Adding upstream version 1.19.8.upstream/1.19.8upstream

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 2166 insertions, 0 deletions

diff --git a/src/runtime/mheap.go b/src/runtime/mheap.go
new file mode 100644
index 0000000..d655f2d
--- /dev/null
+++ b/src/runtime/mheap.go
@@ -0,0 +1,2166 @@
+// 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"
+	"internal/goarch"
+	"runtime/internal/atomic"
+	"unsafe"
+)
+
+const (
+	// 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.
+	minPhysPageSize = 4096
+
+	// maxPhysPageSize is the maximum page size the runtime supports.
+	maxPhysPageSize = 512 << 10
+
+	// maxPhysHugePageSize sets an upper-bound on the maximum huge page size
+	// that the runtime supports.
+	maxPhysHugePageSize = pallocChunkBytes
+
+	// pagesPerReclaimerChunk indicates how many pages to scan from the
+	// pageInUse bitmap at a time. Used by the page reclaimer.
+	//
+	// Higher values reduce contention on scanning indexes (such as
+	// h.reclaimIndex), but increase the minimum latency of the
+	// operation.
+	//
+	// The time required to scan this many pages can vary a lot depending
+	// on how many spans are actually freed. Experimentally, it can
+	// scan for pages at ~300 GB/ms on a 2.6GHz Core i7, but can only
+	// free spans at ~32 MB/ms. Using 512 pages bounds this at
+	// roughly 100µs.
+	//
+	// Must be a multiple of the pageInUse bitmap element size and
+	// must also evenly divide pagesPerArena.
+	pagesPerReclaimerChunk = 512
+
+	// physPageAlignedStacks indicates whether stack allocations must be
+	// physical page aligned. This is a requirement for MAP_STACK on
+	// OpenBSD.
+	physPageAlignedStacks = GOOS == "openbsd"
+)
+
+// Main malloc heap.
+// The heap itself is the "free" and "scav" treaps,
+// 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 must only be acquired on the system stack, otherwise a g
+	// could self-deadlock if its stack grows with the lock held.
+	lock mutex
+
+	_ uint32 // 8-byte align pages so its alignment is consistent with tests.
+
+	pages pageAlloc // page allocation data structure
+
+	sweepgen uint32 // sweep generation, see comment in mspan; written during STW
+
+	// 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
+
+	// _ uint32 // align uint64 fields on 32-bit for atomics
+
+	// Proportional sweep
+	//
+	// These parameters represent a linear function from gcController.heapLive
+	// 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 gcController.heapLive.
+	//
+	// The line has slope sweepPagesPerByte and passes through a
+	// basis point at (sweepHeapLiveBasis, pagesSweptBasis). At
+	// any given time, the system is at (gcController.heapLive,
+	// pagesSwept) in this space.
+	//
+	// It is 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         atomic.Uint64 // pages of spans in stats mSpanInUse
+	pagesSwept         atomic.Uint64 // pages swept this cycle
+	pagesSweptBasis    atomic.Uint64 // pagesSwept to use as the origin of the sweep ratio
+	sweepHeapLiveBasis uint64        // value of gcController.heapLive 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.
+
+	// Page reclaimer state
+
+	// reclaimIndex is the page index in allArenas of next page to
+	// reclaim. Specifically, it refers to page (i %
+	// pagesPerArena) of arena allArenas[i / pagesPerArena].
+	//
+	// If this is >= 1<<63, the page reclaimer is done scanning
+	// the page marks.
+	reclaimIndex atomic.Uint64
+
+	// reclaimCredit is spare credit for extra pages swept. Since
+	// the page reclaimer works in large chunks, it may reclaim
+	// more than requested. Any spare pages released go to this
+	// credit pool.
+	reclaimCredit atomic.Uintptr
+
+	// 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
+
+	// allArenas is the arenaIndex of every mapped arena. This can
+	// be used to iterate through the address space.
+	//
+	// Access is protected by mheap_.lock. However, since this is
+	// append-only and old backing arrays are never freed, it is
+	// safe to acquire mheap_.lock, copy the slice header, and
+	// then release mheap_.lock.
+	allArenas []arenaIdx
+
+	// sweepArenas is a snapshot of allArenas taken at the
+	// beginning of the sweep cycle. This can be read safely by
+	// simply blocking GC (by disabling preemption).
+	sweepArenas []arenaIdx
+
+	// markArenas is a snapshot of allArenas taken at the beginning
+	// of the mark cycle. Because allArenas is append-only, neither
+	// this slice nor its contents will change during the mark, so
+	// it can be read safely.
+	markArenas []arenaIdx
+
+	// curArena is the arena that the heap is currently growing
+	// into. This should always be physPageSize-aligned.
+	curArena struct {
+		base, end uintptr
+	}
+
+	_ 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*
+	specialfinalizeralloc fixalloc // allocator for specialfinalizer*
+	specialprofilealloc   fixalloc // allocator for specialprofile*
+	specialReachableAlloc fixalloc // allocator for specialReachable
+	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.
+//
+//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
+
+	// pageInUse is a bitmap that indicates which spans are in
+	// state mSpanInUse. This bitmap is indexed by page number,
+	// but only the bit corresponding to the first page in each
+	// span is used.
+	//
+	// Reads and writes are atomic.
+	pageInUse [pagesPerArena / 8]uint8
+
+	// pageMarks is a bitmap that indicates which spans have any
+	// marked objects on them. Like pageInUse, only the bit
+	// corresponding to the first page in each span is used.
+	//
+	// Writes are done atomically during marking. Reads are
+	// non-atomic and lock-free since they only occur during
+	// sweeping (and hence never race with writes).
