import DynFlags
import SimplMonad
-import Type hiding ( substTy, extendTvSubst )
-import SimplEnv
+import Type hiding ( substTy, extendTvSubst )
+import SimplEnv
import SimplUtils
+import MkId ( rUNTIME_ERROR_ID )
+import FamInstEnv ( FamInstEnv )
import Id
import Var
import IdInfo
import Coercion
-import FamInstEnv ( topNormaliseType )
-import DataCon ( dataConRepStrictness, dataConUnivTyVars )
+import FamInstEnv ( topNormaliseType )
+import DataCon ( dataConRepStrictness, dataConUnivTyVars )
import CoreSyn
-import NewDemand ( isStrictDmd )
-import PprCore ( pprParendExpr, pprCoreExpr )
-import CoreUnfold ( mkUnfolding, callSiteInline )
+import NewDemand ( isStrictDmd, splitStrictSig )
+import PprCore ( pprParendExpr, pprCoreExpr )
+import CoreUnfold ( mkUnfolding, callSiteInline, CallCtxt(..) )
import CoreUtils
-import Rules ( lookupRule )
-import BasicTypes ( isMarkedStrict )
-import CostCentre ( currentCCS )
-import TysPrim ( realWorldStatePrimTy )
-import PrelInfo ( realWorldPrimId )
-import BasicTypes ( TopLevelFlag(..), isTopLevel,
- RecFlag(..), isNonRuleLoopBreaker )
-import Maybes ( orElse )
+import Rules ( lookupRule, getRules )
+import BasicTypes ( isMarkedStrict )
+import CostCentre ( currentCCS )
+import TysPrim ( realWorldStatePrimTy )
+import PrelInfo ( realWorldPrimId )
+import BasicTypes ( TopLevelFlag(..), isTopLevel,
+ RecFlag(..), isNonRuleLoopBreaker )
+import Maybes ( orElse )
+import Data.List ( mapAccumL )
+import MonadUtils ( foldlM )
+import StaticFlags ( opt_PassCaseBndrToJoinPoints )
import Outputable
-import Util
+import FastString
\end{code}
-----------------------------------------
- *** IMPORTANT NOTE ***
+ *** IMPORTANT NOTE ***
-----------------------------------------
The simplifier used to guarantee that the output had no shadowing, but
it does not do so any more. (Actually, it never did!) The reason is
-----------------------------------------
- *** IMPORTANT NOTE ***
+ *** IMPORTANT NOTE ***
-----------------------------------------
Many parts of the simplifier return a bunch of "floats" as well as an
expression. This is wrapped as a datatype SimplUtils.FloatsWith.
-----------------------------------------
- ORGANISATION OF FUNCTIONS
+ ORGANISATION OF FUNCTIONS
-----------------------------------------
simplTopBinds
- simplify all top-level binders
- for NonRec, call simplRecOrTopPair
- for Rec, call simplRecBind
-
- ------------------------------
-simplExpr (applied lambda) ==> simplNonRecBind
+
+ ------------------------------
+simplExpr (applied lambda) ==> simplNonRecBind
simplExpr (Let (NonRec ...) ..) ==> simplNonRecBind
simplExpr (Let (Rec ...) ..) ==> simplify binders; simplRecBind
- ------------------------------
-simplRecBind [binders already simplfied]
+ ------------------------------
+simplRecBind [binders already simplfied]
- use simplRecOrTopPair on each pair in turn
simplRecOrTopPair [binder already simplified]
Used for: recursive bindings (top level and nested)
- top-level non-recursive bindings
- Returns:
+ top-level non-recursive bindings
+ Returns:
- check for PreInlineUnconditionally
- simplLazyBind
simplNonRecBind
Used for: non-top-level non-recursive bindings
- beta reductions (which amount to the same thing)
- Because it can deal with strict arts, it takes a
- "thing-inside" and returns an expression
+ beta reductions (which amount to the same thing)
+ Because it can deal with strict arts, it takes a
+ "thing-inside" and returns an expression
- check for PreInlineUnconditionally
- simplify binder, including its IdInfo
- if strict binding
- simplStrictArg
- mkAtomicArgs
- completeNonRecX
+ simplStrictArg
+ mkAtomicArgs
+ completeNonRecX
else
- simplLazyBind
- addFloats
+ simplLazyBind
+ addFloats
-simplNonRecX: [given a *simplified* RHS, but an *unsimplified* binder]
+simplNonRecX: [given a *simplified* RHS, but an *unsimplified* binder]
Used for: binding case-binder and constr args in a known-constructor case
- check for PreInLineUnconditionally
- simplify binder
- completeNonRecX
-
- ------------------------------
-simplLazyBind: [binder already simplified, RHS not]
+
+ ------------------------------
+simplLazyBind: [binder already simplified, RHS not]
Used for: recursive bindings (top level and nested)
- top-level non-recursive bindings
- non-top-level, but *lazy* non-recursive bindings
- [must not be strict or unboxed]
+ top-level non-recursive bindings
+ non-top-level, but *lazy* non-recursive bindings
+ [must not be strict or unboxed]
Returns floats + an augmented environment, not an expression
- - substituteIdInfo and add result to in-scope
- [so that rules are available in rec rhs]
+ - substituteIdInfo and add result to in-scope
+ [so that rules are available in rec rhs]
- simplify rhs
- mkAtomicArgs
- float if exposes constructor or PAP
- completeBind
-completeNonRecX: [binder and rhs both simplified]
+completeNonRecX: [binder and rhs both simplified]
- if the the thing needs case binding (unlifted and not ok-for-spec)
- build a Case
+ build a Case
else
- completeBind
- addFloats
+ completeBind
+ addFloats
-completeBind: [given a simplified RHS]
- [used for both rec and non-rec bindings, top level and not]
+completeBind: [given a simplified RHS]
+ [used for both rec and non-rec bindings, top level and not]
- try PostInlineUnconditionally
- add unfolding [this is the only place we add an unfolding]
- add arity
Right hand sides and arguments
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-In many ways we want to treat
- (a) the right hand side of a let(rec), and
- (b) a function argument
+In many ways we want to treat
+ (a) the right hand side of a let(rec), and
+ (b) a function argument
in the same way. But not always! In particular, we would
like to leave these arguments exactly as they are, so they
will match a RULE more easily.
