Pointer Tagging

  
This patch implements pointer tagging as per our ICFP'07 paper "Faster
laziness using dynamic pointer tagging".  It improves performance by
10-15% for most workloads, including GHC itself.

The original patches were by Alexey Rodriguez Yakushev
<mrchebas@gmail.com>, with additions and improvements by me.  I've
re-recorded the development as a single patch.

The basic idea is this: we use the low 2 bits of a pointer to a heap
object (3 bits on a 64-bit architecture) to encode some information
about the object pointed to.  For a constructor, we encode the "tag"
of the constructor (e.g. True vs. False), for a function closure its
arity.  This enables some decisions to be made without dereferencing
the pointer, which speeds up some common operations.  In particular it
enables us to avoid costly indirect jumps in many cases.

More information in the commentary:

http://hackage.haskell.org/trac/ghc/wiki/Commentary/Rts/HaskellExecution/PointerTagging

1 files changed, 8 insertions, 3 deletions

diff --git a/rts/Stable.c b/rts/Stable.c
index e5e8dfbdd0..0ed18bcec2 100644
--- a/rts/Stable.c
+++ b/rts/Stable.c
@@ -177,6 +177,9 @@ exitStablePtrTable(void)
 
 /*
  * get at the real stuff...remove indirections.
+ * It untags pointers before dereferencing and
+ * retags the real stuff with its tag (if there
+ * is any) when returning.
  *
  * ToDo: move to a better home.
  */
@@ -184,16 +187,18 @@ static
 StgClosure*
 removeIndirections(StgClosure* p)
 {
-  StgClosure* q = p;
+  StgWord tag = GET_CLOSURE_TAG(p);
+  StgClosure* q = UNTAG_CLOSURE(p);
 
   while (get_itbl(q)->type == IND ||
          get_itbl(q)->type == IND_STATIC ||
          get_itbl(q)->type == IND_OLDGEN ||
          get_itbl(q)->type == IND_PERM ||
          get_itbl(q)->type == IND_OLDGEN_PERM ) {
-      q = ((StgInd *)q)->indirectee;
+      tag = GET_CLOSURE_TAG(q);
+      q = UNTAG_CLOSURE(((StgInd *)q)->indirectee);
   }
-  return q;
+  return TAG_CLOSURE(tag,q);
 }
 
 static StgWord