runtime: correctly account for allocated objects in allocfree trace

The current implementation doesn't handle s.freeindex at all, which
means it'll skip a whole bunch of recently-made allocations, because the
span may have not been swept yet.

Change-Id: I8c5e360f5927ffe7e9abb448b352a59875e31b02
Reviewed-on: https://go-review.googlesource.com/c/go/+/586996
LUCI-TryBot-Result: Go LUCI <[email protected]>
Auto-Submit: Michael Knyszek <[email protected]>
Reviewed-by: Michael Pratt <[email protected]>

Author: mknyszek

src/runtime/traceallocfree.go

@@ -45,11 +45,10 @@ func traceSnapshotMemory() {
 		// Find all allocated objects.
 		abits := s.allocBitsForIndex(0)
 		for i := uintptr(0); i < uintptr(s.nelems); i++ {
-			if !abits.isMarked() {
-				continue
+			if abits.index < uintptr(s.freeindex) || abits.isMarked() {
+				x := s.base() + i*s.elemsize
+				trace.HeapObjectExists(x, s.typePointersOfUnchecked(x).typ)
 			}
-			x := s.base() + i*s.elemsize
-			trace.HeapObjectExists(x, s.typePointersOfUnchecked(x).typ)
 			abits.advance()
 		}
 	}