runtime: simplify/optimize allocate-black a bit

Currently allocating black switches to the system stack (which is
probably a historical accident) and atomically updates the global
bytes marked stat. Since we're about to depend on this much more,
optimize it a bit by putting it back on the regular stack and updating
the per-P bytes marked stat, which gets lazily folded into the global
bytes marked stat.

Change-Id: Ibbe16e5382d3fd2256e4381f88af342bf7020b04
Reviewed-on: https://go-review.googlesource.com/22170Reviewed-by: Rick Hudson <rlh@golang.org>
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/runtime/malloc.go

@@ -695,9 +695,7 @@ func mallocgc(size uintptr, typ *_type, needzero bool) unsafe.Pointer {
 	// This may be racing with GC so do it atomically if there can be
 	// a race marking the bit.
 	if gcphase == _GCmarktermination || gcBlackenPromptly {
-		systemstack(func() {
-			gcmarknewobject_m(uintptr(x), size, scanSize)
-		})
+		gcmarknewobject(uintptr(x), size, scanSize)
 	}
 
 	if raceenabled {

src/runtime/mgcmark.go

@@ -1132,15 +1132,20 @@ func gcDumpObject(label string, obj, off uintptr) {
 	}
 }
 
-// If gcBlackenPromptly is true we are in the second mark phase phase so we allocate black.
+// gcmarknewobject marks a newly allocated object black. obj must
+// not contain any non-nil pointers.
+//
+// This is nosplit so it can manipulate a gcWork without preemption.
+//
 //go:nowritebarrier
-func gcmarknewobject_m(obj, size, scanSize uintptr) {
+//go:nosplit
+func gcmarknewobject(obj, size, scanSize uintptr) {
 	if useCheckmark && !gcBlackenPromptly { // The world should be stopped so this should not happen.
 		throw("gcmarknewobject called while doing checkmark")
 	}
 	heapBitsForAddr(obj).setMarked()
-	atomic.Xadd64(&work.bytesMarked, int64(size))
 	gcw := &getg().m.p.ptr().gcw
+	gcw.bytesMarked += uint64(size)
 	gcw.scanWork += int64(scanSize)
 }