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Source file src/runtime/mgcmark.go

  // 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 (
  	"runtime/internal/atomic"
  	"runtime/internal/sys"
  	"unsafe"
  )
  
  const (
  	fixedRootFinalizers = iota
  	fixedRootFreeGStacks
  	fixedRootCount
  
  	// rootBlockBytes is the number of bytes to scan per data or
  	// BSS root.
  	rootBlockBytes = 256 << 10
  
  	// rootBlockSpans is the number of spans to scan per span
  	// root.
  	rootBlockSpans = 8 * 1024 // 64MB worth of spans
  
  	// 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
  
  	// idleCheckThreshold specifies how many units of work to do
  	// between run queue checks in an idle worker. 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.
  	idleCheckThreshold = 100000
  )
  
  // gcMarkRootPrepare queues root scanning jobs (stacks, globals, and
  // some miscellany) and initializes scanning-related state.
  //
  // The caller must have call gcCopySpans().
  //
  // The world must be stopped.
  //
  //go:nowritebarrier
  func gcMarkRootPrepare() {
  	if gcphase == _GCmarktermination {
  		work.nFlushCacheRoots = int(gomaxprocs)
  	} else {
  		work.nFlushCacheRoots = 0
  	}
  
  	// Compute how many data and BSS root blocks there are.
  	nBlocks := func(bytes uintptr) int {
  		return int((bytes + rootBlockBytes - 1) / rootBlockBytes)
  	}
  
  	work.nDataRoots = 0
  	work.nBSSRoots = 0
  
  	// Only scan globals once per cycle; preferably concurrently.
  	if !work.markrootDone {
  		for _, datap := range activeModules() {
  			nDataRoots := nBlocks(datap.edata - datap.data)
  			if nDataRoots > work.nDataRoots {
  				work.nDataRoots = nDataRoots
  			}
  		}
  
  		for _, datap := range activeModules() {
  			nBSSRoots := nBlocks(datap.ebss - datap.bss)
  			if nBSSRoots > work.nBSSRoots {
  				work.nBSSRoots = nBSSRoots
  			}
  		}
  	}
  
  	if !work.markrootDone {
  		// On the first markroot, we need to scan span roots.
  		// In concurrent GC, this happens during concurrent
  		// mark and we depend on addfinalizer to ensure the
  		// above invariants for objects that get finalizers
  		// after concurrent mark. In STW GC, this will happen
  		// during mark termination.
  		//
  		// We're only interested in scanning the in-use spans,
  		// which will all be swept at this point. More spans
  		// may be added to this list during concurrent GC, but
  		// we only care about spans that were allocated before
  		// this mark phase.
  		work.nSpanRoots = mheap_.sweepSpans[mheap_.sweepgen/2%2].numBlocks()
  
  		// On the first markroot, we need to scan all Gs. 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
  		// scanned during mark termination. During mark
  		// termination, allglen isn't changing, so we'll scan
  		// all Gs.
  		work.nStackRoots = int(atomic.Loaduintptr(&allglen))
  		work.nRescanRoots = 0
  	} else {
  		// We've already scanned span roots and kept the scan
  		// up-to-date during concurrent mark.
  		work.nSpanRoots = 0
  
  		// On the second pass of markroot, we're just scanning
  		// dirty stacks. It's safe to access rescan since the
  		// world is stopped.
  		work.nStackRoots = 0
  		work.nRescanRoots = len(work.rescan.list)
  	}
  
  	work.markrootNext = 0
  	work.markrootJobs = uint32(fixedRootCount + work.nFlushCacheRoots + work.nDataRoots + work.nBSSRoots + work.nSpanRoots + work.nStackRoots + work.nRescanRoots)
  }
  
  // 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")
  	}
  
  	lock(&allglock)
  	// Check that stacks have been scanned.
  	var gp *g
  	if gcphase == _GCmarktermination && debug.gcrescanstacks > 0 {
  		for i := 0; i < len(allgs); i++ {
  			gp = allgs[i]
  			if !(gp.gcscandone && gp.gcscanvalid) && readgstatus(gp) != _Gdead {
  				goto fail
  			}
  		}
  	} else {
  		for i := 0; i < work.nStackRoots; i++ {
  			gp = allgs[i]
  			if !gp.gcscandone {
  				goto fail
  			}
  		}
  	}
  	unlock(&allglock)
  	return
  
  fail:
  	println("gp", gp, "goid", gp.goid,
  		"status", readgstatus(gp),
  		"gcscandone", gp.gcscandone,
  		"gcscanvalid", gp.gcscanvalid)
  	unlock(&allglock) // Avoid self-deadlock with traceback.
  	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).
  //
  // nowritebarrier is only advisory here.
  //
  //go:nowritebarrier
  func markroot(gcw *gcWork, i uint32) {
  	// TODO(austin): This is a bit ridiculous. Compute and store
  	// the bases in gcMarkRootPrepare instead of the counts.
  	baseFlushCache := uint32(fixedRootCount)
  	baseData := baseFlushCache + uint32(work.nFlushCacheRoots)
  	baseBSS := baseData + uint32(work.nDataRoots)
  	baseSpans := baseBSS + uint32(work.nBSSRoots)
  	baseStacks := baseSpans + uint32(work.nSpanRoots)
  	baseRescan := baseStacks + uint32(work.nStackRoots)
  	end := baseRescan + uint32(work.nRescanRoots)
  
