[dev.garbage] all: merge dev.cc into dev.garbage

The garbage collector is now written in Go.
There is plenty to clean up (just like on dev.cc).

all.bash passes on darwin/amd64, darwin/386, linux/amd64, linux/386.

TBR=rlh
R=austin, rlh, bradfitz
CC=golang-codereviews
https://codereview.appspot.com/173250043

1 files changed, 3186 insertions, 0 deletions

diff --git a/src/runtime/proc1.go b/src/runtime/proc1.go
new file mode 100644
index 000000000..8c941dd35
--- /dev/null
+++ b/src/runtime/proc1.go
@@ -0,0 +1,3186 @@
+// 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.
+
+package runtime
+
+import "unsafe"
+
+var (
+	m0 m
+	g0 g
+)
+
+// Goroutine scheduler
+// The scheduler's job is to distribute ready-to-run goroutines over worker threads.
+//
+// The main concepts are:
+// G - goroutine.
+// M - worker thread, or machine.
+// P - processor, a resource that is required to execute Go code.
+//     M must have an associated P to execute Go code, however it can be
+//     blocked or in a syscall w/o an associated P.
+//
+// Design doc at http://golang.org/s/go11sched.
+
+const (
+	// Number of goroutine ids to grab from sched.goidgen to local per-P cache at once.
+	// 16 seems to provide enough amortization, but other than that it's mostly arbitrary number.
+	_GoidCacheBatch = 16
+)
+
+/*
+SchedT	sched;
+int32	gomaxprocs;
+uint32	needextram;
+bool	iscgo;
+M	m0;
+G	g0;	// idle goroutine for m0
+G*	lastg;
+M*	allm;
+M*	extram;
+P*	allp[MaxGomaxprocs+1];
+int8*	goos;
+int32	ncpu;
+int32	newprocs;
+
+Mutex allglock;	// the following vars are protected by this lock or by stoptheworld
+G**	allg;
+Slice	allgs;
+uintptr allglen;
+ForceGCState	forcegc;
+
+void mstart(void);
+static void runqput(P*, G*);
+static G* runqget(P*);
+static bool runqputslow(P*, G*, uint32, uint32);
+static G* runqsteal(P*, P*);
+static void mput(M*);
+static M* mget(void);
+static void mcommoninit(M*);
+static void schedule(void);
+static void procresize(int32);
+static void acquirep(P*);
+static P* releasep(void);
+static void newm(void(*)(void), P*);
+static void stopm(void);
+static void startm(P*, bool);
+static void handoffp(P*);
+static void wakep(void);
+static void stoplockedm(void);
+static void startlockedm(G*);
+static void sysmon(void);
+static uint32 retake(int64);
+static void incidlelocked(int32);
+static void checkdead(void);
+static void exitsyscall0(G*);
+void park_m(G*);
+static void goexit0(G*);
+static void gfput(P*, G*);
+static G* gfget(P*);
+static void gfpurge(P*);
+static void globrunqput(G*);
+static void globrunqputbatch(G*, G*, int32);
+static G* globrunqget(P*, int32);
+static P* pidleget(void);
+static void pidleput(P*);
+static void injectglist(G*);
+static bool preemptall(void);
+static bool preemptone(P*);
+static bool exitsyscallfast(void);
+static bool haveexperiment(int8*);
+void allgadd(G*);
+static void dropg(void);
+
+extern String buildVersion;
+*/
+
+// The bootstrap sequence is:
+//
+//	call osinit
+//	call schedinit
+//	make & queue new G
+//	call runtime·mstart
+//
+// The new G calls runtime·main.
+func schedinit() {
+	// raceinit must be the first call to race detector.
+	// In particular, it must be done before mallocinit below calls racemapshadow.
+	_g_ := getg()
+	if raceenabled {
+		_g_.racectx = raceinit()
+	}
+
+	sched.maxmcount = 10000
+
+	tracebackinit()
+	symtabinit()
+	stackinit()
+	mallocinit()
+	mcommoninit(_g_.m)
+
+	goargs()
+	goenvs()
+	parsedebugvars()
+	gcinit()
+
+	sched.lastpoll = uint64(nanotime())
+	procs := 1
+	if n := goatoi(gogetenv("GOMAXPROCS")); n > 0 {
+		if n > _MaxGomaxprocs {
+			n = _MaxGomaxprocs
+		}
+		procs = n
+	}
+	procresize(int32(procs))
+
+	if buildVersion == "" {
+		// Condition should never trigger.  This code just serves
+		// to ensure runtime·buildVersion is kept in the resulting binary.
+		buildVersion = "unknown"
+	}
+}
+
+func newsysmon() {
+	_newm(sysmon, nil)
+}
+
+func dumpgstatus(gp *g) {
+	_g_ := getg()
+	print("runtime: gp: gp=", gp, ", goid=", gp.goid, ", gp->atomicstatus=", readgstatus(gp), "\n")
+	print("runtime:  g:  g=", _g_, ", goid=", _g_.goid, ",  g->atomicstatus=", readgstatus(_g_), "\n")
+}
+
+func checkmcount() {
+	// sched lock is held
+	if sched.mcount > sched.maxmcount {
+		print("runtime: program exceeds ", sched.maxmcount, "-thread limit\n")
+		gothrow("thread exhaustion")
+	}
+}
+
+func mcommoninit(mp *m) {
+	_g_ := getg()
+
+	// g0 stack won't make sense for user (and is not necessary unwindable).
+	if _g_ != _g_.m.g0 {
+		callers(1, &mp.createstack[0], len(mp.createstack))
+	}
+
+	mp.fastrand = 0x49f6428a + uint32(mp.id) + uint32(cputicks())
+	if mp.fastrand == 0 {
+		mp.fastrand = 0x49f6428a
+	}
+
+	lock(&sched.lock)
+	mp.id = sched.mcount
+	sched.mcount++
+	checkmcount()
+	mpreinit(mp)
+	if mp.gsignal != nil {
+		mp.gsignal.stackguard1 = mp.gsignal.stack.lo + _StackGuard
+	}
+
+	// Add to allm so garbage collector doesn't free g->m
+	// when it is just in a register or thread-local storage.
+	mp.alllink = allm
+
+	// NumCgoCall() iterates over allm w/o schedlock,
+	// so we need to publish it safely.
+	atomicstorep(unsafe.Pointer(&allm), unsafe.Pointer(mp))
+	unlock(&sched.lock)
+}
+
+// Mark gp ready to run.
+func ready(gp *g) {
+	status := readgstatus(gp)
+
+	// Mark runnable.
+	_g_ := getg()
+	_g_.m.locks++ // disable preemption because it can be holding p in a local var
+	if status&^_Gscan != _Gwaiting {
+		dumpgstatus(gp)
+		gothrow("bad g->status in ready")
+	}
+
+	// status is Gwaiting or Gscanwaiting, make Grunnable and put on runq
+	casgstatus(gp, _Gwaiting, _Grunnable)
+	runqput(_g_.m.p, gp)
+	if atomicload(&sched.npidle) != 0 && atomicload(&sched.nmspinning) == 0 { // TODO: fast atomic
+		wakep()
+	}
+	_g_.m.locks--
+	if _g_.m.locks == 0 && _g_.preempt { // restore the preemption request in case we've cleared it in newstack
+		_g_.stackguard0 = stackPreempt
+	}
+}
+
+func gcprocs() int32 {
+	// Figure out how many CPUs to use during GC.
+	// Limited by gomaxprocs, number of actual CPUs, and MaxGcproc.
+	lock(&sched.lock)
+	n := gomaxprocs
+	if n > ncpu {
+		n = ncpu
+	}
+	if n > _MaxGcproc {
+		n = _MaxGcproc
+	}
+	if n > sched.nmidle+1 { // one M is currently running
+		n = sched.nmidle + 1
+	}
+	unlock(&sched.lock)
+	return n
+}
+
+func needaddgcproc() bool {
+	lock(&sched.lock)
+	n := gomaxprocs
+	if n > ncpu {
+		n = ncpu
+	}
+	if n > _MaxGcproc {
+		n = _MaxGcproc
+	}
+	n -= sched.nmidle + 1 // one M is currently running
+	unlock(&sched.lock)
+	return n > 0
+}
+
+func helpgc(nproc int32) {
+	_g_ := getg()
+	lock(&sched.lock)
+	pos := 0
+	for n := int32(1); n < nproc; n++ { // one M is currently running
+		if allp[pos].mcache == _g_.m.mcache {
+			pos++
+		}
+		mp := mget()
+		if mp == nil {
+			gothrow("gcprocs inconsistency")
+		}
+		mp.helpgc = n
+		mp.mcache = allp[pos].mcache
+		pos++
+		notewakeup(&mp.park)
+	}
+	unlock(&sched.lock)
+}
+
+// Similar to stoptheworld but best-effort and can be called several times.
+// There is no reverse operation, used during crashing.
+// This function must not lock any mutexes.
+func freezetheworld() {
+	if gomaxprocs == 1 {
+		return
+	}
+	// stopwait and preemption requests can be lost
+	// due to races with concurrently executing threads,
+	// so try several times
+	for i := 0; i < 5; i++ {
+		// this should tell the scheduler to not start any new goroutines
+		sched.stopwait = 0x7fffffff
+		atomicstore(&sched.gcwaiting, 1)
+		// this should stop running goroutines
+		if !preemptall() {
+			break // no running goroutines
+		}
+		usleep(1000)
+	}
+	// to be sure
+	usleep(1000)
+	preemptall()
+	usleep(1000)
+}
+
+func isscanstatus(status uint32) bool {
+	if status == _Gscan {
+		gothrow("isscanstatus: Bad status Gscan")
+	}
+	return status&_Gscan == _Gscan
+}
+
+// All reads and writes of g's status go through readgstatus, casgstatus
+// castogscanstatus, casfrom_Gscanstatus.
+//go:nosplit
+func readgstatus(gp *g) uint32 {
+	return atomicload(&gp.atomicstatus)
+}
+
+// The Gscanstatuses are acting like locks and this releases them.
+// If it proves to be a performance hit we should be able to make these
+// simple atomic stores but for now we are going to throw if
+// we see an inconsistent state.
+func casfrom_Gscanstatus(gp *g, oldval, newval uint32) {
+	success := false
+
+	// Check that transition is valid.
+	switch oldval {
+	case _Gscanrunnable,
+		_Gscanwaiting,
+		_Gscanrunning,
+		_Gscansyscall:
+		if newval == oldval&^_Gscan {
+			success = cas(&gp.atomicstatus, oldval, newval)
+		}
+	case _Gscanenqueue:
+		if newval == _Gwaiting {
+			success = cas(&gp.atomicstatus, oldval, newval)
+		}
+	}
+	if !success {
+		print("runtime: casfrom_Gscanstatus failed gp=", gp, ", oldval=", hex(oldval), ", newval=", hex(newval), "\n")
+		dumpgstatus(gp)
+		gothrow("casfrom_Gscanstatus: gp->status is not in scan state")
+	}
+}
+
+// This will return false if the gp is not in the expected status and the cas fails.
+// This acts like a lock acquire while the casfromgstatus acts like a lock release.
+func castogscanstatus(gp *g, oldval, newval uint32) bool {
+	switch oldval {
+	case _Grunnable,
+		_Gwaiting,
+		_Gsyscall:
+		if newval == oldval|_Gscan {
+			return cas(&gp.atomicstatus, oldval, newval)
+		}
+	case _Grunning:
+		if newval == _Gscanrunning || newval == _Gscanenqueue {
+			return cas(&gp.atomicstatus, oldval, newval)
+		}
+	}
+	print("runtime: castogscanstatus oldval=", hex(oldval), " newval=", hex(newval), "\n")
+	gothrow("castogscanstatus")
+	panic("not reached")
+}
+
+// If asked to move to or from a Gscanstatus this will throw. Use the castogscanstatus
+// and casfrom_Gscanstatus instead.
+// casgstatus will loop if the g->atomicstatus is in a Gscan status until the routine that
+// put it in the Gscan state is finished.
+//go:nosplit
+func casgstatus(gp *g, oldval, newval uint32) {
+	if (oldval&_Gscan != 0) || (newval&_Gscan != 0) || oldval == newval {
+		systemstack(func() {
+			print("casgstatus: oldval=", hex(oldval), " newval=", hex(newval), "\n")
+			gothrow("casgstatus: bad incoming values")
+		})
+	}
+
+	// loop if gp->atomicstatus is in a scan state giving
+	// GC time to finish and change the state to oldval.
+	for !cas(&gp.atomicstatus, oldval, newval) {
+	}
+}
+
+// stopg ensures that gp is stopped at a GC safe point where its stack can be scanned
+// or in the context of a moving collector the pointers can be flipped from pointing
+// to old object to pointing to new objects.
+// If stopg returns true, the caller knows gp is at a GC safe point and will remain there until
+// the caller calls restartg.
+// If stopg returns false, the caller is not responsible for calling restartg. This can happen
+// if another thread, either the gp itself or another GC thread is taking the responsibility
+// to do the GC work related to this thread.
+func stopg(gp *g) bool {
+	for {
+		if gp.gcworkdone {
+			return false
+		}
+
+		switch s := readgstatus(gp); s {
+		default:
+			dumpgstatus(gp)
+			gothrow("stopg: gp->atomicstatus is not valid")
+
+		case _Gdead:
+			return false
+
+		case _Gcopystack:
+			// Loop until a new stack is in place.
