runtime: trivial clean ups to greyobject

Previously, the typeDead check in greyobject was under a separate
!useCheckmark conditional.  Put it with the rest of the !useCheckmark
code.  Also move a comment about atomic update of the marked bit to
where we actually do that update now.

Change-Id: Ief5f16401a25739ad57d959607b8d81ffe0bc211
Reviewed-on: https://go-review.googlesource.com/6271Reviewed-by: Rick Hudson <rlh@golang.org>

src/runtime/mbitmap.go

@@ -234,6 +234,10 @@ func (h heapBits) isMarked() bool {
 
 // setMarked sets the marked bit in the heap bits, atomically.
 func (h heapBits) setMarked() {
+	// Each byte of GC bitmap holds info for two words.
+	// Might be racing with other updates, so use atomic update always.
+	// We used to be clever here and use a non-atomic update in certain
+	// cases, but it's not worth the risk.
 	atomicor8(h.bitp, bitMarked<<h.shift)
 }
 

src/runtime/mgcmark.go

@@ -577,15 +577,13 @@ func greyobject(obj, base, off uintptr, hbits heapBits, gcw *gcWorkProducer) {
 			return
 		}
 
-		// Each byte of GC bitmap holds info for two words.
-		// Might be racing with other updates, so use atomic update always.
-		// We used to be clever here and use a non-atomic update in certain
-		// cases, but it's not worth the risk.
 		hbits.setMarked()
-	}
 
-	if !useCheckmark && hbits.typeBits() == typeDead {
-		return // noscan object
+		// If this is a noscan object, fast-track it to black
+		// instead of greying it.
+		if hbits.typeBits() == typeDead {
+			return
+		}
 	}
 
 	// Queue the obj for scanning. The PREFETCH(obj) logic has been removed but