runtime: implement experiment to replace heap bitmap with alloc headers

[gostls13.git] / 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 (
        "internal/abi"
        "internal/goarch"
        "internal/goexperiment"
        "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

        // 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.
        nBlocks := func(bytes uintptr) int {
                return int(divRoundUp(bytes, rootBlockBytes))
        }

        work.nDataRoots = 0
        work.nBSSRoots = 0

        // Scan globals.
        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
                }
        }

        // 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.
        i := 0
        forEachGRace(func(gp *g) {
                if i >= work.nStackRoots {
                        return
                }

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

                i++
        })
}

// 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(gcw *gcWork, i uint32, flushBgCredit bool) int64 {
        // Note: if you add a case here, please also update heapdump.go:dumproots.
        var workDone int64
        var workCounter *atomic.Int64
        switch {
        case work.baseData <= i && i < work.baseBSS:
                workCounter = &gcController.globalsScanWork
                for _, datap := range activeModules() {
                        workDone += markrootBlock(datap.data, datap.edata-datap.data, datap.gcdatamask.bytedata, gcw, int(i-work.baseData))
                }

        case work.baseBSS <= i && i < work.baseSpans:
                workCounter = &gcController.globalsScanWork
                for _, datap := range activeModules() {
                        workDone += markrootBlock(datap.bss, datap.ebss-datap.bss, datap.gcbssmask.bytedata, gcw, int(i-work.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, nil)
                }

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

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

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

                // 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
                }

                // 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.
                        userG := getg().m.curg
                        selfScan := gp == userG && readgstatus(userG) == _Grunning
                        if selfScan {
                                casGToWaiting(userG, _Grunning, 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.
                        stopped := suspendG(gp)
                        if stopped.dead {
                                gp.gcscandone = true
                                return
                        }
                        if gp.gcscandone {
                                throw("g already scanned")
                        }
                        workDone += scanstack(gp, gcw)
                        gp.gcscandone = true
                        resumeG(stopped)

                        if selfScan {
                                casgstatus(userG, _Gwaiting, _Grunning)
                        }
                })
        }
        if workCounter != nil && workDone != 0 {
                workCounter.Add(workDone)
                if flushBgCredit {
                        gcFlushBgCredit(workDone)
                }
        }
        return workDone
}

// 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(b0, n0 uintptr, ptrmask0 *uint8, gcw *gcWork, shard 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.
        off := uintptr(shard) * rootBlockBytes
        if off >= n0 {
                return 0
        }
        b := b0 + off
        ptrmask := (*uint8)(add(unsafe.Pointer(ptrmask0), uintptr(shard)*(rootBlockBytes/(8*goarch.PtrSize))))
        n := uintptr(rootBlockBytes)
        if off+n > n0 {
                n = n0 - off
        }

        // Scan this shard.
        scanblock(b, n, ptrmask, gcw, nil)
        return int64(n)
}

// 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)
        list := sched.gFree.stack
        sched.gFree.stack = gList{}
        unlock(&sched.gFree.lock)
        if list.empty() {
                return
        }

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

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

// markrootSpans marks roots for one shard of markArenas.
//
//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.
        sg := mheap_.sweepgen

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

        // Construct slice of bitmap which we'll iterate over.
        specialsbits := ha.pageSpecials[arenaPage/8:]
        specialsbits = specialsbits[:pagesPerSpanRoot/8]
        for i := range specialsbits {
                // Find set bits, which correspond to spans with specials.
                specials := atomic.Load8(&specialsbits[i])
                if specials == 0 {
                        continue
                }
                for j := uint(0); j < 8; j++ {
                        if specials&(1<<j) == 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.
                        s := ha.spans[arenaPage+uint(i)*8+j]

                        // The state must be mSpanInUse if the specials bit is set, so
                        // sanity check that.
                        if state := s.state.get(); state != mSpanInUse {
                                print("s.state = ", 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 && !(s.sweepgen == sg || s.sweepgen == sg+3) {
                                // sweepgen was updated (+2) during non-checkmark GC pass
                                print("sweep ", s.sweepgen, " ", sg, "\n")
                                throw("gc: unswept span")
                        }

                        // 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).
                                if !s.spanclass.noscan() {
                                        scanobject(p, gcw)
                                }

