internal/runtime/cgobench: add cgo callback benchmark

Change-Id: I43ea575aff87a3e420477cb26d35185d03df5ccc
Reviewed-on: https://go-review.googlesource.com/c/go/+/713283
Reviewed-by: Michael Pratt <mpratt@google.com>
Auto-Submit: Michael Knyszek <mknyszek@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

src/internal/runtime/cgobench/bench_test.go

@@ -24,3 +24,17 @@ func BenchmarkCgoCallParallel(b *testing.B) {
 		}
 	})
 }
+
+func BenchmarkCgoCallWithCallback(b *testing.B) {
+	for b.Loop() {
+		cgobench.Callback()
+	}
+}
+
+func BenchmarkCgoCallParallelWithCallback(b *testing.B) {
+	b.RunParallel(func(pb *testing.PB) {
+		for pb.Next() {
+			cgobench.Callback()
+		}
+	})
+}

src/internal/runtime/cgobench/funcs.go

@@ -9,9 +9,24 @@ package cgobench
 /*
 static void empty() {
 }
+
+void go_empty_callback();
+
+static void callback() {
+	go_empty_callback();
+}
+
 */
 import "C"
 
 func Empty() {
 	C.empty()
 }
+
+func Callback() {
+	C.callback()
+}
+
+//export go_empty_callback
+func go_empty_callback() {
+}