+
+		case _Grunnable,
+			_Gsyscall,
+			_Gwaiting:
+			// Claim goroutine by setting scan bit.
+			if !castogscanstatus(gp, s, s|_Gscan) {
+				break
+			}
+			// In scan state, do work.
+			gcphasework(gp)
+			return true
+
+		case _Gscanrunnable,
+			_Gscanwaiting,
+			_Gscansyscall:
+			// Goroutine already claimed by another GC helper.
+			return false
+
+		case _Grunning:
+			if gcphase == _GCscan {
+				// Running routines not scanned during
+				// GCscan phase, we only scan non-running routines.
+				gp.gcworkdone = true
+				return false
+			}
+
+			// Claim goroutine, so we aren't racing with a status
+			// transition away from Grunning.
+			if !castogscanstatus(gp, _Grunning, _Gscanrunning) {
+				break
+			}
+
+			// Mark gp for preemption.
+			if !gp.gcworkdone {
+				gp.preemptscan = true
+				gp.preempt = true
+				gp.stackguard0 = stackPreempt
+			}
+
+			// Unclaim.
+			casfrom_Gscanstatus(gp, _Gscanrunning, _Grunning)
+			return false
+		}
+	}
+}
+
+// The GC requests that this routine be moved from a scanmumble state to a mumble state.
+func restartg(gp *g) {
+	s := readgstatus(gp)
+	switch s {
+	default:
+		dumpgstatus(gp)
+		gothrow("restartg: unexpected status")
+
+	case _Gdead:
+		// ok
+
+	case _Gscanrunnable,
+		_Gscanwaiting,
+		_Gscansyscall:
+		casfrom_Gscanstatus(gp, s, s&^_Gscan)
+
+	// Scan is now completed.
+	// Goroutine now needs to be made runnable.
+	// We put it on the global run queue; ready blocks on the global scheduler lock.
+	case _Gscanenqueue:
+		casfrom_Gscanstatus(gp, _Gscanenqueue, _Gwaiting)
+		if gp != getg().m.curg {
+			gothrow("processing Gscanenqueue on wrong m")
+		}
+		dropg()
+		ready(gp)
+	}
+}
+
+func stopscanstart(gp *g) {
+	_g_ := getg()
+	if _g_ == gp {
+		gothrow("GC not moved to G0")
+	}
+	if stopg(gp) {
+		if !isscanstatus(readgstatus(gp)) {
+			dumpgstatus(gp)
+			gothrow("GC not in scan state")
+		}
+		restartg(gp)
+	}
+}
+
+// Runs on g0 and does the actual work after putting the g back on the run queue.
+func mquiesce(gpmaster *g) {
+	// enqueue the calling goroutine.
+	restartg(gpmaster)
+
+	activeglen := len(allgs)
+	for i := 0; i < activeglen; i++ {
+		gp := allgs[i]
+		if readgstatus(gp) == _Gdead {
+			gp.gcworkdone = true // noop scan.
+		} else {
+			gp.gcworkdone = false
+		}
+		stopscanstart(gp)
+	}
+
+	// Check that the G's gcwork (such as scanning) has been done. If not do it now.
+	// You can end up doing work here if the page trap on a Grunning Goroutine has
+	// not been sprung or in some race situations. For example a runnable goes dead
+	// and is started up again with a gp->gcworkdone set to false.
+	for i := 0; i < activeglen; i++ {
+		gp := allgs[i]
+		for !gp.gcworkdone {
+			status := readgstatus(gp)
+			if status == _Gdead {
+				//do nothing, scan not needed.
+				gp.gcworkdone = true // scan is a noop
+				break
+			}
+			if status == _Grunning && gp.stackguard0 == uintptr(stackPreempt) && notetsleep(&sched.stopnote, 100*1000) { // nanosecond arg
+				noteclear(&sched.stopnote)
+			} else {
+				stopscanstart(gp)
+			}
+		}
+	}
+
+	for i := 0; i < activeglen; i++ {
+		gp := allgs[i]
+		status := readgstatus(gp)
+		if isscanstatus(status) {
+			print("mstopandscang:bottom: post scan bad status gp=", gp, " has status ", hex(status), "\n")
+			dumpgstatus(gp)
+		}
+		if !gp.gcworkdone && status != _Gdead {
+			print("mstopandscang:bottom: post scan gp=", gp, "->gcworkdone still false\n")
+			dumpgstatus(gp)
+		}
+	}
+
+	schedule() // Never returns.
+}
+
+// quiesce moves all the goroutines to a GC safepoint which for now is a at preemption point.
+// If the global gcphase is GCmark quiesce will ensure that all of the goroutine's stacks
+// have been scanned before it returns.
+func quiesce(mastergp *g) {
+	castogscanstatus(mastergp, _Grunning, _Gscanenqueue)
+	// Now move this to the g0 (aka m) stack.
+	// g0 will potentially scan this thread and put mastergp on the runqueue
+	mcall(mquiesce)
+}
+
+// This is used by the GC as well as the routines that do stack dumps. In the case
+// of GC all the routines can be reliably stopped. This is not always the case
+// when the system is in panic or being exited.
+func stoptheworld() {
+	_g_ := getg()
+
+	// If we hold a lock, then we won't be able to stop another M
+	// that is blocked trying to acquire the lock.
+	if _g_.m.locks > 0 {
+		gothrow("stoptheworld: holding locks")
+	}
+
+	lock(&sched.lock)
+	sched.stopwait = gomaxprocs
+	atomicstore(&sched.gcwaiting, 1)
+	preemptall()
+	// stop current P
+	_g_.m.p.status = _Pgcstop // Pgcstop is only diagnostic.
+	sched.stopwait--
+	// try to retake all P's in Psyscall status
+	for i := 0; i < int(gomaxprocs); i++ {
+		p := allp[i]
+		s := p.status
+		if s == _Psyscall && cas(&p.status, s, _Pgcstop) {
+			sched.stopwait--
+		}
+	}
+	// stop idle P's
+	for {
+		p := pidleget()
+		if p == nil {
+			break
+		}
+		p.status = _Pgcstop
+		sched.stopwait--
+	}
+	wait := sched.stopwait > 0
+	unlock(&sched.lock)
+
+	// wait for remaining P's to stop voluntarily
+	if wait {
+		for {
+			// wait for 100us, then try to re-preempt in case of any races
+			if notetsleep(&sched.stopnote, 100*1000) {
+				noteclear(&sched.stopnote)
+				break
+			}
+			preemptall()
+		}
+	}
+	if sched.stopwait != 0 {
+		gothrow("stoptheworld: not stopped")
+	}
+	for i := 0; i < int(gomaxprocs); i++ {
+		p := allp[i]
+		if p.status != _Pgcstop {
+			gothrow("stoptheworld: not stopped")
+		}
+	}
+}
+
+func mhelpgc() {
+	_g_ := getg()
+	_g_.m.helpgc = -1
+}
+
+func starttheworld() {
+	_g_ := getg()
+
+	_g_.m.locks++        // disable preemption because it can be holding p in a local var
+	gp := netpoll(false) // non-blocking
+	injectglist(gp)
+	add := needaddgcproc()
+	lock(&sched.lock)
+	if newprocs != 0 {
+		procresize(newprocs)
+		newprocs = 0
+	} else {
+		procresize(gomaxprocs)
+	}
+	sched.gcwaiting = 0
+
+	var p1 *p
+	for {
+		p := pidleget()
+		if p == nil {
+			break
+		}
+		// procresize() puts p's with work at the beginning of the list.
+		// Once we reach a p without a run queue, the rest don't have one either.
+		if p.runqhead == p.runqtail {
+			pidleput(p)
+			break
+		}
+		p.m = mget()
+		p.link = p1
+		p1 = p
+	}
+	if sched.sysmonwait != 0 {
+		sched.sysmonwait = 0
+		notewakeup(&sched.sysmonnote)
+	}
+	unlock(&sched.lock)
+
+	for p1 != nil {
+		p := p1
+		p1 = p1.link
+		if p.m != nil {
+			mp := p.m
+			p.m = nil
+			if mp.nextp != nil {
+				gothrow("starttheworld: inconsistent mp->nextp")
+			}
+			mp.nextp = p
+			notewakeup(&mp.park)
+		} else {
+			// Start M to run P.  Do not start another M below.
+			_newm(nil, p)
+			add = false
+		}
+	}
+
+	if add {
+		// If GC could have used another helper proc, start one now,
+		// in the hope that it will be available next time.
+		// It would have been even better to start it before the collection,
+		// but doing so requires allocating memory, so it's tricky to
+		// coordinate.  This lazy approach works out in practice:
+		// we don't mind if the first couple gc rounds don't have quite
+		// the maximum number of procs.
+		_newm(mhelpgc, nil)
+	}
+	_g_.m.locks--
+	if _g_.m.locks == 0 && _g_.preempt { // restore the preemption request in case we've cleared it in newstack
+		_g_.stackguard0 = stackPreempt
+	}
+}
+
+// Called to start an M.
+//go:nosplit
+func mstart() {
+	_g_ := getg()
+
+	if _g_.stack.lo == 0 {
+		// Initialize stack bounds from system stack.
+		// Cgo may have left stack size in stack.hi.
+		size := _g_.stack.hi
+		if size == 0 {
+			size = 8192
+		}
+		_g_.stack.hi = uintptr(noescape(unsafe.Pointer(&size)))
+		_g_.stack.lo = _g_.stack.hi - size + 1024
+	}
+	// Initialize stack guards so that we can start calling
+	// both Go and C functions with stack growth prologues.
+	_g_.stackguard0 = _g_.stack.lo + _StackGuard
+	_g_.stackguard1 = _g_.stackguard0
+	mstart1()
+}
+
+func mstart1() {
+	_g_ := getg()
+
+	if _g_ != _g_.m.g0 {
+		gothrow("bad runtime·mstart")
+	}
+
+	// Record top of stack for use by mcall.
+	// Once we call schedule we're never coming back,
+	// so other calls can reuse this stack space.
+	gosave(&_g_.m.g0.sched)
+	_g_.m.g0.sched.pc = ^uintptr(0) // make sure it is never used
+	asminit()
+	minit()
+
+	// Install signal handlers; after minit so that minit can
+	// prepare the thread to be able to handle the signals.
+	if _g_.m == &m0 {
+		initsig()
+	}
+
+	if _g_.m.mstartfn != nil {
+		fn := *(*func())(unsafe.Pointer(&_g_.m.mstartfn))
+		fn()
+	}
+
+	if _g_.m.helpgc != 0 {
+		_g_.m.helpgc = 0
+		stopm()
+	} else if _g_.m != &m0 {
+		acquirep(_g_.m.nextp)
+		_g_.m.nextp = nil
+	}
+	schedule()
+
+	// TODO(brainman): This point is never reached, because scheduler
+	// does not release os threads at the moment. But once this path
+	// is enabled, we must remove our seh here.
+}
+
+// When running with cgo, we call _cgo_thread_start
+// to start threads for us so that we can play nicely with
+// foreign code.
+var cgoThreadStart unsafe.Pointer
+
+type cgothreadstart struct {
+	g   *g
+	tls *uint64
+	fn  unsafe.Pointer
+}
+
+// Allocate a new m unassociated with any thread.
+// Can use p for allocation context if needed.
+func allocm(_p_ *p) *m {
+	_g_ := getg()
+	_g_.m.locks++ // disable GC because it can be called from sysmon
+	if _g_.m.p == nil {
+		acquirep(_p_) // temporarily borrow p for mallocs in this function
+	}
+	mp := newM()
+	mcommoninit(mp)
+
+	// In case of cgo or Solaris, pthread_create will make us a stack.
+	// Windows and Plan 9 will layout sched stack on OS stack.
+	if iscgo || GOOS == "solaris" || GOOS == "windows" || GOOS == "plan9" {
+		mp.g0 = malg(-1)
+	} else {
+		mp.g0 = malg(8192)
+	}
+	mp.g0.m = mp
+
+	if _p_ == _g_.m.p {
+		releasep()
+	}
+	_g_.m.locks--
+	if _g_.m.locks == 0 && _g_.preempt { // restore the preemption request in case we've cleared it in newstack
+		_g_.stackguard0 = stackPreempt
+	}
+
+	return mp
+}
+
+func allocg() *g {
+	return newG()
+}
+
+// needm is called when a cgo callback happens on a
+// thread without an m (a thread not created by Go).
+// In this case, needm is expected to find an m to use
+// and return with m, g initialized correctly.
+// Since m and g are not set now (likely nil, but see below)
+// needm is limited in what routines it can call. In particular
+// it can only call nosplit functions (textflag 7) and cannot
+// do any scheduling that requires an m.
+//
+// In order to avoid needing heavy lifting here, we adopt
+// the following strategy: there is a stack of available m's
+// that can be stolen. Using compare-and-swap
+// to pop from the stack has ABA races, so we simulate
+// a lock by doing an exchange (via casp) to steal the stack
+// head and replace the top pointer with MLOCKED (1).
+// This serves as a simple spin lock that we can use even
+// without an m. The thread that locks the stack in this way
+// unlocks the stack by storing a valid stack head pointer.
+//
+// In order to make sure that there is always an m structure
+// available to be stolen, we maintain the invariant that there
+// is always one more than needed. At the beginning of the
+// program (if cgo is in use) the list is seeded with a single m.
+// If needm finds that it has taken the last m off the list, its job
+// is - once it has installed its own m so that it can do things like
+// allocate memory - to create a spare m and put it on the list.
