Remove unused imports

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

diff --git a/compiler/simplCore/SetLevels.lhs b/compiler/simplCore/SetLevels.lhs
index 6f48272..0797ad7 100644 (file)
--- a/compiler/simplCore/SetLevels.lhs
+++ b/compiler/simplCore/SetLevels.lhs
@@ -60,7 +60,7 @@ import CoreUtils      ( exprType, exprIsTrivial, mkPiTypes )
 import CoreFVs         -- all of it
 import CoreSubst       ( Subst, emptySubst, extendInScope, extendIdSubst,
                          cloneIdBndr, cloneRecIdBndrs )
-import Id              ( Id, idType, mkSysLocal, isOneShotLambda,
+import Id              ( idType, mkSysLocal, isOneShotLambda,
                          zapDemandIdInfo, transferPolyIdInfo,
                          idSpecialisation, idWorkerInfo, setIdInfo
                        )
@@ -366,6 +366,14 @@ For example:
 where h :: Int -> Int# is expensive. We'd like to float the (h y) outside
 the \x, but we don't because it's unboxed.  Possible solution: box it.
 
+Note [Case MFEs]
+~~~~~~~~~~~~~~~~
+We don't float a case expression as an MFE from a strict context.  Why not?
+Because in doing so we share a tiny bit of computation (the switch) but
+in exchange we build a thunk, which is bad.  This case reduces allocation 
+by 7% in spectral/puzzle (a rather strange benchmark) and 1.2% in real/fem.
+Doesn't change any other allocation at all.
+
 \begin{code}
 lvlMFE ::  Bool                        -- True <=> strict context [body of case or let]
        -> Level                -- Level of innermost enclosing lambda/tylam
@@ -384,6 +392,10 @@ lvlMFE strict_ctxt ctxt_lvl env (_, AnnCast e co)
   = do { expr' <- lvlMFE strict_ctxt ctxt_lvl env e
        ; return (Cast expr' co) }
 
+-- Note [Case MFEs]
+lvlMFE True ctxt_lvl env e@(_, AnnCase {})
+  = lvlExpr ctxt_lvl env e     -- Don't share cases
+
 lvlMFE strict_ctxt ctxt_lvl env ann_expr@(fvs, _)
   |  isUnLiftedType ty                 -- Can't let-bind it; see Note [Unlifted MFEs]
   || isInlineCtxt ctxt_lvl             -- Don't float out of an __inline__ context
@@ -851,7 +863,7 @@ newPolyBndrs dest_lvl env abs_vars bndrs = do
     let new_bndrs = zipWith mk_poly_bndr bndrs uniqs
     return (extendPolyLvlEnv dest_lvl env abs_vars (bndrs `zip` new_bndrs), new_bndrs)
   where
-    mk_poly_bndr bndr uniq = transferPolyIdInfo bndr $         -- Note [transferPolyIdInfo] in Id.lhs
+    mk_poly_bndr bndr uniq = transferPolyIdInfo bndr abs_vars $        -- Note [transferPolyIdInfo] in Id.lhs
                             mkSysLocal (mkFastString str) uniq poly_ty
                           where
                             str     = "poly_" ++ occNameString (getOccName bndr)