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

// Garbage collector: marking and scanning

package runtime

import (
	
	
	
	
	
)

const (
	fixedRootFinalizers = iota
	fixedRootFreeGStacks
	fixedRootCount

	// rootBlockBytes is the number of bytes to scan per data or
	// BSS root.
	rootBlockBytes = 256 << 10

	// maxObletBytes is the maximum bytes of an object to scan at
	// once. Larger objects will be split up into "oblets" of at
	// most this size. Since we can scan 1–2 MB/ms, 128 KB bounds
	// scan preemption at ~100 µs.
	//
	// This must be > _MaxSmallSize so that the object base is the
	// span base.
	maxObletBytes = 128 << 10

	// drainCheckThreshold specifies how many units of work to do
	// between self-preemption checks in gcDrain. Assuming a scan
	// rate of 1 MB/ms, this is ~100 µs. Lower values have higher
	// overhead in the scan loop (the scheduler check may perform
	// a syscall, so its overhead is nontrivial). Higher values
	// make the system less responsive to incoming work.
	drainCheckThreshold = 100000

	// pagesPerSpanRoot indicates how many pages to scan from a span root
	// at a time. Used by special root marking.
	//
	// Higher values improve throughput by increasing locality, but
	// increase the minimum latency of a marking operation.
	//
	// Must be a multiple of the pageInUse bitmap element size and
	// must also evenly divide pagesPerArena.
	pagesPerSpanRoot = 512
)

// gcMarkRootPrepare queues root scanning jobs (stacks, globals, and
// some miscellany) and initializes scanning-related state.
//
// The world must be stopped.
func gcMarkRootPrepare() {
	assertWorldStopped()

	// Compute how many data and BSS root blocks there are.
	 := func( uintptr) int {
		return int(divRoundUp(, rootBlockBytes))
	}

	work.nDataRoots = 0
	work.nBSSRoots = 0

	// Scan globals.
	for ,  := range activeModules() {
		 := (.edata - .data)
		if  > work.nDataRoots {
			work.nDataRoots = 
		}
	}

	for ,  := range activeModules() {
		 := (.ebss - .bss)
		if  > work.nBSSRoots {
			work.nBSSRoots = 
		}
	}

	// Scan span roots for finalizer specials.
	//
	// We depend on addfinalizer to mark objects that get
	// finalizers after root marking.
	//
	// We're going to scan the whole heap (that was available at the time the
	// mark phase started, i.e. markArenas) for in-use spans which have specials.
	//
	// Break up the work into arenas, and further into chunks.
	//
	// Snapshot allArenas as markArenas. This snapshot is safe because allArenas
	// is append-only.
	mheap_.markArenas = mheap_.allArenas[:len(mheap_.allArenas):len(mheap_.allArenas)]
	work.nSpanRoots = len(mheap_.markArenas) * (pagesPerArena / pagesPerSpanRoot)

	// Scan stacks.
	//
	// Gs may be created after this point, but it's okay that we
	// ignore them because they begin life without any roots, so
	// there's nothing to scan, and any roots they create during
	// the concurrent phase will be caught by the write barrier.
	work.stackRoots = allGsSnapshot()
	work.nStackRoots = len(work.stackRoots)

	work.markrootNext = 0
	work.markrootJobs = uint32(fixedRootCount + work.nDataRoots + work.nBSSRoots + work.nSpanRoots + work.nStackRoots)

	// Calculate base indexes of each root type
	work.baseData = uint32(fixedRootCount)
	work.baseBSS = work.baseData + uint32(work.nDataRoots)
	work.baseSpans = work.baseBSS + uint32(work.nBSSRoots)
	work.baseStacks = work.baseSpans + uint32(work.nSpanRoots)
	work.baseEnd = work.baseStacks + uint32(work.nStackRoots)
}

// gcMarkRootCheck checks that all roots have been scanned. It is
// purely for debugging.
func gcMarkRootCheck() {
	if work.markrootNext < work.markrootJobs {
		print(work.markrootNext, " of ", work.markrootJobs, " markroot jobs done\n")
		throw("left over markroot jobs")
	}

	// Check that stacks have been scanned.
	//
	// We only check the first nStackRoots Gs that we should have scanned.
	// Since we don't care about newer Gs (see comment in
	// gcMarkRootPrepare), no locking is required.
	 := 0
	forEachGRace(func( *g) {
		if  >= work.nStackRoots {
			return
		}

		if !.gcscandone {
			println("gp", , "goid", .goid,
				"status", readgstatus(),
				"gcscandone", .gcscandone)
			throw("scan missed a g")
		}

		++
	})
}

// ptrmask for an allocation containing a single pointer.
var oneptrmask = [...]uint8{1}

// markroot scans the i'th root.
//
// Preemption must be disabled (because this uses a gcWork).
//
// Returns the amount of GC work credit produced by the operation.
// If flushBgCredit is true, then that credit is also flushed
// to the background credit pool.
//
// nowritebarrier is only advisory here.
//
//go:nowritebarrier
func markroot( *gcWork,  uint32,  bool) int64 {
	// Note: if you add a case here, please also update heapdump.go:dumproots.
	var  int64
	var  *atomic.Int64
	switch {
	case work.baseData <=  &&  < work.baseBSS:
		 = &gcController.globalsScanWork
		for ,  := range activeModules() {
			 += markrootBlock(.data, .edata-.data, .gcdatamask.bytedata, , int(-work.baseData))
		}

	case work.baseBSS <=  &&  < work.baseSpans:
		 = &gcController.globalsScanWork
		for ,  := range activeModules() {
			 += markrootBlock(.bss, .ebss-.bss, .gcbssmask.bytedata, , int(-work.baseBSS))
		}

	case  == fixedRootFinalizers:
		for  := allfin;  != nil;  = .alllink {
			 := uintptr(atomic.Load(&.cnt))
			scanblock(uintptr(unsafe.Pointer(&.fin[0])), *unsafe.Sizeof(.fin[0]), &finptrmask[0], , nil)
		}

	case  == fixedRootFreeGStacks:
		// Switch to the system stack so we can call
		// stackfree.
		systemstack(markrootFreeGStacks)

	case work.baseSpans <=  &&  < work.baseStacks:
		// mark mspan.specials
		markrootSpans(, int(-work.baseSpans))

	default:
		// the rest is scanning goroutine stacks
		 = &gcController.stackScanWork
		if  < work.baseStacks || work.baseEnd <=  {
			printlock()
			print("runtime: markroot index ", , " not in stack roots range [", work.baseStacks, ", ", work.baseEnd, ")\n")
			throw("markroot: bad index")
		}
		 := work.stackRoots[-work.baseStacks]

