runtime: make next_gc ^0 when GC is disabled

When GC is disabled, we set gcpercent to -1. However, we still use
gcpercent to compute several values, such as next_gc and gc_trigger.
These calculations are meaningless when gcpercent is -1 and result in
meaningless values. This is okay in a sense because we also never use
these values if gcpercent is -1, but they're confusing when exposed to
the user, for example via MemStats or the execution trace. It's
particularly unfortunate in the execution trace because it attempts to
plot the underflowed value of next_gc, which scales all useful
information in the heap row into oblivion.

Fix this by making next_gc ^0 when gcpercent < 0. This has the
advantage of being true in a way: next_gc is effectively infinite when
gcpercent < 0. We can also detect this special value when updating the
execution trace and report next_gc as 0 so it doesn't blow up the
display of the heap line.

Change-Id: I4f366e4451f8892a4908da7b2b6086bdc67ca9a9
Reviewed-on: https://go-review.googlesource.com/30016Reviewed-by: Rick Hudson <rlh@golang.org>

src/runtime/mgc.go

@@ -187,6 +187,9 @@ func gcinit() {
 	//   goal = marked * (1 + GOGC/100)
 	//        = trigger / (1 + triggerRatio) * (1 + GOGC/100)
 	memstats.next_gc = uint64(float64(memstats.gc_trigger) / (1 + gcController.triggerRatio) * (1 + float64(gcpercent)/100))
+	if gcpercent < 0 {
+		memstats.next_gc = ^uint64(0)
+	}
 	work.startSema = 1
 	work.markDoneSema = 1
 }
@@ -434,6 +437,9 @@ func (c *gcControllerState) startCycle() {
 	// Re-compute the heap goal for this cycle in case something
 	// changed. This is the same calculation we use elsewhere.
 	memstats.next_gc = memstats.heap_marked + memstats.heap_marked*uint64(gcpercent)/100
+	if gcpercent < 0 {
+		memstats.next_gc = ^uint64(0)
+	}
 
 	// Ensure that the heap goal is at least a little larger than
 	// the current live heap size. This may not be the case if GC
@@ -1658,6 +1664,9 @@ func gcMark(start_time int64) {
 	// The next GC cycle should finish before the allocated heap
 	// has grown by GOGC/100.
 	memstats.next_gc = memstats.heap_marked + memstats.heap_marked*uint64(gcpercent)/100
+	if gcpercent < 0 {
+		memstats.next_gc = ^uint64(0)
+	}
 	if memstats.next_gc < memstats.gc_trigger {
 		memstats.next_gc = memstats.gc_trigger
 	}

src/runtime/mstats.go

@@ -71,7 +71,7 @@ type mstats struct {
 
 	// Statistics about garbage collector.
 	// Protected by mheap or stopping the world during GC.
-	next_gc         uint64 // goal heap_live for when next GC ends
+	next_gc         uint64 // goal heap_live for when next GC ends; ^0 if disabled
 	last_gc         uint64 // last gc (in absolute time)
 	pause_total_ns  uint64
 	pause_ns        [256]uint64 // circular buffer of recent gc pause lengths

src/runtime/trace.go

@@ -1006,5 +1006,10 @@ func traceHeapAlloc() {
 }
 
 func traceNextGC() {
-	traceEvent(traceEvNextGC, -1, memstats.next_gc)
+	if memstats.next_gc == ^uint64(0) {
+		// Heap-based triggering is disabled.
+		traceEvent(traceEvNextGC, -1, 0)
+	} else {
+		traceEvent(traceEvNextGC, -1, memstats.next_gc)
+	}
 }