A benchmark on the trivial cgo call, to assess overhead of different

ways of calling out to cgo.

usb/libusb.go

@@ -381,3 +381,8 @@ func xfer_callback(cptr unsafe.Pointer) {
 	ch := *(*chan struct{})(cptr)
 	close(ch)
 }
+
+// for benchmarking
+func libusbSetDebug(c *libusbContext, lvl int) {
+	C.libusb_set_debug((*C.libusb_context)(c), C.int(lvl))
+}

usb/libusb_cgo_benchmark_test.go

@@ -0,0 +1,46 @@
+package usb
+
+import "testing"
+
+func BenchmarkCGo(b *testing.B) {
+	for _, bc := range []struct {
+		name  string
+		bfunc func(*libusbContext, int)
+	}{
+		{
+			name: "simple function",
+			bfunc: func(ctx *libusbContext, N int) {
+				for i := 0; i < N; i++ {
+					libusbSetDebug(ctx, i&1)
+				}
+			},
+		},
+		{
+			name: "method",
+			bfunc: func(ctx *libusbContext, N int) {
+				impl := libusbImpl{}
+				for i := 0; i < N; i++ {
+					impl.setDebug(ctx, i&1)
+				}
+			},
+		},
+		{
+			name: "interface",
+			bfunc: func(ctx *libusbContext, N int) {
+				var intf libusbIntf = libusbImpl{}
+				for i := 0; i < N; i++ {
+					intf.setDebug(ctx, i&1)
+				}
+			},
+		},
+	} {
+		b.Run(bc.name, func(b *testing.B) {
+			ctx, err := libusbImpl{}.init()
+			if err != nil {
+				b.Fatalf("libusb_init() failed: %v", err)
+			}
+			b.ResetTimer()
+			bc.bfunc(ctx, b.N)
+		})
+	}
+}