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

        traced := false
retry:
        if gcCPULimiter.limiting() {
                // If the CPU limiter is enabled, intentionally don't
                // assist to reduce the amount of CPU time spent in the GC.
                if traced {
                        traceGCMarkAssistDone()
                }
                return
        }
        // 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.
        assistWorkPerByte := gcController.assistWorkPerByte.Load()
        assistBytesPerWork := gcController.assistBytesPerWork.Load()
        debtBytes := -gp.gcAssistBytes
        scanWork := int64(assistWorkPerByte * float64(debtBytes))
        if scanWork < gcOverAssistWork {
                scanWork = gcOverAssistWork
                debtBytes = int64(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 := gcController.bgScanCredit.Load()
        stolen := int64(0)
        if bgScanCredit > 0 {
                if bgScanCredit < scanWork {
                        stolen = bgScanCredit
                        gp.gcAssistBytes += 1 + int64(assistBytesPerWork*float64(stolen))
                } else {
                        stolen = scanWork
                        gp.gcAssistBytes += debtBytes
                }
                gcController.bgScanCredit.Add(-stolen)

                scanWork -= stolen

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

        if traceEnabled() && !traced {
                traced = true
                traceGCMarkAssistStart()
        }

        // 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.
        }
        if traced {
                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(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-preemptible system
                // stack to determine if we should perform 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.
        //
        // Limiter event tracking might be disabled if we end up here
        // while on a mark worker.
        startTime := nanotime()
        trackLimiterEvent := gp.m.p.ptr().limiterEvent.start(limiterEventMarkAssist, startTime)

        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.
        casGToWaiting(gp, _Grunning, waitReasonGCAssistMarking)

        // 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)

        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.
        assistBytesPerWork := gcController.assistBytesPerWork.Load()
        gp.gcAssistBytes += 1 + int64(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)
                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)
        }
        now := nanotime()
        duration := now - startTime
        pp := gp.m.p.ptr()
        pp.gcAssistTime += duration
        if trackLimiterEvent {
                pp.limiterEvent.stop(limiterEventMarkAssist, now)
        }
        if pp.gcAssistTime > gcAssistTimeSlack {
                gcController.assistTime.Add(pp.gcAssistTime)
                gcCPULimiter.update(now)
                pp.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)
        list := work.assistQueue.q.popList()
        injectglist(&list)
        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
        }

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

        // 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 gcController.bgScanCredit.Load() > 0 {
                work.assistQueue.q = oldList
                if oldList.tail != 0 {
                        oldList.tail.ptr().schedlink.set(nil)
                }
                unlock(&work.assistQueue.lock)
                return false
        }
        // Park.
        goparkunlock(&work.assistQueue.lock, waitReasonGCAssistWait, traceBlockGCMarkAssist, 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.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.
                gcController.bgScanCredit.Add(scanWork)
                return
        }

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

        lock(&work.assistQueue.lock)
        for !work.assistQueue.q.empty() && scanBytes > 0 {
                gp := work.assistQueue.q.pop()
                // 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
                        // 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(gp, 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.
                        work.assistQueue.q.pushBack(gp)
                        break
                }
        }

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

// scanstack scans gp's stack, greying all pointers found on the stack.
//
// 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(gp *g, gcw *gcWork) int64 {
        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 0
        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")
        }

        // scannedSize is the amount of work we'll be reporting.
        //
        // It is less than the allocated size (which is hi-lo).
        var sp uintptr
        if gp.syscallsp != 0 {
                sp = gp.syscallsp // If in a system call this is the stack pointer (gp.sched.sp can be 0 in this case on Windows).
        } else {
                sp = gp.sched.sp
        }
        scannedSize := gp.stack.hi - sp

        // Keep statistics for initial stack size calculation.
        // Note that this accumulates the scanned size, not the allocated size.
        p := getg().m.p.ptr()
        p.scannedStackSize += uint64(scannedSize)
        p.scannedStacks++

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

        var state stackScanState
        state.stack = gp.stack

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

        if debugScanConservative && gp.asyncSafePoint {
                print("scanning async preempted goroutine ", gp.goid, " stack [", hex(gp.stack.lo), ",", hex(gp.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 gp.sched.ctxt != nil {
                scanblock(uintptr(unsafe.Pointer(&gp.sched.ctxt)), goarch.PtrSize, &oneptrmask[0], gcw, &state)
        }

