\begin{code}
module SimplUtils (
- mkLam, mkCase,
+ -- Rebuilding
+ mkLam, mkCase, prepareAlts, bindCaseBndr,
-- Inlining,
- preInlineUnconditionally, postInlineUnconditionally, activeInline, activeRule,
- inlineMode,
+ preInlineUnconditionally, postInlineUnconditionally,
+ activeInline, activeRule, inlineMode,
-- The continuation type
- SimplCont(..), DupFlag(..), LetRhsFlag(..),
- contIsDupable, contResultType,
- countValArgs, countArgs, pushContArgs,
- mkBoringStop, mkLazyArgStop, mkRhsStop, contIsRhs, contIsRhsOrArg,
- getContArgs, interestingCallContext, interestingArgContext,
- interestingArg, isStrictType
+ SimplCont(..), DupFlag(..), ArgInfo(..),
+ contIsDupable, contResultType, contIsTrivial, contArgs, dropArgs,
+ countValArgs, countArgs, splitInlineCont,
+ mkBoringStop, mkLazyArgStop, contIsRhsOrArg,
+ interestingCallContext, interestingArgContext,
+ interestingArg, mkArgInfo,
+
+ abstractFloats
) where
#include "HsVersions.h"
import SimplEnv
-import DynFlags ( SimplifierSwitch(..), SimplifierMode(..),
- DynFlags, DynFlag(..), dopt )
-import StaticFlags ( opt_UF_UpdateInPlace, opt_SimplNoPreInlining,
- opt_RulesOff )
+import DynFlags
+import StaticFlags
import CoreSyn
-import CoreFVs ( exprFreeVars )
-import CoreUtils ( cheapEqExpr, exprType, exprIsTrivial,
- etaExpand, exprEtaExpandArity, bindNonRec, mkCoerce2,
- findDefault, exprOkForSpeculation, exprIsHNF, mergeAlts
- )
-import Literal ( mkStringLit )
-import CoreUnfold ( smallEnoughToInline )
-import MkId ( eRROR_ID )
-import Id ( Id, idType, isDataConWorkId, idOccInfo, isDictId,
- isDeadBinder, idNewDemandInfo, isExportedId,
- idUnfolding, idNewStrictness, idInlinePragma, idHasRules
- )
-import NewDemand ( isStrictDmd, isBotRes, splitStrictSig )
+import qualified CoreSubst
+import PprCore
+import CoreFVs
+import CoreUtils
+import CoreArity ( etaExpand, exprEtaExpandArity )
+import CoreUnfold
+import Name
+import Id
+import Var ( isCoVar )
+import NewDemand
import SimplMonad
-import Type ( Type, splitFunTys, dropForAlls, isStrictType,
- splitTyConApp_maybe, tyConAppArgs
- )
-import TyCon ( tyConDataCons_maybe )
-import DataCon ( dataConRepArity )
+import Type hiding( substTy )
+import Coercion ( coercionKind )
+import TyCon
+import Unify ( dataConCannotMatch )
import VarSet
-import BasicTypes ( TopLevelFlag(..), isNotTopLevel, OccInfo(..), isLoopBreaker, isOneOcc,
- Activation, isAlwaysActive, isActive )
-import Util ( lengthExceeds )
+import BasicTypes
+import Util
+import MonadUtils
import Outputable
+import FastString
+
+import List( nub )
\end{code}
%************************************************************************
%* *
-\subsection{The continuation data type}
+ The SimplCont type
%* *
%************************************************************************
-\begin{code}
-data SimplCont -- Strict contexts
- = Stop OutType -- Type of the result
- LetRhsFlag
- Bool -- True <=> There is something interesting about
- -- the context, and hence the inliner
- -- should be a bit keener (see interestingCallContext)
- -- Two cases:
- -- (a) This is the RHS of a thunk whose type suggests
- -- that update-in-place would be possible
- -- (b) This is an argument of a function that has RULES
- -- Inlining the call might allow the rule to fire
-
- | CoerceIt OutType -- The To-type, simplified
- SimplCont
+A SimplCont allows the simplifier to traverse the expression in a
+zipper-like fashion. The SimplCont represents the rest of the expression,
+"above" the point of interest.
- | ApplyTo DupFlag
- InExpr SimplEnv -- The argument, as yet unsimplified,
- SimplCont -- and its environment
+You can also think of a SimplCont as an "evaluation context", using
+that term in the way it is used for operational semantics. This is the
+way I usually think of it, For example you'll often see a syntax for
+evaluation context looking like
+ C ::= [] | C e | case C of alts | C `cast` co
+That's the kind of thing we are doing here, and I use that syntax in
+the comments.
- | Select DupFlag
- InId [InAlt] SimplEnv -- The case binder, alts, and subst-env
- SimplCont
- | ArgOf LetRhsFlag -- An arbitrary strict context: the argument
- -- of a strict function, or a primitive-arg fn
- -- or a PrimOp
- -- No DupFlag, because we never duplicate it
- OutType -- arg_ty: type of the argument itself
- OutType -- cont_ty: the type of the expression being sought by the context
- -- f (error "foo") ==> coerce t (error "foo")
- -- when f is strict
- -- We need to know the type t, to which to coerce.
+Key points:
+ * A SimplCont describes a *strict* context (just like
+ evaluation contexts do). E.g. Just [] is not a SimplCont
- (SimplEnv -> OutExpr -> SimplM FloatsWithExpr) -- What to do with the result
- -- The result expression in the OutExprStuff has type cont_ty
+ * A SimplCont describes a context that *does not* bind
+ any variables. E.g. \x. [] is not a SimplCont
-data LetRhsFlag = AnArg -- It's just an argument not a let RHS
- | AnRhs -- It's the RHS of a let (so please float lets out of big lambdas)
+\begin{code}
+data SimplCont
+ = Stop -- An empty context, or hole, []
+ CallCtxt -- True <=> There is something interesting about
+ -- the context, and hence the inliner
+ -- should be a bit keener (see interestingCallContext)
+ -- Specifically:
+ -- This is an argument of a function that has RULES
+ -- Inlining the call might allow the rule to fire
-instance Outputable LetRhsFlag where
- ppr AnArg = ptext SLIT("arg")
- ppr AnRhs = ptext SLIT("rhs")
+ | CoerceIt -- C `cast` co
+ OutCoercion -- The coercion simplified
+ SimplCont
+
+ | ApplyTo -- C arg
+ DupFlag
+ InExpr SimplEnv -- The argument and its static env
+ SimplCont
+
+ | Select -- case C of alts
+ DupFlag
+ InId [InAlt] SimplEnv -- The case binder, alts, and subst-env
+ SimplCont
+
+ -- The two strict forms have no DupFlag, because we never duplicate them
+ | StrictBind -- (\x* \xs. e) C
+ InId [InBndr] -- let x* = [] in e
+ InExpr SimplEnv -- is a special case
+ SimplCont
+
+ | StrictArg -- e C
+ OutExpr -- e
+ CallCtxt -- Whether *this* argument position is interesting
+ ArgInfo -- Whether the function at the head of e has rules, etc
+ SimplCont -- plus strictness flags for *further* args
+
+data ArgInfo
+ = ArgInfo {
+ ai_rules :: Bool, -- Function has rules (recursively)
+ -- => be keener to inline in all args
+ ai_strs :: [Bool], -- Strictness of arguments
+ -- Usually infinite, but if it is finite it guarantees
+ -- that the function diverges after being given
+ -- that number of args
+ ai_discs :: [Int] -- Discounts for arguments; non-zero => be keener to inline
+ -- Always infinite
+ }
instance Outputable SimplCont where
- ppr (Stop ty is_rhs _) = ptext SLIT("Stop") <> brackets (ppr is_rhs) <+> ppr ty
- ppr (ApplyTo dup arg se cont) = (ptext SLIT("ApplyTo") <+> ppr dup <+> ppr arg) $$ ppr cont
- ppr (ArgOf _ _ _ _) = ptext SLIT("ArgOf...")
