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, 12 insertions, 7 deletions

diff --git a/rts/Task.c b/rts/Task.c
index a5de804418..e77a030f39 100644
--- a/rts/Task.c
+++ b/rts/Task.c
@@ -318,25 +318,30 @@ void
 taskTimeStamp (Task *task USED_IF_THREADS)
 {
 #if defined(THREADED_RTS)
-    Ticks currentElapsedTime, currentUserTime, elapsedGCTime;
+    Ticks currentElapsedTime, currentUserTime;
 
     currentUserTime = getThreadCPUTime();
     currentElapsedTime = getProcessElapsedTime();
 
-    // XXX this is wrong; we want elapsed GC time since the
-    // Task started.
-    elapsedGCTime = stat_getElapsedGCTime();
-    
-    task->mut_time = 
+    task->mut_time =
 	currentUserTime - task->muttimestart - task->gc_time;
     task->mut_etime = 
-	currentElapsedTime - task->elapsedtimestart - elapsedGCTime;
+        currentElapsedTime - task->elapsedtimestart - task->gc_etime;
 
+    if (task->gc_time   < 0) { task->gc_time   = 0; }
+    if (task->gc_etime  < 0) { task->gc_etime  = 0; }
     if (task->mut_time  < 0) { task->mut_time  = 0; }
     if (task->mut_etime < 0) { task->mut_etime = 0; }
 #endif
 }
 
+void
+taskDoneGC (Task *task, Ticks cpu_time, Ticks elapsed_time)
+{
+    task->gc_time  += cpu_time;
+    task->gc_etime += elapsed_time;
+}
+
 #if defined(THREADED_RTS)
 
 void