		// remember when we've first observed the G blocked
		// needed only to output in traceback
		 := readgstatus() // We are not in a scan state
		if ( == _Gwaiting ||  == _Gsyscall) && .waitsince == 0 {
			.waitsince = work.tstart
		}

		// scanstack must be done on the system stack in case
		// we're trying to scan our own stack.
		systemstack(func() {
			// If this is a self-scan, put the user G in
			// _Gwaiting to prevent self-deadlock. It may
			// already be in _Gwaiting if this is a mark
			// worker or we're in mark termination.
			 := getg().m.curg
			 :=  ==  && readgstatus() == _Grunning
			if  {
				casgstatus(, _Grunning, _Gwaiting)
				.waitreason = waitReasonGarbageCollectionScan
			}

			// TODO: suspendG blocks (and spins) until gp
			// stops, which may take a while for
			// running goroutines. Consider doing this in
			// two phases where the first is non-blocking:
			// we scan the stacks we can and ask running
			// goroutines to scan themselves; and the
			// second blocks.
			 := suspendG()
			if .dead {
				.gcscandone = true
				return
			}
			if .gcscandone {
				throw("g already scanned")
			}
			 += scanstack(, )
			.gcscandone = true
			resumeG()

			if  {
				casgstatus(, _Gwaiting, _Grunning)
			}
		})
	}
	if goexperiment.PacerRedesign {
		if  != nil &&  != 0 {
			.Add()
			if  {
				gcFlushBgCredit()
			}
		}
	}
	return 
}

// markrootBlock scans the shard'th shard of the block of memory [b0,
// b0+n0), with the given pointer mask.
//
// Returns the amount of work done.
//
//go:nowritebarrier
func markrootBlock(,  uintptr,  *uint8,  *gcWork,  int) int64 {
	if rootBlockBytes%(8*goarch.PtrSize) != 0 {
		// This is necessary to pick byte offsets in ptrmask0.
		throw("rootBlockBytes must be a multiple of 8*ptrSize")
	}

	// Note that if b0 is toward the end of the address space,
	// then b0 + rootBlockBytes might wrap around.
	// These tests are written to avoid any possible overflow.
	 := uintptr() * rootBlockBytes
	if  >=  {
		return 0
	}
	 :=  + 
	 := (*uint8)(add(unsafe.Pointer(), uintptr()*(rootBlockBytes/(8*goarch.PtrSize))))
	 := uintptr(rootBlockBytes)
	if + >  {
		 =  - 
	}

	// Scan this shard.
	scanblock(, , , , nil)
	return int64()
}

// markrootFreeGStacks frees stacks of dead Gs.
//
// This does not free stacks of dead Gs cached on Ps, but having a few
// cached stacks around isn't a problem.
func markrootFreeGStacks() {
	// Take list of dead Gs with stacks.
	lock(&sched.gFree.lock)
	 := sched.gFree.stack
	sched.gFree.stack = gList{}
	unlock(&sched.gFree.lock)
	if .empty() {
		return
	}

	// Free stacks.
	 := gQueue{.head, .head}
	for  := .head.ptr();  != nil;  = .schedlink.ptr() {
		stackfree(.stack)
		.stack.lo = 0
		.stack.hi = 0
		// Manipulate the queue directly since the Gs are
		// already all linked the right way.
		.tail.set()
	}

	// Put Gs back on the free list.
	lock(&sched.gFree.lock)
	sched.gFree.noStack.pushAll()
	unlock(&sched.gFree.lock)
}

// markrootSpans marks roots for one shard of markArenas.
//
//go:nowritebarrier
func markrootSpans( *gcWork,  int) {
	// Objects with finalizers have two GC-related invariants:
	//
	// 1) Everything reachable from the object must be marked.
	// This ensures that when we pass the object to its finalizer,
	// everything the finalizer can reach will be retained.
	//
	// 2) Finalizer specials (which are not in the garbage
	// collected heap) are roots. In practice, this means the fn
	// field must be scanned.
	 := mheap_.sweepgen

	// Find the arena and page index into that arena for this shard.
	 := mheap_.markArenas[/(pagesPerArena/pagesPerSpanRoot)]
	 := mheap_.arenas[.l1()][.l2()]
	 := uint(uintptr() * pagesPerSpanRoot % pagesPerArena)

	// Construct slice of bitmap which we'll iterate over.
	 := .pageSpecials[/8:]
	 = [:pagesPerSpanRoot/8]
	for  := range  {
		// Find set bits, which correspond to spans with specials.
		 := atomic.Load8(&[])
		if  == 0 {
			continue
		}
		for  := uint(0);  < 8; ++ {
			if &(1<<) == 0 {
				continue
			}
			// Find the span for this bit.
			//
			// This value is guaranteed to be non-nil because having
			// specials implies that the span is in-use, and since we're
			// currently marking we can be sure that we don't have to worry
			// about the span being freed and re-used.
			 := .spans[+uint()*8+]

			// The state must be mSpanInUse if the specials bit is set, so
			// sanity check that.
			if  := .state.get();  != mSpanInUse {
				print("s.state = ", , "\n")
				throw("non in-use span found with specials bit set")
			}
			// Check that this span was swept (it may be cached or uncached).
			if !useCheckmark && !(.sweepgen ==  || .sweepgen == +3) {
				// sweepgen was updated (+2) during non-checkmark GC pass
				print("sweep ", .sweepgen, " ", , "\n")
				throw("gc: unswept span")
			}