- ppr (Select dup bndr alts se cont) = (ptext SLIT("Select") <+> ppr dup <+> ppr bndr) $$
- (nest 4 (ppr alts)) $$ ppr cont
- ppr (CoerceIt ty cont) = (ptext SLIT("CoerceIt") <+> ppr ty) $$ ppr cont
+ ppr (Stop interesting) = ptext (sLit "Stop") <> brackets (ppr interesting)
+ ppr (ApplyTo dup arg _ cont) = ((ptext (sLit "ApplyTo") <+> ppr dup <+> pprParendExpr arg)
+ {- $$ nest 2 (pprSimplEnv se) -}) $$ ppr cont
+ ppr (StrictBind b _ _ _ cont) = (ptext (sLit "StrictBind") <+> ppr b) $$ ppr cont
+ ppr (StrictArg f _ _ cont) = (ptext (sLit "StrictArg") <+> ppr f) $$ ppr cont
+ ppr (Select dup bndr alts _ cont) = (ptext (sLit "Select") <+> ppr dup <+> ppr bndr) $$
+ (nest 4 (ppr alts)) $$ ppr cont
+ ppr (CoerceIt co cont) = (ptext (sLit "CoerceIt") <+> ppr co) $$ ppr cont
data DupFlag = OkToDup | NoDup
instance Outputable DupFlag where
- ppr OkToDup = ptext SLIT("ok")
- ppr NoDup = ptext SLIT("nodup")
-
+ ppr OkToDup = ptext (sLit "ok")
+ ppr NoDup = ptext (sLit "nodup")
--------------------
-mkBoringStop :: OutType -> SimplCont
-mkBoringStop ty = Stop ty AnArg False
-mkLazyArgStop :: OutType -> Bool -> SimplCont
-mkLazyArgStop ty has_rules = Stop ty AnArg (canUpdateInPlace ty || has_rules)
-mkRhsStop :: OutType -> SimplCont
-mkRhsStop ty = Stop ty AnRhs (canUpdateInPlace ty)
+-------------------
+mkBoringStop :: SimplCont
+mkBoringStop = Stop BoringCtxt
-contIsRhs :: SimplCont -> Bool
-contIsRhs (Stop _ AnRhs _) = True
-contIsRhs (ArgOf AnRhs _ _ _) = True
-contIsRhs other = False
+mkLazyArgStop :: CallCtxt -> SimplCont
+mkLazyArgStop cci = Stop cci
-contIsRhsOrArg (Stop _ _ _) = True
-contIsRhsOrArg (ArgOf _ _ _ _) = True
-contIsRhsOrArg other = False
+-------------------
+contIsRhsOrArg :: SimplCont -> Bool
+contIsRhsOrArg (Stop {}) = True
+contIsRhsOrArg (StrictBind {}) = True
+contIsRhsOrArg (StrictArg {}) = True
+contIsRhsOrArg _ = False
-------------------
contIsDupable :: SimplCont -> Bool
-contIsDupable (Stop _ _ _) = True
+contIsDupable (Stop {}) = True
contIsDupable (ApplyTo OkToDup _ _ _) = True
contIsDupable (Select OkToDup _ _ _ _) = True
contIsDupable (CoerceIt _ cont) = contIsDupable cont
-contIsDupable other = False
+contIsDupable _ = False
-------------------
-discardableCont :: SimplCont -> Bool
-discardableCont (Stop _ _ _) = False
-discardableCont (CoerceIt _ cont) = discardableCont cont
-discardableCont other = True
-
-discardCont :: SimplCont -- A continuation, expecting
- -> SimplCont -- Replace the continuation with a suitable coerce
-discardCont cont = case cont of
- Stop to_ty is_rhs _ -> cont
- other -> CoerceIt to_ty (mkBoringStop to_ty)
- where
- to_ty = contResultType cont
+contIsTrivial :: SimplCont -> Bool
+contIsTrivial (Stop {}) = True
+contIsTrivial (ApplyTo _ (Type _) _ cont) = contIsTrivial cont
+contIsTrivial (CoerceIt _ cont) = contIsTrivial cont
+contIsTrivial _ = False
-------------------
-contResultType :: SimplCont -> OutType
-contResultType (Stop to_ty _ _) = to_ty
-contResultType (ArgOf _ _ to_ty _) = to_ty
-contResultType (ApplyTo _ _ _ cont) = contResultType cont
-contResultType (CoerceIt _ cont) = contResultType cont
-contResultType (Select _ _ _ _ cont) = contResultType cont
+contResultType :: SimplEnv -> OutType -> SimplCont -> OutType
+contResultType env ty cont
+ = go cont ty
+ where
+ subst_ty se ty = substTy (se `setInScope` env) ty
+
+ go (Stop {}) ty = ty
+ go (CoerceIt co cont) _ = go cont (snd (coercionKind co))
+ go (StrictBind _ bs body se cont) _ = go cont (subst_ty se (exprType (mkLams bs body)))
+ go (StrictArg fn _ _ cont) _ = go cont (funResultTy (exprType fn))
+ go (Select _ _ alts se cont) _ = go cont (subst_ty se (coreAltsType alts))
+ go (ApplyTo _ arg se cont) ty = go cont (apply_to_arg ty arg se)
+
+ apply_to_arg ty (Type ty_arg) se = applyTy ty (subst_ty se ty_arg)
+ apply_to_arg ty _ _ = funResultTy ty
-------------------
countValArgs :: SimplCont -> Int
-countValArgs (ApplyTo _ (Type ty) se cont) = countValArgs cont
-countValArgs (ApplyTo _ val_arg se cont) = 1 + countValArgs cont
-countValArgs other = 0
+countValArgs (ApplyTo _ (Type _) _ cont) = countValArgs cont
+countValArgs (ApplyTo _ _ _ cont) = 1 + countValArgs cont
+countValArgs _ = 0
countArgs :: SimplCont -> Int
-countArgs (ApplyTo _ arg se cont) = 1 + countArgs cont
-countArgs other = 0
+countArgs (ApplyTo _ _ _ cont) = 1 + countArgs cont
+countArgs _ = 0
--------------------
-pushContArgs :: SimplEnv -> [OutArg] -> SimplCont -> SimplCont
--- Pushes args with the specified environment
-pushContArgs env [] cont = cont
-pushContArgs env (arg : args) cont = ApplyTo NoDup arg env (pushContArgs env args cont)
+contArgs :: SimplCont -> ([OutExpr], SimplCont)
+-- Uses substitution to turn each arg into an OutExpr
+contArgs cont = go [] cont
+ where
+ go args (ApplyTo _ arg se cont) = go (substExpr se arg : args) cont
+ go args cont = (reverse args, cont)
+
+dropArgs :: Int -> SimplCont -> SimplCont
+dropArgs 0 cont = cont
+dropArgs n (ApplyTo _ _ _ cont) = dropArgs (n-1) cont
+dropArgs n other = pprPanic "dropArgs" (ppr n <+> ppr other)
+
+--------------------
+splitInlineCont :: SimplCont -> Maybe (SimplCont, SimplCont)
+-- Returns Nothing if the continuation should dissolve an InlineMe Note
+-- Return Just (c1,c2) otherwise,
+-- where c1 is the continuation to put inside the InlineMe
+-- and c2 outside
+
+-- Example: (__inline_me__ (/\a. e)) ty
+-- Here we want to do the beta-redex without dissolving the InlineMe
+-- See test simpl017 (and Trac #1627) for a good example of why this is important
+
+splitInlineCont (ApplyTo dup (Type ty) se c)
+ | Just (c1, c2) <- splitInlineCont c = Just (ApplyTo dup (Type ty) se c1, c2)
+splitInlineCont cont@(Stop {}) = Just (mkBoringStop, cont)
+splitInlineCont cont@(StrictBind {}) = Just (mkBoringStop, cont)
+splitInlineCont _ = Nothing
+ -- NB: we dissolve an InlineMe in any strict context,
+ -- not just function aplication.
+ -- E.g. foldr k z (__inline_me (case x of p -> build ...))
+ -- Here we want to get rid of the __inline_me__ so we
+ -- can float the case, and see foldr/build
+ --
+ -- However *not* in a strict RHS, else we get
+ -- let f = __inline_me__ (\x. e) in ...f...
+ -- Now if f is guaranteed to be called, hence a strict binding
+ -- we don't thereby want to dissolve the __inline_me__; for
+ -- example, 'f' might be a wrapper, so we'd inline the worker
\end{code}
\begin{code}
-getContArgs :: SwitchChecker
- -> OutId -> SimplCont
- -> ([(InExpr, SimplEnv, Bool)], -- Arguments; the Bool is true for strict args
- SimplCont) -- Remaining continuation
--- getContArgs id k = (args, k', inl)
--- args are the leading ApplyTo items in k
--- (i.e. outermost comes first)
--- augmented with demand info from the functionn
-getContArgs chkr fun orig_cont
- = let
- -- Ignore strictness info if the no-case-of-case
- -- flag is on. Strictness changes evaluation order
- -- and that can change full laziness
- stricts | switchIsOn chkr NoCaseOfCase = vanilla_stricts
- | otherwise = computed_stricts
- in
- go [] stricts orig_cont
- where
- ----------------------------
-
- -- Type argument
- go acc ss (ApplyTo _ arg@(Type _) se cont)
- = go ((arg,se,False) : acc) ss cont
- -- NB: don't bother to instantiate the function type
-
- -- Value argument
- go acc (s:ss) (ApplyTo _ arg se cont)
- = go ((arg,se,s) : acc) ss cont
-
- -- We're run out of arguments, or else we've run out of demands
- -- The latter only happens if the result is guaranteed bottom
- -- This is the case for
- -- * case (error "hello") of { ... }
- -- * (error "Hello") arg
- -- * f (error "Hello") where f is strict
- -- etc
- -- Then, especially in the first of these cases, we'd like to discard
- -- the continuation, leaving just the bottoming expression. But the
- -- type might not be right, so we may have to add a coerce.
- go acc ss cont
- | null ss && discardableCont cont = (reverse acc, discardCont cont)
- | otherwise = (reverse acc, cont)
-
- ----------------------------
- vanilla_stricts, computed_stricts :: [Bool]
- vanilla_stricts = repeat False
- computed_stricts = zipWith (||) fun_stricts arg_stricts
-
- ----------------------------
- (val_arg_tys, _) = splitFunTys (dropForAlls (idType fun))
- arg_stricts = map isStrictType val_arg_tys ++ repeat False
- -- These argument types are used as a cheap and cheerful way to find
- -- unboxed arguments, which must be strict. But it's an InType
- -- and so there might be a type variable where we expect a function
- -- type (the substitution hasn't happened yet). And we don't bother
- -- doing the type applications for a polymorphic function.
