Fix Trac #1718: interaction of error, unlifted tuples, and casts

[ghc-hetmet.git] / compiler / simplCore / Simplify.lhs

diff --git a/compiler/simplCore/Simplify.lhs b/compiler/simplCore/Simplify.lhs
index 5d40071..e73d0ac 100644 (file)
--- a/compiler/simplCore/Simplify.lhs
+++ b/compiler/simplCore/Simplify.lhs
@@ -4,6 +4,13 @@
 \section[Simplify]{The main module of the simplifier}
 
 \begin{code}
+{-# OPTIONS -w #-}
+-- The above warning supression flag is a temporary kludge.
+-- While working on this module you are encouraged to remove it and fix
+-- any warnings in the module. See
+--     http://hackage.haskell.org/trac/ghc/wiki/Commentary/CodingStyle#Warnings
+-- for details
+
 module Simplify ( simplTopBinds, simplExpr ) where
 
 #include "HsVersions.h"
@@ -415,6 +422,8 @@ That's what the 'go' loop in prepareRhs does
 prepareRhs :: SimplEnv -> OutExpr -> SimplM (SimplEnv, OutExpr)
 -- Adds new floats to the env iff that allows us to return a good RHS
 prepareRhs env (Cast rhs co)   -- Note [Float coercions]
+  | (ty1, ty2) <- coercionKind co      -- Do *not* do this if rhs has an unlifted type
+  , not (isUnLiftedType ty1)           -- see Note [Float coercions (unlifted)]
   = do { (env', rhs') <- makeTrivial env rhs
        ; return (env', Cast rhs' co) }
 
@@ -467,6 +476,22 @@ and lead to further optimisation.  Example:
           go n = case x of { T m -> go (n-m) }
                -- This case should optimise
 
+Note [Float coercions (unlifted)]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+BUT don't do [Float coercions] if 'e' has an unlifted type. 
+This *can* happen:
+
+     foo :: Int = (error (# Int,Int #) "urk") 
+                 `cast` CoUnsafe (# Int,Int #) Int
+
+If do the makeTrivial thing to the error call, we'll get
+    foo = case error (# Int,Int #) "urk" of v -> v `cast` ...
+But 'v' isn't in scope!  
+
+These strange casts can happen as a result of case-of-case
+       bar = case (case x of { T -> (# 2,3 #); F -> error "urk" }) of
+               (# p,q #) -> p+q
+
 
 \begin{code}
 makeTrivial :: SimplEnv -> OutExpr -> SimplM (SimplEnv, OutExpr)