        // Scan the stack. Accumulate a list of stack objects.
        var u unwinder
        for u.init(gp, 0); u.valid(); u.next() {
                scanframeworker(&u.frame, &state, gcw)
        }

        // 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 d := gp._defer; d != nil; d = d.link {
                if d.fn != nil {
                        // Scan the func value, which could be a stack allocated closure.
                        // See issue 30453.
                        scanblock(uintptr(unsafe.Pointer(&d.fn)), goarch.PtrSize, &oneptrmask[0], gcw, &state)
                }
                if d.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(&d.link)), goarch.PtrSize, &oneptrmask[0], gcw, &state)
                }
                // 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 d.heap {
                        scanblock(uintptr(unsafe.Pointer(&d)), goarch.PtrSize, &oneptrmask[0], gcw, &state)
                }
        }
        if gp._panic != nil {
                // Panics are always stack allocated.
                state.putPtr(uintptr(unsafe.Pointer(gp._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.
        state.buildIndex()
        for {
                p, conservative := state.getPtr()
                if p == 0 {
                        break
                }
                obj := state.findObject(p)
                if obj == nil {
                        continue
                }
                r := obj.r
                if r == nil {
                        // We've already scanned this object.
                        continue
                }
                obj.setRecord(nil) // Don't scan it again.
                if stackTraceDebug {
                        printlock()
                        print("  live stkobj at", hex(state.stack.lo+uintptr(obj.off)), "of size", obj.size)
                        if conservative {
                                print(" (conservative)")
                        }
                        println()
                        printunlock()
                }
                gcdata := r.gcdata()
                var s *mspan
                if r.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).
                        s = materializeGCProg(r.ptrdata(), gcdata)
                        gcdata = (*byte)(unsafe.Pointer(s.startAddr))
                }

                b := state.stack.lo + uintptr(obj.off)
                if conservative {
                        scanConservative(b, r.ptrdata(), gcdata, gcw, &state)
                } else {
                        scanblock(b, r.ptrdata(), gcdata, gcw, &state)
                }

                if s != nil {
                        dematerializeGCProg(s)
                }
        }

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

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

        isAsyncPreempt := frame.fn.valid() && frame.fn.funcID == abi.FuncID_asyncPreempt
        isDebugCall := frame.fn.valid() && frame.fn.funcID == abi.FuncID_debugCallV2
        if state.conservative || isAsyncPreempt || isDebugCall {
                if debugScanConservative {
                        println("conservatively scanning function", funcname(frame.fn), "at PC", hex(frame.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 frame.varp != 0 {
                        size := frame.varp - frame.sp
                        if size > 0 {
                                scanConservative(frame.sp, size, nil, gcw, state)
                        }
                }

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

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

        locals, args, objs := frame.getStackMap(false)

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

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

        // Add all stack objects to the stack object list.
        if frame.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 i := range objs {
                        obj := &objs[i]
                        off := obj.off
                        base := frame.varp // locals base pointer
                        if off >= 0 {
                                base = frame.argp // arguments and return values base pointer
                        }
                        ptr := base + uintptr(off)
                        if ptr < frame.sp {
                                // object hasn't been allocated in the frame yet.
                                continue
                        }
                        if stackTraceDebug {
                                println("stkobj at", hex(ptr), "of size", obj.size)
                        }
                        state.addObject(ptr, obj)
                }
        }
}

type gcDrainFlags int

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

// gcDrainMarkWorkerIdle is a wrapper for gcDrain that exists to better account
// mark time in profiles.
func gcDrainMarkWorkerIdle(gcw *gcWork) {
        gcDrain(gcw, gcDrainIdle|gcDrainUntilPreempt|gcDrainFlushBgCredit)
}

// gcDrainMarkWorkerDedicated is a wrapper for gcDrain that exists to better account
// mark time in profiles.
func gcDrainMarkWorkerDedicated(gcw *gcWork, untilPreempt bool) {
        flags := gcDrainFlushBgCredit
        if untilPreempt {
                flags |= gcDrainUntilPreempt
        }
        gcDrain(gcw, flags)
}

