Add notSCCNote, and use it

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

diff --git a/compiler/coreSyn/CorePrep.lhs b/compiler/coreSyn/CorePrep.lhs
index 3a6d037..4db4c53 100644 (file)
--- a/compiler/coreSyn/CorePrep.lhs
+++ b/compiler/coreSyn/CorePrep.lhs
@@ -11,15 +11,17 @@ module CorePrep (
 
 #include "HsVersions.h"
 
 
 #include "HsVersions.h"
 

+import PrelNames       ( lazyIdKey, hasKey )

 import CoreUtils
 import CoreArity
 import CoreFVs
 import CoreUtils
 import CoreArity
 import CoreFVs

-import CoreLint
+import CoreMonad       ( endPass, CoreToDo(..) )

 import CoreSyn
 import CoreSyn

+import CoreSubst

 import Type
 import Coercion
 import TyCon
 import Type
 import Coercion
 import TyCon

-import NewDemand
+import Demand

 import Var
 import VarSet
 import VarEnv
 import Var
 import VarSet
 import VarEnv

@@ -37,6 +39,7 @@ import Util
 import Outputable
 import MonadUtils
 import FastString
 import Outputable
 import MonadUtils
 import FastString

+import Data.List       ( mapAccumL )

 import Control.Monad
 \end{code}
 
 import Control.Monad
 \end{code}
 

@@ -85,10 +88,11 @@ The goal of this pass is to prepare for code generation.
 8.  Inject bindings for the "implicit" Ids:
        * Constructor wrappers
        * Constructor workers
 8.  Inject bindings for the "implicit" Ids:
        * Constructor wrappers
        * Constructor workers

-       * Record selectors

     We want curried definitions for all of these in case they
     aren't inlined by some caller.
        
     We want curried definitions for all of these in case they
     aren't inlined by some caller.
        

+9.  Replace (lazy e) by e.  See Note [lazyId magic] in MkId.lhs
+

 This is all done modulo type applications and abstractions, so that
 when type erasure is done for conversion to STG, we don't end up with
 any trivial or useless bindings.
 This is all done modulo type applications and abstractions, so that
 when type erasure is done for conversion to STG, we don't end up with
 any trivial or useless bindings.

@@ -145,7 +149,7 @@ corePrepPgm dflags binds data_tycons = do
                       floats2 <- corePrepTopBinds implicit_binds
                       return (deFloatTop (floats1 `appendFloats` floats2))
 
                       floats2 <- corePrepTopBinds implicit_binds
                       return (deFloatTop (floats1 `appendFloats` floats2))
 

-    endPass dflags "CorePrep" Opt_D_dump_prep binds_out
+    endPass dflags CorePrep binds_out []

     return binds_out
 
 corePrepExpr :: DynFlags -> CoreExpr -> IO CoreExpr
     return binds_out
 
 corePrepExpr :: DynFlags -> CoreExpr -> IO CoreExpr

@@ -193,24 +197,38 @@ And then x will actually end up case-bound
 
 Note [CafInfo and floating]
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 Note [CafInfo and floating]
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~

-What happens to the CafInfo on the floated bindings?  By default, all
-the CafInfos will be set to MayHaveCafRefs, which is safe.
-
-This might be pessimistic, because the floated binding might not refer
-to any CAFs and the GC will end up doing more traversal than is
-necessary, but it's still better than not floating the bindings at
-all, because then the GC would have to traverse the structure in the
-heap instead.  Given this, we decided not to try to get the CafInfo on
-the floated bindings correct, because it looks difficult.
-
-But that means we can't float anything out of a NoCafRefs binding.
-Consider       f = g (h x)
-If f is NoCafRefs, we don't want to convert to
-              sat = h x
-               f = g sat
-where sat conservatively says HasCafRefs, because now f's info
-is wrong.  I don't think this is common, so we simply switch off
-floating in this case.
+What happens when we try to float bindings to the top level?  At this
+point all the CafInfo is supposed to be correct, and we must make certain
+that is true of the new top-level bindings.  There are two cases
+to consider
+
+a) The top-level binding is marked asCafRefs.  In that case we are
+   basically fine.  The floated bindings had better all be lazy lets,
+   so they can float to top level, but they'll all have HasCafRefs
+   (the default) which is safe.
+
+b) The top-level binding is marked NoCafRefs.  This really happens
+   Example.  CoreTidy produces
+      $fApplicativeSTM [NoCafRefs] = D:Alternative retry# ...blah...
+   Now CorePrep has to eta-expand to
+      $fApplicativeSTM = let sat = \xy. retry x y
+                         in D:Alternative sat ...blah...
+   So what we *want* is
+      sat [NoCafRefs] = \xy. retry x y
+      $fApplicativeSTM [NoCafRefs] = D:Alternative sat ...blah...
+   
+   So, gruesomely, we must set the NoCafRefs flag on the sat bindings,
+   *and* substutite the modified 'sat' into the old RHS.  
+
+   It should be the case that 'sat' is itself [NoCafRefs] (a value, no
+   cafs) else the original top-level binding would not itself have been
+   marked [NoCafRefs].  The DEBUG check in CoreToStg for
+   consistentCafInfo will find this.
+
+This is all very gruesome and horrible. It would be better to figure
+out CafInfo later, after CorePrep.  We'll do that in due course. 
+Meanwhile this horrible hack works.
+

