runtime: eliminate heapBits.hasPointers

This is no longer necessary now that we can more efficiently consult
the span's noscan bit.

This is a cherry-pick of dev.garbage commit 312aa09996.

Change-Id: Id0b00b278533660973f45eb6efa5b00f373d58af
Reviewed-on: https://go-review.googlesource.com/41252
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rick Hudson <rlh@golang.org>

src/runtime/mbitmap.go

@@ -495,17 +495,6 @@ func (h heapBits) isPointer() bool {
 	return h.bits()&bitPointer != 0
 }
 
-// hasPointers reports whether the given object has any pointers.
-// It must be told how large the object at h is for efficiency.
-// h must describe the initial word of the object.
-func (h heapBits) hasPointers(size uintptr) bool {
-	// TODO: Use span.noScan instead of the heap bitmap.
-	if size == sys.PtrSize { // 1-word objects are always pointers
-		return true
-	}
-	return (*h.bitp>>h.shift)&bitScan != 0
-}
-
 // isCheckmarked reports whether the heap bits have the checkmarked bit set.
 // It must be told how large the object at h is, because the encoding of the
 // checkmark bit varies by size.

src/runtime/mgcmark.go

@@ -1127,7 +1127,7 @@ func scanobject(b uintptr, gcw *gcWork) {
 			// paths), in which case we must *not* enqueue
 			// oblets since their bitmaps will be
 			// uninitialized.
-			if !hbits.hasPointers(n) {
+			if s.spanclass.noscan() {
 				// Bypass the whole scan.
 				gcw.bytesMarked += uint64(n)
 				return
@@ -1257,7 +1257,7 @@ func greyobject(obj, base, off uintptr, hbits heapBits, span *mspan, gcw *gcWork
 		atomic.Or8(mbits.bytep, mbits.mask)
 		// If this is a noscan object, fast-track it to black
 		// instead of greying it.
-		if !hbits.hasPointers(span.elemsize) {
+		if span.spanclass.noscan() {
 			gcw.bytesMarked += uint64(span.elemsize)
 			return
 		}