  	// Note: if you add a case here, please also update heapdump.go:dumproots.
  	switch {
  	case baseFlushCache <= i && i < baseData:
  		flushmcache(int(i - baseFlushCache))
  
  	case baseData <= i && i < baseBSS:
  		for _, datap := range activeModules() {
  			markrootBlock(datap.data, datap.edata-datap.data, datap.gcdatamask.bytedata, gcw, int(i-baseData))
  		}
  
  	case baseBSS <= i && i < baseSpans:
  		for _, datap := range activeModules() {
  			markrootBlock(datap.bss, datap.ebss-datap.bss, datap.gcbssmask.bytedata, gcw, int(i-baseBSS))
  		}
  
  	case i == fixedRootFinalizers:
  		for fb := allfin; fb != nil; fb = fb.alllink {
  			cnt := uintptr(atomic.Load(&fb.cnt))
  			scanblock(uintptr(unsafe.Pointer(&fb.fin[0])), cnt*unsafe.Sizeof(fb.fin[0]), &finptrmask[0], gcw)
  		}
  
  	case i == fixedRootFreeGStacks:
  		// Only do this once per GC cycle; preferably
  		// concurrently.
  		if !work.markrootDone {
  			// Switch to the system stack so we can call
  			// stackfree.
  			systemstack(markrootFreeGStacks)
  		}
  
  	case baseSpans <= i && i < baseStacks:
  		// mark MSpan.specials
  		markrootSpans(gcw, int(i-baseSpans))
  
  	default:
  		// the rest is scanning goroutine stacks
  		var gp *g
  		if baseStacks <= i && i < baseRescan {
  			gp = allgs[i-baseStacks]
  		} else if baseRescan <= i && i < end {
  			gp = work.rescan.list[i-baseRescan].ptr()
  			if gp.gcRescan != int32(i-baseRescan) {
  				// Looking for issue #17099.
  				println("runtime: gp", gp, "found at rescan index", i-baseRescan, "but should be at", gp.gcRescan)
  				throw("bad g rescan index")
  			}
  		} else {
  			throw("markroot: bad index")
  		}
  
  		// remember when we've first observed the G blocked
  		// needed only to output in traceback
  		status := readgstatus(gp) // We are not in a scan state
  		if (status == _Gwaiting || status == _Gsyscall) && gp.waitsince == 0 {
  			gp.waitsince = work.tstart
  		}
  
  		// scang 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.
  			userG := getg().m.curg
  			selfScan := gp == userG && readgstatus(userG) == _Grunning
  			if selfScan {
  				casgstatus(userG, _Grunning, _Gwaiting)
  				userG.waitreason = "garbage collection scan"
  			}
  
  			// TODO: scang blocks until gp's stack has
  			// been scanned, 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.
  			scang(gp, gcw)
  
  			if selfScan {
  				casgstatus(userG, _Gwaiting, _Grunning)
  			}
  		})
  	}
  }
  
  // markrootBlock scans the shard'th shard of the block of memory [b0,
  // b0+n0), with the given pointer mask.
  //
  //go:nowritebarrier
  func markrootBlock(b0, n0 uintptr, ptrmask0 *uint8, gcw *gcWork, shard int) {
  	if rootBlockBytes%(8*sys.PtrSize) != 0 {
  		// This is necessary to pick byte offsets in ptrmask0.
  		throw("rootBlockBytes must be a multiple of 8*ptrSize")
  	}
  
  	b := b0 + uintptr(shard)*rootBlockBytes
  	if b >= b0+n0 {
  		return
  	}
  	ptrmask := (*uint8)(add(unsafe.Pointer(ptrmask0), uintptr(shard)*(rootBlockBytes/(8*sys.PtrSize))))
  	n := uintptr(rootBlockBytes)
  	if b+n > b0+n0 {
  		n = b0 + n0 - b
  	}
  
  	// Scan this shard.
  	scanblock(b, n, ptrmask, gcw)
  }
  
  // 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.
  //
  //TODO go:nowritebarrier
  func markrootFreeGStacks() {
  	// Take list of dead Gs with stacks.
  	lock(&sched.gflock)
  	list := sched.gfreeStack
  	sched.gfreeStack = nil
  	unlock(&sched.gflock)
  	if list == nil {
  		return
  	}
  
  	// Free stacks.
  	tail := list
  	for gp := list; gp != nil; gp = gp.schedlink.ptr() {
  		shrinkstack(gp)
  		tail = gp
  	}
  
  	// Put Gs back on the free list.
  	lock(&sched.gflock)
  	tail.schedlink.set(sched.gfreeNoStack)
  	sched.gfreeNoStack = list
  	unlock(&sched.gflock)
  }
  
  // markrootSpans marks roots for one shard of work.spans.
  //
  //go:nowritebarrier
  func markrootSpans(gcw *gcWork, shard 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.
  	//
  	// TODO(austin): There are several ideas for making this more
  	// efficient in issue #11485.
  
  	if work.markrootDone {
  		throw("markrootSpans during second markroot")
  	}
  
  	sg := mheap_.sweepgen
  	spans := mheap_.sweepSpans[mheap_.sweepgen/2%2].block(shard)
  	// Note that work.spans may not include spans that were
  	// allocated between entering the scan phase and now. This is
  	// okay because any objects with finalizers in those spans
  	// must have been allocated and given finalizers after we
  	// entered the scan phase, so addfinalizer will have ensured
  	// the above invariants for them.
  	for _, s := range spans {
  		if s.state != mSpanInUse {
  			continue
  		}
  		if !useCheckmark && s.sweepgen != sg {
  			// sweepgen was updated (+2) during non-checkmark GC pass
  			print("sweep ", s.sweepgen, " ", sg, "\n")
  			throw("gc: unswept span")
  		}
  
  		// Speculatively check if there are any specials
  		// without acquiring the span lock. This may race with
  		// adding the first special to a span, but in that
  		// case addfinalizer will observe that the GC is
  		// active (which is globally synchronized) and ensure
  		// the above invariants. We may also ensure the
  		// invariants, but it's okay to scan an object twice.
  		if s.specials == nil {
  			continue
  		}
  
