/compiler/stranal/DmdAnal.lhs
Haskell | 1317 lines | 825 code | 189 blank | 303 comment | 46 complexity | 249166ce8795e0f17dae8835ce333ee5 MD5 | raw file
- %
- % (c) The GRASP/AQUA Project, Glasgow University, 1993-1998
- %
- -----------------
- A demand analysis
- -----------------
- \begin{code}
- {-# OPTIONS -fno-warn-tabs #-}
- -- The above warning supression flag is a temporary kludge.
- -- While working on this module you are encouraged to remove it and
- -- detab the module (please do the detabbing in a separate patch). See
- -- http://hackage.haskell.org/trac/ghc/wiki/Commentary/CodingStyle#TabsvsSpaces
- -- for details
- module DmdAnal ( dmdAnalPgm, dmdAnalTopRhs,
- both {- needed by WwLib -}
- ) where
- #include "HsVersions.h"
- import DynFlags
- import Demand -- All of it
- import CoreSyn
- import PprCore
- import Coercion ( isCoVarType )
- import CoreUtils ( exprIsHNF, exprIsTrivial )
- import CoreArity ( exprArity )
- import DataCon ( dataConTyCon, dataConRepStrictness )
- import TyCon ( isProductTyCon, isRecursiveTyCon )
- import Id ( Id, idType, idInlineActivation,
- isDataConWorkId, isGlobalId, idArity,
- idStrictness,
- setIdStrictness, idDemandInfo, idUnfolding,
- idDemandInfo_maybe, setIdDemandInfo
- )
- import Var ( Var, isTyVar )
- import VarEnv
- import TysWiredIn ( unboxedPairDataCon )
- import TysPrim ( realWorldStatePrimTy )
- import UniqFM ( addToUFM_Directly, lookupUFM_Directly,
- minusUFM, filterUFM )
- import Type ( isUnLiftedType, eqType, tyConAppTyCon_maybe )
- import Coercion ( coercionKind )
- import Util
- import BasicTypes ( Arity, TopLevelFlag(..), isTopLevel, isNeverActive,
- RecFlag(..), isRec, isMarkedStrict )
- import Maybes ( orElse, expectJust )
- import Outputable
- import Pair
- import Data.List
- import FastString
- \end{code}
- To think about
- * set a noinline pragma on bottoming Ids
- * Consider f x = x+1 `fatbar` error (show x)
- We'd like to unbox x, even if that means reboxing it in the error case.
- %************************************************************************
- %* *
- \subsection{Top level stuff}
- %* *
- %************************************************************************
- \begin{code}
- dmdAnalPgm :: DynFlags -> CoreProgram -> IO CoreProgram
- dmdAnalPgm dflags binds
- = do {
- let { binds_plus_dmds = do_prog binds } ;
- return binds_plus_dmds
- }
- where
- do_prog :: CoreProgram -> CoreProgram
- do_prog binds = snd $ mapAccumL (dmdAnalTopBind dflags) emptySigEnv binds
- dmdAnalTopBind :: DynFlags
- -> SigEnv
- -> CoreBind
- -> (SigEnv, CoreBind)
- dmdAnalTopBind dflags sigs (NonRec id rhs)
- = (sigs2, NonRec id2 rhs2)
- where
- ( _, _, (_, rhs1)) = dmdAnalRhs dflags TopLevel NonRecursive (virgin sigs) (id, rhs)
- (sigs2, _, (id2, rhs2)) = dmdAnalRhs dflags TopLevel NonRecursive (nonVirgin sigs) (id, rhs1)
- -- Do two passes to improve CPR information
- -- See comments with ignore_cpr_info in mk_sig_ty
- -- and with extendSigsWithLam
- dmdAnalTopBind dflags sigs (Rec pairs)
- = (sigs', Rec pairs')
- where
- (sigs', _, pairs') = dmdFix dflags TopLevel (virgin sigs) pairs
- -- We get two iterations automatically
- -- c.f. the NonRec case above
- \end{code}
- \begin{code}
- dmdAnalTopRhs :: DynFlags -> CoreExpr -> (StrictSig, CoreExpr)
- -- Analyse the RHS and return
- -- a) appropriate strictness info
- -- b) the unfolding (decorated with strictness info)
- dmdAnalTopRhs dflags rhs
- = (sig, rhs2)
- where
- call_dmd = vanillaCall (exprArity rhs)
- (_, rhs1) = dmdAnal dflags (virgin emptySigEnv) call_dmd rhs
- (rhs_ty, rhs2) = dmdAnal dflags (nonVirgin emptySigEnv) call_dmd rhs1
- sig = mkTopSigTy dflags rhs rhs_ty
- -- Do two passes; see notes with extendSigsWithLam
- -- Otherwise we get bogus CPR info for constructors like
- -- newtype T a = MkT a
- -- The constructor looks like (\x::T a -> x), modulo the coerce
- -- extendSigsWithLam will optimistically give x a CPR tag the
- -- first time, which is wrong in the end.
- \end{code}
- %************************************************************************
- %* *
- \subsection{The analyser itself}
- %* *
- %************************************************************************
- \begin{code}
- dmdAnal :: DynFlags -> AnalEnv -> Demand -> CoreExpr -> (DmdType, CoreExpr)
- dmdAnal _ _ Abs e = (topDmdType, e)
- dmdAnal dflags env dmd e
- | not (isStrictDmd dmd)
- = let
- (res_ty, e') = dmdAnal dflags env evalDmd e
- in
- (deferType res_ty, e')
- -- It's important not to analyse e with a lazy demand because
- -- a) When we encounter case s of (a,b) ->
- -- we demand s with U(d1d2)... but if the overall demand is lazy
- -- that is wrong, and we'd need to reduce the demand on s,
- -- which is inconvenient
- -- b) More important, consider
- -- f (let x = R in x+x), where f is lazy
- -- We still want to mark x as demanded, because it will be when we
- -- enter the let. If we analyse f's arg with a Lazy demand, we'll
- -- just mark x as Lazy
- -- c) The application rule wouldn't be right either
- -- Evaluating (f x) in a L demand does *not* cause
- -- evaluation of f in a C(L) demand!
