[project @ 2001-01-02 16:42:10 by simonpj]

Add a necessary isUnliftedType to completeBinding (coerce case)

diff --git a/ghc/compiler/simplCore/Simplify.lhs b/ghc/compiler/simplCore/Simplify.lhs
index f15edf8..23f6f93 100644 (file)
--- a/ghc/compiler/simplCore/Simplify.lhs
+++ b/ghc/compiler/simplCore/Simplify.lhs
@@ -544,7 +544,7 @@ completeBinding old_bndr new_bndr top_lvl black_listed new_rhs thing_inside
     thing_inside
 
   | Note coercion@(Coerce _ inner_ty) inner_rhs <- new_rhs,
-    not trivial_rhs
+    not trivial_rhs && not (isUnLiftedType inner_ty)
        -- x = coerce t e  ==>  c = e; x = inline_me (coerce t c)
        -- Now x can get inlined, which moves the coercion
        -- to the usage site.  This is a bit like worker/wrapper stuff,
@@ -558,6 +558,16 @@ completeBinding old_bndr new_bndr top_lvl black_listed new_rhs thing_inside
        -- will inline x.  
        -- Also the full-blown w/w thing isn't set up for non-functions
        --
+       -- The (not (isUnLiftedType inner_ty)) avoids the nasty case of
+       --      x::Int = coerce Int Int# (foo y)
+       -- ==>
+       --      v::Int# = foo y
+       --      x::Int  = coerce Int Int# v
+       -- which would be bogus because then v will be evaluated strictly.
+       -- How can this arise?  Via 
+       --      x::Int = case (foo y) of { ... }
+       -- followed by case elimination.
+       --
        -- The inline_me note is so that the simplifier doesn't 
        -- just substitute c back inside x's rhs!  (Typically, x will
        -- get substituted away, but not if it's exported.)