runtime: report marked heap size in gctrace

When the gctrace GODEBUG option is enabled, it will now report three
heap sizes: the heap size at the beginning of the GC cycle, the heap
size at the end of the GC cycle before sweeping, and marked heap size,
which is the amount of heap that will be retained until the next GC
cycle.

Change-Id: Ie13f8a6d5c609bc9cc47c7555960ab55b37b5f1c
Reviewed-on: https://go-review.googlesource.com/8430Reviewed-by: Rick Hudson <rlh@golang.org>

src/runtime/mgc.go

@@ -294,7 +294,7 @@ func gc(mode int) {
 	// debug.gctrace variables
 	var stwprocs, maxprocs int32
 	var tSweepTerm, tScan, tInstallWB, tMark, tMarkTerm int64
-	var heap0, heap1 uint64
+	var heap0, heap1, heap2 uint64
 
 	// Ok, we're doing it!  Stop everybody else
 	semacquire(&worldsema, false)
@@ -414,6 +414,9 @@ func gc(mode int) {
 	// need to switch to g0 so we can shrink the stack.
 	systemstack(func() {
 		gcMark(startTime)
+		if debug.gctrace > 0 {
+			heap2 = work.bytesMarked
+		}
 		if debug.gccheckmark > 0 {
 			// Run a full stop-the-world mark using checkmark bits,
 			// to check that we didn't forget to mark anything during
@@ -481,7 +484,6 @@ func gc(mode int) {
 
 	memstats.numgc++
 	if debug.gctrace > 0 {
-		// TODO(austin): Marked heap size at end
 		tEnd := nanotime()
 
 		// Update work.totaltime
@@ -512,7 +514,7 @@ func gc(mode int) {
 			"+", installWBCpu/1e6,
 			"+", markCpu/1e6,
 			"+", markTermCpu/1e6, " ms cpu, ",
-			heap0>>20, "->", heap1>>20, " MB, ",
+			heap0>>20, "->", heap1>>20, "->", heap2>>20, " MB, ",
 			maxprocs, " P")
 		if mode != gcBackgroundMode {
 			print(" (forced)")