+//
+// Each of these extra m's also has a g0 and a curg that are
+// pressed into service as the scheduling stack and current
+// goroutine for the duration of the cgo callback.
+//
+// When the callback is done with the m, it calls dropm to
+// put the m back on the list.
+//go:nosplit
+func needm(x byte) {
+	if needextram != 0 {
+		// Can happen if C/C++ code calls Go from a global ctor.
+		// Can not throw, because scheduler is not initialized yet.
+		// XXX
+		// write(2, unsafe.Pointer("fatal error: cgo callback before cgo call\n"), sizeof("fatal error: cgo callback before cgo call\n") - 1)
+		exit(1)
+	}
+
+	// Lock extra list, take head, unlock popped list.
+	// nilokay=false is safe here because of the invariant above,
+	// that the extra list always contains or will soon contain
+	// at least one m.
+	mp := lockextra(false)
+
+	// Set needextram when we've just emptied the list,
+	// so that the eventual call into cgocallbackg will
+	// allocate a new m for the extra list. We delay the
+	// allocation until then so that it can be done
+	// after exitsyscall makes sure it is okay to be
+	// running at all (that is, there's no garbage collection
+	// running right now).
+	mp.needextram = mp.schedlink == nil
+	unlockextra(mp.schedlink)
+
+	// Install g (= m->g0) and set the stack bounds
+	// to match the current stack. We don't actually know
+	// how big the stack is, like we don't know how big any
+	// scheduling stack is, but we assume there's at least 32 kB,
+	// which is more than enough for us.
+	setg(mp.g0)
+	_g_ := getg()
+	_g_.stack.hi = uintptr(noescape(unsafe.Pointer(&x))) + 1024
+	_g_.stack.lo = uintptr(noescape(unsafe.Pointer(&x))) - 32*1024
+	_g_.stackguard0 = _g_.stack.lo + _StackGuard
+
+	// Initialize this thread to use the m.
+	asminit()
+	minit()
+}
+
+// newextram allocates an m and puts it on the extra list.
+// It is called with a working local m, so that it can do things
+// like call schedlock and allocate.
+func newextram() {
+	// Create extra goroutine locked to extra m.
+	// The goroutine is the context in which the cgo callback will run.
+	// The sched.pc will never be returned to, but setting it to
+	// goexit makes clear to the traceback routines where
+	// the goroutine stack ends.
+	mp := allocm(nil)
+	gp := malg(4096)
+	gp.sched.pc = funcPC(goexit) + _PCQuantum
+	gp.sched.sp = gp.stack.hi
+	gp.sched.sp -= 4 * regSize // extra space in case of reads slightly beyond frame
+	gp.sched.lr = 0
+	gp.sched.g = gp
+	gp.syscallpc = gp.sched.pc
+	gp.syscallsp = gp.sched.sp
+	// malg returns status as Gidle, change to Gsyscall before adding to allg
+	// where GC will see it.
+	casgstatus(gp, _Gidle, _Gsyscall)
+	gp.m = mp
+	mp.curg = gp
+	mp.locked = _LockInternal
+	mp.lockedg = gp
+	gp.lockedm = mp
+	gp.goid = int64(xadd64(&sched.goidgen, 1))
+	if raceenabled {
+		gp.racectx = racegostart(funcPC(newextram))
+	}
+	// put on allg for garbage collector
+	allgadd(gp)
+
+	// Add m to the extra list.
+	mnext := lockextra(true)
+	mp.schedlink = mnext
+	unlockextra(mp)
+}
+
+// dropm is called when a cgo callback has called needm but is now
+// done with the callback and returning back into the non-Go thread.
+// It puts the current m back onto the extra list.
+//
+// The main expense here is the call to signalstack to release the
+// m's signal stack, and then the call to needm on the next callback
+// from this thread. It is tempting to try to save the m for next time,
+// which would eliminate both these costs, but there might not be
+// a next time: the current thread (which Go does not control) might exit.
+// If we saved the m for that thread, there would be an m leak each time
+// such a thread exited. Instead, we acquire and release an m on each
+// call. These should typically not be scheduling operations, just a few
+// atomics, so the cost should be small.
+//
+// TODO(rsc): An alternative would be to allocate a dummy pthread per-thread
+// variable using pthread_key_create. Unlike the pthread keys we already use
+// on OS X, this dummy key would never be read by Go code. It would exist
+// only so that we could register at thread-exit-time destructor.
+// That destructor would put the m back onto the extra list.
+// This is purely a performance optimization. The current version,
+// in which dropm happens on each cgo call, is still correct too.
+// We may have to keep the current version on systems with cgo
+// but without pthreads, like Windows.
+func dropm() {
+	// Undo whatever initialization minit did during needm.
+	unminit()
+
+	// Clear m and g, and return m to the extra list.
+	// After the call to setmg we can only call nosplit functions.
+	mp := getg().m
+	setg(nil)
+
+	mnext := lockextra(true)
+	mp.schedlink = mnext
+	unlockextra(mp)
+}
+
+var extram uintptr
+
+// lockextra locks the extra list and returns the list head.
+// The caller must unlock the list by storing a new list head
+// to extram. If nilokay is true, then lockextra will
+// return a nil list head if that's what it finds. If nilokay is false,
+// lockextra will keep waiting until the list head is no longer nil.
+//go:nosplit
+func lockextra(nilokay bool) *m {
+	const locked = 1
+
+	for {
+		old := atomicloaduintptr(&extram)
+		if old == locked {
+			yield := osyield
+			yield()
+			continue
+		}
+		if old == 0 && !nilokay {
+			usleep(1)
+			continue
+		}
+		if casuintptr(&extram, old, locked) {
+			return (*m)(unsafe.Pointer(old))
+		}
+		yield := osyield
+		yield()
+		continue
+	}
+}
+
+//go:nosplit
+func unlockextra(mp *m) {
+	atomicstoreuintptr(&extram, uintptr(unsafe.Pointer(mp)))
+}
+
+// Create a new m.  It will start off with a call to fn, or else the scheduler.
+func _newm(fn func(), _p_ *p) {
+	mp := allocm(_p_)
+	mp.nextp = _p_
+	mp.mstartfn = *(*unsafe.Pointer)(unsafe.Pointer(&fn))
+
+	if iscgo {
+		var ts cgothreadstart
+		if _cgo_thread_start == nil {
+			gothrow("_cgo_thread_start missing")
+		}
+		ts.g = mp.g0
+		ts.tls = (*uint64)(unsafe.Pointer(&mp.tls[0]))
+		ts.fn = unsafe.Pointer(funcPC(mstart))
+		asmcgocall(_cgo_thread_start, unsafe.Pointer(&ts))
+		return
+	}
+	newosproc(mp, unsafe.Pointer(mp.g0.stack.hi))
+}
+
+// Stops execution of the current m until new work is available.
+// Returns with acquired P.
+func stopm() {
+	_g_ := getg()
+
+	if _g_.m.locks != 0 {
+		gothrow("stopm holding locks")
+	}
+	if _g_.m.p != nil {
+		gothrow("stopm holding p")
+	}
+	if _g_.m.spinning {
+		_g_.m.spinning = false
+		xadd(&sched.nmspinning, -1)
+	}
+
+retry:
+	lock(&sched.lock)
+	mput(_g_.m)
+	unlock(&sched.lock)
+	notesleep(&_g_.m.park)
+	noteclear(&_g_.m.park)
+	if _g_.m.helpgc != 0 {
+		gchelper()
+		_g_.m.helpgc = 0
+		_g_.m.mcache = nil
+		goto retry
+	}
+	acquirep(_g_.m.nextp)
+	_g_.m.nextp = nil
+}
+
+func mspinning() {
+	getg().m.spinning = true
+}
+
+// Schedules some M to run the p (creates an M if necessary).
+// If p==nil, tries to get an idle P, if no idle P's does nothing.
+func startm(_p_ *p, spinning bool) {
+	lock(&sched.lock)
+	if _p_ == nil {
+		_p_ = pidleget()
+		if _p_ == nil {
+			unlock(&sched.lock)
+			if spinning {
+				xadd(&sched.nmspinning, -1)
+			}
+			return
+		}
+	}
+	mp := mget()
+	unlock(&sched.lock)
+	if mp == nil {
+		var fn func()
+		if spinning {
+			fn = mspinning
+		}
+		_newm(fn, _p_)
+		return
+	}
+	if mp.spinning {
+		gothrow("startm: m is spinning")
+	}
+	if mp.nextp != nil {
+		gothrow("startm: m has p")
+	}
+	mp.spinning = spinning
+	mp.nextp = _p_
+	notewakeup(&mp.park)
+}
+
+// Hands off P from syscall or locked M.
+func handoffp(_p_ *p) {
+	// if it has local work, start it straight away
+	if _p_.runqhead != _p_.runqtail || sched.runqsize != 0 {
+		startm(_p_, false)
+		return
+	}
+	// no local work, check that there are no spinning/idle M's,
+	// otherwise our help is not required
+	if atomicload(&sched.nmspinning)+atomicload(&sched.npidle) == 0 && cas(&sched.nmspinning, 0, 1) { // TODO: fast atomic
+		startm(_p_, true)
+		return
+	}
+	lock(&sched.lock)
+	if sched.gcwaiting != 0 {
+		_p_.status = _Pgcstop
+		sched.stopwait--
+		if sched.stopwait == 0 {
+			notewakeup(&sched.stopnote)
+		}
+		unlock(&sched.lock)
+		return
+	}
+	if sched.runqsize != 0 {
+		unlock(&sched.lock)
+		startm(_p_, false)
+		return
+	}
+	// If this is the last running P and nobody is polling network,
+	// need to wakeup another M to poll network.
+	if sched.npidle == uint32(gomaxprocs-1) && atomicload64(&sched.lastpoll) != 0 {
+		unlock(&sched.lock)
+		startm(_p_, false)
+		return
+	}
+	pidleput(_p_)
+	unlock(&sched.lock)
+}
+
+// Tries to add one more P to execute G's.
+// Called when a G is made runnable (newproc, ready).
+func wakep() {
+	// be conservative about spinning threads
+	if !cas(&sched.nmspinning, 0, 1) {
+		return
+	}
+	startm(nil, true)
+}
+
+// Stops execution of the current m that is locked to a g until the g is runnable again.
+// Returns with acquired P.
+func stoplockedm() {
+	_g_ := getg()
+
+	if _g_.m.lockedg == nil || _g_.m.lockedg.lockedm != _g_.m {
+		gothrow("stoplockedm: inconsistent locking")
+	}
+	if _g_.m.p != nil {
+		// Schedule another M to run this p.
+		_p_ := releasep()
+		handoffp(_p_)
+	}
+	incidlelocked(1)
+	// Wait until another thread schedules lockedg again.
+	notesleep(&_g_.m.park)
+	noteclear(&_g_.m.park)
+	status := readgstatus(_g_.m.lockedg)
+	if status&^_Gscan != _Grunnable {
+		print("runtime:stoplockedm: g is not Grunnable or Gscanrunnable\n")
+		dumpgstatus(_g_)
+		gothrow("stoplockedm: not runnable")
+	}
+	acquirep(_g_.m.nextp)
+	_g_.m.nextp = nil
+}
+
+// Schedules the locked m to run the locked gp.
+func startlockedm(gp *g) {
+	_g_ := getg()
+
+	mp := gp.lockedm
+	if mp == _g_.m {
+		gothrow("startlockedm: locked to me")
+	}
+	if mp.nextp != nil {
+		gothrow("startlockedm: m has p")
+	}
+	// directly handoff current P to the locked m
+	incidlelocked(-1)
+	_p_ := releasep()
+	mp.nextp = _p_
+	notewakeup(&mp.park)
+	stopm()
+}
+
+// Stops the current m for stoptheworld.
+// Returns when the world is restarted.
+func gcstopm() {
+	_g_ := getg()
+
+	if sched.gcwaiting == 0 {
+		gothrow("gcstopm: not waiting for gc")
+	}
+	if _g_.m.spinning {
+		_g_.m.spinning = false
+		xadd(&sched.nmspinning, -1)
+	}
+	_p_ := releasep()
+	lock(&sched.lock)
+	_p_.status = _Pgcstop
+	sched.stopwait--
+	if sched.stopwait == 0 {
+		notewakeup(&sched.stopnote)
+	}
+	unlock(&sched.lock)
+	stopm()
+}
+
+// Schedules gp to run on the current M.
+// Never returns.
+func execute(gp *g) {
+	_g_ := getg()
+
+	casgstatus(gp, _Grunnable, _Grunning)
+	gp.waitsince = 0
+	gp.preempt = false
+	gp.stackguard0 = gp.stack.lo + _StackGuard
+	_g_.m.p.schedtick++
+	_g_.m.curg = gp
+	gp.m = _g_.m
+
+	// Check whether the profiler needs to be turned on or off.
+	hz := sched.profilehz
+	if _g_.m.profilehz != hz {
+		resetcpuprofiler(hz)
+	}
+
+	gogo(&gp.sched)
+}
+
+// Finds a runnable goroutine to execute.
+// Tries to steal from other P's, get g from global queue, poll network.