-
- f (g x, h x)
- g (+ x)
+
+ f (g x, h x)
+ g (+ x)
It's harder to make the rule match if we ANF-ise the constructor,
or eta-expand the PAP:
- f (let { a = g x; b = h x } in (a,b))
- g (\y. + x y)
+ f (let { a = g x; b = h x } in (a,b))
+ g (\y. + x y)
On the other hand if we see the let-defns
- p = (g x, h x)
- q = + x
+ p = (g x, h x)
+ q = + x
then we *do* want to ANF-ise and eta-expand, so that p and q
-can be safely inlined.
+can be safely inlined.
Even floating lets out is a bit dubious. For let RHS's we float lets
out if that exposes a value, so that the value can be inlined more vigorously.
For example
- r = let x = e in (x,x)
+ r = let x = e in (x,x)
Here, if we float the let out we'll expose a nice constructor. We did experiments
that showed this to be a generally good thing. But it was a bad thing to float
~~~~~~~~~~~~~~
For eta expansion, we want to catch things like
- case e of (a,b) -> \x -> case a of (p,q) -> \y -> r
+ case e of (a,b) -> \x -> case a of (p,q) -> \y -> r
If the \x was on the RHS of a let, we'd eta expand to bring the two
lambdas together. And in general that's a good thing to do. Perhaps
%************************************************************************
-%* *
+%* *
\subsection{Bindings}
-%* *
+%* *
%************************************************************************
\begin{code}
simplTopBinds :: SimplEnv -> [InBind] -> SimplM [OutBind]
-simplTopBinds env binds
- = do { -- Put all the top-level binders into scope at the start
- -- so that if a transformation rule has unexpectedly brought
- -- anything into scope, then we don't get a complaint about that.
- -- It's rather as if the top-level binders were imported.
- ; env <- simplRecBndrs env (bindersOfBinds binds)
- ; dflags <- getDOptsSmpl
- ; let dump_flag = dopt Opt_D_dump_inlinings dflags ||
- dopt Opt_D_dump_rule_firings dflags
- ; env' <- simpl_binds dump_flag env binds
- ; freeTick SimplifierDone
- ; return (getFloats env') }
+simplTopBinds env0 binds0
+ = do { -- Put all the top-level binders into scope at the start
+ -- so that if a transformation rule has unexpectedly brought
+ -- anything into scope, then we don't get a complaint about that.
+ -- It's rather as if the top-level binders were imported.
+ ; env1 <- simplRecBndrs env0 (bindersOfBinds binds0)
+ ; dflags <- getDOptsSmpl
+ ; let dump_flag = dopt Opt_D_dump_inlinings dflags ||
+ dopt Opt_D_dump_rule_firings dflags
+ ; env2 <- simpl_binds dump_flag env1 binds0
+ ; freeTick SimplifierDone
+ ; return (getFloats env2) }
where
- -- We need to track the zapped top-level binders, because
- -- they should have their fragile IdInfo zapped (notably occurrence info)
- -- That's why we run down binds and bndrs' simultaneously.
- --
- -- The dump-flag emits a trace for each top-level binding, which
- -- helps to locate the tracing for inlining and rule firing
+ -- We need to track the zapped top-level binders, because
+ -- they should have their fragile IdInfo zapped (notably occurrence info)
+ -- That's why we run down binds and bndrs' simultaneously.
+ --
+ -- The dump-flag emits a trace for each top-level binding, which
+ -- helps to locate the tracing for inlining and rule firing
simpl_binds :: Bool -> SimplEnv -> [InBind] -> SimplM SimplEnv
- simpl_binds dump env [] = return env
- simpl_binds dump env (bind:binds) = do { env' <- trace dump bind $
- simpl_bind env bind
- ; simpl_binds dump env' binds }
+ simpl_binds _ env [] = return env
+ simpl_binds dump env (bind:binds) = do { env' <- trace_bind dump bind $
+ simpl_bind env bind
+ ; simpl_binds dump env' binds }
- trace True bind = pprTrace "SimplBind" (ppr (bindersOf bind))
- trace False bind = \x -> x
+ trace_bind True bind = pprTrace "SimplBind" (ppr (bindersOf bind))
+ trace_bind False _ = \x -> x
- simpl_bind env (NonRec b r) = simplRecOrTopPair env TopLevel b r
- simpl_bind env (Rec pairs) = simplRecBind env TopLevel pairs
+ simpl_bind env (Rec pairs) = simplRecBind env TopLevel pairs
+ simpl_bind env (NonRec b r) = simplRecOrTopPair env' TopLevel b b' r
+ where
+ (env', b') = addBndrRules env b (lookupRecBndr env b)
\end{code}
%************************************************************************
-%* *
+%* *
\subsection{Lazy bindings}
-%* *
+%* *
%************************************************************************
simplRecBind is used for
- * recursive bindings only
+ * recursive bindings only
\begin{code}
simplRecBind :: SimplEnv -> TopLevelFlag
- -> [(InId, InExpr)]
- -> SimplM SimplEnv
-simplRecBind env top_lvl pairs
- = do { env' <- go (zapFloats env) pairs
- ; return (env `addRecFloats` env') }
- -- addFloats adds the floats from env',
- -- *and* updates env with the in-scope set from env'
+ -> [(InId, InExpr)]
+ -> SimplM SimplEnv
+simplRecBind env0 top_lvl pairs0
+ = do { let (env_with_info, triples) = mapAccumL add_rules env0 pairs0
+ ; env1 <- go (zapFloats env_with_info) triples
+ ; return (env0 `addRecFloats` env1) }
+ -- addFloats adds the floats from env1,
+ -- _and_ updates env0 with the in-scope set from env1
where
+ add_rules :: SimplEnv -> (InBndr,InExpr) -> (SimplEnv, (InBndr, OutBndr, InExpr))
+ -- Add the (substituted) rules to the binder
+ add_rules env (bndr, rhs) = (env', (bndr, bndr', rhs))
+ where
+ (env', bndr') = addBndrRules env bndr (lookupRecBndr env bndr)
+
go env [] = return env
-
- go env ((bndr, rhs) : pairs)
- = do { env <- simplRecOrTopPair env top_lvl bndr rhs
- ; go env pairs }
+
+ go env ((old_bndr, new_bndr, rhs) : pairs)
+ = do { env' <- simplRecOrTopPair env top_lvl old_bndr new_bndr rhs
+ ; go env' pairs }
\end{code}
simplOrTopPair is used for
- * recursive bindings (whether top level or not)
- * top-level non-recursive bindings
+ * recursive bindings (whether top level or not)
+ * top-level non-recursive bindings
It assumes the binder has already been simplified, but not its IdInfo.
