SetLevels.lhs 44 KB
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%
% (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
%
\section{SetLevels}

		***************************
			Overview
		***************************

1. We attach binding levels to Core bindings, in preparation for floating
   outwards (@FloatOut@).

2. We also let-ify many expressions (notably case scrutinees), so they
   will have a fighting chance of being floated sensible.

3. We clone the binders of any floatable let-binding, so that when it is
   floated out it will be unique.  (This used to be done by the simplifier
   but the latter now only ensures that there's no shadowing; indeed, even 
   that may not be true.)

   NOTE: this can't be done using the uniqAway idea, because the variable
 	 must be unique in the whole program, not just its current scope,
	 because two variables in different scopes may float out to the
	 same top level place

   NOTE: Very tiresomely, we must apply this substitution to
	 the rules stored inside a variable too.

   We do *not* clone top-level bindings, because some of them must not change,
   but we *do* clone bindings that are heading for the top level

4. In the expression
	case x of wild { p -> ...wild... }
   we substitute x for wild in the RHS of the case alternatives:
	case x of wild { p -> ...x... }
   This means that a sub-expression involving x is not "trapped" inside the RHS.
   And it's not inconvenient because we already have a substitution.

  Note that this is EXACTLY BACKWARDS from the what the simplifier does.
  The simplifier tries to get rid of occurrences of x, in favour of wild,
  in the hope that there will only be one remaining occurrence of x, namely
  the scrutinee of the case, and we can inline it.  

\begin{code}
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{-# 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://hackage.haskell.org/trac/ghc/wiki/Commentary/CodingStyle#TabsvsSpaces
-- for details

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module SetLevels (
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	setLevels, 
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	Level(..), tOP_LEVEL,
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	LevelledBind, LevelledExpr, LevelledBndr,
	FloatSpec(..), floatSpecLevel,
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	incMinorLvl, ltMajLvl, ltLvl, isTopLvl
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    ) where

#include "HsVersions.h"

import CoreSyn
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import CoreMonad	( FloatOutSwitches(..) )
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import CoreUtils	( exprType, exprOkForSpeculation )
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import CoreArity	( exprBotStrictness_maybe )
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import CoreFVs		-- all of it
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import Coercion         ( isCoVar )
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import CoreSubst	( Subst, emptySubst, extendInScope, substBndr, substRecBndrs,
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			  extendIdSubst, extendSubstWithVar, cloneBndr, cloneRecIdBndrs, substTy, substCo )
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import Id
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import IdInfo
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import Var
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import VarSet
import VarEnv
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import Literal		( litIsTrivial )
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import Demand		( StrictSig, increaseStrictSigArity )
import Name		( getOccName, mkSystemVarName )
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import OccName		( occNameString )
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import Type		( isUnLiftedType, Type, sortQuantVars, mkPiTypes )
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import Kind		( kiVarsOfKinds )
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import BasicTypes	( Arity )
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import UniqSupply
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import Util
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import MonadUtils
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import Outputable
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import FastString
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\end{code}

%************************************************************************
%*									*
\subsection{Level numbers}
%*									*
%************************************************************************

\begin{code}
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type LevelledExpr = TaggedExpr FloatSpec
type LevelledBind = TaggedBind FloatSpec
type LevelledBndr = TaggedBndr FloatSpec

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data Level = Level Int	-- Level number of enclosing lambdas
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	  	   Int	-- Number of big-lambda and/or case expressions between
			-- here and the nearest enclosing lambda
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data FloatSpec 
  = FloatMe Level	-- Float to just inside the binding 
    	    		--    tagged with this level
  | StayPut Level	-- Stay where it is; binding is
    	    		--     tagged with tihs level

floatSpecLevel :: FloatSpec -> Level
floatSpecLevel (FloatMe l) = l
floatSpecLevel (StayPut l) = l
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\end{code}

The {\em level number} on a (type-)lambda-bound variable is the
nesting depth of the (type-)lambda which binds it.  The outermost lambda
has level 1, so (Level 0 0) means that the variable is bound outside any lambda.

On an expression, it's the maximum level number of its free
(type-)variables.  On a let(rec)-bound variable, it's the level of its
RHS.  On a case-bound variable, it's the number of enclosing lambdas.

Top-level variables: level~0.  Those bound on the RHS of a top-level
definition but ``before'' a lambda; e.g., the \tr{x} in (levels shown
as ``subscripts'')...
\begin{verbatim}
a_0 = let  b_? = ...  in
	   x_1 = ... b ... in ...
\end{verbatim}

The main function @lvlExpr@ carries a ``context level'' (@ctxt_lvl@).
That's meant to be the level number of the enclosing binder in the
final (floated) program.  If the level number of a sub-expression is
less than that of the context, then it might be worth let-binding the
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sub-expression so that it will indeed float.  

If you can float to level @Level 0 0@ worth doing so because then your
allocation becomes static instead of dynamic.  We always start with
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context @Level 0 0@.  
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Note [FloatOut inside INLINE]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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@InlineCtxt@ very similar to @Level 0 0@, but is used for one purpose:
to say "don't float anything out of here".  That's exactly what we
want for the body of an INLINE, where we don't want to float anything
out at all.  See notes with lvlMFE below.

But, check this out:

-- At one time I tried the effect of not float anything out of an InlineMe,
-- but it sometimes works badly.  For example, consider PrelArr.done.  It
-- has the form 	__inline (\d. e)
-- where e doesn't mention d.  If we float this to 
--	__inline (let x = e in \d. x)
-- things are bad.  The inliner doesn't even inline it because it doesn't look
-- like a head-normal form.  So it seems a lesser evil to let things float.
-- In SetLevels we do set the context to (Level 0 0) when we get to an InlineMe
-- which discourages floating out.

So the conclusion is: don't do any floating at all inside an InlineMe.
(In the above example, don't float the {x=e} out of the \d.)

One particular case is that of workers: we don't want to float the
call to the worker outside the wrapper, otherwise the worker might get
inlined into the floated expression, and an importing module won't see
the worker at all.

