return;
}
-// This is the Dijkstra barrier coarsened to shade grey to white whereas
-// the original Dijkstra barrier only shaded black to white.
+// This is the Dijkstra barrier coarsened to always shade the ptr (dst) object.
+// The original Dijkstra barrier only shaded ptrs being placed in black slots.
//
// Shade indicates that it has seen a white pointer by adding the referent
-// to wbuf.
+// to wbuf as well as marking it.
+//
// slot is the destination (dst) in go code
// ptr is the value that goes into the slot (src) in the go code
+//
+// Dijkstra pointed out that maintaining the no black to white
+// pointers means that white to white pointers not need
+// to be noted by the write barrier. Furthermore if either
+// white object dies before it is reached by the
+// GC then the object can be collected during this GC cycle
+// instead of waiting for the next cycle. Unfortunately the cost of
+// ensure that the object holding the slot doesn't concurrently
+// change to black without the mutator noticing seems prohibitive.
+//
+// Consider the following example where the mutator writes into
+// a slot and then loads the slot's mark bit while the GC thread
+// writes to the slot's mark bit and then as part of scanning reads
+// the slot.
+//
+// Initially both [slot] and [slotmark] are 0 (nil)
+// Mutator thread GC thread
+// st [slot], ptr st [slotmark], 1
+//
+// ld r1, [slotmark] ld r2, [slot]
+//
+// This is a classic example of independent reads of independent writes,
+// aka IRIW. The question is if r1==r2==0 is allowed and for most HW the
+// answer is yes without inserting a memory barriers between the st and the ld.
+// These barriers are expensive so we have decided that we will
+// always grey the ptr object regardless of the slot's color.
+//
void
runtime·gcmarkwb_m()
{
case GCscan:
break;
case GCmark:
- if(ptr != nil && inheap(ptr) && shaded((byte*)slot))
+ if(ptr != nil && inheap(ptr))
shade(ptr);
break;
case GCmarktermination:
- if(ptr != nil && inheap(ptr) && shaded((byte*)slot))
+ if(ptr != nil && inheap(ptr))
shade(ptr);
break;
}