stranal/WorkWrap.lhs

% % (c) The GRASP/AQUA Project, Glasgow University, 1993-1998 % \section[WorkWrap]{Worker/wrapper-generating back-end of strictness analyser} \begin{code}

    {-# OPTIONS -fno-warn-tabs #-}
    -- The above warning supression flag is a temporary kludge.
    -- While working on this module you are encouraged to remove it and
    -- detab the module (please do the detabbing in a separate patch). See
    --     http://ghc.haskell.org/trac/ghc/wiki/Commentary/CodingStyle#TabsvsSpaces
    -- for details

    module WorkWrap ( wwTopBinds ) where

    import CoreSyn
    import CoreUnfold	( certainlyWillInline, mkInlineUnfolding, mkWwInlineRule )
    import CoreUtils	( exprType, exprIsHNF )
    import CoreArity	( exprArity )
    import Var
    import Id
    import IdInfo
    import UniqSupply
    import BasicTypes
    import DynFlags
    import VarEnv		( isEmptyVarEnv )
    import Demand
    import WwLib
    import Util
    import Outputable
    import FamInstEnv
    import MonadUtils

    #include "HsVersions.h"

\end{code} We take Core bindings whose binders have: \begin{enumerate} \item Strictness attached (by the front-end of the strictness analyser), and / or \item Constructed Product Result information attached by the CPR analysis pass. \end{enumerate} and we return some ``plain'' bindings which have been worker/wrapper-ified, meaning: \begin{enumerate} \item Functions have been split into workers and wrappers where appropriate. If a function has both strictness and CPR properties then only one worker/wrapper doing both transformations is produced; \item Binders' @IdInfos@ have been updated to reflect the existence of these workers/wrappers (this is where we get STRICTNESS and CPR pragma info for exported values). \end{enumerate} \begin{code}

    wwTopBinds :: DynFlags -> FamInstEnvs -> UniqSupply -> CoreProgram -> CoreProgram

    wwTopBinds dflags fam_envs us top_binds
      = initUs_ us $ do
        top_binds' <- mapM (wwBind dflags fam_envs) top_binds
        return (concat top_binds')

\end{code} %************************************************************************ %* * \subsection[wwBind-wwExpr]{@wwBind@ and @wwExpr@} %* * %************************************************************************ @wwBind@ works on a binding, trying each \tr{(binder, expr)} pair in turn. Non-recursive case first, then recursive... \begin{code}

    wwBind  :: DynFlags
            -> FamInstEnvs
            -> CoreBind
    	-> UniqSM [CoreBind]	-- returns a WwBinding intermediate form;
    				-- the caller will convert to Expr/Binding,
    				-- as appropriate.

    wwBind dflags fam_envs (NonRec binder rhs) = do
        new_rhs <- wwExpr dflags fam_envs rhs
        new_pairs <- tryWW dflags fam_envs NonRecursive binder new_rhs
        return [NonRec b e | (b,e) <- new_pairs]
          -- Generated bindings must be non-recursive
          -- because the original binding was.

    wwBind dflags fam_envs (Rec pairs)
      = return . Rec <$> concatMapM do_one pairs
      where
        do_one (binder, rhs) = do new_rhs <- wwExpr dflags fam_envs rhs
                                  tryWW dflags fam_envs Recursive binder new_rhs

\end{code} @wwExpr@ basically just walks the tree, looking for appropriate annotations that can be used. Remember it is @wwBind@ that does the matching by looking for strict arguments of the correct type. @wwExpr@ is a version that just returns the ``Plain'' Tree. \begin{code}

    wwExpr :: DynFlags -> FamInstEnvs -> CoreExpr -> UniqSM CoreExpr

    wwExpr _      _ e@(Type {}) = return e
    wwExpr _      _ e@(Coercion {}) = return e
    wwExpr _      _ e@(Lit  {}) = return e
    wwExpr _      _ e@(Var  {}) = return e

    wwExpr dflags fam_envs (Lam binder expr)
      = Lam binder <$> wwExpr dflags fam_envs expr

    wwExpr dflags fam_envs (App f a)
      = App <$> wwExpr dflags fam_envs f <*> wwExpr dflags fam_envs a

    wwExpr dflags fam_envs (Tick note expr)
      = Tick note <$> wwExpr dflags fam_envs expr

    wwExpr dflags fam_envs (Cast expr co) = do
        new_expr <- wwExpr dflags fam_envs expr
        return (Cast new_expr co)

    wwExpr dflags fam_envs (Let bind expr)
      = mkLets <$> wwBind dflags fam_envs bind <*> wwExpr dflags fam_envs expr

    wwExpr dflags fam_envs (Case expr binder ty alts) = do
        new_expr <- wwExpr dflags fam_envs expr
        new_alts <- mapM ww_alt alts
        return (Case new_expr binder ty new_alts)
      where
        ww_alt (con, binders, rhs) = do
            new_rhs <- wwExpr dflags fam_envs rhs
            return (con, binders, new_rhs)