  		// Lock the specials to prevent a special from being
  		// removed from the list while we're traversing it.
  		lock(&s.speciallock)
  
  		for sp := s.specials; sp != nil; sp = sp.next {
  			if sp.kind != _KindSpecialFinalizer {
  				continue
  			}
  			// don't mark finalized object, but scan it so we
  			// retain everything it points to.
  			spf := (*specialfinalizer)(unsafe.Pointer(sp))
  			// A finalizer can be set for an inner byte of an object, find object beginning.
  			p := s.base() + uintptr(spf.special.offset)/s.elemsize*s.elemsize
  
  			// Mark everything that can be reached from
  			// the object (but *not* the object itself or
  			// we'll never collect it).
  			scanobject(p, gcw)
  
  			// The special itself is a root.
  			scanblock(uintptr(unsafe.Pointer(&spf.fn)), sys.PtrSize, &oneptrmask[0], gcw)
  		}
  
  		unlock(&s.speciallock)
  	}
  }
  
  // gcAssistAlloc performs GC work to make gp's assist debt positive.
  // gp must be the calling user gorountine.
  //
  // This must be called with preemption enabled.
  func gcAssistAlloc(gp *g) {
  	// Don't assist in non-preemptible contexts. These are
  	// generally fragile and won't allow the assist to block.
  	if getg() == gp.m.g0 {
  		return
  	}
  	if mp := getg().m; mp.locks > 0 || mp.preemptoff != "" {
  		return
  	}
  
  retry:
  	// 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.
  	debtBytes := -gp.gcAssistBytes
  	scanWork := int64(gcController.assistWorkPerByte * float64(debtBytes))
  	if scanWork < gcOverAssistWork {
  		scanWork = gcOverAssistWork
  		debtBytes = int64(gcController.assistBytesPerWork * float64(scanWork))
  	}
  
  	// 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.
  	bgScanCredit := atomic.Loadint64(&gcController.bgScanCredit)
  	stolen := int64(0)
  	if bgScanCredit > 0 {
  		if bgScanCredit < scanWork {
  			stolen = bgScanCredit
  			gp.gcAssistBytes += 1 + int64(gcController.assistBytesPerWork*float64(stolen))
  		} else {
  			stolen = scanWork
  			gp.gcAssistBytes += debtBytes
  		}
  		atomic.Xaddint64(&gcController.bgScanCredit, -stolen)
  
  		scanWork -= stolen
  
  		if scanWork == 0 {
  			// We were able to steal all of the credit we
  			// needed.
  			return
  		}
  	}
  
  	// Perform assist work
  	systemstack(func() {
  		gcAssistAlloc1(gp, scanWork)
  		// The user stack may have moved, so this can't touch
  		// anything on it until it returns from systemstack.
  	})
  
  	completed := gp.param != nil
  	gp.param = nil
  	if completed {
  		gcMarkDone()
  	}
  
  	if gp.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 gp.preempt {
  			Gosched()
  			goto retry
  		}
  
  		// 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 retry
  		}
  
  		// At this point either background GC has satisfied
  		// this G's assist debt, or the GC cycle is over.
  	}
  }
  
  // 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(gp *g, scanWork int64) {
  	// Clear the flag indicating that this assist completed the
  	// mark phase.
  	gp.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 preform an assist.
  
  		// GC is done, so ignore any remaining debt.
  		gp.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.
  	startTime := nanotime()
  
  	decnwait := atomic.Xadd(&work.nwait, -1)
  	if decnwait == work.nproc {
  		println("runtime: work.nwait =", decnwait, "work.nproc=", work.nproc)
  		throw("nwait > work.nprocs")
  	}
  
  	// gcDrainN requires the caller to be preemptible.
  	casgstatus(gp, _Grunning, _Gwaiting)
  	gp.waitreason = "GC assist marking"
  
  	// drain own cached work first in the hopes that it
  	// will be more cache friendly.
  	gcw := &getg().m.p.ptr().gcw
  	workDone := gcDrainN(gcw, scanWork)
  	// If we are near the end of the mark phase
  	// dispose of the gcw.
  	if gcBlackenPromptly {
  		gcw.dispose()
  	}
  
  	casgstatus(gp, _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.
  	gp.gcAssistBytes += 1 + int64(gcController.assistBytesPerWork*float64(workDone))
  
  	// If this is the last worker and we ran out of work,
  	// signal a completion point.
  	incnwait := atomic.Xadd(&work.nwait, +1)
  	if incnwait > work.nproc {
  		println("runtime: work.nwait=", incnwait,
  			"work.nproc=", work.nproc,
  			"gcBlackenPromptly=", gcBlackenPromptly)
  		throw("work.nwait > work.nproc")
  	}
  
  	if incnwait == 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).
  		gp.param = unsafe.Pointer(gp)
  	}
  	duration := nanotime() - startTime
  	_p_ := gp.m.p.ptr()
  	_p_.gcAssistTime += duration
  	if _p_.gcAssistTime > gcAssistTimeSlack {
  		atomic.Xaddint64(&gcController.assistTime, _p_.gcAssistTime)
  		_p_.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)
  	injectglist(work.assistQueue.head.ptr())
  	work.assistQueue.head.set(nil)
  	work.assistQueue.tail.set(nil)
  	unlock(&work.assistQueue.lock)
  }
  
  // gcParkAssist puts the current goroutine on the assist queue and parks.
  //
  // gcParkAssist returns whether the assist is now satisfied. If it
  // returns false, the caller must retry the assist.
  //
  //go:nowritebarrier
  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
  	}
  
  	gp := getg()
  	oldHead, oldTail := work.assistQueue.head, work.assistQueue.tail
  	if oldHead == 0 {
  		work.assistQueue.head.set(gp)
  	} else {
  		oldTail.ptr().schedlink.set(gp)
  	}
  	work.assistQueue.tail.set(gp)
  	gp.schedlink.set(nil)
  
