runtime: change p.runq from []*g to []guintptr

This eliminates many write barriers in the scheduler code that are
unnecessary and will interfere with upcoming changes where the garbage
collector will have to invoke run queue functions in contexts that
must not have write barriers.

Change-Id: I702d0ac99cfd00ffff406e7362917db6a43e7e55
Reviewed-on: https://go-review.googlesource.com/16556Reviewed-by: Russ Cox <rsc@golang.org>
Run-TryBot: Austin Clements <austin@google.com>

src/runtime/proc.go

@@ -3064,7 +3064,7 @@ func procresize(nprocs int32) *p {
 		for p.runqhead != p.runqtail {
 			// pop from tail of local queue
 			p.runqtail--
-			gp := p.runq[p.runqtail%uint32(len(p.runq))]
+			gp := p.runq[p.runqtail%uint32(len(p.runq))].ptr()
 			// push onto head of global queue
 			globrunqputhead(gp)
 		}
@@ -3753,7 +3753,7 @@ retry:
 	h := atomicload(&_p_.runqhead) // load-acquire, synchronize with consumers
 	t := _p_.runqtail
 	if t-h < uint32(len(_p_.runq)) {
-		_p_.runq[t%uint32(len(_p_.runq))] = gp
+		_p_.runq[t%uint32(len(_p_.runq))].set(gp)
 		atomicstore(&_p_.runqtail, t+1) // store-release, makes the item available for consumption
 		return
 	}
@@ -3776,7 +3776,7 @@ func runqputslow(_p_ *p, gp *g, h, t uint32) bool {
 		throw("runqputslow: queue is not full")
 	}
 	for i := uint32(0); i < n; i++ {
-		batch[i] = _p_.runq[(h+i)%uint32(len(_p_.runq))]
+		batch[i] = _p_.runq[(h+i)%uint32(len(_p_.runq))].ptr()
 	}
 	if !cas(&_p_.runqhead, h, h+n) { // cas-release, commits consume
 		return false
@@ -3824,7 +3824,7 @@ func runqget(_p_ *p) (gp *g, inheritTime bool) {
 		if t == h {
 			return nil, false
 		}
-		gp := _p_.runq[h%uint32(len(_p_.runq))]
+		gp := _p_.runq[h%uint32(len(_p_.runq))].ptr()
 		if cas(&_p_.runqhead, h, h+1) { // cas-release, commits consume
 			return gp, false
 		}
@@ -3835,7 +3835,7 @@ func runqget(_p_ *p) (gp *g, inheritTime bool) {
 // Batch is a ring buffer starting at batchHead.
 // Returns number of grabbed goroutines.
 // Can be executed by any P.
-func runqgrab(_p_ *p, batch *[256]*g, batchHead uint32, stealRunNextG bool) uint32 {
+func runqgrab(_p_ *p, batch *[256]guintptr, batchHead uint32, stealRunNextG bool) uint32 {
 	for {
 		h := atomicload(&_p_.runqhead) // load-acquire, synchronize with other consumers
 		t := atomicload(&_p_.runqtail) // load-acquire, synchronize with the producer
@@ -3856,7 +3856,7 @@ func runqgrab(_p_ *p, batch *[256]*g, batchHead uint32, stealRunNextG bool) uint
 					if !_p_.runnext.cas(next, 0) {
 						continue
 					}
-					batch[batchHead%uint32(len(batch))] = next.ptr()
+					batch[batchHead%uint32(len(batch))] = next
 					return 1
 				}
 			}
@@ -3885,7 +3885,7 @@ func runqsteal(_p_, p2 *p, stealRunNextG bool) *g {
 		return nil
 	}
 	n--
-	gp := _p_.runq[(t+n)%uint32(len(_p_.runq))]
+	gp := _p_.runq[(t+n)%uint32(len(_p_.runq))].ptr()
 	if n == 0 {
 		return gp
 	}

src/runtime/runtime2.go

@@ -358,7 +358,7 @@ type p struct {
 	// Queue of runnable goroutines. Accessed without lock.
 	runqhead uint32
 	runqtail uint32
-	runq     [256]*g
+	runq     [256]guintptr
 	// runnext, if non-nil, is a runnable G that was ready'd by
 	// the current G and should be run next instead of what's in
 	// runq if there's time remaining in the running G's time