runtime: abstract specials list iteration

The specials processing loop in mspan.sweep is about to get more
complicated and I'm too allergic to list manipulation to open code
more of it there.

Change-Id: I767a0889739da85fb2878fc06a5c55b73bf2ba7d
Reviewed-on: https://go-review.googlesource.com/c/go/+/305551
Trust: Austin Clements <austin@google.com>
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Michael Pratt <mpratt@google.com>

src/runtime/mgcsweep.go

@@ -356,11 +356,10 @@ func (s *mspan) sweep(preserve bool) bool {
 	//    If such object is not marked, we need to queue all finalizers at once.
 	// Both 1 and 2 are possible at the same time.
 	hadSpecials := s.specials != nil
-	specialp := &s.specials
-	special := *specialp
-	for special != nil {
+	siter := newSpecialsIter(s)
+	for siter.valid() {
 		// A finalizer can be set for an inner byte of an object, find object beginning.
-		objIndex := uintptr(special.offset) / size
+		objIndex := uintptr(siter.s.offset) / size
 		p := s.base() + objIndex*size
 		mbits := s.markBitsForIndex(objIndex)
 		if !mbits.isMarked() {
@@ -368,7 +367,7 @@ func (s *mspan) sweep(preserve bool) bool {
 			// Pass 1: see if it has at least one finalizer.
 			hasFin := false
 			endOffset := p - s.base() + size
-			for tmp := special; tmp != nil && uintptr(tmp.offset) < endOffset; tmp = tmp.next {
+			for tmp := siter.s; tmp != nil && uintptr(tmp.offset) < endOffset; tmp = tmp.next {
 				if tmp.kind == _KindSpecialFinalizer {
 					// Stop freeing of object if it has a finalizer.
 					mbits.setMarkedNonAtomic()
@@ -377,27 +376,23 @@ func (s *mspan) sweep(preserve bool) bool {
 				}
 			}
 			// Pass 2: queue all finalizers _or_ handle profile record.
-			for special != nil && uintptr(special.offset) < endOffset {
+			for siter.valid() && uintptr(siter.s.offset) < endOffset {
 				// Find the exact byte for which the special was setup
 				// (as opposed to object beginning).
+				special := siter.s
 				p := s.base() + uintptr(special.offset)
 				if special.kind == _KindSpecialFinalizer || !hasFin {
-					// Splice out special record.
-					y := special
-					special = special.next
-					*specialp = special
-					freespecial(y, unsafe.Pointer(p), size)
+					siter.unlinkAndNext()
+					freeSpecial(special, unsafe.Pointer(p), size)
 				} else {
 					// This is profile record, but the object has finalizers (so kept alive).
 					// Keep special record.
-					specialp = &special.next
-					special = *specialp
+					siter.next()
 				}
 			}
 		} else {
 			// object is still live: keep special record
-			specialp = &special.next
-			special = *specialp
+			siter.next()
 		}
 	}
 	if hadSpecials && s.specials == nil {

src/runtime/mheap.go

@@ -1854,9 +1854,37 @@ func setprofilebucket(p unsafe.Pointer, b *bucket) {
 	}
 }
 
-// Do whatever cleanup needs to be done to deallocate s. It has
-// already been unlinked from the mspan specials list.
-func freespecial(s *special, p unsafe.Pointer, size uintptr) {
+// specialsIter helps iterate over specials lists.
+type specialsIter struct {
+	pprev **special
+	s     *special
+}
+
+func newSpecialsIter(span *mspan) specialsIter {
+	return specialsIter{&span.specials, span.specials}
+}
+
+func (i *specialsIter) valid() bool {
+	return i.s != nil
+}
+
+func (i *specialsIter) next() {
+	i.pprev = &i.s.next
+	i.s = *i.pprev
+}
+
+// unlinkAndNext removes the current special from the list and moves
+// the iterator to the next special. It returns the unlinked special.
+func (i *specialsIter) unlinkAndNext() *special {
+	cur := i.s
+	i.s = cur.next
+	*i.pprev = i.s
+	return cur
+}
+
+// freeSpecial performs any cleanup on special s and deallocates it.
+// s must already be unlinked from the specials list.
+func freeSpecial(s *special, p unsafe.Pointer, size uintptr) {
 	switch s.kind {
 	case _KindSpecialFinalizer:
 		sf := (*specialfinalizer)(unsafe.Pointer(s))