			// Lock the specials to prevent a special from being
			// removed from the list while we're traversing it.
			lock(&.speciallock)
			for  := .specials;  != nil;  = .next {
				if .kind != _KindSpecialFinalizer {
					continue
				}
				// don't mark finalized object, but scan it so we
				// retain everything it points to.
				 := (*specialfinalizer)(unsafe.Pointer())
				// A finalizer can be set for an inner byte of an object, find object beginning.
				 := .base() + uintptr(.special.offset)/.elemsize*.elemsize

				// Mark everything that can be reached from
				// the object (but *not* the object itself or
				// we'll never collect it).
				scanobject(, )

				// The special itself is a root.
				scanblock(uintptr(unsafe.Pointer(&.fn)), goarch.PtrSize, &oneptrmask[0], , nil)
			}
			unlock(&.speciallock)
		}
	}
}

// gcAssistAlloc performs GC work to make gp's assist debt positive.
// gp must be the calling user goroutine.
//
// This must be called with preemption enabled.
func gcAssistAlloc( *g) {
	// Don't assist in non-preemptible contexts. These are
	// generally fragile and won't allow the assist to block.
	if getg() == .m.g0 {
		return
	}
	if  := getg().m; .locks > 0 || .preemptoff != "" {
		return
	}

	 := false
:
	// Compute the amount of scan work we need to do to make the
	// balance positive. When the required amount of work is low,
	// we over-assist to build up credit for future allocations
	// and amortize the cost of assisting.
	 := gcController.assistWorkPerByte.Load()
	 := gcController.assistBytesPerWork.Load()
	 := -.gcAssistBytes
	 := int64( * float64())
	if  < gcOverAssistWork {
		 = gcOverAssistWork
		 = int64( * float64())
	}

	// Steal as much credit as we can from the background GC's
	// scan credit. This is racy and may drop the background
	// credit below 0 if two mutators steal at the same time. This
	// will just cause steals to fail until credit is accumulated
	// again, so in the long run it doesn't really matter, but we
	// do have to handle the negative credit case.
	 := atomic.Loadint64(&gcController.bgScanCredit)
	 := int64(0)
	if  > 0 {
		if  <  {
			 = 
			.gcAssistBytes += 1 + int64(*float64())
		} else {
			 = 
			.gcAssistBytes += 
		}
		atomic.Xaddint64(&gcController.bgScanCredit, -)

		 -= 

		if  == 0 {
			// We were able to steal all of the credit we
			// needed.
			if  {
				traceGCMarkAssistDone()
			}
			return
		}
	}

	if trace.enabled && ! {
		 = true
		traceGCMarkAssistStart()
	}

	// Perform assist work
	systemstack(func() {
		gcAssistAlloc1(, )
		// The user stack may have moved, so this can't touch
		// anything on it until it returns from systemstack.
	})

	 := .param != nil
	.param = nil
	if  {
		gcMarkDone()
	}

	if .gcAssistBytes < 0 {
		// We were unable steal enough credit or perform
		// enough work to pay off the assist debt. We need to
		// do one of these before letting the mutator allocate
		// more to prevent over-allocation.
		//
		// If this is because we were preempted, reschedule
		// and try some more.
		if .preempt {
			Gosched()
			goto 
		}

		// Add this G to an assist queue and park. When the GC
		// has more background credit, it will satisfy queued
		// assists before flushing to the global credit pool.
		//
		// Note that this does *not* get woken up when more
		// work is added to the work list. The theory is that
		// there wasn't enough work to do anyway, so we might
		// as well let background marking take care of the
		// work that is available.
		if !gcParkAssist() {
			goto 
		}

		// At this point either background GC has satisfied
		// this G's assist debt, or the GC cycle is over.
	}
	if  {
		traceGCMarkAssistDone()
	}
}

// gcAssistAlloc1 is the part of gcAssistAlloc that runs on the system
// stack. This is a separate function to make it easier to see that
// we're not capturing anything from the user stack, since the user
// stack may move while we're in this function.
//
// gcAssistAlloc1 indicates whether this assist completed the mark
// phase by setting gp.param to non-nil. This can't be communicated on
// the stack since it may move.
//
//go:systemstack
func gcAssistAlloc1( *g,  int64) {
	// Clear the flag indicating that this assist completed the
	// mark phase.
	.param = nil

	if atomic.Load(&gcBlackenEnabled) == 0 {
		// The gcBlackenEnabled check in malloc races with the
		// store that clears it but an atomic check in every malloc
		// would be a performance hit.
		// Instead we recheck it here on the non-preemptable system
		// stack to determine if we should perform an assist.

		// GC is done, so ignore any remaining debt.
		.gcAssistBytes = 0
		return
	}
	// Track time spent in this assist. Since we're on the
	// system stack, this is non-preemptible, so we can
	// just measure start and end time.
	 := nanotime()

	 := atomic.Xadd(&work.nwait, -1)
	if  == work.nproc {
		println("runtime: work.nwait =", , "work.nproc=", work.nproc)
		throw("nwait > work.nprocs")
	}

	// gcDrainN requires the caller to be preemptible.
	casgstatus(, _Grunning, _Gwaiting)
	.waitreason = waitReasonGCAssistMarking

	// drain own cached work first in the hopes that it
	// will be more cache friendly.
	 := &getg().m.p.ptr().gcw
	 := gcDrainN(, )

	casgstatus(, _Gwaiting, _Grunning)

	// Record that we did this much scan work.
	//
	// Back out the number of bytes of assist credit that
	// this scan work counts for. The "1+" is a poor man's
	// round-up, to ensure this adds credit even if
	// assistBytesPerWork is very low.
	 := gcController.assistBytesPerWork.Load()
	.gcAssistBytes += 1 + int64(*float64())

	// If this is the last worker and we ran out of work,
	// signal a completion point.
	 := atomic.Xadd(&work.nwait, +1)
	if  > work.nproc {
		println("runtime: work.nwait=", ,
			"work.nproc=", work.nproc)
		throw("work.nwait > work.nproc")
	}

	if  == work.nproc && !gcMarkWorkAvailable(nil) {
		// This has reached a background completion point. Set
		// gp.param to a non-nil value to indicate this. It
		// doesn't matter what we set it to (it just has to be
		// a valid pointer).
		.param = unsafe.Pointer()
	}
	 := nanotime() - 
	 := .m.p.ptr()
	.gcAssistTime += 
	if .gcAssistTime > gcAssistTimeSlack {
		atomic.Xaddint64(&gcController.assistTime, .gcAssistTime)
		.gcAssistTime = 0
	}
}