// gcDrainMarkWorkerFractional is a wrapper for gcDrain that exists to better account
// mark time in profiles.
func gcDrainMarkWorkerFractional(gcw *gcWork) {
        gcDrain(gcw, gcDrainFractional|gcDrainUntilPreempt|gcDrainFlushBgCredit)
}

// 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.
//
// Disabling write barriers is necessary to ensure that after we've
// confirmed that we've drained gcw, that we don't accidentally end
// up flipping that condition by immediately adding work in the form
// of a write barrier buffer flush.
//
// Don't set nowritebarrierrec because it's safe for some callees to
// have write barriers enabled.
//
//go:nowritebarrier
func gcDrain(gcw *gcWork, flags gcDrainFlags) {
        if !writeBarrier.enabled {
                throw("gcDrain phase incorrect")
        }

        gp := getg().m.curg
        preemptible := flags&gcDrainUntilPreempt != 0
        flushBgCredit := flags&gcDrainFlushBgCredit != 0
        idle := flags&gcDrainIdle != 0

        initScanWork := gcw.heapScanWork

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

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

        // Drain heap marking jobs.
        // Stop if we're preemptible or if someone wants to STW.
        for !(gp.preempt && (preemptible || sched.gcwaiting.Load())) {
                // 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()
                }

                b := gcw.tryGetFast()
                if b == 0 {
                        b = gcw.tryGet()
                        if b == 0 {
                                // Flush the write barrier
                                // buffer; this may create
                                // more work.
                                wbBufFlush()
                                b = gcw.tryGet()
                        }
                }
                if b == 0 {
                        // Unable to get work.
                        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.heapScanWork >= gcCreditSlack {
                        gcController.heapScanWork.Add(gcw.heapScanWork)
                        if flushBgCredit {
                                gcFlushBgCredit(gcw.heapScanWork - initScanWork)
                                initScanWork = 0
                        }
                        checkWork -= gcw.heapScanWork
                        gcw.heapScanWork = 0

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

done:
        // Flush remaining scan work credit.
        if gcw.heapScanWork > 0 {
                gcController.heapScanWork.Add(gcw.heapScanWork)
                if flushBgCredit {
                        gcFlushBgCredit(gcw.heapScanWork - initScanWork)
                }
                gcw.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(gcw *gcWork, scanWork int64) int64 {
        if !writeBarrier.enabled {
                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.heapScanWork

        // In addition to backing out because of a preemption, back out
        // if the GC CPU limiter is enabled.
        gp := getg().m.curg
        for !gp.preempt && !gcCPULimiter.limiting() && workFlushed+gcw.heapScanWork < scanWork {
                // See gcDrain comment.
                if work.full == 0 {
                        gcw.balance()
                }

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

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

                scanobject(b, gcw)

                // Flush background scan work credit.
                if gcw.heapScanWork >= gcCreditSlack {
                        gcController.heapScanWork.Add(gcw.heapScanWork)
                        workFlushed += gcw.heapScanWork
                        gcw.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 workFlushed + gcw.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(b0, n0 uintptr, ptrmask *uint8, gcw *gcWork, stk *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.
        b := b0
        n := n0

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

        // 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.
        s := spanOfUnchecked(b)
        n := s.elemsize
        if n == 0 {
                throw("scanobject n == 0")
        }
        if s.spanclass.noscan() {
                // Correctness-wise this is ok, but it's inefficient
                // if noscan objects reach here.
                throw("scanobject of a noscan object")
        }

        var tp typePointers
        if n > maxObletBytes {
                // Large object. Break into oblets for better
                // parallelism and lower latency.
                if b == s.base() {
                        // 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
                n = min(n, maxObletBytes)
                if goexperiment.AllocHeaders {
                        tp = s.typePointersOfUnchecked(s.base())
                        tp = tp.fastForward(b-tp.addr, b+n)
                }
        } else {
                if goexperiment.AllocHeaders {
                        tp = s.typePointersOfUnchecked(b)
                }
        }

        var hbits heapBits
        if !goexperiment.AllocHeaders {
                hbits = heapBitsForAddr(b, n)
        }
        var scanSize uintptr
        for {
                var addr uintptr
                if goexperiment.AllocHeaders {
                        if tp, addr = tp.nextFast(); addr == 0 {
                                if tp, addr = tp.next(b + n); addr == 0 {
                                        break
                                }
                        }
                } else {
                        if hbits, addr = hbits.nextFast(); addr == 0 {
                                if hbits, addr = hbits.next(); addr == 0 {
                                        break
                                }
                        }
                }

