X-Git-Url: http://git.megacz.com/?a=blobdiff_plain;f=compiler%2FsimplCore%2FSetLevels.lhs;h=0797ad7727eb1fd5a37d22e7a9be2970f8c74def;hb=48196c3c2a365ce085d0b0567cbd4ea047af59df;hp=0235981a804b6171c25b6bf3da1eb2f5f65ec1bf;hpb=6561069ad5d0b11de223686be59372a3b1e6aed7;p=ghc-hetmet.git

diff --git a/compiler/simplCore/SetLevels.lhs b/compiler/simplCore/SetLevels.lhs
index 0235981..0797ad7 100644
--- 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