var (
	wbSink interface{}
	wbPtr  *int
)

func BenchmarkWriteBarrier(b *testing.B) {
	if runtime.GOMAXPROCS(-1) < 2 {
		// We don't want to time the allocation goroutine.
		b.Skip("need GOMAXPROCS >= 2")
	}

	// Give the GC something interesting to chew on so it runs for
	// a while.
	type node struct{ l, r *node }
	var mkTree func(int) *node
	mkTree = func(level int) *node {
		if level == 0 {
			return nil
		}
		return &node{mkTree(level - 1), mkTree(level - 1)}
	}
	wbSink = mkTree(22) // 64 MB
	runtime.GC()
	var ms runtime.MemStats
	runtime.ReadMemStats(&ms)
	b.Logf("heap size: %d MB", ms.HeapAlloc>>20)
	var stop uint32
	done := make(chan bool)
	go func() {
		for atomic.LoadUint32(&stop) == 0 {
			println("force GC")
			runtime.GC()
		}
		close(done)
	}()
	var i1, i2 int
	b.ResetTimer()

	for i := 0; i < b.N; i++ {
		if i%2 == 0 {
			wbPtr = &i1
		} else {
			wbPtr = &i2
		}
	}

	b.StopTimer()
	atomic.StoreUint32(&stop, 1)
	<-done
	wbSink = nil
}