runtime: include heap goal in gctrace line

This may or may not be useful to the end user, but it's incredibly
useful for us to understand the behavior of the pacer. Currently this
is fairly easy (though not trivial) to derive from the other heap
stats we print, but we're about to change how we compute the goal,
which will make it much harder to derive.

Change-Id: I796ef233d470c01f606bd9929820c01ece1f585a
Reviewed-on: https://go-review.googlesource.com/9176Reviewed-by: Rick Hudson <rlh@golang.org>

src/runtime/mgc.go

@@ -684,7 +684,7 @@ func gc(mode int) {
 	// debug.gctrace variables
 	var stwprocs, maxprocs int32
 	var tSweepTerm, tScan, tInstallWB, tMark, tMarkTerm int64
-	var heap0, heap1, heap2 uint64
+	var heap0, heap1, heap2, heapGoal uint64
 
 	// Ok, we're doing it!  Stop everybody else
 	semacquire(&worldsema, false)
@@ -732,6 +732,7 @@ func gc(mode int) {
 
 	if mode == gcBackgroundMode { // Do as much work concurrently as possible
 		gcController.startCycle()
+		heapGoal = gcController.heapGoal
 
 		systemstack(func() {
 			gcphase = _GCscan
@@ -791,6 +792,7 @@ func gc(mode int) {
 		if debug.gctrace > 0 {
 			t := nanotime()
 			tScan, tInstallWB, tMark, tMarkTerm = t, t, t, t
+			heapGoal = heap0
 		}
 	}
 
@@ -923,6 +925,7 @@ func gc(mode int) {
 			"/", gcController.idleMarkTime/1e6,
 			"+", markTermCpu/1e6, " ms cpu, ",
 			heap0>>20, "->", heap1>>20, "->", heap2>>20, " MB, ",
+			heapGoal>>20, " MB goal, ",
 			maxprocs, " P")
 		if mode != gcBackgroundMode {
 			print(" (forced)")