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\begin{code}
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instance Outputable FloatSpec where
  ppr (FloatMe l) = char 'F' <> ppr l
  ppr (StayPut l) = ppr l
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tOP_LEVEL :: Level
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tOP_LEVEL   = Level 0 0
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incMajorLvl :: Level -> Level
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incMajorLvl (Level major _) = Level (major + 1) 0
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incMinorLvl :: Level -> Level
incMinorLvl (Level major minor) = Level major (minor+1)

maxLvl :: Level -> Level -> Level
maxLvl l1@(Level maj1 min1) l2@(Level maj2 min2)
  | (maj1 > maj2) || (maj1 == maj2 && min1 > min2) = l1
  | otherwise					   = l2

ltLvl :: Level -> Level -> Bool
ltLvl (Level maj1 min1) (Level maj2 min2)
  = (maj1 < maj2) || (maj1 == maj2 && min1 < min2)

ltMajLvl :: Level -> Level -> Bool
    -- Tells if one level belongs to a difft *lambda* level to another
ltMajLvl (Level maj1 _) (Level maj2 _) = maj1 < maj2

isTopLvl :: Level -> Bool
isTopLvl (Level 0 0) = True
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isTopLvl _           = False
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instance Outputable Level where
  ppr (Level maj min) = hcat [ char '<', int maj, char ',', int min, char '>' ]

instance Eq Level where
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  (Level maj1 min1) == (Level maj2 min2) = maj1 == maj2 && min1 == min2
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\end{code}

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%************************************************************************
%*									*
\subsection{Main level-setting code}
%*									*
%************************************************************************

\begin{code}
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setLevels :: FloatOutSwitches
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	  -> CoreProgram
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	  -> UniqSupply
	  -> [LevelledBind]

setLevels float_lams binds us
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  = initLvl us (do_them init_env binds)
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  where
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    init_env = initialEnv float_lams
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    do_them :: LevelEnv -> [CoreBind] -> LvlM [LevelledBind]
    do_them _ [] = return []
    do_them env (b:bs)
      = do { (lvld_bind, env') <- lvlTopBind env b
           ; lvld_binds <- do_them env' bs
           ; return (lvld_bind : lvld_binds) }

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lvlTopBind :: LevelEnv -> Bind Id -> LvlM (LevelledBind, LevelEnv)
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lvlTopBind env (NonRec bndr rhs)
  = do rhs' <- lvlExpr tOP_LEVEL env (freeVars rhs)
       let  bndr' = TB bndr (StayPut tOP_LEVEL)
            env'  = extendLvlEnv env [bndr']
       return (NonRec bndr' rhs', env')
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lvlTopBind env (Rec pairs)
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  = do let (bndrs,rhss) = unzip pairs
           bndrs' = [TB b (StayPut tOP_LEVEL) | b <- bndrs]
           env'   = extendLvlEnv env bndrs'
       rhss' <- mapM (lvlExpr tOP_LEVEL env' . freeVars) rhss
       return (Rec (bndrs' `zip` rhss'), env')
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\end{code}

%************************************************************************
%*									*
\subsection{Setting expression levels}
%*									*
%************************************************************************

\begin{code}
lvlExpr :: Level		-- ctxt_lvl: Level of enclosing expression
	-> LevelEnv		-- Level of in-scope names/tyvars
	-> CoreExprWithFVs	-- input expression
	-> LvlM LevelledExpr	-- Result expression
\end{code}

The @ctxt_lvl@ is, roughly, the level of the innermost enclosing
binder.  Here's an example

	v = \x -> ...\y -> let r = case (..x..) of
					..x..
			   in ..

When looking at the rhs of @r@, @ctxt_lvl@ will be 1 because that's
the level of @r@, even though it's inside a level-2 @\y@.  It's
important that @ctxt_lvl@ is 1 and not 2 in @r@'s rhs, because we
don't want @lvlExpr@ to turn the scrutinee of the @case@ into an MFE
--- because it isn't a *maximal* free expression.

If there were another lambda in @r@'s rhs, it would get level-2 as well.

\begin{code}
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lvlExpr _ env (_, AnnType ty) = return (Type (substTy (le_subst env) ty))
lvlExpr _ env (_, AnnCoercion co) = return (Coercion (substCo (le_subst env) co))
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lvlExpr _ env (_, AnnVar v)   = return (lookupVar env v)
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lvlExpr _ _   (_, AnnLit lit) = return (Lit lit)
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lvlExpr ctxt_lvl env expr@(_, AnnApp _ _) = do
    let
      (fun, args) = collectAnnArgs expr
    --
    case fun of
         -- float out partial applications.  This is very beneficial
         -- in some cases (-7% runtime -4% alloc over nofib -O2).
         -- In order to float a PAP, there must be a function at the
         -- head of the application, and the application must be
         -- over-saturated with respect to the function's arity.
      (_, AnnVar f) | floatPAPs env &&
                      arity > 0 && arity < n_val_args ->
        do
         let (lapp, rargs) = left (n_val_args - arity) expr []
         rargs' <- mapM (lvlMFE False ctxt_lvl env) rargs
         lapp' <- lvlMFE False ctxt_lvl env lapp
         return (foldl App lapp' rargs')
        where
         n_val_args = count (isValArg . deAnnotate) args
         arity = idArity f

         -- separate out the PAP that we are floating from the extra
         -- arguments, by traversing the spine until we have collected
         -- (n_val_args - arity) value arguments.
         left 0 e               rargs = (e, rargs)
         left n (_, AnnApp f a) rargs
            | isValArg (deAnnotate a) = left (n-1) f (a:rargs)
            | otherwise               = left n     f (a:rargs)
         left _ _ _                   = panic "SetLevels.lvlExpr.left"

         -- No PAPs that we can float: just carry on with the
         -- arguments and the function.
      _otherwise -> do
         args' <- mapM (lvlMFE False ctxt_lvl env) args
         fun'  <- lvlExpr ctxt_lvl env fun
         return (foldl App fun' args')
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lvlExpr ctxt_lvl env (_, AnnTick tickish expr) = do
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    expr' <- lvlExpr ctxt_lvl env expr
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    return (Tick tickish expr')
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lvlExpr ctxt_lvl env (_, AnnCast expr (_, co)) = do
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    expr' <- lvlExpr ctxt_lvl env expr
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    return (Cast expr' (substCo (le_subst env) co))
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-- We don't split adjacent lambdas.  That is, given
--	\x y -> (x+1,y)
-- we don't float to give 
--	\x -> let v = x+y in \y -> (v,y)
-- Why not?  Because partial applications are fairly rare, and splitting
-- lambdas makes them more expensive.

