diff --git a/src/runtime/export_test.go b/src/runtime/export_test.go
index 929bb35..e3d6441 100644
--- a/src/runtime/export_test.go
+++ b/src/runtime/export_test.go
@@ -983,9 +983,31 @@
 	return x, y
 }
 
-func MSpanCountAlloc(bits []byte) int {
-	s := (*mspan)(mheap_.spanalloc.alloc())
+// mspan wrapper for testing.
+//go:notinheap
+type MSpan mspan
+
+// Allocate an mspan for testing.
+func AllocMSpan() *MSpan {
+	var s *mspan
+	systemstack(func() {
+		s = (*mspan)(mheap_.spanalloc.alloc())
+	})
+	return (*MSpan)(s)
+}
+
+// Free an allocated mspan.
+func FreeMSpan(s *MSpan) {
+	systemstack(func() {
+		mheap_.spanalloc.free(unsafe.Pointer(s))
+	})
+}
+
+func MSpanCountAlloc(ms *MSpan, bits []byte) int {
+	s := (*mspan)(ms)
 	s.nelems = uintptr(len(bits) * 8)
 	s.gcmarkBits = (*gcBits)(unsafe.Pointer(&bits[0]))
-	return s.countAlloc()
+	result := s.countAlloc()
+	s.gcmarkBits = nil
+	return result
 }
diff --git a/src/runtime/gc_test.go b/src/runtime/gc_test.go
index c5c8a4c..9edebda 100644
--- a/src/runtime/gc_test.go
+++ b/src/runtime/gc_test.go
@@ -763,6 +763,10 @@
 }
 
 func BenchmarkMSpanCountAlloc(b *testing.B) {
+	// Allocate one dummy mspan for the whole benchmark.
+	s := runtime.AllocMSpan()
+	defer runtime.FreeMSpan(s)
+
 	// n is the number of bytes to benchmark against.
 	// n must always be a multiple of 8, since gcBits is
 	// always rounded up 8 bytes.
@@ -774,7 +778,7 @@
 
 			b.ResetTimer()
 			for i := 0; i < b.N; i++ {
-				runtime.MSpanCountAlloc(bits)
+				runtime.MSpanCountAlloc(s, bits)
 			}
 		})
 	}