diff --git a/.last_go_version b/.last_go_version
index 531e0b896..1055b8669 100644
--- a/.last_go_version
+++ b/.last_go_version
@@ -1 +1 @@
-6g version weekly.2011-06-02 8633+
+6g version weekly.2011-05-22 8613+
diff --git a/lib/go/camli/rollsum/rollsum.go b/lib/go/camli/rollsum/rollsum.go
index 6fec3f52b..0c6b07b90 100644
--- a/lib/go/camli/rollsum/rollsum.go
+++ b/lib/go/camli/rollsum/rollsum.go
@@ -16,6 +16,9 @@ limitations under the License.
 
 // Package rollsum implements rolling checksums similar to apenwarr's bup, which
 // is similar to librsync.
+//
+// The bup project is at https://github.com/apenwarr/bup and its splitting in
+// particular is at https://github.com/apenwarr/bup/blob/master/lib/bup/bupsplit.c
 package rollsum
 
 import (
diff --git a/lib/go/camli/rollsum/rollsum_test.go b/lib/go/camli/rollsum/rollsum_test.go
index 0993fdce4..728bed688 100644
--- a/lib/go/camli/rollsum/rollsum_test.go
+++ b/lib/go/camli/rollsum/rollsum_test.go
@@ -53,3 +53,19 @@ func TestSum(t *testing.T) {
 		t.Errorf("sum3a=%d sum3b=%d", sum3a, sum3b)
 	}
 }
+
+func BenchmarkRollsum(b *testing.B) {
+	bytesSize := int64(1024 * 1024 * 5)
+	rs := New()
+	bits := 0
+	for i := 0; i < b.N; i++ {
+		for j := int64(0); j < bytesSize; j++ {
+			rs.Roll(byte(rand.Int63() & 0xff))
+			if rs.OnSplit() {
+				bits = rs.Bits()
+			}
+		}
+	}
+	b.SetBytes(bytesSize)
+	_ = bits
+}