// gcWakeAllAssists wakes all currently blocked assists. This is used
// at the end of a GC cycle. gcBlackenEnabled must be false to prevent
// new assists from going to sleep after this point.
func gcWakeAllAssists() {
	lock(&work.assistQueue.lock)
	 := work.assistQueue.q.popList()
	injectglist(&)
	unlock(&work.assistQueue.lock)
}

// gcParkAssist puts the current goroutine on the assist queue and parks.
//
// gcParkAssist reports whether the assist is now satisfied. If it
// returns false, the caller must retry the assist.
func gcParkAssist() bool {
	lock(&work.assistQueue.lock)
	// If the GC cycle finished while we were getting the lock,
	// exit the assist. The cycle can't finish while we hold the
	// lock.
	if atomic.Load(&gcBlackenEnabled) == 0 {
		unlock(&work.assistQueue.lock)
		return true
	}

	 := getg()
	 := work.assistQueue.q
	work.assistQueue.q.pushBack()

	// Recheck for background credit now that this G is in
	// the queue, but can still back out. This avoids a
	// race in case background marking has flushed more
	// credit since we checked above.
	if atomic.Loadint64(&gcController.bgScanCredit) > 0 {
		work.assistQueue.q = 
		if .tail != 0 {
			.tail.ptr().schedlink.set(nil)
		}
		unlock(&work.assistQueue.lock)
		return false
	}
	// Park.
	goparkunlock(&work.assistQueue.lock, waitReasonGCAssistWait, traceEvGoBlockGC, 2)
	return true
}

// gcFlushBgCredit flushes scanWork units of background scan work
// credit. This first satisfies blocked assists on the
// work.assistQueue and then flushes any remaining credit to
// gcController.bgScanCredit.
//
// Write barriers are disallowed because this is used by gcDrain after
// it has ensured that all work is drained and this must preserve that
// condition.
//
//go:nowritebarrierrec
func gcFlushBgCredit( int64) {
	if work.assistQueue.q.empty() {
		// Fast path; there are no blocked assists. There's a
		// small window here where an assist may add itself to
		// the blocked queue and park. If that happens, we'll
		// just get it on the next flush.
		atomic.Xaddint64(&gcController.bgScanCredit, )
		return
	}

	 := gcController.assistBytesPerWork.Load()
	 := int64(float64() * )

	lock(&work.assistQueue.lock)
	for !work.assistQueue.q.empty() &&  > 0 {
		 := work.assistQueue.q.pop()
		// Note that gp.gcAssistBytes is negative because gp
		// is in debt. Think carefully about the signs below.
		if +.gcAssistBytes >= 0 {
			// Satisfy this entire assist debt.
			 += .gcAssistBytes
			.gcAssistBytes = 0
			// It's important that we *not* put gp in
			// runnext. Otherwise, it's possible for user
			// code to exploit the GC worker's high
			// scheduler priority to get itself always run
			// before other goroutines and always in the
			// fresh quantum started by GC.
			ready(, 0, false)
		} else {
			// Partially satisfy this assist.
			.gcAssistBytes += 
			 = 0
			// As a heuristic, we move this assist to the
			// back of the queue so that large assists
			// can't clog up the assist queue and
			// substantially delay small assists.
			work.assistQueue.q.pushBack()
			break
		}
	}

	if  > 0 {
		// Convert from scan bytes back to work.
		 := gcController.assistWorkPerByte.Load()
		 = int64(float64() * )
		atomic.Xaddint64(&gcController.bgScanCredit, )
	}
	unlock(&work.assistQueue.lock)
}

// scanstack scans gp's stack, greying all pointers found on the stack.
//
// For goexperiment.PacerRedesign:
// Returns the amount of scan work performed, but doesn't update
// gcController.stackScanWork or flush any credit. Any background credit produced
// by this function should be flushed by its caller. scanstack itself can't
// safely flush because it may result in trying to wake up a goroutine that
// was just scanned, resulting in a self-deadlock.
//
// scanstack will also shrink the stack if it is safe to do so. If it
// is not, it schedules a stack shrink for the next synchronous safe
// point.
//
// scanstack is marked go:systemstack because it must not be preempted
// while using a workbuf.
//
//go:nowritebarrier
//go:systemstack
func scanstack( *g,  *gcWork) int64 {
	if readgstatus()&_Gscan == 0 {
		print("runtime:scanstack: gp=", , ", goid=", .goid, ", gp->atomicstatus=", hex(readgstatus()), "\n")
		throw("scanstack - bad status")
	}

	switch readgstatus() &^ _Gscan {
	default:
		print("runtime: gp=", , ", goid=", .goid, ", gp->atomicstatus=", readgstatus(), "\n")
		throw("mark - bad status")
	case _Gdead:
		return 0
	case _Grunning:
		print("runtime: gp=", , ", goid=", .goid, ", gp->atomicstatus=", readgstatus(), "\n")
		throw("scanstack: goroutine not stopped")
	case _Grunnable, _Gsyscall, _Gwaiting:
		// ok
	}

	if  == getg() {
		throw("can't scan our own stack")
	}

	// stackSize is the amount of work we'll be reporting.
	//
	// We report the total stack size, more than we scan,
	// because this number needs to line up with gcControllerState's
	// stackScan and scannableStackSize fields.
	//
	// See the documentation on those fields for more information.
	 := .stack.hi - .stack.lo

	if isShrinkStackSafe() {
		// Shrink the stack if not much of it is being used.
		shrinkstack()
	} else {
		// Otherwise, shrink the stack at the next sync safe point.
		.preemptShrink = true
	}

	var  stackScanState
	.stack = .stack

	if stackTraceDebug {
		println("stack trace goroutine", .goid)
	}

	if debugScanConservative && .asyncSafePoint {
		print("scanning async preempted goroutine ", .goid, " stack [", hex(.stack.lo), ",", hex(.stack.hi), ")\n")
	}

	// Scan the saved context register. This is effectively a live
	// register that gets moved back and forth between the
	// register and sched.ctxt without a write barrier.
	if .sched.ctxt != nil {
		scanblock(uintptr(unsafe.Pointer(&.sched.ctxt)), goarch.PtrSize, &oneptrmask[0], , &)
	}

	// Scan the stack. Accumulate a list of stack objects.
	 := func( *stkframe,  unsafe.Pointer) bool {
		scanframeworker(, &, )
		return true
	}
	gentraceback(^uintptr(0), ^uintptr(0), 0, , 0, nil, 0x7fffffff, , nil, 0)

	// Find additional pointers that point into the stack from the heap.
	// Currently this includes defers and panics. See also function copystack.