- -- Hence the splitFunTys*IgnoringForAlls*
-
- ----------------------------
- -- If fun_stricts is finite, it means the function returns bottom
- -- after that number of value args have been consumed
- -- Otherwise it's infinite, extended with False
- fun_stricts
- = case splitStrictSig (idNewStrictness fun) of
- (demands, result_info)
- | not (demands `lengthExceeds` countValArgs orig_cont)
- -> -- Enough args, use the strictness given.
- -- For bottoming functions we used to pretend that the arg
- -- is lazy, so that we don't treat the arg as an
- -- interesting context. This avoids substituting
- -- top-level bindings for (say) strings into
- -- calls to error. But now we are more careful about
- -- inlining lone variables, so its ok (see SimplUtils.analyseCont)
- if isBotRes result_info then
- map isStrictDmd demands -- Finite => result is bottom
- else
- map isStrictDmd demands ++ vanilla_stricts
-
- other -> vanilla_stricts -- Not enough args, or no strictness
-
--------------------
interestingArg :: OutExpr -> Bool
-- An argument is interesting if it has *some* structure
-- We are here trying to avoid unfolding a function that
interestingArg (Type _) = False
interestingArg (App fn (Type _)) = interestingArg fn
interestingArg (Note _ a) = interestingArg a
-interestingArg other = True
+
+-- Idea (from Sam B); I'm not sure if it's a good idea, so commented out for now
+-- interestingArg expr | isUnLiftedType (exprType expr)
+-- -- Unlifted args are only ever interesting if we know what they are
+-- = case expr of
+-- Lit lit -> True
+-- _ -> False
+
+interestingArg _ = True
-- Consider let x = 3 in f x
-- The substitution will contain (x -> ContEx 3), and we want to
-- to say that x is an interesting argument.
-- that x is not interesting (assuming y has no unfolding)
\end{code}
+
Comment about interestingCallContext
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
We want to avoid inlining an expression where there can't possibly be
contIsInteresting looks for case expressions with just a single
default case.
+
\begin{code}
-interestingCallContext :: Bool -- False <=> no args at all
- -> Bool -- False <=> no value args
- -> SimplCont -> Bool
- -- The "lone-variable" case is important. I spent ages
- -- messing about with unsatisfactory varaints, but this is nice.
- -- The idea is that if a variable appear all alone
- -- as an arg of lazy fn, or rhs Stop
- -- as scrutinee of a case Select
- -- as arg of a strict fn ArgOf
- -- then we should not inline it (unless there is some other reason,
- -- e.g. is is the sole occurrence). We achieve this by making
- -- interestingCallContext return False for a lone variable.
- --
- -- Why? At least in the case-scrutinee situation, turning
- -- let x = (a,b) in case x of y -> ...
- -- into
- -- let x = (a,b) in case (a,b) of y -> ...
- -- and thence to
- -- let x = (a,b) in let y = (a,b) in ...
- -- is bad if the binding for x will remain.
- --
- -- Another example: I discovered that strings
- -- were getting inlined straight back into applications of 'error'
- -- because the latter is strict.
- -- s = "foo"
- -- f = \x -> ...(error s)...
-
- -- Fundamentally such contexts should not ecourage inlining because
- -- the context can ``see'' the unfolding of the variable (e.g. case or a RULE)
- -- so there's no gain.
- --
- -- However, even a type application or coercion isn't a lone variable.
- -- Consider
- -- case $fMonadST @ RealWorld of { :DMonad a b c -> c }
- -- We had better inline that sucker! The case won't see through it.
- --
- -- For now, I'm treating treating a variable applied to types
- -- in a *lazy* context "lone". The motivating example was
- -- f = /\a. \x. BIG
- -- g = /\a. \y. h (f a)
- -- There's no advantage in inlining f here, and perhaps
- -- a significant disadvantage. Hence some_val_args in the Stop case
-
-interestingCallContext some_args some_val_args cont
+interestingCallContext :: SimplCont -> CallCtxt
+interestingCallContext cont
= interesting cont
where
- interesting (Select _ _ _ _ _) = some_args
- interesting (ApplyTo _ _ _ _) = True -- Can happen if we have (coerce t (f x)) y
- -- Perhaps True is a bit over-keen, but I've
- -- seen (coerce f) x, where f has an INLINE prag,
- -- So we have to give some motivaiton for inlining it
- interesting (ArgOf _ _ _ _) = some_val_args
- interesting (Stop ty _ interesting) = some_val_args && interesting
- interesting (CoerceIt _ cont) = interesting cont
+ interesting (Select _ bndr _ _ _)
+ | isDeadBinder bndr = CaseCtxt
+ | otherwise = ArgCtxt False 2 -- If the binder is used, this
+ -- is like a strict let
+
+ interesting (ApplyTo _ arg _ cont)
+ | isTypeArg arg = interesting cont
+ | otherwise = ValAppCtxt -- Can happen if we have (f Int |> co) y
+ -- If f has an INLINE prag we need to give it some
+ -- motivation to inline. See Note [Cast then apply]
+ -- in CoreUnfold
+
+ interesting (StrictArg _ cci _ _) = cci
+ interesting (StrictBind {}) = BoringCtxt
+ interesting (Stop cci) = cci
+ interesting (CoerceIt _ cont) = interesting cont
-- If this call is the arg of a strict function, the context
-- is a bit interesting. If we inline here, we may get useful
-- evaluation information to avoid repeated evals: e.g.
-------------------
+mkArgInfo :: Id
+ -> Int -- Number of value args
+ -> SimplCont -- Context of the cal
+ -> ArgInfo
+
+mkArgInfo fun n_val_args call_cont
+ | n_val_args < idArity fun -- Note [Unsaturated functions]
+ = ArgInfo { ai_rules = False
+ , ai_strs = vanilla_stricts
+ , ai_discs = vanilla_discounts }
+ | otherwise
+ = ArgInfo { ai_rules = interestingArgContext fun call_cont
+ , ai_strs = add_type_str (idType fun) arg_stricts
+ , ai_discs = arg_discounts }
+ where
+ vanilla_discounts, arg_discounts :: [Int]
+ vanilla_discounts = repeat 0
+ arg_discounts = case idUnfolding fun of
+ CoreUnfolding _ _ _ _ _ (UnfoldIfGoodArgs _ discounts _ _)
+ -> discounts ++ vanilla_discounts
+ _ -> vanilla_discounts
+
+ vanilla_stricts, arg_stricts :: [Bool]
+ vanilla_stricts = repeat False
+
+ arg_stricts
+ = case splitStrictSig (idNewStrictness fun) of
+ (demands, result_info)
+ | not (demands `lengthExceeds` n_val_args)
+ -> -- Enough args, use the strictness given.
+ -- For bottoming functions we used to pretend that the arg
+ -- is lazy, so that we don't treat the arg as an
+ -- interesting context. This avoids substituting
+ -- top-level bindings for (say) strings into
+ -- calls to error. But now we are more careful about
+ -- inlining lone variables, so its ok (see SimplUtils.analyseCont)
+ if isBotRes result_info then
+ map isStrictDmd demands -- Finite => result is bottom
+ else
+ map isStrictDmd demands ++ vanilla_stricts
+ | otherwise
+ -> WARN( True, text "More demands than arity" <+> ppr fun <+> ppr (idArity fun)
+ <+> ppr n_val_args <+> ppr demands )
+ vanilla_stricts -- Not enough args, or no strictness
+
+ add_type_str :: Type -> [Bool] -> [Bool]
+ -- If the function arg types are strict, record that in the 'strictness bits'
+ -- No need to instantiate because unboxed types (which dominate the strict
+ -- types) can't instantiate type variables.
+ -- add_type_str is done repeatedly (for each call); might be better
+ -- once-for-all in the function
+ -- But beware primops/datacons with no strictness
+ add_type_str _ [] = []
+ add_type_str fun_ty strs -- Look through foralls
+ | Just (_, fun_ty') <- splitForAllTy_maybe fun_ty -- Includes coercions
+ = add_type_str fun_ty' strs
+ add_type_str fun_ty (str:strs) -- Add strict-type info
+ | Just (arg_ty, fun_ty') <- splitFunTy_maybe fun_ty
+ = (str || isStrictType arg_ty) : add_type_str fun_ty' strs
+ add_type_str _ strs
+ = strs
+
+{- Note [Unsaturated functions]
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Consider (test eyeball/inline4)
+ x = a:as
+ y = f x
+where f has arity 2. Then we do not want to inline 'x', because
+it'll just be floated out again. Even if f has lots of discounts
+on its first argument -- it must be saturated for these to kick in
+-}
+
interestingArgContext :: Id -> SimplCont -> Bool
-- If the argument has form (f x y), where x,y are boring,
-- and f is marked INLINE, then we don't want to inline f.
-- where g has rules, then we *do* want to inline f, in case it
-- exposes a rule that might fire. Similarly, if the context is
-- h (g (f x x))
--- where h has rules, then we do want to inline f.
+-- where h has rules, then we do want to inline f; hence the
+-- call_cont argument to interestingArgContext
+--
-- The interesting_arg_ctxt flag makes this happen; if it's
-- set, the inliner gets just enough keener to inline f
-- regardless of how boring f's arguments are, if it's marked INLINE
-- The alternative would be to *always* inline an INLINE function,
-- regardless of how boring its context is; but that seems overkill
-- For example, it'd mean that wrapper functions were always inlined
-interestingArgContext fn cont
- = idHasRules fn || go cont
+interestingArgContext fn call_cont
+ = idHasRules fn || go call_cont
where
- go (Select {}) = False
- go (ApplyTo {}) = False
- go (ArgOf {}) = True
- go (CoerceIt _ c) = go c
- go (Stop _ _ interesting) = interesting
-
--------------------
-canUpdateInPlace :: Type -> Bool
--- Consider let x = <wurble> in ...
