runtime: make work.bytesMarked 8-byte aligned

Make atomic access on 32-bit architectures happy.

Updates #17786.

Change-Id: I42de63ff1381af42124dc51befc887160f71797d
Reviewed-on: https://go-review.googlesource.com/33235
Run-TryBot: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Austin Clements <austin@google.com>

src/runtime/mgc.go

@@ -781,6 +781,22 @@ var work struct {
 	empty uint64                   // lock-free list of empty blocks workbuf
 	pad0  [sys.CacheLineSize]uint8 // prevents false-sharing between full/empty and nproc/nwait
 
+	// bytesMarked is the number of bytes marked this cycle. This
+	// includes bytes blackened in scanned objects, noscan objects
+	// that go straight to black, and permagrey objects scanned by
+	// markroot during the concurrent scan phase. This is updated
+	// atomically during the cycle. Updates may be batched
+	// arbitrarily, since the value is only read at the end of the
+	// cycle.
+	//
+	// Because of benign races during marking, this number may not
+	// be the exact number of marked bytes, but it should be very
+	// close.
+	//
+	// Put this field here because it needs 64-bit atomic access
+	// (and thus 8-byte alignment even on 32-bit architectures).
+	bytesMarked uint64
+
 	markrootNext uint32 // next markroot job
 	markrootJobs uint32 // number of markroot jobs
 
@@ -842,19 +858,6 @@ var work struct {
 	// program started if debug.gctrace > 0.
 	totaltime int64
 
-	// bytesMarked is the number of bytes marked this cycle. This
-	// includes bytes blackened in scanned objects, noscan objects
-	// that go straight to black, and permagrey objects scanned by
-	// markroot during the concurrent scan phase. This is updated
-	// atomically during the cycle. Updates may be batched
-	// arbitrarily, since the value is only read at the end of the
-	// cycle.
-	//
-	// Because of benign races during marking, this number may not
-	// be the exact number of marked bytes, but it should be very
-	// close.
-	bytesMarked uint64
-
 	// initialHeapLive is the value of memstats.heap_live at the
 	// beginning of this GC cycle.
 	initialHeapLive uint64