- dmdAnal _ _ _ (Lit lit) = (topDmdType, Lit lit)
- dmdAnal _ _ _ (Type ty) = (topDmdType, Type ty) -- Doesn't happen, in fact
- dmdAnal _ _ _ (Coercion co) = (topDmdType, Coercion co)
- dmdAnal _ env dmd (Var var)
- = (dmdTransform env var dmd, Var var)
- dmdAnal dflags env dmd (Cast e co)
- = (dmd_ty, Cast e' co)
- where
- (dmd_ty, e') = dmdAnal dflags env dmd' e
- to_co = pSnd (coercionKind co)
- dmd'
- | Just tc <- tyConAppTyCon_maybe to_co
- , isRecursiveTyCon tc = evalDmd
- | otherwise = dmd
- -- This coerce usually arises from a recursive
- -- newtype, and we don't want to look inside them
- -- for exactly the same reason that we don't look
- -- inside recursive products -- we might not reach
- -- a fixpoint. So revert to a vanilla Eval demand
- dmdAnal dflags env dmd (Tick t e)
- = (dmd_ty, Tick t e')
- where
- (dmd_ty, e') = dmdAnal dflags env dmd e
- dmdAnal dflags env dmd (App fun (Type ty))
- = (fun_ty, App fun' (Type ty))
- where
- (fun_ty, fun') = dmdAnal dflags env dmd fun
- dmdAnal dflags sigs dmd (App fun (Coercion co))
- = (fun_ty, App fun' (Coercion co))
- where
- (fun_ty, fun') = dmdAnal dflags sigs dmd fun
- -- Lots of the other code is there to make this
- -- beautiful, compositional, application rule :-)
- dmdAnal dflags env dmd (App fun arg) -- Non-type arguments
- = let -- [Type arg handled above]
- (fun_ty, fun') = dmdAnal dflags env (Call dmd) fun
- (arg_ty, arg') = dmdAnal dflags env arg_dmd arg
- (arg_dmd, res_ty) = splitDmdTy fun_ty
- in
- (res_ty `bothType` arg_ty, App fun' arg')
- dmdAnal dflags env dmd (Lam var body)
- | isTyVar var
- = let
- (body_ty, body') = dmdAnal dflags env dmd body
- in
- (body_ty, Lam var body')
- | Call body_dmd <- dmd -- A call demand: good!
- = let
- env' = extendSigsWithLam env var
- (body_ty, body') = dmdAnal dflags env' body_dmd body
- (lam_ty, var') = annotateLamIdBndr dflags env body_ty var
- in
- (lam_ty, Lam var' body')
- | otherwise -- Not enough demand on the lambda; but do the body
- = let -- anyway to annotate it and gather free var info
- (body_ty, body') = dmdAnal dflags env evalDmd body
- (lam_ty, var') = annotateLamIdBndr dflags env body_ty var
- in
- (deferType lam_ty, Lam var' body')
- dmdAnal dflags env dmd (Case scrut case_bndr ty [alt@(DataAlt dc, _, _)])
- | let tycon = dataConTyCon dc
- , isProductTyCon tycon
- , not (isRecursiveTyCon tycon)
- = let
- env_alt = extendAnalEnv NotTopLevel env case_bndr case_bndr_sig
- (alt_ty, alt') = dmdAnalAlt dflags env_alt dmd alt
- (alt_ty1, case_bndr') = annotateBndr alt_ty case_bndr
- (_, bndrs', _) = alt'
- case_bndr_sig = cprSig
- -- Inside the alternative, the case binder has the CPR property.
- -- Meaning that a case on it will successfully cancel.
- -- Example:
- -- f True x = case x of y { I# x' -> if x' ==# 3 then y else I# 8 }
- -- f False x = I# 3
- --
- -- We want f to have the CPR property:
- -- f b x = case fw b x of { r -> I# r }
- -- fw True x = case x of y { I# x' -> if x' ==# 3 then x' else 8 }
- -- fw False x = 3
- -- Figure out whether the demand on the case binder is used, and use
- -- that to set the scrut_dmd. This is utterly essential.
- -- Consider f x = case x of y { (a,b) -> k y a }
- -- If we just take scrut_demand = U(L,A), then we won't pass x to the
- -- worker, so the worker will rebuild
- -- x = (a, absent-error)
- -- and that'll crash.
- -- So at one stage I had:
- -- dead_case_bndr = isAbsentDmd (idDemandInfo case_bndr')
- -- keepity | dead_case_bndr = Drop
- -- | otherwise = Keep
- --
- -- But then consider
- -- case x of y { (a,b) -> h y + a }
- -- where h : U(LL) -> T
- -- The above code would compute a Keep for x, since y is not Abs, which is silly
- -- The insight is, of course, that a demand on y is a demand on the
- -- scrutinee, so we need to `both` it with the scrut demand
- alt_dmd = Eval (Prod [idDemandInfo b | b <- bndrs', isId b])
- scrut_dmd = alt_dmd `both`
- idDemandInfo case_bndr'
- (scrut_ty, scrut') = dmdAnal dflags env scrut_dmd scrut
- res_ty = alt_ty1 `bothType` scrut_ty
- in
- -- pprTrace "dmdAnal:Case1" (vcat [ text "scrut" <+> ppr scrut
- -- , text "scrut_ty" <+> ppr scrut_ty
- -- , text "alt_ty" <+> ppr alt_ty1
- -- , text "res_ty" <+> ppr res_ty ]) $
- (res_ty, Case scrut' case_bndr' ty [alt'])
- dmdAnal dflags env dmd (Case scrut case_bndr ty alts)
- = let
- (alt_tys, alts') = mapAndUnzip (dmdAnalAlt dflags env dmd) alts
- (scrut_ty, scrut') = dmdAnal dflags env evalDmd scrut
- (alt_ty, case_bndr') = annotateBndr (foldr lubType botDmdType alt_tys) case_bndr
- res_ty = alt_ty `bothType` scrut_ty
- in
- -- pprTrace "dmdAnal:Case2" (vcat [ text "scrut" <+> ppr scrut
- -- , text "scrut_ty" <+> ppr scrut_ty
- -- , text "alt_ty" <+> ppr alt_ty
- -- , text "res_ty" <+> ppr res_ty ]) $
- (res_ty, Case scrut' case_bndr' ty alts')
- dmdAnal dflags env dmd (Let (NonRec id rhs) body)
- = let
- (sigs', lazy_fv, (id1, rhs')) = dmdAnalRhs dflags NotTopLevel NonRecursive env (id, rhs)
- (body_ty, body') = dmdAnal dflags (updSigEnv env sigs') dmd body
- (body_ty1, id2) = annotateBndr body_ty id1
- body_ty2 = addLazyFVs body_ty1 lazy_fv
- in
- -- If the actual demand is better than the vanilla call
- -- demand, you might think that we might do better to re-analyse
- -- the RHS with the stronger demand.
- -- But (a) That seldom happens, because it means that *every* path in
- -- the body of the let has to use that stronger demand
- -- (b) It often happens temporarily in when fixpointing, because
- -- the recursive function at first seems to place a massive demand.
- -- But we don't want to go to extra work when the function will
- -- probably iterate to something less demanding.
- -- In practice, all the times the actual demand on id2 is more than
- -- the vanilla call demand seem to be due to (b). So we don't
- -- bother to re-analyse the RHS.
- (body_ty2, Let (NonRec id2 rhs') body')
- dmdAnal dflags env dmd (Let (Rec pairs) body)
- = let
- bndrs = map fst pairs
- (sigs', lazy_fv, pairs') = dmdFix dflags NotTopLevel env pairs
- (body_ty, body') = dmdAnal dflags (updSigEnv env sigs') dmd body
- body_ty1 = addLazyFVs body_ty lazy_fv
- in
- sigs' `seq` body_ty `seq`
- let
- (body_ty2, _) = annotateBndrs body_ty1 bndrs
- -- Don't bother to add demand info to recursive
- -- binders as annotateBndr does;
- -- being recursive, we can't treat them strictly.
