runtime: mark heapBits.bits nosplit

heapBits.bits is used during bulkBarrierPreWrite via
heapBits.isPointer, which means it must not be preempted. If it is
preempted, several bad things can happen:

1. This could allow a GC phase change, and the resulting shear between
the barriers and the memory writes could result in a lost pointer.

2. Since bulkBarrierPreWrite uses the P's local write barrier buffer,
if it also migrates to a different P, it could try to append to the
write barrier buffer concurrently with another write barrier. This can
result in the buffer's next pointer skipping over its end pointer,
which results in a buffer overflow that can corrupt arbitrary other
fields in the Ps (or anything in the heap, really, but it'll probably
crash from the corrupted P quickly).

Fix this by marking heapBits.bits go:nosplit. This would be the
perfect use for a recursive no-preempt annotation (#21314).

This doesn't actually affect any binaries because this function was
always inlined anyway. (I discovered it when I was modifying heapBits
and make h.bits() no longer inline, which led to rampant crashes from
problem 2 above.)

Updates #22987 and #22988 (but doesn't fix because it doesn't actually
change the generated code).

Change-Id: I60ebb928b1233b0613361ac3d0558d7b1cb65610
Reviewed-on: https://go-review.googlesource.com/83015
Run-TryBot: Austin Clements <austin@google.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Rick Hudson <rlh@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

Author: aclements

src/runtime/mbitmap.go

@@ -475,6 +475,9 @@ func (h heapBits) forward(n uintptr) heapBits {
 // The caller can test morePointers and isPointer by &-ing with bitScan and bitPointer.
 // The result includes in its higher bits the bits for subsequent words
 // described by the same bitmap byte.
+//
+// nosplit because it is used during write barriers and must not be preempted.
+//go:nosplit
 func (h heapBits) bits() uint32 {
 	// The (shift & 31) eliminates a test and conditional branch
 	// from the generated code.