Add (a) CoreM monad, (b) new Annotations feature

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

diff --git a/compiler/simplCore/LiberateCase.lhs b/compiler/simplCore/LiberateCase.lhs
index 67d2e5c..4b1055b 100644 (file)
--- a/compiler/simplCore/LiberateCase.lhs
+++ b/compiler/simplCore/LiberateCase.lhs
@@ -8,18 +8,16 @@ module LiberateCase ( liberateCase ) where
 
 #include "HsVersions.h"
 
 
 #include "HsVersions.h"
 

-import DynFlags                ( DynFlags, DynFlag(..) )
-import StaticFlags     ( opt_LiberateCaseThreshold )
-import CoreLint                ( showPass, endPass )
+import DynFlags

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

-import Id              ( Id, setIdName, idName, setIdNotExported )
+import Id

 import VarEnv
 import VarEnv

-import Name            ( localiseName )
-import Outputable

 import Util             ( notNull )
 \end{code}
 
 import Util             ( notNull )
 \end{code}
 

+The liberate-case transformation
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

 This module walks over @Core@, and looks for @case@ on free variables.
 The criterion is:
        if there is case on a free on the route to the recursive call,
 This module walks over @Core@, and looks for @case@ on free variables.
 The criterion is:
        if there is case on a free on the route to the recursive call,

@@ -27,59 +25,35 @@ The criterion is:
 
 Example
 
 
 Example
 

-\begin{verbatim}
-f = \ t -> case v of
-              V a b -> a : f t
-\end{verbatim}
+   f = \ t -> case v of
+                V a b -> a : f t

 
 => the inner f is replaced.
 
 
 => the inner f is replaced.
 

-\begin{verbatim}
-f = \ t -> case v of
-              V a b -> a : (letrec
+   f = \ t -> case v of
+                V a b -> a : (letrec

                                f =  \ t -> case v of
                                               V a b -> a : f t
                                f =  \ t -> case v of
                                               V a b -> a : f t

-                            in f) t
-\end{verbatim}
+                              in f) t

 (note the NEED for shadowing)
 
 => Simplify
 
 (note the NEED for shadowing)
 
 => Simplify
 

-\begin{verbatim}
-f = \ t -> case v of
-              V a b -> a : (letrec
+  f = \ t -> case v of
+                V a b -> a : (letrec

                                f = \ t -> a : f t
                                f = \ t -> a : f t

-                            in f t)
-\begin{verbatim}
+                              in f t)

 
 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]
 ~~~~~~~~~~~~~~~~~~~~~~~~~~

@@ -87,13 +61,30 @@ Consider this:
     f = \ t -> case (v `cast` co) of
                 V a b -> a : f t
 
     f = \ t -> case (v `cast` co) of
                 V a b -> a : f t
 

-Exactly the same optimistaion (unrolling one call to f) will work here, 
+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

@@ -105,7 +96,6 @@ big.
 
 Data types
 ~~~~~~~~~~
 