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lvlExpr ctxt_lvl env expr@(_, AnnLam {}) = do
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    new_body <- lvlMFE True new_lvl new_env body
    return (mkLams new_bndrs new_body)
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  where 
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    (bndrs, body)	 = collectAnnBndrs expr
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    (new_lvl, new_bndrs) = lvlLamBndrs ctxt_lvl bndrs
    new_env 		 = extendLvlEnv env new_bndrs
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	-- At one time we called a special verion of collectBinders,
	-- which ignored coercions, because we don't want to split
	-- a lambda like this (\x -> coerce t (\s -> ...))
	-- This used to happen quite a bit in state-transformer programs,
	-- but not nearly so much now non-recursive newtypes are transparent.
	-- [See SetLevels rev 1.50 for a version with this approach.]
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lvlExpr ctxt_lvl env (_, AnnLet bind body) = do
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    (bind', new_lvl, new_env) <- lvlBind ctxt_lvl env bind
    body' <- lvlExpr new_lvl new_env body
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    return (Let bind' body')
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lvlExpr ctxt_lvl env (_, AnnCase scrut@(scrut_fvs,_) case_bndr ty alts)
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  = do { scrut' <- lvlMFE True ctxt_lvl env scrut
       ; lvlCase ctxt_lvl env scrut_fvs scrut' case_bndr ty alts }

-------------------------------------------
lvlCase :: Level		-- ctxt_lvl: Level of enclosing expression
	-> LevelEnv		-- Level of in-scope names/tyvars
        -> VarSet		-- Free vars of input scrutinee
        -> LevelledExpr		-- Processed scrutinee
	-> Id -> Type		-- Case binder and result type
	-> [AnnAlt Id VarSet]	-- Input alternatives
	-> LvlM LevelledExpr	-- Result expression
lvlCase ctxt_lvl env scrut_fvs scrut' case_bndr ty alts
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  | [(con@(DataAlt {}), bs, rhs)] <- alts
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  , exprOkForSpeculation scrut'	  -- See Note [Check the output scrutinee for okForSpec]
  , not (isTopLvl dest_lvl)	  -- Can't have top-level cases
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  =     -- See Note [Floating cases]
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    	-- Always float the case if possible
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  	-- Unlike lets we don't insist that it escapes a value lambda
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    do { (rhs_env, (case_bndr':bs')) <- cloneVars env (case_bndr:bs) dest_lvl
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       	 	   -- We don't need to use extendCaseBndrLvlEnv here
		   -- because we are floating the case outwards so
		   -- no need to do the binder-swap thing
       ; rhs' <- lvlMFE True ctxt_lvl rhs_env rhs
       ; let alt' = (con, [TB b (StayPut dest_lvl) | b <- bs'], rhs')
       ; return (Case scrut' (TB case_bndr' (FloatMe dest_lvl)) ty [alt']) }

  | otherwise	  -- Stays put
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  = do { let case_bndr' = TB case_bndr bndr_spec
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             alts_env   = extendCaseBndrLvlEnv env scrut' case_bndr'
       ; alts' <- mapM (lvl_alt alts_env) alts
       ; return (Case scrut' case_bndr' ty alts') }
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  where
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      incd_lvl  = incMinorLvl ctxt_lvl
      bndr_spec = StayPut incd_lvl
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      dest_lvl = maxFvLevel (const True) env scrut_fvs
   	      -- Don't abstact over type variables, hence const True

      lvl_alt alts_env (con, bs, rhs)
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        = do { rhs' <- lvlMFE True incd_lvl new_env rhs
             ; return (con, bs', rhs') }
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        where
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          bs'     = [ TB b bndr_spec | b <- bs ]
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          new_env = extendLvlEnv alts_env bs'
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\end{code}

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Note [Floating cases]
~~~~~~~~~~~~~~~~~~~~~
Consider this:
  data T a = MkT !a
  f :: T Int -> blah
  f x vs = case x of { MkT y -> 
             let f vs = ...(case y of I# w -> e)...f..
             in f vs
Here we can float the (case y ...) out , because y is sure
to be evaluated, to give
  f x vs = case x of { MkT y -> 
           caes y of I# w ->
             let f vs = ...(e)...f..
             in f vs

That saves unboxing it every time round the loop.  It's important in
some DPH stuff where we really want to avoid that repeated unboxing in
the inner loop.

Things to note
 * We can't float a case to top level
 * It's worth doing this float even if we don't float
   the case outside a value lambda
 * We only do this with a single-alternative case

Note [Check the output scrutinee for okForSpec]
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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Consider this:
  case x of y { 
    A -> ....(case y of alts)....
  }
Because of the binder-swap, the inner case will get substituted to
(case x of ..).  So when testing whether the scrutinee is
okForSpecuation we must be careful to test the *result* scrutinee ('x'
in this case), not the *input* one 'y'.  The latter *is* ok for
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speculation here, but the former is not -- and indeed we can't float
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the inner case out, at least not unless x is also evaluated at its
binding site.

That's why we apply exprOkForSpeculation to scrut' and not to scrut.
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\begin{code}
lvlMFE ::  Bool			-- True <=> strict context [body of case or let]
	-> Level		-- Level of innermost enclosing lambda/tylam
	-> LevelEnv		-- Level of in-scope names/tyvars
	-> CoreExprWithFVs	-- input expression
	-> LvlM LevelledExpr	-- Result expression
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-- lvlMFE is just like lvlExpr, except that it might let-bind
-- the expression, so that it can itself be floated.
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lvlMFE _ _ env (_, AnnType ty)
  = return (Type (substTy (le_subst env) ty))
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-- No point in floating out an expression wrapped in a coercion or note
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-- If we do we'll transform  lvl = e |> co 
--			 to  lvl' = e; lvl = lvl' |> co
-- and then inline lvl.  Better just to float out the payload.
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lvlMFE strict_ctxt ctxt_lvl env (_, AnnTick t e)
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  = do { e' <- lvlMFE strict_ctxt ctxt_lvl env e
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       ; return (Tick t e') }
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lvlMFE strict_ctxt ctxt_lvl env (_, AnnCast e (_, co))
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  = do	{ e' <- lvlMFE strict_ctxt ctxt_lvl env e
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	; return (Cast e' (substCo (le_subst env) co)) }
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-- Note [Case MFEs]
lvlMFE True ctxt_lvl env e@(_, AnnCase {})
  = lvlExpr ctxt_lvl env e     -- Don't share cases

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lvlMFE strict_ctxt ctxt_lvl env ann_expr@(fvs, _)
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  |  isUnLiftedType ty		-- Can't let-bind it; see Note [Unlifted MFEs]
     		    		-- This includes coercions, which we don't
				-- want to float anyway
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  || notWorthFloating ann_expr abs_vars
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  || not float_me
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  = 	-- Don't float it out
    lvlExpr ctxt_lvl env ann_expr

  | otherwise	-- Float it out!
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  = do expr' <- lvlFloatRhs abs_vars dest_lvl env ann_expr
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       var <- newLvlVar abs_vars ty mb_bot
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       return (Let (NonRec (TB var (FloatMe dest_lvl)) expr') 
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                   (mkVarApps (Var var) abs_vars))
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  where
    expr     = deAnnotate ann_expr
    ty       = exprType expr
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    mb_bot   = exprBotStrictness_maybe expr
    dest_lvl = destLevel env fvs (isFunction ann_expr) mb_bot
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    abs_vars = abstractVars dest_lvl env fvs

	-- A decision to float entails let-binding this thing, and we only do 
	-- that if we'll escape a value lambda, or will go to the top level.
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    float_me = dest_lvl `ltMajLvl` ctxt_lvl		-- Escapes a value lambda
    	     	-- OLD CODE: not (exprIsCheap expr) || isTopLvl dest_lvl
		-- 	     see Note [Escaping a value lambda]

            || (isTopLvl dest_lvl 	-- Only float if we are going to the top level
         	&& floatConsts env	--   and the floatConsts flag is on
              	&& not strict_ctxt)	-- Don't float from a strict context	
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	  -- We are keen to float something to the top level, even if it does not
	  -- escape a lambda, because then it needs no allocation.  But it's controlled
	  -- by a flag, because doing this too early loses opportunities for RULES
	  -- which (needless to say) are important in some nofib programs
	  -- (gcd is an example).
	  --
	  -- Beware:
	  --	concat = /\ a -> foldr ..a.. (++) []
	  -- was getting turned into
	  --	lvl    = /\ a -> foldr ..a.. (++) []
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	  --	concat = /\ a -> lvl a
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	  -- which is pretty stupid.  Hence the strict_ctxt test
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	  -- 
	  -- Also a strict contxt includes uboxed values, and they
	  -- can't be bound at top level
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\end{code}

Note [Unlifted MFEs]
~~~~~~~~~~~~~~~~~~~~
We don't float unlifted MFEs, which potentially loses big opportunites.
For example:
	\x -> f (h y)
where h :: Int -> Int# is expensive. We'd like to float the (h y) outside
the \x, but we don't because it's unboxed.  Possible solution: box it.