 
 Note [Data constructor workers]
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 Note [Data constructor workers]
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

@@ -242,7 +260,7 @@ cpeBind :: TopLevelFlag
        -> UniqSM (CorePrepEnv, Floats)
 cpeBind top_lvl env (NonRec bndr rhs)
   = do { (_, bndr1) <- cloneBndr env bndr
        -> UniqSM (CorePrepEnv, Floats)
 cpeBind top_lvl env (NonRec bndr rhs)
   = do { (_, bndr1) <- cloneBndr env bndr

-       ; let is_strict   = isStrictDmd (idNewDemandInfo bndr)
+       ; let is_strict   = isStrictDmd (idDemandInfo bndr)

              is_unlifted = isUnLiftedType (idType bndr)
        ; (floats, bndr2, rhs2) <- cpePair top_lvl NonRecursive 
                                                  (is_strict || is_unlifted) 
              is_unlifted = isUnLiftedType (idType bndr)
        ; (floats, bndr2, rhs2) <- cpePair top_lvl NonRecursive 
                                                  (is_strict || is_unlifted) 

@@ -274,28 +292,20 @@ cpeBind top_lvl env (Rec pairs)
 ---------------
 cpePair :: TopLevelFlag -> RecFlag -> RhsDemand
        -> CorePrepEnv -> Id -> CoreExpr
 ---------------
 cpePair :: TopLevelFlag -> RecFlag -> RhsDemand
        -> CorePrepEnv -> Id -> CoreExpr

-       -> UniqSM (Floats, Id, CoreExpr)
+       -> UniqSM (Floats, Id, CpeRhs)

 -- Used for all bindings
 cpePair top_lvl is_rec is_strict_or_unlifted env bndr rhs
   = do { (floats1, rhs1) <- cpeRhsE env rhs
 -- Used for all bindings
 cpePair top_lvl is_rec is_strict_or_unlifted env bndr rhs
   = do { (floats1, rhs1) <- cpeRhsE env rhs

-       ; let (rhs1_bndrs, _) = collectBinders rhs1
-       ; (floats2, rhs2)
-                   <- if want_float floats1 rhs1 
-                      then return (floats1, rhs1)
-                      else -- Non-empty floats will wrap rhs1
-                    -- But: rhs1 might have lambdas, and we can't
-                   --      put them inside a wrapBinds
-              if valBndrCount rhs1_bndrs <= arity 
-              then    -- Lambdas in rhs1 will be nuked by eta expansion
-                   return (emptyFloats, wrapBinds floats1 rhs1)
-          
-              else do { body1 <- rhsToBodyNF rhs1
-                      ; return (emptyFloats, wrapBinds floats1 body1) } 
-
-       ; (floats3, rhs')   -- Note [Silly extra arguments]
-            <- if manifestArity rhs2 <= arity 
+
+       -- See if we are allowed to float this stuff out of the RHS
+       ; (floats2, rhs2) <- float_from_rhs floats1 rhs1
+
+       -- Make the arity match up
+       ; (floats3, rhs')
+            <- if manifestArity rhs1 <= arity 

               then return (floats2, cpeEtaExpand arity rhs2)
               else WARN(True, text "CorePrep: silly extra arguments:" <+> ppr bndr)
               then return (floats2, cpeEtaExpand arity rhs2)
               else WARN(True, text "CorePrep: silly extra arguments:" <+> ppr bndr)

+                              -- Note [Silly extra arguments]