+	//
+	// This is used to quickly find whole spans that can be freed.
+	//
+	// TODO(austin): It would be nice if this was uint64 for
+	// faster scanning, but we don't have 64-bit atomic bit
+	// operations.
+	pageMarks [pagesPerArena / 8]uint8
+
+	// pageSpecials is a bitmap that indicates which spans have
+	// specials (finalizers or other). Like pageInUse, only the bit
+	// corresponding to the first page in each span is used.
+	//
+	// Writes are done atomically whenever a special is added to
+	// a span and whenever the last special is removed from a span.
+	// Reads are done atomically to find spans containing specials
+	// during marking.
+	pageSpecials [pagesPerArena / 8]uint8
+
+	// checkmarks stores the debug.gccheckmark state. It is only
+	// used if debug.gccheckmark > 0.
+	checkmarks *checkmarksMap
+
+	// zeroedBase marks the first byte of the first page in this
+	// arena which hasn't been used yet and is therefore already
+	// zero. zeroedBase is relative to the arena base.
+	// Increases monotonically until it hits heapArenaBytes.
+	//
+	// This field is sufficient to determine if an allocation
+	// needs to be zeroed because the page allocator follows an
+	// address-ordered first-fit policy.
+	//
+	// Read atomically and written with an atomic CAS.
+	zeroedBase uintptr
+}
+
+// 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 treap, state == mSpanFree
+// and heapmap(s->start) == span, heapmap(s->start+s->npages-1) == span.
+// If the mspan is in the heap scav treap, then in addition to the
+// above scavenged == true. scavenged == false in all other cases.
+//
+// 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 in the mheap's
+// busy list 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.
+//
+// Setting mspan.state to mSpanInUse or mSpanManual must be done
+// atomically and only after all other span fields are valid.
+// Likewise, if inspecting a span is contingent on it being
+// mSpanInUse, the state should be loaded atomically and checked
+// before depending on other fields. This allows the garbage collector
+// to safely deal with potentially invalid pointers, since resolving
+// such pointers may race with a span being allocated.
+type mSpanState uint8
+
+const (
+	mSpanDead   mSpanState = iota
+	mSpanInUse             // allocated for garbage collected heap
+	mSpanManual            // allocated for manual management (e.g., stack allocator)
+)
+
+// mSpanStateNames are the names of the span states, indexed by
+// mSpanState.
+var mSpanStateNames = []string{
+	"mSpanDead",
+	"mSpanInUse",
+	"mSpanManual",
+	"mSpanFree",
+}
+
+// mSpanStateBox holds an mSpanState and provides atomic operations on
+// it. This is a separate type to disallow accidental comparison or
+// assignment with mSpanState.
+type mSpanStateBox struct {
+	s mSpanState
+}
+
+func (b *mSpanStateBox) set(s mSpanState) {
+	atomic.Store8((*uint8)(&b.s), uint8(s))
+}
+
+func (b *mSpanStateBox) get() mSpanState {
+	return mSpanState(atomic.Load8((*uint8)(&b.s)))
+}
+
+// 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                uint32        // for divide by elemsize
+	allocCount            uint16        // number of allocated objects
+	spanclass             spanClass     // size class and noscan (uint8)
+	state                 mSpanStateBox // mSpanInUse etc; accessed atomically (get/set methods)
+	needzero              uint8         // needs to be zeroed before allocation
+	allocCountBeforeCache uint16        // a copy of allocCount that is stored just before this span is cached
+	elemsize              uintptr       // computed from sizeclass or from npages
+	limit                 uintptr       // end of data in span
+	speciallock           mutex         // guards specials list
+	specials              *special      // linked list of special records sorted by offset.
+
+	// freeIndexForScan is like freeindex, except that freeindex is
+	// used by the allocator whereas freeIndexForScan is used by the
+	// GC scanner. They are two fields so that the GC sees the object
+	// is allocated only when the object and the heap bits are
+	// initialized (see also the assignment of freeIndexForScan in
+	// mallocgc, and issue 54596).
+	freeIndexForScan uintptr
+}
+
+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
+}
+
+// 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.
+//
+// The heap lock must be held.
+//
+//go:nowritebarrierrec
+func recordspan(vh unsafe.Pointer, p unsafe.Pointer) {
+	h := (*mheap)(vh)
+	s := (*mspan)(p)
+
+	assertLockHeld(&h.lock)
+
+	if len(h.allspans) >= cap(h.allspans) {
+		n := 64 * 1024 / goarch.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)*goarch.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.get() {
+	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)
+	// s is nil if it's never been allocated. Otherwise, we check
+	// its state first because we don't trust this pointer, so we
+	// have to synchronize with span initialization. Then, it's
+	// still possible we picked up a stale span pointer, so we
+	// have to check the span's bounds.
+	if s == nil || s.state.get() != mSpanInUse || p < s.base() || p >= s.limit {
+		return nil
+	}
+	return s
+}
+
+// pageIndexOf returns the arena, page index, and page mask for pointer p.
+// The caller must ensure p is in the heap.
+func pageIndexOf(p uintptr) (arena *heapArena, pageIdx uintptr, pageMask uint8) {
+	ai := arenaIndex(p)
+	arena = mheap_.arenas[ai.l1()][ai.l2()]
+	pageIdx = ((p / pageSize) / 8) % uintptr(len(arena.pageInUse))
+	pageMask = byte(1 << ((p / pageSize) % 8))
+	return
+}
+
+// Initialize the heap.
