path: root/compiler/stranal/WorkWrap.lhs

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

\begin{code}
module WorkWrap ( wwTopBinds, mkWrapper ) where

import CoreSyn
import CoreUnfold	( certainlyWillInline, mkInlineRule, mkWwInlineRule )
import CoreUtils	( exprType, exprIsHNF )
import CoreArity	( exprArity )
import Var
import Id		( idType, isOneShotLambda, idUnfolding,
			  setIdStrictness, mkWorkerId,
			  setInlineActivation, setIdUnfolding,
			  setIdArity )
import Type		( Type )
import IdInfo
import Demand           ( Demand(..), StrictSig(..), DmdType(..), DmdResult(..), 
			  Demands(..), mkTopDmdType, isBotRes, returnsCPR, topSig, isAbsent
			)
import UniqSupply
import BasicTypes	( RecFlag(..), isNonRec, isNeverActive,
                          Activation, inlinePragmaActivation )
import VarEnv		( isEmptyVarEnv )
import Maybes		( orElse )
import WwLib
import Util		( lengthIs, notNull )
import Outputable
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 :: UniqSupply -> [CoreBind] -> [CoreBind]

wwTopBinds us top_binds
  = initUs_ us $ do
    top_binds' <- mapM wwBind 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	:: CoreBind
	-> UniqSM [CoreBind]	-- returns a WwBinding intermediate form;
				-- the caller will convert to Expr/Binding,
				-- as appropriate.

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

wwBind (Rec pairs)
  = return . Rec <$> concatMapM do_one pairs
  where
    do_one (binder, rhs) = do new_rhs <- wwExpr rhs
                              tryWW 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 :: CoreExpr -> UniqSM CoreExpr

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

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

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

wwExpr (Note note expr)
  = Note note <$> wwExpr expr

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

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

wwExpr (Case expr binder ty alts) = do
    new_expr <- wwExpr 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 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.

We have to BE CAREFUL that we don't worker-wrapperize an Id that has
already been w-w'd!  (You can end up with several liked-named Ids
bouncing around at the same time---absolute mischief.)  So the
criterion we use is: if an Id already has an unfolding (for whatever
reason), then we don't w-w it.

The only reason this is monadised is for the unique supply.

Note [Don't w/w inline things (a)]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
It's very important to refrain from w/w-ing an INLINE function
because the wrapper will then overwrite the InlineRule unfolding.

It was wrong with the old InlineMe Note too: if we do so by mistake 
we transform
	f = __inline (\x -> E)
into
	f = __inline (\x -> case x of (a,b) -> fw E)
	fw = \ab -> (__inline (\x -> E)) (a,b)
and the original __inline now vanishes, so E is no longer
inside its __inline wrapper.  Death!  Disaster!

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.

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 inline things (b)]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In general, therefore, we refrain from w/w-ing *small* functions,
because they'll inline anyway.  But we must take care: it may look
small now, but get to be big later after other inling 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.  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.


\begin{code}
tryWW	:: 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 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) ]

  | is_thunk && worthSplittingThunk maybe_fn_dmd res_info
  = ASSERT2( isNonRec is_rec, ppr new_fn_id )	-- The thunk must be non-recursive
    checkSize new_fn_id rhs $ 
    splitThunk new_fn_id rhs

  | is_fun && worthSplittingFun wrap_dmds res_info
  = checkSize new_fn_id rhs $
    splitFun new_fn_id fn_info wrap_dmds res_info inline_act rhs

  | otherwise
  = return [ (new_fn_id, rhs) ]

  where
    fn_info   	 = idInfo fn_id
    maybe_fn_dmd = demandInfo fn_info
    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 `orElse` topSig
    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` 
				     StrictSig (mkTopDmdType wrap_dmds res_info)

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

---------------------
checkSize :: Id -> CoreExpr
	  -> UniqSM [(Id,CoreExpr)] -> UniqSM [(Id,CoreExpr)]
 -- See Note [Don't w/w inline things (a) and (b)]
checkSize fn_id rhs thing_inside
  | isStableUnfolding unfolding	   -- For DFuns and INLINE things, leave their
  = return [ (fn_id, rhs) ]	   -- unfolding unchanged; but still attach 
    	     	     	  	   -- strictness info to the Id	

  | certainlyWillInline unfolding
  = return [ (fn_id `setIdUnfolding` inline_rule, rhs) ]
		-- Note [Don't w/w inline things (b)]

  | otherwise = thing_inside
  where
    unfolding = idUnfolding fn_id
    inline_rule = mkInlineRule unSaturatedOk rhs (unfoldingArity unfolding)

