Completely new treatment of INLINE pragmas (big patch)

[ghc-hetmet.git] / compiler / coreSyn / CoreUnfold.lhs

diff --git a/compiler/coreSyn/CoreUnfold.lhs b/compiler/coreSyn/CoreUnfold.lhs
index b6706c1..258cd46 100644 (file)
--- a/compiler/coreSyn/CoreUnfold.lhs
+++ b/compiler/coreSyn/CoreUnfold.lhs
@@ -18,12 +18,10 @@ find, unsurprisingly, a Core expression.
 module CoreUnfold (
        Unfolding, UnfoldingGuidance,   -- Abstract types
 
 module CoreUnfold (
        Unfolding, UnfoldingGuidance,   -- Abstract types
 

-       noUnfolding, mkTopUnfolding, mkImplicitUnfolding, mkUnfolding, 
-       mkCompulsoryUnfolding, seqUnfolding,
-       evaldUnfolding, mkOtherCon, otherCons,
-       unfoldingTemplate, maybeUnfoldingTemplate,
-       isEvaldUnfolding, isValueUnfolding, isCheapUnfolding, isCompulsoryUnfolding,
-       hasUnfolding, hasSomeUnfolding, neverUnfold,
+       noUnfolding, mkImplicitUnfolding, 
+       mkTopUnfolding, mkUnfolding, 
+       mkInlineRule, mkWwInlineRule,
+       mkCompulsoryUnfolding, 

 
        couldBeSmallEnoughToInline, 
        certainlyWillInline, smallEnoughToInline,
 
        couldBeSmallEnoughToInline, 
        certainlyWillInline, smallEnoughToInline,

@@ -37,7 +35,7 @@ import DynFlags
 import CoreSyn
 import PprCore         ()      -- Instances
 import OccurAnal
 import CoreSyn
 import PprCore         ()      -- Instances
 import OccurAnal

-import CoreSubst       ( Subst, emptySubst, substTy, extendIdSubst, extendTvSubst
+import CoreSubst       ( emptySubst, substTy, extendIdSubst, extendTvSubst

                        , lookupIdSubst, substBndr, substBndrs, substRecBndrs )
 import CoreUtils
 import Id
                        , lookupIdSubst, substBndr, substBndrs, substRecBndrs )
 import CoreUtils
 import Id

@@ -45,7 +43,9 @@ import DataCon
 import Literal
 import PrimOp
 import IdInfo
 import Literal
 import PrimOp
 import IdInfo

+import BasicTypes      ( Arity )

 import Type hiding( substTy, extendTvSubst )
 import Type hiding( substTy, extendTvSubst )

+import Maybes

 import PrelNames
 import Bag
 import FastTypes
 import PrelNames
 import Bag
 import FastTypes

@@ -68,24 +68,37 @@ mkTopUnfolding expr = mkUnfolding True {- Top level -} expr
 mkImplicitUnfolding :: CoreExpr -> Unfolding
 -- For implicit Ids, do a tiny bit of optimising first
 mkImplicitUnfolding expr 
 mkImplicitUnfolding :: CoreExpr -> Unfolding
 -- For implicit Ids, do a tiny bit of optimising first
 mkImplicitUnfolding expr 

-  = CoreUnfolding (simpleOptExpr emptySubst expr)
+  = CoreUnfolding (simpleOptExpr expr)

                  True
                  (exprIsHNF expr)
                  (exprIsCheap expr)
                  (calcUnfoldingGuidance opt_UF_CreationThreshold expr)
 
                  True
                  (exprIsHNF expr)
                  (exprIsCheap expr)
                  (calcUnfoldingGuidance opt_UF_CreationThreshold expr)
 

+mkInlineRule :: CoreExpr -> Arity -> Unfolding
+mkInlineRule expr arity 
+  = InlineRule { uf_tmpl = simpleOptExpr expr, 
+                uf_is_top = True,       -- Conservative; this gets set more
+                                        -- accuately by the simplifier (slight hack)
+                                        -- in SimplEnv.substUnfolding
+                 uf_arity = arity, 
+                uf_is_value = exprIsHNF expr,
+                uf_worker = Nothing }
+
+mkWwInlineRule :: CoreExpr -> Arity -> Id -> Unfolding
+mkWwInlineRule expr arity wkr 
+  = InlineRule { uf_tmpl = simpleOptExpr expr, 
+                uf_is_top = True,       -- Conservative; see mkInlineRule
+                 uf_arity = arity, 
+                uf_is_value = exprIsHNF expr,
+                uf_worker = Just wkr }
+

 mkUnfolding :: Bool -> CoreExpr -> Unfolding
 mkUnfolding top_lvl expr
 mkUnfolding :: Bool -> CoreExpr -> Unfolding
 mkUnfolding top_lvl expr