+func findrunnable() *g {
+	_g_ := getg()
+
+top:
+	if sched.gcwaiting != 0 {
+		gcstopm()
+		goto top
+	}
+	if fingwait && fingwake {
+		if gp := wakefing(); gp != nil {
+			ready(gp)
+		}
+	}
+
+	// local runq
+	if gp := runqget(_g_.m.p); gp != nil {
+		return gp
+	}
+
+	// global runq
+	if sched.runqsize != 0 {
+		lock(&sched.lock)
+		gp := globrunqget(_g_.m.p, 0)
+		unlock(&sched.lock)
+		if gp != nil {
+			return gp
+		}
+	}
+
+	// poll network - returns list of goroutines
+	if gp := netpoll(false); gp != nil { // non-blocking
+		injectglist(gp.schedlink)
+		casgstatus(gp, _Gwaiting, _Grunnable)
+		return gp
+	}
+
+	// If number of spinning M's >= number of busy P's, block.
+	// This is necessary to prevent excessive CPU consumption
+	// when GOMAXPROCS>>1 but the program parallelism is low.
+	if !_g_.m.spinning && 2*atomicload(&sched.nmspinning) >= uint32(gomaxprocs)-atomicload(&sched.npidle) { // TODO: fast atomic
+		goto stop
+	}
+	if !_g_.m.spinning {
+		_g_.m.spinning = true
+		xadd(&sched.nmspinning, 1)
+	}
+	// random steal from other P's
+	for i := 0; i < int(2*gomaxprocs); i++ {
+		if sched.gcwaiting != 0 {
+			goto top
+		}
+		_p_ := allp[fastrand1()%uint32(gomaxprocs)]
+		var gp *g
+		if _p_ == _g_.m.p {
+			gp = runqget(_p_)
+		} else {
+			gp = runqsteal(_g_.m.p, _p_)
+		}
+		if gp != nil {
+			return gp
+		}
+	}
+stop:
+
+	// return P and block
+	lock(&sched.lock)
+	if sched.gcwaiting != 0 {
+		unlock(&sched.lock)
+		goto top
+	}
+	if sched.runqsize != 0 {
+		gp := globrunqget(_g_.m.p, 0)
+		unlock(&sched.lock)
+		return gp
+	}
+	_p_ := releasep()
+	pidleput(_p_)
+	unlock(&sched.lock)
+	if _g_.m.spinning {
+		_g_.m.spinning = false
+		xadd(&sched.nmspinning, -1)
+	}
+
+	// check all runqueues once again
+	for i := 0; i < int(gomaxprocs); i++ {
+		_p_ := allp[i]
+		if _p_ != nil && _p_.runqhead != _p_.runqtail {
+			lock(&sched.lock)
+			_p_ = pidleget()
+			unlock(&sched.lock)
+			if _p_ != nil {
+				acquirep(_p_)
+				goto top
+			}
+			break
+		}
+	}
+
+	// poll network
+	if xchg64(&sched.lastpoll, 0) != 0 {
+		if _g_.m.p != nil {
+			gothrow("findrunnable: netpoll with p")
+		}
+		if _g_.m.spinning {
+			gothrow("findrunnable: netpoll with spinning")
+		}
+		gp := netpoll(true) // block until new work is available
+		atomicstore64(&sched.lastpoll, uint64(nanotime()))
+		if gp != nil {
+			lock(&sched.lock)
+			_p_ = pidleget()
+			unlock(&sched.lock)
+			if _p_ != nil {
+				acquirep(_p_)
+				injectglist(gp.schedlink)
+				casgstatus(gp, _Gwaiting, _Grunnable)
+				return gp
+			}
+			injectglist(gp)
+		}
+	}
+	stopm()
+	goto top
+}
+
+func resetspinning() {
+	_g_ := getg()
+
+	var nmspinning uint32
+	if _g_.m.spinning {
+		_g_.m.spinning = false
+		nmspinning = xadd(&sched.nmspinning, -1)
+		if nmspinning < 0 {
+			gothrow("findrunnable: negative nmspinning")
+		}
+	} else {
+		nmspinning = atomicload(&sched.nmspinning)
+	}
+
+	// M wakeup policy is deliberately somewhat conservative (see nmspinning handling),
+	// so see if we need to wakeup another P here.
+	if nmspinning == 0 && atomicload(&sched.npidle) > 0 {
+		wakep()
+	}
+}
+
+// Injects the list of runnable G's into the scheduler.
+// Can run concurrently with GC.
+func injectglist(glist *g) {
+	if glist == nil {
+		return
+	}
+	lock(&sched.lock)
+	var n int
+	for n = 0; glist != nil; n++ {
+		gp := glist
+		glist = gp.schedlink
+		casgstatus(gp, _Gwaiting, _Grunnable)
+		globrunqput(gp)
+	}
+	unlock(&sched.lock)
+	for ; n != 0 && sched.npidle != 0; n-- {
+		startm(nil, false)
+	}
+}
+
+// One round of scheduler: find a runnable goroutine and execute it.
+// Never returns.
+func schedule() {
+	_g_ := getg()
+
+	if _g_.m.locks != 0 {
+		gothrow("schedule: holding locks")
+	}
+
+	if _g_.m.lockedg != nil {
+		stoplockedm()
+		execute(_g_.m.lockedg) // Never returns.
+	}
+
+top:
+	if sched.gcwaiting != 0 {
+		gcstopm()
+		goto top
+	}
+
+	var gp *g
+	// Check the global runnable queue once in a while to ensure fairness.
+	// Otherwise two goroutines can completely occupy the local runqueue
+	// by constantly respawning each other.
+	tick := _g_.m.p.schedtick
+	// This is a fancy way to say tick%61==0,
+	// it uses 2 MUL instructions instead of a single DIV and so is faster on modern processors.
+	if uint64(tick)-((uint64(tick)*0x4325c53f)>>36)*61 == 0 && sched.runqsize > 0 {
+		lock(&sched.lock)
+		gp = globrunqget(_g_.m.p, 1)
+		unlock(&sched.lock)
+		if gp != nil {
+			resetspinning()
+		}
+	}
+	if gp == nil {
+		gp = runqget(_g_.m.p)
+		if gp != nil && _g_.m.spinning {
+			gothrow("schedule: spinning with local work")
+		}
+	}
+	if gp == nil {
+		gp = findrunnable() // blocks until work is available
+		resetspinning()
+	}
+
+	if gp.lockedm != nil {
+		// Hands off own p to the locked m,
+		// then blocks waiting for a new p.
+		startlockedm(gp)
+		goto top
+	}
+
+	execute(gp)
+}
+
+// dropg removes the association between m and the current goroutine m->curg (gp for short).
+// Typically a caller sets gp's status away from Grunning and then
+// immediately calls dropg to finish the job. The caller is also responsible
+// for arranging that gp will be restarted using ready at an
+// appropriate time. After calling dropg and arranging for gp to be
+// readied later, the caller can do other work but eventually should
+// call schedule to restart the scheduling of goroutines on this m.
+func dropg() {
+	_g_ := getg()
+
+	if _g_.m.lockedg == nil {
+		_g_.m.curg.m = nil
+		_g_.m.curg = nil
+	}
+}
+
+// Puts the current goroutine into a waiting state and calls unlockf.
+// If unlockf returns false, the goroutine is resumed.
+func park(unlockf func(*g, unsafe.Pointer) bool, lock unsafe.Pointer, reason string) {
+	_g_ := getg()
+
+	_g_.m.waitlock = lock
+	_g_.m.waitunlockf = *(*unsafe.Pointer)(unsafe.Pointer(&unlockf))
+	_g_.waitreason = reason
+	mcall(park_m)
+}
+
+func parkunlock_c(gp *g, lock unsafe.Pointer) bool {
+	unlock((*mutex)(lock))
+	return true
+}
+
+// Puts the current goroutine into a waiting state and unlocks the lock.
+// The goroutine can be made runnable again by calling ready(gp).
+func parkunlock(lock *mutex, reason string) {
+	park(parkunlock_c, unsafe.Pointer(lock), reason)
+}
+
+// park continuation on g0.
+func park_m(gp *g) {
+	_g_ := getg()
+
+	casgstatus(gp, _Grunning, _Gwaiting)
+	dropg()
+
+	if _g_.m.waitunlockf != nil {
+		fn := *(*func(*g, unsafe.Pointer) bool)(unsafe.Pointer(&_g_.m.waitunlockf))
+		ok := fn(gp, _g_.m.waitlock)
+		_g_.m.waitunlockf = nil
+		_g_.m.waitlock = nil
+		if !ok {
+			casgstatus(gp, _Gwaiting, _Grunnable)
+			execute(gp) // Schedule it back, never returns.
+		}
+	}
+	schedule()
+}
+
+// Gosched continuation on g0.
+func gosched_m(gp *g) {
+	status := readgstatus(gp)
+	if status&^_Gscan != _Grunning {
+		dumpgstatus(gp)
+		gothrow("bad g status")
+	}
+	casgstatus(gp, _Grunning, _Grunnable)
+	dropg()
+	lock(&sched.lock)
+	globrunqput(gp)
+	unlock(&sched.lock)
+
+	schedule()
+}
+
+// Finishes execution of the current goroutine.
+// Must be NOSPLIT because it is called from Go. (TODO - probably not anymore)
+//go:nosplit
+func goexit1() {
+	if raceenabled {
+		racegoend()
+	}
+	mcall(goexit0)
+}
+
+// goexit continuation on g0.
+func goexit0(gp *g) {
+	_g_ := getg()
+
+	casgstatus(gp, _Grunning, _Gdead)
+	gp.m = nil
+	gp.lockedm = nil
+	_g_.m.lockedg = nil
+	gp.paniconfault = false
+	gp._defer = nil // should be true already but just in case.
+	gp._panic = nil // non-nil for Goexit during panic. points at stack-allocated data.
+	gp.writebuf = nil
+	gp.waitreason = ""
+	gp.param = nil
+
+	dropg()
+
+	if _g_.m.locked&^_LockExternal != 0 {
+		print("invalid m->locked = ", _g_.m.locked, "\n")
+		gothrow("internal lockOSThread error")
+	}
+	_g_.m.locked = 0
+	gfput(_g_.m.p, gp)
+	schedule()
+}
+
+//go:nosplit
+func save(pc, sp uintptr) {
+	_g_ := getg()
+
+	_g_.sched.pc = pc
+	_g_.sched.sp = sp
+	_g_.sched.lr = 0
+	_g_.sched.ret = 0
+	_g_.sched.ctxt = nil
+	// write as uintptr to avoid write barrier, which will smash _g_.sched.
+	*(*uintptr)(unsafe.Pointer(&_g_.sched.g)) = uintptr(unsafe.Pointer(_g_))
+}
+
+// The goroutine g is about to enter a system call.
+// Record that it's not using the cpu anymore.
+// This is called only from the go syscall library and cgocall,
+// not from the low-level system calls used by the
+//
+// Entersyscall cannot split the stack: the gosave must
+// make g->sched refer to the caller's stack segment, because
+// entersyscall is going to return immediately after.
+//
+// Nothing entersyscall calls can split the stack either.
+// We cannot safely move the stack during an active call to syscall,
+// because we do not know which of the uintptr arguments are
+// really pointers (back into the stack).
+// In practice, this means that we make the fast path run through
+// entersyscall doing no-split things, and the slow path has to use systemstack
+// to run bigger things on the system stack.
+//
+// reentersyscall is the entry point used by cgo callbacks, where explicitly
+// saved SP and PC are restored. This is needed when exitsyscall will be called
+// from a function further up in the call stack than the parent, as g->syscallsp
+// must always point to a valid stack frame. entersyscall below is the normal
+// entry point for syscalls, which obtains the SP and PC from the caller.
+//go:nosplit
+func reentersyscall(pc, sp uintptr) {
+	_g_ := getg()
+
+	// Disable preemption because during this function g is in Gsyscall status,
+	// but can have inconsistent g->sched, do not let GC observe it.
+	_g_.m.locks++
+
+	// Entersyscall must not call any function that might split/grow the stack.
+	// (See details in comment above.)
+	// Catch calls that might, by replacing the stack guard with something that
+	// will trip any stack check and leaving a flag to tell newstack to die.
+	_g_.stackguard0 = stackPreempt
+	_g_.throwsplit = true
+
+	// Leave SP around for GC and traceback.
+	save(pc, sp)
+	_g_.syscallsp = sp
+	_g_.syscallpc = pc
+	casgstatus(_g_, _Grunning, _Gsyscall)
+	if _g_.syscallsp < _g_.stack.lo || _g_.stack.hi < _g_.syscallsp {
+		systemstack(func() {
+			print("entersyscall inconsistent ", hex(_g_.syscallsp), " [", hex(_g_.stack.lo), ",", hex(_g_.stack.hi), "]\n")
+			gothrow("entersyscall")
+		})
+	}
+
+	if atomicload(&sched.sysmonwait) != 0 { // TODO: fast atomic
+		systemstack(entersyscall_sysmon)
+		save(pc, sp)
+	}
+
+	_g_.m.mcache = nil
+	_g_.m.p.m = nil
+	atomicstore(&_g_.m.p.status, _Psyscall)
+	if sched.gcwaiting != 0 {
+		systemstack(entersyscall_gcwait)
+		save(pc, sp)
+	}
+
+	// Goroutines must not split stacks in Gsyscall status (it would corrupt g->sched).
+	// We set _StackGuard to StackPreempt so that first split stack check calls morestack.
+	// Morestack detects this case and throws.
+	_g_.stackguard0 = stackPreempt
+	_g_.m.locks--
+}
+
+// Standard syscall entry used by the go syscall library and normal cgo calls.
