`systemstack`, `mcall`, or `asmcgocall` to perform tasks that must not
be preempted, that must not grow the user stack, or that switch user
goroutines. Code running on the system stack is implicitly
non-preemptible and the garbage collector does not scan system stacks.
While running on the system stack, the current user stack is not used
for execution.

nosplit functions
-----------------

					// during the execution of the function prologue, the traceback
					// code will not see a half-updated stack frame.
					// This sequence is not async preemptible, as if we open a frame
					// at the current SP, it will clobber the saved LR.
					q = obj.Appendp(q, c.newprog)
					q.As = AMOVD
					q.Pos = p.Pos
					q.From.Type = obj.TYPE_REG
					q.From.Reg = REG_LR

				// Store link register before decrement SP, so if a signal comes
				// during the execution of the function prologue, the traceback
				// code will not see a half-updated stack frame.
				// This sequence is not async preemptible, as if we open a frame
				// at the current SP, it will clobber the saved LR.
				q = c.ctxt.StartUnsafePoint(q, c.newprog)

				q = obj.Appendp(q, newprog)
				q.As = mov
				q.Pos = p.Pos

// weight allocation. If it is a heavy weight allocation the caller must
// determine whether a new GC cycle needs to be started or if the GC is active
// whether this goroutine needs to assist the GC.
//
// Must run in a non-preemptible context since otherwise the owner of
// c could change.
func (c *mcache) nextFree(spc spanClass) (v gclinkptr, s *mspan, shouldhelpgc bool) {
	s = c.alloc[spc]
	shouldhelpgc = false
	freeIndex := s.nextFreeIndex()

	// hits a traceAcquire, it sees that the trace is enabled.
	//
	// Note also that stopping the world is necessary to make sure sweep-related events are
	// coherent. Since the world is stopped and sweeps are non-preemptible, we can never start
	// the world and see an unpaired sweep 'end' event. Other parts of the tracer rely on this.
	stw := stopTheWorld(stwStartTrace)

	// Prevent sysmon from running any code that could generate events.

			n := stack[tos]
			if n.l == nil {
				tos--
			} else {
				n.l, n.r = n.r, n.l
				stack[tos] = n.l
				stack[tos+1] = n.r
				tos++
			}

			if i%(1<<12) == 0 {
				// Avoid non-preemptible loops (see issue #10958).
				runtime.Gosched()
			}
		}
	})

	runtime.KeepAlive(wbRoots)
}

func BenchmarkBulkWriteBarrier(b *testing.B) {
	if runtime.GOMAXPROCS(-1) < 2 {

	execution tracer to double-check stack ownership before taking a stack trace.

	asyncpreemptoff: asyncpreemptoff=1 disables signal-based
	asynchronous goroutine preemption. This makes some loops
	non-preemptible for long periods, which may delay GC and
	goroutine scheduling. This is useful for debugging GC issues
	because it also disables the conservative stack scanning used
	for asynchronously preempted goroutines.

				// Store link register before decrementing SP, so if a signal comes
				// during the execution of the function prologue, the traceback
				// code will not see a half-updated stack frame.
				// This sequence is not async preemptible, as if we open a frame
				// at the current SP, it will clobber the saved LR.
				q = c.ctxt.StartUnsafePoint(p, c.newprog)

				q = obj.Appendp(q, c.newprog)
				q.As = AMOVD
				q.From.Type = obj.TYPE_REG

//
// Note that this condition may transition from false to true at any
// time as the sweeper runs. It may transition from true to false if a
// GC runs; to prevent that the caller must be non-preemptible or must
// somehow block GC progress.
func isSweepDone() bool {
	return sweep.active.isDone()
}

// Returns only when span s has been swept.
//
//go:nowritebarrier
func (s *mspan) ensureSwept() {

				// Store link register before decrement SP, so if a signal comes
				// during the execution of the function prologue, the traceback
				// code will not see a half-updated stack frame.
				// This sequence is not async preemptible, as if we open a frame
				// at the current SP, it will clobber the saved LR.
				q = c.ctxt.StartUnsafePoint(q, c.newprog)

				q = obj.Appendp(q, newprog)
				q.As = mov
				q.Pos = p.Pos