+func (h *mheap) init() {
+	lockInit(&h.lock, lockRankMheap)
+	lockInit(&h.speciallock, lockRankMheapSpecial)
+
+	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.specialReachableAlloc.init(unsafe.Sizeof(specialReachable{}), 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
+
+	for i := range h.central {
+		h.central[i].mcentral.init(spanClass(i))
+	}
+
+	h.pages.init(&h.lock, &memstats.gcMiscSys)
+}
+
+// reclaim sweeps and reclaims at least npage pages into the heap.
+// It is called before allocating npage pages to keep growth in check.
+//
+// reclaim implements the page-reclaimer half of the sweeper.
+//
+// h.lock must NOT be held.
+func (h *mheap) reclaim(npage uintptr) {
+	// TODO(austin): Half of the time spent freeing spans is in
+	// locking/unlocking the heap (even with low contention). We
+	// could make the slow path here several times faster by
+	// batching heap frees.
+
+	// Bail early if there's no more reclaim work.
+	if h.reclaimIndex.Load() >= 1<<63 {
+		return
+	}
+
+	// Disable preemption so the GC can't start while we're
+	// sweeping, so we can read h.sweepArenas, and so
+	// traceGCSweepStart/Done pair on the P.
+	mp := acquirem()
+
+	if trace.enabled {
+		traceGCSweepStart()
+	}
+
+	arenas := h.sweepArenas
+	locked := false
+	for npage > 0 {
+		// Pull from accumulated credit first.
+		if credit := h.reclaimCredit.Load(); credit > 0 {
+			take := credit
+			if take > npage {
+				// Take only what we need.
+				take = npage
+			}
+			if h.reclaimCredit.CompareAndSwap(credit, credit-take) {
+				npage -= take
+			}
+			continue
+		}
+
+		// Claim a chunk of work.
+		idx := uintptr(h.reclaimIndex.Add(pagesPerReclaimerChunk) - pagesPerReclaimerChunk)
+		if idx/pagesPerArena >= uintptr(len(arenas)) {
+			// Page reclaiming is done.
+			h.reclaimIndex.Store(1 << 63)
+			break
+		}
+
+		if !locked {
+			// Lock the heap for reclaimChunk.
+			lock(&h.lock)
+			locked = true
+		}
+
+		// Scan this chunk.
+		nfound := h.reclaimChunk(arenas, idx, pagesPerReclaimerChunk)
+		if nfound <= npage {
+			npage -= nfound
+		} else {
+			// Put spare pages toward global credit.
+			h.reclaimCredit.Add(nfound - npage)
+			npage = 0
+		}
+	}
+	if locked {
+		unlock(&h.lock)
+	}
+
+	if trace.enabled {
+		traceGCSweepDone()
+	}
+	releasem(mp)
+}
+
+// reclaimChunk sweeps unmarked spans that start at page indexes [pageIdx, pageIdx+n).
+// It returns the number of pages returned to the heap.
+//
+// h.lock must be held and the caller must be non-preemptible. Note: h.lock may be
+// temporarily unlocked and re-locked in order to do sweeping or if tracing is
+// enabled.
+func (h *mheap) reclaimChunk(arenas []arenaIdx, pageIdx, n uintptr) uintptr {
+	// The heap lock must be held because this accesses the
+	// heapArena.spans arrays using potentially non-live pointers.
+	// In particular, if a span were freed and merged concurrently
+	// with this probing heapArena.spans, it would be possible to
+	// observe arbitrary, stale span pointers.
+	assertLockHeld(&h.lock)
+
+	n0 := n
+	var nFreed uintptr
+	sl := sweep.active.begin()
+	if !sl.valid {
+		return 0
+	}
+	for n > 0 {
+		ai := arenas[pageIdx/pagesPerArena]
+		ha := h.arenas[ai.l1()][ai.l2()]
+
+		// Get a chunk of the bitmap to work on.
+		arenaPage := uint(pageIdx % pagesPerArena)
+		inUse := ha.pageInUse[arenaPage/8:]
+		marked := ha.pageMarks[arenaPage/8:]
+		if uintptr(len(inUse)) > n/8 {
+			inUse = inUse[:n/8]
+			marked = marked[:n/8]
+		}
+
+		// Scan this bitmap chunk for spans that are in-use
+		// but have no marked objects on them.
+		for i := range inUse {
+			inUseUnmarked := atomic.Load8(&inUse[i]) &^ marked[i]
+			if inUseUnmarked == 0 {
+				continue
+			}
+
+			for j := uint(0); j < 8; j++ {
+				if inUseUnmarked&(1<<j) != 0 {
+					s := ha.spans[arenaPage+uint(i)*8+j]
+					if s, ok := sl.tryAcquire(s); ok {
+						npages := s.npages
+						unlock(&h.lock)
+						if s.sweep(false) {
+							nFreed += npages
+						}
+						lock(&h.lock)
+						// Reload inUse. It's possible nearby
+						// spans were freed when we dropped the
+						// lock and we don't want to get stale
+						// pointers from the spans array.
+						inUseUnmarked = atomic.Load8(&inUse[i]) &^ marked[i]
+					}
+				}
+			}
+		}
+
+		// Advance.
+		pageIdx += uintptr(len(inUse) * 8)
+		n -= uintptr(len(inUse) * 8)
+	}
+	sweep.active.end(sl)
+	if trace.enabled {
+		unlock(&h.lock)
+		// Account for pages scanned but not reclaimed.
+		traceGCSweepSpan((n0 - nFreed) * pageSize)
+		lock(&h.lock)
+	}
+
+	assertLockHeld(&h.lock) // Must be locked on return.
+	return nFreed
+}
+
+// spanAllocType represents the type of allocation to make, or
+// the type of allocation to be freed.
