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[dev.garbage] runtime: Coarsen the write barrier to always grey the destination.
LGTM=rsc R=rsc CC=golang-codereviews https://golang.org/cl/174820043
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@ -1056,13 +1056,41 @@ shade(byte *b)
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return;
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}
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// This is the Dijkstra barrier coarsened to shade grey to white whereas
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// the original Dijkstra barrier only shaded black to white.
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// This is the Dijkstra barrier coarsened to always shade the ptr (dst) object.
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// The original Dijkstra barrier only shaded ptrs being placed in black slots.
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//
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// Shade indicates that it has seen a white pointer by adding the referent
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// to wbuf.
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// to wbuf as well as marking it.
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//
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// slot is the destination (dst) in go code
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// ptr is the value that goes into the slot (src) in the go code
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//
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// Dijkstra pointed out that maintaining the no black to white
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// pointers means that white to white pointers not need
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// to be noted by the write barrier. Furthermore if either
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// white object dies before it is reached by the
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// GC then the object can be collected during this GC cycle
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// instead of waiting for the next cycle. Unfortunately the cost of
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// ensure that the object holding the slot doesn't concurrently
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// change to black without the mutator noticing seems prohibitive.
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//
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// Consider the following example where the mutator writes into
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// a slot and then loads the slot's mark bit while the GC thread
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// writes to the slot's mark bit and then as part of scanning reads
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// the slot.
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//
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// Initially both [slot] and [slotmark] are 0 (nil)
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// Mutator thread GC thread
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// st [slot], ptr st [slotmark], 1
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//
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// ld r1, [slotmark] ld r2, [slot]
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//
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// This is a classic example of independent reads of independent writes,
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// aka IRIW. The question is if r1==r2==0 is allowed and for most HW the
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// answer is yes without inserting a memory barriers between the st and the ld.
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// These barriers are expensive so we have decided that we will
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// always grey the ptr object regardless of the slot's color.
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//
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void
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runtime·gcmarkwb_m()
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{
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@ -1081,11 +1109,11 @@ runtime·gcmarkwb_m()
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case GCscan:
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break;
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case GCmark:
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if(ptr != nil && inheap(ptr) && shaded((byte*)slot))
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if(ptr != nil && inheap(ptr))
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shade(ptr);
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break;
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case GCmarktermination:
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if(ptr != nil && inheap(ptr) && shaded((byte*)slot))
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if(ptr != nil && inheap(ptr))
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shade(ptr);
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break;
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}
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