  	// 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.head = oldHead
  		work.assistQueue.tail = oldTail
  		if oldTail != 0 {
  			oldTail.ptr().schedlink.set(nil)
  		}
  		unlock(&work.assistQueue.lock)
  		return false
  	}
  	// Park.
  	goparkunlock(&work.assistQueue.lock, "GC assist wait", 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(scanWork int64) {
  	if work.assistQueue.head == 0 {
  		// 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, scanWork)
  		return
  	}
  
  	scanBytes := int64(float64(scanWork) * gcController.assistBytesPerWork)
  
  	lock(&work.assistQueue.lock)
  	gp := work.assistQueue.head.ptr()
  	for gp != nil && scanBytes > 0 {
  		// Note that gp.gcAssistBytes is negative because gp
  		// is in debt. Think carefully about the signs below.
  		if scanBytes+gp.gcAssistBytes >= 0 {
  			// Satisfy this entire assist debt.
  			scanBytes += gp.gcAssistBytes
  			gp.gcAssistBytes = 0
  			xgp := gp
  			gp = gp.schedlink.ptr()
  			// It's important that we *not* put xgp 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(xgp, 0, false)
  		} else {
  			// Partially satisfy this assist.
  			gp.gcAssistBytes += scanBytes
  			scanBytes = 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.
  			xgp := gp
  			gp = gp.schedlink.ptr()
  			if gp == nil {
  				// gp is the only assist in the queue.
  				gp = xgp
  			} else {
  				xgp.schedlink = 0
  				work.assistQueue.tail.ptr().schedlink.set(xgp)
  				work.assistQueue.tail.set(xgp)
  			}
  			break
  		}
  	}
  	work.assistQueue.head.set(gp)
  	if gp == nil {
  		work.assistQueue.tail.set(nil)
  	}
  
  	if scanBytes > 0 {
  		// Convert from scan bytes back to work.
  		scanWork = int64(float64(scanBytes) * gcController.assistWorkPerByte)
  		atomic.Xaddint64(&gcController.bgScanCredit, scanWork)
  	}
  	unlock(&work.assistQueue.lock)
  }
  
  // scanstack scans gp's stack, greying all pointers found on the stack.
  //
  // During mark phase, it also installs stack barriers while traversing
  // gp's stack. During mark termination, it stops scanning when it
  // reaches an unhit stack barrier.
  //
  // scanstack is marked go:systemstack because it must not be preempted
  // while using a workbuf.
  //
  //go:nowritebarrier
  //go:systemstack
  func scanstack(gp *g, gcw *gcWork) {
  	if gp.gcscanvalid {
  		return
  	}
  
  	if readgstatus(gp)&_Gscan == 0 {
  		print("runtime:scanstack: gp=", gp, ", goid=", gp.goid, ", gp->atomicstatus=", hex(readgstatus(gp)), "\n")
  		throw("scanstack - bad status")
  	}
  
  	switch readgstatus(gp) &^ _Gscan {
  	default:
  		print("runtime: gp=", gp, ", goid=", gp.goid, ", gp->atomicstatus=", readgstatus(gp), "\n")
  		throw("mark - bad status")
  	case _Gdead:
  		return
  	case _Grunning:
  		print("runtime: gp=", gp, ", goid=", gp.goid, ", gp->atomicstatus=", readgstatus(gp), "\n")
  		throw("scanstack: goroutine not stopped")
  	case _Grunnable, _Gsyscall, _Gwaiting:
  		// ok
  	}
  
  	if gp == getg() {
  		throw("can't scan our own stack")
  	}
  	mp := gp.m
  	if mp != nil && mp.helpgc != 0 {
  		throw("can't scan gchelper stack")
  	}
  
  	// Shrink the stack if not much of it is being used. During
  	// concurrent GC, we can do this during concurrent mark.
  	if !work.markrootDone {
  		shrinkstack(gp)
  	}
  
  	// Prepare for stack barrier insertion/removal.
  	var sp, barrierOffset, nextBarrier uintptr
  	if gp.syscallsp != 0 {
  		sp = gp.syscallsp
  	} else {
  		sp = gp.sched.sp
  	}
  	gcLockStackBarriers(gp) // Not necessary during mark term, but harmless.
  	switch gcphase {
  	case _GCmark:
  		// Install stack barriers during stack scan.
  		barrierOffset = uintptr(firstStackBarrierOffset)
  		nextBarrier = sp + barrierOffset
  
  		if debug.gcstackbarrieroff > 0 {
  			nextBarrier = ^uintptr(0)
  		}
  
  		// Remove any existing stack barriers before we
  		// install new ones.
  		gcRemoveStackBarriers(gp)
  
  	case _GCmarktermination:
  		if !work.markrootDone {
  			// This is a STW GC. There may be stale stack
  			// barriers from an earlier cycle since we
  			// never passed through mark phase.
  			gcRemoveStackBarriers(gp)
  		}
  
  		if int(gp.stkbarPos) == len(gp.stkbar) {
  			// gp hit all of the stack barriers (or there
  			// were none). Re-scan the whole stack.
  			nextBarrier = ^uintptr(0)
  		} else {
  			// Only re-scan up to the lowest un-hit
  			// barrier. Any frames above this have not
  			// executed since the concurrent scan of gp and
  			// any writes through up-pointers to above
  			// this barrier had write barriers.
  			nextBarrier = gp.stkbar[gp.stkbarPos].savedLRPtr
  			if debugStackBarrier {
  				print("rescan below ", hex(nextBarrier), " in [", hex(sp), ",", hex(gp.stack.hi), ") goid=", gp.goid, "\n")
  			}
  		}
  
  	default:
  		throw("scanstack in wrong phase")
  	}
  
  	// Scan the stack.
  	var cache pcvalueCache
  	n := 0
  	scanframe := func(frame *stkframe, unused unsafe.Pointer) bool {
  		scanframeworker(frame, &cache, gcw)
  