Note [Bottoming floats]
~~~~~~~~~~~~~~~~~~~~~~~
If we see
	f = \x. g (error "urk")
we'd like to float the call to error, to get
	lvl = error "urk"
	f = \x. g lvl
Furthermore, we want to float a bottoming expression even if it has free
variables:
	f = \x. g (let v = h x in error ("urk" ++ v))
Then we'd like to abstact over 'x' can float the whole arg of g:
	lvl = \x. let v = h x in error ("urk" ++ v)
	f = \x. g (lvl x)
See Maessen's paper 1999 "Bottom extraction: factoring error handling out
of functional programs" (unpublished I think).

When we do this, we set the strictness and arity of the new bottoming 
Id, so that it's properly exposed as such in the interface file, even if
this is all happening after strictness analysis.  
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Note [Bottoming floats: eta expansion] c.f Note [Bottoming floats]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Tiresomely, though, the simplifier has an invariant that the manifest
arity of the RHS should be the same as the arity; but we can't call
etaExpand during SetLevels because it works over a decorated form of
CoreExpr.  So we do the eta expansion later, in FloatOut.

Note [Case MFEs]
~~~~~~~~~~~~~~~~
We don't float a case expression as an MFE from a strict context.  Why not?
Because in doing so we share a tiny bit of computation (the switch) but
in exchange we build a thunk, which is bad.  This case reduces allocation 
by 7% in spectral/puzzle (a rather strange benchmark) and 1.2% in real/fem.
Doesn't change any other allocation at all.

\begin{code}
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annotateBotStr :: Id -> Maybe (Arity, StrictSig) -> Id
annotateBotStr id Nothing            = id
annotateBotStr id (Just (arity,sig)) = id `setIdArity` arity
				          `setIdStrictness` sig

notWorthFloating :: CoreExprWithFVs -> [Var] -> Bool
-- Returns True if the expression would be replaced by
-- something bigger than it is now.  For example:
--   abs_vars = tvars only:  return True if e is trivial, 
--                           but False for anything bigger
--   abs_vars = [x] (an Id): return True for trivial, or an application (f x)
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--   	      	    	     but False for (f x x)
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--
-- One big goal is that floating should be idempotent.  Eg if
-- we replace e with (lvl79 x y) and then run FloatOut again, don't want
-- to replace (lvl79 x y) with (lvl83 x y)!

notWorthFloating e abs_vars
  = go e (count isId abs_vars)
  where
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    go (_, AnnVar {}) n    = n >= 0
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    go (_, AnnLit lit) n   = ASSERT( n==0 ) 
                             litIsTrivial lit	-- Note [Floating literals]
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    go (_, AnnCast e _)  n = go e n
    go (_, AnnApp e arg) n 
       | (_, AnnType {}) <- arg = go e n
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       | (_, AnnCoercion {}) <- arg = go e n
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       | n==0                   = False
       | is_triv arg       	= go e (n-1)
       | otherwise         	= False
    go _ _                 	= False

    is_triv (_, AnnLit {})   	       	  = True	-- Treat all literals as trivial
    is_triv (_, AnnVar {})   	       	  = True	-- (ie not worth floating)
    is_triv (_, AnnCast e _) 	       	  = is_triv e
    is_triv (_, AnnApp e (_, AnnType {})) = is_triv e
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    is_triv (_, AnnApp e (_, AnnCoercion {})) = is_triv e
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    is_triv _                             = False     
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\end{code}

