runtime: simplify sweep allocation counting

Currently sweep counts the number of allocated objects, computes the
number of free objects from that, then re-computes the number of
allocated objects from that. Simplify and clean this up by skipping
these intermediate steps.

Change-Id: I3ed98e371eb54bbcab7c8530466c4ab5fde35f0a
Reviewed-on: https://go-review.googlesource.com/34935
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Marvin Stenger <marvin.stenger94@gmail.com>
Reviewed-by: Rick Hudson <rlh@golang.org>

src/runtime/mbitmap.go

@@ -823,10 +823,10 @@ var oneBitCount = [256]uint8{
 	4, 5, 5, 6, 5, 6, 6, 7,
 	5, 6, 6, 7, 6, 7, 7, 8}
 
-// countFree runs through the mark bits in a span and counts the number of free objects
-// in the span.
+// countAlloc returns the number of objects allocated in span s by
+// scanning the allocation bitmap.
 // TODO:(rlh) Use popcount intrinsic.
-func (s *mspan) countFree() int {
+func (s *mspan) countAlloc() int {
 	count := 0
 	maxIndex := s.nelems / 8
 	for i := uintptr(0); i < maxIndex; i++ {
@@ -839,7 +839,7 @@ func (s *mspan) countFree() int {
 		bits := mrkBits & mask
 		count += int(oneBitCount[bits])
 	}
-	return int(s.nelems) - count
+	return count
 }
 
 // heapBitsSetType records that the new allocation [x, x+size)

src/runtime/mgcsweep.go

@@ -186,7 +186,6 @@ func (s *mspan) sweep(preserve bool) bool {
 	cl := s.sizeclass
 	size := s.elemsize
 	res := false
-	nfree := 0
 
 	c := _g_.m.mcache
 	freeToHeap := false
@@ -276,15 +275,14 @@ func (s *mspan) sweep(preserve bool) bool {
 	}
 
 	// Count the number of free objects in this span.
-	nfree = s.countFree()
-	if cl == 0 && nfree != 0 {
+	nalloc := uint16(s.countAlloc())
+	if cl == 0 && nalloc == 0 {
 		s.needzero = 1
 		freeToHeap = true
 	}
-	nalloc := uint16(s.nelems) - uint16(nfree)
 	nfreed := s.allocCount - nalloc
 	if nalloc > s.allocCount {
-		print("runtime: nelems=", s.nelems, " nfree=", nfree, " nalloc=", nalloc, " previous allocCount=", s.allocCount, " nfreed=", nfreed, "\n")
+		print("runtime: nelems=", s.nelems, " nalloc=", nalloc, " previous allocCount=", s.allocCount, " nfreed=", nfreed, "\n")
 		throw("sweep increased allocation count")
 	}