--- If <wurble> returns an explicit constructor, we might be able
--- to do update in place. So we treat even a thunk RHS context
--- as interesting if update in place is possible. We approximate
--- this by seeing if the type has a single constructor with a
--- small arity. But arity zero isn't good -- we share the single copy
--- for that case, so no point in sharing.
-
-canUpdateInPlace ty
- | not opt_UF_UpdateInPlace = False
- | otherwise
- = case splitTyConApp_maybe ty of
- Nothing -> False
- Just (tycon, _) -> case tyConDataCons_maybe tycon of
- Just [dc] -> arity == 1 || arity == 2
- where
- arity = dataConRepArity dc
- other -> False
+ go (Select {}) = False
+ go (ApplyTo {}) = False
+ go (StrictArg _ cci _ _) = interesting cci
+ go (StrictBind {}) = False -- ??
+ go (CoerceIt _ c) = go c
+ go (Stop cci) = interesting cci
+
+ interesting (ArgCtxt rules _) = rules
+ interesting _ = False
\end{code}
(d) Simplifying a GHCi expression or Template
Haskell splice
- SimplPhase n Used at all other times
+ SimplPhase n _ Used at all other times
The key thing about SimplGently is that it does no call-site inlining.
Before full laziness we must be careful not to inline wrappers,
might have a BIG rhs, which will now be dup'd at every occurrenc of x.
-Evne RHSs labelled InlineMe aren't caught here, because there might be
+Even RHSs labelled InlineMe aren't caught here, because there might be
no benefit from inlining at the call site.
[Sept 01] Don't unconditionally inline a top-level thing, because that
| otherwise = case idOccInfo bndr of
IAmDead -> True -- Happens in ((\x.1) v)
OneOcc in_lam True int_cxt -> try_once in_lam int_cxt
- other -> False
+ _ -> False
where
phase = getMode env
active = case phase of
- SimplGently -> isAlwaysActive prag
- SimplPhase n -> isActive n prag
- prag = idInlinePragma bndr
+ SimplGently -> isAlwaysActive act
+ SimplPhase n _ -> isActive n act
+ act = idInlineActivation bndr
try_once in_lam int_cxt -- There's one textual occurrence
| not in_lam = isNotTopLevel top_lvl || early_phase
-- canInlineInLam => free vars of rhs are (Once in_lam) or Many,
-- so substituting rhs inside a lambda doesn't change the occ info.
-- Sadly, not quite the same as exprIsHNF.
- canInlineInLam (Lit l) = True
+ canInlineInLam (Lit _) = True
canInlineInLam (Lam b e) = isRuntimeVar b || canInlineInLam e
canInlineInLam (Note _ e) = canInlineInLam e
canInlineInLam _ = False
early_phase = case phase of
- SimplPhase 0 -> False
- other -> True
+ SimplPhase 0 _ -> False
+ _ -> True
-- If we don't have this early_phase test, consider
-- x = length [1,2,3]
-- The full laziness pass carefully floats all the cons cells to
-> Bool
postInlineUnconditionally env top_lvl bndr occ_info rhs unfolding
| not active = False
- | isLoopBreaker occ_info = False
+ | isLoopBreaker occ_info = False -- If it's a loop-breaker of any kind, don't inline
+ -- because it might be referred to "earlier"
| isExportedId bndr = False
| exprIsTrivial rhs = True
| otherwise
-- True -> case x of ...
-- False -> case x of ...
-- I'm not sure how important this is in practice
- OneOcc in_lam one_br int_cxt -- OneOcc => no work-duplication issue
+ OneOcc in_lam _one_br int_cxt -- OneOcc => no code-duplication issue
-> smallEnoughToInline unfolding -- Small enough to dup
-- ToDo: consider discount on smallEnoughToInline if int_cxt is true
--
-- int_cxt to prevent us inlining inside a lambda without some
-- good reason. See the notes on int_cxt in preInlineUnconditionally
- other -> False
+ IAmDead -> True -- This happens; for example, the case_bndr during case of
+ -- known constructor: case (a,b) of x { (p,q) -> ... }
+ -- Here x isn't mentioned in the RHS, so we don't want to
+ -- create the (dead) let-binding let x = (a,b) in ...
+
+ _ -> False
-- Here's an example that we don't handle well:
-- let f = if b then Left (\x.BIG) else Right (\y.BIG)
-- in \y. ....case f of {...} ....
-- Here f is used just once, and duplicating the case work is fine (exprIsCheap).
-- But
--- * We can't preInlineUnconditionally because that woud invalidate
--- the occ info for b.
--- * We can't postInlineUnconditionally because the RHS is big, and
--- that risks exponential behaviour
--- * We can't call-site inline, because the rhs is big
+-- - We can't preInlineUnconditionally because that woud invalidate
+-- the occ info for b.
+-- - We can't postInlineUnconditionally because the RHS is big, and
+-- that risks exponential behaviour
+-- - We can't call-site inline, because the rhs is big
-- Alas!
where
active = case getMode env of
- SimplGently -> isAlwaysActive prag
- SimplPhase n -> isActive n prag
- prag = idInlinePragma bndr
+ SimplGently -> isAlwaysActive act
+ SimplPhase n _ -> isActive n act
+ act = idInlineActivation bndr
-activeInline :: SimplEnv -> OutId -> OccInfo -> Bool
-activeInline env id occ
+activeInline :: SimplEnv -> OutId -> Bool
+activeInline env id
= case getMode env of
- SimplGently -> isOneOcc occ && isAlwaysActive prag
+ SimplGently -> False
-- No inlining at all when doing gentle stuff,
- -- except for local things that occur once
+ -- except for local things that occur once (pre/postInlineUnconditionally)
-- The reason is that too little clean-up happens if you
-- don't inline use-once things. Also a bit of inlining is *good* for
-- full laziness; it can expose constant sub-expressions.
-- and they are now constructed as Compulsory unfoldings (in MkId)
-- so they'll happen anyway.
- SimplPhase n -> isActive n prag
+ SimplPhase n _ -> isActive n act
where
- prag = idInlinePragma id
+ act = idInlineActivation id
-activeRule :: SimplEnv -> Maybe (Activation -> Bool)
+activeRule :: DynFlags -> SimplEnv -> Maybe (Activation -> Bool)
-- Nothing => No rules at all
-activeRule env
- | opt_RulesOff = Nothing
+activeRule dflags env
+ | not (dopt Opt_EnableRewriteRules dflags)
+ = Nothing -- Rewriting is off
| otherwise
= case getMode env of
- SimplGently -> Just isAlwaysActive
+ SimplGently -> Just isAlwaysActive
-- Used to be Nothing (no rules in gentle mode)
-- Main motivation for changing is that I wanted
-- lift String ===> ...
-- to work in Template Haskell when simplifying
-- splices, so we get simpler code for literal strings
- SimplPhase n -> Just (isActive n)
-\end{code}
+ SimplPhase n _ -> Just (isActive n)
+\end{code}
%************************************************************************
%* *
-\subsection{Rebuilding a lambda}
+ Rebuilding a lambda
%* *
%************************************************************************
\begin{code}
-mkLam :: SimplEnv -> [OutBinder] -> OutExpr -> SimplCont -> SimplM FloatsWithExpr
+mkLam :: SimplEnv -> [OutBndr] -> OutExpr -> SimplM OutExpr
+-- mkLam tries three things
+-- a) eta reduction, if that gives a trivial expression
+-- b) eta expansion [only if there are some value lambdas]
+
+mkLam _b [] body
+ = return body
+mkLam _env bndrs body
+ = do { dflags <- getDOptsSmpl
+ ; mkLam' dflags bndrs body }
+ where
+ mkLam' :: DynFlags -> [OutBndr] -> OutExpr -> SimplM OutExpr
+ mkLam' dflags bndrs (Cast body co)
+ | not (any bad bndrs)
+ -- Note [Casts and lambdas]
+ = do { lam <- mkLam' dflags bndrs body
+ ; return (mkCoerce (mkPiTypes bndrs co) lam) }
+ where
+ co_vars = tyVarsOfType co
+ bad bndr = isCoVar bndr && bndr `elemVarSet` co_vars
+
+ mkLam' dflags bndrs body
+ | dopt Opt_DoEtaReduction dflags,
+ Just etad_lam <- tryEtaReduce bndrs body
+ = do { tick (EtaReduction (head bndrs))
+ ; return etad_lam }
+
+ | dopt Opt_DoLambdaEtaExpansion dflags,
+ any isRuntimeVar bndrs
+ = do { let body' = tryEtaExpansion dflags body
+ ; return (mkLams bndrs body') }
+
+ | otherwise
+ = return (mkLams bndrs body)
\end{code}
-Try three things
- a) eta reduction, if that gives a trivial expression
- b) eta expansion [only if there are some value lambdas]
- c) floating lets out through big lambdas
- [only if all tyvar lambdas, and only if this lambda
- is the RHS of a let]
-
-\begin{code}
-mkLam env bndrs body cont
- = getDOptsSmpl `thenSmpl` \dflags ->
- mkLam' dflags env bndrs body cont
- where
- mkLam' dflags env bndrs body cont
- | dopt Opt_DoEtaReduction dflags,
- Just etad_lam <- tryEtaReduce bndrs body
- = tick (EtaReduction (head bndrs)) `thenSmpl_`
- returnSmpl (emptyFloats env, etad_lam)
-
- | dopt Opt_DoLambdaEtaExpansion dflags,
- any isRuntimeVar bndrs
- = tryEtaExpansion dflags body `thenSmpl` \ body' ->
- returnSmpl (emptyFloats env, mkLams bndrs body')
+Note [Casts and lambdas]
+~~~~~~~~~~~~~~~~~~~~~~~~
+Consider
+ (\x. (\y. e) `cast` g1) `cast` g2
+There is a danger here that the two lambdas look separated, and the
+full laziness pass might float an expression to between the two.
