Copyright 2020 The Go Authors. All rights reserved. Use of this source code is governed by a BSD-style license that can be found in the LICENSE file.
GC checkmarks In a concurrent garbage collector, one worries about failing to mark a live object due to mutations without write barriers or bugs in the collector implementation. As a sanity check, the GC has a 'checkmark' mode that retraverses the object graph with the world stopped, to make sure that everything that should be marked is marked.

package runtime

import (
	
	
	
)
A checkmarksMap stores the GC marks in "checkmarks" mode. It is a per-arena bitmap with a bit for every word in the arena. The mark is stored on the bit corresponding to the first word of the marked allocation.go:notinheap
type checkmarksMap [heapArenaBytes / sys.PtrSize / 8]uint8
If useCheckmark is true, marking of an object uses the checkmark bits instead of the standard mark bits.
var useCheckmark = false
startCheckmarks prepares for the checkmarks phase. The world must be stopped.
func () {
	assertWorldStopped()
Clear all checkmarks.
	for ,  := range mheap_.allArenas {
		 := mheap_.arenas[.l1()][.l2()]
		 := .checkmarks
Allocate bitmap on first use.
			 = (*checkmarksMap)(persistentalloc(unsafe.Sizeof(*), 0, &memstats.gcMiscSys))
			if  == nil {
				throw("out of memory allocating checkmarks bitmap")
			}
			.checkmarks =
Otherwise clear the existing bitmap.
			for  := range  {
				[] = 0
			}
		}
Enable checkmarking.
	useCheckmark = true
}
endCheckmarks ends the checkmarks phase.
func () {
	if gcMarkWorkAvailable(nil) {
		throw("GC work not flushed")
	}
	useCheckmark = false
}
setCheckmark throws if marking object is a checkmarks violation, and otherwise sets obj's checkmark. It returns true if obj was already checkmarked.
func (, ,  uintptr,  markBits) bool {
	if !.isMarked() {
		printlock()
		print("runtime: checkmarks found unexpected unmarked object obj=", hex(), "\n")
		print("runtime: found obj at *(", hex(), "+", hex(), ")\n")
Dump the source (base) object
		gcDumpObject("base", , )
Dump the object
		gcDumpObject("obj", , ^uintptr(0))

		getg().m.traceback = 2
		throw("checkmark found unmarked object")
	}

	 := arenaIndex()
	 := mheap_.arenas[.l1()][.l2()]
	 := ( / heapArenaBytes / 8) % uintptr(len(.checkmarks))
	 := byte(1 << (( / heapArenaBytes) % 8))
	 := &.checkmarks[]
Already checkmarked.
		return true
	}

	atomic.Or8(, )
	return false