pruneSparkQueue: check for tagged pointers

This was a bug in 6.12.3.  I think the problem no longer occurs due to
the way sparks are treated as weak pointers, but it doesn't hurt to
test for tagged pointers anyway: better to do the test than have a
subtle invariant.

1 files changed, 33 insertions, 22 deletions

diff --git a/rts/Sparks.c b/rts/Sparks.c
index 857921260b..ad08f3b577 100644
--- a/rts/Sparks.c
+++ b/rts/Sparks.c
@@ -197,31 +197,42 @@ pruneSparkQueue (Capability *cap)
       // We have to be careful here: in the parallel GC, another
       // thread might evacuate this closure while we're looking at it,
       // so grab the info pointer just once.
-      info = spark->header.info;
-      if (IS_FORWARDING_PTR(info)) {
-          tmp = (StgClosure*)UN_FORWARDING_PTR(info);
-          /* if valuable work: shift inside the pool */
-          if (closure_SHOULD_SPARK(tmp)) {
-              elements[botInd] = tmp; // keep entry (new address)
-              botInd++;
-              n++;
-          } else {
-              pruned_sparks++; // discard spark
-              cap->sparks_fizzled++;
-          }
-      } else if (HEAP_ALLOCED(spark) && 
-                 (Bdescr((P_)spark)->flags & BF_EVACUATED)) {
-          if (closure_SHOULD_SPARK(spark)) {
-              elements[botInd] = spark; // keep entry (new address)
-              botInd++;
-              n++;
+      if (GET_CLOSURE_TAG(spark) != 0) {
+          // Tagged pointer is a value, so the spark has fizzled.  It
+          // probably never happens that we get a tagged pointer in
+          // the spark pool, because we would have pruned the spark
+          // during the previous GC cycle if it turned out to be
+          // evaluated, but it doesn't hurt to have this check for
+          // robustness.
+          pruned_sparks++;
+          cap->sparks_fizzled++;
+      } else {
+          info = spark->header.info;
+          if (IS_FORWARDING_PTR(info)) {
+              tmp = (StgClosure*)UN_FORWARDING_PTR(info);
+              /* if valuable work: shift inside the pool */
+              if (closure_SHOULD_SPARK(tmp)) {
+                  elements[botInd] = tmp; // keep entry (new address)
+                  botInd++;
+                  n++;
+              } else {
+                  pruned_sparks++; // discard spark
+                  cap->sparks_fizzled++;
+              }
+          } else if (HEAP_ALLOCED(spark) &&
+                     (Bdescr((P_)spark)->flags & BF_EVACUATED)) {
+              if (closure_SHOULD_SPARK(spark)) {
+                  elements[botInd] = spark; // keep entry (new address)
+                  botInd++;
+                  n++;
+              } else {
+                  pruned_sparks++; // discard spark
+                  cap->sparks_fizzled++;
+              }
           } else {
               pruned_sparks++; // discard spark
-              cap->sparks_fizzled++;
+              cap->sparks_gcd++;
           }
-      } else {
-          pruned_sparks++; // discard spark
-          cap->sparks_gcd++;
       }
 
       currInd++;