cmd/compile: reordering regular loads over atomic add

[aclements]

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

[randall77]

Minimal repro:

package main

import "sync/atomic"

func f(p, q *int32) bool {
	x := *q
	return atomic.AddInt32(p, 1) == x
}

The *q occurs after the atomic. Something is wrong with the schedule pass

    v9 = XADDLlock <uint32,mem> v12 v7 v1
    v5 = Select0 <uint32> v9
    v14 = Select1 <mem> v9
    v11 = MOVLload <int32> v8 v1

There's more than one memory variable live before v11 (v1 and v14).

[gopherbot]

CL https://golang.org/cl/43294 mentions this issue.

[gopherbot]

CL https://golang.org/cl/43311 mentions this issue.

[randall77]

Reopen for 1.8.2.

[gopherbot]

CL https://golang.org/cl/43411 mentions this issue.

[gopherbot]

CL https://golang.org/cl/43495 mentions this issue.