\begin{code}
simplRecOrTopPair :: SimplEnv
- -> TopLevelFlag
- -> InId -> InExpr -- Binder and rhs
- -> SimplM SimplEnv -- Returns an env that includes the binding
+ -> TopLevelFlag
+ -> InId -> OutBndr -> InExpr -- Binder and rhs
+ -> SimplM SimplEnv -- Returns an env that includes the binding
-simplRecOrTopPair env top_lvl bndr rhs
- | preInlineUnconditionally env top_lvl bndr rhs -- Check for unconditional inline
- = do { tick (PreInlineUnconditionally bndr)
- ; return (extendIdSubst env bndr (mkContEx env rhs)) }
+simplRecOrTopPair env top_lvl old_bndr new_bndr rhs
+ | preInlineUnconditionally env top_lvl old_bndr rhs -- Check for unconditional inline
+ = do { tick (PreInlineUnconditionally old_bndr)
+ ; return (extendIdSubst env old_bndr (mkContEx env rhs)) }
| otherwise
- = do { let bndr' = lookupRecBndr env bndr
- (env', bndr'') = addLetIdInfo env bndr bndr'
- ; simplLazyBind env' top_lvl Recursive bndr bndr'' rhs env' }
- -- May not actually be recursive, but it doesn't matter
+ = simplLazyBind env top_lvl Recursive old_bndr new_bndr rhs env
+ -- May not actually be recursive, but it doesn't matter
\end{code}
simplLazyBind is used for
* [simplRecOrTopPair] recursive bindings (whether top level or not)
* [simplRecOrTopPair] top-level non-recursive bindings
- * [simplNonRecE] non-top-level *lazy* non-recursive bindings
+ * [simplNonRecE] non-top-level *lazy* non-recursive bindings
Nota bene:
- 1. It assumes that the binder is *already* simplified,
+ 1. It assumes that the binder is *already* simplified,
and is in scope, and its IdInfo too, except unfolding
2. It assumes that the binder type is lifted.
\begin{code}
simplLazyBind :: SimplEnv
- -> TopLevelFlag -> RecFlag
- -> InId -> OutId -- Binder, both pre-and post simpl
- -- The OutId has IdInfo, except arity, unfolding
- -> InExpr -> SimplEnv -- The RHS and its environment
- -> SimplM SimplEnv
+ -> TopLevelFlag -> RecFlag
+ -> InId -> OutId -- Binder, both pre-and post simpl
+ -- The OutId has IdInfo, except arity, unfolding
+ -> InExpr -> SimplEnv -- The RHS and its environment
+ -> SimplM SimplEnv
simplLazyBind env top_lvl is_rec bndr bndr1 rhs rhs_se
- = do { let rhs_env = rhs_se `setInScope` env
- (tvs, body) = collectTyBinders rhs
- ; (body_env, tvs') <- simplBinders rhs_env tvs
- -- See Note [Floating and type abstraction]
- -- in SimplUtils
-
- -- Simplify the RHS; note the mkRhsStop, which tells
- -- the simplifier that this is the RHS of a let.
- ; let rhs_cont = mkRhsStop (applyTys (idType bndr1) (mkTyVarTys tvs'))
- ; (body_env1, body1) <- simplExprF body_env body rhs_cont
-
- -- ANF-ise a constructor or PAP rhs
- ; (body_env2, body2) <- prepareRhs body_env1 body1
-
- ; (env', rhs')
- <- if not (doFloatFromRhs top_lvl is_rec False body2 body_env2)
- then -- No floating, just wrap up!
- do { rhs' <- mkLam tvs' (wrapFloats body_env2 body2)
- ; return (env, rhs') }
-
- else if null tvs then -- Simple floating
- do { tick LetFloatFromLet
- ; return (addFloats env body_env2, body2) }
-
- else -- Do type-abstraction first
- do { tick LetFloatFromLet
- ; (poly_binds, body3) <- abstractFloats tvs' body_env2 body2
- ; rhs' <- mkLam tvs' body3
- ; return (extendFloats env poly_binds, rhs') }
-
- ; completeBind env' top_lvl bndr bndr1 rhs' }
+ = do { let rhs_env = rhs_se `setInScope` env
+ (tvs, body) = case collectTyBinders rhs of
+ (tvs, body) | not_lam body -> (tvs,body)
+ | otherwise -> ([], rhs)
+ not_lam (Lam _ _) = False
+ not_lam _ = True
+ -- Do not do the "abstract tyyvar" thing if there's
+ -- a lambda inside, becuase it defeats eta-reduction
+ -- f = /\a. \x. g a x
+ -- should eta-reduce
+
+ ; (body_env, tvs') <- simplBinders rhs_env tvs
+ -- See Note [Floating and type abstraction] in SimplUtils
+
+ -- Simplify the RHS
+ ; (body_env1, body1) <- simplExprF body_env body mkBoringStop
+
+ -- ANF-ise a constructor or PAP rhs
+ ; (body_env2, body2) <- prepareRhs body_env1 body1
+
+ ; (env', rhs')
+ <- if not (doFloatFromRhs top_lvl is_rec False body2 body_env2)
+ then -- No floating, just wrap up!
+ do { rhs' <- mkLam tvs' (wrapFloats body_env2 body2)
+ ; return (env, rhs') }
+
+ else if null tvs then -- Simple floating
+ do { tick LetFloatFromLet
+ ; return (addFloats env body_env2, body2) }
+
+ else -- Do type-abstraction first
+ do { tick LetFloatFromLet
+ ; (poly_binds, body3) <- abstractFloats tvs' body_env2 body2
+ ; rhs' <- mkLam tvs' body3
+ ; env' <- foldlM (addPolyBind top_lvl) env poly_binds
+ ; return (env', rhs') }
+
+ ; completeBind env' top_lvl bndr bndr1 rhs' }
\end{code}
-A specialised variant of simplNonRec used when the RHS is already simplified,
+A specialised variant of simplNonRec used when the RHS is already simplified,
notably in knownCon. It uses case-binding where necessary.