-  = CoreUnfolding (occurAnalyseExpr expr)
-                 top_lvl
-
-                 (exprIsHNF expr)
-                       -- Already evaluated
-
-                 (exprIsCheap expr)
-                       -- OK to inline inside a lambda
-
-                 (calcUnfoldingGuidance opt_UF_CreationThreshold expr)
+  = CoreUnfolding { uf_tmpl = occurAnalyseExpr expr,
+                   uf_is_top = top_lvl,
+                   uf_is_value = exprIsHNF expr,
+                   uf_is_cheap = exprIsCheap expr,
+                   uf_guidance = calcUnfoldingGuidance opt_UF_CreationThreshold expr }

        -- Sometimes during simplification, there's a large let-bound thing     
        -- which has been substituted, and so is now dead; so 'expr' contains
        -- two copies of the thing while the occurrence-analysed expression doesn't
        -- Sometimes during simplification, there's a large let-bound thing     
        -- which has been substituted, and so is now dead; so 'expr' contains
        -- two copies of the thing while the occurrence-analysed expression doesn't

@@ -95,14 +108,6 @@ mkUnfolding top_lvl expr
        -- This can occasionally mean that the guidance is very pessimistic;
        -- it gets fixed up next round
 
        -- This can occasionally mean that the guidance is very pessimistic;
        -- it gets fixed up next round
 

-instance Outputable Unfolding where
-  ppr NoUnfolding = ptext (sLit "No unfolding")
-  ppr (OtherCon cs) = ptext (sLit "OtherCon") <+> ppr cs
-  ppr (CompulsoryUnfolding e) = ptext (sLit "Compulsory") <+> ppr e
-  ppr (CoreUnfolding e top hnf cheap g) 
-       = ptext (sLit "Unf") <+> sep [ppr top <+> ppr hnf <+> ppr cheap <+> ppr g, 
-                                    ppr e]
-

 mkCompulsoryUnfolding :: CoreExpr -> Unfolding
 mkCompulsoryUnfolding expr     -- Used for things that absolutely must be unfolded
   = CompulsoryUnfolding (occurAnalyseExpr expr)
 mkCompulsoryUnfolding :: CoreExpr -> Unfolding
 mkCompulsoryUnfolding expr     -- Used for things that absolutely must be unfolded
   = CompulsoryUnfolding (occurAnalyseExpr expr)

@@ -116,75 +121,27 @@ mkCompulsoryUnfolding expr        -- Used for things that absolutely must be unfolded
 %************************************************************************
 
 \begin{code}
 %************************************************************************
 
 \begin{code}

-instance Outputable UnfoldingGuidance where
-    ppr UnfoldNever    = ptext (sLit "NEVER")
-    ppr (UnfoldIfGoodArgs v cs size discount)
-      = hsep [ ptext (sLit "IF_ARGS"), int v,
-              brackets (hsep (map int cs)),
-              int size,
-              int discount ]
-\end{code}
-
-
-\begin{code}

 calcUnfoldingGuidance
        :: Int                  -- bomb out if size gets bigger than this
        -> CoreExpr             -- expression to look at
        -> UnfoldingGuidance
 calcUnfoldingGuidance bOMB_OUT_SIZE expr
 calcUnfoldingGuidance
        :: Int                  -- bomb out if size gets bigger than this
        -> CoreExpr             -- expression to look at
        -> UnfoldingGuidance
 calcUnfoldingGuidance bOMB_OUT_SIZE expr

-  = case collect_val_bndrs expr of { (inline, val_binders, body) ->
+  = case collectBinders expr of { (binders, body) ->

     let
     let

+        val_binders = filter isId binders

        n_val_binders = length val_binders
        n_val_binders = length val_binders

-
-       max_inline_size = n_val_binders+2
-       -- The idea is that if there is an INLINE pragma (inline is True)
-       -- and there's a big body, we give a size of n_val_binders+2.  This
-       -- This is just enough to fail the no-size-increase test in callSiteInline,
-       --   so that INLINE things don't get inlined into entirely boring contexts,
-       --   but no more.
-

     in
     case (sizeExpr (iUnbox bOMB_OUT_SIZE) val_binders body) of
     in
     case (sizeExpr (iUnbox bOMB_OUT_SIZE) val_binders body) of

