runtime: add benchmark of heapBitsSetType

There was an old benchmark that measured this indirectly
via allocation, but I don't understand how to factor out the
allocation cost when interpreting the numbers.

Replace with a benchmark that only calls heapBitsSetType,
that does not allocate. This was not possible when the
benchmark was first written, because heapBitsSetType had
not been factored out of mallocgc.

Change-Id: I30f0f02362efab3465a50769398be859832e6640
Reviewed-on: https://go-review.googlesource.com/9701
Reviewed-by: Austin Clements <austin@google.com>

src/runtime/export_test.go

@@ -128,3 +128,32 @@ func Envs() []string     { return envs }
 func SetEnvs(e []string) { envs = e }
 
 var BigEndian = _BigEndian
+
+// For benchmarking.
+
+func BenchSetType(n int, x interface{}) {
+	e := *(*eface)(unsafe.Pointer(&x))
+	t := e._type
+	var size uintptr
+	var p unsafe.Pointer
+	switch t.kind & kindMask {
+	case _KindPtr:
+		t = (*ptrtype)(unsafe.Pointer(t)).elem
+		size = t.size
+		p = e.data
+	case _KindSlice:
+		slice := *(*struct {
+			ptr      unsafe.Pointer
+			len, cap uintptr
+		})(e.data)
+		t = (*slicetype)(unsafe.Pointer(t)).elem
+		size = t.size * slice.len
+		p = slice.ptr
+	}
+	allocSize := roundupsize(size)
+	systemstack(func() {
+		for i := 0; i < n; i++ {
+			heapBitsSetType(uintptr(p), allocSize, size, t)
+		}
+	})
+}