+
+So this equation in mkLam' floats the g1 out, thus:
+ (\x. e `cast` g1) --> (\x.e) `cast` (tx -> g1)
+where x:tx.
+
+In general, this floats casts outside lambdas, where (I hope) they
+might meet and cancel with some other cast:
+ \x. e `cast` co ===> (\x. e) `cast` (tx -> co)
+ /\a. e `cast` co ===> (/\a. e) `cast` (/\a. co)
+ /\g. e `cast` co ===> (/\g. e) `cast` (/\g. co)
+ (if not (g `in` co))
+
+Notice that it works regardless of 'e'. Originally it worked only
+if 'e' was itself a lambda, but in some cases that resulted in
+fruitless iteration in the simplifier. A good example was when
+compiling Text.ParserCombinators.ReadPrec, where we had a definition
+like (\x. Get `cast` g)
+where Get is a constructor with nonzero arity. Then mkLam eta-expanded
+the Get, and the next iteration eta-reduced it, and then eta-expanded
+it again.
+
+Note also the side condition for the case of coercion binders.
+It does not make sense to transform
+ /\g. e `cast` g ==> (/\g.e) `cast` (/\g.g)
+because the latter is not well-kinded.
+
+-- c) floating lets out through big lambdas
+-- [only if all tyvar lambdas, and only if this lambda
+-- is the RHS of a let]
{- Sept 01: I'm experimenting with getting the
full laziness pass to float out past big lambdsa
-- if this is indeed a right-hand side; otherwise
-- we end up floating the thing out, only for float-in
-- to float it right back in again!
- = tryRhsTyLam env bndrs body `thenSmpl` \ (floats, body') ->
- returnSmpl (floats, mkLams bndrs body')
+ = do (floats, body') <- tryRhsTyLam env bndrs body
+ return (floats, mkLams bndrs body')
-}
- | otherwise
- = returnSmpl (emptyFloats env, mkLams bndrs body)
-\end{code}
-
%************************************************************************
%* *
-\subsection{Eta expansion and reduction}
+ Eta reduction
%* *
%************************************************************************
-We try for eta reduction here, but *only* if we get all the
-way to an exprIsTrivial expression.
-We don't want to remove extra lambdas unless we are going
-to avoid allocating this thing altogether
+Note [Eta reduction conditions]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+We try for eta reduction here, but *only* if we get all the way to an
+trivial expression. We don't want to remove extra lambdas unless we
+are going to avoid allocating this thing altogether.
+
+There are some particularly delicate points here:
+
+* Eta reduction is not valid in general:
+ \x. bot /= bot
+ This matters, partly for old-fashioned correctness reasons but,
+ worse, getting it wrong can yield a seg fault. Consider
+ f = \x.f x
+ h y = case (case y of { True -> f `seq` True; False -> False }) of
+ True -> ...; False -> ...
+
+ If we (unsoundly) eta-reduce f to get f=f, the strictness analyser
+ says f=bottom, and replaces the (f `seq` True) with just
+ (f `cast` unsafe-co). BUT, as thing stand, 'f' got arity 1, and it
+ *keeps* arity 1 (perhaps also wrongly). So CorePrep eta-expands
+ the definition again, so that it does not termninate after all.
+ Result: seg-fault because the boolean case actually gets a function value.
+ See Trac #1947.
+
+ So it's important to to the right thing.
+
+* Note [Arity care]: we need to be careful if we just look at f's
+ arity. Currently (Dec07), f's arity is visible in its own RHS (see
+ Note [Arity robustness] in SimplEnv) so we must *not* trust the
+ arity when checking that 'f' is a value. Otherwise we will
+ eta-reduce
+ f = \x. f x
+ to
+ f = f
+ Which might change a terminiating program (think (f `seq` e)) to a
+ non-terminating one. So we check for being a loop breaker first.
+
+ However for GlobalIds we can look at the arity; and for primops we
+ must, since they have no unfolding.
+
+* Regardless of whether 'f' is a value, we always want to
+ reduce (/\a -> f a) to f
+ This came up in a RULE: foldr (build (/\a -> g a))
+ did not match foldr (build (/\b -> ...something complex...))
+ The type checker can insert these eta-expanded versions,
+ with both type and dictionary lambdas; hence the slightly
+ ad-hoc isDictId
+
+* Never *reduce* arity. For example
+ f = \xy. g x y
+ Then if h has arity 1 we don't want to eta-reduce because then
+ f's arity would decrease, and that is bad
+
+These delicacies are why we don't use exprIsTrivial and exprIsHNF here.
+Alas.
\begin{code}
-tryEtaReduce :: [OutBinder] -> OutExpr -> Maybe OutExpr
+tryEtaReduce :: [OutBndr] -> OutExpr -> Maybe OutExpr
tryEtaReduce bndrs body
- -- We don't use CoreUtils.etaReduce, because we can be more
- -- efficient here:
- -- (a) we already have the binders
- -- (b) we can do the triviality test before computing the free vars
= go (reverse bndrs) body
where
+ incoming_arity = count isId bndrs
+
go (b : bs) (App fun arg) | ok_arg b arg = go bs fun -- Loop round
go [] fun | ok_fun fun = Just fun -- Success!
go _ _ = Nothing -- Failure!
- ok_fun fun = exprIsTrivial fun
- && not (any (`elemVarSet` (exprFreeVars fun)) bndrs)
- && (exprIsHNF fun || all ok_lam bndrs)
+ -- Note [Eta reduction conditions]
+ ok_fun (App fun (Type ty))
+ | not (any (`elemVarSet` tyVarsOfType ty) bndrs)
+ = ok_fun fun
+ ok_fun (Var fun_id)
+ = not (fun_id `elem` bndrs)
+ && (ok_fun_id fun_id || all ok_lam bndrs)
+ ok_fun _fun = False
+
+ ok_fun_id fun = fun_arity fun >= incoming_arity
+
+ fun_arity fun -- See Note [Arity care]
+ | isLocalId fun && isLoopBreaker (idOccInfo fun) = 0
+ | otherwise = idArity fun
+
ok_lam v = isTyVar v || isDictId v
- -- The exprIsHNF is because eta reduction is not
- -- valid in general: \x. bot /= bot
- -- So we need to be sure that the "fun" is a value.
- --
- -- However, we always want to reduce (/\a -> f a) to f
- -- This came up in a RULE: foldr (build (/\a -> g a))
- -- did not match foldr (build (/\b -> ...something complex...))
- -- The type checker can insert these eta-expanded versions,
- -- with both type and dictionary lambdas; hence the slightly
- -- ad-hoc isDictTy
ok_arg b arg = varToCoreExpr b `cheapEqExpr` arg
\end{code}
- Try eta expansion for RHSs
+%************************************************************************
+%* *
+ Eta expansion
+%* *
+%************************************************************************
+
We go for:
f = \x1..xn -> N ==> f = \x1..xn y1..ym -> N y1..ym
* N is a NORMAL FORM (i.e. no redexes anywhere)
wanting a suitable number of extra args.
+The biggest reason for doing this is for cases like
+
+ f = \x -> case x of
+ True -> \y -> e1
+ False -> \y -> e2
+
+Here we want to get the lambdas together. A good exmaple is the nofib
+program fibheaps, which gets 25% more allocation if you don't do this
+eta-expansion.
+
We may have to sandwich some coerces between the lambdas
to make the types work. exprEtaExpandArity looks through coerces
when computing arity; and etaExpand adds the coerces as necessary when
actually computing the expansion.
\begin{code}
-tryEtaExpansion :: DynFlags -> OutExpr -> SimplM OutExpr
+tryEtaExpansion :: DynFlags -> OutExpr -> OutExpr
-- There is at least one runtime binder in the binders
tryEtaExpansion dflags body
- = getUniquesSmpl `thenSmpl` \ us ->
- returnSmpl (etaExpand fun_arity us body (exprType body))
+ = etaExpand fun_arity body
where
fun_arity = exprEtaExpandArity dflags body
\end{code}
%* *
%************************************************************************
-tryRhsTyLam tries this transformation, when the big lambda appears as
-the RHS of a let(rec) binding:
+Note [Floating and type abstraction]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Consider this:
+ x = /\a. C e1 e2
+We'd like to float this to
+ y1 = /\a. e1
+ y2 = /\a. e2
+ x = /\a. C (y1 a) (y2 a)
+for the usual reasons: we want to inline x rather vigorously.