+//go:nosplit
+func entersyscall(dummy int32) {
+	reentersyscall(getcallerpc(unsafe.Pointer(&dummy)), getcallersp(unsafe.Pointer(&dummy)))
+}
+
+func entersyscall_sysmon() {
+	lock(&sched.lock)
+	if atomicload(&sched.sysmonwait) != 0 {
+		atomicstore(&sched.sysmonwait, 0)
+		notewakeup(&sched.sysmonnote)
+	}
+	unlock(&sched.lock)
+}
+
+func entersyscall_gcwait() {
+	_g_ := getg()
+
+	lock(&sched.lock)
+	if sched.stopwait > 0 && cas(&_g_.m.p.status, _Psyscall, _Pgcstop) {
+		if sched.stopwait--; sched.stopwait == 0 {
+			notewakeup(&sched.stopnote)
+		}
+	}
+	unlock(&sched.lock)
+}
+
+// The same as entersyscall(), but with a hint that the syscall is blocking.
+//go:nosplit
+func entersyscallblock(dummy int32) {
+	_g_ := getg()
+
+	_g_.m.locks++ // see comment in entersyscall
+	_g_.throwsplit = true
+	_g_.stackguard0 = stackPreempt // see comment in entersyscall
+
+	// Leave SP around for GC and traceback.
+	pc := getcallerpc(unsafe.Pointer(&dummy))
+	sp := getcallersp(unsafe.Pointer(&dummy))
+	save(pc, sp)
+	_g_.syscallsp = _g_.sched.sp
+	_g_.syscallpc = _g_.sched.pc
+	if _g_.syscallsp < _g_.stack.lo || _g_.stack.hi < _g_.syscallsp {
+		sp1 := sp
+		sp2 := _g_.sched.sp
+		sp3 := _g_.syscallsp
+		systemstack(func() {
+			print("entersyscallblock inconsistent ", hex(sp1), " ", hex(sp2), " ", hex(sp3), " [", hex(_g_.stack.lo), ",", hex(_g_.stack.hi), "]\n")
+			gothrow("entersyscallblock")
+		})
+	}
+	casgstatus(_g_, _Grunning, _Gsyscall)
+	if _g_.syscallsp < _g_.stack.lo || _g_.stack.hi < _g_.syscallsp {
+		systemstack(func() {
+			print("entersyscallblock inconsistent ", hex(sp), " ", hex(_g_.sched.sp), " ", hex(_g_.syscallsp), " [", hex(_g_.stack.lo), ",", hex(_g_.stack.hi), "]\n")
+			gothrow("entersyscallblock")
+		})
+	}
+
+	systemstack(entersyscallblock_handoff)
+
+	// Resave for traceback during blocked call.
+	save(getcallerpc(unsafe.Pointer(&dummy)), getcallersp(unsafe.Pointer(&dummy)))
+
+	_g_.m.locks--
+}
+
+func entersyscallblock_handoff() {
+	handoffp(releasep())
+}
+
+// The goroutine g exited its system call.
+// Arrange for it to run on a cpu again.
+// This is called only from the go syscall library, not
+// from the low-level system calls used by the
+//go:nosplit
+func exitsyscall(dummy int32) {
+	_g_ := getg()
+
+	_g_.m.locks++ // see comment in entersyscall
+	if getcallersp(unsafe.Pointer(&dummy)) > _g_.syscallsp {
+		gothrow("exitsyscall: syscall frame is no longer valid")
+	}
+
+	_g_.waitsince = 0
+	if exitsyscallfast() {
+		if _g_.m.mcache == nil {
+			gothrow("lost mcache")
+		}
+		// There's a cpu for us, so we can run.
+		_g_.m.p.syscalltick++
+		// We need to cas the status and scan before resuming...
+		casgstatus(_g_, _Gsyscall, _Grunning)
+
+		// Garbage collector isn't running (since we are),
+		// so okay to clear syscallsp.
+		_g_.syscallsp = 0
+		_g_.m.locks--
+		if _g_.preempt {
+			// restore the preemption request in case we've cleared it in newstack
+			_g_.stackguard0 = stackPreempt
+		} else {
+			// otherwise restore the real _StackGuard, we've spoiled it in entersyscall/entersyscallblock
+			_g_.stackguard0 = _g_.stack.lo + _StackGuard
+		}
+		_g_.throwsplit = false
+		return
+	}
+
+	_g_.m.locks--
+
+	// Call the scheduler.
+	mcall(exitsyscall0)
+
+	if _g_.m.mcache == nil {
+		gothrow("lost mcache")
+	}
+
+	// Scheduler returned, so we're allowed to run now.
+	// Delete the syscallsp information that we left for
+	// the garbage collector during the system call.
+	// Must wait until now because until gosched returns
+	// we don't know for sure that the garbage collector
+	// is not running.
+	_g_.syscallsp = 0
+	_g_.m.p.syscalltick++
+	_g_.throwsplit = false
+}
+
+//go:nosplit
+func exitsyscallfast() bool {
+	_g_ := getg()
+
+	// Freezetheworld sets stopwait but does not retake P's.
+	if sched.stopwait != 0 {
+		_g_.m.mcache = nil
+		_g_.m.p = nil
+		return false
+	}
+
+	// Try to re-acquire the last P.
+	if _g_.m.p != nil && _g_.m.p.status == _Psyscall && cas(&_g_.m.p.status, _Psyscall, _Prunning) {
+		// There's a cpu for us, so we can run.
+		_g_.m.mcache = _g_.m.p.mcache
+		_g_.m.p.m = _g_.m
+		return true
+	}
+
+	// Try to get any other idle P.
+	_g_.m.mcache = nil
+	_g_.m.p = nil
+	if sched.pidle != nil {
+		var ok bool
+		systemstack(func() {
+			ok = exitsyscallfast_pidle()
+		})
+		if ok {
+			return true
+		}
+	}
+	return false
+}
+
+func exitsyscallfast_pidle() bool {
+	lock(&sched.lock)
+	_p_ := pidleget()
+	if _p_ != nil && atomicload(&sched.sysmonwait) != 0 {
+		atomicstore(&sched.sysmonwait, 0)
+		notewakeup(&sched.sysmonnote)
+	}
+	unlock(&sched.lock)
+	if _p_ != nil {
+		acquirep(_p_)
+		return true
+	}
+	return false
+}
+
+// exitsyscall slow path on g0.
+// Failed to acquire P, enqueue gp as runnable.
+func exitsyscall0(gp *g) {
+	_g_ := getg()
+
+	casgstatus(gp, _Gsyscall, _Grunnable)
+	dropg()
+	lock(&sched.lock)
+	_p_ := pidleget()
+	if _p_ == nil {
+		globrunqput(gp)
+	} else if atomicload(&sched.sysmonwait) != 0 {
+		atomicstore(&sched.sysmonwait, 0)
+		notewakeup(&sched.sysmonnote)
+	}
+	unlock(&sched.lock)
+	if _p_ != nil {
+		acquirep(_p_)
+		execute(gp) // Never returns.
+	}
+	if _g_.m.lockedg != nil {
+		// Wait until another thread schedules gp and so m again.
+		stoplockedm()
+		execute(gp) // Never returns.
+	}
+	stopm()
+	schedule() // Never returns.
+}
+
+func beforefork() {
+	gp := getg().m.curg
+
+	// Fork can hang if preempted with signals frequently enough (see issue 5517).
+	// Ensure that we stay on the same M where we disable profiling.
+	gp.m.locks++
+	if gp.m.profilehz != 0 {
+		resetcpuprofiler(0)
+	}
+
+	// This function is called before fork in syscall package.
+	// Code between fork and exec must not allocate memory nor even try to grow stack.
+	// Here we spoil g->_StackGuard to reliably detect any attempts to grow stack.
+	// runtime_AfterFork will undo this in parent process, but not in child.
+	gp.stackguard0 = stackFork
+}
+
+// Called from syscall package before fork.
+//go:nosplit
+func syscall_BeforeFork() {
+	systemstack(beforefork)
+}
+
+func afterfork() {
+	gp := getg().m.curg
+
+	// See the comment in beforefork.
+	gp.stackguard0 = gp.stack.lo + _StackGuard
+
+	hz := sched.profilehz
+	if hz != 0 {
+		resetcpuprofiler(hz)
+	}
+	gp.m.locks--
+}
+
+// Called from syscall package after fork in parent.
+//go:nosplit
+func syscall_AfterFork() {
+	systemstack(afterfork)
+}
+
+// Allocate a new g, with a stack big enough for stacksize bytes.
+func malg(stacksize int32) *g {
+	newg := allocg()
+	if stacksize >= 0 {
+		stacksize = round2(_StackSystem + stacksize)
+		systemstack(func() {
+			newg.stack = stackalloc(uint32(stacksize))
+		})
+		newg.stackguard0 = newg.stack.lo + _StackGuard
+		newg.stackguard1 = ^uintptr(0)
+	}
+	return newg
+}
+
+// Create a new g running fn with siz bytes of arguments.
+// Put it on the queue of g's waiting to run.
+// The compiler turns a go statement into a call to this.
+// Cannot split the stack because it assumes that the arguments
+// are available sequentially after &fn; they would not be
+// copied if a stack split occurred.
+//go:nosplit
+func newproc(siz int32, fn *funcval) {
+	argp := add(unsafe.Pointer(&fn), ptrSize)
+	if hasLinkRegister {
+		argp = add(argp, ptrSize) // skip caller's saved LR
+	}
+
+	pc := getcallerpc(unsafe.Pointer(&siz))
+	systemstack(func() {
+		newproc1(fn, (*uint8)(argp), siz, 0, pc)
+	})
+}
+
+// Create a new g running fn with narg bytes of arguments starting
+// at argp and returning nret bytes of results.  callerpc is the
+// address of the go statement that created this.  The new g is put
+// on the queue of g's waiting to run.
+func newproc1(fn *funcval, argp *uint8, narg int32, nret int32, callerpc uintptr) *g {
+	_g_ := getg()
+
+	if fn == nil {
+		_g_.m.throwing = -1 // do not dump full stacks
+		gothrow("go of nil func value")
+	}
+	_g_.m.locks++ // disable preemption because it can be holding p in a local var
+	siz := narg + nret
+	siz = (siz + 7) &^ 7
+
+	// We could allocate a larger initial stack if necessary.
+	// Not worth it: this is almost always an error.
+	// 4*sizeof(uintreg): extra space added below
+	// sizeof(uintreg): caller's LR (arm) or return address (x86, in gostartcall).
+	if siz >= _StackMin-4*regSize-regSize {
+		gothrow("newproc: function arguments too large for new goroutine")
+	}
+
+	_p_ := _g_.m.p
+	newg := gfget(_p_)
+	if newg == nil {
+		newg = malg(_StackMin)
+		casgstatus(newg, _Gidle, _Gdead)
+		allgadd(newg) // publishes with a g->status of Gdead so GC scanner doesn't look at uninitialized stack.
+	}
+	if newg.stack.hi == 0 {
+		gothrow("newproc1: newg missing stack")
+	}
+
+	if readgstatus(newg) != _Gdead {
+		gothrow("newproc1: new g is not Gdead")
+	}
+
+	sp := newg.stack.hi
+	sp -= 4 * regSize // extra space in case of reads slightly beyond frame
+	sp -= uintptr(siz)
+	memmove(unsafe.Pointer(sp), unsafe.Pointer(argp), uintptr(narg))
+	if hasLinkRegister {
+		// caller's LR
+		sp -= ptrSize
+		*(*unsafe.Pointer)(unsafe.Pointer(sp)) = nil
+	}
+
+	memclr(unsafe.Pointer(&newg.sched), unsafe.Sizeof(newg.sched))
+	newg.sched.sp = sp
+	newg.sched.pc = funcPC(goexit) + _PCQuantum // +PCQuantum so that previous instruction is in same function
+	newg.sched.g = newg
+	gostartcallfn(&newg.sched, fn)
+	newg.gopc = callerpc
+	casgstatus(newg, _Gdead, _Grunnable)
+
+	if _p_.goidcache == _p_.goidcacheend {
+		// Sched.goidgen is the last allocated id,
+		// this batch must be [sched.goidgen+1, sched.goidgen+GoidCacheBatch].
+		// At startup sched.goidgen=0, so main goroutine receives goid=1.
+		_p_.goidcache = xadd64(&sched.goidgen, _GoidCacheBatch)
+		_p_.goidcache -= _GoidCacheBatch - 1
+		_p_.goidcacheend = _p_.goidcache + _GoidCacheBatch
+	}
+	newg.goid = int64(_p_.goidcache)
+	_p_.goidcache++
+	if raceenabled {
+		newg.racectx = racegostart(callerpc)
+	}
+	runqput(_p_, newg)
+
+	if atomicload(&sched.npidle) != 0 && atomicload(&sched.nmspinning) == 0 && unsafe.Pointer(fn.fn) != unsafe.Pointer(funcPC(main)) { // TODO: fast atomic
+		wakep()
+	}
+	_g_.m.locks--
+	if _g_.m.locks == 0 && _g_.preempt { // restore the preemption request in case we've cleared it in newstack
+		_g_.stackguard0 = stackPreempt
+	}
+	return newg
+}
+
+// Put on gfree list.
+// If local list is too long, transfer a batch to the global list.
+func gfput(_p_ *p, gp *g) {
+	if readgstatus(gp) != _Gdead {
+		gothrow("gfput: bad status (not Gdead)")
+	}
+
+	stksize := gp.stack.hi - gp.stack.lo
+
+	if stksize != _FixedStack {
+		// non-standard stack size - free it.