- -- But we do need to remove the binders from the result demand env
- in
- (body_ty2, Let (Rec pairs') body')
- dmdAnalAlt :: DynFlags -> AnalEnv -> Demand -> Alt Var -> (DmdType, Alt Var)
- dmdAnalAlt dflags env dmd (con,bndrs,rhs)
- = let
- (rhs_ty, rhs') = dmdAnal dflags env dmd rhs
- rhs_ty' = addDataConPatDmds con bndrs rhs_ty
- (alt_ty, bndrs') = annotateBndrs rhs_ty' bndrs
- final_alt_ty | io_hack_reqd = alt_ty `lubType` topDmdType
- | otherwise = alt_ty
- -- There's a hack here for I/O operations. Consider
- -- case foo x s of { (# s, r #) -> y }
- -- Is this strict in 'y'. Normally yes, but what if 'foo' is an I/O
- -- operation that simply terminates the program (not in an erroneous way)?
- -- In that case we should not evaluate y before the call to 'foo'.
- -- Hackish solution: spot the IO-like situation and add a virtual branch,
- -- as if we had
- -- case foo x s of
- -- (# s, r #) -> y
- -- other -> return ()
- -- So the 'y' isn't necessarily going to be evaluated
- --
- -- A more complete example (Trac #148, #1592) where this shows up is:
- -- do { let len = <expensive> ;
- -- ; when (...) (exitWith ExitSuccess)
- -- ; print len }
- io_hack_reqd = con == DataAlt unboxedPairDataCon &&
- idType (head bndrs) `eqType` realWorldStatePrimTy
- in
- (final_alt_ty, (con, bndrs', rhs'))
- addDataConPatDmds :: AltCon -> [Var] -> DmdType -> DmdType
- -- See Note [Add demands for strict constructors]
- addDataConPatDmds DEFAULT _ dmd_ty = dmd_ty
- addDataConPatDmds (LitAlt _) _ dmd_ty = dmd_ty
- addDataConPatDmds (DataAlt con) bndrs dmd_ty
- = foldr add dmd_ty str_bndrs
- where
- add bndr dmd_ty = addVarDmd dmd_ty bndr seqDmd
- str_bndrs = [ b | (b,s) <- zipEqual "addDataConPatBndrs"
- (filter isId bndrs)
- (dataConRepStrictness con)
- , isMarkedStrict s ]
- \end{code}
- Note [Add demands for strict constructors]
- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
- Consider this program (due to Roman):
- data X a = X !a
- foo :: X Int -> Int -> Int
- foo (X a) n = go 0
- where
- go i | i < n = a + go (i+1)
- | otherwise = 0
- We want the worker for 'foo' too look like this:
- $wfoo :: Int# -> Int# -> Int#
- with the first argument unboxed, so that it is not eval'd each time
- around the loop (which would otherwise happen, since 'foo' is not
- strict in 'a'. It is sound for the wrapper to pass an unboxed arg
- because X is strict, so its argument must be evaluated. And if we
- *don't* pass an unboxed argument, we can't even repair it by adding a
- `seq` thus:
- foo (X a) n = a `seq` go 0
- because the seq is discarded (very early) since X is strict!
- There is the usual danger of reboxing, which as usual we ignore. But
- if X is monomorphic, and has an UNPACK pragma, then this optimisation
- is even more important. We don't want the wrapper to rebox an unboxed
- argument, and pass an Int to $wfoo!
- %************************************************************************
- %* *
- Demand transformer
- %* *
- %************************************************************************
- \begin{code}
- dmdTransform :: AnalEnv -- The strictness environment
- -> Id -- The function
- -> Demand -- The demand on the function
- -> DmdType -- The demand type of the function in this context
- -- Returned DmdEnv includes the demand on
- -- this function plus demand on its free variables
- dmdTransform env var dmd
- ------ DATA CONSTRUCTOR
- | isDataConWorkId var -- Data constructor
- = let
- StrictSig dmd_ty = idStrictness var -- It must have a strictness sig
- DmdType _ _ con_res = dmd_ty
- arity = idArity var
- in
- if arity == call_depth then -- Saturated, so unleash the demand
- let
- -- Important! If we Keep the constructor application, then
- -- we need the demands the constructor places (always lazy)
- -- If not, we don't need to. For example:
- -- f p@(x,y) = (p,y) -- S(AL)
- -- g a b = f (a,b)
- -- It's vital that we don't calculate Absent for a!
- dmd_ds = case res_dmd of
- Box (Eval ds) -> mapDmds box ds
- Eval ds -> ds
- _ -> Poly Top
- -- ds can be empty, when we are just seq'ing the thing
- -- If so we must make up a suitable bunch of demands
- arg_ds = case dmd_ds of
- Poly d -> replicate arity d
- Prod ds -> ASSERT( ds `lengthIs` arity ) ds
- in
- mkDmdType emptyDmdEnv arg_ds con_res
- -- Must remember whether it's a product, hence con_res, not TopRes
- else
- topDmdType
- ------ IMPORTED FUNCTION
- | isGlobalId var, -- Imported function
- let StrictSig dmd_ty = idStrictness var
- = -- pprTrace "strict-sig" (ppr var $$ ppr dmd_ty) $
- if dmdTypeDepth dmd_ty <= call_depth then -- Saturated, so unleash the demand
- dmd_ty
- else
- topDmdType
- ------ LOCAL LET/REC BOUND THING
- | Just (StrictSig dmd_ty, top_lvl) <- lookupSigEnv env var
- = let
- fn_ty | dmdTypeDepth dmd_ty <= call_depth = dmd_ty
- | otherwise = deferType dmd_ty
- -- NB: it's important to use deferType, and not just return topDmdType
- -- Consider let { f x y = p + x } in f 1
- -- The application isn't saturated, but we must nevertheless propagate
- -- a lazy demand for p!
- in
- if isTopLevel top_lvl then fn_ty -- Don't record top level things
- else addVarDmd fn_ty var dmd
- ------ LOCAL NON-LET/REC BOUND THING
- | otherwise -- Default case
- = unitVarDmd var dmd
- where
- (call_depth, res_dmd) = splitCallDmd dmd
- \end{code}
- %************************************************************************
- %* *
- \subsection{Bindings}
- %* *
- %************************************************************************
- \begin{code}
- dmdFix :: DynFlags
- -> TopLevelFlag
- -> AnalEnv -- Does not include bindings for this binding
- -> [(Id,CoreExpr)]
- -> (SigEnv, DmdEnv,
- [(Id,CoreExpr)]) -- Binders annotated with stricness info
- dmdFix dflags top_lvl env orig_pairs
- = loop 1 initial_env orig_pairs
- where
- bndrs = map fst orig_pairs
- initial_env = addInitialSigs top_lvl env bndrs
-
- loop :: Int
- -> AnalEnv -- Already contains the current sigs
- -> [(Id,CoreExpr)]
- -> (SigEnv, DmdEnv, [(Id,CoreExpr)])
- loop n env pairs
- = -- pprTrace "dmd loop" (ppr n <+> ppr bndrs $$ ppr env) $
- loop' n env pairs
- loop' n env pairs
- | found_fixpoint
- = (sigs', lazy_fv, pairs')
- -- Note: return pairs', not pairs. pairs' is the result of
- -- processing the RHSs with sigs (= sigs'), whereas pairs
- -- is the result of processing the RHSs with the *previous*
- -- iteration of sigs.
