runtime: remove useless heap_objects accounting

This change removes useless additional heap_objects accounting for large
objects. heap_objects is computed from scratch at ReadMemStats time
(which stops the world) by using nlargealloc and nlargefree, so mutating
heap_objects turns out to be pointless.

As a result, the "large" parameter on "mheap_.freeSpan" is no longer
necessary and so this change cleans that up too.

Change-Id: I7d6b486d9b57c018e3db46221d81b55fe4c1b021
Reviewed-on: https://go-review.googlesource.com/c/go/+/196637
Run-TryBot: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>

src/runtime/mcentral.go

@@ -243,7 +243,7 @@ func (c *mcentral) freeSpan(s *mspan, preserve bool, wasempty bool) bool {
 
 	c.nonempty.remove(s)
 	unlock(&c.lock)
-	mheap_.freeSpan(s, false)
+	mheap_.freeSpan(s)
 	return true
 }
 

src/runtime/mgcsweep.go

@@ -386,7 +386,7 @@ 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, true)
+			mheap_.freeSpan(s)
 		}
 		c.local_nlargefree++
 		c.local_largefree += size

src/runtime/mheap.go

@@ -926,7 +926,6 @@ func (h *mheap) alloc_m(npage uintptr, spanclass spanClass, large bool) *mspan {
 		// update stats, sweep lists
 		h.pagesInUse += uint64(npage)
 		if large {
-			memstats.heap_objects++
 			mheap_.largealloc += uint64(s.elemsize)
 			mheap_.nlargealloc++
 			atomic.Xadd64(&memstats.heap_live, int64(npage<<_PageShift))
@@ -1201,10 +1200,7 @@ func (h *mheap) grow(npage uintptr) bool {
 }
 
 // Free the span back into the heap.
-//
-// large must match the value of large passed to mheap.alloc. This is
-// used for accounting.
-func (h *mheap) freeSpan(s *mspan, large bool) {
+func (h *mheap) freeSpan(s *mspan) {
 	systemstack(func() {
 		mp := getg().m
 		lock(&h.lock)
@@ -1218,10 +1214,6 @@ func (h *mheap) freeSpan(s *mspan, large bool) {
 			bytes := s.npages << _PageShift
 			msanfree(base, bytes)
 		}
-		if large {
-			// Match accounting done in mheap.alloc.
-			memstats.heap_objects--
-		}
 		if gcBlackenEnabled != 0 {
 			// heap_scan changed.
 			gcController.revise()

src/runtime/mstats.go

@@ -40,7 +40,10 @@ type mstats struct {
 	heap_idle     uint64 // bytes in idle spans
 	heap_inuse    uint64 // bytes in mSpanInUse spans
 	heap_released uint64 // bytes released to the os
-	heap_objects  uint64 // total number of allocated objects
+
+	// heap_objects is not used by the runtime directly and instead
+	// computed on the fly by updatememstats.
+	heap_objects uint64 // total number of allocated objects
 
 	// Statistics about allocation of low-level fixed-size structures.
 	// Protected by FixAlloc locks.