	// Find and trace other pointers in defer records.
	for  := ._defer;  != nil;  = .link {
		if .fn != nil {
			// Scan the func value, which could be a stack allocated closure.
			// See issue 30453.
			scanblock(uintptr(unsafe.Pointer(&.fn)), goarch.PtrSize, &oneptrmask[0], , &)
		}
		if .link != nil {
			// The link field of a stack-allocated defer record might point
			// to a heap-allocated defer record. Keep that heap record live.
			scanblock(uintptr(unsafe.Pointer(&.link)), goarch.PtrSize, &oneptrmask[0], , &)
		}
		// Retain defers records themselves.
		// Defer records might not be reachable from the G through regular heap
		// tracing because the defer linked list might weave between the stack and the heap.
		if .heap {
			scanblock(uintptr(unsafe.Pointer(&)), goarch.PtrSize, &oneptrmask[0], , &)
		}
	}
	if ._panic != nil {
		// Panics are always stack allocated.
		.putPtr(uintptr(unsafe.Pointer(._panic)), false)
	}

	// Find and scan all reachable stack objects.
	//
	// The state's pointer queue prioritizes precise pointers over
	// conservative pointers so that we'll prefer scanning stack
	// objects precisely.
	.buildIndex()
	for {
		,  := .getPtr()
		if  == 0 {
			break
		}
		 := .findObject()
		if  == nil {
			continue
		}
		 := .r
		if  == nil {
			// We've already scanned this object.
			continue
		}
		.setRecord(nil) // Don't scan it again.
		if stackTraceDebug {
			printlock()
			print("  live stkobj at", hex(.stack.lo+uintptr(.off)), "of size", .size)
			if  {
				print(" (conservative)")
			}
			println()
			printunlock()
		}
		 := .gcdata()
		var  *mspan
		if .useGCProg() {
			// This path is pretty unlikely, an object large enough
			// to have a GC program allocated on the stack.
			// We need some space to unpack the program into a straight
			// bitmask, which we allocate/free here.
			// TODO: it would be nice if there were a way to run a GC
			// program without having to store all its bits. We'd have
			// to change from a Lempel-Ziv style program to something else.
			// Or we can forbid putting objects on stacks if they require
			// a gc program (see issue 27447).
			 = materializeGCProg(.ptrdata(), )
			 = (*byte)(unsafe.Pointer(.startAddr))
		}

		 := .stack.lo + uintptr(.off)
		if  {
			scanConservative(, .ptrdata(), , , &)
		} else {
			scanblock(, .ptrdata(), , , &)
		}

		if  != nil {
			dematerializeGCProg()
		}
	}

	// Deallocate object buffers.
	// (Pointer buffers were all deallocated in the loop above.)
	for .head != nil {
		 := .head
		.head = .next
		if stackTraceDebug {
			for  := 0;  < .nobj; ++ {
				 := &.obj[]
				if .r == nil { // reachable
					continue
				}
				println("  dead stkobj at", hex(.stack.lo+uintptr(.off)), "of size", .r.size)
				// Note: not necessarily really dead - only reachable-from-ptr dead.
			}
		}
		.nobj = 0
		putempty((*workbuf)(unsafe.Pointer()))
	}
	if .buf != nil || .cbuf != nil || .freeBuf != nil {
		throw("remaining pointer buffers")
	}
	return int64()
}

// Scan a stack frame: local variables and function arguments/results.
//go:nowritebarrier
func scanframeworker( *stkframe,  *stackScanState,  *gcWork) {
	if _DebugGC > 1 && .continpc != 0 {
		print("scanframe ", funcname(.fn), "\n")
	}

	 := .fn.valid() && .fn.funcID == funcID_asyncPreempt
	 := .fn.valid() && .fn.funcID == funcID_debugCallV2
	if .conservative ||  ||  {
		if debugScanConservative {
			println("conservatively scanning function", funcname(.fn), "at PC", hex(.continpc))
		}

		// Conservatively scan the frame. Unlike the precise
		// case, this includes the outgoing argument space
		// since we may have stopped while this function was
		// setting up a call.
		//
		// TODO: We could narrow this down if the compiler
		// produced a single map per function of stack slots
		// and registers that ever contain a pointer.
		if .varp != 0 {
			 := .varp - .sp
			if  > 0 {
				scanConservative(.sp, , nil, , )
			}
		}

		// Scan arguments to this frame.
		if .arglen != 0 {
			// TODO: We could pass the entry argument map
			// to narrow this down further.
			scanConservative(.argp, .arglen, nil, , )
		}

		if  ||  {
			// This function's frame contained the
			// registers for the asynchronously stopped
			// parent frame. Scan the parent
			// conservatively.
			.conservative = true
		} else {
			// We only wanted to scan those two frames
			// conservatively. Clear the flag for future
			// frames.
			.conservative = false
		}
		return
	}

	, ,  := getStackMap(, &.cache, false)

	// Scan local variables if stack frame has been allocated.
	if .n > 0 {
		 := uintptr(.n) * goarch.PtrSize
		scanblock(.varp-, , .bytedata, , )
	}

	// Scan arguments.
	if .n > 0 {
		scanblock(.argp, uintptr(.n)*goarch.PtrSize, .bytedata, , )
	}

