gcw
// Mark everything reachable from the object
// so it's retained for the finalizer.
scanobject(base, gcw)
// Mark the finalizer itself, since the
// special isn't part of the GC'd heap.
scanblock(uintptr(unsafe.Pointer(&s.fn)), sys.PtrSize, &oneptrmask[0], gcw, nil)
releasem(mp)
}
return true
}
// There was an old finalizer
lock(&mheap_.speciallock)
mheap_.specialfinalizeralloc.free(unsafe.Pointer(s))
unlock(&mheap_.speciallock)
return false
}
// Removes the finalizer (if any) from the object p.
func removefinalizer(p unsafe.Pointer) {
s := (*specialfinalizer)(unsafe.Pointer(removespecial(p, _KindSpecialFinalizer)))
if s == nil {
return // there wasn't a finalizer to remove
}
lock(&mheap_.speciallock)
mheap_.specialfinalizeralloc.free(unsafe.Pointer(s))
unlock(&mheap_.speciallock)
}
// The described object is being heap profiled.
//
//go:notinheap
type specialprofile struct {
special special
b *bucket
}
// Set the heap profile bucket associated with addr to b.
func setprofilebucket(p unsafe.Pointer, b *bucket) {
lock(&mheap_.speciallock)
s := (*specialprofile)(mheap_.specialprofilealloc.alloc())
unlock(&mheap_.speciallock)
s.special.kind = _KindSpecialProfile
s.b = b
if !addspecial(p, &s.special) {
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.
func freespecial(s *special, p unsafe.Pointer, size uintptr) {
switch s.kind {
case _KindSpecialFinalizer:
sf := (*specialfinalizer)(unsafe.Pointer(s))
queuefinalizer(p, sf.fn, sf.nret, sf.fint, sf.ot)
lock(&mheap_.speciallock)
mheap_.specialfinalizeralloc.free(unsafe.Pointer(sf))
unlock(&mheap_.speciallock)
case _KindSpecialProfile:
sp := (*specialprofile)(unsafe.Pointer(s))
mProf_Free(sp.b, size)
lock(&mheap_.speciallock)
mheap_.specialprofilealloc.free(unsafe.Pointer(sp))
unlock(&mheap_.speciallock)
default:
throw("bad special kind")
panic("not reached")
}
}
// gcBits is an alloc/mark bitmap. This is always used as *gcBits.
//
//go:notinheap
type gcBits uint8
// bytep returns a pointer to the n'th byte of b.
func (b *gcBits) bytep(n uintptr) *uint8 {
return addb((*uint8)(b), n)
}
// bitp returns a pointer to the byte containing bit n and a mask for
// selecting that bit from *bytep.
func (b *gcBits) bitp(n uintptr) (bytep *uint8, mask uint8) {
return b.bytep(n / 8), 1 << (n % 8)
}
const gcBitsChunkBytes = uintptr(64 << 10)
const gcBitsHeaderBytes = unsafe.Sizeof(gcBitsHeader{})
type gcBitsHeader struct {
free uintptr // free is the index into bits of the next free byte.
next uintptr // *gcBits triggers recursive type bug. (issue 14620)
}
//go:notinheap
type gcBitsArena struct {
// gcBitsHeader // side step recursive type bug (issue 14620) by including fields by hand.
free uintptr // free is the index into bits of the next free byte; read/write atomically
next *gcBitsArena
bits [gcBitsChunkBytes - gcBitsHeaderBytes]gcBits
}
var gcBitsArenas struct {
lock mutex
free *gcBitsArena
next *gcBitsArena // Read atomically. Write atomically under lock.
current *gcBitsArena
previous *gcBitsArena
}
// tryAlloc allocates from b or returns nil if b does not have enough room.
// This is safe to call concurrently.
func (b *gcBitsArena) tryAlloc(bytes uintptr) *gcBits {
if b == nil || atomic.Loaduintptr(&b.free)+bytes > uintptr(len(b.bits)) {
return nil
}
// Try to allocate from this block.
end := atomic.Xadduintptr(&b.free, bytes)
if end > uintptr(len(b.bits)) {
return nil
}
// There was enough room.
start := end - bytes
return &b.bits[start]
}
// newMarkBits returns a pointer to 8 byte aligned bytes
// to be used for a span's mark bits.
func newMarkBits(nelems uintptr) *gcBits {
blocksNeeded := uintptr((nelems + 63) / 64)
bytesNeeded := blocksNeeded * 8
// Try directly allocating from the current head arena.
