diff --git a/src/runtime/mgc0.h b/src/runtime/mgc0.h
new file mode 100644
index 0000000000..62726b4f0f
--- /dev/null
+++ b/src/runtime/mgc0.h
@@ -0,0 +1,20 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Used by cmd/gc.
+
+enum {
+	gcBits = 4,
+	BitsPerPointer = 2,
+	BitsDead = 0,
+	BitsScalar = 1,
+	BitsPointer = 2,
+	BitsMask = 3,
+	PointersPerByte = 8/BitsPerPointer,
+	MaxGCMask = 64,
+	insData = 1,
+	insArray,
+	insArrayEnd,
+	insEnd,
+};