	// Add all stack objects to the stack object list.
	if .varp != 0 {
		// varp is 0 for defers, where there are no locals.
		// In that case, there can't be a pointer to its args, either.
		// (And all args would be scanned above anyway.)
		for  := range  {
			 := &[]
			 := .off
			 := .varp // locals base pointer
			if  >= 0 {
				 = .argp // arguments and return values base pointer
			}
			 :=  + uintptr()
			if  < .sp {
				// object hasn't been allocated in the frame yet.
				continue
			}
			if stackTraceDebug {
				println("stkobj at", hex(), "of size", .size)
			}
			.addObject(, )
		}
	}
}

type gcDrainFlags int

const (
	gcDrainUntilPreempt gcDrainFlags = 1 << iota
	gcDrainFlushBgCredit
	gcDrainIdle
	gcDrainFractional
)

// gcDrain scans roots and objects in work buffers, blackening grey
// objects until it is unable to get more work. It may return before
// GC is done; it's the caller's responsibility to balance work from
// other Ps.
//
// If flags&gcDrainUntilPreempt != 0, gcDrain returns when g.preempt
// is set.
//
// If flags&gcDrainIdle != 0, gcDrain returns when there is other work
// to do.
//
// If flags&gcDrainFractional != 0, gcDrain self-preempts when
// pollFractionalWorkerExit() returns true. This implies
// gcDrainNoBlock.
//
// If flags&gcDrainFlushBgCredit != 0, gcDrain flushes scan work
// credit to gcController.bgScanCredit every gcCreditSlack units of
// scan work.
//
// gcDrain will always return if there is a pending STW.
//
//go:nowritebarrier
func gcDrain( *gcWork,  gcDrainFlags) {
	if !writeBarrier.needed {
		throw("gcDrain phase incorrect")
	}

	 := getg().m.curg
	 := &gcDrainUntilPreempt != 0
	 := &gcDrainFlushBgCredit != 0
	 := &gcDrainIdle != 0

	 := .heapScanWork

	// checkWork is the scan work before performing the next
	// self-preempt check.
	 := int64(1<<63 - 1)
	var  func() bool
	if &(gcDrainIdle|gcDrainFractional) != 0 {
		 =  + drainCheckThreshold
		if  {
			 = pollWork
		} else if &gcDrainFractional != 0 {
			 = pollFractionalWorkerExit
		}
	}

	// Drain root marking jobs.
	if work.markrootNext < work.markrootJobs {
		// Stop if we're preemptible or if someone wants to STW.
		for !(.preempt && ( || atomic.Load(&sched.gcwaiting) != 0)) {
			 := atomic.Xadd(&work.markrootNext, +1) - 1
			if  >= work.markrootJobs {
				break
			}
			markroot(, , )
			if  != nil && () {
				goto 
			}
		}
	}

	// Drain heap marking jobs.
	// Stop if we're preemptible or if someone wants to STW.
	for !(.preempt && ( || atomic.Load(&sched.gcwaiting) != 0)) {
		// Try to keep work available on the global queue. We used to
		// check if there were waiting workers, but it's better to
		// just keep work available than to make workers wait. In the
		// worst case, we'll do O(log(_WorkbufSize)) unnecessary
		// balances.
		if work.full == 0 {
			.balance()
		}

		 := .tryGetFast()
		if  == 0 {
			 = .tryGet()
			if  == 0 {
				// Flush the write barrier
				// buffer; this may create
				// more work.
				wbBufFlush(nil, 0)
				 = .tryGet()
			}
		}
		if  == 0 {
			// Unable to get work.
			break
		}
		scanobject(, )

		// Flush background scan work credit to the global
		// account if we've accumulated enough locally so
		// mutator assists can draw on it.
		if .heapScanWork >= gcCreditSlack {
			gcController.heapScanWork.Add(.heapScanWork)
			if  {
				gcFlushBgCredit(.heapScanWork - )
				 = 0
			}
			 -= .heapScanWork
			.heapScanWork = 0

			if  <= 0 {
				 += drainCheckThreshold
				if  != nil && () {
					break
				}
			}
		}
	}

:
	// Flush remaining scan work credit.
	if .heapScanWork > 0 {
		gcController.heapScanWork.Add(.heapScanWork)
		if  {
			gcFlushBgCredit(.heapScanWork - )
		}
		.heapScanWork = 0
	}
}

// gcDrainN blackens grey objects until it has performed roughly
// scanWork units of scan work or the G is preempted. This is
// best-effort, so it may perform less work if it fails to get a work
// buffer. Otherwise, it will perform at least n units of work, but
// may perform more because scanning is always done in whole object
// increments. It returns the amount of scan work performed.
//
// The caller goroutine must be in a preemptible state (e.g.,
// _Gwaiting) to prevent deadlocks during stack scanning. As a
// consequence, this must be called on the system stack.
//
//go:nowritebarrier
//go:systemstack
func gcDrainN( *gcWork,  int64) int64 {
	if !writeBarrier.needed {
		throw("gcDrainN phase incorrect")
	}

	// There may already be scan work on the gcw, which we don't
	// want to claim was done by this call.
	 := -.heapScanWork

	 := getg().m.curg
	for !.preempt && +.heapScanWork <  {
		// See gcDrain comment.
		if work.full == 0 {
			.balance()
		}

		 := .tryGetFast()
		if  == 0 {
			 = .tryGet()
			if  == 0 {
				// Flush the write barrier buffer;
				// this may create more work.
				wbBufFlush(nil, 0)
				 = .tryGet()
			}
		}

		if  == 0 {
			// Try to do a root job.
			if work.markrootNext < work.markrootJobs {
				 := atomic.Xadd(&work.markrootNext, +1) - 1
				if  < work.markrootJobs {
					 := markroot(, , false)
					if goexperiment.PacerRedesign {
						 += 
					}
					continue
				}
			}
			// No heap or root jobs.
			break
		}

		scanobject(, )

		// Flush background scan work credit.
		if .heapScanWork >= gcCreditSlack {
			gcController.heapScanWork.Add(.heapScanWork)
			 += .heapScanWork
			.heapScanWork = 0
		}
	}

