Remove CPP from nativeGen/RegAlloc/Graph/TrivColorable.hs

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

diff --git a/compiler/simplCore/LiberateCase.lhs b/compiler/simplCore/LiberateCase.lhs
index 01e410d..fe1f758 100644 (file)
--- a/compiler/simplCore/LiberateCase.lhs
+++ b/compiler/simplCore/LiberateCase.lhs
@@ -9,23 +9,11 @@ module LiberateCase ( liberateCase ) where
 #include "HsVersions.h"
 
 import DynFlags
 #include "HsVersions.h"
 
 import DynFlags

-import HscTypes
-import CoreLint                ( showPass, endPass )

 import CoreSyn
 import CoreUnfold      ( couldBeSmallEnoughToInline )
 import CoreSyn
 import CoreUnfold      ( couldBeSmallEnoughToInline )

-import Rules           ( RuleBase )
-import UniqSupply      ( UniqSupply )
-import SimplMonad      ( SimplCount, zeroSimplCount )

 import Id
 import Id

-import FamInstEnv
-import Type
-import Coercion
-import TyCon

 import VarEnv
 import VarEnv

-import Name            ( localiseName )
-import Outputable

 import Util             ( notNull )
 import Util             ( notNull )

-import Data.IORef      ( readIORef )

 \end{code}
 
 The liberate-case transformation
 \end{code}
 
 The liberate-case transformation

@@ -59,30 +47,13 @@ Example
 Better code, because 'a' is  free inside the inner letrec, rather
 than needing projection from v.
 
 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
 
 
        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]
 ~~~~~~~~~~~~~~~~~~~~~~~~~~
        
 Note [Scrutinee with cast]
 ~~~~~~~~~~~~~~~~~~~~~~~~~~

@@ -94,9 +65,26 @@ 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.
 
