runtime: eliminate mheap.nspan and use range loops

This was necessary in the C days when allspans was an mspan**, but now
that allspans is a Go slice, this is redundant with len(allspans) and
we can use range loops over allspans.

Change-Id: Ie1dc39611e574e29a896e01690582933f4c5be7e
Reviewed-on: https://go-review.googlesource.com/30531
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rick Hudson <rlh@golang.org>

src/runtime/heapdump.go

@@ -437,9 +437,7 @@ func dumproots() {
 	dumpfields(firstmoduledata.gcbssmask)
 
 	// MSpan.types
-	allspans := mheap_.allspans
-	for spanidx := uint32(0); spanidx < mheap_.nspan; spanidx++ {
-		s := allspans[spanidx]
+	for _, s := range mheap_.allspans {
 		if s.state == _MSpanInUse {
 			// Finalizers
 			for sp := s.specials; sp != nil; sp = sp.next {
@@ -462,8 +460,7 @@ func dumproots() {
 var freemark [_PageSize / 8]bool
 
 func dumpobjs() {
-	for i := uintptr(0); i < uintptr(mheap_.nspan); i++ {
-		s := mheap_.allspans[i]
+	for _, s := range mheap_.allspans {
 		if s.state != _MSpanInUse {
 			continue
 		}
@@ -608,9 +605,7 @@ func dumpmemprof_callback(b *bucket, nstk uintptr, pstk *uintptr, size, allocs,
 
 func dumpmemprof() {
 	iterate_memprof(dumpmemprof_callback)
-	allspans := mheap_.allspans
-	for spanidx := uint32(0); spanidx < mheap_.nspan; spanidx++ {
-		s := allspans[spanidx]
+	for _, s := range mheap_.allspans {
 		if s.state != _MSpanInUse {
 			continue
 		}
@@ -631,8 +626,7 @@ var dumphdr = []byte("go1.7 heap dump\n")
 
 func mdump() {
 	// make sure we're done sweeping
-	for i := uintptr(0); i < uintptr(mheap_.nspan); i++ {
-		s := mheap_.allspans[i]
+	for _, s := range mheap_.allspans {
 		if s.state == _MSpanInUse {
 			s.ensureSwept()
 		}

src/runtime/mheap.go

@@ -48,14 +48,11 @@ type mheap struct {
 	// must ensure that allocation cannot happen around the
 	// access (since that may free the backing store).
 	allspans []*mspan // all spans out there
-	nspan    uint32
 
 	// span lookup
 	spans        **mspan
 	spans_mapped uintptr
 
-	_ uint32 // align uint64 fields on 32-bit for atomics
-
 	// Proportional sweep
 	pagesInUse        uint64  // pages of spans in stats _MSpanInUse; R/W with mheap.lock
 	spanBytesAlloc    uint64  // bytes of spans allocated this cycle; updated atomically
@@ -282,7 +279,6 @@ func recordspan(vh unsafe.Pointer, p unsafe.Pointer) {
 		}
 	}
 	h.allspans = append(h.allspans, s)
-	h.nspan = uint32(len(h.allspans))
 }
 
 // inheap reports whether b is a pointer into a (potentially dead) heap object.

src/runtime/mstats.go

@@ -528,8 +528,7 @@ func updatememstats(stats *gcstats) {
 
 	// Scan all spans and count number of alive objects.
 	lock(&mheap_.lock)
-	for i := uint32(0); i < mheap_.nspan; i++ {
-		s := mheap_.allspans[i]
+	for _, s := range mheap_.allspans {
 		if s.state != mSpanInUse {
 			continue
 		}