head := (*gcBitsArena)(atomic.Loadp(unsafe.Pointer(&gcBitsArenas.next)))
if p := head.tryAlloc(bytesNeeded); p != nil {
return p
}
// There's not enough room in the head arena. We may need to
// allocate a new arena.
lock(&gcBitsArenas.lock)
// Try the head arena again, since it may have changed. Now
// that we hold the lock, the list head can't change, but its
// free position still can.
if p := gcBitsArenas.next.tryAlloc(bytesNeeded); p != nil {
unlock(&gcBitsArenas.lock)
return p
}
// Allocate a new arena. This may temporarily drop the lock.
fresh := newArenaMayUnlock()
// If newArenaMayUnlock dropped the lock, another thread may
// have put a fresh arena on the "next" list. Try allocating
// from next again.
if p := gcBitsArenas.next.tryAlloc(bytesNeeded); p != nil {
// Put fresh back on the free list.
// TODO: Mark it "already zeroed"
fresh.next = gcBitsArenas.free
gcBitsArenas.free = fresh
unlock(&gcBitsArenas.lock)
return p
}
// Allocate from the fresh arena. We haven't linked it in yet, so
// this cannot race and is guaranteed to succeed.
p := fresh.tryAlloc(bytesNeeded)
if p == nil {
throw("markBits overflow")
}
// Add the fresh arena to the "next" list.
fresh.next = gcBitsArenas.next
atomic.StorepNoWB(unsafe.Pointer(&gcBitsArenas.next), unsafe.Pointer(fresh))
unlock(&gcBitsArenas.lock)
return p
}
// newAllocBits returns a pointer to 8 byte aligned bytes
// to be used for this span's alloc bits.
// newAllocBits is used to provide newly initialized spans
// allocation bits. For spans not being initialized the
// mark bits are repurposed as allocation bits when
// the span is swept.
func newAllocBits(nelems uintptr) *gcBits {
return newMarkBits(nelems)
}
// nextMarkBitArenaEpoch establishes a new epoch for the arenas
// holding the mark bits. The arenas are named relative to the
// current GC cycle which is demarcated by the call to finishweep_m.
//
// All current spans have been swept.
// During that sweep each span allocated room for its gcmarkBits in
// gcBitsArenas.next block. gcBitsArenas.next becomes the gcBitsArenas.current
// where the GC will mark objects and after each span is swept these bits
// will be used to allocate objects.
// gcBitsArenas.current becomes gcBitsArenas.previous where the span's
// gcAllocBits live until all the spans have been swept during this GC cycle.
// The span's sweep extinguishes all the references to gcBitsArenas.previous
// by pointing gcAllocBits into the gcBitsArenas.current.
// The gcBitsArenas.previous is released to the gcBitsArenas.free list.
func nextMarkBitArenaEpoch() {
lock(&gcBitsArenas.lock)
if gcBitsArenas.previous != nil {
if gcBitsArenas.free == nil {
gcBitsArenas.free = gcBitsArenas.previous
} else {
// Find end of previous arenas.
last := gcBitsArenas.previous
for last = gcBitsArenas.previous; last.next != nil; last = last.next {
}
last.next = gcBitsArenas.free
gcBitsArenas.free = gcBitsArenas.previous
}
}
gcBitsArenas.previous = gcBitsArenas.current
gcBitsArenas.current = gcBitsArenas.next
atomic.StorepNoWB(unsafe.Pointer(&gcBitsArenas.next), nil) // newMarkBits calls newArena when needed
unlock(&gcBitsArenas.lock)
}
// newArenaMayUnlock allocates and zeroes a gcBits arena.
// The caller must hold gcBitsArena.lock. This may temporarily release it.
func newArenaMayUnlock() *gcBitsArena {
var result *gcBitsArena
if gcBitsArenas.free == nil {
unlock(&gcBitsArenas.lock)
result = (*gcBitsArena)(sysAlloc(gcBitsChunkBytes, &memstats.gc_sys))
if result == nil {
throw("runtime: cannot allocate memory")
}
lock(&gcBitsArenas.lock)
} else {
result = gcBitsArenas.free
gcBitsArenas.free = gcBitsArenas.free.next
memclrNoHeapPointers(unsafe.Pointer(result), gcBitsChunkBytes)
}
result.next = nil
// If result.bits is not 8 byte aligned adjust index so
// that &result.bits[result.free] is 8 byte aligned.
if uintptr(unsafe.Offsetof(gcBitsArena{}.bits))&7 == 0 {
result.free = 0
} else {
result.free = 8 - (uintptr(unsafe.Pointer(&result.bits[0])) & 7)
}
return result
}