 
 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)
 ~~~~~~~~~~~~~~
 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
 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

@@ -121,43 +109,6 @@ scope.  For example:
 Here, the level of @f@ is zero, the level of @g@ is one,
 and the level of @h@ is zero (NB not one).
 
 Here, the level of @f@ is zero, the level of @g@ is one,
 and the level of @h@ is zero (NB not one).
 

-Note [Indexed data types]
-~~~~~~~~~~~~~~~~~~~~~~~~~
-Consider
-       data family T :: * -> *
-       data T Int = TI Int
-
-       f :: T Int -> Bool
-       f x = case x of { DEFAULT -> <body> }
-
-We would like to change this to
-       f x = case x `cast` co of { TI p -> <body> }
-
-so that <body> can make use of the fact that x is already evaluated to
-a TI; and a case on a known data type may be more efficient than a
-polymorphic one (not sure this is true any longer).  Anyway the former
-showed up in Roman's experiments.  Example:
-  foo :: FooT Int -> Int -> Int
-  foo t n = t `seq` bar n
-     where
-       bar 0 = 0
-       bar n = bar (n - case t of TI i -> i)
-Here we'd like to avoid repeated evaluating t inside the loop, by 
-taking advantage of the `seq`.
-
-We implement this as part of the liberate-case transformation by 
-spotting
-       case <scrut> of (x::T) tys { DEFAULT ->  <body> }
-where x :: T tys, and T is a indexed family tycon.  Find the
-representation type (T77 tys'), and coercion co, and transform to
-       case <scrut> `cast` co of (y::T77 tys')
-           DEFAULT -> let x = y `cast` sym co in <body>
-
-The "find the representation type" part is done by looking up in the
-family-instance environment.
-
-NB: in fact we re-use x (changing its type) to avoid making a fresh y;
-this entails shadowing, but that's ok.

 
 %************************************************************************
 %*                                                                     *
 
 %************************************************************************
 %*                                                                     *

@@ -166,21 +117,10 @@ this entails shadowing, but that's ok.
 %************************************************************************
 
 \begin{code}
 %************************************************************************
 
 \begin{code}

-liberateCase :: HscEnv -> UniqSupply -> RuleBase -> ModGuts
-            -> IO (SimplCount, ModGuts)
-liberateCase hsc_env _ _ guts
-  = do { let dflags = hsc_dflags hsc_env
-       ; eps <- readIORef (hsc_EPS hsc_env)
-       ; let fam_envs = (eps_fam_inst_env eps, mg_fam_inst_env guts)
-
-       ; showPass dflags "Liberate case"
-       ; let { env = initEnv dflags fam_envs
-             ; binds' = do_prog env (mg_binds guts) }
-       ; endPass dflags "Liberate case" Opt_D_verbose_core2core binds'
-                       {- no specific flag for dumping -} 
-       ; return (zeroSimplCount dflags, guts { mg_binds = binds' }) }
+liberateCase :: DynFlags -> [CoreBind] -> [CoreBind]
+liberateCase dflags binds = do_prog (initEnv dflags) binds

   where
   where

-    do_prog env [] = []
+    do_prog _   [] = []

     do_prog env (bind:binds) = bind' : do_prog env' binds
                             where
                               (env', bind') = libCaseBind env bind
     do_prog env (bind:binds) = bind' : do_prog env' binds
                             where
                               (env', bind') = libCaseBind env bind

@@ -204,37 +144,49 @@ libCaseBind env (NonRec binder rhs)
 libCaseBind env (Rec pairs)
   = (env_body, Rec pairs')
   where
 libCaseBind env (Rec pairs)
   = (env_body, Rec pairs')
   where

-    (binders, rhss) = unzip pairs
+    binders = map fst pairs

 
     env_body = addBinders env binders
 
     pairs' = [(binder, libCase env_rhs rhs) | (binder,rhs) <- pairs]
 
 
     env_body = addBinders env binders
 
     pairs' = [(binder, libCase env_rhs rhs) | (binder,rhs) <- pairs]
 

-    env_rhs = if all rhs_small_enough rhss 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
        -- that the same process doesn't occur for ever!
        -- We extend the rec-env by binding each Id to its rhs, first
        -- processing the rhs with an *un-extended* environment, so
        -- that the same process doesn't occur for ever!

-       --
-    extended_env = addRecBinds env [ (adjust binder, libCase env_body rhs)
-                                  | (binder, rhs) <- pairs ]
-
-       -- Two subtle things: 
-       -- (a)  Reset the export flags on the binders so
-       --      that we don't get name clashes on exported things if the 
-       --      local binding floats out to top level.  This is most unlikely
-       --      to happen, since the whole point concerns free variables. 
-       --      But resetting the export flag is right regardless.
-       -- 
-       -- (b)  Make the name an Internal one.  External Names should never be
-       --      nested; if it were floated to the top level, we'd get a name
-       --      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
+    env_rhs = addRecBinds env [ (localiseId binder, libCase env_body rhs)
+                             | (binder, rhs) <- pairs
+                             , rhs_small_enough binder rhs ]
+       -- localiseID : see Note [Need to localiseId in libCaseBind]
+                
+
+    rhs_small_enough id rhs    -- Note [Small enough]
+       =  idArity id > 0       -- Note [Only functions!]
+       && maybe True (\size -> couldBeSmallEnoughToInline size rhs)
+                      (bombOutSize env)

 \end{code}
 
 \end{code}
 

+Note [Need to localiseId in libCaseBind]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+The call to localiseId is needed for two subtle reasons
+(a)  Reset the export flags on the binders so
+       that we don't get name clashes on exported things if the 
+       local binding floats out to top level.  This is most unlikely
+       to happen, since the whole point concerns free variables. 
+       But resetting the export flag is right regardless.
+
+(b)  Make the name an Internal one.  External Names should never be
+       nested; if it were floated to the top level, we'd get a name
+       clash at code generation time.
+
+Note [Small enough]
+~~~~~~~~~~~~~~~~~~~
+Consider
+  \fv. letrec
+        f = \x. BIG...(case fv of { (a,b) -> ...g.. })...
+        g = \y. SMALL...f...
+Then we *can* do liberate-case on g (small RHS) but not for f (too big).
+But we can choose on a item-by-item basis, and that's what the
+rhs_small_enough call in the comprehension for env_rhs does.

 
 Expressions
 ~~~~~~~~~~~
 