- | n >= 10
- = pprTrace "dmdFix loop" (ppr n <+> (vcat
- [ text "Sigs:" <+> ppr [ (id,lookupVarEnv sigs id, lookupVarEnv sigs' id)
- | (id,_) <- pairs],
- text "env:" <+> ppr env,
- text "binds:" <+> pprCoreBinding (Rec pairs)]))
- (sigEnv env, lazy_fv, orig_pairs) -- Safe output
- -- The lazy_fv part is really important! orig_pairs has no strictness
- -- info, including nothing about free vars. But if we have
- -- letrec f = ....y..... in ...f...
- -- where 'y' is free in f, we must record that y is mentioned,
- -- otherwise y will get recorded as absent altogether
- | otherwise
- = loop (n+1) (nonVirgin sigs') pairs'
- where
- sigs = sigEnv env
- found_fixpoint = all (same_sig sigs sigs') bndrs
- ((sigs',lazy_fv), pairs') = mapAccumL my_downRhs (sigs, emptyDmdEnv) pairs
- -- mapAccumL: Use the new signature to do the next pair
- -- The occurrence analyser has arranged them in a good order
- -- so this can significantly reduce the number of iterations needed
-
- my_downRhs (sigs,lazy_fv) (id,rhs)
- = ((sigs', lazy_fv'), pair')
- where
- (sigs', lazy_fv1, pair') = dmdAnalRhs dflags top_lvl Recursive (updSigEnv env sigs) (id,rhs)
- lazy_fv' = plusVarEnv_C both lazy_fv lazy_fv1
-
- same_sig sigs sigs' var = lookup sigs var == lookup sigs' var
- lookup sigs var = case lookupVarEnv sigs var of
- Just (sig,_) -> sig
- Nothing -> pprPanic "dmdFix" (ppr var)
- dmdAnalRhs :: DynFlags -> TopLevelFlag -> RecFlag
- -> AnalEnv -> (Id, CoreExpr)
- -> (SigEnv, DmdEnv, (Id, CoreExpr))
- -- Process the RHS of the binding, add the strictness signature
- -- to the Id, and augment the environment with the signature as well.
- dmdAnalRhs dflags top_lvl rec_flag env (id, rhs)
- = (sigs', lazy_fv, (id', rhs'))
- where
- arity = idArity id -- The idArity should be up to date
- -- The simplifier was run just beforehand
- (rhs_dmd_ty, rhs') = dmdAnal dflags env (vanillaCall arity) rhs
- (lazy_fv, sig_ty) = WARN( arity /= dmdTypeDepth rhs_dmd_ty && not (exprIsTrivial rhs), ppr id )
- -- The RHS can be eta-reduced to just a variable,
- -- in which case we should not complain.
- mkSigTy dflags top_lvl rec_flag id rhs rhs_dmd_ty
- id' = id `setIdStrictness` sig_ty
- sigs' = extendSigEnv top_lvl (sigEnv env) id sig_ty
- \end{code}
- %************************************************************************
- %* *
- \subsection{Strictness signatures and types}
- %* *
- %************************************************************************
- \begin{code}
- mkTopSigTy :: DynFlags -> CoreExpr -> DmdType -> StrictSig
- -- Take a DmdType and turn it into a StrictSig
- -- NB: not used for never-inline things; hence False
- mkTopSigTy dflags rhs dmd_ty = snd (mk_sig_ty dflags False False rhs dmd_ty)
- mkSigTy :: DynFlags -> TopLevelFlag -> RecFlag -> Id -> CoreExpr -> DmdType -> (DmdEnv, StrictSig)
- mkSigTy dflags top_lvl rec_flag id rhs dmd_ty
- = mk_sig_ty dflags never_inline thunk_cpr_ok rhs dmd_ty
- where
- never_inline = isNeverActive (idInlineActivation id)
- maybe_id_dmd = idDemandInfo_maybe id
- -- Is Nothing the first time round
- thunk_cpr_ok -- See Note [CPR for thunks]
- | isTopLevel top_lvl = False -- Top level things don't get
- -- their demandInfo set at all
- | isRec rec_flag = False -- Ditto recursive things
- | Just dmd <- maybe_id_dmd = isStrictDmd dmd
- | otherwise = True -- Optimistic, first time round
- -- See notes below
- \end{code}
- Note [CPR for thunks]
- ~~~~~~~~~~~~~~~~~~~~~
- If the rhs is a thunk, we usually forget the CPR info, because
- it is presumably shared (else it would have been inlined, and
- so we'd lose sharing if w/w'd it into a function). E.g.
- let r = case expensive of
- (a,b) -> (b,a)
- in ...
- If we marked r as having the CPR property, then we'd w/w into
- let $wr = \() -> case expensive of
- (a,b) -> (# b, a #)
- r = case $wr () of
- (# b,a #) -> (b,a)
- in ...
- But now r is a thunk, which won't be inlined, so we are no further ahead.
- But consider
- f x = let r = case expensive of (a,b) -> (b,a)
- in if foo r then r else (x,x)
- Does f have the CPR property? Well, no.
- However, if the strictness analyser has figured out (in a previous
- iteration) that it's strict, then we DON'T need to forget the CPR info.
- Instead we can retain the CPR info and do the thunk-splitting transform
- (see WorkWrap.splitThunk).
- This made a big difference to PrelBase.modInt, which had something like
- modInt = \ x -> let r = ... -> I# v in
- ...body strict in r...
- r's RHS isn't a value yet; but modInt returns r in various branches, so
- if r doesn't have the CPR property then neither does modInt
- Another case I found in practice (in Complex.magnitude), looks like this:
- let k = if ... then I# a else I# b
- in ... body strict in k ....
- (For this example, it doesn't matter whether k is returned as part of
- the overall result; but it does matter that k's RHS has the CPR property.)
- Left to itself, the simplifier will make a join point thus:
- let $j k = ...body strict in k...
- if ... then $j (I# a) else $j (I# b)
- With thunk-splitting, we get instead
- let $j x = let k = I#x in ...body strict in k...
- in if ... then $j a else $j b
- This is much better; there's a good chance the I# won't get allocated.
