runtime: avoid redundant scans

During a concurrent GC stacks are scanned in
an initial scan phase informing the GC of all
pointers on the stack. The GC only needs to rescan
the stack if it potentially changes which can only
happen if the goroutine runs.
This CL tracks whether the Goroutine has run
since it was last scanned and thus may have changed
its stack. If necessary the stack is rescanned.

Change-Id: I5fb1c4338d42e3f61ab56c9beb63b7b2da25f4f1
Reviewed-on: https://go-review.googlesource.com/3275
Reviewed-by: Russ Cox <rsc@golang.org>

src/runtime/runtime2.go

@@ -203,6 +203,7 @@ type g struct {
 	paniconfault bool // panic (instead of crash) on unexpected fault address
 	preemptscan  bool // preempted g does scan for gc
 	gcworkdone   bool // debug: cleared at begining of gc work phase cycle, set by gcphasework, tested at end of cycle
+	gcscanvalid  bool // false at start of gc cycle, true if G has not run since last scan
 	throwsplit   bool // must not split stack
 	raceignore   int8 // ignore race detection events
 	m            *m   // for debuggers, but offset not hard-coded