+		stackfree(gp.stack)
+		gp.stack.lo = 0
+		gp.stack.hi = 0
+		gp.stackguard0 = 0
+	}
+
+	gp.schedlink = _p_.gfree
+	_p_.gfree = gp
+	_p_.gfreecnt++
+	if _p_.gfreecnt >= 64 {
+		lock(&sched.gflock)
+		for _p_.gfreecnt >= 32 {
+			_p_.gfreecnt--
+			gp = _p_.gfree
+			_p_.gfree = gp.schedlink
+			gp.schedlink = sched.gfree
+			sched.gfree = gp
+			sched.ngfree++
+		}
+		unlock(&sched.gflock)
+	}
+}
+
+// Get from gfree list.
+// If local list is empty, grab a batch from global list.
+func gfget(_p_ *p) *g {
+retry:
+	gp := _p_.gfree
+	if gp == nil && sched.gfree != nil {
+		lock(&sched.gflock)
+		for _p_.gfreecnt < 32 && sched.gfree != nil {
+			_p_.gfreecnt++
+			gp = sched.gfree
+			sched.gfree = gp.schedlink
+			sched.ngfree--
+			gp.schedlink = _p_.gfree
+			_p_.gfree = gp
+		}
+		unlock(&sched.gflock)
+		goto retry
+	}
+	if gp != nil {
+		_p_.gfree = gp.schedlink
+		_p_.gfreecnt--
+		if gp.stack.lo == 0 {
+			// Stack was deallocated in gfput.  Allocate a new one.
+			systemstack(func() {
+				gp.stack = stackalloc(_FixedStack)
+			})
+			gp.stackguard0 = gp.stack.lo + _StackGuard
+		} else {
+			if raceenabled {
+				racemalloc(unsafe.Pointer(gp.stack.lo), gp.stack.hi-gp.stack.lo)
+			}
+		}
+	}
+	return gp
+}
+
+// Purge all cached G's from gfree list to the global list.
+func gfpurge(_p_ *p) {
+	lock(&sched.gflock)
+	for _p_.gfreecnt != 0 {
+		_p_.gfreecnt--
+		gp := _p_.gfree
+		_p_.gfree = gp.schedlink
+		gp.schedlink = sched.gfree
+		sched.gfree = gp
+		sched.ngfree++
+	}
+	unlock(&sched.gflock)
+}
+
+// Breakpoint executes a breakpoint trap.
+func Breakpoint() {
+	breakpoint()
+}
+
+// dolockOSThread is called by LockOSThread and lockOSThread below
+// after they modify m.locked. Do not allow preemption during this call,
+// or else the m might be different in this function than in the caller.
+//go:nosplit
+func dolockOSThread() {
+	_g_ := getg()
+	_g_.m.lockedg = _g_
+	_g_.lockedm = _g_.m
+}
+
+//go:nosplit
+
+// LockOSThread wires the calling goroutine to its current operating system thread.
+// Until the calling goroutine exits or calls UnlockOSThread, it will always
+// execute in that thread, and no other goroutine can.
+func LockOSThread() {
+	getg().m.locked |= _LockExternal
+	dolockOSThread()
+}
+
+//go:nosplit
+func lockOSThread() {
+	getg().m.locked += _LockInternal
+	dolockOSThread()
+}
+
+// dounlockOSThread is called by UnlockOSThread and unlockOSThread below
+// after they update m->locked. Do not allow preemption during this call,
+// or else the m might be in different in this function than in the caller.
+//go:nosplit
+func dounlockOSThread() {
+	_g_ := getg()
+	if _g_.m.locked != 0 {
+		return
+	}
+	_g_.m.lockedg = nil
+	_g_.lockedm = nil
+}
+
+//go:nosplit
+
+// UnlockOSThread unwires the calling goroutine from its fixed operating system thread.
+// If the calling goroutine has not called LockOSThread, UnlockOSThread is a no-op.
+func UnlockOSThread() {
+	getg().m.locked &^= _LockExternal
+	dounlockOSThread()
+}
+
+//go:nosplit
+func unlockOSThread() {
+	_g_ := getg()
+	if _g_.m.locked < _LockInternal {
+		systemstack(badunlockosthread)
+	}
+	_g_.m.locked -= _LockInternal
+	dounlockOSThread()
+}
+
+func badunlockosthread() {
+	gothrow("runtime: internal error: misuse of lockOSThread/unlockOSThread")
+}
+
+func gcount() int32 {
+	n := int32(allglen) - sched.ngfree
+	for i := 0; ; i++ {
+		_p_ := allp[i]
+		if _p_ == nil {
+			break
+		}
+		n -= _p_.gfreecnt
+	}
+
+	// All these variables can be changed concurrently, so the result can be inconsistent.
+	// But at least the current goroutine is running.
+	if n < 1 {
+		n = 1
+	}
+	return n
+}
+
+func mcount() int32 {
+	return sched.mcount
+}
+
+var prof struct {
+	lock uint32
+	hz   int32
+}
+
+func _System()       { _System() }
+func _ExternalCode() { _ExternalCode() }
+func _GC()           { _GC() }
+
+var etext struct{}
+
+// Called if we receive a SIGPROF signal.
+func sigprof(pc *uint8, sp *uint8, lr *uint8, gp *g, mp *m) {
+	var n int32
+	var traceback bool
+	var stk [100]uintptr
+
+	if prof.hz == 0 {
+		return
+	}
+
+	// Profiling runs concurrently with GC, so it must not allocate.
+	mp.mallocing++
+
+	// Define that a "user g" is a user-created goroutine, and a "system g"
+	// is one that is m->g0 or m->gsignal. We've only made sure that we
+	// can unwind user g's, so exclude the system g's.
+	//
+	// It is not quite as easy as testing gp == m->curg (the current user g)
+	// because we might be interrupted for profiling halfway through a
+	// goroutine switch. The switch involves updating three (or four) values:
+	// g, PC, SP, and (on arm) LR. The PC must be the last to be updated,
+	// because once it gets updated the new g is running.
+	//
+	// When switching from a user g to a system g, LR is not considered live,
+	// so the update only affects g, SP, and PC. Since PC must be last, there
+	// the possible partial transitions in ordinary execution are (1) g alone is updated,
+	// (2) both g and SP are updated, and (3) SP alone is updated.
+	// If g is updated, we'll see a system g and not look closer.
+	// If SP alone is updated, we can detect the partial transition by checking
+	// whether the SP is within g's stack bounds. (We could also require that SP
+	// be changed only after g, but the stack bounds check is needed by other
+	// cases, so there is no need to impose an additional requirement.)
+	//
+	// There is one exceptional transition to a system g, not in ordinary execution.
+	// When a signal arrives, the operating system starts the signal handler running
+	// with an updated PC and SP. The g is updated last, at the beginning of the
+	// handler. There are two reasons this is okay. First, until g is updated the
+	// g and SP do not match, so the stack bounds check detects the partial transition.
+	// Second, signal handlers currently run with signals disabled, so a profiling
+	// signal cannot arrive during the handler.
+	//
+	// When switching from a system g to a user g, there are three possibilities.
+	//
+	// First, it may be that the g switch has no PC update, because the SP
+	// either corresponds to a user g throughout (as in asmcgocall)
+	// or because it has been arranged to look like a user g frame
+	// (as in cgocallback_gofunc). In this case, since the entire
+	// transition is a g+SP update, a partial transition updating just one of
+	// those will be detected by the stack bounds check.
+	//
+	// Second, when returning from a signal handler, the PC and SP updates
+	// are performed by the operating system in an atomic update, so the g
+	// update must be done before them. The stack bounds check detects
+	// the partial transition here, and (again) signal handlers run with signals
+	// disabled, so a profiling signal cannot arrive then anyway.
+	//
+	// Third, the common case: it may be that the switch updates g, SP, and PC
+	// separately, as in gogo.
+	//
+	// Because gogo is the only instance, we check whether the PC lies
+	// within that function, and if so, not ask for a traceback. This approach
+	// requires knowing the size of the gogo function, which we
+	// record in arch_*.h and check in runtime_test.go.
+	//
+	// There is another apparently viable approach, recorded here in case
+	// the "PC within gogo" check turns out not to be usable.
+	// It would be possible to delay the update of either g or SP until immediately
+	// before the PC update instruction. Then, because of the stack bounds check,
+	// the only problematic interrupt point is just before that PC update instruction,
+	// and the sigprof handler can detect that instruction and simulate stepping past
+	// it in order to reach a consistent state. On ARM, the update of g must be made
+	// in two places (in R10 and also in a TLS slot), so the delayed update would
+	// need to be the SP update. The sigprof handler must read the instruction at
+	// the current PC and if it was the known instruction (for example, JMP BX or
+	// MOV R2, PC), use that other register in place of the PC value.
+	// The biggest drawback to this solution is that it requires that we can tell
+	// whether it's safe to read from the memory pointed at by PC.
+	// In a correct program, we can test PC == nil and otherwise read,
+	// but if a profiling signal happens at the instant that a program executes
+	// a bad jump (before the program manages to handle the resulting fault)
+	// the profiling handler could fault trying to read nonexistent memory.
+	//
+	// To recap, there are no constraints on the assembly being used for the
+	// transition. We simply require that g and SP match and that the PC is not
+	// in gogo.
+	traceback = true
+	usp := uintptr(unsafe.Pointer(sp))
+	gogo := funcPC(gogo)
+	if gp == nil || gp != mp.curg ||
+		usp < gp.stack.lo || gp.stack.hi < usp ||
+		(gogo <= uintptr(unsafe.Pointer(pc)) && uintptr(unsafe.Pointer(pc)) < gogo+_RuntimeGogoBytes) {
+		traceback = false
+	}
+
+	n = 0
+	if traceback {
+		n = int32(gentraceback(uintptr(unsafe.Pointer(pc)), uintptr(unsafe.Pointer(sp)), uintptr(unsafe.Pointer(lr)), gp, 0, &stk[0], len(stk), nil, nil, _TraceTrap))
+	}
+	if !traceback || n <= 0 {
+		// Normal traceback is impossible or has failed.
+		// See if it falls into several common cases.
+		n = 0
+		if mp.ncgo > 0 && mp.curg != nil && mp.curg.syscallpc != 0 && mp.curg.syscallsp != 0 {
+			// Cgo, we can't unwind and symbolize arbitrary C code,
+			// so instead collect Go stack that leads to the cgo call.
+			// This is especially important on windows, since all syscalls are cgo calls.
+			n = int32(gentraceback(mp.curg.syscallpc, mp.curg.syscallsp, 0, mp.curg, 0, &stk[0], len(stk), nil, nil, 0))
+		}
+		if GOOS == "windows" && n == 0 && mp.libcallg != nil && mp.libcallpc != 0 && mp.libcallsp != 0 {
+			// Libcall, i.e. runtime syscall on windows.
+			// Collect Go stack that leads to the call.
+			n = int32(gentraceback(mp.libcallpc, mp.libcallsp, 0, mp.libcallg, 0, &stk[0], len(stk), nil, nil, 0))
+		}
+		if n == 0 {
+			// If all of the above has failed, account it against abstract "System" or "GC".
+			n = 2
+			// "ExternalCode" is better than "etext".
+			if uintptr(unsafe.Pointer(pc)) > uintptr(unsafe.Pointer(&etext)) {
+				pc = (*uint8)(unsafe.Pointer(uintptr(funcPC(_ExternalCode) + _PCQuantum)))
+			}
+			stk[0] = uintptr(unsafe.Pointer(pc))
+			if mp.gcing != 0 || mp.helpgc != 0 {
+				stk[1] = funcPC(_GC) + _PCQuantum
+			} else {
+				stk[1] = funcPC(_System) + _PCQuantum
+			}
+		}
+	}
+
+	if prof.hz != 0 {
+		// Simple cas-lock to coordinate with setcpuprofilerate.
+		for !cas(&prof.lock, 0, 1) {
+			osyield()
+		}
+		if prof.hz != 0 {
+			cpuproftick(&stk[0], n)
+		}
+		atomicstore(&prof.lock, 0)
+	}
+	mp.mallocing--
+}
+
+// Arrange to call fn with a traceback hz times a second.
+func setcpuprofilerate_m(hz int32) {
+	// Force sane arguments.
+	if hz < 0 {
+		hz = 0
+	}
+
+	// Disable preemption, otherwise we can be rescheduled to another thread
+	// that has profiling enabled.
+	_g_ := getg()
+	_g_.m.locks++
+
+	// Stop profiler on this thread so that it is safe to lock prof.
+	// if a profiling signal came in while we had prof locked,
+	// it would deadlock.
+	resetcpuprofiler(0)
+
+	for !cas(&prof.lock, 0, 1) {
+		osyield()
+	}
+	prof.hz = hz
+	atomicstore(&prof.lock, 0)
+
+	lock(&sched.lock)
+	sched.profilehz = hz
+	unlock(&sched.lock)
+
+	if hz != 0 {
+		resetcpuprofiler(hz)
+	}
+
+	_g_.m.locks--
+}
+
+// Change number of processors.  The world is stopped, sched is locked.
+// gcworkbufs are not being modified by either the GC or
+// the write barrier code.