	// Unlike gcDrain, there's no need to flush remaining work
	// here because this never flushes to bgScanCredit and
	// gcw.dispose will flush any remaining work to scanWork.

	return  + .heapScanWork
}

// scanblock scans b as scanobject would, but using an explicit
// pointer bitmap instead of the heap bitmap.
//
// This is used to scan non-heap roots, so it does not update
// gcw.bytesMarked or gcw.heapScanWork.
//
// If stk != nil, possible stack pointers are also reported to stk.putPtr.
//go:nowritebarrier
func scanblock(,  uintptr,  *uint8,  *gcWork,  *stackScanState) {
	// Use local copies of original parameters, so that a stack trace
	// due to one of the throws below shows the original block
	// base and extent.
	 := 
	 := 

	for  := uintptr(0);  < ; {
		// Find bits for the next word.
		 := uint32(*addb(, /(goarch.PtrSize*8)))
		if  == 0 {
			 += goarch.PtrSize * 8
			continue
		}
		for  := 0;  < 8 &&  < ; ++ {
			if &1 != 0 {
				// Same work as in scanobject; see comments there.
				 := *(*uintptr)(unsafe.Pointer( + ))
				if  != 0 {
					if , ,  := findObject(, , );  != 0 {
						greyobject(, , , , , )
					} else if  != nil &&  >= .stack.lo &&  < .stack.hi {
						.putPtr(, false)
					}
				}
			}
			 >>= 1
			 += goarch.PtrSize
		}
	}
}

// scanobject scans the object starting at b, adding pointers to gcw.
// b must point to the beginning of a heap object or an oblet.
// scanobject consults the GC bitmap for the pointer mask and the
// spans for the size of the object.
//
//go:nowritebarrier
func scanobject( uintptr,  *gcWork) {
	// Prefetch object before we scan it.
	//
	// This will overlap fetching the beginning of the object with initial
	// setup before we start scanning the object.
	sys.Prefetch()

	// Find the bits for b and the size of the object at b.
	//
	// b is either the beginning of an object, in which case this
	// is the size of the object to scan, or it points to an
	// oblet, in which case we compute the size to scan below.
	 := heapBitsForAddr()
	 := spanOfUnchecked()
	 := .elemsize
	if  == 0 {
		throw("scanobject n == 0")
	}

	if  > maxObletBytes {
		// Large object. Break into oblets for better
		// parallelism and lower latency.
		if  == .base() {
			// It's possible this is a noscan object (not
			// from greyobject, but from other code
			// paths), in which case we must *not* enqueue
			// oblets since their bitmaps will be
			// uninitialized.
			if .spanclass.noscan() {
				// Bypass the whole scan.
				.bytesMarked += uint64()
				return
			}

			// Enqueue the other oblets to scan later.
			// Some oblets may be in b's scalar tail, but
			// these will be marked as "no more pointers",
			// so we'll drop out immediately when we go to
			// scan those.
			for  :=  + maxObletBytes;  < .base()+.elemsize;  += maxObletBytes {
				if !.putFast() {
					.put()
				}
			}
		}

		// Compute the size of the oblet. Since this object
		// must be a large object, s.base() is the beginning
		// of the object.
		 = .base() + .elemsize - 
		if  > maxObletBytes {
			 = maxObletBytes
		}
	}

	var  uintptr
	for  = 0;  < ; ,  = +goarch.PtrSize, .next() {
		// Load bits once. See CL 22712 and issue 16973 for discussion.
		 := .bits()
		if &bitScan == 0 {
			break // no more pointers in this object
		}
		if &bitPointer == 0 {
			continue // not a pointer
		}

		// Work here is duplicated in scanblock and above.
		// If you make changes here, make changes there too.
		 := *(*uintptr)(unsafe.Pointer( + ))

		// At this point we have extracted the next potential pointer.
		// Quickly filter out nil and pointers back to the current object.
		if  != 0 && - >=  {
			// Test if obj points into the Go heap and, if so,
			// mark the object.
			//
			// Note that it's possible for findObject to
			// fail if obj points to a just-allocated heap
			// object because of a race with growing the
			// heap. In this case, we know the object was
			// just allocated and hence will be marked by
			// allocation itself.
			if , ,  := findObject(, , );  != 0 {
				greyobject(, , , , , )
			}
		}
	}
	.bytesMarked += uint64()
	.heapScanWork += int64()
}

// scanConservative scans block [b, b+n) conservatively, treating any
// pointer-like value in the block as a pointer.
//
// If ptrmask != nil, only words that are marked in ptrmask are
// considered as potential pointers.
//
// If state != nil, it's assumed that [b, b+n) is a block in the stack
// and may contain pointers to stack objects.
func scanConservative(,  uintptr,  *uint8,  *gcWork,  *stackScanState) {
	if debugScanConservative {
		printlock()
		print("conservatively scanning [", hex(), ",", hex(+), ")\n")
		hexdumpWords(, +, func( uintptr) byte {
			if  != nil {
				 := ( - ) / goarch.PtrSize
				 := *addb(, /8)
				if (>>(%8))&1 == 0 {
					return '$'
				}
			}

			 := *(*uintptr)(unsafe.Pointer())
			if  != nil && .stack.lo <=  &&  < .stack.hi {
				return '@'
			}

			 := spanOfHeap()
			if  == nil {
				return ' '
			}
			 := .objIndex()
			if .isFree() {
				return ' '
			}
			return '*'
		})
		printunlock()
	}

	for  := uintptr(0);  < ;  += goarch.PtrSize {
		if  != nil {
			 :=  / goarch.PtrSize
			 := *addb(, /8)
			if  == 0 {
				// Skip 8 words (the loop increment will do the 8th)
				//
				// This must be the first time we've
				// seen this word of ptrmask, so i
				// must be 8-word-aligned, but check
				// our reasoning just in case.
				if %(goarch.PtrSize*8) != 0 {
					throw("misaligned mask")
				}
				 += goarch.PtrSize*8 - goarch.PtrSize
				continue
			}
			if (>>(%8))&1 == 0 {
				continue
			}
		}