-
-      TooBig 
-       | not inline -> UnfoldNever
-               -- A big function with an INLINE pragma must
-               -- have an UnfoldIfGoodArgs guidance
-       | otherwise  -> UnfoldIfGoodArgs n_val_binders
-                                        (map (const 0) val_binders)
-                                        max_inline_size 0
-
+      TooBig -> UnfoldNever

       SizeIs size cased_args scrut_discount
       SizeIs size cased_args scrut_discount

-       -> UnfoldIfGoodArgs
-                       n_val_binders
-                       (map discount_for val_binders)
-                       final_size
-                       (iBox scrut_discount)
+       -> UnfoldIfGoodArgs { ug_arity = n_val_binders
+                           , ug_args  = map discount_for val_binders
+                           , ug_size  = iBox size
+                           , ug_res   = iBox scrut_discount }

        where        
        where        

-           boxed_size    = iBox size
-
-           final_size | inline     = boxed_size `min` max_inline_size
-                      | otherwise  = boxed_size
-
-               -- Sometimes an INLINE thing is smaller than n_val_binders+2.
-               -- A particular case in point is a constructor, which has size 1.
-               -- We want to inline this regardless, hence the `min`
-

            discount_for b = foldlBag (\acc (b',n) -> if b==b' then acc+n else acc) 
                                      0 cased_args
        }
            discount_for b = foldlBag (\acc (b',n) -> if b==b' then acc+n else acc) 
                                      0 cased_args
        }

-  where
-    collect_val_bndrs e = go False [] e
-       -- We need to be a bit careful about how we collect the
-       -- value binders.  In ptic, if we see 
-       --      __inline_me (\x y -> e)
-       -- We want to say "2 value binders".  Why?  So that 
-       -- we take account of information given for the arguments
-
-    go _      rev_vbs (Note InlineMe e)     = go True   rev_vbs     e
-    go inline rev_vbs (Lam b e) | isId b    = go inline (b:rev_vbs) e
-                               | otherwise = go inline rev_vbs     e
-    go inline rev_vbs e                            = (inline, reverse rev_vbs, e)

 \end{code}
 
 \begin{code}
 \end{code}
 
 \begin{code}

@@ -197,21 +154,10 @@ sizeExpr :: FastInt           -- Bomb out if it gets bigger than this
 sizeExpr bOMB_OUT_SIZE top_args expr
   = size_up expr
   where
 sizeExpr bOMB_OUT_SIZE top_args expr
   = size_up expr
   where

-    size_up (Type _)           = sizeZero        -- Types cost nothing
+    size_up (Type _)           = sizeZero      -- Types cost nothing

     size_up (Var _)            = sizeOne
     size_up (Var _)            = sizeOne

-
-    size_up (Note InlineMe _)  = sizeOne         -- Inline notes make it look very small
-       -- This can be important.  If you have an instance decl like this:
-       --      instance Foo a => Foo [a] where
-       --         {-# INLINE op1, op2 #-}
-       --         op1 = ...
-       --         op2 = ...
-       -- then we'll get a dfun which is a pair of two INLINE lambdas
-
-    size_up (Note _      body) = size_up body  -- Other notes cost nothing
-    
+    size_up (Note _ body)      = size_up body  -- Notes cost nothing

     size_up (Cast e _)         = size_up e
     size_up (Cast e _)         = size_up e

-

     size_up (App fun (Type _)) = size_up fun
     size_up (App fun arg)      = size_up_app fun [arg]
 
     size_up (App fun (Type _)) = size_up fun
     size_up (App fun arg)      = size_up_app fun [arg]
 

@@ -484,13 +430,17 @@ couldBeSmallEnoughToInline threshold rhs = case calcUnfoldingGuidance threshold
 
 certainlyWillInline :: Unfolding -> Bool
   -- Sees if the unfolding is pretty certain to inline 
 
 certainlyWillInline :: Unfolding -> Bool
   -- Sees if the unfolding is pretty certain to inline 