 Expressions
 ~~~~~~~~~~~

@@ -244,9 +196,10 @@ libCase :: LibCaseEnv
        -> CoreExpr
        -> CoreExpr
 
        -> CoreExpr
        -> CoreExpr
 

-libCase env (Var v)            = libCaseId env v
-libCase env (Lit lit)          = Lit lit
-libCase env (Type ty)          = Type ty
+libCase env (Var v)             = libCaseId env v
+libCase _   (Lit lit)           = Lit lit
+libCase _   (Type ty)           = Type ty
+libCase _   (Coercion co)       = Coercion co

 libCase env (App fun arg)       = App (libCase env fun) (libCase env arg)
 libCase env (Note note body)    = Note note (libCase env body)
 libCase env (Cast e co)         = Cast (libCase env e) co
 libCase env (App fun arg)       = App (libCase env fun) (libCase env arg)
 libCase env (Note note body)    = Note note (libCase env body)
 libCase env (Cast e co)         = Cast (libCase env e) co

@@ -260,33 +213,18 @@ libCase env (Let bind body)
     (env_body, bind') = libCaseBind env bind
 
 libCase env (Case scrut bndr ty alts)
     (env_body, bind') = libCaseBind env bind
 
 libCase env (Case scrut bndr ty alts)

-  = mkCase env (libCase env scrut) bndr ty (map (libCaseAlt env_alts) alts)
+  = Case (libCase env scrut) bndr ty (map (libCaseAlt env_alts) alts)

   where
     env_alts = addBinders (mk_alt_env scrut) [bndr]
     mk_alt_env (Var scrut_var) = addScrutedVar env scrut_var
     mk_alt_env (Cast scrut _)  = mk_alt_env scrut      -- Note [Scrutinee with cast]
   where
     env_alts = addBinders (mk_alt_env scrut) [bndr]
     mk_alt_env (Var scrut_var) = addScrutedVar env scrut_var
     mk_alt_env (Cast scrut _)  = mk_alt_env scrut      -- Note [Scrutinee with cast]

-    mk_alt_env otehr          = env
+    mk_alt_env _              = env

 
 

+libCaseAlt :: LibCaseEnv -> (AltCon, [CoreBndr], CoreExpr)
+                         -> (AltCon, [CoreBndr], CoreExpr)

 libCaseAlt env (con,args,rhs) = (con, args, libCase (addBinders env args) rhs)
 \end{code}
 
 libCaseAlt env (con,args,rhs) = (con, args, libCase (addBinders env args) rhs)
 \end{code}
 

-\begin{code}
-mkCase :: LibCaseEnv -> CoreExpr -> Id -> Type -> [CoreAlt] -> CoreExpr
--- See Note [Indexed data types]
-mkCase env scrut bndr ty [(DEFAULT,_,rhs)]
-  | Just (tycon, tys)   <- splitTyConApp_maybe (idType bndr)
-  , [(subst, fam_inst)] <- lookupFamInstEnv (lc_fams env) tycon tys
-  = let 
-       rep_tc     = famInstTyCon fam_inst
-       rep_tys    = map (substTyVar subst) (tyConTyVars rep_tc)
-       bndr'      = setIdType bndr (mkTyConApp rep_tc rep_tys)
-       Just co_tc = tyConFamilyCoercion_maybe rep_tc
-       co         = mkTyConApp co_tc rep_tys
-       bind       = NonRec bndr (Cast (Var bndr') (mkSymCoercion co))
-    in mkCase env (Cast scrut co) bndr' ty [(DEFAULT,[],Let bind rhs)]
-mkCase env scrut bndr ty alts
-  = Case scrut bndr ty alts
-\end{code}

 
 Ids
 ~~~
 
