[project @ 2001-10-31 15:22:53 by simonpj]

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

diff --git a/ghc/compiler/simplCore/OccurAnal.lhs b/ghc/compiler/simplCore/OccurAnal.lhs
index 3ac0dcc..895d743 100644 (file)
--- a/ghc/compiler/simplCore/OccurAnal.lhs
+++ b/ghc/compiler/simplCore/OccurAnal.lhs
@@ -507,6 +507,20 @@ occAnalRhs env id rhs
   = (final_usage, rhs')
   where
     (rhs_usage, rhs') = occAnal (rhsCtxt env) rhs
+       -- Note that we use an rhsCtxt.  This tells the occ anal that it's
+       -- looking at an RHS, which has an effect in occAnalApp
+       --
+       -- But there's a problem.  Consider
+       --      x1 = a0 : []
+       --      x2 = a1 : x1
+       --      x3 = a2 : x2
+       --      g  = f x2
+       -- First time round, it looks as if x1 and x2 occur as an arg of a 
+       -- let-bound constructor ==> give them a many-occurrence.
+       -- But then x3 is inlined (unconditionally as it happens) and
+       -- next time round, x2 will be, and the next time round x1 will be
+       -- Result: multiple simplifier iterations.  Sigh.  
+       -- Possible solution: use rhsCtxt for things that occur just once...
 
        -- [March 98] A new wrinkle is that if the binder has specialisations inside
        -- it then we count the specialised Ids as "extra rhs's".  That way