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Note [Floating literals]
~~~~~~~~~~~~~~~~~~~~~~~~
It's important to float Integer literals, so that they get shared,
rather than being allocated every time round the loop.
Hence the litIsTrivial.

We'd *like* to share MachStr literal strings too, mainly so we could
CSE them, but alas can't do so directly because they are unlifted.


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Note [Escaping a value lambda]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
We want to float even cheap expressions out of value lambdas, 
because that saves allocation.  Consider
	f = \x.  .. (\y.e) ...
Then we'd like to avoid allocating the (\y.e) every time we call f,
(assuming e does not mention x).   

An example where this really makes a difference is simplrun009.

Another reason it's good is because it makes SpecContr fire on functions.
Consider
	f = \x. ....(f (\y.e))....
After floating we get
	lvl = \y.e
	f = \x. ....(f lvl)...
and that is much easier for SpecConstr to generate a robust specialisation for.

The OLD CODE (given where this Note is referred to) prevents floating
of the example above, so I just don't understand the old code.  I
don't understand the old comment either (which appears below).  I
measured the effect on nofib of changing OLD CODE to 'True', and got
zeros everywhere, but a 4% win for 'puzzle'.  Very small 0.5% loss for
'cse'; turns out to be because our arity analysis isn't good enough
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yet (mentioned in Simon-nofib-notes).
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OLD comment was:
	 Even if it escapes a value lambda, we only
	 float if it's not cheap (unless it'll get all the
	 way to the top).  I've seen cases where we
	 float dozens of tiny free expressions, which cost
	 more to allocate than to evaluate.
	 NB: exprIsCheap is also true of bottom expressions, which
	     is good; we don't want to share them

	It's only Really Bad to float a cheap expression out of a
	strict context, because that builds a thunk that otherwise
	would never be built.  So another alternative would be to
	add 
		|| (strict_ctxt && not (exprIsBottom expr))
	to the condition above. We should really try this out.

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%************************************************************************
%*									*
\subsection{Bindings}
%*									*
%************************************************************************

The binding stuff works for top level too.

\begin{code}
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lvlBind :: Level		-- Context level; might be Top even for bindings 
				-- nested in the RHS of a top level binding
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	-> LevelEnv
	-> CoreBindWithFVs
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	-> LvlM (LevelledBind, Level, LevelEnv)

lvlBind ctxt_lvl env (AnnNonRec bndr rhs@(rhs_fvs,_))
  | isTyVar bndr    -- Don't do anything for TyVar binders
	            --   (simplifier gets rid of them pronto)
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  || isCoVar bndr   -- Difficult to fix up CoVar occurrences (see extendPolyLvlEnv)
                    -- so we will ignore this case for now
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  || not (profitableFloat ctxt_lvl dest_lvl)
  || (isTopLvl dest_lvl && isUnLiftedType (idType bndr))
	  -- We can't float an unlifted binding to top level, so we don't 
	  -- float it at all.  It's a bit brutal, but unlifted bindings 
	  -- aren't expensive either
  = -- No float
    do rhs' <- lvlExpr ctxt_lvl env rhs
       let  (env', bndr') = substLetBndrNonRec env bndr bind_lvl
            bind_lvl      = incMinorLvl ctxt_lvl
            tagged_bndr   = TB bndr' (StayPut bind_lvl)
       return (NonRec tagged_bndr rhs', bind_lvl, env')

  -- Otherwise we are going to float
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  | null abs_vars
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  = do  -- No type abstraction; clone existing binder
       rhs' <- lvlExpr dest_lvl env rhs
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       (env', bndr') <- cloneVar env bndr dest_lvl
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       return (NonRec (TB bndr' (FloatMe dest_lvl)) rhs', ctxt_lvl, env') 
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  | otherwise
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  = do  -- Yes, type abstraction; create a new binder, extend substitution, etc
       rhs' <- lvlFloatRhs abs_vars dest_lvl env rhs
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       (env', [bndr']) <- newPolyBndrs dest_lvl env abs_vars [bndr_w_str]
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       return (NonRec (TB bndr' (FloatMe dest_lvl)) rhs', ctxt_lvl, env')
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  where
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    bind_fvs   = rhs_fvs `unionVarSet` idFreeVars bndr
    abs_vars   = abstractVars dest_lvl env bind_fvs
    dest_lvl   = destLevel env bind_fvs (isFunction rhs) mb_bot
    mb_bot     = exprBotStrictness_maybe (deAnnotate rhs)
    bndr_w_str = annotateBotStr bndr mb_bot
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lvlBind ctxt_lvl env (AnnRec pairs)
  | not (profitableFloat ctxt_lvl dest_lvl)
  = do let bind_lvl = incMinorLvl ctxt_lvl
           (env', bndrs') = substLetBndrsRec env bndrs bind_lvl
           tagged_bndrs = [ TB bndr' (StayPut bind_lvl) 
                          | bndr' <- bndrs' ] 
       rhss' <- mapM (lvlExpr bind_lvl env') rhss
       return (Rec (tagged_bndrs `zip` rhss'), bind_lvl, env')
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  | null abs_vars 
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  = do (new_env, new_bndrs) <- cloneRecVars env bndrs dest_lvl
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       new_rhss <- mapM (lvlExpr ctxt_lvl new_env) rhss
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       return ( Rec ([TB b (FloatMe dest_lvl) | b <- new_bndrs] `zip` new_rhss)
              , ctxt_lvl, new_env)
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-- ToDo: when enabling the floatLambda stuff,
--       I think we want to stop doing this
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  | isSingleton pairs && count isId abs_vars > 1
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  = do	-- Special case for self recursion where there are
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	-- several variables carried around: build a local loop:	
	--	poly_f = \abs_vars. \lam_vars . letrec f = \lam_vars. rhs in f lam_vars
	-- This just makes the closures a bit smaller.  If we don't do
	-- this, allocation rises significantly on some programs
	--
	-- We could elaborate it for the case where there are several
	-- mutually functions, but it's quite a bit more complicated
	-- 
	-- This all seems a bit ad hoc -- sigh
    let
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        (bndr,rhs) = head pairs
        (rhs_lvl, abs_vars_w_lvls) = lvlLamBndrs dest_lvl abs_vars
        rhs_env = extendLvlEnv env abs_vars_w_lvls
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    (rhs_env', new_bndr) <- cloneVar rhs_env bndr rhs_lvl
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    let
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        (lam_bndrs, rhs_body)     = collectAnnBndrs rhs
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        (body_lvl, new_lam_bndrs) = lvlLamBndrs rhs_lvl lam_bndrs
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        body_env                  = extendLvlEnv rhs_env' new_lam_bndrs
    new_rhs_body <- lvlExpr body_lvl body_env rhs_body
