Refactoring of the GC in preparation for parallel GC

  
This patch localises the state of the GC into a gc_thread structure,
and reorganises the inner loop of the GC to scavenge one block at a
time from global work lists in each "step".  The gc_thread structure
has a "workspace" for each step, in which it collects evacuated
objects until it has a full block to push out to the step's global
list.  Details of the algorithm will be on the wiki in due course.

At the moment, THREADED_RTS does not compile, but the single-threaded
GC works (and is 10-20% slower than before).

1 files changed, 26 insertions, 2 deletions

diff --git a/rts/sm/GCUtils.h b/rts/sm/GCUtils.h
index 70dd7a882f..2b22407d31 100644
--- a/rts/sm/GCUtils.h
+++ b/rts/sm/GCUtils.h
@@ -11,5 +11,29 @@
  *
  * --------------------------------------------------------------------------*/
 
-bdescr *gc_alloc_block(step *stp);
-bdescr *gc_alloc_scavd_block(step *stp);
+#include "SMP.h"
+
+#ifdef THREADED_RTS
+extern SpinLock gc_alloc_block_sync;
+#endif
+
+bdescr *allocBlock_sync(void);
+
+void    push_scan_block      (bdescr *bd, step_workspace *ws);
+bdescr *grab_todo_block      (step_workspace *ws);
+bdescr *gc_alloc_todo_block  (step_workspace *ws);
+bdescr *gc_alloc_scavd_block (step_workspace *ws);
+
+// Returns true if a block is 3/4 full.  This predicate is used to try
+// to re-use partial blocks wherever possible, and to reduce wastage.
+// We might need to tweak the actual value.
+INLINE_HEADER rtsBool
+isPartiallyFull(bdescr *bd)
+{
+    return (bd->free + BLOCK_SIZE_W/4 < bd->start + BLOCK_SIZE_W);
+}
+
+
+#if DEBUG
+void printMutableList (generation *gen);
+#endif