                    (do { v <- newVar (idType bndr)
                        ; let float = mkFloat False False v rhs2
                        ; return (addFloat floats2 float, cpeEtaExpand arity (Var v)) })
                    (do { v <- newVar (idType bndr)
                        ; let float = mkFloat False False v rhs2
                        ; return (addFloat floats2 float, cpeEtaExpand arity (Var v)) })

@@ -307,9 +317,40 @@ cpePair top_lvl is_rec is_strict_or_unlifted env bndr rhs
        ; return (floats3, bndr', rhs') }
   where
     arity = idArity bndr       -- We must match this arity
        ; return (floats3, bndr', rhs') }
   where
     arity = idArity bndr       -- We must match this arity

-    want_float floats rhs 
-     | isTopLevel top_lvl = wantFloatTop bndr floats
-     | otherwise          = wantFloatNested is_rec is_strict_or_unlifted floats rhs
+
+    ---------------------
+    float_from_rhs floats rhs
+      | isEmptyFloats floats = return (emptyFloats, rhs)
+      | isTopLevel top_lvl    = float_top    floats rhs
+      | otherwise             = float_nested floats rhs
+
+    ---------------------
+    float_nested floats rhs
+      | wantFloatNested is_rec is_strict_or_unlifted floats rhs
+                  = return (floats, rhs)
+      | otherwise = dont_float floats rhs
+
+    ---------------------
+    float_top floats rhs       -- Urhgh!  See Note [CafInfo and floating]
+      | mayHaveCafRefs (idCafInfo bndr)
+      , allLazyTop floats
+      = return (floats, rhs)
+
+      -- So the top-level binding is marked NoCafRefs
+      | Just (floats', rhs') <- canFloatFromNoCaf floats rhs
+      = return (floats', rhs')
+
+      | otherwise
+      = dont_float floats rhs
+
+    ---------------------
+    dont_float floats rhs
+      -- Non-empty floats, but do not want to float from rhs
+      -- So wrap the rhs in the floats
+      -- But: rhs1 might have lambdas, and we can't
+      --      put them inside a wrapBinds
+      = do { body <- rhsToBodyNF rhs
+          ; return (emptyFloats, wrapBinds floats body) } 

 
 {- Note [Silly extra arguments]
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 {- Note [Silly extra arguments]
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

@@ -341,9 +382,14 @@ cpeRhsE :: CorePrepEnv -> CoreExpr -> UniqSM (Floats, CpeRhs)
 
 cpeRhsE _env expr@(Type _) = return (emptyFloats, expr)
 cpeRhsE _env expr@(Lit _)  = return (emptyFloats, expr)
 
 cpeRhsE _env expr@(Type _) = return (emptyFloats, expr)
 cpeRhsE _env expr@(Lit _)  = return (emptyFloats, expr)

-cpeRhsE env expr@(App {})  = cpeApp env expr

 cpeRhsE env expr@(Var {})  = cpeApp env expr
 
 cpeRhsE env expr@(Var {})  = cpeApp env expr
 

+cpeRhsE env (Var f `App` _ `App` arg)
+  | f `hasKey` lazyIdKey         -- Replace (lazy a) by a
+  = cpeRhsE env arg              -- See Note [lazyId magic] in MkId
+
+cpeRhsE env expr@(App {}) = cpeApp env expr
+

 cpeRhsE env (Let bind expr)
   = do { (env', new_binds) <- cpeBind NotTopLevel env bind
        ; (floats, body) <- cpeRhsE env' expr
 cpeRhsE env (Let bind expr)
   = do { (env', new_binds) <- cpeBind NotTopLevel env bind
        ; (floats, body) <- cpeRhsE env' expr

@@ -420,9 +466,9 @@ rhsToBody (Cast e co)
        ; return (floats, Cast e' co) }
 
 rhsToBody expr@(Lam {})
        ; return (floats, Cast e' co) }
 
 rhsToBody expr@(Lam {})

-  | Just no_lam_result <- tryEtaReduce bndrs body
+  | Just no_lam_result <- tryEtaReducePrep bndrs body

   = return (emptyFloats, no_lam_result)
   = return (emptyFloats, no_lam_result)

-  | all isTyVar bndrs          -- Type lambdas are ok
+  | all isTyCoVar bndrs                -- Type lambdas are ok

   = return (emptyFloats, expr)
   | otherwise                  -- Some value lambdas
   = do { fn <- newVar (exprType expr)
   = return (emptyFloats, expr)
   | otherwise                  -- Some value lambdas
   = do { fn <- newVar (exprType expr)