  		if frame.fp > nextBarrier {
  			// We skip installing a barrier on bottom-most
  			// frame because on LR machines this LR is not
  			// on the stack.
  			if gcphase == _GCmark && n != 0 {
  				if gcInstallStackBarrier(gp, frame) {
  					barrierOffset *= 2
  					nextBarrier = sp + barrierOffset
  				}
  			} else if gcphase == _GCmarktermination {
  				// We just scanned a frame containing
  				// a return to a stack barrier. Since
  				// this frame never returned, we can
  				// stop scanning.
  				return false
  			}
  		}
  		n++
  
  		return true
  	}
  	gentraceback(^uintptr(0), ^uintptr(0), 0, gp, 0, nil, 0x7fffffff, scanframe, nil, 0)
  	tracebackdefers(gp, scanframe, nil)
  	gcUnlockStackBarriers(gp)
  	if gcphase == _GCmark {
  		// gp may have added itself to the rescan list between
  		// when GC started and now. It's clean now, so remove
  		// it. This isn't safe during mark termination because
  		// mark termination is consuming this list, but it's
  		// also not necessary.
  		dequeueRescan(gp)
  	}
  	gp.gcscanvalid = true
  }
  
  // Scan a stack frame: local variables and function arguments/results.
  //go:nowritebarrier
  func scanframeworker(frame *stkframe, cache *pcvalueCache, gcw *gcWork) {
  
  	f := frame.fn
  	targetpc := frame.continpc
  	if targetpc == 0 {
  		// Frame is dead.
  		return
  	}
  	if _DebugGC > 1 {
  		print("scanframe ", funcname(f), "\n")
  	}
  	if targetpc != f.entry {
  		targetpc--
  	}
  	pcdata := pcdatavalue(f, _PCDATA_StackMapIndex, targetpc, cache)
  	if pcdata == -1 {
  		// We do not have a valid pcdata value but there might be a
  		// stackmap for this function. It is likely that we are looking
  		// at the function prologue, assume so and hope for the best.
  		pcdata = 0
  	}
  
  	// Scan local variables if stack frame has been allocated.
  	size := frame.varp - frame.sp
  	var minsize uintptr
  	switch sys.ArchFamily {
  	case sys.ARM64:
  		minsize = sys.SpAlign
  	default:
  		minsize = sys.MinFrameSize
  	}
  	if size > minsize {
  		stkmap := (*stackmap)(funcdata(f, _FUNCDATA_LocalsPointerMaps))
  		if stkmap == nil || stkmap.n <= 0 {
  			print("runtime: frame ", funcname(f), " untyped locals ", hex(frame.varp-size), "+", hex(size), "\n")
  			throw("missing stackmap")
  		}
  
  		// Locals bitmap information, scan just the pointers in locals.
  		if pcdata < 0 || pcdata >= stkmap.n {
  			// don't know where we are
  			print("runtime: pcdata is ", pcdata, " and ", stkmap.n, " locals stack map entries for ", funcname(f), " (targetpc=", targetpc, ")\n")
  			throw("scanframe: bad symbol table")
  		}
  		bv := stackmapdata(stkmap, pcdata)
  		size = uintptr(bv.n) * sys.PtrSize
  		scanblock(frame.varp-size, size, bv.bytedata, gcw)
  	}
  
  	// Scan arguments.
  	if frame.arglen > 0 {
  		var bv bitvector
  		if frame.argmap != nil {
  			bv = *frame.argmap
  		} else {
  			stkmap := (*stackmap)(funcdata(f, _FUNCDATA_ArgsPointerMaps))
  			if stkmap == nil || stkmap.n <= 0 {
  				print("runtime: frame ", funcname(f), " untyped args ", hex(frame.argp), "+", hex(frame.arglen), "\n")
  				throw("missing stackmap")
  			}
  			if pcdata < 0 || pcdata >= stkmap.n {
  				// don't know where we are
  				print("runtime: pcdata is ", pcdata, " and ", stkmap.n, " args stack map entries for ", funcname(f), " (targetpc=", targetpc, ")\n")
  				throw("scanframe: bad symbol table")
  			}
  			bv = stackmapdata(stkmap, pcdata)
  		}
  		scanblock(frame.argp, uintptr(bv.n)*sys.PtrSize, bv.bytedata, gcw)
  	}
  }
  
  // queueRescan adds gp to the stack rescan list and clears
  // gp.gcscanvalid. The caller must own gp and ensure that gp isn't
  // already on the rescan list.
  func queueRescan(gp *g) {
  	if debug.gcrescanstacks == 0 {
  		// Clear gcscanvalid to keep assertions happy.
  		//
  		// TODO: Remove gcscanvalid entirely when we remove
  		// stack rescanning.
  		gp.gcscanvalid = false
  		return
  	}
  
  	if gcphase == _GCoff {
  		gp.gcscanvalid = false
  		return
  	}
  	if gp.gcRescan != -1 {
  		throw("g already on rescan list")
  	}
  
  	lock(&work.rescan.lock)
  	gp.gcscanvalid = false
  
  	// Recheck gcphase under the lock in case there was a phase change.
  	if gcphase == _GCoff {
  		unlock(&work.rescan.lock)
  		return
  	}
  	if len(work.rescan.list) == cap(work.rescan.list) {
  		throw("rescan list overflow")
  	}
  	n := len(work.rescan.list)
  	gp.gcRescan = int32(n)
  	work.rescan.list = work.rescan.list[:n+1]
  	work.rescan.list[n].set(gp)
  	unlock(&work.rescan.lock)
  }
  