+func procresize(new int32) {
+	old := gomaxprocs
+	if old < 0 || old > _MaxGomaxprocs || new <= 0 || new > _MaxGomaxprocs {
+		gothrow("procresize: invalid arg")
+	}
+
+	// initialize new P's
+	for i := int32(0); i < new; i++ {
+		p := allp[i]
+		if p == nil {
+			p = newP()
+			p.id = i
+			p.status = _Pgcstop
+			atomicstorep(unsafe.Pointer(&allp[i]), unsafe.Pointer(p))
+		}
+		if p.mcache == nil {
+			if old == 0 && i == 0 {
+				if getg().m.mcache == nil {
+					gothrow("missing mcache?")
+				}
+				p.mcache = getg().m.mcache // bootstrap
+			} else {
+				p.mcache = allocmcache()
+			}
+		}
+	}
+
+	// redistribute runnable G's evenly
+	// collect all runnable goroutines in global queue preserving FIFO order
+	// FIFO order is required to ensure fairness even during frequent GCs
+	// see http://golang.org/issue/7126
+	empty := false
+	for !empty {
+		empty = true
+		for i := int32(0); i < old; i++ {
+			p := allp[i]
+			if p.runqhead == p.runqtail {
+				continue
+			}
+			empty = false
+			// pop from tail of local queue
+			p.runqtail--
+			gp := p.runq[p.runqtail%uint32(len(p.runq))]
+			// push onto head of global queue
+			gp.schedlink = sched.runqhead
+			sched.runqhead = gp
+			if sched.runqtail == nil {
+				sched.runqtail = gp
+			}
+			sched.runqsize++
+		}
+	}
+
+	// fill local queues with at most len(p.runq)/2 goroutines
+	// start at 1 because current M already executes some G and will acquire allp[0] below,
+	// so if we have a spare G we want to put it into allp[1].
+	var _p_ p
+	for i := int32(1); i < new*int32(len(_p_.runq))/2 && sched.runqsize > 0; i++ {
+		gp := sched.runqhead
+		sched.runqhead = gp.schedlink
+		if sched.runqhead == nil {
+			sched.runqtail = nil
+		}
+		sched.runqsize--
+		runqput(allp[i%new], gp)
+	}
+
+	// free unused P's
+	for i := new; i < old; i++ {
+		p := allp[i]
+		freemcache(p.mcache)
+		p.mcache = nil
+		gfpurge(p)
+		p.status = _Pdead
+		// can't free P itself because it can be referenced by an M in syscall
+	}
+
+	_g_ := getg()
+	if _g_.m.p != nil {
+		_g_.m.p.m = nil
+	}
+	_g_.m.p = nil
+	_g_.m.mcache = nil
+	p := allp[0]
+	p.m = nil
+	p.status = _Pidle
+	acquirep(p)
+	for i := new - 1; i > 0; i-- {
+		p := allp[i]
+		p.status = _Pidle
+		pidleput(p)
+	}
+	var int32p *int32 = &gomaxprocs // make compiler check that gomaxprocs is an int32
+	atomicstore((*uint32)(unsafe.Pointer(int32p)), uint32(new))
+}
+
+// Associate p and the current m.
+func acquirep(_p_ *p) {
+	_g_ := getg()
+
+	if _g_.m.p != nil || _g_.m.mcache != nil {
+		gothrow("acquirep: already in go")
+	}
+	if _p_.m != nil || _p_.status != _Pidle {
+		id := int32(0)
+		if _p_.m != nil {
+			id = _p_.m.id
+		}
+		print("acquirep: p->m=", _p_.m, "(", id, ") p->status=", _p_.status, "\n")
+		gothrow("acquirep: invalid p state")
+	}
+	_g_.m.mcache = _p_.mcache
+	_g_.m.p = _p_
+	_p_.m = _g_.m
+	_p_.status = _Prunning
+}
+
+// Disassociate p and the current m.
+func releasep() *p {
+	_g_ := getg()
+
+	if _g_.m.p == nil || _g_.m.mcache == nil {
+		gothrow("releasep: invalid arg")
+	}
+	_p_ := _g_.m.p
+	if _p_.m != _g_.m || _p_.mcache != _g_.m.mcache || _p_.status != _Prunning {
+		print("releasep: m=", _g_.m, " m->p=", _g_.m.p, " p->m=", _p_.m, " m->mcache=", _g_.m.mcache, " p->mcache=", _p_.mcache, " p->status=", _p_.status, "\n")
+		gothrow("releasep: invalid p state")
+	}
+	_g_.m.p = nil
+	_g_.m.mcache = nil
+	_p_.m = nil
+	_p_.status = _Pidle
+	return _p_
+}
+
+func incidlelocked(v int32) {
+	lock(&sched.lock)
+	sched.nmidlelocked += v
+	if v > 0 {
+		checkdead()
+	}
+	unlock(&sched.lock)
+}
+
+// Check for deadlock situation.
+// The check is based on number of running M's, if 0 -> deadlock.
+func checkdead() {
+	// If we are dying because of a signal caught on an already idle thread,
+	// freezetheworld will cause all running threads to block.
+	// And runtime will essentially enter into deadlock state,
+	// except that there is a thread that will call exit soon.
+	if panicking > 0 {
+		return
+	}
+
+	// -1 for sysmon
+	run := sched.mcount - sched.nmidle - sched.nmidlelocked - 1
+	if run > 0 {
+		return
+	}
+	if run < 0 {
+		print("runtime: checkdead: nmidle=", sched.nmidle, " nmidlelocked=", sched.nmidlelocked, " mcount=", sched.mcount, "\n")
+		gothrow("checkdead: inconsistent counts")
+	}
+
+	grunning := 0
+	lock(&allglock)
+	for i := 0; i < len(allgs); i++ {
+		gp := allgs[i]
+		if gp.issystem {
+			continue
+		}
+		s := readgstatus(gp)
+		switch s &^ _Gscan {
+		case _Gwaiting:
+			grunning++
+		case _Grunnable,
+			_Grunning,
+			_Gsyscall:
+			unlock(&allglock)
+			print("runtime: checkdead: find g ", gp.goid, " in status ", s, "\n")
+			gothrow("checkdead: runnable g")
+		}
+	}
+	unlock(&allglock)
+	if grunning == 0 { // possible if main goroutine calls runtime·Goexit()
+		gothrow("no goroutines (main called runtime.Goexit) - deadlock!")
+	}
+
+	// Maybe jump time forward for playground.
+	gp := timejump()
+	if gp != nil {
+		casgstatus(gp, _Gwaiting, _Grunnable)
+		globrunqput(gp)
+		_p_ := pidleget()
+		if _p_ == nil {
+			gothrow("checkdead: no p for timer")
+		}
+		mp := mget()
+		if mp == nil {
+			_newm(nil, _p_)
+		} else {
+			mp.nextp = _p_
+			notewakeup(&mp.park)
+		}
+		return
+	}
+
+	getg().m.throwing = -1 // do not dump full stacks
+	gothrow("all goroutines are asleep - deadlock!")
+}
+
+func sysmon() {
+	// If we go two minutes without a garbage collection, force one to run.
+	forcegcperiod := int64(2 * 60 * 1e9)
+
+	// If a heap span goes unused for 5 minutes after a garbage collection,
+	// we hand it back to the operating system.
+	scavengelimit := int64(5 * 60 * 1e9)
+
+	if debug.scavenge > 0 {
+		// Scavenge-a-lot for testing.
+		forcegcperiod = 10 * 1e6
+		scavengelimit = 20 * 1e6
+	}
+
+	lastscavenge := nanotime()
+	nscavenge := 0
+
+	// Make wake-up period small enough for the sampling to be correct.
+	maxsleep := forcegcperiod / 2
+	if scavengelimit < forcegcperiod {
+		maxsleep = scavengelimit / 2
+	}
+
+	lasttrace := int64(0)
+	idle := 0 // how many cycles in succession we had not wokeup somebody
+	delay := uint32(0)
+	for {
+		if idle == 0 { // start with 20us sleep...
+			delay = 20
+		} else if idle > 50 { // start doubling the sleep after 1ms...
+			delay *= 2
+		}
+		if delay > 10*1000 { // up to 10ms
+			delay = 10 * 1000
+		}
+		usleep(delay)
+		if debug.schedtrace <= 0 && (sched.gcwaiting != 0 || atomicload(&sched.npidle) == uint32(gomaxprocs)) { // TODO: fast atomic
+			lock(&sched.lock)
+			if atomicload(&sched.gcwaiting) != 0 || atomicload(&sched.npidle) == uint32(gomaxprocs) {
+				atomicstore(&sched.sysmonwait, 1)
+				unlock(&sched.lock)
+				notetsleep(&sched.sysmonnote, maxsleep)
+				lock(&sched.lock)
+				atomicstore(&sched.sysmonwait, 0)
+				noteclear(&sched.sysmonnote)
+				idle = 0
+				delay = 20
+			}
+			unlock(&sched.lock)
+		}
+		// poll network if not polled for more than 10ms
+		lastpoll := int64(atomicload64(&sched.lastpoll))
+		now := nanotime()
+		unixnow := unixnanotime()
+		if lastpoll != 0 && lastpoll+10*1000*1000 < now {
+			cas64(&sched.lastpoll, uint64(lastpoll), uint64(now))
+			gp := netpoll(false) // non-blocking - returns list of goroutines
+			if gp != nil {
+				// Need to decrement number of idle locked M's
+				// (pretending that one more is running) before injectglist.
+				// Otherwise it can lead to the following situation:
+				// injectglist grabs all P's but before it starts M's to run the P's,
+				// another M returns from syscall, finishes running its G,
+				// observes that there is no work to do and no other running M's
+				// and reports deadlock.
+				incidlelocked(-1)
+				injectglist(gp)
+				incidlelocked(1)
+			}
+		}
+		// retake P's blocked in syscalls
+		// and preempt long running G's
+		if retake(now) != 0 {
+			idle = 0
+		} else {
+			idle++
+		}
+		// check if we need to force a GC
+		lastgc := int64(atomicload64(&memstats.last_gc))
+		if lastgc != 0 && unixnow-lastgc > forcegcperiod && atomicload(&forcegc.idle) != 0 {
+			lock(&forcegc.lock)
+			forcegc.idle = 0
+			forcegc.g.schedlink = nil
+			injectglist(forcegc.g)
+			unlock(&forcegc.lock)
+		}
+		// scavenge heap once in a while
+		if lastscavenge+scavengelimit/2 < now {
+			mHeap_Scavenge(int32(nscavenge), uint64(now), uint64(scavengelimit))
+			lastscavenge = now
+			nscavenge++
+		}
+		if debug.schedtrace > 0 && lasttrace+int64(debug.schedtrace*1000000) <= now {
+			lasttrace = now
+			schedtrace(debug.scheddetail > 0)
+		}
+	}
+}
+
+var pdesc [_MaxGomaxprocs]struct {
+	schedtick   uint32
+	schedwhen   int64
+	syscalltick uint32
+	syscallwhen int64
+}
+
+func retake(now int64) uint32 {
+	n := 0
+	for i := int32(0); i < gomaxprocs; i++ {
+		_p_ := allp[i]
+		if _p_ == nil {
+			continue
+		}
+		pd := &pdesc[i]
+		s := _p_.status
+		if s == _Psyscall {
+			// Retake P from syscall if it's there for more than 1 sysmon tick (at least 20us).
+			t := int64(_p_.syscalltick)
+			if int64(pd.syscalltick) != t {
+				pd.syscalltick = uint32(t)
+				pd.syscallwhen = now
+				continue
+			}
+			// On the one hand we don't want to retake Ps if there is no other work to do,
+			// but on the other hand we want to retake them eventually
+			// because they can prevent the sysmon thread from deep sleep.
+			if _p_.runqhead == _p_.runqtail && atomicload(&sched.nmspinning)+atomicload(&sched.npidle) > 0 && pd.syscallwhen+10*1000*1000 > now {
+				continue
+			}
+			// Need to decrement number of idle locked M's
+			// (pretending that one more is running) before the CAS.
+			// Otherwise the M from which we retake can exit the syscall,
+			// increment nmidle and report deadlock.
+			incidlelocked(-1)
+			if cas(&_p_.status, s, _Pidle) {
+				n++
+				handoffp(_p_)
+			}
+			incidlelocked(1)
+		} else if s == _Prunning {
+			// Preempt G if it's running for more than 10ms.
+			t := int64(_p_.schedtick)
+			if int64(pd.schedtick) != t {
+				pd.schedtick = uint32(t)
+				pd.schedwhen = now
+				continue
+			}
+			if pd.schedwhen+10*1000*1000 > now {
+				continue
+			}
+			preemptone(_p_)
+		}
+	}
+	return uint32(n)
+}
+
+// Tell all goroutines that they have been preempted and they should stop.
+// This function is purely best-effort.  It can fail to inform a goroutine if a
+// processor just started running it.
+// No locks need to be held.
+// Returns true if preemption request was issued to at least one goroutine.
+func preemptall() bool {
+	res := false
+	for i := int32(0); i < gomaxprocs; i++ {
+		_p_ := allp[i]
+		if _p_ == nil || _p_.status != _Prunning {
+			continue
+		}
+		if preemptone(_p_) {
+			res = true
+		}
+	}
+	return res
+}
+
+// Tell the goroutine running on processor P to stop.
+// This function is purely best-effort.  It can incorrectly fail to inform the
+// goroutine.  It can send inform the wrong goroutine.  Even if it informs the
+// correct goroutine, that goroutine might ignore the request if it is
+// simultaneously executing newstack.
+// No lock needs to be held.
+// Returns true if preemption request was issued.