@@ -475,7 +521,7 @@ cpeApp env expr
 
     collect_args (App fun arg) depth
       = do { (fun',hd,fun_ty,floats,ss) <- collect_args fun (depth+1)
 
     collect_args (App fun arg) depth
       = do { (fun',hd,fun_ty,floats,ss) <- collect_args fun (depth+1)

-           ; let
+          ; let

               (ss1, ss_rest)   = case ss of
                                    (ss1:ss_rest) -> (ss1,     ss_rest)
                                    []            -> (lazyDmd, [])
               (ss1, ss_rest)   = case ss of
                                    (ss1:ss_rest) -> (ss1,     ss_rest)
                                    []            -> (lazyDmd, [])

@@ -490,7 +536,7 @@ cpeApp env expr
            ; let v2 = lookupCorePrepEnv env v1
            ; return (Var v2, (Var v2, depth), idType v2, emptyFloats, stricts) }
        where
            ; let v2 = lookupCorePrepEnv env v1
            ; return (Var v2, (Var v2, depth), idType v2, emptyFloats, stricts) }
        where

-         stricts = case idNewStrictness v of
+         stricts = case idStrictness v of

                        StrictSig (DmdType _ demands _)
                            | listLengthCmp demands depth /= GT -> demands
                                    -- length demands <= depth
                        StrictSig (DmdType _ demands _)
                            | listLengthCmp demands depth /= GT -> demands
                                    -- length demands <= depth

@@ -633,7 +679,6 @@ ignoreNote :: Note -> Bool
 -- want to get this:
 --     unzip = /\ab \xs. (__inline_me__ ...) a b xs
 ignoreNote (CoreNote _) = True 
 -- want to get this:
 --     unzip = /\ab \xs. (__inline_me__ ...) a b xs
 ignoreNote (CoreNote _) = True 

-ignoreNote InlineMe     = True

 ignoreNote _other       = False
 
 
 ignoreNote _other       = False
 
 

@@ -643,10 +688,9 @@ cpe_ExprIsTrivial (Var _)                  = True
 cpe_ExprIsTrivial (Type _)                 = True
 cpe_ExprIsTrivial (Lit _)                  = True
 cpe_ExprIsTrivial (App e arg)              = isTypeArg arg && cpe_ExprIsTrivial e
 cpe_ExprIsTrivial (Type _)                 = True
 cpe_ExprIsTrivial (Lit _)                  = True
 cpe_ExprIsTrivial (App e arg)              = isTypeArg arg && cpe_ExprIsTrivial e

-cpe_ExprIsTrivial (Note (SCC _) _)         = False
-cpe_ExprIsTrivial (Note _ e)               = cpe_ExprIsTrivial e
+cpe_ExprIsTrivial (Note n e)               = notSccNote n  && cpe_ExprIsTrivial e

 cpe_ExprIsTrivial (Cast e _)               = cpe_ExprIsTrivial e
 cpe_ExprIsTrivial (Cast e _)               = cpe_ExprIsTrivial e

-cpe_ExprIsTrivial (Lam b body) | isTyVar b = cpe_ExprIsTrivial body
+cpe_ExprIsTrivial (Lam b body) | isTyCoVar b = cpe_ExprIsTrivial body

 cpe_ExprIsTrivial _                        = False
 \end{code}
 
 cpe_ExprIsTrivial _                        = False
 \end{code}
 

@@ -691,7 +735,7 @@ Instead CoreArity.etaExpand gives
                f = /\a -> \y -> let s = h 3 in g s y
 
 \begin{code}
                f = /\a -> \y -> let s = h 3 in g s y
 
 \begin{code}

-cpeEtaExpand :: Arity -> CoreExpr -> CoreExpr
+cpeEtaExpand :: Arity -> CpeRhs -> CpeRhs

 cpeEtaExpand arity expr
   | arity == 0 = expr
   | otherwise  = etaExpand arity expr
 cpeEtaExpand arity expr
   | arity == 0 = expr
   | otherwise  = etaExpand arity expr