  // dequeueRescan removes gp from the stack rescan list, if gp is on
  // the rescan list. The caller must own gp.
  func dequeueRescan(gp *g) {
  	if debug.gcrescanstacks == 0 {
  		return
  	}
  
  	if gp.gcRescan == -1 {
  		return
  	}
  	if gcphase == _GCoff {
  		gp.gcRescan = -1
  		return
  	}
  
  	lock(&work.rescan.lock)
  	if work.rescan.list[gp.gcRescan].ptr() != gp {
  		throw("bad dequeueRescan")
  	}
  	// Careful: gp may itself be the last G on the list.
  	last := work.rescan.list[len(work.rescan.list)-1]
  	work.rescan.list[gp.gcRescan] = last
  	last.ptr().gcRescan = gp.gcRescan
  	gp.gcRescan = -1
  	work.rescan.list = work.rescan.list[:len(work.rescan.list)-1]
  	unlock(&work.rescan.lock)
  }
  
  type gcDrainFlags int
  
  const (
  	gcDrainUntilPreempt gcDrainFlags = 1 << iota
  	gcDrainNoBlock
  	gcDrainFlushBgCredit
  	gcDrainIdle
  
  	// gcDrainBlock means neither gcDrainUntilPreempt or
  	// gcDrainNoBlock. It is the default, but callers should use
  	// the constant for documentation purposes.
  	gcDrainBlock gcDrainFlags = 0
  )
  
  // gcDrain scans roots and objects in work buffers, blackening grey
  // objects until all roots and work buffers have been drained.
  //
  // If flags&gcDrainUntilPreempt != 0, gcDrain returns when g.preempt
  // is set. This implies gcDrainNoBlock.
  //
  // If flags&gcDrainIdle != 0, gcDrain returns when there is other work
  // to do. This implies gcDrainNoBlock.
  //
  // If flags&gcDrainNoBlock != 0, gcDrain returns as soon as it is
  // unable to get more work. Otherwise, it will block until all
  // blocking calls are blocked in gcDrain.
  //
  // If flags&gcDrainFlushBgCredit != 0, gcDrain flushes scan work
  // credit to gcController.bgScanCredit every gcCreditSlack units of
  // scan work.
  //
  //go:nowritebarrier
  func gcDrain(gcw *gcWork, flags gcDrainFlags) {
  	if !writeBarrier.needed {
  		throw("gcDrain phase incorrect")
  	}
  
  	gp := getg().m.curg
  	preemptible := flags&gcDrainUntilPreempt != 0
  	blocking := flags&(gcDrainUntilPreempt|gcDrainIdle|gcDrainNoBlock) == 0
  	flushBgCredit := flags&gcDrainFlushBgCredit != 0
  	idle := flags&gcDrainIdle != 0
  
  	initScanWork := gcw.scanWork
  	// idleCheck is the scan work at which to perform the next
  	// idle check with the scheduler.
  	idleCheck := initScanWork + idleCheckThreshold
  
  	// Drain root marking jobs.
  	if work.markrootNext < work.markrootJobs {
  		for !(preemptible && gp.preempt) {
  			job := atomic.Xadd(&work.markrootNext, +1) - 1
  			if job >= work.markrootJobs {
  				break
  			}
  			markroot(gcw, job)
  			if idle && pollWork() {
  				goto done
  			}
  		}
  	}
  
  	// Drain heap marking jobs.
  	for !(preemptible && gp.preempt) {
  		// 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 {
  			gcw.balance()
  		}
  
  		var b uintptr
  		if blocking {
  			b = gcw.get()
  		} else {
  			b = gcw.tryGetFast()
  			if b == 0 {
  				b = gcw.tryGet()
  			}
  		}
  		if b == 0 {
  			// work barrier reached or tryGet failed.
  			break
  		}
  		scanobject(b, gcw)
  
  		// Flush background scan work credit to the global
  		// account if we've accumulated enough locally so
  		// mutator assists can draw on it.
  		if gcw.scanWork >= gcCreditSlack {
  			atomic.Xaddint64(&gcController.scanWork, gcw.scanWork)
  			if flushBgCredit {
  				gcFlushBgCredit(gcw.scanWork - initScanWork)
  				initScanWork = 0
  			}
  			idleCheck -= gcw.scanWork
  			gcw.scanWork = 0
  
  			if idle && idleCheck <= 0 {
  				idleCheck += idleCheckThreshold
  				if pollWork() {
  					break
  				}
  			}
  		}
  	}
  
  	// In blocking mode, write barriers are not allowed after this
  	// point because we must preserve the condition that the work
  	// buffers are empty.
  
  done:
  	// Flush remaining scan work credit.
  	if gcw.scanWork > 0 {
  		atomic.Xaddint64(&gcController.scanWork, gcw.scanWork)
  		if flushBgCredit {
  			gcFlushBgCredit(gcw.scanWork - initScanWork)
  		}
  		gcw.scanWork = 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(gcw *gcWork, scanWork 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.
  	workFlushed := -gcw.scanWork
  
  	gp := getg().m.curg
  	for !gp.preempt && workFlushed+gcw.scanWork < scanWork {
  		// See gcDrain comment.
  		if work.full == 0 {
  			gcw.balance()
  		}
  
  		// This might be a good place to add prefetch code...
  		// if(wbuf.nobj > 4) {
  		//         PREFETCH(wbuf->obj[wbuf.nobj - 3];
  		//  }
  		//
  		b := gcw.tryGetFast()
  		if b == 0 {
  			b = gcw.tryGet()
  		}
  
  		if b == 0 {
  			// Try to do a root job.
  			//
  			// TODO: Assists should get credit for this
  			// work.
  			if work.markrootNext < work.markrootJobs {
  				job := atomic.Xadd(&work.markrootNext, +1) - 1
  				if job < work.markrootJobs {
  					markroot(gcw, job)
  					continue
  				}
  			}
  			// No heap or root jobs.
  			break
  		}
  		scanobject(b, gcw)
  
  		// Flush background scan work credit.
  		if gcw.scanWork >= gcCreditSlack {
  			atomic.Xaddint64(&gcController.scanWork, gcw.scanWork)
  			workFlushed += gcw.scanWork
  			gcw.scanWork = 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 workFlushed + gcw.scanWork
  }
  