+
+You may think that this kind of thing is rare. But in some programs it is
+common. For example, if you do closure conversion you might get:
+
+ data a :-> b = forall e. (e -> a -> b) :$ e
+
+ f_cc :: forall a. a :-> a
+ f_cc = /\a. (\e. id a) :$ ()
+
+Now we really want to inline that f_cc thing so that the
+construction of the closure goes away.
+
+So I have elaborated simplLazyBind to understand right-hand sides that look
+like
+ /\ a1..an. body
+
+and treat them specially. The real work is done in SimplUtils.abstractFloats,
+but there is quite a bit of plumbing in simplLazyBind as well.
+
+The same transformation is good when there are lets in the body:
/\abc -> let(rec) x = e in b
==>
This optimisation is CRUCIAL in eliminating the junk introduced by
desugaring mutually recursive definitions. Don't eliminate it lightly!
-So far as the implementation is concerned:
-
- Invariant: go F e = /\tvs -> F e
-
- Equalities:
- go F (Let x=e in b)
- = Let x' = /\tvs -> F e
- in
- go G b
- where
- G = F . Let x = x' tvs
-
- go F (Letrec xi=ei in b)
- = Letrec {xi' = /\tvs -> G ei}
- in
- go G b
- where
- G = F . Let {xi = xi' tvs}
-
[May 1999] If we do this transformation *regardless* then we can
end up with some pretty silly stuff. For example,
If we abstract this wrt the tyvar we then can't do the case inline
as we would normally do.
+That's why the whole transformation is part of the same process that
+floats let-bindings and constructor arguments out of RHSs. In particular,
+it is guarded by the doFloatFromRhs call in simplLazyBind.
-\begin{code}
-{- Trying to do this in full laziness
-
-tryRhsTyLam :: SimplEnv -> [OutTyVar] -> OutExpr -> SimplM FloatsWithExpr
--- Call ensures that all the binders are type variables
-
-tryRhsTyLam env tyvars body -- Only does something if there's a let
- | not (all isTyVar tyvars)
- || not (worth_it body) -- inside a type lambda,
- = returnSmpl (emptyFloats env, body) -- and a WHNF inside that
-
- | otherwise
- = go env (\x -> x) body
+\begin{code}
+abstractFloats :: [OutTyVar] -> SimplEnv -> OutExpr -> SimplM ([OutBind], OutExpr)
+abstractFloats main_tvs body_env body
+ = ASSERT( notNull body_floats )
+ do { (subst, float_binds) <- mapAccumLM abstract empty_subst body_floats
+ ; return (float_binds, CoreSubst.substExpr subst body) }
where
- worth_it e@(Let _ _) = whnf_in_middle e
- worth_it e = False
-
- whnf_in_middle (Let (NonRec x rhs) e) | isUnLiftedType (idType x) = False
- whnf_in_middle (Let _ e) = whnf_in_middle e
- whnf_in_middle e = exprIsCheap e
-
- main_tyvar_set = mkVarSet tyvars
-
- go env fn (Let bind@(NonRec var rhs) body)
- | exprIsTrivial rhs
- = go env (fn . Let bind) body
-
- go env fn (Let (NonRec var rhs) body)
- = mk_poly tyvars_here var `thenSmpl` \ (var', rhs') ->
- addAuxiliaryBind env (NonRec var' (mkLams tyvars_here (fn rhs))) $ \ env ->
- go env (fn . Let (mk_silly_bind var rhs')) body
-
+ main_tv_set = mkVarSet main_tvs
+ body_floats = getFloats body_env
+ empty_subst = CoreSubst.mkEmptySubst (seInScope body_env)
+
+ abstract :: CoreSubst.Subst -> OutBind -> SimplM (CoreSubst.Subst, OutBind)
+ abstract subst (NonRec id rhs)
+ = do { (poly_id, poly_app) <- mk_poly tvs_here id
+ ; let poly_rhs = mkLams tvs_here rhs'
+ subst' = CoreSubst.extendIdSubst subst id poly_app
+ ; return (subst', (NonRec poly_id poly_rhs)) }
where
-
- tyvars_here = varSetElems (main_tyvar_set `intersectVarSet` exprSomeFreeVars isTyVar rhs)
+ rhs' = CoreSubst.substExpr subst rhs
+ tvs_here | any isCoVar main_tvs = main_tvs -- Note [Abstract over coercions]
+ | otherwise
+ = varSetElems (main_tv_set `intersectVarSet` exprSomeFreeVars isTyVar rhs')
+
-- Abstract only over the type variables free in the rhs
-- wrt which the new binding is abstracted. But the naive
-- approach of abstract wrt the tyvars free in the Id's type
-- abstracting wrt *all* the tyvars. We'll see if that
-- gives rise to problems. SLPJ June 98
- go env fn (Let (Rec prs) body)
- = mapAndUnzipSmpl (mk_poly tyvars_here) vars `thenSmpl` \ (vars', rhss') ->
- let
- gn body = fn (foldr Let body (zipWith mk_silly_bind vars rhss'))
- pairs = vars' `zip` [mkLams tyvars_here (gn rhs) | rhs <- rhss]
- in
- addAuxiliaryBind env (Rec pairs) $ \ env ->
- go env gn body
+ abstract subst (Rec prs)
+ = do { (poly_ids, poly_apps) <- mapAndUnzipM (mk_poly tvs_here) ids
+ ; let subst' = CoreSubst.extendSubstList subst (ids `zip` poly_apps)
+ poly_rhss = [mkLams tvs_here (CoreSubst.substExpr subst' rhs) | rhs <- rhss]
+ ; return (subst', Rec (poly_ids `zip` poly_rhss)) }
where
- (vars,rhss) = unzip prs
- tyvars_here = varSetElems (main_tyvar_set `intersectVarSet` exprsSomeFreeVars isTyVar (map snd prs))
- -- See notes with tyvars_here above
-
- go env fn body = returnSmpl (emptyFloats env, fn body)
-
- mk_poly tyvars_here var
- = getUniqueSmpl `thenSmpl` \ uniq ->
- let
- poly_name = setNameUnique (idName var) uniq -- Keep same name
- poly_ty = mkForAllTys tyvars_here (idType var) -- But new type of course
- poly_id = mkLocalId poly_name poly_ty
-
+ (ids,rhss) = unzip prs
+ -- For a recursive group, it's a bit of a pain to work out the minimal
+ -- set of tyvars over which to abstract:
+ -- /\ a b c. let x = ...a... in
+ -- letrec { p = ...x...q...
+ -- q = .....p...b... } in
+ -- ...
+ -- Since 'x' is abstracted over 'a', the {p,q} group must be abstracted
+ -- over 'a' (because x is replaced by (poly_x a)) as well as 'b'.
+ -- Since it's a pain, we just use the whole set, which is always safe
+ --
+ -- If you ever want to be more selective, remember this bizarre case too:
+ -- x::a = x
+ -- Here, we must abstract 'x' over 'a'.
+ tvs_here = main_tvs
+
+ mk_poly tvs_here var
+ = do { uniq <- getUniqueM
+ ; let poly_name = setNameUnique (idName var) uniq -- Keep same name
+ poly_ty = mkForAllTys tvs_here (idType var) -- But new type of course
+ poly_id = transferPolyIdInfo var tvs_here $ -- Note [transferPolyIdInfo] in Id.lhs
+ mkLocalId poly_name poly_ty
+ ; return (poly_id, mkTyApps (Var poly_id) (mkTyVarTys tvs_here)) }
-- In the olden days, it was crucial to copy the occInfo of the original var,
-- because we were looking at occurrence-analysed but as yet unsimplified code!
-- In particular, we mustn't lose the loop breakers. BUT NOW we are looking
-- where x* has an INLINE prag on it. Now, once x* is inlined,
-- the occurrences of x' will be just the occurrences originally
-- pinned on x.
- in
- returnSmpl (poly_id, mkTyApps (Var poly_id) (mkTyVarTys tyvars_here))
+\end{code}
+
+Note [Abstract over coercions]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+If a coercion variable (g :: a ~ Int) is free in the RHS, then so is the
+type variable a. Rather than sort this mess out, we simply bale out and abstract
+wrt all the type variables if any of them are coercion variables.
+
+
+Historical note: if you use let-bindings instead of a substitution, beware of this:
- mk_silly_bind var rhs = NonRec var (Note InlineMe rhs)
-- Suppose we start with:
--
-- x = /\ a -> let g = G in E
-- Solution: put an INLINE note on g's RHS, so that poly_g seems
-- to appear many times. (NB: mkInlineMe eliminates
-- such notes on trivial RHSs, so do it manually.)
--}
-\end{code}
%************************************************************************
%* *
-\subsection{Case absorption and identity-case elimination}
+ prepareAlts
%* *
%************************************************************************
-mkCase puts a case expression back together, trying various transformations first.
-
-\begin{code}
-mkCase :: OutExpr -> OutId -> OutType
- -> [OutAlt] -- Increasing order
- -> SimplM OutExpr
-
-mkCase scrut case_bndr ty alts
- = getDOptsSmpl `thenSmpl` \dflags ->
- mkAlts dflags scrut case_bndr alts `thenSmpl` \ better_alts ->
- mkCase1 scrut case_bndr ty better_alts
-\end{code}
-
-
-mkAlts tries these things:
+prepareAlts tries these things:
1. If several alternatives are identical, merge them into
a single DEFAULT alternative. I've occasionally seen this
and similarly in cascade for all the join points!