+type spanAllocType uint8
+
+const (
+	spanAllocHeap          spanAllocType = iota // heap span
+	spanAllocStack                              // stack span
+	spanAllocPtrScalarBits                      // unrolled GC prog bitmap span
+	spanAllocWorkBuf                            // work buf span
+)
+
+// manual returns true if the span allocation is manually managed.
+func (s spanAllocType) manual() bool {
+	return s != spanAllocHeap
+}
+
+// alloc allocates a new span of npage pages from the GC'd heap.
+//
+// spanclass indicates the span's size class and scannability.
+//
+// Returns a span that has been fully initialized. span.needzero indicates
+// whether the span has been zeroed. Note that it may not be.
+func (h *mheap) alloc(npages uintptr, spanclass spanClass) *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() {
+		// To prevent excessive heap growth, before allocating n pages
+		// we need to sweep and reclaim at least n pages.
+		if !isSweepDone() {
+			h.reclaim(npages)
+		}
+		s = h.allocSpan(npages, spanAllocHeap, spanclass)
+	})
+	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 heapInUse.
+//
+// The memory backing the returned span may not be zeroed if
+// span.needzero is set.
+//
+// allocManual must be called on the system stack because it may
+// acquire the heap lock via allocSpan. See mheap for details.
+//
+// If new code is written to call allocManual, do NOT use an
+// existing spanAllocType value and instead declare a new one.
+//
+//go:systemstack
+func (h *mheap) allocManual(npages uintptr, typ spanAllocType) *mspan {
+	if !typ.manual() {
+		throw("manual span allocation called with non-manually-managed type")
+	}
+	return h.allocSpan(npages, typ, 0)
+}
+
+// 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
+	}
+}
+
+// allocNeedsZero checks if the region of address space [base, base+npage*pageSize),
+// assumed to be allocated, needs to be zeroed, updating heap arena metadata for
+// future allocations.
+//
+// This must be called each time pages are allocated from the heap, even if the page
+// allocator can otherwise prove the memory it's allocating is already zero because
+// they're fresh from the operating system. It updates heapArena metadata that is
+// critical for future page allocations.
+//
+// There are no locking constraints on this method.
+func (h *mheap) allocNeedsZero(base, npage uintptr) (needZero bool) {
+	for npage > 0 {
+		ai := arenaIndex(base)
+		ha := h.arenas[ai.l1()][ai.l2()]
+
+		zeroedBase := atomic.Loaduintptr(&ha.zeroedBase)
+		arenaBase := base % heapArenaBytes
+		if arenaBase < zeroedBase {
+			// We extended into the non-zeroed part of the
+			// arena, so this region needs to be zeroed before use.
+			//
+			// zeroedBase is monotonically increasing, so if we see this now then
+			// we can be sure we need to zero this memory region.
+			//
+			// We still need to update zeroedBase for this arena, and
+			// potentially more arenas.
+			needZero = true
+		}
+		// We may observe arenaBase > zeroedBase if we're racing with one or more
+		// allocations which are acquiring memory directly before us in the address
+		// space. But, because we know no one else is acquiring *this* memory, it's
+		// still safe to not zero.
+
+		// Compute how far into the arena we extend into, capped
+		// at heapArenaBytes.
+		arenaLimit := arenaBase + npage*pageSize
+		if arenaLimit > heapArenaBytes {
+			arenaLimit = heapArenaBytes
+		}
+		// Increase ha.zeroedBase so it's >= arenaLimit.
+		// We may be racing with other updates.
+		for arenaLimit > zeroedBase {
+			if atomic.Casuintptr(&ha.zeroedBase, zeroedBase, arenaLimit) {
+				break
+			}
+			zeroedBase = atomic.Loaduintptr(&ha.zeroedBase)
+			// Double check basic conditions of zeroedBase.
+			if zeroedBase <= arenaLimit && zeroedBase > arenaBase {
+				// The zeroedBase moved into the space we were trying to
+				// claim. That's very bad, and indicates someone allocated
+				// the same region we did.
+				throw("potentially overlapping in-use allocations detected")
+			}
+		}
+
+		// Move base forward and subtract from npage to move into
+		// the next arena, or finish.
+		base += arenaLimit - arenaBase
+		npage -= (arenaLimit - arenaBase) / pageSize
+	}
+	return
+}
+
+// tryAllocMSpan attempts to allocate an mspan object from
+// the P-local cache, but may fail.
+//
+// h.lock need not be held.
+//
+// This caller must ensure that its P won't change underneath
+// it during this function. Currently to ensure that we enforce
+// that the function is run on the system stack, because that's
+// the only place it is used now. In the future, this requirement
+// may be relaxed if its use is necessary elsewhere.
+//
+//go:systemstack
+func (h *mheap) tryAllocMSpan() *mspan {
+	pp := getg().m.p.ptr()
+	// If we don't have a p or the cache is empty, we can't do
+	// anything here.
+	if pp == nil || pp.mspancache.len == 0 {
+		return nil
+	}
+	// Pull off the last entry in the cache.
+	s := pp.mspancache.buf[pp.mspancache.len-1]
+	pp.mspancache.len--
+	return s
+}
+
+// allocMSpanLocked allocates an mspan object.
+//
+// h.lock must be held.
+//
+// allocMSpanLocked must be called on the system stack because
+// its caller holds the heap lock. See mheap for details.
+// Running on the system stack also ensures that we won't
+// switch Ps during this function. See tryAllocMSpan for details.
+//
+//go:systemstack
+func (h *mheap) allocMSpanLocked() *mspan {
+	assertLockHeld(&h.lock)
+
+	pp := getg().m.p.ptr()
+	if pp == nil {
+		// We don't have a p so just do the normal thing.
+		return (*mspan)(h.spanalloc.alloc())
+	}
+	// Refill the cache if necessary.
