Two more wibbles to CorePrep (fixes HTTP package and DPH)

Ensuring that
  a) lambdas show up only on the RHSs of binding after CorePrep
  b) the arity of a binding exactly matches the maifest lambdas
is surprisingly tricky.

I got it wrong (again) in my recent CorePrep shuffling, which broke
packages HTTP and DPH.  This patch fixes both.

diff --git a/compiler/coreSyn/CorePrep.lhs b/compiler/coreSyn/CorePrep.lhs
index b8dd80f..908c90c 100644 (file)
--- a/compiler/coreSyn/CorePrep.lhs
+++ b/compiler/coreSyn/CorePrep.lhs
@@ -277,35 +277,59 @@ cpePair :: TopLevelFlag -> RecFlag -> RhsDemand
        -> UniqSM (Floats, Id, CoreExpr)
 -- Used for all bindings
 cpePair top_lvl is_rec is_strict_or_unlifted env bndr rhs
-  = do { (floats, rhs') <- cpeRhs want_float (idArity bndr) env 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 
+              then return (floats2, cpeEtaExpand arity rhs2)
+              else WARN(True, text "CorePrep: silly extra arguments:" <+> ppr bndr)
+                   (do { v <- newVar (idType bndr)
+                       ; let float = mkFloat False False v rhs2
+                       ; return (addFloat floats2 float, cpeEtaExpand arity (Var v)) })
 
                -- Record if the binder is evaluated
        ; let bndr' | exprIsHNF rhs' = bndr `setIdUnfolding` evaldUnfolding
                           | otherwise      = bndr
 
-       ; return (floats, bndr', rhs') }
+       ; 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
 
-
+{- Note [Silly extra arguments]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Suppose we had this
+       f{arity=1} = \x\y. e
+We *must* match the arity on the Id, so we have to generate
+        f' = \x\y. e
+       f  = \x. f' x
+
+It's a bizarre case: why is the arity on the Id wrong?  Reason
+(in the days of __inline_me__): 
+        f{arity=0} = __inline_me__ (let v = expensive in \xy. e)
+When InlineMe notes go away this won't happen any more.  But
+it seems good for CorePrep to be robust.
+-}
 
 -- ---------------------------------------------------------------------------
 --             CpeRhs: produces a result satisfying CpeRhs
 -- ---------------------------------------------------------------------------
 
-cpeRhs :: (Floats -> CpeRhs -> Bool)   -- Float the floats out
-       -> Arity                -- Guarantees an Rhs with this manifest arity
-       -> CorePrepEnv
-       -> CoreExpr     -- Expression and its type
-       -> UniqSM (Floats, CpeRhs)
-cpeRhs want_float arity env expr
-  = do { (floats, rhs) <- cpeRhsE env expr
-       ; if want_float floats rhs
-                then return (floats,      cpeEtaExpand arity rhs)
-                else return (emptyFloats, cpeEtaExpand arity (wrapBinds floats rhs)) }
-
 cpeRhsE :: CorePrepEnv -> CoreExpr -> UniqSM (Floats, CpeRhs)
 -- If
 --     e  ===>  (bs, e')
@@ -377,6 +401,11 @@ cpeBody env expr
        ; return (floats1 `appendFloats` floats2, body) }
 
 --------
+rhsToBodyNF :: CpeRhs -> UniqSM CpeBody
+rhsToBodyNF rhs = do { (floats,body) <- rhsToBody rhs
+                    ; return (wrapBinds floats body) }
+
+--------
 rhsToBody :: CpeRhs -> UniqSM (Floats, CpeBody)
 -- Remove top level lambdas by let-bindinig
 
@@ -498,14 +527,21 @@ cpeApp env expr
 cpeArg :: CorePrepEnv -> RhsDemand -> CoreArg -> Type
        -> UniqSM (Floats, CpeTriv)
 cpeArg env is_strict arg arg_ty
-  | cpe_ExprIsTrivial arg      -- Do not eta expand etc a trivial argument
-  = cpeBody env arg    -- Must still do substitution though
+  | cpe_ExprIsTrivial arg   -- Do not eta expand etc a trivial argument
+  = cpeBody env arg        -- Must still do substitution though
   | otherwise
-  = do { (floats, arg') <- cpeRhs want_float
-                                         (exprArity arg) env arg
+  = do { (floats1, arg1) <- cpeRhsE env arg    -- arg1 can be a lambda
+       ; (floats2, arg2) <- if want_float floats1 arg1 
+                                   then return (floats1, arg1)
+                                   else do { body1 <- rhsToBodyNF arg1
+                                   ; return (emptyFloats, wrapBinds floats1 body1) } 
+               -- Else case: arg1 might have lambdas, and we can't
+               --            put them inside a wrapBinds
+
        ; v <- newVar arg_ty
-       ; let arg_float = mkFloat is_strict is_unlifted v arg'
-       ; return (addFloat floats arg_float, Var v) }
+       ; let arg3      = cpeEtaExpand (exprArity arg2) arg2
+                    arg_float = mkFloat is_strict is_unlifted v arg3
+       ; return (addFloat floats2 arg_float, Var v) }
   where
     is_unlifted = isUnLiftedType arg_ty
     want_float = wantFloatNested NonRecursive (is_strict || is_unlifted)
@@ -748,6 +784,9 @@ mkFloat is_strict is_unlifted bndr rhs
 emptyFloats :: Floats
 emptyFloats = Floats OkToSpec nilOL
 
+isEmptyFloats :: Floats -> Bool
+isEmptyFloats (Floats _ bs) = isNilOL bs
+
 wrapBinds :: Floats -> CoreExpr -> CoreExpr
 wrapBinds (Floats _ binds) body
   = foldrOL mk_bind body binds
@@ -800,12 +839,14 @@ deFloatTop (Floats _ floats)
 -------------------------------------------
 wantFloatTop :: Id -> Floats -> Bool
        -- Note [CafInfo and floating]
-wantFloatTop bndr floats = mayHaveCafRefs (idCafInfo bndr)
-                          && allLazyTop floats
+wantFloatTop bndr floats = isEmptyFloats floats
+                        || (mayHaveCafRefs (idCafInfo bndr)
+                            && allLazyTop floats)
 
 wantFloatNested :: RecFlag -> Bool -> Floats -> CpeRhs -> Bool
 wantFloatNested is_rec strict_or_unlifted floats rhs
-  = strict_or_unlifted
+  =  isEmptyFloats floats
+  || strict_or_unlifted
   || (allLazyNested is_rec floats && exprIsHNF rhs)
        -- Why the test for allLazyNested? 
        --      v = f (x `divInt#` y)