runtime: tidy mheap.freeSpan

freeSpan currently takes a mysterious "acct int32" argument. This is
really just a boolean and actually just needs to match the "large"
argument to alloc in order to balance out accounting.

To make this clearer, replace acct with a "large bool" argument that
must match the call to mheap.alloc.

Change-Id: Ibc81faefdf9f0583114e1953fcfb362e9c3c76de
Reviewed-on: https://go-review.googlesource.com/c/138655
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/runtime/mcentral.go

@@ -244,7 +244,7 @@ func (c *mcentral) freeSpan(s *mspan, preserve bool, wasempty bool) bool {
 
 	c.nonempty.remove(s)
 	unlock(&c.lock)
-	mheap_.freeSpan(s, 0)
+	mheap_.freeSpan(s, false)
 	return true
 }
 

src/runtime/mgcsweep.go

@@ -366,7 +366,7 @@ func (s *mspan) sweep(preserve bool) bool {
 			s.limit = 0 // prevent mlookup from finding this span
 			sysFault(unsafe.Pointer(s.base()), size)
 		} else {
-			mheap_.freeSpan(s, 1)
+			mheap_.freeSpan(s, true)
 		}
 		c.local_nlargefree++
 		c.local_largefree += size

src/runtime/mheap.go

@@ -945,7 +945,10 @@ func (h *mheap) grow(npage uintptr) bool {
 }
 
 // Free the span back into the heap.
-func (h *mheap) freeSpan(s *mspan, acct int32) {
+//
+// large must match the value of large passed to mheap.alloc. This is
+// used for accounting.
+func (h *mheap) freeSpan(s *mspan, large bool) {
 	systemstack(func() {
 		mp := getg().m
 		lock(&h.lock)
@@ -959,7 +962,8 @@ func (h *mheap) freeSpan(s *mspan, acct int32) {
 			bytes := s.npages << _PageShift
 			msanfree(base, bytes)
 		}
-		if acct != 0 {
+		if large {
+			// Match accounting done in mheap.alloc.
 			memstats.heap_objects--
 		}
 		if gcBlackenEnabled != 0 {