runtime: enable precise GC checks when using stack maps

In the runtime there are bad pointer checks that currently don't
    work with the concervative collector. With stack maps, the GC is
    precise and the checks should work. Enable them.
    
    Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/153871

From-SVN: r269406

gcc/go/gofrontend/MERGE

@@ -1,4 +1,4 @@
-14e48e756af205a68374c872f3bd03d62ccd70bb
+3f8ddaa1d773309b6a4b8e4640f8b9675d9764c6
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

libgo/go/runtime/mgcmark.go

@@ -1106,9 +1106,9 @@ func scanstackblockwithmap(pc, b0, n0 uintptr, ptrmask *uint8, gcw *gcWork) {
 // Preemption must be disabled.
 //go:nowritebarrier
 func shade(b uintptr) {
-	if obj, span, objIndex := findObject(b, 0, 0, true); obj != 0 {
+	if obj, span, objIndex := findObject(b, 0, 0, !usestackmaps); obj != 0 {
 		gcw := &getg().m.p.ptr().gcw
-		greyobject(obj, 0, 0, span, gcw, objIndex, true)
+		greyobject(obj, 0, 0, span, gcw, objIndex, !usestackmaps)
 	}
 }
 

libgo/go/runtime/mgcsweep.go

@@ -342,8 +342,10 @@ func (s *mspan) sweep(preserve bool) bool {
 	// it is not otherwise a problem. So we disable the test for gccgo.
 	nfreedSigned := int(nfreed)
 	if nalloc > s.allocCount {
-		// print("runtime: nelems=", s.nelems, " nalloc=", nalloc, " previous allocCount=", s.allocCount, " nfreed=", nfreed, "\n")
-		// throw("sweep increased allocation count")
+		if usestackmaps {
+			print("runtime: nelems=", s.nelems, " nalloc=", nalloc, " previous allocCount=", s.allocCount, " nfreed=", nfreed, "\n")
+			throw("sweep increased allocation count")
+		}
 
 		// For gccgo, adjust the freed count as a signed number.
 		nfreedSigned = int(s.allocCount) - int(nalloc)

libgo/go/runtime/runtime1.go

@@ -362,7 +362,9 @@ func parsedebugvars() {
 	// At that point, if debug.invalidptr is set, we crash.
 	// This is not a problem, assuming that M1 really is dead and
 	// the pointer we discovered to it will not be used.
-	// debug.invalidptr = 1
+	if usestackmaps {
+		debug.invalidptr = 1
+	}
 
 	for p := gogetenv("GODEBUG"); p != ""; {
 		field := ""