diff --git a/src/runtime/mgcmark.go b/src/runtime/mgcmark.go
index 5d3a9efbb3..4a3cc4d769 100644
--- a/src/runtime/mgcmark.go
+++ b/src/runtime/mgcmark.go
@@ -522,45 +522,16 @@ func greyobject(obj, base, off uintptr, hbits heapBits, gcw *gcWorkProducer) {
 
 	if useCheckmark {
 		if !hbits.isMarked() {
+			printlock()
 			print("runtime:greyobject: checkmarks finds unexpected unmarked object obj=", hex(obj), "\n")
 			print("runtime: found obj at *(", hex(base), "+", hex(off), ")\n")
 
 			// Dump the source (base) object
-
-			kb := base >> _PageShift
-			xb := kb
-			xb -= mheap_.arena_start >> _PageShift
-			sb := h_spans[xb]
-			printlock()
-			print("runtime:greyobject Span: base=", hex(base), " kb=", hex(kb))
-			if sb == nil {
-				print(" sb=nil\n")
-			} else {
-				print(" sb.start*_PageSize=", hex(sb.start*_PageSize), " sb.limit=", hex(sb.limit), " sb.sizeclass=", sb.sizeclass, " sb.elemsize=", sb.elemsize, "\n")
-				// base is (a pointer to) the source object holding the reference to object. Create a pointer to each of the fields
-				// fields in base and print them out as hex values.
-				for i := 0; i < int(sb.elemsize/ptrSize); i++ {
-					print(" *(base+", i*ptrSize, ") = ", hex(*(*uintptr)(unsafe.Pointer(base + uintptr(i)*ptrSize))), "\n")
-				}
-			}
+			gcDumpObject("base", base, off)
 
 			// Dump the object
+			gcDumpObject("obj", obj, ^uintptr(0))
 
-			k := obj >> _PageShift
-			x := k
-			x -= mheap_.arena_start >> _PageShift
-			s := h_spans[x]
-			print("runtime:greyobject Span: obj=", hex(obj), " k=", hex(k))
-			if s == nil {
-				print(" s=nil\n")
-			} else {
-				print(" s.start=", hex(s.start*_PageSize), " s.limit=", hex(s.limit), " s.sizeclass=", s.sizeclass, " s.elemsize=", s.elemsize, "\n")
-				// NOTE(rsc): This code is using s.sizeclass as an approximation of the
-				// number of pointer-sized words in an object. Perhaps not what was intended.
-				for i := 0; i < int(s.sizeclass); i++ {
-					print(" *(obj+", i*ptrSize, ") = ", hex(*(*uintptr)(unsafe.Pointer(obj + uintptr(i)*ptrSize))), "\n")
-				}
-			}
 			throw("checkmark found unmarked object")
 		}
 		if !hbits.isCheckmarked() {
@@ -595,6 +566,28 @@ func greyobject(obj, base, off uintptr, hbits heapBits, gcw *gcWorkProducer) {
 	gcw.put(obj)
 }
 
+// gcDumpObject dumps the contents of obj for debugging and marks the
+// field at byte offset off in obj.
+func gcDumpObject(label string, obj, off uintptr) {
+	k := obj >> _PageShift
+	x := k
+	x -= mheap_.arena_start >> _PageShift
+	s := h_spans[x]
+	print(label, "=", hex(obj), " k=", hex(k))
+	if s == nil {
+		print(" s=nil\n")
+		return
+	}
+	print(" s.start*_PageSize=", hex(s.start*_PageSize), " s.limit=", hex(s.limit), " s.sizeclass=", s.sizeclass, " s.elemsize=", s.elemsize, "\n")
+	for i := uintptr(0); i < s.elemsize; i += ptrSize {
+		print(" *(", label, "+", i, ") = ", hex(*(*uintptr)(unsafe.Pointer(obj + uintptr(i)))))
+		if i == off {
+			print(" <==")
+		}
+		print("\n")
+	}
+}
+
 // When in GCmarkterminate phase we allocate black.
 //go:nowritebarrier
 func gcmarknewobject_m(obj uintptr) {