  // 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.scanWork.
  //
  //go:nowritebarrier
  func scanblock(b0, n0 uintptr, ptrmask *uint8, gcw *gcWork) {
  	// 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.
  	b := b0
  	n := n0
  
  	arena_start := mheap_.arena_start
  	arena_used := mheap_.arena_used
  
  	for i := uintptr(0); i < n; {
  		// Find bits for the next word.
  		bits := uint32(*addb(ptrmask, i/(sys.PtrSize*8)))
  		if bits == 0 {
  			i += sys.PtrSize * 8
  			continue
  		}
  		for j := 0; j < 8 && i < n; j++ {
  			if bits&1 != 0 {
  				// Same work as in scanobject; see comments there.
  				obj := *(*uintptr)(unsafe.Pointer(b + i))
  				if obj != 0 && arena_start <= obj && obj < arena_used {
  					if obj, hbits, span, objIndex := heapBitsForObject(obj, b, i); obj != 0 {
  						greyobject(obj, b, i, hbits, span, gcw, objIndex)
  					}
  				}
  			}
  			bits >>= 1
  			i += sys.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(b uintptr, gcw *gcWork) {
  	// Note that arena_used may change concurrently during
  	// scanobject and hence scanobject may encounter a pointer to
  	// a newly allocated heap object that is *not* in
  	// [start,used). It will not mark this object; however, we
  	// know that it was just installed by a mutator, which means
  	// that mutator will execute a write barrier and take care of
  	// marking it. This is even more pronounced on relaxed memory
  	// architectures since we access arena_used without barriers
  	// or synchronization, but the same logic applies.
  	arena_start := mheap_.arena_start
  	arena_used := mheap_.arena_used
  
  	// 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.
  	hbits := heapBitsForAddr(b)
  	s := spanOfUnchecked(b)
  	n := s.elemsize
  	if n == 0 {
  		throw("scanobject n == 0")
  	}
  
  	if n > maxObletBytes {
  		// Large object. Break into oblets for better
  		// parallelism and lower latency.
  		if b == s.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 !hbits.hasPointers(n) {
  				// Bypass the whole scan.
  				gcw.bytesMarked += uint64(n)
  				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 oblet := b + maxObletBytes; oblet < s.base()+s.elemsize; oblet += maxObletBytes {
  				if !gcw.putFast(oblet) {
  					gcw.put(oblet)
  				}
  			}
  		}
  
  		// Compute the size of the oblet. Since this object
  		// must be a large object, s.base() is the beginning
  		// of the object.
  		n = s.base() + s.elemsize - b
  		if n > maxObletBytes {
  			n = maxObletBytes
  		}
  	}
  
  	var i uintptr
  	for i = 0; i < n; i += sys.PtrSize {
  		// Find bits for this word.
  		if i != 0 {
  			// Avoid needless hbits.next() on last iteration.
  			hbits = hbits.next()
  		}
  		// Load bits once. See CL 22712 and issue 16973 for discussion.
  		bits := hbits.bits()
  		// During checkmarking, 1-word objects store the checkmark
  		// in the type bit for the one word. The only one-word objects
  		// are pointers, or else they'd be merged with other non-pointer
  		// data into larger allocations.
  		if i != 1*sys.PtrSize && bits&bitScan == 0 {
  			break // no more pointers in this object
  		}
  		if bits&bitPointer == 0 {
  			continue // not a pointer
  		}
  
  		// Work here is duplicated in scanblock and above.
  		// If you make changes here, make changes there too.
  		obj := *(*uintptr)(unsafe.Pointer(b + i))
  
  		// At this point we have extracted the next potential pointer.
  		// Check if it points into heap and not back at the current object.
  		if obj != 0 && arena_start <= obj && obj < arena_used && obj-b >= n {
  			// Mark the object.
  			if obj, hbits, span, objIndex := heapBitsForObject(obj, b, i); obj != 0 {
  				greyobject(obj, b, i, hbits, span, gcw, objIndex)
  			}
  		}
  	}
  	gcw.bytesMarked += uint64(n)
  	gcw.scanWork += int64(i)
  }
  
  // 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(b uintptr) {
  	if obj, hbits, span, objIndex := heapBitsForObject(b, 0, 0); obj != 0 {
  		gcw := &getg().m.p.ptr().gcw
  		greyobject(obj, 0, 0, hbits, span, gcw, objIndex)
  		if gcphase == _GCmarktermination || gcBlackenPromptly {
  			// Ps aren't allowed to cache work during mark
  			// termination.
  			gcw.dispose()
  		}
  	}
  }
  
  // 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.
  //go:nowritebarrierrec
  func greyobject(obj, base, off uintptr, hbits heapBits, span *mspan, gcw *gcWork, objIndex uintptr) {
  	// obj should be start of allocation, and so must be at least pointer-aligned.
  	if obj&(sys.PtrSize-1) != 0 {
  		throw("greyobject: obj not pointer-aligned")
  	}
  	mbits := span.markBitsForIndex(objIndex)
  
  	if useCheckmark {
  		if !mbits.isMarked() {
  			printlock()
  			print("runtime:greyobject: checkmarks finds unexpected unmarked object obj=", hex(obj), "\n")
  			print("runtime: found obj at *(", hex(base), "+", hex(off), ")\n")
  
  			// Dump the source (base) object
  			gcDumpObject("base", base, off)
  