\begin{code}
simplNonRecX :: SimplEnv
- -> InId -- Old binder
- -> OutExpr -- Simplified RHS
- -> SimplM SimplEnv
+ -> InId -- Old binder
+ -> OutExpr -- Simplified RHS
+ -> SimplM SimplEnv
simplNonRecX env bndr new_rhs
- = do { (env, bndr') <- simplBinder env bndr
- ; completeNonRecX env NotTopLevel NonRecursive
- (isStrictId bndr) bndr bndr' new_rhs }
+ | isDeadBinder bndr -- Not uncommon; e.g. case (a,b) of b { (p,q) -> p }
+ = return env -- Here b is dead, and we avoid creating
+ | otherwise -- the binding b = (a,b)
+ = do { (env', bndr') <- simplBinder env bndr
+ ; completeNonRecX env' (isStrictId bndr) bndr bndr' new_rhs }
completeNonRecX :: SimplEnv
- -> TopLevelFlag -> RecFlag -> Bool
- -> InId -- Old binder
- -> OutId -- New binder
- -> OutExpr -- Simplified RHS
- -> SimplM SimplEnv
-
-completeNonRecX env top_lvl is_rec is_strict old_bndr new_bndr new_rhs
- = do { (env1, rhs1) <- prepareRhs (zapFloats env) new_rhs
- ; (env2, rhs2) <-
- if doFloatFromRhs top_lvl is_rec is_strict rhs1 env1
- then do { tick LetFloatFromLet
- ; return (addFloats env env1, rhs1) } -- Add the floats to the main env
- else return (env, wrapFloats env1 rhs1) -- Wrap the floats around the RHS
- ; completeBind env2 NotTopLevel old_bndr new_bndr rhs2 }
+ -> Bool
+ -> InId -- Old binder
+ -> OutId -- New binder
+ -> OutExpr -- Simplified RHS
+ -> SimplM SimplEnv
+
+completeNonRecX env is_strict old_bndr new_bndr new_rhs
+ = do { (env1, rhs1) <- prepareRhs (zapFloats env) new_rhs
+ ; (env2, rhs2) <-
+ if doFloatFromRhs NotTopLevel NonRecursive is_strict rhs1 env1
+ then do { tick LetFloatFromLet
+ ; return (addFloats env env1, rhs1) } -- Add the floats to the main env
+ else return (env, wrapFloats env1 rhs1) -- Wrap the floats around the RHS
+ ; completeBind env2 NotTopLevel old_bndr new_bndr rhs2 }
\end{code}
{- No, no, no! Do not try preInlineUnconditionally in completeNonRecX
Doing so risks exponential behaviour, because new_rhs has been simplified once already
- In the cases described by the folowing commment, postInlineUnconditionally will
+ In the cases described by the folowing commment, postInlineUnconditionally will
catch many of the relevant cases.
- -- 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 ...
- --
- -- Similarly, single occurrences can be inlined vigourously
- -- e.g. case (f x, g y) of (a,b) -> ....
- -- If a,b occur once we can avoid constructing the let binding for them.
+ -- 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 ...
+ --
+ -- Similarly, single occurrences can be inlined vigourously
+ -- e.g. case (f x, g y) of (a,b) -> ....
+ -- If a,b occur once we can avoid constructing the let binding for them.
Furthermore in the case-binding case preInlineUnconditionally risks extra thunks
- -- Consider case I# (quotInt# x y) of
- -- I# v -> let w = J# v in ...
- -- If we gaily inline (quotInt# x y) for v, we end up building an
- -- extra thunk:
- -- let w = J# (quotInt# x y) in ...
- -- because quotInt# can fail.
+ -- Consider case I# (quotInt# x y) of
+ -- I# v -> let w = J# v in ...
+ -- If we gaily inline (quotInt# x y) for v, we end up building an
+ -- extra thunk:
+ -- let w = J# (quotInt# x y) in ...
+ -- because quotInt# can fail.
| preInlineUnconditionally env NotTopLevel bndr new_rhs
= thing_inside (extendIdSubst env bndr (DoneEx new_rhs))
----------------------------------
prepareRhs takes a putative RHS, checks whether it's a PAP or
-constructor application and, if so, converts it to ANF, so that the
+constructor application and, if so, converts it to ANF, so that the
resulting thing can be inlined more easily. Thus
- x = (f a, g b)
+ x = (f a, g b)
becomes
- t1 = f a
- t2 = g b
- x = (t1,t2)
+ t1 = f a
+ t2 = g b
+ x = (t1,t2)
We also want to deal well cases like this
- v = (f e1 `cast` co) e2
+ v = (f e1 `cast` co) e2
Here we want to make e1,e2 trivial and get
- x1 = e1; x2 = e2; v = (f x1 `cast` co) v2
+ x1 = e1; x2 = e2; v = (f x1 `cast` co) v2
That's what the 'go' loop in prepareRhs does
\begin{code}
prepareRhs :: SimplEnv -> OutExpr -> SimplM (SimplEnv, OutExpr)
-- Adds new floats to the env iff that allows us to return a good RHS
-prepareRhs env (Cast rhs co) -- Note [Float coercions]
- = do { (env', rhs') <- makeTrivial env rhs
- ; return (env', Cast rhs' co) }
-
-prepareRhs env rhs
- = do { (is_val, env', rhs') <- go 0 env rhs
- ; return (env', rhs') }
+prepareRhs env (Cast rhs co) -- Note [Float coercions]
+ | (ty1, _ty2) <- coercionKind co -- Do *not* do this if rhs has an unlifted type
+ , not (isUnLiftedType ty1) -- see Note [Float coercions (unlifted)]
+ = do { (env', rhs') <- makeTrivial env rhs
+ ; return (env', Cast rhs' co) }
+
+prepareRhs env0 rhs0
+ = do { (_is_val, env1, rhs1) <- go 0 env0 rhs0
+ ; return (env1, rhs1) }
where
go n_val_args env (Cast rhs co)
- = do { (is_val, env', rhs') <- go n_val_args env rhs
- ; return (is_val, env', Cast rhs' co) }
+ = do { (is_val, env', rhs') <- go n_val_args env rhs
+ ; return (is_val, env', Cast rhs' co) }
go n_val_args env (App fun (Type ty))
- = do { (is_val, env', rhs') <- go n_val_args env fun
- ; return (is_val, env', App rhs' (Type ty)) }
+ = do { (is_val, env', rhs') <- go n_val_args env fun
+ ; return (is_val, env', App rhs' (Type ty)) }
go n_val_args env (App fun arg)
- = do { (is_val, env', fun') <- go (n_val_args+1) env fun
- ; case is_val of
- True -> do { (env'', arg') <- makeTrivial env' arg
- ; return (True, env'', App fun' arg') }
- False -> return (False, env, App fun arg) }
+ = do { (is_val, env', fun') <- go (n_val_args+1) env fun
+ ; case is_val of
+ True -> do { (env'', arg') <- makeTrivial env' arg
+ ; return (True, env'', App fun' arg') }
+ False -> return (False, env, App fun arg) }
go n_val_args env (Var fun)
- = return (is_val, env, Var fun)
- where
- is_val = n_val_args > 0 -- There is at least one arg
- -- ...and the fun a constructor or PAP
- && (isDataConWorkId fun || n_val_args < idArity fun)
- go n_val_args env other
- = return (False, env, other)
+ = return (is_val, env, Var fun)
+ where
+ is_val = n_val_args > 0 -- There is at least one arg
+ -- ...and the fun a constructor or PAP
+ && (isDataConWorkId fun || n_val_args < idArity fun)
+ go _ env other
+ = return (False, env, other)
\end{code}
Note [Float coercions]
~~~~~~~~~~~~~~~~~~~~~~
When we find the binding
- x = e `cast` co
+ x = e `cast` co
we'd like to transform it to
- x' = e
- x = x `cast` co -- A trivial binding
+ x' = e
+ x = x `cast` co -- A trivial binding
There's a chance that e will be a constructor application or function, or something
like that, so moving the coerion to the usage site may well cancel the coersions
and lead to further optimisation. Example:
x = T m
go 0 = 0
go n = case x of { T m -> go (n-m) }
- -- This case should optimise
+ -- This case should optimise
+
+Note [Float coercions (unlifted)]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+BUT don't do [Float coercions] if 'e' has an unlifted type.
