X-Git-Url: http://git.megacz.com/?a=blobdiff_plain;f=compiler%2FsimplCore%2FLiberateCase.lhs;h=9c51103d7636b4b4d550d079eba0caff61371b88;hb=ac4a2563ee5b8d6bb9d0a366fe0ff3ed3fde4bb2;hp=0df9b3736ee8aaa44ad49d7f6b924020af6185ab;hpb=9670d6643e55adeb15f998a0efd5799d499ea2a4;p=ghc-hetmet.git

diff --git a/compiler/simplCore/LiberateCase.lhs b/compiler/simplCore/LiberateCase.lhs
index 0df9b37..9c51103 100644
--- a/compiler/simplCore/LiberateCase.lhs
+++ b/compiler/simplCore/LiberateCase.lhs
@@ -4,6 +4,13 @@
 \section[LiberateCase]{Unroll recursion to allow evals to be lifted from a loop}
 
 \begin{code}
+{-# OPTIONS -w #-}
+-- The above warning supression flag is a temporary kludge.
+-- While working on this module you are encouraged to remove it and fix
+-- any warnings in the module. See
+--     http://hackage.haskell.org/trac/ghc/wiki/Commentary/CodingStyle#Warnings
+-- for details
+
 module LiberateCase ( liberateCase ) where
 
 #include "HsVersions.h"
@@ -53,30 +60,13 @@ Example
 Better code, because 'a' is  free inside the inner letrec, rather
 than needing projection from v.
 
-Other examples we'd like to catch with this kind of transformation
+Note that this deals with *free variables*.  SpecConstr deals with
+*arguments* that are of known form.  E.g.
 
 	last []     = error 
 	last (x:[]) = x
 	last (x:xs) = last xs
 
-We'd like to avoid the redundant pattern match, transforming to
-
-	last [] = error
-	last (x:[]) = x
-	last (x:(y:ys)) = last' y ys
-		where
-		  last' y []     = y
-		  last' _ (y:ys) = last' y ys
-
-	(is this necessarily an improvement)
-
-Similarly drop:
-
-	drop n [] = []
-	drop 0 xs = xs
-	drop n (x:xs) = drop (n-1) xs
-
-Would like to pass n along unboxed.
 	
 Note [Scrutinee with cast]
 ~~~~~~~~~~~~~~~~~~~~~~~~~~
@@ -88,9 +78,26 @@ Exactly the same optimisation (unrolling one call to f) will work here,
 despite the cast.  See mk_alt_env in the Case branch of libCase.
 
 
+Note [Only functions!]
+~~~~~~~~~~~~~~~~~~~~~~
+Consider the following code
+
+       f = g (case v of V a b -> a : t f)
+
+where g is expensive. If we aren't careful, liberate case will turn this into
+
+       f = g (case v of
+               V a b -> a : t (letrec f = g (case v of V a b -> a : f t)
+                                in f)
+             )
+
+Yikes! We evaluate g twice. This leads to a O(2^n) explosion
+if g calls back to the same code recursively.
+
+Solution: make sure that we only do the liberate-case thing on *functions*
+
 To think about (Apr 94)
 ~~~~~~~~~~~~~~
-
 Main worry: duplicating code excessively.  At the moment we duplicate
 the entire binding group once at each recursive call.  But there may
 be a group of recursive calls which share a common set of evaluated
@@ -165,7 +172,7 @@ libCaseBind env (Rec pairs)
 
     pairs' = [(binder, libCase env_rhs rhs) | (binder,rhs) <- pairs]
 
-    env_rhs = if all rhs_small_enough rhss then extended_env else env
+    env_rhs = if all rhs_small_enough pairs then extended_env else env
 
 	-- We extend the rec-env by binding each Id to its rhs, first
 	-- processing the rhs with an *un-extended* environment, so
@@ -186,8 +193,10 @@ libCaseBind env (Rec pairs)
 	-- 	clash at code generation time.
     adjust bndr = setIdNotExported (setIdName bndr (localiseName (idName bndr)))
 
-    rhs_small_enough rhs = couldBeSmallEnoughToInline lIBERATE_BOMB_SIZE rhs
-    lIBERATE_BOMB_SIZE   = bombOutSize env
+    rhs_small_enough (id,rhs)
+	=  idArity id > 0	-- Note [Only functions!]
+	&& maybe True (\size -> couldBeSmallEnoughToInline size rhs)
+                      (bombOutSize env)
 \end{code}
 