\end{code} %************************************************************************ %* * \subsection[tryWW]{@tryWW@: attempt a worker/wrapper pair} %* * %************************************************************************ @tryWW@ just accumulates arguments, converts strictness info from the front-end into the proper form, then calls @mkWwBodies@ to do the business. The only reason this is monadised is for the unique supply. Note [Don't w/w INLINE things] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ It's very important to refrain from w/w-ing an INLINE function (ie one with an InlineRule) because the wrapper will then overwrite the InlineRule unfolding. Furthermore, if the programmer has marked something as INLINE, we may lose by w/w'ing it. If the strictness analyser is run twice, this test also prevents wrappers (which are INLINEd) from being re-done. (You can end up with several liked-named Ids bouncing around at the same time---absolute mischief.) Notice that we refrain from w/w'ing an INLINE function even if it is in a recursive group. It might not be the loop breaker. (We could test for loop-breaker-hood, but I'm not sure that ever matters.) Note [Don't w/w INLINABLE things] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ If we have {-# INLINABLE f #-} f x y = .... then in principle we might get a more efficient loop by w/w'ing f. But that would make a new unfolding which would overwrite the old one. So we leave INLINABLE things alone too. This is a slight infelicity really, because it means that adding an INLINABLE pragma could make a program a bit less efficient, because you lose the worker/wrapper stuff. But I don't see a way to avoid that. Note [Don't w/w inline small non-loop-breaker things] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ In general, we refrain from w/w-ing *small* functions, which are not loop breakers, because they'll inline anyway. But we must take care: it may look small now, but get to be big later after other inlining has happened. So we take the precaution of adding an INLINE pragma to any such functions. I made this change when I observed a big function at the end of compilation with a useful strictness signature but no w-w. (It was small during demand analysis, we refrained from w/w, and then got big when something was inlined in its rhs.) When I measured it on nofib, it didn't make much difference; just a few percent improved allocation on one benchmark (bspt/Euclid.space). But nothing got worse. There is an infelicity though. We may get something like f = g val ==> g x = case gw x of r -> I# r f {- InlineStable, Template = g val -} f = case gw x of r -> I# r The code for f duplicates that for g, without any real benefit. It won't really be executed, because calls to f will go via the inlining. Note [Wrapper activation] ~~~~~~~~~~~~~~~~~~~~~~~~~ When should the wrapper inlining be active? It must not be active earlier than the current Activation of the Id (eg it might have a NOINLINE pragma). But in fact strictness analysis happens fairly late in the pipeline, and we want to prioritise specialisations over strictness. Eg if we have module Foo where f :: Num a => a -> Int -> a f n 0 = n -- Strict in the Int, hence wrapper f n x = f (n+n) (x-1) g :: Int -> Int g x = f x x -- Provokes a specialisation for f module Bsr where import Foo h :: Int -> Int h x = f 3 x Then we want the specialisation for 'f' to kick in before the wrapper does. Now in fact the 'gentle' simplification pass encourages this, by having rules on, but inlinings off. But that's kind of lucky. It seems more robust to give the wrapper an Activation of (ActiveAfter 0), so that it becomes active in an importing module at the same time that it appears in the first place in the defining module. \begin{code}

    tryWW   :: DynFlags
            -> FamInstEnvs
            -> RecFlag
    	-> Id				-- The fn binder
    	-> CoreExpr			-- The bound rhs; its innards
    					--   are already ww'd
    	-> UniqSM [(Id, CoreExpr)]	-- either *one* or *two* pairs;
    					-- if one, then no worker (only
    					-- the orig "wrapper" lives on);
    					-- if two, then a worker and a
    					-- wrapper.
    tryWW dflags fam_envs is_rec fn_id rhs
      | isNeverActive inline_act
    	-- No point in worker/wrappering if the thing is never inlined!
    	-- Because the no-inline prag will prevent the wrapper ever
    	-- being inlined at a call site.
    	--
    	-- Furthermore, don't even expose strictness info
      = return [ (fn_id, rhs) ]