+This *can* happen:
+
+ foo :: Int = (error (# Int,Int #) "urk")
+ `cast` CoUnsafe (# Int,Int #) Int
+
+If do the makeTrivial thing to the error call, we'll get
+ foo = case error (# Int,Int #) "urk" of v -> v `cast` ...
+But 'v' isn't in scope!
+
+These strange casts can happen as a result of case-of-case
+ bar = case (case x of { T -> (# 2,3 #); F -> error "urk" }) of
+ (# p,q #) -> p+q
\begin{code}
makeTrivial env expr
| exprIsTrivial expr
= return (env, expr)
- | otherwise -- See Note [Take care] below
- = do { var <- newId FSLIT("a") (exprType expr)
- ; env <- completeNonRecX env NotTopLevel NonRecursive
- False var var expr
- ; return (env, substExpr env (Var var)) }
+ | otherwise -- See Note [Take care] below
+ = do { var <- newId (fsLit "a") (exprType expr)
+ ; env' <- completeNonRecX env False var var expr
+-- pprTrace "makeTrivial" (vcat [ppr var <+> ppr (exprArity (substExpr env' (Var var)))
+-- , ppr expr
+-- , ppr (substExpr env' (Var var))
+-- , ppr (idArity (fromJust (lookupInScope (seInScope env') var))) ]) $
+ ; return (env', substExpr env' (Var var)) }
+ -- The substitution is needed becase we're constructing a new binding
+ -- a = rhs
+ -- And if rhs is of form (rhs1 |> co), then we might get
+ -- a1 = rhs1
+ -- a = a1 |> co
+ -- and now a's RHS is trivial and can be substituted out, and that
+ -- is what completeNonRecX will do
\end{code}
%************************************************************************
-%* *
+%* *
\subsection{Completing a lazy binding}
-%* *
+%* *
%************************************************************************
completeBind
- add arity
It does *not* attempt to do let-to-case. Why? Because it is used for
- - top-level bindings (when let-to-case is impossible)
+ - top-level bindings (when let-to-case is impossible)
- many situations where the "rhs" is known to be a WHNF
- (so let-to-case is inappropriate).
+ (so let-to-case is inappropriate).
Nor does it do the atomic-argument thing
\begin{code}
completeBind :: SimplEnv
- -> TopLevelFlag -- Flag stuck into unfolding
- -> InId -- Old binder
- -> OutId -> OutExpr -- New binder and RHS
- -> SimplM SimplEnv
--- completeBind may choose to do its work
--- * by extending the substitution (e.g. let x = y in ...)
--- * or by adding to the floats in the envt
+ -> TopLevelFlag -- Flag stuck into unfolding
+ -> InId -- Old binder
+ -> OutId -> OutExpr -- New binder and RHS
+ -> SimplM SimplEnv
+-- completeBind may choose to do its work
+-- * by extending the substitution (e.g. let x = y in ...)
+-- * or by adding to the floats in the envt
completeBind env top_lvl old_bndr new_bndr new_rhs
- | postInlineUnconditionally env top_lvl new_bndr occ_info new_rhs unfolding
- -- Inline and discard the binding
- = do { tick (PostInlineUnconditionally old_bndr)
- ; -- pprTrace "postInlineUnconditionally" (ppr old_bndr <+> ppr new_bndr <+> ppr new_rhs) $
- return (extendIdSubst env old_bndr (DoneEx new_rhs)) }
- -- Use the substitution to make quite, quite sure that the
- -- substitution will happen, since we are going to discard the binding
-
- | otherwise
- = let
- -- Arity info
- new_bndr_info = idInfo new_bndr `setArityInfo` exprArity new_rhs
-
- -- Unfolding info
- -- Add the unfolding *only* for non-loop-breakers
- -- Making loop breakers not have an unfolding at all
- -- means that we can avoid tests in exprIsConApp, for example.
- -- This is important: if exprIsConApp says 'yes' for a recursive
- -- thing, then we can get into an infinite loop
-
- -- Demand info
- -- If the unfolding is a value, the demand info may
- -- go pear-shaped, so we nuke it. Example:
- -- let x = (a,b) in
- -- case x of (p,q) -> h p q x
- -- Here x is certainly demanded. But after we've nuked
- -- the case, we'll get just
- -- let x = (a,b) in h a b x
- -- and now x is not demanded (I'm assuming h is lazy)
- -- This really happens. Similarly
- -- let f = \x -> e in ...f..f...
- -- After inlining f at some of its call sites the original binding may
- -- (for example) be no longer strictly demanded.
- -- The solution here is a bit ad hoc...