+// The actual preemption will happen at some point in the future
+// and will be indicated by the gp->status no longer being
+// Grunning
+func preemptone(_p_ *p) bool {
+	mp := _p_.m
+	if mp == nil || mp == getg().m {
+		return false
+	}
+	gp := mp.curg
+	if gp == nil || gp == mp.g0 {
+		return false
+	}
+
+	gp.preempt = true
+
+	// Every call in a go routine checks for stack overflow by
+	// comparing the current stack pointer to gp->stackguard0.
+	// Setting gp->stackguard0 to StackPreempt folds
+	// preemption into the normal stack overflow check.
+	gp.stackguard0 = stackPreempt
+	return true
+}
+
+var starttime int64
+
+func schedtrace(detailed bool) {
+	now := nanotime()
+	if starttime == 0 {
+		starttime = now
+	}
+
+	lock(&sched.lock)
+	print("SCHED ", (now-starttime)/1e6, "ms: gomaxprocs=", gomaxprocs, " idleprocs=", sched.npidle, " threads=", sched.mcount, " spinningthreads=", sched.nmspinning, " idlethreads=", sched.nmidle, " runqueue=", sched.runqsize)
+	if detailed {
+		print(" gcwaiting=", sched.gcwaiting, " nmidlelocked=", sched.nmidlelocked, " stopwait=", sched.stopwait, " sysmonwait=", sched.sysmonwait, "\n")
+	}
+	// We must be careful while reading data from P's, M's and G's.
+	// Even if we hold schedlock, most data can be changed concurrently.
+	// E.g. (p->m ? p->m->id : -1) can crash if p->m changes from non-nil to nil.
+	for i := int32(0); i < gomaxprocs; i++ {
+		_p_ := allp[i]
+		if _p_ == nil {
+			continue
+		}
+		mp := _p_.m
+		h := atomicload(&_p_.runqhead)
+		t := atomicload(&_p_.runqtail)
+		if detailed {
+			id := int32(-1)
+			if mp != nil {
+				id = mp.id
+			}
+			print("  P", i, ": status=", _p_.status, " schedtick=", _p_.schedtick, " syscalltick=", _p_.syscalltick, " m=", id, " runqsize=", t-h, " gfreecnt=", _p_.gfreecnt, "\n")
+		} else {
+			// In non-detailed mode format lengths of per-P run queues as:
+			// [len1 len2 len3 len4]
+			print(" ")
+			if i == 0 {
+				print("[")
+			}
+			print(t - h)
+			if i == gomaxprocs-1 {
+				print("]\n")
+			}
+		}
+	}
+
+	if !detailed {
+		unlock(&sched.lock)
+		return
+	}
+
+	for mp := allm; mp != nil; mp = mp.alllink {
+		_p_ := mp.p
+		gp := mp.curg
+		lockedg := mp.lockedg
+		id1 := int32(-1)
+		if _p_ != nil {
+			id1 = _p_.id
+		}
+		id2 := int64(-1)
+		if gp != nil {
+			id2 = gp.goid
+		}
+		id3 := int64(-1)
+		if lockedg != nil {
+			id3 = lockedg.goid
+		}
+		print("  M", mp.id, ": p=", id1, " curg=", id2, " mallocing=", mp.mallocing, " throwing=", mp.throwing, " gcing=", mp.gcing, ""+" locks=", mp.locks, " dying=", mp.dying, " helpgc=", mp.helpgc, " spinning=", mp.spinning, " blocked=", getg().m.blocked, " lockedg=", id3, "\n")
+	}
+
+	lock(&allglock)
+	for gi := 0; gi < len(allgs); gi++ {
+		gp := allgs[gi]
+		mp := gp.m
+		lockedm := gp.lockedm
+		id1 := int32(-1)
+		if mp != nil {
+			id1 = mp.id
+		}
+		id2 := int32(-1)
+		if lockedm != nil {
+			id2 = lockedm.id
+		}
+		print("  G", gp.goid, ": status=", readgstatus(gp), "(", gp.waitreason, ") m=", id1, " lockedm=", id2, "\n")
+	}
+	unlock(&allglock)
+	unlock(&sched.lock)
+}
+
+// Put mp on midle list.
+// Sched must be locked.
+func mput(mp *m) {
+	mp.schedlink = sched.midle
+	sched.midle = mp
+	sched.nmidle++
+	checkdead()
+}
+
+// Try to get an m from midle list.
+// Sched must be locked.
+func mget() *m {
+	mp := sched.midle
+	if mp != nil {
+		sched.midle = mp.schedlink
+		sched.nmidle--
+	}
+	return mp
+}
+
+// Put gp on the global runnable queue.
+// Sched must be locked.
+func globrunqput(gp *g) {
+	gp.schedlink = nil
+	if sched.runqtail != nil {
+		sched.runqtail.schedlink = gp
+	} else {
+		sched.runqhead = gp
+	}
+	sched.runqtail = gp
+	sched.runqsize++
+}
+
+// Put a batch of runnable goroutines on the global runnable queue.
+// Sched must be locked.
+func globrunqputbatch(ghead *g, gtail *g, n int32) {
+	gtail.schedlink = nil
+	if sched.runqtail != nil {
+		sched.runqtail.schedlink = ghead
+	} else {
+		sched.runqhead = ghead
+	}
+	sched.runqtail = gtail
+	sched.runqsize += n
+}
+
+// Try get a batch of G's from the global runnable queue.
+// Sched must be locked.
+func globrunqget(_p_ *p, max int32) *g {
+	if sched.runqsize == 0 {
+		return nil
+	}
+
+	n := sched.runqsize/gomaxprocs + 1
+	if n > sched.runqsize {
+		n = sched.runqsize
+	}
+	if max > 0 && n > max {
+		n = max
+	}
+	if n > int32(len(_p_.runq))/2 {
+		n = int32(len(_p_.runq)) / 2
+	}
+
+	sched.runqsize -= n
+	if sched.runqsize == 0 {
+		sched.runqtail = nil
+	}
+
+	gp := sched.runqhead
+	sched.runqhead = gp.schedlink
+	n--
+	for ; n > 0; n-- {
+		gp1 := sched.runqhead
+		sched.runqhead = gp1.schedlink
+		runqput(_p_, gp1)
+	}
+	return gp
+}
+
+// Put p to on _Pidle list.
+// Sched must be locked.
+func pidleput(_p_ *p) {
+	_p_.link = sched.pidle
+	sched.pidle = _p_
+	xadd(&sched.npidle, 1) // TODO: fast atomic
+}
+
+// Try get a p from _Pidle list.
+// Sched must be locked.
+func pidleget() *p {
+	_p_ := sched.pidle
+	if _p_ != nil {
+		sched.pidle = _p_.link
+		xadd(&sched.npidle, -1) // TODO: fast atomic
+	}
+	return _p_
+}
+
+// Try to put g on local runnable queue.
+// If it's full, put onto global queue.
+// Executed only by the owner P.
+func runqput(_p_ *p, gp *g) {
+retry:
+	h := atomicload(&_p_.runqhead) // load-acquire, synchronize with consumers
+	t := _p_.runqtail
+	if t-h < uint32(len(_p_.runq)) {
+		_p_.runq[t%uint32(len(_p_.runq))] = gp
+		atomicstore(&_p_.runqtail, t+1) // store-release, makes the item available for consumption
+		return
+	}
+	if runqputslow(_p_, gp, h, t) {
+		return
+	}
+	// the queue is not full, now the put above must suceed
+	goto retry
+}
+
+// Put g and a batch of work from local runnable queue on global queue.
+// Executed only by the owner P.
+func runqputslow(_p_ *p, gp *g, h, t uint32) bool {
+	var batch [len(_p_.runq)/2 + 1]*g
+
+	// First, grab a batch from local queue.
+	n := t - h
+	n = n / 2
+	if n != uint32(len(_p_.runq)/2) {
+		gothrow("runqputslow: queue is not full")
+	}
+	for i := uint32(0); i < n; i++ {
+		batch[i] = _p_.runq[(h+i)%uint32(len(_p_.runq))]
+	}
+	if !cas(&_p_.runqhead, h, h+n) { // cas-release, commits consume
+		return false
+	}
+	batch[n] = gp
+
+	// Link the goroutines.
+	for i := uint32(0); i < n; i++ {
+		batch[i].schedlink = batch[i+1]
+	}
+
+	// Now put the batch on global queue.
+	lock(&sched.lock)
+	globrunqputbatch(batch[0], batch[n], int32(n+1))
+	unlock(&sched.lock)
+	return true
+}
+
+// Get g from local runnable queue.
+// Executed only by the owner P.
+func runqget(_p_ *p) *g {
+	for {
+		h := atomicload(&_p_.runqhead) // load-acquire, synchronize with other consumers
+		t := _p_.runqtail
+		if t == h {
+			return nil
+		}
+		gp := _p_.runq[h%uint32(len(_p_.runq))]
+		if cas(&_p_.runqhead, h, h+1) { // cas-release, commits consume
+			return gp
+		}
+	}
+}
+
+// Grabs a batch of goroutines from local runnable queue.
+// batch array must be of size len(p->runq)/2. Returns number of grabbed goroutines.
+// Can be executed by any P.
+func runqgrab(_p_ *p, batch []*g) uint32 {
+	for {
+		h := atomicload(&_p_.runqhead) // load-acquire, synchronize with other consumers
+		t := atomicload(&_p_.runqtail) // load-acquire, synchronize with the producer
+		n := t - h
+		n = n - n/2
+		if n == 0 {
+			return 0
+		}
+		if n > uint32(len(_p_.runq)/2) { // read inconsistent h and t
+			continue
+		}
+		for i := uint32(0); i < n; i++ {
+			batch[i] = _p_.runq[(h+i)%uint32(len(_p_.runq))]
+		}
+		if cas(&_p_.runqhead, h, h+n) { // cas-release, commits consume
+			return n
+		}
+	}
+}
+
+// Steal half of elements from local runnable queue of p2
+// and put onto local runnable queue of p.
+// Returns one of the stolen elements (or nil if failed).
+func runqsteal(_p_, p2 *p) *g {
+	var batch [len(_p_.runq) / 2]*g
+
+	n := runqgrab(p2, batch[:])
+	if n == 0 {
+		return nil
+	}
+	n--
+	gp := batch[n]
+	if n == 0 {
+		return gp
+	}
+	h := atomicload(&_p_.runqhead) // load-acquire, synchronize with consumers
+	t := _p_.runqtail
+	if t-h+n >= uint32(len(_p_.runq)) {
+		gothrow("runqsteal: runq overflow")
+	}
+	for i := uint32(0); i < n; i++ {
+		_p_.runq[(t+i)%uint32(len(_p_.runq))] = batch[i]
+	}
+	atomicstore(&_p_.runqtail, t+n) // store-release, makes the item available for consumption
+	return gp
+}
+
+func testSchedLocalQueue() {
+	_p_ := new(p)
+	gs := make([]g, len(_p_.runq))
+	for i := 0; i < len(_p_.runq); i++ {
+		if runqget(_p_) != nil {
+			gothrow("runq is not empty initially")
+		}
+		for j := 0; j < i; j++ {
+			runqput(_p_, &gs[i])
+		}
+		for j := 0; j < i; j++ {
+			if runqget(_p_) != &gs[i] {
+				print("bad element at iter ", i, "/", j, "\n")
+				gothrow("bad element")
+			}
+		}
+		if runqget(_p_) != nil {
+			gothrow("runq is not empty afterwards")
+		}
+	}
+}
+
+func testSchedLocalQueueSteal() {
+	p1 := new(p)
+	p2 := new(p)
+	gs := make([]g, len(p1.runq))
+	for i := 0; i < len(p1.runq); i++ {
+		for j := 0; j < i; j++ {
+			gs[j].sig = 0
+			runqput(p1, &gs[j])
+		}
+		gp := runqsteal(p2, p1)
+		s := 0
+		if gp != nil {
+			s++
+			gp.sig++
+		}
+		for {
+			gp = runqget(p2)
+			if gp == nil {
+				break
+			}
+			s++
+			gp.sig++
+		}
+		for {
+			gp = runqget(p1)
+			if gp == nil {
+				break
+			}
+			gp.sig++
+		}
+		for j := 0; j < i; j++ {
+			if gs[j].sig != 1 {
+				print("bad element ", j, "(", gs[j].sig, ") at iter ", i, "\n")
+				gothrow("bad element")
+			}
+		}
+		if s != i/2 && s != i/2+1 {
+			print("bad steal ", s, ", want ", i/2, " or ", i/2+1, ", iter ", i, "\n")
+			gothrow("bad steal")
+		}
+	}
+}
+
+func setMaxThreads(in int) (out int) {
+	lock(&sched.lock)
+	out = int(sched.maxmcount)
+	sched.maxmcount = int32(in)
+	checkmcount()
+	unlock(&sched.lock)
+	return
+}
+
+var goexperiment string = "GOEXPERIMENT" // TODO: defined in zaexperiment.h
+
+func haveexperiment(name string) bool {
+	x := goexperiment
+	for x != "" {
+		xname := ""
+		i := index(x, ",")
+		if i < 0 {
+			xname, x = x, ""
+		} else {
+			xname, x = x[:i], x[i+1:]
+		}
+		if xname == name {
+			return true
+		}
+	}
+	return false
+}
+
+//go:nosplit
+func sync_procPin() int {
+	_g_ := getg()
+	mp := _g_.m
+
+	mp.locks++
+	return int(mp.p.id)
+}
+
+//go:nosplit
+func sync_procUnpin() {
+	_g_ := getg()
+	_g_.m.locks--
+}