@@ -709,8 +753,8 @@ get to a partial application:
     ==> case x of { p -> map f }
 
 \begin{code}
     ==> case x of { p -> map f }
 
 \begin{code}

-tryEtaReduce :: [CoreBndr] -> CoreExpr -> Maybe CoreExpr
-tryEtaReduce bndrs expr@(App _ _)
+tryEtaReducePrep :: [CoreBndr] -> CoreExpr -> Maybe CoreExpr
+tryEtaReducePrep bndrs expr@(App _ _)

   | ok_to_eta_reduce f &&
     n_remaining >= 0 &&
     and (zipWith ok bndrs last_args) &&
   | ok_to_eta_reduce f &&
     n_remaining >= 0 &&
     and (zipWith ok bndrs last_args) &&

@@ -730,15 +774,15 @@ tryEtaReduce bndrs expr@(App _ _)
     ok_to_eta_reduce (Var f) = not (hasNoBinding f)
     ok_to_eta_reduce _       = False --safe. ToDo: generalise
 
     ok_to_eta_reduce (Var f) = not (hasNoBinding f)
     ok_to_eta_reduce _       = False --safe. ToDo: generalise
 

-tryEtaReduce bndrs (Let bind@(NonRec _ r) body)
+tryEtaReducePrep bndrs (Let bind@(NonRec _ r) body)

   | not (any (`elemVarSet` fvs) bndrs)
   | not (any (`elemVarSet` fvs) bndrs)

-  = case tryEtaReduce bndrs body of
+  = case tryEtaReducePrep bndrs body of

        Just e -> Just (Let bind e)
        Nothing -> Nothing
   where
     fvs = exprFreeVars r
 
        Just e -> Just (Let bind e)
        Nothing -> Nothing
   where
     fvs = exprFreeVars r
 

-tryEtaReduce _ _ = Nothing
+tryEtaReducePrep _ _ = Nothing

 \end{code}
 
 
 \end{code}
 
 

@@ -758,18 +802,37 @@ type RhsDemand = Bool  -- True => used strictly; hence not top-level, non-recurs
 
 \begin{code}
 data FloatingBind 
 
 \begin{code}
 data FloatingBind 

-  = FloatLet CoreBind          -- Rhs of bindings are CpeRhss
-  | FloatCase Id CpeBody Bool   -- The bool indicates "ok-for-speculation"
+  = FloatLet CoreBind   -- Rhs of bindings are CpeRhss
+                        -- They are always of lifted type;
+                        -- unlifted ones are done with FloatCase
+ 
+ | FloatCase 
+      Id CpeBody 
+      Bool             -- The bool indicates "ok-for-speculation"

 
 data Floats = Floats OkToSpec (OrdList FloatingBind)
 
 
 data Floats = Floats OkToSpec (OrdList FloatingBind)
 

+instance Outputable FloatingBind where
+  ppr (FloatLet b) = ppr b
+  ppr (FloatCase b r ok) = brackets (ppr ok) <+> ppr b <+> equals <+> ppr r
+
+instance Outputable Floats where
+  ppr (Floats flag fs) = ptext (sLit "Floats") <> brackets (ppr flag) <+>
+                         braces (vcat (map ppr (fromOL fs)))
+
+instance Outputable OkToSpec where
+  ppr OkToSpec    = ptext (sLit "OkToSpec")
+  ppr IfUnboxedOk = ptext (sLit "IfUnboxedOk")
+  ppr NotOkToSpec = ptext (sLit "NotOkToSpec")
+ 

 -- Can we float these binds out of the rhs of a let?  We cache this decision
 -- to avoid having to recompute it in a non-linear way when there are
 -- deeply nested lets.
 data OkToSpec
 -- Can we float these binds out of the rhs of a let?  We cache this decision
 -- to avoid having to recompute it in a non-linear way when there are
 -- deeply nested lets.
 data OkToSpec

-   = NotOkToSpec       -- definitely not
-   | OkToSpec          -- yes
-   | IfUnboxedOk       -- only if floating an unboxed binding is ok
+   = OkToSpec          -- Lazy bindings of lifted type
+   | IfUnboxedOk       -- A mixture of lazy lifted bindings and n
+                       -- ok-to-speculate unlifted bindings
+   | NotOkToSpec       -- Some not-ok-to-speculate unlifted bindings

 
 mkFloat :: Bool -> Bool -> Id -> CpeRhs -> FloatingBind
 mkFloat is_strict is_unlifted bndr rhs
 
 mkFloat :: Bool -> Bool -> Id -> CpeRhs -> FloatingBind
 mkFloat is_strict is_unlifted bndr rhs