- info_w_unf = new_bndr_info `setUnfoldingInfo` unfolding
- final_info | loop_breaker = new_bndr_info
- | isEvaldUnfolding unfolding = zapDemandInfo info_w_unf `orElse` info_w_unf
- | otherwise = info_w_unf
-
- final_id = new_bndr `setIdInfo` final_info
+ = do { let old_info = idInfo old_bndr
+ old_unf = unfoldingInfo old_info
+ occ_info = occInfo old_info
+
+ ; new_unfolding <- simplUnfolding env top_lvl old_bndr occ_info old_unf new_rhs
+
+ ; if postInlineUnconditionally env top_lvl new_bndr occ_info new_rhs new_unfolding
+ -- Inline and discard the binding
+ then do { tick (PostInlineUnconditionally old_bndr)
+ ; return (extendIdSubst env old_bndr (DoneEx new_rhs)) }
+ -- Use the substitution to make quite, quite sure that the
+ -- substitution will happen, since we are going to discard the binding
+
+ else return (addNonRecWithUnf env new_bndr new_rhs new_unfolding) }
+
+------------------------------
+addPolyBind :: TopLevelFlag -> SimplEnv -> OutBind -> SimplM SimplEnv
+-- Add a new binding to the environment, complete with its unfolding
+-- but *do not* do postInlineUnconditionally, because we have already
+-- processed some of the scope of the binding
+-- We still want the unfolding though. Consider
+-- let
+-- x = /\a. let y = ... in Just y
+-- in body
+-- Then we float the y-binding out (via abstractFloats and addPolyBind)
+-- but 'x' may well then be inlined in 'body' in which case we'd like the
+-- opportunity to inline 'y' too.
+
+addPolyBind top_lvl env (NonRec poly_id rhs)
+ = do { unfolding <- simplUnfolding env top_lvl poly_id NoOccInfo noUnfolding rhs
+ -- Assumes that poly_id did not have an INLINE prag
+ -- which is perhaps wrong. ToDo: think about this
+ ; return (addNonRecWithUnf env poly_id rhs unfolding) }
+
+addPolyBind _ env bind@(Rec _) = return (extendFloats env bind)
+ -- Hack: letrecs are more awkward, so we extend "by steam"
+ -- without adding unfoldings etc. At worst this leads to
+ -- more simplifier iterations
+
+------------------------------
+addNonRecWithUnf :: SimplEnv
+ -> OutId -> OutExpr -- New binder and RHS
+ -> Unfolding -- New unfolding
+ -> SimplEnv
+addNonRecWithUnf env new_bndr new_rhs new_unfolding
+ = let new_arity = exprArity new_rhs
+ old_arity = idArity new_bndr
+ info1 = idInfo new_bndr `setArityInfo` new_arity
+
+ -- Unfolding info: Note [Setting the new unfolding]
+ info2 = info1 `setUnfoldingInfo` new_unfolding
+
+ -- Demand info: Note [Setting the demand info]
+ info3 | isEvaldUnfolding new_unfolding = zapDemandInfo info2 `orElse` info2
+ | otherwise = info2
+
+ final_id = new_bndr `setIdInfo` info3
+ dmd_arity = length $ fst $ splitStrictSig $ idNewStrictness new_bndr
in
- -- These seqs forces the Id, and hence its IdInfo,
- -- and hence any inner substitutions
- final_id `seq`
- -- pprTrace "Binding" (ppr final_id <+> ppr unfolding) $
- return (addNonRec env final_id new_rhs)
- where
- unfolding = mkUnfolding (isTopLevel top_lvl) new_rhs
- loop_breaker = isNonRuleLoopBreaker occ_info
- old_info = idInfo old_bndr
- occ_info = occInfo old_info
+ ASSERT( isId new_bndr )
+ WARN( new_arity < old_arity || new_arity < dmd_arity,
+ (ppr final_id <+> ppr old_arity <+> ppr new_arity <+> ppr dmd_arity) $$ ppr new_rhs )
+
+ final_id `seq` -- This seq forces the Id, and hence its IdInfo,
+ -- and hence any inner substitutions
+ -- pprTrace "Binding" (ppr final_id <+> ppr unfolding) $
+ addNonRec env final_id new_rhs
+ -- The addNonRec adds it to the in-scope set too
+
+
+------------------------------
+simplUnfolding :: SimplEnv-> TopLevelFlag
+ -> Id -- Debug output only
+ -> OccInfo -> Unfolding -> OutExpr
+ -> SimplM Unfolding
+simplUnfolding env top_lvl bndr occ_info old_unf new_rhs -- Note [Setting the new unfolding]
+ | omit_unfolding = WARN( is_inline_rule, ppr bndr ) return NoUnfolding
+ | is_inline_rule = return (substUnfolding env is_top_lvl old_unf)
+ | otherwise = return (mkUnfolding is_top_lvl new_rhs)
+ where
+ is_top_lvl = isTopLevel top_lvl
+ is_inline_rule = isInlineRule old_unf
+ omit_unfolding = isNonRuleLoopBreaker occ_info
\end{code}
+Note [Setting the new unfolding]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+* If there's an INLINE pragma, we use substUnfolding to retain the
+ supplied inlining
+
+* If not, we make an unfolding from the new RHS. But *only* for
+ non-loop-breakers. Making loop breakers not have an unfolding at all
+ means that we can avoid tests in exprIsConApp, for example. This is
+ important: if exprIsConApp says 'yes' for a recursive thing, then we
+ can get into an infinite loop
+
+If there's an INLINE pragma on a loop breaker, we simply discard it
+(with a DEBUG warning). The desugarer complains about binding groups
+that look likely to trigger this behaviour.
+
+
+Note [Setting the demand info]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+If the unfolding is a value, the demand info may
+go pear-shaped, so we nuke it. Example:
+ let x = (a,b) in
+ case x of (p,q) -> h p q x
+Here x is certainly demanded. But after we've nuked
+the case, we'll get just
+ let x = (a,b) in h a b x
+and now x is not demanded (I'm assuming h is lazy)
+This really happens. Similarly
+ let f = \x -> e in ...f..f...
+After inlining f at some of its call sites the original binding may
+(for example) be no longer strictly demanded.
+The solution here is a bit ad hoc...
+
%************************************************************************
-%* *
+%* *
\subsection[Simplify-simplExpr]{The main function: simplExpr}
-%* *
+%* *
%************************************************************************
The reason for this OutExprStuff stuff is that we want to float *after*
To see why it's important to do it after, consider this (real) example:
- let t = f x
- in fst t
+ let t = f x
+ in fst t
==>
- let t = let a = e1
- b = e2
- in (a,b)
- in fst t
+ let t = let a = e1
+ b = e2
+ in (a,b)
+ in fst t
==>
- let a = e1
- b = e2
- t = (a,b)
- in
- a -- Can't inline a this round, cos it appears twice
+ let a = e1
+ b = e2
+ t = (a,b)
+ in
+ a -- Can't inline a this round, cos it appears twice
==>
- e1
+ e1
Each of the ==> steps is a round of simplification. We'd save a
whole round if we float first. This can cascade. Consider
- let f = g d
- in \x -> ...f...
