What version of Go are you using (go version)?

Tested on both go version go1.8.1 linux/amd64 and go version devel +8c49c06b48 Mon May 8 15:16:21 2017 +0000 linux/amd64.

What operating system and processor architecture are you using (go env)?

linux/amd64

What did you do?

runtime.gchelper ends with the following few lines:

	nproc := work.nproc // work.nproc can change right after we increment work.ndone
	if atomic.Xadd(&work.ndone, +1) == nproc-1 {
		notewakeup(&work.alldone)
	}

What did you expect to see?

I expected the atomic.Xadd to act as a memory fence and force the regular load of work.nproc to happen before the atomic.

What did you see instead?

The compiler reorders the regular load to occur after the atomic.Xadd. I believe this is leading to #19305.

/cc @dr2chase @cherrymui