-certainlyWillInline (CoreUnfolding _ _ _ is_cheap (UnfoldIfGoodArgs n_vals _ size _))
+certainlyWillInline (CompulsoryUnfolding {}) = True
+certainlyWillInline (InlineRule {})          = True
+certainlyWillInline (CoreUnfolding 
+    { uf_is_cheap = is_cheap
+    , uf_guidance = UnfoldIfGoodArgs {ug_arity = n_vals, ug_size = size}})

   = is_cheap && size - (n_vals +1) <= opt_UF_UseThreshold
 certainlyWillInline _
   = False
 
 smallEnoughToInline :: Unfolding -> Bool
   = is_cheap && size - (n_vals +1) <= opt_UF_UseThreshold
 certainlyWillInline _
   = False
 
 smallEnoughToInline :: Unfolding -> Bool

-smallEnoughToInline (CoreUnfolding _ _ _ _ (UnfoldIfGoodArgs _ _ size _))
+smallEnoughToInline (CoreUnfolding {uf_guidance = UnfoldIfGoodArgs {ug_size = size}})

   = size <= opt_UF_UseThreshold
 smallEnoughToInline _
   = False
   = size <= opt_UF_UseThreshold
 smallEnoughToInline _
   = False

@@ -536,6 +486,10 @@ data CallCtxt = BoringCtxt
                                --      => be keener to inline
                -- INVARIANT: ArgCtxt False 0 ==> BoringCtxt
 
                                --      => be keener to inline
                -- INVARIANT: ArgCtxt False 0 ==> BoringCtxt
 

+             | ValAppCtxt      -- We're applied to at least one value arg
+                               -- This arises when we have ((f x |> co) y)
+                               -- Then the (f x) has argument 'x' but in a ValAppCtxt
+

              | CaseCtxt        -- We're the scrutinee of a case
                                -- that decomposes its scrutinee
 
              | CaseCtxt        -- We're the scrutinee of a case
                                -- that decomposes its scrutinee
 

@@ -543,9 +497,13 @@ instance Outputable CallCtxt where
   ppr BoringCtxt    = ptext (sLit "BoringCtxt")
   ppr (ArgCtxt _ _) = ptext (sLit "ArgCtxt")
   ppr CaseCtxt             = ptext (sLit "CaseCtxt")
   ppr BoringCtxt    = ptext (sLit "BoringCtxt")
   ppr (ArgCtxt _ _) = ptext (sLit "ArgCtxt")
   ppr CaseCtxt             = ptext (sLit "CaseCtxt")

+  ppr ValAppCtxt    = ptext (sLit "ValAppCtxt")

 
 callSiteInline dflags active_inline id lone_variable arg_infos cont_info
 
 callSiteInline dflags active_inline id lone_variable arg_infos cont_info

-  = case idUnfolding id of {
+  = let
+       n_val_args  = length arg_infos
+    in
+    case idUnfolding id of {

        NoUnfolding -> Nothing ;
        OtherCon _  -> Nothing ;
 
        NoUnfolding -> Nothing ;
        OtherCon _  -> Nothing ;
 

@@ -556,14 +514,45 @@ callSiteInline dflags active_inline id lone_variable arg_infos cont_info
                -- compulsory unfoldings (see MkId.lhs).
                -- We don't allow them to be inactive
 
                -- compulsory unfoldings (see MkId.lhs).
                -- We don't allow them to be inactive
 

-       CoreUnfolding unf_template is_top is_value is_cheap guidance ->
+       InlineRule { uf_tmpl = unf_template, uf_arity = arity, uf_is_top = is_top
+                  , uf_is_value = is_value, uf_worker = mb_worker }
+           -> let yes_or_no | not active_inline   = False
+                            | n_val_args <  arity = yes_unsat  -- Not enough value args
+                            | n_val_args == arity = yes_exact  -- Exactly saturated
+                            | otherwise           = True       -- Over-saturated
+                  result | yes_or_no = Just unf_template
+                         | otherwise = Nothing
+                  
+                  -- See Note [Inlining an InlineRule]
+                  is_wrapper = isJust mb_worker 
+                  yes_unsat | is_wrapper  = or arg_infos
+                            | otherwise   = False
+
+                  yes_exact = or arg_infos || interesting_saturated_call
+                  interesting_saturated_call 
+                       = case cont_info of
+                           BoringCtxt -> not is_top                            -- Note [Nested functions]
+                           CaseCtxt   -> not lone_variable || not is_value     -- Note [Lone variables]
+                           ArgCtxt {} -> arity > 0                             -- Note [Inlining in ArgCtxt]
+                           ValAppCtxt -> True                                  -- Note [Cast then apply]
+              in
+              if dopt Opt_D_dump_inlinings dflags then
+               pprTrace ("Considering InlineRule for: " ++ showSDoc (ppr id))
+                        (vcat [text "active:" <+> ppr active_inline,
+                               text "arg infos" <+> ppr arg_infos,
+                               text "interesting call" <+> ppr interesting_saturated_call,
+                               text "is value:" <+> ppr is_value,
+                               text "ANSWER =" <+> if yes_or_no then text "YES" else text "NO"])
+                         result
+               else result ;
+
+       CoreUnfolding { uf_tmpl = unf_template, uf_is_top = is_top, uf_is_value = is_value,
+                       uf_is_cheap = is_cheap, uf_guidance = guidance } ->

