runtime: add benchmark for write barriers

For #22460.

Change-Id: I798f26d45bbe1efd16b632e201413cb26cb3e6c7
Reviewed-on: https://go-review.googlesource.com/73811
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rick Hudson <rlh@golang.org>

src/runtime/gc_test.go

@@ -10,6 +10,7 @@ import (
 	"reflect"
 	"runtime"
 	"runtime/debug"
+	"sync/atomic"
 	"testing"
 	"time"
 	"unsafe"
@@ -515,3 +516,83 @@ func TestUserForcedGC(t *testing.T) {
 		t.Fatalf("runtime.GC() was not accounted in NumForcedGC")
 	}
 }
+
+func BenchmarkWriteBarrier(b *testing.B) {
+	if runtime.GOMAXPROCS(-1) < 2 {
+		// We don't want GC to take our time.
+		b.Skip("need GOMAXPROCS >= 2")
+	}
+
+	// Construct a large tree both so the GC runs for a while and
+	// so we have a data structure to manipulate the pointers of.
+	type node struct {
+		l, r *node
+	}
+	var wbRoots []*node
+	var mkTree func(level int) *node
+	mkTree = func(level int) *node {
+		if level == 0 {
+			return nil
+		}
+		n := &node{mkTree(level - 1), mkTree(level - 1)}
+		if level == 10 {
+			// Seed GC with enough early pointers so it
+			// doesn't accidentally switch to mark 2 when
+			// it only has the top of the tree.
+			wbRoots = append(wbRoots, n)
+		}
+		return n
+	}
+	const depth = 22 // 64 MB
+	root := mkTree(22)
+
+	runtime.GC()
+	var ms runtime.MemStats
+	runtime.ReadMemStats(&ms)
+	//b.Logf("heap size: %d MB", ms.HeapAlloc>>20)
+
+	// Keep GC running continuously during the benchmark.
+	var stop uint32
+	done := make(chan bool)
+	go func() {
+		for atomic.LoadUint32(&stop) == 0 {
+			runtime.GC()
+		}
+		close(done)
+	}()
+
+	b.ResetTimer()
+
+	var stack [depth]*node
+	tos := -1
+
+	// There are two write barriers per iteration, so i+=2.
+	for i := 0; i < b.N; i += 2 {
+		if tos == -1 {
+			stack[0] = root
+			tos = 0
+		}
+
+		// Perform one step of reversing the tree.
+		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()
+		}
+	}
+
+	b.StopTimer()
+	atomic.StoreUint32(&stop, 1)
+	<-done
+
+	runtime.KeepAlive(wbRoots)
+}