- The difficulty with this is that we need the strictness type to
- look at the body... but we now need the body to calculate the demand
- on the variable, so we can decide whether its strictness type should
- have a CPR in it or not. Simple solution:
- a) use strictness info from the previous iteration
- b) make sure we do at least 2 iterations, by doing a second
- round for top-level non-recs. Top level recs will get at
- least 2 iterations except for totally-bottom functions
- which aren't very interesting anyway.
- NB: strictly_demanded is never true of a top-level Id, or of a recursive Id.
- Note [Optimistic in the Nothing case]
- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
- Demand info now has a 'Nothing' state, just like strictness info.
- The analysis works from 'dangerous' towards a 'safe' state; so we
- start with botSig for 'Nothing' strictness infos, and we start with
- "yes, it's demanded" for 'Nothing' in the demand info. The
- fixpoint iteration will sort it all out.
- We can't start with 'not-demanded' because then consider
- f x = let
- t = ... I# x
- in
- if ... then t else I# y else f x'
- In the first iteration we'd have no demand info for x, so assume
- not-demanded; then we'd get TopRes for f's CPR info. Next iteration
- we'd see that t was demanded, and so give it the CPR property, but by
- now f has TopRes, so it will stay TopRes. Instead, with the Nothing
- setting the first time round, we say 'yes t is demanded' the first
- time.
- However, this does mean that for non-recursive bindings we must
- iterate twice to be sure of not getting over-optimistic CPR info,
- in the case where t turns out to be not-demanded. This is handled
- by dmdAnalTopBind.
- Note [NOINLINE and strictness]
- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
- The strictness analyser used to have a HACK which ensured that NOINLNE
- things were not strictness-analysed. The reason was unsafePerformIO.
- Left to itself, the strictness analyser would discover this strictness
- for unsafePerformIO:
- unsafePerformIO: C(U(AV))
- But then consider this sub-expression
- unsafePerformIO (\s -> let r = f x in
- case writeIORef v r s of (# s1, _ #) ->
- (# s1, r #)
- The strictness analyser will now find that r is sure to be eval'd,
- and may then hoist it out. This makes tests/lib/should_run/memo002
- deadlock.
- Solving this by making all NOINLINE things have no strictness info is overkill.
- In particular, it's overkill for runST, which is perfectly respectable.
- Consider
- f x = runST (return x)
- This should be strict in x.
- So the new plan is to define unsafePerformIO using the 'lazy' combinator:
- unsafePerformIO (IO m) = lazy (case m realWorld# of (# _, r #) -> r)
- Remember, 'lazy' is a wired-in identity-function Id, of type a->a, which is
- magically NON-STRICT, and is inlined after strictness analysis. So
- unsafePerformIO will look non-strict, and that's what we want.
- Now we don't need the hack in the strictness analyser. HOWEVER, this
- decision does mean that even a NOINLINE function is not entirely
- opaque: some aspect of its implementation leaks out, notably its
- strictness. For example, if you have a function implemented by an
- error stub, but which has RULES, you may want it not to be eliminated
- in favour of error!
- \begin{code}
- mk_sig_ty :: DynFlags -> Bool -> Bool -> CoreExpr
- -> DmdType -> (DmdEnv, StrictSig)
- mk_sig_ty dflags _never_inline thunk_cpr_ok rhs (DmdType fv dmds res)
- = (lazy_fv, mkStrictSig dmd_ty)
- -- Re unused never_inline, see Note [NOINLINE and strictness]
- where
- dmd_ty = DmdType strict_fv final_dmds res'
- lazy_fv = filterUFM (not . isStrictDmd) fv
- strict_fv = filterUFM isStrictDmd fv
- -- We put the strict FVs in the DmdType of the Id, so
- -- that at its call sites we unleash demands on its strict fvs.
- -- An example is 'roll' in imaginary/wheel-sieve2
- -- Something like this:
- -- roll x = letrec
- -- go y = if ... then roll (x-1) else x+1
- -- in
- -- go ms
- -- We want to see that roll is strict in x, which is because
- -- go is called. So we put the DmdEnv for x in go's DmdType.
- --
- -- Another example:
- -- f :: Int -> Int -> Int
- -- f x y = let t = x+1
- -- h z = if z==0 then t else
- -- if z==1 then x+1 else
- -- x + h (z-1)
- -- in
- -- h y
- -- Calling h does indeed evaluate x, but we can only see
- -- that if we unleash a demand on x at the call site for t.
- --
- -- Incidentally, here's a place where lambda-lifting h would
- -- lose the cigar --- we couldn't see the joint strictness in t/x
- --
- -- ON THE OTHER HAND
- -- We don't want to put *all* the fv's from the RHS into the
- -- DmdType, because that makes fixpointing very slow --- the
- -- DmdType gets full of lazy demands that are slow to converge.
- final_dmds = setUnpackStrategy dflags dmds
- -- Set the unpacking strategy
-
- res' = case res of
- RetCPR | ignore_cpr_info -> TopRes
- _ -> res
- ignore_cpr_info = not (exprIsHNF rhs || thunk_cpr_ok)
- \end{code}
- The unpack strategy determines whether we'll *really* unpack the argument,
- or whether we'll just remember its strictness. If unpacking would give
- rise to a *lot* of worker args, we may decide not to unpack after all.
- \begin{code}
- setUnpackStrategy :: DynFlags -> [Demand] -> [Demand]
- setUnpackStrategy dflags ds
- = snd (go (maxWorkerArgs dflags - nonAbsentArgs ds) ds)
- where
- go :: Int -- Max number of args available for sub-components of [Demand]
- -> [Demand]
- -> (Int, [Demand]) -- Args remaining after subcomponents of [Demand] are unpacked
- go n (Eval (Prod cs) : ds)
- | n' >= 0 = Eval (Prod cs') `cons` go n'' ds
- | otherwise = Box (Eval (Prod cs)) `cons` go n ds
- where
- (n'',cs') = go n' cs
- n' = n + 1 - non_abs_args
- -- Add one to the budget 'cos we drop the top-level arg
- non_abs_args = nonAbsentArgs cs
- -- Delete # of non-absent args to which we'll now be committed
-
- go n (d:ds) = d `cons` go n ds
- go n [] = (n,[])
- cons d (n,ds) = (n, d:ds)
- nonAbsentArgs :: [Demand] -> Int
- nonAbsentArgs [] = 0
- nonAbsentArgs (Abs : ds) = nonAbsentArgs ds
- nonAbsentArgs (_ : ds) = 1 + nonAbsentArgs ds
- \end{code}
- %************************************************************************
- %* *
- \subsection{Strictness signatures and types}
- %* *
- %************************************************************************
- \begin{code}
- unitVarDmd :: Var -> Demand -> DmdType
- unitVarDmd var dmd = DmdType (unitVarEnv var dmd) [] TopRes
- addVarDmd :: DmdType -> Var -> Demand -> DmdType
- addVarDmd (DmdType fv ds res) var dmd
- = DmdType (extendVarEnv_C both fv var dmd) ds res
- addLazyFVs :: DmdType -> DmdEnv -> DmdType
- addLazyFVs (DmdType fv ds res) lazy_fvs
- = DmdType both_fv1 ds res
- where
- both_fv = plusVarEnv_C both fv lazy_fvs
- both_fv1 = modifyEnv (isBotRes res) (`both` Bot) lazy_fvs fv both_fv
- -- This modifyEnv is vital. Consider
- -- let f = \x -> (x,y)
- -- in error (f 3)
- -- Here, y is treated as a lazy-fv of f, but we must `both` that L
- -- demand with the bottom coming up from 'error'
- --
- -- I got a loop in the fixpointer without this, due to an interaction
- -- with the lazy_fv filtering in mkSigTy. Roughly, it was
- -- letrec f n x
- -- = letrec g y = x `fatbar`
- -- letrec h z = z + ...g...