    (poly_env, [poly_bndr]) <- newPolyBndrs dest_lvl env abs_vars [bndr]
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    return (Rec [(TB poly_bndr (FloatMe dest_lvl) 
               	 , mkLams abs_vars_w_lvls $
               	   mkLams new_lam_bndrs $
               	   Let (Rec [( TB new_bndr (StayPut rhs_lvl)
               	             , mkLams new_lam_bndrs new_rhs_body)]) 
               	       (mkVarApps (Var new_bndr) lam_bndrs))]
           , ctxt_lvl
           , poly_env)
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  | otherwise = do  -- Non-null abs_vars
    (new_env, new_bndrs) <- newPolyBndrs dest_lvl env abs_vars bndrs
    new_rhss <- mapM (lvlFloatRhs abs_vars dest_lvl new_env) rhss
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    return ( Rec ([TB b (FloatMe dest_lvl) | b <- new_bndrs] `zip` new_rhss)
           , ctxt_lvl, new_env)
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  where
    (bndrs,rhss) = unzip pairs

	-- Finding the free vars of the binding group is annoying
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    bind_fvs = (unionVarSets [ idFreeVars bndr `unionVarSet` rhs_fvs
	     		    | (bndr, (rhs_fvs,_)) <- pairs])
	       `minusVarSet`
	       mkVarSet bndrs
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    dest_lvl = destLevel env bind_fvs (all isFunction rhss) Nothing
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    abs_vars = abstractVars dest_lvl env bind_fvs

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profitableFloat :: Level -> Level -> Bool
profitableFloat ctxt_lvl dest_lvl
  =  (dest_lvl `ltMajLvl` ctxt_lvl)	-- Escapes a value lambda
  || isTopLvl dest_lvl    		-- Going all the way to top level

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----------------------------------------------------
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-- Three help functions for the type-abstraction case
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lvlFloatRhs :: [CoreBndr] -> Level -> LevelEnv -> CoreExprWithFVs
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            -> UniqSM (Expr LevelledBndr)
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lvlFloatRhs abs_vars dest_lvl env rhs = do
    rhs' <- lvlExpr rhs_lvl rhs_env rhs
    return (mkLams abs_vars_w_lvls rhs')
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  where
    (rhs_lvl, abs_vars_w_lvls) = lvlLamBndrs dest_lvl abs_vars
    rhs_env = extendLvlEnv env abs_vars_w_lvls
\end{code}


%************************************************************************
%*									*
\subsection{Deciding floatability}
%*									*
%************************************************************************

\begin{code}
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lvlLamBndrs :: Level -> [CoreBndr] -> (Level, [LevelledBndr])
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-- Compute the levels for the binders of a lambda group
-- The binders returned are exactly the same as the ones passed,
-- but they are now paired with a level
lvlLamBndrs lvl [] 
  = (lvl, [])

lvlLamBndrs lvl bndrs
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  = (new_lvl, [TB bndr (StayPut new_lvl) | bndr <- bndrs])
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  -- All the new binders get the same level, because
  -- any floating binding is either going to float past 
  -- all or none.  We never separate binders
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  where
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    new_lvl | any is_major bndrs = incMajorLvl lvl
            | otherwise          = incMinorLvl lvl
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    is_major bndr = isId bndr && not (isOneShotLambda bndr)
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\end{code}

\begin{code}
  -- Destintion level is the max Id level of the expression
  -- (We'll abstract the type variables, if any.)
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destLevel :: LevelEnv -> VarSet -> Bool -> Maybe (Arity, StrictSig) -> Level
destLevel env fvs is_function mb_bot
  | Just {} <- mb_bot = tOP_LEVEL	-- Send bottoming bindings to the top 
					-- regardless; see Note [Bottoming floats]
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  | Just n_args <- floatLams env
  , n_args > 0	-- n=0 case handled uniformly by the 'otherwise' case
  , is_function
  , countFreeIds fvs <= n_args
  = tOP_LEVEL	-- Send functions to top level; see
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		-- the comments with isFunction
  | otherwise = maxFvLevel isId env fvs  -- Max over Ids only; the tyvars
    	      		   	    	 -- will be abstracted
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isFunction :: CoreExprWithFVs -> Bool
-- The idea here is that we want to float *functions* to
-- the top level.  This saves no work, but 
--	(a) it can make the host function body a lot smaller, 
--		and hence inlinable.  
--	(b) it can also save allocation when the function is recursive:
--	    h = \x -> letrec f = \y -> ...f...y...x...
--		      in f x
--     becomes
--	    f = \x y -> ...(f x)...y...x...
--	    h = \x -> f x x
--     No allocation for f now.
-- We may only want to do this if there are sufficiently few free 
-- variables.  We certainly only want to do it for values, and not for
-- constructors.  So the simple thing is just to look for lambdas
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isFunction (_, AnnLam b e) | isId b    = True
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                           | otherwise = isFunction e
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-- isFunction (_, AnnTick _ e)          = isFunction e  -- dubious
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isFunction _                           = False
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countFreeIds :: VarSet -> Int
countFreeIds = foldVarSet add 0
  where
    add :: Var -> Int -> Int
    add v n | isId v    = n+1
            | otherwise = n 
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\end{code}


%************************************************************************
%*									*
\subsection{Free-To-Level Monad}
%*									*
%************************************************************************

\begin{code}
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data LevelEnv 
  = LE { le_switches :: FloatOutSwitches
       , le_lvl_env  :: VarEnv Level	-- Domain is *post-cloned* TyVars and Ids
       , le_subst    :: Subst 		-- Domain is pre-cloned Ids; tracks the in-scope set
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					-- 	so that substitution is capture-avoiding
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                                        -- The Id -> CoreExpr in the Subst is ignored
                                        -- (since we want to substitute in LevelledExpr
                                        -- instead) but we do use the Co/TyVar substs
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       , le_env      :: IdEnv ([Var], LevelledExpr)	-- Domain is pre-cloned Ids
    }
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	-- We clone let-bound variables so that they are still
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	-- distinct when floated out; hence the le_subst/le_env.
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        -- (see point 3 of the module overview comment).
	-- We also use these envs when making a variable polymorphic
	-- because we want to float it out past a big lambda.
	--
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	-- The le_subst and le_env always implement the same mapping, but the
	-- le_subst maps to CoreExpr and the le_env to LevelledExpr