+	if pp.mspancache.len == 0 {
+		const refillCount = len(pp.mspancache.buf) / 2
+		for i := 0; i < refillCount; i++ {
+			pp.mspancache.buf[i] = (*mspan)(h.spanalloc.alloc())
+		}
+		pp.mspancache.len = refillCount
+	}
+	// Pull off the last entry in the cache.
+	s := pp.mspancache.buf[pp.mspancache.len-1]
+	pp.mspancache.len--
+	return s
+}
+
+// freeMSpanLocked free an mspan object.
+//
+// h.lock must be held.
+//
+// freeMSpanLocked must be called on the system stack because
+// its caller holds the heap lock. See mheap for details.
+// Running on the system stack also ensures that we won't
+// switch Ps during this function. See tryAllocMSpan for details.
+//
+//go:systemstack
+func (h *mheap) freeMSpanLocked(s *mspan) {
+	assertLockHeld(&h.lock)
+
+	pp := getg().m.p.ptr()
+	// First try to free the mspan directly to the cache.
+	if pp != nil && pp.mspancache.len < len(pp.mspancache.buf) {
+		pp.mspancache.buf[pp.mspancache.len] = s
+		pp.mspancache.len++
+		return
+	}
+	// Failing that (or if we don't have a p), just free it to
+	// the heap.
+	h.spanalloc.free(unsafe.Pointer(s))
+}
+
+// allocSpan allocates an mspan which owns npages worth of memory.
+//
+// If typ.manual() == false, allocSpan allocates a heap span of class spanclass
+// and updates heap accounting. If manual == true, allocSpan allocates a
+// manually-managed span (spanclass is ignored), and the caller is
+// responsible for any accounting related to its use of the span. Either
+// way, allocSpan will atomically add the bytes in the newly allocated
+// span to *sysStat.
+//
+// The returned span is fully initialized.
+//
+// h.lock must not be held.
+//
+// allocSpan must be called on the system stack both because it acquires
+// the heap lock and because it must block GC transitions.
+//
+//go:systemstack
+func (h *mheap) allocSpan(npages uintptr, typ spanAllocType, spanclass spanClass) (s *mspan) {
+	// Function-global state.
+	gp := getg()
+	base, scav := uintptr(0), uintptr(0)
+	growth := uintptr(0)
+
+	// On some platforms we need to provide physical page aligned stack
+	// allocations. Where the page size is less than the physical page
+	// size, we already manage to do this by default.
+	needPhysPageAlign := physPageAlignedStacks && typ == spanAllocStack && pageSize < physPageSize
+
+	// If the allocation is small enough, try the page cache!
+	// The page cache does not support aligned allocations, so we cannot use
+	// it if we need to provide a physical page aligned stack allocation.
+	pp := gp.m.p.ptr()
+	if !needPhysPageAlign && pp != nil && npages < pageCachePages/4 {
+		c := &pp.pcache
+
+		// If the cache is empty, refill it.
+		if c.empty() {
+			lock(&h.lock)
+			*c = h.pages.allocToCache()
+			unlock(&h.lock)
+		}
+
+		// Try to allocate from the cache.
+		base, scav = c.alloc(npages)
+		if base != 0 {
+			s = h.tryAllocMSpan()
+			if s != nil {
+				goto HaveSpan
+			}
+			// We have a base but no mspan, so we need
+			// to lock the heap.
+		}
+	}
+
+	// For one reason or another, we couldn't get the
+	// whole job done without the heap lock.
+	lock(&h.lock)
+
+	if needPhysPageAlign {
+		// Overallocate by a physical page to allow for later alignment.
+		extraPages := physPageSize / pageSize
+
+		// Find a big enough region first, but then only allocate the
+		// aligned portion. We can't just allocate and then free the
+		// edges because we need to account for scavenged memory, and
+		// that's difficult with alloc.
+		//
+		// Note that we skip updates to searchAddr here. It's OK if
+		// it's stale and higher than normal; it'll operate correctly,
+		// just come with a performance cost.
+		base, _ = h.pages.find(npages + extraPages)
+		if base == 0 {
+			var ok bool
+			growth, ok = h.grow(npages + extraPages)
+			if !ok {
+				unlock(&h.lock)
+				return nil
+			}
+			base, _ = h.pages.find(npages + extraPages)
+			if base == 0 {
+				throw("grew heap, but no adequate free space found")
+			}
+		}
+		base = alignUp(base, physPageSize)
+		scav = h.pages.allocRange(base, npages)
+	}
+	if base == 0 {
+		// Try to acquire a base address.
+		base, scav = h.pages.alloc(npages)
+		if base == 0 {
+			var ok bool
+			growth, ok = h.grow(npages)
+			if !ok {
+				unlock(&h.lock)
+				return nil
+			}
+			base, scav = h.pages.alloc(npages)
+			if base == 0 {
+				throw("grew heap, but no adequate free space found")
+			}
+		}
+	}
+	if s == nil {
+		// We failed to get an mspan earlier, so grab
+		// one now that we have the heap lock.
+		s = h.allocMSpanLocked()
+	}
+	unlock(&h.lock)
+
+HaveSpan:
+	// At this point, both s != nil and base != 0, and the heap
+	// lock is no longer held. Initialize the span.
+	s.init(base, npages)
+	if h.allocNeedsZero(base, npages) {
+		s.needzero = 1
+	}
+	nbytes := npages * pageSize
+	if typ.manual() {
+		s.manualFreeList = 0
+		s.nelems = 0
+		s.limit = s.base() + s.npages*pageSize
+		s.state.set(mSpanManual)
+	} else {
+		// We must set span properties before the span is published anywhere
+		// since we're not holding the heap lock.
