%
% (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 )
import CoreUtils ( exprType, exprIsHNF, mkInlineMe )
import CoreArity ( exprArity )
import Var
import Id
import Type ( Type )
import IdInfo
import NewDemand ( 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]
wwBind (NonRec binder rhs) = do
new_rhs <- wwExpr rhs
new_pairs <- tryWW NonRecursive binder new_rhs
return [NonRec b e | (b,e) <- new_pairs]
wwBind (Rec pairs)
= return . Rec <$> concatMapM do_one pairs
where
do_one (binder, rhs) = do new_rhs <- wwExpr rhs
tryWW Recursive binder new_rhs
\end{code}
@wwExpr@ basically just walks the tree, looking for appropriate
annotations that can be used. Remember it is @wwBind@ that does the
matching by looking for strict arguments of the correct type.
@wwExpr@ is a version that just returns the ``Plain'' Tree.
\begin{code}
wwExpr :: CoreExpr -> UniqSM CoreExpr
wwExpr e@(Type {}) = return e
wwExpr e@(Lit {}) = return e
wwExpr e@(Var {}) = return e
wwExpr e@(Note InlineMe _) = return e
wwExpr (Lam binder expr)
= Lam binder <$> 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
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
-> CoreExpr
-> UniqSM [(Id, CoreExpr)]
tryWW is_rec fn_id rhs
| isNeverActive inline_act
= return [ (fn_id, rhs) ]
| is_thunk && worthSplittingThunk maybe_fn_dmd res_info
= ASSERT2( isNonRec is_rec, ppr new_fn_id )
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 = newDemandInfo fn_info
inline_act = inlinePragmaActivation (inlinePragInfo fn_info)
strict_sig = newStrictnessInfo fn_info `orElse` topSig
StrictSig (DmdType env wrap_dmds res_info) = strict_sig
new_fn_id | isEmptyVarEnv env = fn_id
| otherwise = fn_id `setIdNewStrictness`
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)]
checkSize fn_id rhs thing_inside
| certainlyWillInline unfolding = return [ (fn_id, mkInlineMe rhs) ]
| otherwise = thing_inside
where
unfolding = idUnfolding fn_id
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 {
(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
`setIdNewStrictness` StrictSig (mkTopDmdType work_demands work_res_info)
`setIdArity` (exprArity work_rhs)
wrap_rhs = wrap_fn work_id
wrap_id = fn_id `setIdWorkerInfo` HasWorker work_id arity
; return ([(work_id, work_rhs), (wrap_id, wrap_rhs)]) })
where
fun_ty = idType fn_id
arity = arityInfo fn_info
work_res_info | isBotRes res_info = BotRes
| otherwise = TopRes
one_shots = get_one_shots rhs
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 :: 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
worthSplittingFun ds res
= any worth_it ds || returnsCPR res
where
worth_it Abs = True
worth_it (Eval (Prod _)) = True
worth_it _ = False
worthSplittingThunk :: Maybe Demand
-> DmdResult
-> Bool
worthSplittingThunk maybe_dmd res
= worth_it maybe_dmd || returnsCPR res
where
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
-> StrictSig
-> UniqSM (Id -> CoreExpr)
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}