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	-- Since the range is always a variable or type application,
	-- there is never any difference between the two, but sadly
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	-- the types differ.  The le_subst is used when substituting in
	-- a variable's IdInfo; the le_env when we find a Var.
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	--
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	-- In addition the le_env records a list of tyvars free in the
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	-- type application, just so we don't have to call freeVars on
	-- the type application repeatedly.
	--
	-- The domain of the both envs is *pre-cloned* Ids, though
	--
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	-- The domain of the le_lvl_env is the *post-cloned* Ids
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initialEnv :: FloatOutSwitches -> LevelEnv
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initialEnv float_lams 
  = LE { le_switches = float_lams, le_lvl_env = emptyVarEnv
       , le_subst = emptySubst, le_env = emptyVarEnv }
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floatLams :: LevelEnv -> Maybe Int
floatLams le = floatOutLambdas (le_switches le)
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floatConsts :: LevelEnv -> Bool
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floatConsts le = floatOutConstants (le_switches le)
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floatPAPs :: LevelEnv -> Bool
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floatPAPs le = floatOutPartialApplications (le_switches le)
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extendLvlEnv :: LevelEnv -> [LevelledBndr] -> LevelEnv
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-- Used when *not* cloning
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extendLvlEnv le@(LE { le_lvl_env = lvl_env, le_subst = subst, le_env = id_env }) 
             prs
  = le { le_lvl_env = foldl add_lvl lvl_env prs
       , le_subst   = foldl del_subst subst prs
       , le_env     = foldl del_id id_env prs }
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  where
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    add_lvl   env (TB v s) = extendVarEnv env v (floatSpecLevel s)
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    del_subst env (TB v _) = extendInScope env v
    del_id    env (TB v _) = delVarEnv env v
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  -- We must remove any clone for this variable name in case of
  -- shadowing.  This bit me in the following case
  -- (in nofib/real/gg/Spark.hs):
  -- 
  --   case ds of wild {
  --     ... -> case e of wild {
  --              ... -> ... wild ...
  --            }
  --   }
  -- 
  -- The inside occurrence of @wild@ was being replaced with @ds@,
  -- incorrectly, because the SubstEnv was still lying around.  Ouch!
  -- KSW 2000-07.

-- extendCaseBndrLvlEnv adds the mapping case-bndr->scrut-var if it can
-- (see point 4 of the module overview comment)
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extendCaseBndrLvlEnv :: LevelEnv -> Expr LevelledBndr
                     -> LevelledBndr -> LevelEnv
extendCaseBndrLvlEnv le@(LE { le_subst = subst, le_env = id_env }) 
                     (Var scrut_var) (TB case_bndr _)
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  = le { le_subst   = extendSubstWithVar subst case_bndr scrut_var
       , le_env     = extendVarEnv id_env case_bndr ([scrut_var], ASSERT(not (isCoVar scrut_var)) Var scrut_var) }
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extendCaseBndrLvlEnv env _scrut case_bndr
  = extendLvlEnv env [case_bndr]
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extendPolyLvlEnv :: Level -> LevelEnv -> [Var] -> [(Var {- :: t -}, Var {- :: mkPiTypes abs_vars t -})] -> LevelEnv
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extendPolyLvlEnv dest_lvl 
                 le@(LE { le_lvl_env = lvl_env, le_subst = subst, le_env = id_env }) 
                 abs_vars bndr_pairs
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   = ASSERT( all (not . isCoVar . fst) bndr_pairs ) -- What would we add to the CoSubst in this case. No easy answer, so avoid floating 
    le { le_lvl_env = foldl add_lvl   lvl_env bndr_pairs
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       , le_subst   = foldl add_subst subst   bndr_pairs
       , le_env     = foldl add_id    id_env  bndr_pairs }
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  where
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     add_lvl   env (_, v') = extendVarEnv env v' dest_lvl
     add_subst env (v, v') = extendIdSubst env v (mkVarApps (Var v') abs_vars)
     add_id    env (v, v') = extendVarEnv env v ((v':abs_vars), mkVarApps (Var v') abs_vars)
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extendCloneLvlEnv :: Level -> LevelEnv -> Subst -> [(Var, Var)] -> LevelEnv
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extendCloneLvlEnv lvl le@(LE { le_lvl_env = lvl_env, le_env = id_env }) 
                  new_subst bndr_pairs
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  = le { le_lvl_env = foldl add_lvl lvl_env bndr_pairs
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       , le_subst   = new_subst
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       , le_env     = foldl add_id  id_env  bndr_pairs }
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  where
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     add_lvl env (_, v_cloned) = extendVarEnv env v_cloned lvl
     add_id  env (v, v_cloned) = if isTyVar v
                                 then delVarEnv    env v
                                 else extendVarEnv env v ([v_cloned], ASSERT(not (isCoVar v_cloned)) Var v_cloned)
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maxFvLevel :: (Var -> Bool) -> LevelEnv -> VarSet -> Level
maxFvLevel max_me (LE { le_lvl_env = lvl_env, le_env = id_env }) var_set
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  = foldVarSet max_in tOP_LEVEL var_set
  where
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    max_in in_var lvl 
       = foldr max_out lvl (case lookupVarEnv id_env in_var of
				Just (abs_vars, _) -> abs_vars
				Nothing		   -> [in_var])
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    max_out out_var lvl 
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	| max_me out_var = case lookupVarEnv lvl_env out_var of
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				Just lvl' -> maxLvl lvl' lvl
				Nothing   -> lvl 
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	| otherwise = lvl	-- Ignore some vars depending on max_me
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lookupVar :: LevelEnv -> Id -> LevelledExpr
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lookupVar le v = case lookupVarEnv (le_env le) v of
		    Just (_, expr) -> expr
		    _              -> Var v
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abstractVars :: Level -> LevelEnv -> VarSet -> [Var]
	-- Find the variables in fvs, free vars of the target expresion,
	-- whose level is greater than the destination level
	-- These are the ones we are going to abstract out
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abstractVars dest_lvl (LE { le_lvl_env = lvl_env, le_env = id_env }) fvs
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  = map zap $ uniq $ sortQuantVars  -- IA0_NOTE: centralizing sorting on variables
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	[var | fv <- varSetElems fvs
	     , var <- absVarsOf id_env fv
	     , abstract_me var ]
	-- NB: it's important to call abstract_me only on the OutIds the
	-- come from absVarsOf (not on fv, which is an InId)