-
+Note [Dead binders]
+~~~~~~~~~~~~~~~~~~~~
+We do this *here*, looking at un-simplified alternatives, because we
+have to check that r doesn't mention the variables bound by the
+pattern in each alternative, so the binder-info is rather useful.
\begin{code}
+prepareAlts :: SimplEnv -> OutExpr -> OutId -> [InAlt] -> SimplM ([AltCon], [InAlt])
+prepareAlts env scrut case_bndr' alts
+ = do { dflags <- getDOptsSmpl
+ ; alts <- combineIdenticalAlts case_bndr' alts
+
+ ; let (alts_wo_default, maybe_deflt) = findDefault alts
+ alt_cons = [con | (con,_,_) <- alts_wo_default]
+ imposs_deflt_cons = nub (imposs_cons ++ alt_cons)
+ -- "imposs_deflt_cons" are handled
+ -- EITHER by the context,
+ -- OR by a non-DEFAULT branch in this case expression.
+
+ ; default_alts <- prepareDefault dflags env case_bndr' mb_tc_app
+ imposs_deflt_cons maybe_deflt
+
+ ; let trimmed_alts = filterOut impossible_alt alts_wo_default
+ merged_alts = mergeAlts trimmed_alts default_alts
+ -- We need the mergeAlts in case the new default_alt
+ -- has turned into a constructor alternative.
+ -- The merge keeps the inner DEFAULT at the front, if there is one
+ -- and interleaves the alternatives in the right order
+
+ ; return (imposs_deflt_cons, merged_alts) }
+ where
+ mb_tc_app = splitTyConApp_maybe (idType case_bndr')
+ Just (_, inst_tys) = mb_tc_app
+
+ imposs_cons = case scrut of
+ Var v -> otherCons (idUnfolding v)
+ _ -> []
+
+ impossible_alt :: CoreAlt -> Bool
+ impossible_alt (con, _, _) | con `elem` imposs_cons = True
+ impossible_alt (DataAlt con, _, _) = dataConCannotMatch inst_tys con
+ impossible_alt _ = False
+
+
--------------------------------------------------
-- 1. Merge identical branches
--------------------------------------------------
-mkAlts dflags scrut case_bndr alts@((con1,bndrs1,rhs1) : con_alts)
+combineIdenticalAlts :: OutId -> [InAlt] -> SimplM [InAlt]
+
+combineIdenticalAlts case_bndr ((_con1,bndrs1,rhs1) : con_alts)
| all isDeadBinder bndrs1, -- Remember the default
length filtered_alts < length con_alts -- alternative comes first
- = tick (AltMerge case_bndr) `thenSmpl_`
- returnSmpl better_alts
+ -- Also Note [Dead binders]
+ = do { tick (AltMerge case_bndr)
+ ; return ((DEFAULT, [], rhs1) : filtered_alts) }
where
filtered_alts = filter keep con_alts
- keep (con,bndrs,rhs) = not (all isDeadBinder bndrs && rhs `cheapEqExpr` rhs1)
- better_alts = (DEFAULT, [], rhs1) : filtered_alts
-
-
---------------------------------------------------
--- 2. Merge nested cases
---------------------------------------------------
-
-mkAlts dflags scrut outer_bndr outer_alts
- | dopt Opt_CaseMerge dflags,
- (outer_alts_without_deflt, maybe_outer_deflt) <- findDefault outer_alts,
- Just (Case (Var scrut_var) inner_bndr _ inner_alts) <- maybe_outer_deflt,
- scruting_same_var scrut_var
- = let
- munged_inner_alts = [(con, args, munge_rhs rhs) | (con, args, rhs) <- inner_alts]
- munge_rhs rhs = bindCaseBndr inner_bndr (Var outer_bndr) rhs
-
- new_alts = mergeAlts outer_alts_without_deflt munged_inner_alts
- -- The merge keeps the inner DEFAULT at the front, if there is one
- -- and eliminates any inner_alts that are shadowed by the outer_alts
- in
- tick (CaseMerge outer_bndr) `thenSmpl_`
- returnSmpl new_alts
- -- Warning: don't call mkAlts recursively!
+ keep (_con,bndrs,rhs) = not (all isDeadBinder bndrs && rhs `cheapEqExpr` rhs1)
+
+combineIdenticalAlts _ alts = return alts
+
+-------------------------------------------------------------------------
+-- Prepare the default alternative
+-------------------------------------------------------------------------
+prepareDefault :: DynFlags
+ -> SimplEnv
+ -> OutId -- Case binder; need just for its type. Note that as an
+ -- OutId, it has maximum information; this is important.
+ -- Test simpl013 is an example
+ -> Maybe (TyCon, [Type]) -- Type of scrutinee, decomposed
+ -> [AltCon] -- These cons can't happen when matching the default
+ -> Maybe InExpr -- Rhs
+ -> SimplM [InAlt] -- Still unsimplified
+ -- We use a list because it's what mergeAlts expects,
+ -- And becuase case-merging can cause many to show up
+
+------- Merge nested cases ----------
+prepareDefault dflags env outer_bndr _bndr_ty imposs_cons (Just deflt_rhs)
+ | dopt Opt_CaseMerge dflags
+ , Case (Var inner_scrut_var) inner_bndr _ inner_alts <- deflt_rhs
+ , DoneId inner_scrut_var' <- substId env inner_scrut_var
+ -- Remember, inner_scrut_var is an InId, but outer_bndr is an OutId
+ , inner_scrut_var' == outer_bndr
+ -- NB: the substId means that if the outer scrutinee was a
+ -- variable, and inner scrutinee is the same variable,
+ -- then inner_scrut_var' will be outer_bndr
+ -- via the magic of simplCaseBinder
+ = do { tick (CaseMerge outer_bndr)
+
+ ; let munge_rhs rhs = bindCaseBndr inner_bndr (Var outer_bndr) rhs
+ ; return [(con, args, munge_rhs rhs) | (con, args, rhs) <- inner_alts,
+ not (con `elem` imposs_cons) ]
+ -- NB: filter out any imposs_cons. Example:
+ -- case x of
+ -- A -> e1
+ -- DEFAULT -> case x of
+ -- A -> e2
+ -- B -> e3
+ -- When we merge, we must ensure that e1 takes
+ -- precedence over e2 as the value for A!
+ }
+ -- Warning: don't call prepareAlts recursively!
-- Firstly, there's no point, because inner alts have already had
-- mkCase applied to them, so they won't have a case in their default
-- Secondly, if you do, you get an infinite loop, because the bindCaseBndr
-- in munge_rhs may put a case into the DEFAULT branch!
- where
- -- We are scrutinising the same variable if it's
- -- the outer case-binder, or if the outer case scrutinises a variable
- -- (and it's the same). Testing both allows us not to replace the
- -- outer scrut-var with the outer case-binder (Simplify.simplCaseBinder).
- scruting_same_var = case scrut of
- Var outer_scrut -> \ v -> v == outer_bndr || v == outer_scrut
- other -> \ v -> v == outer_bndr
-
-------------------------------------------------
--- Catch-all
-------------------------------------------------
-mkAlts dflags scrut case_bndr other_alts = returnSmpl other_alts
+
+--------- Fill in known constructor -----------
+prepareDefault _ _ case_bndr (Just (tycon, inst_tys)) imposs_cons (Just deflt_rhs)
+ | -- This branch handles the case where we are
+ -- scrutinisng an algebraic data type
+ isAlgTyCon tycon -- It's a data type, tuple, or unboxed tuples.
+ , not (isNewTyCon tycon) -- We can have a newtype, if we are just doing an eval:
+ -- case x of { DEFAULT -> e }
+ -- and we don't want to fill in a default for them!
+ , Just all_cons <- tyConDataCons_maybe tycon
+ , not (null all_cons) -- This is a tricky corner case. If the data type has no constructors,
+ -- which GHC allows, then the case expression will have at most a default
+ -- alternative. We don't want to eliminate that alternative, because the
+ -- invariant is that there's always one alternative. It's more convenient
+ -- to leave
+ -- case x of { DEFAULT -> e }
+ -- as it is, rather than transform it to
+ -- error "case cant match"
+ -- which would be quite legitmate. But it's a really obscure corner, and
+ -- not worth wasting code on.
+ , let imposs_data_cons = [con | DataAlt con <- imposs_cons] -- We now know it's a data type
+ impossible con = con `elem` imposs_data_cons || dataConCannotMatch inst_tys con
+ = case filterOut impossible all_cons of
+ [] -> return [] -- Eliminate the default alternative
+ -- altogether if it can't match
+
+ [con] -> -- It matches exactly one constructor, so fill it in
+ do { tick (FillInCaseDefault case_bndr)
+ ; us <- getUniquesM
+ ; let (ex_tvs, co_tvs, arg_ids) =
+ dataConRepInstPat us con inst_tys
+ ; return [(DataAlt con, ex_tvs ++ co_tvs ++ arg_ids, deflt_rhs)] }
+
+ _ -> return [(DEFAULT, [], deflt_rhs)]
+
+ | debugIsOn, isAlgTyCon tycon, not (isOpenTyCon tycon), null (tyConDataCons tycon)
+ -- This can legitimately happen for type families, so don't report that
+ = pprTrace "prepareDefault" (ppr case_bndr <+> ppr tycon)
+ $ return [(DEFAULT, [], deflt_rhs)]
+
+--------- Catch-all cases -----------
+prepareDefault _dflags _env _case_bndr _bndr_ty _imposs_cons (Just deflt_rhs)
+ = return [(DEFAULT, [], deflt_rhs)]
+
+prepareDefault _dflags _env _case_bndr _bndr_ty _imposs_cons Nothing
+ = return [] -- No default branch
\end{code}
=================================================================================
-mkCase1 tries these things
+mkCase tries these things
1. Eliminate the case altogether if possible
and similar friends.