+ let f = g d
+ in \x -> ...f...
==>
- let f = let d1 = ..d.. in \y -> e
- in \x -> ...f...
+ let f = let d1 = ..d.. in \y -> e
+ in \x -> ...f...
==>
- let d1 = ..d..
- in \x -> ...(\y ->e)...
+ let d1 = ..d..
+ in \x -> ...(\y ->e)...
-Only in this second round can the \y be applied, and it
+Only in this second round can the \y be applied, and it
might do the same again.
\begin{code}
simplExpr :: SimplEnv -> CoreExpr -> SimplM CoreExpr
-simplExpr env expr = simplExprC env expr (mkBoringStop expr_ty')
- where
- expr_ty' = substTy env (exprType expr)
- -- The type in the Stop continuation, expr_ty', is usually not used
- -- It's only needed when discarding continuations after finding
- -- a function that returns bottom.
- -- Hence the lazy substitution
-
+simplExpr env expr = simplExprC env expr mkBoringStop
simplExprC :: SimplEnv -> CoreExpr -> SimplCont -> SimplM CoreExpr
- -- Simplify an expression, given a continuation
-simplExprC env expr cont
+ -- Simplify an expression, given a continuation
+simplExprC env expr cont
= -- pprTrace "simplExprC" (ppr expr $$ ppr cont {- $$ ppr (seIdSubst env) -} $$ ppr (seFloats env) ) $
- do { (env', expr') <- simplExprF (zapFloats env) expr cont
- ; -- pprTrace "simplExprC ret" (ppr expr $$ ppr expr') $
- -- pprTrace "simplExprC ret3" (ppr (seInScope env')) $
- -- pprTrace "simplExprC ret4" (ppr (seFloats env')) $
+ do { (env', expr') <- simplExprF (zapFloats env) expr cont
+ ; -- pprTrace "simplExprC ret" (ppr expr $$ ppr expr') $
+ -- pprTrace "simplExprC ret3" (ppr (seInScope env')) $
+ -- pprTrace "simplExprC ret4" (ppr (seFloats env')) $
return (wrapFloats env' expr') }
--------------------------------------------------
simplExprF :: SimplEnv -> InExpr -> SimplCont
- -> SimplM (SimplEnv, OutExpr)
+ -> SimplM (SimplEnv, OutExpr)
-simplExprF env e cont
+simplExprF env e cont
= -- pprTrace "simplExprF" (ppr e $$ ppr cont $$ ppr (seTvSubst env) $$ ppr (seIdSubst env) {- $$ ppr (seFloats env) -} ) $
simplExprF' env e cont
-
-simplExprF' env (Var v) cont = simplVar env v cont
+
+simplExprF' :: SimplEnv -> InExpr -> SimplCont
+ -> SimplM (SimplEnv, OutExpr)
+simplExprF' env (Var v) cont = simplVar env v cont
simplExprF' env (Lit lit) cont = rebuild env (Lit lit) cont
simplExprF' env (Note n expr) cont = simplNote env n expr cont
simplExprF' env (Cast body co) cont = simplCast env body co cont
simplExprF' env (App fun arg) cont = simplExprF env fun $
- ApplyTo NoDup arg env cont
+ ApplyTo NoDup arg env cont
-simplExprF' env expr@(Lam _ _) cont
+simplExprF' env expr@(Lam _ _) cont
= simplLam env (map zap bndrs) body cont
- -- The main issue here is under-saturated lambdas
- -- (\x1. \x2. e) arg1
- -- Here x1 might have "occurs-once" occ-info, because occ-info
- -- is computed assuming that a group of lambdas is applied
- -- all at once. If there are too few args, we must zap the
- -- occ-info.
+ -- The main issue here is under-saturated lambdas
+ -- (\x1. \x2. e) arg1
+ -- Here x1 might have "occurs-once" occ-info, because occ-info
+ -- is computed assuming that a group of lambdas is applied
+ -- all at once. If there are too few args, we must zap the
+ -- occ-info.
where
n_args = countArgs cont
n_params = length bndrs
(bndrs, body) = collectBinders expr
- zap | n_args >= n_params = \b -> b
- | otherwise = \b -> if isTyVar b then b
- else zapLamIdInfo b
- -- NB: we count all the args incl type args
- -- so we must count all the binders (incl type lambdas)
+ zap | n_args >= n_params = \b -> b
+ | otherwise = \b -> if isTyVar b then b
+ else zapLamIdInfo b
+ -- NB: we count all the args incl type args
+ -- so we must count all the binders (incl type lambdas)
simplExprF' env (Type ty) cont
= ASSERT( contIsRhsOrArg cont )
- do { ty' <- simplType env ty
- ; rebuild env (Type ty') cont }
+ do { ty' <- simplType env ty
+ ; rebuild env (Type ty') cont }
-simplExprF' env (Case scrut bndr case_ty alts) cont
+simplExprF' env (Case scrut bndr _ alts) cont
| not (switchIsOn (getSwitchChecker env) NoCaseOfCase)
- = -- Simplify the scrutinee with a Select continuation
+ = -- Simplify the scrutinee with a Select continuation
simplExprF env scrut (Select NoDup bndr alts env cont)
| otherwise
- = -- If case-of-case is off, simply simplify the case expression
- -- in a vanilla Stop context, and rebuild the result around it
- do { case_expr' <- simplExprC env scrut case_cont
- ; rebuild env case_expr' cont }
+ = -- If case-of-case is off, simply simplify the case expression
+ -- in a vanilla Stop context, and rebuild the result around it
+ do { case_expr' <- simplExprC env scrut case_cont
+ ; rebuild env case_expr' cont }
where
- case_cont = Select NoDup bndr alts env (mkBoringStop case_ty')
- case_ty' = substTy env case_ty -- c.f. defn of simplExpr
+ case_cont = Select NoDup bndr alts env mkBoringStop
simplExprF' env (Let (Rec pairs) body) cont
- = do { env <- simplRecBndrs env (map fst pairs)
- -- NB: bndrs' don't have unfoldings or rules
- -- We add them as we go down
+ = do { env' <- simplRecBndrs env (map fst pairs)
+ -- NB: bndrs' don't have unfoldings or rules
+ -- We add them as we go down
- ; env <- simplRecBind env NotTopLevel pairs