		 := *(*uintptr)(unsafe.Pointer( + ))

		// Check if val points into the stack.
		if  != nil && .stack.lo <=  &&  < .stack.hi {
			// val may point to a stack object. This
			// object may be dead from last cycle and
			// hence may contain pointers to unallocated
			// objects, but unlike heap objects we can't
			// tell if it's already dead. Hence, if all
			// pointers to this object are from
			// conservative scanning, we have to scan it
			// defensively, too.
			.putPtr(, true)
			continue
		}

		// Check if val points to a heap span.
		 := spanOfHeap()
		if  == nil {
			continue
		}

		// Check if val points to an allocated object.
		 := .objIndex()
		if .isFree() {
			continue
		}

		// val points to an allocated object. Mark it.
		 := .base() + *.elemsize
		greyobject(, , , , , )
	}
}

// Shade the object if it isn't already.
// The object is not nil and known to be in the heap.
// Preemption must be disabled.
//go:nowritebarrier
func shade( uintptr) {
	if , ,  := findObject(, 0, 0);  != 0 {
		 := &getg().m.p.ptr().gcw
		greyobject(, 0, 0, , , )
	}
}

// obj is the start of an object with mark mbits.
// If it isn't already marked, mark it and enqueue into gcw.
// base and off are for debugging only and could be removed.
//
// See also wbBufFlush1, which partially duplicates this logic.
//
//go:nowritebarrierrec
func greyobject(, ,  uintptr,  *mspan,  *gcWork,  uintptr) {
	// obj should be start of allocation, and so must be at least pointer-aligned.
	if &(goarch.PtrSize-1) != 0 {
		throw("greyobject: obj not pointer-aligned")
	}
	 := .markBitsForIndex()

	if useCheckmark {
		if setCheckmark(, , , ) {
			// Already marked.
			return
		}
	} else {
		if debug.gccheckmark > 0 && .isFree() {
			print("runtime: marking free object ", hex(), " found at *(", hex(), "+", hex(), ")\n")
			gcDumpObject("base", , )
			gcDumpObject("obj", , ^uintptr(0))
			getg().m.traceback = 2
			throw("marking free object")
		}

		// If marked we have nothing to do.
		if .isMarked() {
			return
		}
		.setMarked()

		// Mark span.
		, ,  := pageIndexOf(.base())
		if .pageMarks[]& == 0 {
			atomic.Or8(&.pageMarks[], )
		}

		// If this is a noscan object, fast-track it to black
		// instead of greying it.
		if .spanclass.noscan() {
			.bytesMarked += uint64(.elemsize)
			return
		}
	}

	// We're adding obj to P's local workbuf, so it's likely
	// this object will be processed soon by the same P.
	// Even if the workbuf gets flushed, there will likely still be
	// some benefit on platforms with inclusive shared caches.
	sys.Prefetch()
	// Queue the obj for scanning.
	if !.putFast() {
		.put()
	}
}

// gcDumpObject dumps the contents of obj for debugging and marks the
// field at byte offset off in obj.
func gcDumpObject( string, ,  uintptr) {
	 := spanOf()
	print(, "=", hex())
	if  == nil {
		print(" s=nil\n")
		return
	}
	print(" s.base()=", hex(.base()), " s.limit=", hex(.limit), " s.spanclass=", .spanclass, " s.elemsize=", .elemsize, " s.state=")
	if  := .state.get(); 0 <=  && int() < len(mSpanStateNames) {
		print(mSpanStateNames[], "\n")
	} else {
		print("unknown(", , ")\n")
	}

	 := false
	 := .elemsize
	if .state.get() == mSpanManual &&  == 0 {
		// We're printing something from a stack frame. We
		// don't know how big it is, so just show up to an
		// including off.
		 =  + goarch.PtrSize
	}
	for  := uintptr(0);  < ;  += goarch.PtrSize {
		// For big objects, just print the beginning (because
		// that usually hints at the object's type) and the
		// fields around off.
		if !( < 128*goarch.PtrSize || -16*goarch.PtrSize <  &&  < +16*goarch.PtrSize) {
			 = true
			continue
		}
		if  {
			print(" ...\n")
			 = false
		}
		print(" *(", , "+", , ") = ", hex(*(*uintptr)(unsafe.Pointer( + ))))
		if  ==  {
			print(" <==")
		}
		print("\n")
	}
	if  {
		print(" ...\n")
	}
}

// gcmarknewobject marks a newly allocated object black. obj must
// not contain any non-nil pointers.
//
// This is nosplit so it can manipulate a gcWork without preemption.
//
//go:nowritebarrier
//go:nosplit
func gcmarknewobject( *mspan, , ,  uintptr) {
	if useCheckmark { // The world should be stopped so this should not happen.
		throw("gcmarknewobject called while doing checkmark")
	}

	// Mark object.
	 := .objIndex()
	.markBitsForIndex().setMarked()

	// Mark span.
	, ,  := pageIndexOf(.base())
	if .pageMarks[]& == 0 {
		atomic.Or8(&.pageMarks[], )
	}

	 := &getg().m.p.ptr().gcw
	.bytesMarked += uint64()
	if !goexperiment.PacerRedesign {
		// The old pacer counts newly allocated memory toward
		// heapScanWork because heapScan is continuously updated
		// throughout the GC cycle with newly allocated memory. However,
		// these objects are never actually scanned, so we need
		// to account for them in heapScanWork here, "faking" their work.
		// Otherwise the pacer will think it's always behind, potentially
		// by a large margin.
		//
		// The new pacer doesn't care about this because it ceases to updated
		// heapScan once a GC cycle starts, effectively snapshotting it.
		.heapScanWork += int64()
	}
}

// gcMarkTinyAllocs greys all active tiny alloc blocks.
//
// The world must be stopped.
func gcMarkTinyAllocs() {
	assertWorldStopped()

	for ,  := range allp {
		 := .mcache
		if  == nil || .tiny == 0 {
			continue
		}
		, ,  := findObject(.tiny, 0, 0)
		 := &.gcw
		greyobject(.tiny, 0, 0, , , )
	}
}