 
@@ -241,8 +250,39 @@ libCaseId env v
   where
     rec_id_level = lookupLevel env v
     free_scruts  = freeScruts env rec_id_level
+
+freeScruts :: LibCaseEnv
+	   -> LibCaseLevel 	-- Level of the recursive Id
+	   -> [Id]		-- Ids that are scrutinised between the binding
+				-- of the recursive Id and here
+freeScruts env rec_bind_lvl
+  = [v | (v,scrut_bind_lvl) <- lc_scruts env
+       , scrut_bind_lvl <= rec_bind_lvl]
+	-- Note [When to specialise]
 \end{code}
 
+Note [When to specialise]
+~~~~~~~~~~~~~~~~~~~~~~~~~
+Consider
+  f = \x. letrec g = \y. case x of
+			   True  -> ... (f a) ...
+			   False -> ... (g b) ...
+
+We get the following levels
+	  f  0
+	  x  1
+	  g  1
+	  y  2  
+
+Then 'x' is being scrutinised at a deeper level than its binding, so
+it's added to lc_sruts:  [(x,1)]  
+
+We do *not* want to specialise the call to 'f', becuase 'x' is not free 
+in 'f'.  So here the bind-level of 'x' (=1) is not <= the bind-level of 'f' (=0).
+
+We *do* want to specialise the call to 'g', because 'x' is free in g.
+Here the bind-level of 'x' (=1) is <= the bind-level of 'g' (=1).
+
 
 %************************************************************************
 %*									*
@@ -279,7 +319,7 @@ addScrutedVar env@(LibCaseEnv { lc_lvl = lvl, lc_lvl_env = lvl_env,
 
   | otherwise = env
   where
-    scruts'  = (scrut_var, lvl) : scruts
+    scruts'  = (scrut_var, bind_lvl) : scruts
     bind_lvl = case lookupVarEnv lvl_env scrut_var of
 		 Just lvl -> lvl
 		 Nothing  -> topLevel
@@ -292,13 +332,6 @@ lookupLevel env id
   = case lookupVarEnv (lc_lvl_env env) id of
       Just lvl -> lvl
       Nothing  -> topLevel
-
-freeScruts :: LibCaseEnv
-	   -> LibCaseLevel 	-- Level of the recursive Id
-	   -> [Id]		-- Ids that are scrutinised between the binding
-				-- of the recursive Id and here
-freeScruts env rec_bind_lvl
-  = [v | (v,scrut_lvl) <- lc_scruts env, scrut_lvl > rec_bind_lvl]
 \end{code}
 
 %************************************************************************
@@ -317,35 +350,35 @@ topLevel = 0
 \begin{code}
 data LibCaseEnv
   = LibCaseEnv {
-	lc_size :: Int,		-- Bomb-out size for deciding if
+	lc_size :: Maybe Int,	-- Bomb-out size for deciding if
 				-- potential liberatees are too big.
 				-- (passed in from cmd-line args)
 
 	lc_lvl :: LibCaseLevel,	-- Current level
+		-- The level is incremented when (and only when) going
+		-- inside the RHS of a (sufficiently small) recursive
+		-- function.
 
 	lc_lvl_env :: IdEnv LibCaseLevel,  
-			-- Binds all non-top-level in-scope Ids
-			-- (top-level and imported things have
-			-- a level of zero)
+		-- Binds all non-top-level in-scope Ids (top-level and
+		-- imported things have a level of zero)
 
 	lc_rec_env :: IdEnv CoreBind, 
-			-- Binds *only* recursively defined ids, 
-			-- to their own binding group,
-			-- and *only* in their own RHSs
+		-- Binds *only* recursively defined ids, to their own
+		-- binding group, and *only* in their own RHSs
 
 	lc_scruts :: [(Id,LibCaseLevel)]
-     			-- Each of these Ids was scrutinised by an
-			-- enclosing case expression, with the
-			-- specified number of enclosing
-			-- recursive bindings; furthermore,
-			-- the Id is bound at a lower level
-			-- than the case expression.  The order is
-			-- insignificant; it's a bag really
+     		-- Each of these Ids was scrutinised by an enclosing
+		-- case expression, at a level deeper than its binding
+		-- level.  The LibCaseLevel recorded here is the *binding
+		-- level* of the scrutinised Id.
+		-- 
+		-- The order is insignificant; it's a bag really
 	}
 
 initEnv :: DynFlags -> LibCaseEnv
 initEnv dflags 
-  = LibCaseEnv { lc_size = specThreshold dflags,
+  = LibCaseEnv { lc_size = liberateCaseThreshold dflags,
 		 lc_lvl = 0,
 		 lc_lvl_env = emptyVarEnv, 
 		 lc_rec_env = emptyVarEnv,