  			// Dump the object
  			gcDumpObject("obj", obj, ^uintptr(0))
  
  			throw("checkmark found unmarked object")
  		}
  		if hbits.isCheckmarked(span.elemsize) {
  			return
  		}
  		hbits.setCheckmarked(span.elemsize)
  		if !hbits.isCheckmarked(span.elemsize) {
  			throw("setCheckmarked and isCheckmarked disagree")
  		}
  	} else {
  		if debug.gccheckmark > 0 && span.isFree(objIndex) {
  			print("runtime: marking free object ", hex(obj), " found at *(", hex(base), "+", hex(off), ")\n")
  			gcDumpObject("base", base, off)
  			gcDumpObject("obj", obj, ^uintptr(0))
  			throw("marking free object")
  		}
  
  		// If marked we have nothing to do.
  		if mbits.isMarked() {
  			return
  		}
  		// mbits.setMarked() // Avoid extra call overhead with manual inlining.
  		atomic.Or8(mbits.bytep, mbits.mask)
  		// If this is a noscan object, fast-track it to black
  		// instead of greying it.
  		if !hbits.hasPointers(span.elemsize) {
  			gcw.bytesMarked += uint64(span.elemsize)
  			return
  		}
  	}
  
  	// Queue the obj for scanning. The PREFETCH(obj) logic has been removed but
  	// seems like a nice optimization that can be added back in.
  	// There needs to be time between the PREFETCH and the use.
  	// Previously we put the obj in an 8 element buffer that is drained at a rate
  	// to give the PREFETCH time to do its work.
  	// Use of PREFETCHNTA might be more appropriate than PREFETCH
  	if !gcw.putFast(obj) {
  		gcw.put(obj)
  	}
  }
  
  // gcDumpObject dumps the contents of obj for debugging and marks the
  // field at byte offset off in obj.
  func gcDumpObject(label string, obj, off uintptr) {
  	if obj < mheap_.arena_start || obj >= mheap_.arena_used {
  		print(label, "=", hex(obj), " is not in the Go heap\n")
  		return
  	}
  	k := obj >> _PageShift
  	x := k
  	x -= mheap_.arena_start >> _PageShift
  	s := mheap_.spans[x]
  	print(label, "=", hex(obj), " k=", hex(k))
  	if s == nil {
  		print(" s=nil\n")
  		return
  	}
  	print(" s.base()=", hex(s.base()), " s.limit=", hex(s.limit), " s.sizeclass=", s.sizeclass, " s.elemsize=", s.elemsize, " s.state=")
  	if 0 <= s.state && int(s.state) < len(mSpanStateNames) {
  		print(mSpanStateNames[s.state], "\n")
  	} else {
  		print("unknown(", s.state, ")\n")
  	}
  
  	skipped := false
  	size := s.elemsize
  	if s.state == _MSpanStack && size == 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.
  		size = off + sys.PtrSize
  	}
  	for i := uintptr(0); i < size; i += sys.PtrSize {
  		// For big objects, just print the beginning (because
  		// that usually hints at the object's type) and the
  		// fields around off.
  		if !(i < 128*sys.PtrSize || off-16*sys.PtrSize < i && i < off+16*sys.PtrSize) {
  			skipped = true
  			continue
  		}
  		if skipped {
  			print(" ...\n")
  			skipped = false
  		}
  		print(" *(", label, "+", i, ") = ", hex(*(*uintptr)(unsafe.Pointer(obj + i))))
  		if i == off {
  			print(" <==")
  		}
  		print("\n")
  	}
  	if skipped {
  		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(obj, size, scanSize uintptr) {
  	if useCheckmark && !gcBlackenPromptly { // The world should be stopped so this should not happen.
  		throw("gcmarknewobject called while doing checkmark")
  	}
  	markBitsForAddr(obj).setMarked()
  	gcw := &getg().m.p.ptr().gcw
  	gcw.bytesMarked += uint64(size)
  	gcw.scanWork += int64(scanSize)
  	if gcBlackenPromptly {
  		// There shouldn't be anything in the work queue, but
  		// we still need to flush stats.
  		gcw.dispose()
  	}
  }
  
  // gcMarkTinyAllocs greys all active tiny alloc blocks.
  //
  // The world must be stopped.
  func gcMarkTinyAllocs() {
  	for _, p := range &allp {
  		if p == nil || p.status == _Pdead {
  			break
  		}
  		c := p.mcache
  		if c == nil || c.tiny == 0 {
  			continue
  		}
  		_, hbits, span, objIndex := heapBitsForObject(c.tiny, 0, 0)
  		gcw := &p.gcw
  		greyobject(c.tiny, 0, 0, hbits, span, gcw, objIndex)
  		if gcBlackenPromptly {
  			gcw.dispose()
  		}
  	}
  }
  
  // Checkmarking
  
  // To help debug the concurrent GC we remark with the world
  // stopped ensuring that any object encountered has their normal
  // mark bit set. To do this we use an orthogonal bit
  // pattern to indicate the object is marked. The following pattern
  // uses the upper two bits in the object's boundary nibble.
  // 01: scalar  not marked
  // 10: pointer not marked
  // 11: pointer     marked
  // 00: scalar      marked
  // Xoring with 01 will flip the pattern from marked to unmarked and vica versa.
  // The higher bit is 1 for pointers and 0 for scalars, whether the object
  // is marked or not.
  // The first nibble no longer holds the typeDead pattern indicating that the
  // there are no more pointers in the object. This information is held
  // in the second nibble.
  
  // If useCheckmark is true, marking of an object uses the
  // checkmark bits (encoding above) instead of the standard
  // mark bits.
  var useCheckmark = false
  
  //go:nowritebarrier
  func initCheckmarks() {
  	useCheckmark = true
  	for _, s := range mheap_.allspans {
  		if s.state == _MSpanInUse {
  			heapBitsForSpan(s.base()).initCheckmarkSpan(s.layout())
  		}
  	}
  }
  
  func clearCheckmarks() {
  	useCheckmark = false
  	for _, s := range mheap_.allspans {
  		if s.state == _MSpanInUse {
  			heapBitsForSpan(s.base()).clearCheckmarkSpan(s.layout())
  		}
  	}
  }
  

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