[project @ 2003-04-11 08:27:53 by simonpj]

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

diff --git a/ghc/compiler/simplCore/Simplify.lhs b/ghc/compiler/simplCore/Simplify.lhs
index 7a75c05..144ff75 100644 (file)
--- a/ghc/compiler/simplCore/Simplify.lhs
+++ b/ghc/compiler/simplCore/Simplify.lhs
@@ -453,26 +453,36 @@ simplLazyBind :: SimplEnv
              -> SimplM (FloatsWith SimplEnv)
 
 simplLazyBind env top_lvl is_rec bndr bndr1 rhs rhs_se
-  =    -- Substitute the rules for this binder in the light
-       -- of earlier substitutions in this very letrec group,
-       -- add the substituted rules to the IdInfo, and 
-       -- extend the in-scope env, so that the IdInfo for this 
-       -- binder extends  over the RHS for the binder itself.
+  = let        -- Transfer the IdInfo of the original binder to the new binder
+       -- This is crucial: we must preserve
+       --      strictness
+       --      rules
+       --      worker info
+       -- etc.  To do this we must apply the current substitution, 
+       -- which incorporates earlier substitutions in this very letrec group.
        --
+       -- NB 1.  We do this *before* processing the RHS of the binder, so that
+       -- its substituted rules are visible in its own RHS.
        -- This is important.  Manuel found cases where he really, really
        -- wanted a RULE for a recursive function to apply in that function's
        -- own right-hand side.
        --
-       -- NB: does no harm for non-recursive bindings
-       --
-       -- NB2: just rules!  In particular, the arity of an Id is not visible
+       -- NB 2: We do not transfer the arity (see Subst.substIdInfo)
+       -- The arity of an Id should not be visible
        -- in its own RHS, else we eta-reduce
        --      f = \x -> f x
        -- to
        --      f = f
        -- which isn't sound.  And it makes the arity in f's IdInfo greater than
        -- the manifest arity, which isn't good.
-    let
+       -- The arity will get added later.
+       --
+       -- NB 3: It's important that we *do* transer the loop-breaker OccInfo,
+       -- because that's what stops the Id getting inlined infinitely, in the body
+       -- of the letrec.
+
+       -- NB 4: does no harm for non-recursive bindings
+
        bndr2             = bndr1 `setIdInfo` simplIdInfo (getSubst env) (idInfo bndr)
        env1              = modifyInScope env bndr2 bndr2
        rhs_env           = setInScope rhs_se env1