- -- in h (f (n-1) x)
- -- in ...
- -- In the initial iteration for f, f=Bot
- -- Suppose h is found to be strict in z, but the occurrence of g in its RHS
- -- is lazy. Now consider the fixpoint iteration for g, esp the demands it
- -- places on its free variables. Suppose it places none. Then the
- -- x `fatbar` ...call to h...
- -- will give a x->V demand for x. That turns into a L demand for x,
- -- which floats out of the defn for h. Without the modifyEnv, that
- -- L demand doesn't get both'd with the Bot coming up from the inner
- -- call to f. So we just get an L demand for x for g.
- --
- -- A better way to say this is that the lazy-fv filtering should give the
- -- same answer as putting the lazy fv demands in the function's type.
- annotateBndr :: DmdType -> Var -> (DmdType, Var)
- -- The returned env has the var deleted
- -- The returned var is annotated with demand info
- -- No effect on the argument demands
- annotateBndr dmd_ty@(DmdType fv ds res) var
- | isTyVar var = (dmd_ty, var)
- | otherwise = (DmdType fv' ds res, setIdDemandInfo var dmd)
- where
- (fv', dmd) = removeFV fv var res
- annotateBndrs :: DmdType -> [Var] -> (DmdType, [Var])
- annotateBndrs = mapAccumR annotateBndr
- annotateLamIdBndr :: DynFlags
- -> AnalEnv
- -> DmdType -- Demand type of body
- -> Id -- Lambda binder
- -> (DmdType, -- Demand type of lambda
- Id) -- and binder annotated with demand
- annotateLamIdBndr dflags env (DmdType fv ds res) id
- -- For lambdas we add the demand to the argument demands
- -- Only called for Ids
- = ASSERT( isId id )
- (final_ty, setIdDemandInfo id hacked_dmd)
- where
- -- Watch out! See note [Lambda-bound unfoldings]
- final_ty = case maybeUnfoldingTemplate (idUnfolding id) of
- Nothing -> main_ty
- Just unf -> main_ty `bothType` unf_ty
- where
- (unf_ty, _) = dmdAnal dflags env dmd unf
-
- main_ty = DmdType fv' (hacked_dmd:ds) res
- (fv', dmd) = removeFV fv id res
- hacked_dmd = argDemand dmd
- -- This call to argDemand is vital, because otherwise we label
- -- a lambda binder with demand 'B'. But in terms of calling
- -- conventions that's Abs, because we don't pass it. But
- -- when we do a w/w split we get
- -- fw x = (\x y:B -> ...) x (error "oops")
- -- And then the simplifier things the 'B' is a strict demand
- -- and evaluates the (error "oops"). Sigh
- removeFV :: DmdEnv -> Var -> DmdResult -> (DmdEnv, Demand)
- removeFV fv id res = (fv', zapUnlifted id dmd)
- where
- fv' = fv `delVarEnv` id
- dmd = lookupVarEnv fv id `orElse` deflt
- deflt | isBotRes res = Bot
- | otherwise = Abs
- zapUnlifted :: Id -> Demand -> Demand
- -- For unlifted-type variables, we are only
- -- interested in Bot/Abs/Box Abs
- zapUnlifted id dmd
- = case dmd of
- _ | isCoVarType ty -> lazyDmd -- For coercions, ignore str/abs totally
- Bot -> Bot
- Abs -> Abs
- _ | isUnLiftedType ty -> lazyDmd -- For unlifted types, ignore strictness
- | otherwise -> dmd
- where
- ty = idType id
- \end{code}
- Note [Lamba-bound unfoldings]
- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
- We allow a lambda-bound variable to carry an unfolding, a facility that is used
- exclusively for join points; see Note [Case binders and join points]. If so,
- we must be careful to demand-analyse the RHS of the unfolding! Example
- \x. \y{=Just x}. <body>
- Then if <body> uses 'y', then transitively it uses 'x', and we must not
- forget that fact, otherwise we might make 'x' absent when it isn't.
- %************************************************************************
- %* *
- \subsection{Strictness signatures}
- %* *
- %************************************************************************
- \begin{code}
- data AnalEnv
- = AE { ae_sigs :: SigEnv
- , ae_virgin :: Bool } -- True on first iteration only
- -- See Note [Initialising strictness]
- -- We use the se_env to tell us whether to
- -- record info about a variable in the DmdEnv
- -- We do so if it's a LocalId, but not top-level
- --
- -- The DmdEnv gives the demand on the free vars of the function
- -- when it is given enough args to satisfy the strictness signature
- type SigEnv = VarEnv (StrictSig, TopLevelFlag)
- instance Outputable AnalEnv where
- ppr (AE { ae_sigs = env, ae_virgin = virgin })
- = ptext (sLit "AE") <+> braces (vcat
- [ ptext (sLit "ae_virgin =") <+> ppr virgin
- , ptext (sLit "ae_sigs =") <+> ppr env ])
- emptySigEnv :: SigEnv
- emptySigEnv = emptyVarEnv
- sigEnv :: AnalEnv -> SigEnv
- sigEnv = ae_sigs
- updSigEnv :: AnalEnv -> SigEnv -> AnalEnv
- updSigEnv env sigs = env { ae_sigs = sigs }
- extendAnalEnv :: TopLevelFlag -> AnalEnv -> Id -> StrictSig -> AnalEnv
- extendAnalEnv top_lvl env var sig
- = env { ae_sigs = extendSigEnv top_lvl (ae_sigs env) var sig }
- extendSigEnv :: TopLevelFlag -> SigEnv -> Id -> StrictSig -> SigEnv
- extendSigEnv top_lvl sigs var sig = extendVarEnv sigs var (sig, top_lvl)
- lookupSigEnv :: AnalEnv -> Id -> Maybe (StrictSig, TopLevelFlag)
- lookupSigEnv env id = lookupVarEnv (ae_sigs env) id
- addInitialSigs :: TopLevelFlag -> AnalEnv -> [Id] -> AnalEnv
- -- See Note [Initialising strictness]
- addInitialSigs top_lvl env@(AE { ae_sigs = sigs, ae_virgin = virgin }) ids
- = env { ae_sigs = extendVarEnvList sigs [ (id, (init_sig id, top_lvl))
- | id <- ids ] }
- where
- init_sig | virgin = \_ -> botSig
- | otherwise = idStrictness
- virgin, nonVirgin :: SigEnv -> AnalEnv
- virgin sigs = AE { ae_sigs = sigs, ae_virgin = True }
- nonVirgin sigs = AE { ae_sigs = sigs, ae_virgin = False }
- extendSigsWithLam :: AnalEnv -> Id -> AnalEnv
- -- Extend the AnalEnv when we meet a lambda binder
- -- If the binder is marked demanded with a product demand, then give it a CPR
- -- signature, because in the likely event that this is a lambda on a fn defn
- -- [we only use this when the lambda is being consumed with a call demand],
- -- it'll be w/w'd and so it will be CPR-ish. E.g.