 Data types
 ~~~~~~~~~~

-

 The ``level'' of a binder tells how many
 recursive defns lexically enclose the binding
 A recursive defn "encloses" its RHS, not its
 The ``level'' of a binder tells how many
 recursive defns lexically enclose the binding
 A recursive defn "encloses" its RHS, not its

@@ -119,76 +109,32 @@ 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).
 

-\begin{code}
-type LibCaseLevel = Int

 
 

-topLevel :: LibCaseLevel
-topLevel = 0
-\end{code}
-
-\begin{code}
-data LibCaseEnv
-  = LibCaseEnv {
-       lc_size :: Int,         -- Bomb-out size for deciding if
-                               -- potential liberatees are too big.
-                               -- (passed in from cmd-line args)
-
-       lc_lvl :: LibCaseLevel, -- Current level
-
-       lc_lvl_env :: IdEnv LibCaseLevel,  
-                       -- Binds all non-top-level in-scope Ids
-                       -- (top-level and imported things have
-                       -- a level of zero)
+%************************************************************************
+%*                                                                     *
+        Top-level code
+%*                                                                     *
+%************************************************************************

 
 

-       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 
-       }
-
-initEnv :: Int -> LibCaseEnv
-initEnv bomb_size
-  = LibCaseEnv { lc_size = bomb_size, lc_lvl = 0,
-                lc_lvl_env = emptyVarEnv, lc_rec_env = emptyVarEnv,
-                lc_scruts = [] }
-
-bombOutSize = lc_size
-\end{code}
-
-
-Programs
-~~~~~~~~

 \begin{code}
 \begin{code}

-liberateCase :: DynFlags -> [CoreBind] -> IO [CoreBind]
-liberateCase dflags binds
-  = do {
-       showPass dflags "Liberate case" ;
-       let { binds' = do_prog (initEnv opt_LiberateCaseThreshold) binds } ;
-       endPass dflags "Liberate case" Opt_D_verbose_core2core binds'
-                               {- no specific flag for dumping -} 
-    }
+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
 \end{code}
 
     do_prog env (bind:binds) = bind' : do_prog env' binds
                             where
                               (env', bind') = libCaseBind env bind
 \end{code}
 

+
+%************************************************************************
+%*                                                                     *
+        Main payload
+%*                                                                     *
+%************************************************************************
+

 Bindings
 ~~~~~~~~
 Bindings
 ~~~~~~~~

-

 \begin{code}
 libCaseBind :: LibCaseEnv -> CoreBind -> (LibCaseEnv, CoreBind)
 
 \begin{code}
 libCaseBind :: LibCaseEnv -> CoreBind -> (LibCaseEnv, CoreBind)
 

@@ -198,22 +144,22 @@ 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, _rhss) = unzip 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
+    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
        -- 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)
+    extended_env = addRecBinds env [ (localiseId binder, libCase env_body rhs)

                                   | (binder, rhs) <- pairs ]
 
                                   | (binder, rhs) <- pairs ]
 

-       -- Two subtle things: 
+       -- 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
        -- (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

@@ -223,10 +169,11 @@ libCaseBind env (Rec pairs)
        -- (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.
        -- (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
+    rhs_small_enough (id,rhs)
+       =  idArity id > 0       -- Note [Only functions!]
+       && maybe True (\size -> couldBeSmallEnoughToInline size rhs)
+                      (bombOutSize env)

 \end{code}
 
 
 \end{code}
 
 

@@ -238,9 +185,9 @@ 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 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

@@ -259,11 +206,14 @@ libCase env (Case scrut bndr ty alts)
     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]
     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}
 

+

 Ids
 ~~~
 \begin{code}
 Ids
 ~~~
 \begin{code}

@@ -279,12 +229,46 @@ 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) <- lc_scruts env
+       , scrut_bind_lvl <= rec_bind_lvl]
+       -- Note [When to specialise]

 \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).
+
+
+%************************************************************************
+%*                                                                     *
+       Utility functions
+%*                                                                     *
+%************************************************************************

 
 

-Utility functions
-~~~~~~~~~~~~~~~~~

 \begin{code}
 addBinders :: LibCaseEnv -> [CoreBndr] -> LibCaseEnv
 addBinders env@(LibCaseEnv { lc_lvl = lvl, lc_lvl_env = lvl_env }) binders
 \begin{code}
 addBinders :: LibCaseEnv -> [CoreBndr] -> LibCaseEnv
 addBinders env@(LibCaseEnv { lc_lvl = lvl, lc_lvl_env = lvl_env }) binders

@@ -314,7 +298,7 @@ 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) : 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

@@ -325,13 +309,62 @@ 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

+\end{code}

 
 

-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]
+%************************************************************************
+%*                                                                     *
+        The environment
+%*                                                                     *
+%************************************************************************
+
+\begin{code}
+type LibCaseLevel = Int
+
+topLevel :: LibCaseLevel
+topLevel = 0
+\end{code}
+
+\begin{code}
+data LibCaseEnv
+  = LibCaseEnv {
+       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)
+
+       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, 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 = liberateCaseThreshold dflags,
+                lc_lvl = 0,
+                lc_lvl_env = emptyVarEnv, 
+                lc_rec_env = emptyVarEnv,
+                lc_scruts = [] }
+
+bombOutSize :: LibCaseEnv -> Maybe Int
+bombOutSize = lc_size

 \end{code}
 \end{code}

+
+