-Start with a simple situation:
-
- case x# of ===> e[x#/y#]
- y# -> e
-
-(when x#, y# are of primitive type, of course). We can't (in general)
-do this for algebraic cases, because we might turn bottom into
-non-bottom!
-
-Actually, we generalise this idea to look for a case where we're
-scrutinising a variable, and we know that only the default case can
-match. For example:
-\begin{verbatim}
- case x of
- 0# -> ...
- other -> ...(case x of
- 0# -> ...
- other -> ...) ...
-\end{code}
-Here the inner case can be eliminated. This really only shows up in
-eliminating error-checking code.
-
-We also make sure that we deal with this very common case:
-
- case e of
- x -> ...x...
-
-Here we are using the case as a strict let; if x is used only once
-then we want to inline it. We have to be careful that this doesn't
-make the program terminate when it would have diverged before, so we
-check that
- - x is used strictly, or
- - e is already evaluated (it may so if e is a variable)
-
-Lastly, we generalise the transformation to handle this:
-
- case e of ===> r
- True -> r
- False -> r
-
-We only do this for very cheaply compared r's (constructors, literals
-and variables). If pedantic bottoms is on, we only do it when the
-scrutinee is a PrimOp which can't fail.
-
-We do it *here*, looking at un-simplified alternatives, because we
-have to check that r doesn't mention the variables bound by the
-pattern in each alternative, so the binder-info is rather useful.
-
-So the case-elimination algorithm is:
-
- 1. Eliminate alternatives which can't match
-
- 2. Check whether all the remaining alternatives
- (a) do not mention in their rhs any of the variables bound in their pattern
- and (b) have equal rhss
-
- 3. Check we can safely ditch the case:
- * PedanticBottoms is off,
- or * the scrutinee is an already-evaluated variable
- or * the scrutinee is a primop which is ok for speculation
- -- ie we want to preserve divide-by-zero errors, and
- -- calls to error itself!
-
- or * [Prim cases] the scrutinee is a primitive variable
-
- or * [Alg cases] the scrutinee is a variable and
- either * the rhs is the same variable
- (eg case x of C a b -> x ===> x)
- or * there is only one alternative, the default alternative,
- and the binder is used strictly in its scope.
- [NB this is helped by the "use default binder where
- possible" transformation; see below.]
-
-
-If so, then we can replace the case with one of the rhss.
-
-Further notes about case elimination
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-Consider: test :: Integer -> IO ()
- test = print
-
-Turns out that this compiles to:
- Print.test
- = \ eta :: Integer
- eta1 :: State# RealWorld ->
- case PrelNum.< eta PrelNum.zeroInteger of wild { __DEFAULT ->
- case hPutStr stdout
- (PrelNum.jtos eta ($w[] @ Char))
- eta1
- of wild1 { (# new_s, a4 #) -> PrelIO.lvl23 new_s }}
-
-Notice the strange '<' which has no effect at all. This is a funny one.
-It started like this:
-
-f x y = if x < 0 then jtos x
- else if y==0 then "" else jtos x
-
-At a particular call site we have (f v 1). So we inline to get
-
- if v < 0 then jtos x
- else if 1==0 then "" else jtos x
-
-Now simplify the 1==0 conditional:
-
- if v<0 then jtos v else jtos v
-
-Now common-up the two branches of the case:
-
- case (v<0) of DEFAULT -> jtos v
-
-Why don't we drop the case? Because it's strict in v. It's technically
-wrong to drop even unnecessary evaluations, and in practice they
-may be a result of 'seq' so we *definitely* don't want to drop those.
-I don't really know how to improve this situation.
-
-
\begin{code}
---------------------------------------------------
--- 0. Check for empty alternatives
---------------------------------------------------
-
--- This isn't strictly an error. It's possible that the simplifer might "see"
--- that an inner case has no accessible alternatives before it "sees" that the
--- entire branch of an outer case is inaccessible. So we simply
--- put an error case here insteadd
-mkCase1 scrut case_bndr ty []
- = pprTrace "mkCase1: null alts" (ppr case_bndr <+> ppr scrut) $
- return (mkApps (Var eRROR_ID)
- [Type ty, Lit (mkStringLit "Impossible alternative")])
-
---------------------------------------------------
--- 1. Eliminate the case altogether if poss
---------------------------------------------------
-
-mkCase1 scrut case_bndr ty [(con,bndrs,rhs)]
- -- See if we can get rid of the case altogether
- -- See the extensive notes on case-elimination above
- -- mkCase made sure that if all the alternatives are equal,
- -- then there is now only one (DEFAULT) rhs
- | all isDeadBinder bndrs,
-
- -- Check that the scrutinee can be let-bound instead of case-bound
- exprOkForSpeculation scrut
- -- OK not to evaluate it
- -- This includes things like (==# a# b#)::Bool
- -- so that we simplify
- -- case ==# a# b# of { True -> x; False -> x }
- -- to just
- -- x
- -- This particular example shows up in default methods for
- -- comparision operations (e.g. in (>=) for Int.Int32)
- || exprIsHNF scrut -- It's already evaluated
- || var_demanded_later scrut -- It'll be demanded later
-
--- || not opt_SimplPedanticBottoms) -- Or we don't care!
--- We used to allow improving termination by discarding cases, unless -fpedantic-bottoms was on,
--- but that breaks badly for the dataToTag# primop, which relies on a case to evaluate
--- its argument: case x of { y -> dataToTag# y }
--- Here we must *not* discard the case, because dataToTag# just fetches the tag from
--- the info pointer. So we'll be pedantic all the time, and see if that gives any
--- other problems
--- Also we don't want to discard 'seq's
- = tick (CaseElim case_bndr) `thenSmpl_`
- returnSmpl (bindCaseBndr case_bndr scrut rhs)
-
- where
- -- The case binder is going to be evaluated later,
- -- and the scrutinee is a simple variable
- var_demanded_later (Var v) = isStrictDmd (idNewDemandInfo case_bndr)
- var_demanded_later other = False
-
+mkCase :: OutExpr -> OutId -> [OutAlt] -- Increasing order
+ -> SimplM OutExpr
--------------------------------------------------
-- 2. Identity case
--------------------------------------------------
-mkCase1 scrut case_bndr ty alts -- Identity case
+mkCase scrut case_bndr alts -- Identity case
| all identity_alt alts
- = tick (CaseIdentity case_bndr) `thenSmpl_`
- returnSmpl (re_note scrut)
+ = do tick (CaseIdentity case_bndr)
+ return (re_cast scrut)
where
- identity_alt (con, args, rhs) = de_note rhs `cheapEqExpr` identity_rhs con args
+ identity_alt (con, args, rhs) = check_eq con args (de_cast rhs)
- identity_rhs (DataAlt con) args = mkConApp con (arg_tys ++ map varToCoreExpr args)
- identity_rhs (LitAlt lit) _ = Lit lit
- identity_rhs DEFAULT _ = Var case_bndr
+ check_eq DEFAULT _ (Var v) = v == case_bndr
+ check_eq (LitAlt lit') _ (Lit lit) = lit == lit'
+ check_eq (DataAlt con) args rhs = rhs `cheapEqExpr` mkConApp con (arg_tys ++ varsToCoreExprs args)
+ || rhs `cheapEqExpr` Var case_bndr
+ check_eq _ _ _ = False
arg_tys = map Type (tyConAppArgs (idType case_bndr))
-- We've seen this:
- -- case coerce T e of x { _ -> coerce T' x }
- -- And we definitely want to eliminate this case!
- -- So we throw away notes from the RHS, and reconstruct
- -- (at least an approximation) at the other end
- de_note (Note _ e) = de_note e
- de_note e = e
+ -- case e of x { _ -> x `cast` c }
+ -- And we definitely want to eliminate this case, to give
+ -- e `cast` c
+ -- So we throw away the cast from the RHS, and reconstruct
+ -- it at the other end. All the RHS casts must be the same
+ -- if (all identity_alt alts) holds.
+ --
+ -- Don't worry about nested casts, because the simplifier combines them
+ de_cast (Cast e _) = e
+ de_cast e = e
+
+ re_cast scrut = case head alts of
+ (_,_,Cast _ co) -> Cast scrut co
+ _ -> scrut
- -- re_note wraps a coerce if it might be necessary
- re_note scrut = case head alts of
- (_,_,rhs1@(Note _ _)) -> mkCoerce2 (exprType rhs1) (idType case_bndr) scrut
- other -> scrut
--------------------------------------------------
-- Catch-all
--------------------------------------------------
-mkCase1 scrut bndr ty alts = returnSmpl (Case scrut bndr ty alts)
+mkCase scrut bndr alts = return (Case scrut bndr (coreAltsType alts) alts)
\end{code}
cascade rather nicely.
\begin{code}
+bindCaseBndr :: Id -> CoreExpr -> CoreExpr -> CoreExpr
bindCaseBndr bndr rhs body
| isDeadBinder bndr = body
- | otherwise = bindNonRec bndr rhs body
+ | otherwise = bindNonRec bndr rhs body
\end{code}
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