@@ -787,7 +850,7 @@ emptyFloats = Floats OkToSpec nilOL
 isEmptyFloats :: Floats -> Bool
 isEmptyFloats (Floats _ bs) = isNilOL bs
 
 isEmptyFloats :: Floats -> Bool
 isEmptyFloats (Floats _ bs) = isNilOL bs
 

-wrapBinds :: Floats -> CoreExpr -> CoreExpr
+wrapBinds :: Floats -> CpeBody -> CpeBody

 wrapBinds (Floats _ binds) body
   = foldrOL mk_bind body binds
   where
 wrapBinds (Floats _ binds) body
   = foldrOL mk_bind body binds
   where

@@ -824,10 +887,6 @@ combine IfUnboxedOk _ = IfUnboxedOk
 combine _ IfUnboxedOk = IfUnboxedOk
 combine _ _           = OkToSpec
     
 combine _ IfUnboxedOk = IfUnboxedOk
 combine _ _           = OkToSpec
     

-instance Outputable FloatingBind where
-  ppr (FloatLet bind)        = text "FloatLet" <+> ppr bind
-  ppr (FloatCase b rhs spec) = text "FloatCase" <+> ppr b <+> ppr spec <+> equals <+> ppr rhs
-

 deFloatTop :: Floats -> [CoreBind]
 -- For top level only; we don't expect any FloatCases
 deFloatTop (Floats _ floats)
 deFloatTop :: Floats -> [CoreBind]
 -- For top level only; we don't expect any FloatCases
 deFloatTop (Floats _ floats)

@@ -837,11 +896,52 @@ deFloatTop (Floats _ floats)
     get b            _  = pprPanic "corePrepPgm" (ppr b)
 
 -------------------------------------------
     get b            _  = pprPanic "corePrepPgm" (ppr b)
 
 -------------------------------------------

-wantFloatTop :: Id -> Floats -> Bool
+canFloatFromNoCaf ::  Floats -> CpeRhs -> Maybe (Floats, CpeRhs)

        -- Note [CafInfo and floating]
        -- Note [CafInfo and floating]

-wantFloatTop bndr floats = isEmptyFloats floats
-                        || (mayHaveCafRefs (idCafInfo bndr)
-                            && allLazyTop floats)
+canFloatFromNoCaf (Floats ok_to_spec fs) rhs
+  | OkToSpec <- ok_to_spec          -- Worth trying
+  , Just (subst, fs') <- go (emptySubst, nilOL) (fromOL fs)
+  = Just (Floats OkToSpec fs', subst_expr subst rhs)
+  | otherwise              
+  = Nothing
+  where
+    subst_expr = substExpr (text "CorePrep")
+
+    go :: (Subst, OrdList FloatingBind) -> [FloatingBind]
+       -> Maybe (Subst, OrdList FloatingBind)
+
+    go (subst, fbs_out) [] = Just (subst, fbs_out)
+    
+    go (subst, fbs_out) (FloatLet (NonRec b r) : fbs_in) 
+      | rhs_ok r
+      = go (subst', fbs_out `snocOL` new_fb) fbs_in
+      where
+        (subst', b') = set_nocaf_bndr subst b
+        new_fb = FloatLet (NonRec b' (subst_expr subst r))
+
+    go (subst, fbs_out) (FloatLet (Rec prs) : fbs_in)
+      | all rhs_ok rs
+      = go (subst', fbs_out `snocOL` new_fb) fbs_in
+      where
+        (bs,rs) = unzip prs
+        (subst', bs') = mapAccumL set_nocaf_bndr subst bs
+        rs' = map (subst_expr subst') rs
+        new_fb = FloatLet (Rec (bs' `zip` rs'))
+
+    go _ _ = Nothing     -- Encountered a caffy binding
+
+    ------------
+    set_nocaf_bndr subst bndr 
+      = (extendIdSubst subst bndr (Var bndr'), bndr')
+      where
+        bndr' = bndr `setIdCafInfo` NoCafRefs
+
+    ------------
+    rhs_ok :: CoreExpr -> Bool
+    -- We can only float to top level from a NoCaf thing if
+    -- the new binding is static. However it can't mention
+    -- any non-static things or it would *already* be Caffy
+    rhs_ok = rhsIsStatic (\_ -> False)

 
 wantFloatNested :: RecFlag -> Bool -> Floats -> CpeRhs -> Bool
 wantFloatNested is_rec strict_or_unlifted floats rhs
 
 wantFloatNested :: RecFlag -> Bool -> Floats -> CpeRhs -> Bool
 wantFloatNested is_rec strict_or_unlifted floats rhs