---------------------
splitFun :: Id -> IdInfo -> [Demand] -> DmdResult -> Activation -> Expr Var
         -> UniqSM [(Id, CoreExpr)]
splitFun fn_id fn_info wrap_dmds res_info inline_act 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
      (work_demands, wrap_fn, work_fn) <- mkWwBodies fun_ty wrap_dmds res_info one_shots
    ; work_uniq <- getUniqueM
    ; let
	work_rhs = work_fn rhs
	work_id  = mkWorkerId work_uniq fn_id (exprType work_rhs) 
			`setInlineActivation` inline_act
				-- Any inline activation (which sets when inlining is active) 
				-- on the original function is duplicated on the worker and wrapper
				-- 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` StrictSig (mkTopDmdType 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_id  = fn_id `setIdUnfolding` mkWwInlineRule work_id wrap_rhs arity

    ; return ([(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
  where
    fun_ty = idType fn_id

    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 -> [Bool]
get_one_shots (Lam b e)
  | isId b    = isOneShotLambda b : get_one_shots e
  | otherwise = get_one_shots e
get_one_shots (Note _ e) = get_one_shots e
get_one_shots _    	 = noOneShotInfo
\end{code}

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}
-- 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?

splitThunk :: Var -> Expr Var -> UniqSM [(Var, Expr Var)]
splitThunk fn_id rhs = do
    (_, wrap_fn, work_fn) <- mkWWstr [fn_id]
    return [ (fn_id, Let (NonRec fn_id rhs) (wrap_fn (work_fn (Var fn_id)))) ]
\end{code}


%************************************************************************
%*									*
\subsection{Functions over Demands}
%*									*
%************************************************************************

\begin{code}
worthSplittingFun :: [Demand] -> DmdResult -> Bool
		-- True <=> the wrapper would not be an identity function
worthSplittingFun ds res
  = any worth_it ds || returnsCPR res
	-- worthSplitting returns False for an empty list of demands,
	-- and hence do_strict_ww is False if arity is zero and there is no CPR
  -- See Note [Worker-wrapper for bottoming functions]
  where
    worth_it Abs	      = True	-- Absent arg
    worth_it (Eval (Prod _)) = True	-- Product arg to evaluate
    worth_it _    	      = False

worthSplittingThunk :: Maybe Demand	-- Demand on the thunk
		    -> DmdResult	-- CPR info for the thunk
		    -> Bool
worthSplittingThunk maybe_dmd res
  = worth_it maybe_dmd || returnsCPR res
  where
	-- Split if the thing is unpacked
    worth_it (Just (Eval (Prod ds))) = not (all isAbsent ds)
    worth_it _    	   	     = False
\end{code}

Note [Worker-wrapper for bottoming functions]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
We used not to split if the result is bottom.
[Justification:  there's no efficiency to be gained.]

But it's sometimes bad not to make a wrapper.  Consider
	fw = \x# -> let x = I# x# in case e of
					p1 -> error_fn x
					p2 -> error_fn x
					p3 -> the real stuff
The re-boxing code won't go away unless error_fn gets a wrapper too.
[We don't do reboxing now, but in general it's better to pass an
unboxed thing to f, and have it reboxed in the error cases....]


%************************************************************************
%*									*
\subsection{The worker wrapper core}
%*									*
%************************************************************************

@mkWrapper@ is called when importing a function.  We have the type of 
the function and the name of its worker, and we want to make its body (the wrapper).

\begin{code}
mkWrapper :: Type		-- Wrapper type
	  -> StrictSig		-- Wrapper strictness info
	  -> UniqSM (Id -> CoreExpr)	-- Wrapper body, missing worker Id

mkWrapper fun_ty (StrictSig (DmdType _ demands res_info)) = do
    (_, wrap_fn, _) <- mkWwBodies fun_ty demands res_info noOneShotInfo
    return wrap_fn

noOneShotInfo :: [Bool]
noOneShotInfo = repeat False
\end{code}