Refactoring and tidy up

This is a port of some of the changes from my private local-GC branch
(which is still in darcs, I haven't converted it to git yet).  There
are a couple of small functional differences in the GC stats: first,
per-thread GC timings should now be more accurate, and secondly we now
report average and maximum pause times. e.g. from minimax +RTS -N8 -s:

                                    Tot time (elapsed)  Avg pause  Max pause
  Gen  0      2755 colls,  2754 par   13.16s    0.93s     0.0003s    0.0150s
  Gen  1       769 colls,   769 par    3.71s    0.26s     0.0003s    0.0059s

1 files changed, 4 insertions, 9 deletions

diff --git a/rts/STM.c b/rts/STM.c
index 3de42e2090..e8d3fc0e0b 100644
--- a/rts/STM.c
+++ b/rts/STM.c
@@ -879,17 +879,12 @@ static StgBool check_read_only(StgTRecHeader *trec STG_UNUSED) {
 
 /************************************************************************/
 
-void stmPreGCHook() {
-  nat i;
-
+void stmPreGCHook (Capability *cap) {
   lock_stm(NO_TREC);
   TRACE("stmPreGCHook");
-  for (i = 0; i < n_capabilities; i ++) {
-    Capability *cap = &capabilities[i];
-    cap -> free_tvar_watch_queues = END_STM_WATCH_QUEUE;
-    cap -> free_trec_chunks = END_STM_CHUNK_LIST;
-    cap -> free_trec_headers = NO_TREC;
-  }
+  cap->free_tvar_watch_queues = END_STM_WATCH_QUEUE;
+  cap->free_trec_chunks = END_STM_CHUNK_LIST;
+  cap->free_trec_headers = NO_TREC;
   unlock_stm(NO_TREC);
 }