                // Keep track of farthest pointer we found, so we can
                // update heapScanWork. TODO: is there a better metric,
                // now that we can skip scalar portions pretty efficiently?
                scanSize = addr - b + goarch.PtrSize

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

                // At this point we have extracted the next potential pointer.
                // Quickly filter out nil and pointers back to the current object.
                if obj != 0 && obj-b >= n {
                        // 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 obj, span, objIndex := findObject(obj, b, addr-b); obj != 0 {
                                greyobject(obj, b, addr-b, span, gcw, objIndex)
                        }
                }
        }
        gcw.bytesMarked += uint64(n)
        gcw.heapScanWork += int64(scanSize)
}

// 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(b, n uintptr, ptrmask *uint8, gcw *gcWork, state *stackScanState) {
        if debugScanConservative {
                printlock()
                print("conservatively scanning [", hex(b), ",", hex(b+n), ")\n")
                hexdumpWords(b, b+n, func(p uintptr) byte {
                        if ptrmask != nil {
                                word := (p - b) / goarch.PtrSize
                                bits := *addb(ptrmask, word/8)
                                if (bits>>(word%8))&1 == 0 {
                                        return '$'
                                }
                        }

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

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

        for i := uintptr(0); i < n; i += goarch.PtrSize {
                if ptrmask != nil {
                        word := i / goarch.PtrSize
                        bits := *addb(ptrmask, word/8)
                        if bits == 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 i%(goarch.PtrSize*8) != 0 {
                                        throw("misaligned mask")
                                }
                                i += goarch.PtrSize*8 - goarch.PtrSize
                                continue
                        }
                        if (bits>>(word%8))&1 == 0 {
                                continue
                        }
                }

                val := *(*uintptr)(unsafe.Pointer(b + i))

                // Check if val points into the stack.
                if state != nil && state.stack.lo <= val && val < state.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.
                        state.putPtr(val, true)
                        continue
                }

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

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

                // val points to an allocated object. Mark it.
                obj := span.base() + idx*span.elemsize
                greyobject(obj, b, i, span, gcw, idx)
        }
}

// 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, span, objIndex := findObject(b, 0, 0); obj != 0 {
                gcw := &getg().m.p.ptr().gcw
                greyobject(obj, 0, 0, span, gcw, objIndex)
        }
}

// 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(obj, base, off uintptr, span *mspan, gcw *gcWork, objIndex uintptr) {
        // obj should be start of allocation, and so must be at least pointer-aligned.
        if obj&(goarch.PtrSize-1) != 0 {
                throw("greyobject: obj not pointer-aligned")
        }
        mbits := span.markBitsForIndex(objIndex)

        if useCheckmark {
                if setCheckmark(obj, base, off, mbits) {
                        // Already marked.
                        return
                }
        } 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))
                        getg().m.traceback = 2
                        throw("marking free object")
                }

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

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

                // If this is a noscan object, fast-track it to black
                // instead of greying it.
                if span.spanclass.noscan() {
                        gcw.bytesMarked += uint64(span.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(obj)
        // Queue the obj for scanning.
        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) {
        s := spanOf(obj)
        print(label, "=", hex(obj))
        if s == nil {
                print(" s=nil\n")
                return
        }
        print(" s.base()=", hex(s.base()), " s.limit=", hex(s.limit), " s.spanclass=", s.spanclass, " s.elemsize=", s.elemsize, " s.state=")
        if state := s.state.get(); 0 <= state && int(state) < len(mSpanStateNames) {
                print(mSpanStateNames[state], "\n")
        } else {
                print("unknown(", state, ")\n")
        }

        skipped := false
        size := s.elemsize
        if s.state.get() == mSpanManual && 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 + goarch.PtrSize
        }
        for i := uintptr(0); i < size; i += goarch.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*goarch.PtrSize || off-16*goarch.PtrSize < i && i < off+16*goarch.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(span *mspan, obj, size uintptr) {
        if useCheckmark { // The world should be stopped so this should not happen.
                throw("gcmarknewobject called while doing checkmark")
        }

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

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

        gcw := &getg().m.p.ptr().gcw
        gcw.bytesMarked += uint64(size)
}

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

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