+		s.spanclass = spanclass
+		if sizeclass := spanclass.sizeclass(); sizeclass == 0 {
+			s.elemsize = nbytes
+			s.nelems = 1
+			s.divMul = 0
+		} else {
+			s.elemsize = uintptr(class_to_size[sizeclass])
+			s.nelems = nbytes / s.elemsize
+			s.divMul = class_to_divmagic[sizeclass]
+		}
+
+		// Initialize mark and allocation structures.
+		s.freeindex = 0
+		s.freeIndexForScan = 0
+		s.allocCache = ^uint64(0) // all 1s indicating all free.
+		s.gcmarkBits = newMarkBits(s.nelems)
+		s.allocBits = newAllocBits(s.nelems)
+
+		// It's safe to access h.sweepgen without the heap lock because it's
+		// only ever updated with the world stopped and we run on the
+		// systemstack which blocks a STW transition.
+		atomic.Store(&s.sweepgen, h.sweepgen)
+
+		// Now that the span is filled in, set its state. This
+		// is a publication barrier for the other fields in
+		// the span. While valid pointers into this span
+		// should never be visible until the span is returned,
+		// if the garbage collector finds an invalid pointer,
+		// access to the span may race with initialization of
+		// the span. We resolve this race by atomically
+		// setting the state after the span is fully
+		// initialized, and atomically checking the state in
+		// any situation where a pointer is suspect.
+		s.state.set(mSpanInUse)
+	}
+
+	// Decide if we need to scavenge in response to what we just allocated.
+	// Specifically, we track the maximum amount of memory to scavenge of all
+	// the alternatives below, assuming that the maximum satisfies *all*
+	// conditions we check (e.g. if we need to scavenge X to satisfy the
+	// memory limit and Y to satisfy heap-growth scavenging, and Y > X, then
+	// it's fine to pick Y, because the memory limit is still satisfied).
+	//
+	// It's fine to do this after allocating because we expect any scavenged
+	// pages not to get touched until we return. Simultaneously, it's important
+	// to do this before calling sysUsed because that may commit address space.
+	bytesToScavenge := uintptr(0)
+	if limit := gcController.memoryLimit.Load(); go119MemoryLimitSupport && !gcCPULimiter.limiting() {
+		// Assist with scavenging to maintain the memory limit by the amount
+		// that we expect to page in.
+		inuse := gcController.mappedReady.Load()
+		// Be careful about overflow, especially with uintptrs. Even on 32-bit platforms
+		// someone can set a really big memory limit that isn't maxInt64.
+		if uint64(scav)+inuse > uint64(limit) {
+			bytesToScavenge = uintptr(uint64(scav) + inuse - uint64(limit))
+		}
+	}
+	if goal := scavenge.gcPercentGoal.Load(); goal != ^uint64(0) && growth > 0 {
+		// We just caused a heap growth, so scavenge down what will soon be used.
+		// By scavenging inline we deal with the failure to allocate out of
+		// memory fragments by scavenging the memory fragments that are least
+		// likely to be re-used.
+		//
+		// Only bother with this because we're not using a memory limit. We don't
+		// care about heap growths as long as we're under the memory limit, and the
+		// previous check for scaving already handles that.
+		if retained := heapRetained(); retained+uint64(growth) > goal {
+			// The scavenging algorithm requires the heap lock to be dropped so it
+			// can acquire it only sparingly. This is a potentially expensive operation
+			// so it frees up other goroutines to allocate in the meanwhile. In fact,
+			// they can make use of the growth we just created.
+			todo := growth
+			if overage := uintptr(retained + uint64(growth) - goal); todo > overage {
+				todo = overage
+			}
+			if todo > bytesToScavenge {
+				bytesToScavenge = todo
+			}
+		}
+	}
+	// There are a few very limited cirumstances where we won't have a P here.
+	// It's OK to simply skip scavenging in these cases. Something else will notice
+	// and pick up the tab.
+	if pp != nil && bytesToScavenge > 0 {
+		// Measure how long we spent scavenging and add that measurement to the assist
+		// time so we can track it for the GC CPU limiter.
+		//
+		// Limiter event tracking might be disabled if we end up here
+		// while on a mark worker.
+		start := nanotime()
+		track := pp.limiterEvent.start(limiterEventScavengeAssist, start)
+
+		// Scavenge, but back out if the limiter turns on.
+		h.pages.scavenge(bytesToScavenge, func() bool {
+			return gcCPULimiter.limiting()
+		})
+
+		// Finish up accounting.
+		now := nanotime()
+		if track {
+			pp.limiterEvent.stop(limiterEventScavengeAssist, now)
+		}
+		h.pages.scav.assistTime.Add(now - start)
+	}
+
+	// Commit and account for any scavenged memory that the span now owns.
+	if scav != 0 {
+		// sysUsed all the pages that are actually available
+		// in the span since some of them might be scavenged.
+		sysUsed(unsafe.Pointer(base), nbytes, scav)
+		gcController.heapReleased.add(-int64(scav))
+	}
+	// Update stats.
+	gcController.heapFree.add(-int64(nbytes - scav))
+	if typ == spanAllocHeap {
+		gcController.heapInUse.add(int64(nbytes))
+	}
+	// Update consistent stats.