- -- f = \x::(Int,Int). if ...strict in x... then
- -- x
- -- else
- -- (a,b)
- -- We want f to have the CPR property because x does, by the time f has been w/w'd
- --
- -- Also note that we only want to do this for something that
- -- definitely has product type, else we may get over-optimistic
- -- CPR results (e.g. from \x -> x!).
- extendSigsWithLam env id
- = case idDemandInfo_maybe id of
- Nothing -> extendAnalEnv NotTopLevel env id cprSig
- -- See Note [Optimistic in the Nothing case]
- Just (Eval (Prod _)) -> extendAnalEnv NotTopLevel env id cprSig
- _ -> env
- \end{code}
- Note [Initialising strictness]
- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
- Our basic plan is to initialise the strictness of each Id in
- a recursive group to "bottom", and find a fixpoint from there.
- However, this group A might be inside an *enclosing* recursive
- group B, in which case we'll do the entire fixpoint shebang on A
- for each iteration of B.
- To speed things up, we initialise each iteration of B from the result
- of the last one, which is neatly recorded in each binder. That way we
- make use of earlier iterations of the fixpoint algorithm. (Cunning
- plan.)
- But on the *first* iteration we want to *ignore* the current strictness
- of the Id, and start from "bottom". Nowadays the Id can have a current
- strictness, because interface files record strictness for nested bindings.
- To know when we are in the first iteration, we look at the ae_virgin
- field of the AnalEnv.
- %************************************************************************
- %* *
- Demands
- %* *
- %************************************************************************
- \begin{code}
- splitDmdTy :: DmdType -> (Demand, DmdType)
- -- Split off one function argument
- -- We already have a suitable demand on all
- -- free vars, so no need to add more!
- splitDmdTy (DmdType fv (dmd:dmds) res_ty) = (dmd, DmdType fv dmds res_ty)
- splitDmdTy ty@(DmdType _ [] res_ty) = (resTypeArgDmd res_ty, ty)
- splitCallDmd :: Demand -> (Int, Demand)
- splitCallDmd (Call d) = case splitCallDmd d of
- (n, r) -> (n+1, r)
- splitCallDmd d = (0, d)
- vanillaCall :: Arity -> Demand
- vanillaCall 0 = evalDmd
- vanillaCall n = Call (vanillaCall (n-1))
- deferType :: DmdType -> DmdType
- deferType (DmdType fv _ _) = DmdType (deferEnv fv) [] TopRes
- -- Notice that we throw away info about both arguments and results
- -- For example, f = let ... in \x -> x
- -- We don't want to get a stricness type V->T for f.
- deferEnv :: DmdEnv -> DmdEnv
- deferEnv fv = mapVarEnv defer fv
- ----------------
- argDemand :: Demand -> Demand
- -- The 'Defer' demands are just Lazy at function boundaries
- -- Ugly! Ask John how to improve it.
- argDemand Top = lazyDmd
- argDemand (Defer _) = lazyDmd
- argDemand (Eval ds) = Eval (mapDmds argDemand ds)
- argDemand (Box Bot) = evalDmd
- argDemand (Box d) = box (argDemand d)
- argDemand Bot = Abs -- Don't pass args that are consumed (only) by bottom
- argDemand d = d
- \end{code}
- \begin{code}
- -------------------------
- lubType :: DmdType -> DmdType -> DmdType
- -- Consider (if x then y else []) with demand V
- -- Then the first branch gives {y->V} and the second
- -- *implicitly* has {y->A}. So we must put {y->(V `lub` A)}
- -- in the result env.
- lubType (DmdType fv1 ds1 r1) (DmdType fv2 ds2 r2)
- = DmdType lub_fv2 (lub_ds ds1 ds2) (r1 `lubRes` r2)
- where
- lub_fv = plusVarEnv_C lub fv1 fv2
- lub_fv1 = modifyEnv (not (isBotRes r1)) absLub fv2 fv1 lub_fv
- lub_fv2 = modifyEnv (not (isBotRes r2)) absLub fv1 fv2 lub_fv1
- -- lub is the identity for Bot
- -- Extend the shorter argument list to match the longer
- lub_ds (d1:ds1) (d2:ds2) = lub d1 d2 : lub_ds ds1 ds2
- lub_ds [] [] = []
- lub_ds ds1 [] = map (`lub` resTypeArgDmd r2) ds1
- lub_ds [] ds2 = map (resTypeArgDmd r1 `lub`) ds2
- -----------------------------------
- bothType :: DmdType -> DmdType -> DmdType
- -- (t1 `bothType` t2) takes the argument/result info from t1,
- -- using t2 just for its free-var info
- -- NB: Don't forget about r2! It might be BotRes, which is
- -- a bottom demand on all the in-scope variables.
- -- Peter: can this be done more neatly?
- bothType (DmdType fv1 ds1 r1) (DmdType fv2 _ r2)
- = DmdType both_fv2 ds1 (r1 `bothRes` r2)
- where
- both_fv = plusVarEnv_C both fv1 fv2
- both_fv1 = modifyEnv (isBotRes r1) (`both` Bot) fv2 fv1 both_fv
- both_fv2 = modifyEnv (isBotRes r2) (`both` Bot) fv1 fv2 both_fv1
- -- both is the identity for Abs
- \end{code}
- \begin{code}
- lubRes :: DmdResult -> DmdResult -> DmdResult
- lubRes BotRes r = r
- lubRes r BotRes = r
- lubRes RetCPR RetCPR = RetCPR
- lubRes _ _ = TopRes
- bothRes :: DmdResult -> DmdResult -> DmdResult
- -- If either diverges, the whole thing does
- -- Otherwise take CPR info from the first
- bothRes _ BotRes = BotRes
- bothRes r1 _ = r1
- \end{code}
- \begin{code}
- modifyEnv :: Bool -- No-op if False
- -> (Demand -> Demand) -- The zapper
- -> DmdEnv -> DmdEnv -- Env1 and Env2
- -> DmdEnv -> DmdEnv -- Transform this env
- -- Zap anything in Env1 but not in Env2
- -- Assume: dom(env) includes dom(Env1) and dom(Env2)
- modifyEnv need_to_modify zapper env1 env2 env
- | need_to_modify = foldr zap env (varEnvKeys (env1 `minusUFM` env2))
- | otherwise = env
- where
- zap uniq env = addToUFM_Directly env uniq (zapper current_val)
- where
- current_val = expectJust "modifyEnv" (lookupUFM_Directly env uniq)
- \end{code}
- %************************************************************************
- %* *
- \subsection{LUB and BOTH}
- %* *
- %************************************************************************
- \begin{code}
- lub :: Demand -> Demand -> Demand
- lub Bot d2 = d2
- lub Abs d2 = absLub d2
- lub Top _ = Top
- lub (Defer ds1) d2 = defer (Eval ds1 `lub` d2)
- lub (Call d1) (Call d2) = Call (d1 `lub` d2)
- lub d1@(Call _) (Box d2) = d1 `lub` d2 -- Just strip the box
- lub (Call _) d2@(Eval _) = d2 -- Presumably seq or vanilla eval
- lub d1@(Call _) d2 = d2 `lub` d1 -- Bot, Abs, Top
- -- For the Eval case, we use these approximation rules
- -- Box Bot <= Eval (Box Bot ...)
