Ensure exprIsCheap/exprIsExpandable deal with Cast properly

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

diff --git a/compiler/coreSyn/CorePrep.lhs b/compiler/coreSyn/CorePrep.lhs
index 8b0499c..540fa2d 100644 (file)
--- a/compiler/coreSyn/CorePrep.lhs
+++ b/compiler/coreSyn/CorePrep.lhs
@@ -577,10 +577,7 @@ 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
-  | otherwise
-  = do { (floats1, arg1) <- cpeRhsE env arg    -- arg1 can be a lambda
+  = 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
@@ -588,10 +585,13 @@ cpeArg env is_strict arg arg_ty
                -- Else case: arg1 might have lambdas, and we can't
                --            put them inside a wrapBinds
 
-       ; v <- newVar arg_ty
+       ; if cpe_ExprIsTrivial arg2    -- Do not eta expand a trivial argument
+         then return (floats2, arg2)
+         else do
+       { v <- newVar arg_ty
        ; let arg3      = cpeEtaExpand (exprArity arg2) arg2
                     arg_float = mkFloat is_strict is_unlifted v arg3
-       ; return (addFloat floats2 arg_float, Var v) }
+       ; return (addFloat floats2 arg_float, Var v) } }
   where
     is_unlifted = isUnLiftedType arg_ty
     want_float = wantFloatNested NonRecursive (is_strict || is_unlifted)
@@ -637,9 +637,7 @@ maybeSaturate fn expr n_args
 
 -------------
 saturateDataToTag :: CpeApp -> UniqSM CpeApp
--- Horrid: ensure that the arg of data2TagOp is evaluated
---   (data2tag x) -->  (case x of y -> data2tag y)
--- (yuk yuk) take into account the lambdas we've now introduced
+-- See Note [dataToTag magic]
 saturateDataToTag sat_expr
   = do { let (eta_bndrs, eta_body) = collectBinders sat_expr
        ; eta_body' <- eval_data2tag_arg eta_body
@@ -663,7 +661,14 @@ saturateDataToTag sat_expr
        = pprPanic "eval_data2tag" (ppr other)
 \end{code}
 
+Note [dataToTag magic]
+~~~~~~~~~~~~~~~~~~~~~~
+Horrid: we must ensure that the arg of data2TagOp is evaluated
+  (data2tag x) -->  (case x of y -> data2tag y)
+(yuk yuk) take into account the lambdas we've now introduced
 
+How might it not be evaluated?  Well, we might have floated it out
+of the scope of a `seq`, or dropped the `seq` altogether.
 
 
 %************************************************************************