      | otherwise
      = do
        let doSplit | is_fun    = splitFun dflags fam_envs new_fn_id fn_info wrap_dmds res_info rhs
                    | is_thunk  = splitThunk dflags fam_envs is_rec new_fn_id rhs
    	                                        -- See Note [Thunk splitting]
                    | otherwise = return Nothing
        try <- doSplit
        case try of
            Nothing ->    return $ [ (new_fn_id, rhs) ]
            Just binds -> checkSize dflags new_fn_id rhs binds

      where
        fn_info	 = idInfo fn_id
        inline_act   = inlinePragmaActivation (inlinePragInfo fn_info)

    	-- In practice it always will have a strictness
    	-- signature, even if it's a uninformative one
        strict_sig  = strictnessInfo fn_info
        StrictSig (DmdType env wrap_dmds res_info) = strict_sig

    	-- new_fn_id has the DmdEnv zapped.
    	--	(a) it is never used again
    	--	(b) it wastes space
    	--	(c) it becomes incorrect as things are cloned, because
    	--	    we don't push the substitution into it
        new_fn_id | isEmptyVarEnv env = fn_id
                  | otherwise         = fn_id `setIdStrictness`
                                         mkClosedStrictSig wrap_dmds res_info

        is_fun    = notNull wrap_dmds
        is_thunk  = not is_fun && not (exprIsHNF rhs)

    ---------------------
    checkSize :: DynFlags -> Id -> CoreExpr -> [(Id,CoreExpr)] -> UniqSM [(Id,CoreExpr)]
    checkSize dflags fn_id rhs thing_inside
      | isStableUnfolding (realIdUnfolding fn_id)
      = return [ (fn_id, rhs) ]
          -- See Note [Don't w/w INLINE things]
          -- and Note [Don't w/w INLINABLE things]
          -- NB: use realIdUnfolding because we want to see the unfolding
          --     even if it's a loop breaker!

      | certainlyWillInline dflags (idUnfolding fn_id)
      = return [ (fn_id `setIdUnfolding` inline_rule, rhs) ]
    	-- Note [Don't w/w inline small non-loop-breaker things]
    	-- NB: use idUnfolding because we don't want to apply
    	--     this criterion to a loop breaker!

      | otherwise = return thing_inside
      where
        inline_rule = mkInlineUnfolding Nothing rhs

    ---------------------
    splitFun :: DynFlags -> FamInstEnvs -> Id -> IdInfo -> [Demand] -> DmdResult -> CoreExpr
             -> UniqSM (Maybe [(Id, CoreExpr)])
    splitFun dflags fam_envs fn_id fn_info wrap_dmds res_info rhs
      = WARN( not (wrap_dmds `lengthIs` arity), ppr fn_id <+> (ppr arity $$ ppr wrap_dmds $$ ppr res_info) ) do
        -- The arity should match the signature
        stuff <- mkWwBodies dflags fam_envs fun_ty wrap_dmds res_info one_shots
        case stuff of
          Just (work_demands, wrap_fn, work_fn) -> do
            work_uniq <- getUniqueM
            let work_rhs = work_fn rhs
    	    work_id  = mkWorkerId work_uniq fn_id (exprType work_rhs)
    		        `setIdOccInfo` occInfo fn_info
    				-- Copy over occurrence info from parent
    				-- Notably whether it's a loop breaker
    				-- Doesn't matter much, since we will simplify next, but
    				-- seems right-er to do so

    			`setInlineActivation` (inlinePragmaActivation inl_prag)
    				-- Any inline activation (which sets when inlining is active)
    				-- on the original function is duplicated on the worker
    				-- It *matters* that the pragma stays on the wrapper
    				-- It seems sensible to have it on the worker too, although we
    				-- can't think of a compelling reason. (In ptic, INLINE things are
    				-- not w/wd). However, the RuleMatchInfo is not transferred since
                                    -- it does not make sense for workers to be constructorlike.

