runtime: add comment to mgc0.h

Missed that comment in CL 153990043.

LGTM=khr
R=khr
CC=golang-codereviews
https://golang.org/cl/156010043

diff --git a/src/runtime/mgc0.h b/src/runtime/mgc0.h
index 10f24d009fa14477f9a7b7af132170adddb2434d..64f8189143b1e8bf90f11cdef4c36d14a1972c76 100644 (file)
--- a/src/runtime/mgc0.h
+++ b/src/runtime/mgc0.h
@@ -42,6 +42,8 @@ enum {
        BitsMask        = (1<<BitsPerPointer)-1,
        PointersPerByte = 8/BitsPerPointer,
 
+       // If you change these, also change scanblock.
+       // scanblock does "if(bits == BitsScalar || bits == BitsDead)" as "if(bits <= BitsScalar)".
        BitsDead        = 0,
        BitsScalar      = 1,
        BitsPointer     = 2,