Completely new treatment of INLINE pragmas (big patch)

This is a major patch, which changes the way INLINE pragmas work.
Although lots of files are touched, the net is only +21 lines of
code -- and I bet that most of those are comments!

HEADS UP: interface file format has changed, so you'll need to
recompile everything.

There is not much effect on overall performance for nofib, 
probably because those programs don't make heavy use of INLINE pragmas.

        Program           Size    Allocs   Runtime   Elapsed
            Min         -11.3%     -6.9%     -9.2%     -8.2%
            Max          -0.1%     +4.6%     +7.5%     +8.9%
 Geometric Mean          -2.2%     -0.2%     -1.0%     -0.8%

(The +4.6% for on allocs is cichelli; see other patch relating to
-fpass-case-bndr-to-join-points.)

The old INLINE system
~~~~~~~~~~~~~~~~~~~~~
The old system worked like this. A function with an INLINE pragam
got a right-hand side which looked like
     f = __inline_me__ (\xy. e)
The __inline_me__ part was an InlineNote, and was treated specially
in various ways.  Notably, the simplifier didn't inline inside an
__inline_me__ note.  

As a result, the code for f itself was pretty crappy. That matters
if you say (map f xs), because then you execute the code for f,
rather than inlining a copy at the call site.

The new story: InlineRules
~~~~~~~~~~~~~~~~~~~~~~~~~~
The new system removes the InlineMe Note altogether.  Instead there
is a new constructor InlineRule in CoreSyn.Unfolding.  This is a 
bit like a RULE, in that it remembers the template to be inlined inside
the InlineRule.  No simplification or inlining is done on an InlineRule,
just like RULEs.  

An Id can have an InlineRule *or* a CoreUnfolding (since these are two
constructors from Unfolding). The simplifier treats them differently:

  - An InlineRule is has the substitution applied (like RULES) but 
    is otherwise left undisturbed.

  - A CoreUnfolding is updated with the new RHS of the definition,
    on each iteration of the simplifier.

An InlineRule fires regardless of size, but *only* when the function
is applied to enough arguments.  The "arity" of the rule is specified
(by the programmer) as the number of args on the LHS of the "=".  So
it makes a difference whether you say
  	{-# INLINE f #-}
	f x = \y -> e     or     f x y = e
This is one of the big new features that InlineRule gives us, and it
is one that Roman really wanted.

In contrast, a CoreUnfolding can fire when it is applied to fewer
args than than the function has lambdas, provided the result is small
enough.


Consequential stuff
~~~~~~~~~~~~~~~~~~~
* A 'wrapper' no longer has a WrapperInfo in the IdInfo.  Instead,
  the InlineRule has a field identifying wrappers.

* Of course, IfaceSyn and interface serialisation changes appropriately.

* Making implication constraints inline nicely was a bit fiddly. In
  the end I added a var_inline field to HsBInd.VarBind, which is why
  this patch affects the type checker slightly

* I made some changes to the way in which eta expansion happens in
  CorePrep, mainly to ensure that *arguments* that become let-bound
  are also eta-expanded.  I'm still not too happy with the clarity
  and robustness fo the result.

* We now complain if the programmer gives an INLINE pragma for
  a recursive function (prevsiously we just ignored it).  Reason for
  change: we don't want an InlineRule on a LoopBreaker, because then
  we'd have to check for loop-breaker-hood at occurrence sites (which
  isn't currenlty done).  Some tests need changing as a result.

This patch has been in my tree for quite a while, so there are
probably some other minor changes.

1 files changed, 18 insertions, 15 deletions

diff --git a/compiler/specialise/Specialise.lhs b/compiler/specialise/Specialise.lhs
index 4d8efdd8c5..4a1cc4c37b 100644
--- a/compiler/specialise/Specialise.lhs
+++ b/compiler/specialise/Specialise.lhs
@@ -16,7 +16,7 @@ module Specialise ( specProgram ) where
 
 import Id		( Id, idName, idType, mkUserLocal, idCoreRules,
 			  idInlinePragma, setInlinePragma, setIdUnfolding,
-			  isLocalId ) 
+			  isLocalId, idUnfolding ) 
 import TcType		( Type, mkTyVarTy, tcSplitSigmaTy, 
 			  tyVarsOfTypes, tyVarsOfTheta, isClassPred,
 			  tcCmpType, isUnLiftedType
@@ -26,7 +26,7 @@ import CoreSubst	( Subst, mkEmptySubst, extendTvSubstList, lookupIdSubst,
 			  cloneIdBndr, cloneIdBndrs, cloneRecIdBndrs,
 			  extendIdSubst
 			) 
-import CoreUnfold	( mkUnfolding )
+import CoreUnfold	( mkUnfolding, mkInlineRule )
 import SimplUtils	( interestingArg )
 import Var		( DictId )
 import VarSet
@@ -43,6 +43,7 @@ import Name
 import MkId		( voidArgId, realWorldPrimId )
 import FiniteMap
 import Maybes		( catMaybes, isJust )
+import BasicTypes	( Arity )
 import Bag
 import Util
 import Outputable
@@ -831,10 +832,14 @@ specDefn subst calls fn rhs
     n_dicts	       = length theta
     inline_prag        = idInlinePragma fn
 
-	-- It's important that we "see past" any INLINE pragma
-	-- else we'll fail to specialise an INLINE thing
-    (inline_rhs, rhs_inside) = dropInline rhs
-    (rhs_tyvars, rhs_ids, rhs_body) = collectTyAndValBinders rhs_inside
+	-- Figure out whether the function has an INLINE pragma
+	-- See Note [Inline specialisations]
+    fn_has_inline_rule :: Maybe Arity	 -- Gives arity of the *specialised* inline rule
+    fn_has_inline_rule = case idUnfolding fn of
+    		       	   InlineRule { uf_arity = arity } -> Just (arity - n_dicts)
+			   _other                          -> Nothing
+
+    (rhs_tyvars, rhs_ids, rhs_body) = collectTyAndValBinders rhs
 
     rhs_dict_ids = take n_dicts rhs_ids
     body         = mkLams (drop n_dicts rhs_ids) rhs_body
@@ -922,10 +927,13 @@ specDefn subst calls fn rhs
 		-- Add the { d1' = dx1; d2' = dx2 } usage stuff
 	   	final_uds = foldr addDictBind rhs_uds dx_binds
 
-	   	spec_pr | inline_rhs = (spec_f `setInlinePragma` inline_prag, Note InlineMe spec_rhs)
-		        | otherwise  = (spec_f, 		 	      spec_rhs)
-
-	   ; return (Just (spec_pr, final_uds, spec_env_rule)) } }
+		-- See Note [Inline specialisations]
+		final_spec_f | Just spec_arity <- fn_has_inline_rule
+			     = spec_f `setInlinePragma` inline_prag
+			       	      `setIdUnfolding`  mkInlineRule spec_rhs spec_arity
+			     | otherwise 
+			     = spec_f
+	   ; return (Just ((final_spec_f, spec_rhs), final_uds, spec_env_rule)) } }
       where
 	my_zipEqual xs ys zs
 	 | debugIsOn && not (equalLength xs ys && equalLength ys zs)
@@ -1090,11 +1098,6 @@ specialised version.
 A case in point is dictionary functions, which are current marked
 INLINE, but which are worth specialising.
 
-\begin{code}
-dropInline :: CoreExpr -> (Bool, CoreExpr)
-dropInline (Note InlineMe rhs) = (True,  rhs)
-dropInline rhs		       = (False, rhs)
-\end{code}
 
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