runtime: perform publication barrier even for noscan objects

Currently we only execute a publication barrier for scan objects (and
skip it for noscan objects). This used to be okay because GC would
never consult the object itself (so it wouldn't observe uninitialized
memory even if it found a pointer to a noscan object), and the heap
bitmap was pre-initialized to noscan.

However, now we explicitly initialize the heap bitmap for noscan
objects when we allocate them. While the GC will still never consult
the contents of a noscan object, it does need to see the initialized
heap bitmap. Hence, we need to execute a publication barrier to make
the bitmap visible before user code can expose a pointer to the newly
allocated object even for noscan objects.

Change-Id: Ie4133c638db0d9055b4f7a8061a634d970627153
Reviewed-on: https://go-review.googlesource.com/23043
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rick Hudson <rlh@golang.org>

src/runtime/malloc.go

@@ -699,16 +699,16 @@ func mallocgc(size uintptr, typ *_type, needzero bool) unsafe.Pointer {
 			scanSize = typ.ptrdata
 		}
 		c.local_scan += scanSize
-
-		// Ensure that the stores above that initialize x to
-		// type-safe memory and set the heap bits occur before
-		// the caller can make x observable to the garbage
-		// collector. Otherwise, on weakly ordered machines,
-		// the garbage collector could follow a pointer to x,
-		// but see uninitialized memory or stale heap bits.
-		publicationBarrier()
 	}
 
+	// Ensure that the stores above that initialize x to
+	// type-safe memory and set the heap bits occur before
+	// the caller can make x observable to the garbage
+	// collector. Otherwise, on weakly ordered machines,
+	// the garbage collector could follow a pointer to x,
+	// but see uninitialized memory or stale heap bits.
+	publicationBarrier()
+
 	// Allocate black during GC.
 	// All slots hold nil so no scanning is needed.
 	// This may be racing with GC so do it atomically if there can be