 
     let
        result | yes_or_no = Just unf_template
               | otherwise = Nothing
 
 
     let
        result | yes_or_no = Just unf_template
               | otherwise = Nothing
 

-       n_val_args  = length arg_infos
-

        yes_or_no = active_inline && is_cheap && consider_safe
                -- We consider even the once-in-one-branch
                -- occurrences, because they won't all have been
        yes_or_no = active_inline && is_cheap && consider_safe
                -- We consider even the once-in-one-branch
                -- occurrences, because they won't all have been

@@ -579,7 +568,8 @@ callSiteInline dflags active_inline id lone_variable arg_infos cont_info
                -- work-duplication issue (the caller checks that).
          = case guidance of
              UnfoldNever  -> False
                -- work-duplication issue (the caller checks that).
          = case guidance of
              UnfoldNever  -> False

-             UnfoldIfGoodArgs n_vals_wanted arg_discounts size res_discount
+             UnfoldIfGoodArgs { ug_arity = n_vals_wanted, ug_args = arg_discounts
+                               , ug_res = res_discount, ug_size = size }

                  | enough_args && size <= (n_vals_wanted + 1)
                        -- Inline unconditionally if there no size increase
                        -- Size of call is n_vals_wanted (+1 for the function)
                  | enough_args && size <= (n_vals_wanted + 1)
                        -- Inline unconditionally if there no size increase
                        -- Size of call is n_vals_wanted (+1 for the function)

@@ -610,8 +600,8 @@ callSiteInline dflags active_inline id lone_variable arg_infos cont_info
                        = case cont_info of
                            BoringCtxt -> not is_top && n_vals_wanted > 0       -- Note [Nested functions] 
                            CaseCtxt   -> not lone_variable || not is_value     -- Note [Lone variables]
                        = case cont_info of
                            BoringCtxt -> not is_top && n_vals_wanted > 0       -- Note [Nested functions] 
                            CaseCtxt   -> not lone_variable || not is_value     -- Note [Lone variables]

-                           ArgCtxt {} -> n_vals_wanted > 0 
-                               -- See Note [Inlining in ArgCtxt]
+                           ArgCtxt {} -> n_vals_wanted > 0                     -- Note [Inlining in ArgCtxt]
+                           ValAppCtxt -> True                                  -- Note [Cast then apply]

 
                    small_enough = (size - discount) <= opt_UF_UseThreshold
                    discount = computeDiscount n_vals_wanted arg_discounts 
 
                    small_enough = (size - discount) <= opt_UF_UseThreshold
                    discount = computeDiscount n_vals_wanted arg_discounts 

@@ -619,7 +609,7 @@ callSiteInline dflags active_inline id lone_variable arg_infos cont_info
                    res_discount' = case cont_info of
                                        BoringCtxt  -> 0
                                        CaseCtxt    -> res_discount
                    res_discount' = case cont_info of
                                        BoringCtxt  -> 0
                                        CaseCtxt    -> res_discount

-                                       ArgCtxt _ _ -> 4 `min` res_discount
+                                       _other      -> 4 `min` res_discount

                        -- res_discount can be very large when a function returns
                        -- construtors; but we only want to invoke that large discount
                        -- when there's a case continuation.
                        -- res_discount can be very large when a function returns
                        -- construtors; but we only want to invoke that large discount
                        -- when there's a case continuation.