 Ids
 ~~~

@@ -303,8 +241,57 @@ libCaseId env v
   where
     rec_id_level = lookupLevel env v
     free_scruts  = freeScruts env rec_id_level
   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, scrut_at_lvl) <- lc_scruts env
+       , scrut_bind_lvl <= rec_bind_lvl
+       , scrut_at_lvl > rec_bind_lvl]
+       -- Note [When to specialise]
+       -- Note [Avoiding fruitless liberate-case]

 \end{code}
 
 \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).
+
+Note [Avoiding fruitless liberate-case]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Consider also:
+  f = \x. case top_lvl_thing of
+                I# _ -> let g = \y. ... g ...
+                        in ...
+
+Here, top_lvl_thing is scrutinised at a level (1) deeper than its
+binding site (0).  Nevertheless, we do NOT want to specialise the call
+to 'g' because all the structure in its free variables is already
+visible at the definition site for g.  Hence, when considering specialising
+an occurrence of 'g', we want to check that there's a scruted-var v st
+
+   a) v's binding site is *outside* g
+   b) v's scrutinisation site is *inside* g
+

 
 %************************************************************************
 %*                                                                     *
 
 %************************************************************************
 %*                                                                     *

@@ -341,7 +328,7 @@ addScrutedVar env@(LibCaseEnv { lc_lvl = lvl, lc_lvl_env = lvl_env,
 
   | otherwise = env
   where
 
   | otherwise = env
   where

-    scruts'  = (scrut_var, lvl) : scruts
+    scruts'  = (scrut_var, bind_lvl, lvl) : scruts

     bind_lvl = case lookupVarEnv lvl_env scrut_var of
                 Just lvl -> lvl
                 Nothing  -> topLevel
     bind_lvl = case lookupVarEnv lvl_env scrut_var of
                 Just lvl -> lvl
                 Nothing  -> topLevel

@@ -352,15 +339,8 @@ lookupRecId env id = lookupVarEnv (lc_rec_env env) id
 lookupLevel :: LibCaseEnv -> Id -> LibCaseLevel
 lookupLevel env id
   = case lookupVarEnv (lc_lvl_env env) id of
 lookupLevel :: LibCaseEnv -> Id -> LibCaseLevel
 lookupLevel env id
   = case lookupVarEnv (lc_lvl_env env) id of

-      Just lvl -> lc_lvl env
+      Just lvl -> lvl

       Nothing  -> topLevel
       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}
 
 %************************************************************************
 \end{code}
 
 %************************************************************************

@@ -379,45 +359,52 @@ topLevel = 0
 \begin{code}
 data LibCaseEnv
   = LibCaseEnv {
 \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
                                -- 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,  
 
        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, 
 
        lc_rec_env :: IdEnv CoreBind, 

-                       -- 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
-
-       lc_fams :: FamInstEnvs
-                       -- Instance env for indexed data types 
+               -- Binds *only* recursively defined ids, to their own
+               -- binding group, and *only* in their own RHSs
+
+       lc_scruts :: [(Id, LibCaseLevel, LibCaseLevel)]
+               -- Each of these Ids was scrutinised by an enclosing
+               -- case expression, at a level deeper than its binding
+               -- level.
+               -- 
+               -- The first LibCaseLevel is the *binding level* of
+               --   the scrutinised Id, 
+               -- The second is the level *at which it was scrutinised*.
+               --   (see Note [Avoiding fruitless liberate-case])
+               -- The former is a bit redundant, since you could always
+               -- look it up in lc_lvl_env, but it's just cached here
+               -- 
+               -- The order is insignificant; it's a bag really
+               -- 
+               -- There's one element per scrutinisation;
+               --    in principle the same Id may appear multiple times,
+               --    although that'd be unusual:
+               --       case x of { (a,b) -> ....(case x of ...) .. }

        }
 
        }
 

-initEnv :: DynFlags -> FamInstEnvs -> LibCaseEnv
-initEnv dflags fams
-  = LibCaseEnv { lc_size = specThreshold dflags,
+initEnv :: DynFlags -> LibCaseEnv
+initEnv dflags 
+  = LibCaseEnv { lc_size = liberateCaseThreshold dflags,

                 lc_lvl = 0,
                 lc_lvl_env = emptyVarEnv, 
                 lc_rec_env = emptyVarEnv,
                 lc_lvl = 0,
                 lc_lvl_env = emptyVarEnv, 
                 lc_rec_env = emptyVarEnv,

-                lc_scruts = [],
-                lc_fams = fams }
+                lc_scruts = [] }

 
 

+bombOutSize :: LibCaseEnv -> Maybe Int

 bombOutSize = lc_size
 \end{code}
 
 bombOutSize = lc_size
 \end{code}
 

-