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  where
    uniq :: [Var] -> [Var]
	-- Remove adjacent duplicates; the sort will have brought them together
    uniq (v1:v2:vs) | v1 == v2  = uniq (v2:vs)
		    | otherwise = v1 : uniq (v2:vs)
    uniq vs = vs

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    abstract_me v = case lookupVarEnv lvl_env v of
			Just lvl -> dest_lvl `ltLvl` lvl
			Nothing  -> False

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	-- We are going to lambda-abstract, so nuke any IdInfo,
	-- and add the tyvars of the Id (if necessary)
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    zap v | isId v = WARN( isStableUnfolding (idUnfolding v) ||
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		           not (isEmptySpecInfo (idSpecialisation v)),
		           text "absVarsOf: discarding info on" <+> ppr v )
		     setIdInfo v vanillaIdInfo
	  | otherwise = v

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absVarsOf :: IdEnv ([Var], LevelledExpr) -> Var -> [Var]
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	-- If f is free in the expression, and f maps to poly_f a b c in the
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	-- current substitution, then we must report a b c as candidate type
	-- variables
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	--
	-- Also, if x::a is an abstracted variable, then so is a; that is,
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	-- we must look in x's type. What's more, if a mentions kind variables,
	-- we must also return those.
	-- 
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	-- And similarly if x is a coercion variable.
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absVarsOf id_env v 
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  | isId v    = [av2 | av1 <- lookup_avs v
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		     , av2 <- add_tyvars av1]
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  | otherwise = ASSERT( isTyVar v ) [v]
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  where
    lookup_avs v = case lookupVarEnv id_env v of
			Just (abs_vars, _) -> abs_vars
			Nothing	           -> [v]

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    add_tyvars v = v : (varSetElems tyvars ++ varSetElems kivars)
    tyvars = varTypeTyVars v
    kivars = kiVarsOfKinds (map tyVarKind (varSetElems tyvars))
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\end{code}

\begin{code}
type LvlM result = UniqSM result

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initLvl :: UniqSupply -> UniqSM a -> a
initLvl = initUs_
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\end{code}

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\begin{code}
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newPolyBndrs :: Level -> LevelEnv -> [Var] -> [Id] -> UniqSM (LevelEnv, [Id])
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newPolyBndrs dest_lvl env abs_vars bndrs = do
    uniqs <- getUniquesM
    let new_bndrs = zipWith mk_poly_bndr bndrs uniqs
    return (extendPolyLvlEnv dest_lvl env abs_vars (bndrs `zip` new_bndrs), new_bndrs)
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  where
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    mk_poly_bndr bndr uniq = transferPolyIdInfo bndr abs_vars $ 	-- Note [transferPolyIdInfo] in Id.lhs
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			     mkSysLocal (mkFastString str) uniq poly_ty
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			   where
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			     str     = "poly_" ++ occNameString (getOccName bndr)
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			     poly_ty = mkPiTypes abs_vars (idType bndr)
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newLvlVar :: [CoreBndr] -> Type 	-- Abstract wrt these bndrs
	  -> Maybe (Arity, StrictSig)   -- Note [Bottoming floats]
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	  -> LvlM Id
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newLvlVar vars body_ty mb_bot
  = do { uniq <- getUniqueM
       ; return (mkLocalIdWithInfo (mk_name uniq) (mkPiTypes vars body_ty) info) }
  where
    mk_name uniq = mkSystemVarName uniq (mkFastString "lvl")
    arity = count isId vars
    info = case mb_bot of
		Nothing               -> vanillaIdInfo
		Just (bot_arity, sig) -> vanillaIdInfo 
					   `setArityInfo`      (arity + bot_arity)
					   `setStrictnessInfo` Just (increaseStrictSigArity arity sig)
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-- The deeply tiresome thing is that we have to apply the substitution
-- to the rules inside each Id.  Grr.  But it matters.

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substLetBndrNonRec :: LevelEnv -> Id -> Level -> (LevelEnv, Id)
substLetBndrNonRec 
    le@(LE { le_lvl_env = lvl_env, le_subst = subst, le_env = id_env }) 
    bndr bind_lvl
  = ASSERT( isId bndr )
    (env', bndr' )
  where
    (subst', bndr') = substBndr subst bndr
    env'	    = le { le_lvl_env = extendVarEnv lvl_env bndr bind_lvl
                         , le_subst = subst'
                         , le_env = delVarEnv id_env bndr }

substLetBndrsRec :: LevelEnv -> [Id] -> Level -> (LevelEnv, [Id])
substLetBndrsRec 
    le@(LE { le_lvl_env = lvl_env, le_subst = subst, le_env = id_env }) 
    bndrs bind_lvl
  = ASSERT( all isId bndrs )
    (env', bndrs')
  where
    (subst', bndrs') = substRecBndrs subst bndrs
    env'	     = le { le_lvl_env = extendVarEnvList lvl_env [(b,bind_lvl) | b <- bndrs]
                          , le_subst = subst'
                          , le_env = delVarEnvList id_env bndrs }
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cloneVar :: LevelEnv -> Var -> Level -> LvlM (LevelEnv, Var)
cloneVar env v dest_lvl -- Works for Ids, TyVars and CoVars
  = do { u <- getUniqueM
       ; let (subst', v1) = cloneBndr (le_subst env) u v
      	     v2     	  = if isId v1 then zapDemandIdInfo v1 else v1
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      	     env'	  = extendCloneLvlEnv dest_lvl env subst' [(v,v2)]
       ; return (env', v2) }

cloneVars :: LevelEnv -> [Var] -> Level -> LvlM (LevelEnv, [Var])
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cloneVars env vs dest_lvl = mapAccumLM (\env v -> cloneVar env v dest_lvl) env vs
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cloneRecVars :: LevelEnv -> [Id] -> Level -> LvlM (LevelEnv, [Id])
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cloneRecVars env vs dest_lvl -- Works for CoVars too (since cloneRecIdBndrs does)
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  = ASSERT( all isId vs ) do
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    us <- getUniqueSupplyM
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    let
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      (subst', vs1) = cloneRecIdBndrs (le_subst env) us vs
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      vs2	    = map zapDemandIdInfo vs1  -- Note [Zapping the demand info]
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      env'	    = extendCloneLvlEnv dest_lvl env subst' (vs `zip` vs2)
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    return (env', vs2)
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\end{code}
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Note [Zapping the demand info]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
VERY IMPORTANT: we must zap the demand info if the thing is going to
float out, becuause it may be less demanded than at its original
binding site.  Eg
   f :: Int -> Int
   f x = let v = 3*4 in v+x
Here v is strict; but if we float v to top level, it isn't any more.

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