Based on Dmitry Vyukov's comments in CL 65210.

Change-Id: I5dce7286b0d180cd43cad3aaf70f537fafcda588
Reviewed-on: https://go-review.googlesource.com/123275Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Rick Hudson <rlh@golang.org>
Reviewed-by: Dmitry Vyukov <dvyukov@google.com>