rts: TraverseHeap: Simplify profiling header

Having a union in the closure profiling header really just complicates
things so get back to basics, we just have a single StgWord there for now.

5 files changed, 18 insertions, 27 deletions

diff --git a/includes/rts/storage/Closures.h b/includes/rts/storage/Closures.h
index 0fff039cc6..ebb836bca2 100644
--- a/includes/rts/storage/Closures.h
+++ b/includes/rts/storage/Closures.h
@@ -20,22 +20,13 @@
 typedef struct {
   CostCentreStack *ccs;
   union {
-
-    union {
-      /* Accessor for the least significant bit of the entire union. Invariant:
-       * This must be at least as large as the largest field in this union for
-       * this to work. If you add more fields make sure you maintain this.
-       *
-       * See Note [Profiling heap traversal visited bit].
-       */
-      StgWord lsb;
-
-      /* Retainer Set */
-      struct _RetainerSet *rs;
-    } trav;
-
+    StgWord trav;             /* Heap traversal */
     StgWord ldvw;             /* Lag/Drag/Void Word */
   } hp;
+    // Heap profiling header. This field is shared among the various heap
+    // profiling modes. Currently it is used by ProfHeap.c for Lag/Drag/Void
+    // profiling and by the heap traversal modes using TraverseHeap.c such as
+    // the retainer profiler.
 } StgProfHeader;
 
 /* -----------------------------------------------------------------------------
diff --git a/rts/RetainerProfile.c b/rts/RetainerProfile.c
index b89519b05d..a996e8d2e2 100644
--- a/rts/RetainerProfile.c
+++ b/rts/RetainerProfile.c
@@ -251,7 +251,7 @@ associate( StgClosure *c, RetainerSet *s )
 {
     // StgWord has the same size as pointers, so the following type
     // casting is okay.
-    RSET(c) = (RetainerSet *)((StgWord)s | flip);
+    RSET(c) = ((StgWord)s | flip);
 }
 
 static bool
diff --git a/rts/RetainerProfile.h b/rts/RetainerProfile.h
index ba0161c98d..e7a280fa5d 100644
--- a/rts/RetainerProfile.h
+++ b/rts/RetainerProfile.h
@@ -21,7 +21,7 @@ void endRetainerProfiling  ( void );
 void retainerProfile       ( void );
 
 // extract the retainer set field from c
-#define RSET(c)   ((c)->header.prof.hp.trav.rs)
+#define RSET(c)   ((c)->header.prof.hp.trav)
 
 static inline RetainerSet *
 retainerSetOf( const StgClosure *c )
diff --git a/rts/TraverseHeap.c b/rts/TraverseHeap.c
index 55334e4104..4873f27c8b 100644
--- a/rts/TraverseHeap.c
+++ b/rts/TraverseHeap.c
@@ -19,18 +19,18 @@
 /** Note [Profiling heap traversal visited bit]
  *
  * If the RTS is compiled with profiling enabled StgProfHeader can be used by
- * profiling code to store per-heap object information.
+ * profiling code to store per-heap object information. Specifically the
+ * 'hp_hdr' field is used to store heap profiling information.
  *
  * The generic heap traversal code reserves the least significant bit of the
- * largest members of the 'trav' union to decide whether we've already visited a
- * given closure in the current pass or not. The rest of the field is free to be
- * used by the calling profiler.
+ * heap profiling word to decide whether we've already visited a given closure
+ * in the current pass or not. The rest of the field is free to be used by the
+ * calling profiler.
  *
- * By doing things this way we implicitly assume that the LSB of the largest
- * field in the 'trav' union is insignificant. This is true at least for the
- * word aligned pointers which the retainer profiler currently stores there and
- * should be maintained by new users of the 'trav' union for example by shifting
- * the real data up by one bit.
+ * By doing things this way we implicitly assume that the LSB is not used by the
+ * user. This is true at least for the word aligned pointers which the retainer
+ * profiler currently stores there and should be maintained by new users for
+ * example by shifting the real data up by one bit.
  *
  * Since we don't want to have to scan the entire heap a second time just to
  * reset the per-object visitied bit before/after the real traversal we make the
@@ -49,7 +49,7 @@
 StgWord flip = 0;
 
 #define setTravDataToZero(c) \
-  (c)->header.prof.hp.trav.lsb = flip
+  (c)->header.prof.hp.trav = flip
 
 typedef enum {
     // Object with fixed layout. Keeps an information about that
diff --git a/rts/TraverseHeap.h b/rts/TraverseHeap.h
index a06a7d2f18..cc217a8858 100644
--- a/rts/TraverseHeap.h
+++ b/rts/TraverseHeap.h
@@ -22,7 +22,7 @@ void resetStaticObjectForProfiling(StgClosure *static_objects);
 extern StgWord flip;
 
 #define isTravDataValid(c) \
-  ((((StgWord)(c)->header.prof.hp.trav.lsb & 1) ^ flip) == 0)
+  ((((StgWord)(c)->header.prof.hp.trav & 1) ^ flip) == 0)
 
 typedef struct traverseState_ traverseState;