+	stats := memstats.heapStats.acquire()
+	atomic.Xaddint64(&stats.committed, int64(scav))
+	atomic.Xaddint64(&stats.released, -int64(scav))
+	switch typ {
+	case spanAllocHeap:
+		atomic.Xaddint64(&stats.inHeap, int64(nbytes))
+	case spanAllocStack:
+		atomic.Xaddint64(&stats.inStacks, int64(nbytes))
+	case spanAllocPtrScalarBits:
+		atomic.Xaddint64(&stats.inPtrScalarBits, int64(nbytes))
+	case spanAllocWorkBuf:
+		atomic.Xaddint64(&stats.inWorkBufs, int64(nbytes))
+	}
+	memstats.heapStats.release()
+
+	// Publish the span in various locations.
+
+	// This is safe to call without the lock held because the slots
+	// related to this span will only ever be read or modified by
+	// this thread until pointers into the span are published (and
+	// we execute a publication barrier at the end of this function
+	// before that happens) or pageInUse is updated.
+	h.setSpans(s.base(), npages, s)
+
+	if !typ.manual() {
+		// Mark in-use span in arena page bitmap.
+		//
+		// This publishes the span to the page sweeper, so
+		// it's imperative that the span be completely initialized
+		// prior to this line.
+		arena, pageIdx, pageMask := pageIndexOf(s.base())
+		atomic.Or8(&arena.pageInUse[pageIdx], pageMask)
+
+		// Update related page sweeper stats.
+		h.pagesInUse.Add(int64(npages))
+	}
+
+	// Make sure the newly allocated span will be observed
+	// by the GC before pointers into the span are published.
+	publicationBarrier()
+
+	return s
+}
+
+// Try to add at least npage pages of memory to the heap,
+// returning how much the heap grew by and whether it worked.
+//
+// h.lock must be held.
+func (h *mheap) grow(npage uintptr) (uintptr, bool) {
+	assertLockHeld(&h.lock)
+
+	// We must grow the heap in whole palloc chunks.
+	// We call sysMap below but note that because we
+	// round up to pallocChunkPages which is on the order
+	// of MiB (generally >= to the huge page size) we
+	// won't be calling it too much.
+	ask := alignUp(npage, pallocChunkPages) * pageSize
+
+	totalGrowth := uintptr(0)
+	// This may overflow because ask could be very large
+	// and is otherwise unrelated to h.curArena.base.
+	end := h.curArena.base + ask
+	nBase := alignUp(end, physPageSize)
+	if nBase > h.curArena.end || /* overflow */ end < h.curArena.base {
+		// Not enough room in the current arena. Allocate more
+		// arena space. This may not be contiguous with the
+		// current arena, so we have to request the full ask.
+		av, asize := h.sysAlloc(ask)
+		if av == nil {
+			inUse := gcController.heapFree.load() + gcController.heapReleased.load() + gcController.heapInUse.load()
+			print("runtime: out of memory: cannot allocate ", ask, "-byte block (", inUse, " in use)\n")
+			return 0, false
+		}
+
+		if uintptr(av) == h.curArena.end {
+			// The new space is contiguous with the old
+			// space, so just extend the current space.
+			h.curArena.end = uintptr(av) + asize
+		} else {
+			// The new space is discontiguous. Track what
+			// remains of the current space and switch to
+			// the new space. This should be rare.
+			if size := h.curArena.end - h.curArena.base; size != 0 {
+				// Transition this space from Reserved to Prepared and mark it
+				// as released since we'll be able to start using it after updating
+				// the page allocator and releasing the lock at any time.
+				sysMap(unsafe.Pointer(h.curArena.base), size, &gcController.heapReleased)
+				// Update stats.
+				stats := memstats.heapStats.acquire()
+				atomic.Xaddint64(&stats.released, int64(size))
+				memstats.heapStats.release()
+				// Update the page allocator's structures to make this
+				// space ready for allocation.
+				h.pages.grow(h.curArena.base, size)
+				totalGrowth += size
+			}
+			// Switch to the new space.
+			h.curArena.base = uintptr(av)
+			h.curArena.end = uintptr(av) + asize
+		}
+
+		// Recalculate nBase.
+		// We know this won't overflow, because sysAlloc returned
+		// a valid region starting at h.curArena.base which is at
+		// least ask bytes in size.
+		nBase = alignUp(h.curArena.base+ask, physPageSize)
+	}
+
+	// Grow into the current arena.
+	v := h.curArena.base
+	h.curArena.base = nBase
+
+	// Transition the space we're going to use from Reserved to Prepared.
+	//
+	// The allocation is always aligned to the heap arena
+	// size which is always > physPageSize, so its safe to
+	// just add directly to heapReleased.
+	sysMap(unsafe.Pointer(v), nBase-v, &gcController.heapReleased)
+
+	// The memory just allocated counts as both released
+	// and idle, even though it's not yet backed by spans.
+	stats := memstats.heapStats.acquire()
+	atomic.Xaddint64(&stats.released, int64(nBase-v))
+	memstats.heapStats.release()
+
+	// Update the page allocator's structures to make this
+	// space ready for allocation.
+	h.pages.grow(v, nBase-v)
+	totalGrowth += nBase - v
+	return totalGrowth, true
+}
+
+// Free the span back into the heap.
+func (h *mheap) freeSpan(s *mspan) {
+	systemstack(func() {
+		lock(&h.lock)
+		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 asanenabled {
+			// Tell asan that this entire span is no longer in use.
+			base := unsafe.Pointer(s.base())
+			bytes := s.npages << _PageShift
+			asanpoison(base, bytes)
+		}
+		h.freeSpanLocked(s, spanAllocHeap)
+		unlock(&h.lock)
+	})
+}
+
+// freeManual frees a manually-managed span returned by allocManual.
+// typ must be the same as the spanAllocType 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 because it acquires
+// the heap lock. See mheap for details.