    			`setIdStrictness` mkClosedStrictSig work_demands work_res_info
    				-- Even though we may not be at top level,
    				-- it's ok to give it an empty DmdEnv

                            `setIdArity` (exprArity work_rhs)
                                    -- Set the arity so that the Core Lint check that the
                                    -- arity is consistent with the demand type goes through

    	    wrap_rhs  = wrap_fn work_id
    	    wrap_prag = InlinePragma { inl_inline = Inline
                                         , inl_sat    = Nothing
                                         , inl_act    = ActiveAfter 0
                                         , inl_rule   = rule_match_info }
    		-- See Note [Wrapper activation]
    		-- The RuleMatchInfo is (and must be) unaffected
    		-- The inl_inline is bound to be False, else we would not be
    		--    making a wrapper

                wrap_id   = fn_id `setIdUnfolding` mkWwInlineRule wrap_rhs arity
    			      `setInlinePragma` wrap_prag
    		              `setIdOccInfo` NoOccInfo
    			        -- Zap any loop-breaker-ness, to avoid bleating from Lint
    				-- about a loop breaker with an INLINE rule
            return $ Just [(work_id, work_rhs), (wrap_id, wrap_rhs)]
                -- Worker first, because wrapper mentions it
                -- mkWwBodies has already built a wrap_rhs with an INLINE pragma wrapped around it

          Nothing ->
            return Nothing
      where
        fun_ty          = idType fn_id
        inl_prag        = inlinePragInfo fn_info
        rule_match_info = inlinePragmaRuleMatchInfo inl_prag
        arity           = arityInfo fn_info
        		    -- The arity is set by the simplifier using exprEtaExpandArity
    		    -- So it may be more than the number of top-level-visible lambdas

        work_res_info | isBotRes res_info = botRes	-- Cpr stuff done by wrapper
    		  | otherwise	      = topRes

        one_shots = get_one_shots rhs

    -- If the original function has one-shot arguments, it is important to
    -- make the wrapper and worker have corresponding one-shot arguments too.
    -- Otherwise we spuriously float stuff out of case-expression join points,
    -- which is very annoying.
    get_one_shots :: Expr Var -> [OneShotInfo]
    get_one_shots (Lam b e)
      | isId b    = idOneShotInfo b : get_one_shots e
      | otherwise = get_one_shots e
    get_one_shots (Tick _ e) = get_one_shots e
    get_one_shots _    	 = []

\end{code} Note [Do not split void functions] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Consider this rather common form of binding: $j = \x:Void# -> ...no use of x... Since x is not used it'll be marked as absent. But there is no point in w/w-ing because we'll simply add (\y:Void#), see WwLib.mkWorerArgs. If x has a more interesting type (eg Int, or Int#), there *is* a point in w/w so that we don't pass the argument at all. Note [Thunk splitting] ~~~~~~~~~~~~~~~~~~~~~~ Suppose x is used strictly (never mind whether it has the CPR property). let x* = x-rhs in body splitThunk transforms like this: let x* = case x-rhs of { I# a -> I# a } in body Now simplifier will transform to case x-rhs of I# a -> let x* = I# a in body which is what we want. Now suppose x-rhs is itself a case: x-rhs = case e of { T -> I# a; F -> I# b } The join point will abstract over a, rather than over (which is what would have happened before) which is fine. Notice that x certainly has the CPR property now! In fact, splitThunk uses the function argument w/w splitting function, so that if x's demand is deeper (say U(U(L,L),L)) then the splitting will go deeper too. \begin{code}

    -- See Note [Thunk splitting]
    -- splitThunk converts the *non-recursive* binding
    --	x = e
    -- into
    --	x = let x = e
    --	    in case x of
    --		 I# y -> let x = I# y in x }
    -- See comments above. Is it not beautifully short?
    -- Moreover, it works just as well when there are
    -- several binders, and if the binders are lifted
    -- E.g.     x = e
    --     -->  x = let x = e in
    --              case x of (a,b) -> let x = (a,b)  in x

    splitThunk :: DynFlags -> FamInstEnvs -> RecFlag -> Var -> Expr Var -> UniqSM (Maybe [(Var, Expr Var)])
    splitThunk dflags fam_envs is_rec fn_id rhs = do
        (useful,_, wrap_fn, work_fn) <- mkWWstr dflags fam_envs [fn_id]
        let res = [ (fn_id, Let (NonRec fn_id rhs) (wrap_fn (work_fn (Var fn_id)))) ]
        if useful then ASSERT2( isNonRec is_rec, ppr fn_id ) -- The thunk must be non-recursive
                       return (Just res)
                  else return Nothing

\end{code}