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, 24 insertions, 10 deletions

diff --git a/rts/Apply.cmm b/rts/Apply.cmm
index e0ca03944c..cf8a108006 100644
--- a/rts/Apply.cmm
+++ b/rts/Apply.cmm
@@ -90,8 +90,6 @@ stg_PAP_apply
   // Enter PAP cost centre 
   ENTER_CCS_PAP_CL(pap);
 
-  R1 = StgPAP_fun(pap);
-
   // Reload the stack 
   W_ i;
   W_ p;
@@ -105,14 +103,30 @@ for:
     goto for;
   }
 
+  R1 = StgPAP_fun(pap);
+
+/* DEBUGGING CODE, ensures that arity 1 and 2 functions are entered tagged
+  if (TO_W_(StgFunInfoExtra_arity(%FUN_INFO(%INFO_PTR(UNTAG(R1))))) == 1 ) {
+    if (GETTAG(R1)!=1) {
+	W_[0]=1;
+    }
+  }
+
+  if (TO_W_(StgFunInfoExtra_arity(%FUN_INFO(%INFO_PTR(UNTAG(R1))))) == 2 ) {
+    if (GETTAG(R1)!=2) {
+	W_[0]=1;
+    }
+  }
+*/
+
   // Off we go! 
   TICK_ENT_VIA_NODE();
 
 #ifdef NO_ARG_REGS
-  jump %GET_ENTRY(R1);
+  jump %GET_ENTRY(UNTAG(R1));
 #else
       W_ info;
-      info = %GET_FUN_INFO(R1);
+      info = %GET_FUN_INFO(UNTAG(R1));
       W_ type;
       type = TO_W_(StgFunInfoExtra_fun_type(info));
       if (type == ARG_GEN) {
@@ -167,8 +181,6 @@ INFO_TABLE(stg_AP,/*special layout*/0,0,AP,"AP","AP")
   // Enter PAP cost centre
   ENTER_CCS_PAP_CL(ap);   // ToDo: ENTER_CC_AP_CL 
 
-  R1 = StgAP_fun(ap);
-
   // Reload the stack 
   W_ i;
   W_ p;
@@ -182,14 +194,16 @@ for:
     goto for;
   }
 
+  R1 = StgAP_fun(ap);
+
   // Off we go! 
   TICK_ENT_VIA_NODE();
 
 #ifdef NO_ARG_REGS
-  jump %GET_ENTRY(R1);
+  jump %GET_ENTRY(UNTAG(R1));
 #else
       W_ info;
-      info = %GET_FUN_INFO(R1);
+      info = %GET_FUN_INFO(UNTAG(R1));
       W_ type;
       type = TO_W_(StgFunInfoExtra_fun_type(info));
       if (type == ARG_GEN) {
@@ -246,8 +260,6 @@ INFO_TABLE(stg_AP_STACK,/*special layout*/0,0,AP_STACK,"AP_STACK","AP_STACK")
   // Enter PAP cost centre
   ENTER_CCS_PAP_CL(ap);   // ToDo: ENTER_CC_AP_CL 
 
-  R1 = StgAP_STACK_fun(ap);
-
   // Reload the stack
   W_ i;
   W_ p;
@@ -264,5 +276,7 @@ for:
   // Off we go!
   TICK_ENT_VIA_NODE();
 
+  R1 = StgAP_STACK_fun(ap);
+
   ENTER();
 }