+//
+//go:systemstack
+func (h *mheap) freeManual(s *mspan, typ spanAllocType) {
+	s.needzero = 1
+	lock(&h.lock)
+	h.freeSpanLocked(s, typ)
+	unlock(&h.lock)
+}
+
+func (h *mheap) freeSpanLocked(s *mspan, typ spanAllocType) {
+	assertLockHeld(&h.lock)
+
+	switch s.state.get() {
+	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.Add(-int64(s.npages))
+
+		// Clear in-use bit in arena page bitmap.
+		arena, pageIdx, pageMask := pageIndexOf(s.base())
+		atomic.And8(&arena.pageInUse[pageIdx], ^pageMask)
+	default:
+		throw("mheap.freeSpanLocked - invalid span state")
+	}
+
+	// Update stats.
+	//
+	// Mirrors the code in allocSpan.
+	nbytes := s.npages * pageSize
+	gcController.heapFree.add(int64(nbytes))
+	if typ == spanAllocHeap {
+		gcController.heapInUse.add(-int64(nbytes))
+	}
+	// Update consistent stats.
+	stats := memstats.heapStats.acquire()
+	switch typ {
+	case spanAllocHeap:
+		atomic.Xaddint64(&stats.inHeap, -int64(nbytes))
+	case spanAllocStack:
+		atomic.Xaddint64(&stats.inStacks, -int64(nbytes))
+	case spanAllocPtrScalarBits:
+		atomic.Xaddint64(&stats.inPtrScalarBits, -int64(nbytes))
+	case spanAllocWorkBuf:
+		atomic.Xaddint64(&stats.inWorkBufs, -int64(nbytes))
+	}
+	memstats.heapStats.release()
+
+	// Mark the space as free.
+	h.pages.free(s.base(), s.npages, false)
+
+	// Free the span structure. We no longer have a use for it.
+	s.state.set(mSpanDead)
+	h.freeMSpanLocked(s)
+}
+
+// scavengeAll acquires the heap lock (blocking any additional
+// manipulation of the page allocator) and iterates over the whole
+// heap, scavenging every free page available.
+func (h *mheap) scavengeAll() {
+	// Disallow malloc or panic while holding the heap lock. We do
+	// this here because this is a non-mallocgc entry-point to
+	// the mheap API.
+	gp := getg()
+	gp.m.mallocing++
+
+	released := h.pages.scavenge(^uintptr(0), nil)
+
+	gp.m.mallocing--
+
+	if debug.scavtrace > 0 {
+		printScavTrace(released, true)
+	}
+}
+
+//go:linkname runtime_debug_freeOSMemory runtime/debug.freeOSMemory
+func runtime_debug_freeOSMemory() {
+	GC()
+	systemstack(func() { mheap_.scavengeAll() })
+}
+
+// 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.speciallock.key = 0
+	span.specials = nil
+	span.needzero = 0
+	span.freeindex = 0
+	span.freeIndexForScan = 0
+	span.allocBits = nil
+	span.gcmarkBits = nil
+	span.state.set(mSpanDead)
+	lockInit(&span.speciallock, lockRankMspanSpecial)
+}
+
+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
+	// _KindSpecialReachable is a special used for tracking
+	// reachability during testing.
+	_KindSpecialReachable = 3
+	// 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
+}
+
+// spanHasSpecials marks a span as having specials in the arena bitmap.
+func spanHasSpecials(s *mspan) {
+	arenaPage := (s.base() / pageSize) % pagesPerArena
+	ai := arenaIndex(s.base())
+	ha := mheap_.arenas[ai.l1()][ai.l2()]
+	atomic.Or8(&ha.pageSpecials[arenaPage/8], uint8(1)<<(arenaPage%8))
+}
+
+// spanHasNoSpecials marks a span as having no specials in the arena bitmap.
+func spanHasNoSpecials(s *mspan) {
+	arenaPage := (s.base() / pageSize) % pagesPerArena
+	ai := arenaIndex(s.base())
+	ha := mheap_.arenas[ai.l1()][ai.l2()]
+	atomic.And8(&ha.pageSpecials[arenaPage/8], ^(uint8(1) << (arenaPage % 8)))
+}
+
+// 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
+	spanHasSpecials(span)
+	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()
+
+	var result *special
+	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
+			result = s
+			break
+		}
+		t = &s.next
+	}
+	if span.specials == nil {
+		spanHasNoSpecials(span)
+	}
+	unlock(&span.speciallock)
+	releasem(mp)
+	return result
+}
+
+// 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)), goarch.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")
+	}
+}
+
+// specialReachable tracks whether an object is reachable on the next
+// GC cycle. This is used by testing.
+type specialReachable struct {
+	special   special
+	done      bool
+	reachable bool
+}
+
+// specialsIter helps iterate over specials lists.
+type specialsIter struct {
+	pprev **special
+	s     *special
+}
+
+func newSpecialsIter(span *mspan) specialsIter {
+	return specialsIter{&span.specials, span.specials}
+}
+
+func (i *specialsIter) valid() bool {
+	return i.s != nil
+}
+
+func (i *specialsIter) next() {
+	i.pprev = &i.s.next
+	i.s = *i.pprev
+}
+
+// unlinkAndNext removes the current special from the list and moves
+// the iterator to the next special. It returns the unlinked special.
+func (i *specialsIter) unlinkAndNext() *special {
+	cur := i.s
+	i.s = cur.next
+	*i.pprev = i.s
+	return cur
+}
+
+// freeSpecial performs any cleanup on special s and deallocates it.
+// s must already be unlinked from the 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)
+	case _KindSpecialReachable:
+		sp := (*specialReachable)(unsafe.Pointer(s))
+		sp.done = true
+		// The creator frees these.
+	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.gcMiscSys))
+		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
+}