[dev.garbage] runtime: determine if an object is public

ROC (request oriented collector) needs to determine
if an object is visible to other goroutines, i.e.
public. In a later CL this will be used by the write
barrier and the publishing logic to distinguish between
local and public objects and act accordingly.

Change-Id: I6a80da9deb21f57e831a2ec04e41477f997a8c33
Reviewed-on: https://go-review.googlesource.com/25056
Reviewed-by: Austin Clements <austin@google.com>

src/runtime/mbitmap.go

@@ -189,6 +189,90 @@ type markBits struct {
 	index uintptr
 }
 
+//go:nosplit
+func inBss(p uintptr) bool {
+	for datap := &firstmoduledata; datap != nil; datap = datap.next {
+		if p >= datap.bss && p < datap.ebss {
+			return true
+		}
+	}
+	return false
+}
+
+//go:nosplit
+func inData(p uintptr) bool {
+	for datap := &firstmoduledata; datap != nil; datap = datap.next {
+		if p >= datap.data && p < datap.edata {
+			return true
+		}
+	}
+	return false
+}
+
+// isPublic checks whether the object has been published.
+// ptr may not point to the start of the object.
+// This is conservative in the sense that it will return true
+// for any object that hasn't been allocated by this
+// goroutine since the last roc checkpoint was performed.
+// Must run on the system stack to prevent stack growth and
+// moving of goroutine stack.
+//go:systemstack
+func isPublic(ptr uintptr) bool {
+	if debug.gcroc == 0 {
+		// Unexpected call to ROC specific routine while not running ROC.
+		// blowup without supressing inlining.
+		_ = *(*int)(nil)
+	}
+
+	if inStack(ptr, getg().stack) {
+		return false
+	}
+
+	if getg().m != nil && getg().m.curg != nil && inStack(ptr, getg().m.curg.stack) {
+		return false
+	}
+
+	if inBss(ptr) {
+		return true
+	}
+	if inData(ptr) {
+		return true
+	}
+	if !inheap(ptr) {
+		// Note: Objects created using persistentalloc are not in the heap
+		// so any pointers from such object to local objects needs to be dealt
+		// with specially. nil is also considered not in the heap.
+		return true
+	}
+	// At this point we know the object is in the heap.
+	s := spanOf(ptr)
+	oldSweepgen := atomic.Load(&s.sweepgen)
+	sg := mheap_.sweepgen
+	if oldSweepgen != sg {
+		// We have an unswept span which means that the pointer points to a public object since it will
+		// be found to be marked once it is swept.
+		return true
+	}
+	abits := s.allocBitsForAddr(ptr)
+	if abits.isMarked() {
+		return true
+	} else if s.freeindex <= abits.index {
+		// Unmarked and beyond freeindex yet reachable object encountered.
+		// blowup without supressing inlining.
+		_ = *(*int)(nil)
+	}
+
+	// The object is not marked. If it is part of the current
+	// ROC epoch then it is not public.
+	if s.startindex*s.elemsize <= ptr-s.base() {
+		// Object allocated in this ROC epoch and since it is
+		// not marked it has not been published.
+		return false
+	}
+	// Object allocated since last GC but in a previous ROC epoch so it is public.
+	return true
+}
+
 //go:nosplit
 func (s *mspan) allocBitsForIndex(allocBitIndex uintptr) markBits {
 	whichByte := allocBitIndex / 8
@@ -197,6 +281,16 @@ func (s *mspan) allocBitsForIndex(allocBitIndex uintptr) markBits {
 	return markBits{bytePtr, uint8(1 << whichBit), allocBitIndex}
 }
 
+//go:nosplit
+func (s *mspan) allocBitsForAddr(p uintptr) markBits {
+	byteOffset := p - s.base()
+	allocBitIndex := byteOffset / s.elemsize
+	whichByte := allocBitIndex / 8
+	whichBit := allocBitIndex % 8
+	bytePtr := addb(s.allocBits, whichByte)
+	return markBits{bytePtr, uint8(1 << whichBit), allocBitIndex}
+}
+
 // refillaCache takes 8 bytes s.allocBits starting at whichByte
 // and negates them so that ctz (count trailing zeros) instructions
 // can be used. It then places these 8 bytes into the cached 64 bit

src/runtime/mheap.go

@@ -195,6 +195,13 @@ type mspan struct {
 	// helps performance.
 	nelems uintptr // number of object in the span.
 
+	// startindex is the object index where the owner G started allocating in this span.
+	//
+	// This is used in conjunction with nextUsedSpan to implement ROC checkpoints and recycles.
+	startindex uintptr
+	// nextUsedSpan links together all spans that have the same span class and owner G.
+	nextUsedSpan *mspan
+
 	// Cache of the allocBits at freeindex. allocCache is shifted
 	// such that the lowest bit corresponds to the bit freeindex.
 	// allocCache holds the complement of allocBits, thus allowing

src/runtime/stack.go

@@ -1225,3 +1225,8 @@ func morestackc() {
 		throw("attempt to execute C code on Go stack")
 	})
 }
+
+//go:nosplit
+func inStack(p uintptr, s stack) bool {
+	return s.lo <= p && p < s.hi
+}