[ghc-hetmet.git] / 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, mkInlineUnfolding, mkWwInlineRule )
import CoreUtils        ( exprType, exprIsHNF )
import CoreArity        ( exprArity )
import Var
import Id
import Type             ( Type )
import IdInfo
import Demand
import UniqSupply
import BasicTypes
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.

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 (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 inline things (b)]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In general, 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.

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 [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   :: 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
        -- See Note [Thunk splitting]
  = 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 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)]
 -- and Note [Don't w/w INLINABLE things]
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   = realIdUnfolding fn_id -- We want to see the unfolding 
                                        -- for loop breakers!
    inline_rule = mkInlineUnfolding Nothing rhs

---------------------
splitFun :: Id -> IdInfo -> [Demand] -> DmdResult -> Expr Var
         -> UniqSM [(Id, CoreExpr)]
splitFun 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
      (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) 
                        `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` 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_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 work_id 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 ([(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
    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 -> [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}

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?

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}