Fixing assumption in arenaread that we are in a trace and the segment is grey.

Copied from Perforce
 Change: 179403
 ServerID: perforce.ravenbrook.com

1 files changed, 8 insertions, 2 deletions

diff --git a/mps/code/global.c b/mps/code/global.c
index d70f9eb8e2b..0f16c5e4475 100644
--- a/mps/code/global.c
+++ b/mps/code/global.c
@@ -940,8 +940,14 @@ Ref ArenaRead(Arena arena, Ref *p)
 
   /* .read.conservative: Scan according to rank phase-of-trace, */
   /* See <code/trace.c#scan.conservative> */
-  rank = TraceRankForAccess(arena, seg);
-  TraceScanSingleRef(arena->flippedTraces, rank, arena, seg, p);
+  /* If the segment isn't grey it doesn't need scanning, and in fact it
+     would be wrong to even ask what rank to scan it at, since there might
+     not be any traces running. */
+  if (TraceSetInter(SegGrey(seg), arena->flippedTraces) != TraceSetEMPTY) {
+    rank = TraceRankForAccess(arena, seg);
+    TraceScanSingleRef(arena->flippedTraces, rank, arena, seg, p);
+  }
+
   /* We don't need to update the Seg Summary as in PoolSingleAccess
    * because we are not changing it after it has been scanned. */