@@ -629,20 +619,35 @@ callSiteInline dflags active_inline id lone_variable arg_infos cont_info
                
     in    
     if dopt Opt_D_dump_inlinings dflags then
                
     in    
     if dopt Opt_D_dump_inlinings dflags then

-       pprTrace "Considering inlining"
-                (ppr id <+> vcat [text "active:" <+> ppr active_inline,
-                                  text "arg infos" <+> ppr arg_infos,
-                                  text "interesting continuation" <+> ppr cont_info,
-                                  text "is value:" <+> ppr is_value,
-                                  text "is cheap:" <+> ppr is_cheap,
-                                  text "guidance" <+> ppr guidance,
-                                  text "ANSWER =" <+> if yes_or_no then text "YES" else text "NO"])
+       pprTrace ("Considering inlining: " ++ showSDoc (ppr id))
+                (vcat [text "active:" <+> ppr active_inline,
+                       text "arg infos" <+> ppr arg_infos,
+                       text "interesting continuation" <+> ppr cont_info,
+                       text "is value:" <+> ppr is_value,
+                       text "is cheap:" <+> ppr is_cheap,
+                       text "guidance" <+> ppr guidance,
+                       text "ANSWER =" <+> if yes_or_no then text "YES" else text "NO"])

                  result
     else
     result
     }
 \end{code}
 
                  result
     else
     result
     }
 \end{code}
 

+Note [Inlining an InlineRule]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+An InlineRules is used for
+  (a) pogrammer INLINE pragmas
+  (b) inlinings from worker/wrapper
+
+For (a) the RHS may be large, and our contract is that we *only* inline
+when the function is applied to all the arguments on the LHS of the
+source-code defn.  (The uf_arity in the rule.)
+
+However for worker/wrapper it may be worth inlining even if the 
+arity is not satisfied (as we do in the CoreUnfolding case) so we don't
+require saturation.
+
+

 Note [Nested functions]
 ~~~~~~~~~~~~~~~~~~~~~~~
 If a function has a nested defn we also record some-benefit, on the
 Note [Nested functions]
 ~~~~~~~~~~~~~~~~~~~~~~~
 If a function has a nested defn we also record some-benefit, on the

@@ -655,6 +660,16 @@ branches.  Then inlining it doesn't increase allocation, but it does
 increase the chance that the constructor won't be allocated at all in
 the branches that don't use it.
 
 increase the chance that the constructor won't be allocated at all in
 the branches that don't use it.
 

+Note [Cast then apply]
+~~~~~~~~~~~~~~~~~~~~~~
+Consider
+   myIndex = __inline_me ( (/\a. <blah>) |> co )
+   co :: (forall a. a -> a) ~ (forall a. T a)
+     ... /\a.\x. case ((myIndex a) |> sym co) x of { ... } ...
+
+We need to inline myIndex to unravel this; but the actual call (myIndex a) has
+no value arguments.  The ValAppCtxt gives it enough incentive to inline.
+

 Note [Inlining in ArgCtxt]
 ~~~~~~~~~~~~~~~~~~~~~~~~~~
 The condition (n_vals_wanted > 0) here is very important, because otherwise
 Note [Inlining in ArgCtxt]
 ~~~~~~~~~~~~~~~~~~~~~~~~~~
 The condition (n_vals_wanted > 0) here is very important, because otherwise

@@ -757,14 +772,14 @@ computeDiscount n_vals_wanted arg_discounts result_discount arg_infos
 
 
 \begin{code}
 
 
 \begin{code}

-simpleOptExpr :: Subst -> CoreExpr -> CoreExpr
+simpleOptExpr :: CoreExpr -> CoreExpr

 -- Return an occur-analysed and slightly optimised expression
 -- The optimisation is very straightforward: just
 -- inline non-recursive bindings that are used only once, 
 -- or wheere the RHS is trivial
 
 -- Return an occur-analysed and slightly optimised expression
 -- The optimisation is very straightforward: just
 -- inline non-recursive bindings that are used only once, 
 -- or wheere the RHS is trivial
 

-simpleOptExpr subst expr
-  = go subst (occurAnalyseExpr expr)
+simpleOptExpr expr
+  = go emptySubst (occurAnalyseExpr expr)

   where
     go subst (Var v)          = lookupIdSubst subst v
     go subst (App e1 e2)      = App (go subst e1) (go subst e2)
   where
     go subst (Var v)          = lookupIdSubst subst v
     go subst (App e1 e2)      = App (go subst e1) (go subst e2)