- -- Box Top <= Defer (Box Bot ...)
- -- Box (Eval ds) <= Eval (map Box ds)
- lub (Eval ds1) (Eval ds2) = Eval (ds1 `lubs` ds2)
- lub (Eval ds1) (Box Bot) = Eval (mapDmds (`lub` Box Bot) ds1)
- lub (Eval ds1) (Box (Eval ds2)) = Eval (ds1 `lubs` mapDmds box ds2)
- lub (Eval ds1) (Box Abs) = deferEval (mapDmds (`lub` Box Bot) ds1)
- lub d1@(Eval _) d2 = d2 `lub` d1 -- Bot,Abs,Top,Call,Defer
- lub (Box d1) (Box d2) = box (d1 `lub` d2)
- lub d1@(Box _) d2 = d2 `lub` d1
- lubs :: Demands -> Demands -> Demands
- lubs ds1 ds2 = zipWithDmds lub ds1 ds2
- ---------------------
- box :: Demand -> Demand
- -- box is the smart constructor for Box
- -- It computes <B,bot> & d
- -- INVARIANT: (Box d) => d = Bot, Abs, Eval
- -- Seems to be no point in allowing (Box (Call d))
- box (Call d) = Call d -- The odd man out. Why?
- box (Box d) = Box d
- box (Defer _) = lazyDmd
- box Top = lazyDmd -- Box Abs and Box Top
- box Abs = lazyDmd -- are the same <B,L>
- box d = Box d -- Bot, Eval
- ---------------
- defer :: Demand -> Demand
- -- defer is the smart constructor for Defer
- -- The idea is that (Defer ds) = <U(ds), L>
- --
- -- It specifies what happens at a lazy function argument
- -- or a lambda; the L* operator
- -- Set the strictness part to L, but leave
- -- the boxity side unaffected
- -- It also ensures that Defer (Eval [LLLL]) = L
- defer Bot = Abs
- defer Abs = Abs
- defer Top = Top
- defer (Call _) = lazyDmd -- Approximation here?
- defer (Box _) = lazyDmd
- defer (Defer ds) = Defer ds
- defer (Eval ds) = deferEval ds
- deferEval :: Demands -> Demand
- -- deferEval ds = defer (Eval ds)
- deferEval ds | allTop ds = Top
- | otherwise = Defer ds
- ---------------------
- absLub :: Demand -> Demand
- -- Computes (Abs `lub` d)
- -- For the Bot case consider
- -- f x y = if ... then x else error x
- -- Then for y we get Abs `lub` Bot, and we really
- -- want Abs overall
- absLub Bot = Abs
- absLub Abs = Abs
- absLub Top = Top
- absLub (Call _) = Top
- absLub (Box _) = Top
- absLub (Eval ds) = Defer (absLubs ds) -- Or (Defer ds)?
- absLub (Defer ds) = Defer (absLubs ds) -- Or (Defer ds)?
- absLubs :: Demands -> Demands
- absLubs = mapDmds absLub
- ---------------
- both :: Demand -> Demand -> Demand
- both Abs d2 = d2
- -- Note [Bottom demands]
- both Bot Bot = Bot
- both Bot Abs = Bot
- both Bot (Eval ds) = Eval (mapDmds (`both` Bot) ds)
- both Bot (Defer ds) = Eval (mapDmds (`both` Bot) ds)
- both Bot _ = errDmd
- both Top Bot = errDmd
- both Top Abs = Top
- both Top Top = Top
- both Top (Box d) = Box d
- both Top (Call d) = Call d
- both Top (Eval ds) = Eval (mapDmds (`both` Top) ds)
- both Top (Defer ds) -- = defer (Top `both` Eval ds)
- -- = defer (Eval (mapDmds (`both` Top) ds))
- = deferEval (mapDmds (`both` Top) ds)
- both (Box d1) (Box d2) = box (d1 `both` d2)
- both (Box d1) d2@(Call _) = box (d1 `both` d2)
- both (Box d1) d2@(Eval _) = box (d1 `both` d2)
- both (Box d1) (Defer _) = Box d1
- both d1@(Box _) d2 = d2 `both` d1
- both (Call d1) (Call d2) = Call (d1 `both` d2)
- both (Call d1) (Eval _) = Call d1 -- Could do better for (Poly Bot)?
- both (Call d1) (Defer _) = Call d1 -- Ditto
- both d1@(Call _) d2 = d2 `both` d1
- both (Eval ds1) (Eval ds2) = Eval (ds1 `boths` ds2)
- both (Eval ds1) (Defer ds2) = Eval (ds1 `boths` mapDmds defer ds2)
- both d1@(Eval _) d2 = d2 `both` d1
- both (Defer ds1) (Defer ds2) = deferEval (ds1 `boths` ds2)
- both d1@(Defer _) d2 = d2 `both` d1
-
- boths :: Demands -> Demands -> Demands
- boths ds1 ds2 = zipWithDmds both ds1 ds2
- \end{code}
- Note [Bottom demands]
- ~~~~~~~~~~~~~~~~~~~~~
- Consider
- f x = error x
- From 'error' itself we get demand Bot on x
- From the arg demand on x we get
- x :-> evalDmd = Box (Eval (Poly Abs))
- So we get Bot `both` Box (Eval (Poly Abs))
- = Seq Keep (Poly Bot)
- Consider also
- f x = if ... then error (fst x) else fst x
- Then we get (Eval (Box Bot, Bot) `lub` Eval (SA))
- = Eval (SA)
- which is what we want.
- Consider also
- f x = error [fst x]
- Then we get
- x :-> Bot `both` Defer [SA]
- and we want the Bot demand to cancel out the Defer
- so that we get Eval [SA]. Otherwise we'd have the odd
- situation that
- f x = error (fst x) -- Strictness U(SA)b
- g x = error ('y':fst x) -- Strictness Tb