- ; simplExprF env body cont }
+ ; env'' <- simplRecBind env' NotTopLevel pairs
+ ; simplExprF env'' body cont }
simplExprF' env (Let (NonRec bndr rhs) body) cont
= simplNonRecE env bndr (rhs, env) ([], body) cont
---------------------------------
simplType :: SimplEnv -> InType -> SimplM OutType
- -- Kept monadic just so we can do the seqType
+ -- Kept monadic just so we can do the seqType
simplType env ty
= -- pprTrace "simplType" (ppr ty $$ ppr (seTvSubst env)) $
- seqType new_ty `seq` returnSmpl new_ty
+ seqType new_ty `seq` return new_ty
where
new_ty = substTy env ty
\end{code}
%************************************************************************
-%* *
+%* *
\subsection{The main rebuilder}
-%* *
+%* *
%************************************************************************
\begin{code}
rebuild :: SimplEnv -> OutExpr -> SimplCont -> SimplM (SimplEnv, OutExpr)
-- At this point the substitution in the SimplEnv should be irrelevant
-- only the in-scope set and floats should matter
-rebuild env expr cont
- = -- pprTrace "rebuild" (ppr expr $$ ppr cont $$ ppr (seFloats env)) $
- case cont of
- Stop {} -> return (env, expr)
- CoerceIt co cont -> rebuild env (mkCoerce co expr) cont
+rebuild env expr cont0
+ = -- pprTrace "rebuild" (ppr expr $$ ppr cont0 $$ ppr (seFloats env)) $
+ case cont0 of
+ Stop {} -> return (env, expr)
+ CoerceIt co cont -> rebuild env (mkCoerce co expr) cont
Select _ bndr alts se cont -> rebuildCase (se `setFloats` env) expr bndr alts cont
- StrictArg fun ty info cont -> rebuildCall env (fun `App` expr) (funResultTy ty) info cont
+ StrictArg fun _ info cont -> rebuildCall env (fun `App` expr) info cont
StrictBind b bs body se cont -> do { env' <- simplNonRecX (se `setFloats` env) b expr
- ; simplLam env' bs body cont }
- ApplyTo _ arg se cont -> do { arg' <- simplExpr (se `setInScope` env) arg
- ; rebuild env (App expr arg') cont }
+ ; simplLam env' bs body cont }
+ ApplyTo _ arg se cont -> do { arg' <- simplExpr (se `setInScope` env) arg
+ ; rebuild env (App expr arg') cont }
\end{code}
%************************************************************************
-%* *
+%* *
\subsection{Lambdas}
-%* *
+%* *
%************************************************************************
\begin{code}
simplCast :: SimplEnv -> InExpr -> Coercion -> SimplCont
- -> SimplM (SimplEnv, OutExpr)
-simplCast env body co cont
- = do { co' <- simplType env co
- ; simplExprF env body (addCoerce co' cont) }
+ -> SimplM (SimplEnv, OutExpr)
+simplCast env body co0 cont0
+ = do { co1 <- simplType env co0
+ ; simplExprF env body (addCoerce co1 cont0) }
where
addCoerce co cont = add_coerce co (coercionKind co) cont
- add_coerce co (s1, k1) cont -- co :: ty~ty
- | s1 `coreEqType` k1 = cont -- is a no-op
-
- add_coerce co1 (s1, k2) (CoerceIt co2 cont)
- | (l1, t1) <- coercionKind co2
- -- coerce T1 S1 (coerce S1 K1 e)
- -- ==>
- -- e, if T1=K1
- -- coerce T1 K1 e, otherwise
- --
- -- For example, in the initial form of a worker
- -- we may find (coerce T (coerce S (\x.e))) y
- -- and we'd like it to simplify to e[y/x] in one round
- -- of simplification
- , s1 `coreEqType` t1 = cont -- The coerces cancel out
+ add_coerce _co (s1, k1) cont -- co :: ty~ty
+ | s1 `coreEqType` k1 = cont -- is a no-op
+
+ add_coerce co1 (s1, _k2) (CoerceIt co2 cont)
+ | (_l1, t1) <- coercionKind co2
+ -- e |> (g1 :: S1~L) |> (g2 :: L~T1)
+ -- ==>
+ -- e, if T1=T2
+ -- e |> (g1 . g2 :: T1~T2) otherwise
+ --
+ -- For example, in the initial form of a worker
+ -- we may find (coerce T (coerce S (\x.e))) y
+ -- and we'd like it to simplify to e[y/x] in one round
+ -- of simplification
+ , s1 `coreEqType` t1 = cont -- The coerces cancel out
| otherwise = CoerceIt (mkTransCoercion co1 co2) cont
-
- add_coerce co (s1s2, t1t2) (ApplyTo dup (Type arg_ty) arg_se cont)
- -- (f `cast` g) ty ---> (f ty) `cast` (g @ ty)
- -- This implements the PushT rule from the paper
- | Just (tyvar,_) <- splitForAllTy_maybe s1s2
- , not (isCoVar tyvar)
- = ApplyTo dup (Type ty') (zapSubstEnv env) (addCoerce (mkInstCoercion co ty') cont)
- where
- ty' = substTy arg_se arg_ty
-
- -- ToDo: the PushC rule is not implemented at all
-
- add_coerce co (s1s2, t1t2) (ApplyTo dup arg arg_se cont)
+
+ add_coerce co (s1s2, _t1t2) (ApplyTo dup (Type arg_ty) arg_se cont)
+ -- (f |> g) ty ---> (f ty) |> (g @ ty)
+ -- This implements the PushT rule from the paper
+ | Just (tyvar,_) <- splitForAllTy_maybe s1s2
+ , not (isCoVar tyvar)
+ = ApplyTo dup (Type ty') (zapSubstEnv env) (addCoerce (mkInstCoercion co ty') cont)
+ where
+ ty' = substTy (arg_se `setInScope` env) arg_ty
+
+ -- ToDo: the PushC rule is not implemented at all
+
+ add_coerce co (s1s2, _t1t2) (ApplyTo dup arg arg_se cont)
| not (isTypeArg arg) -- This implements the Push rule from the paper
- , isFunTy s1s2 -- t1t2 must be a function type, becuase it's applied
- -- co : s1s2 :=: t1t2
- -- (coerce (T1->T2) (S1->S2) F) E
- -- ===>
- -- coerce T2 S2 (F (coerce S1 T1 E))
- --
- -- t1t2 must be a function type, T1->T2, because it's applied
- -- to something but s1s2 might conceivably not be
- --
- -- When we build the ApplyTo we can't mix the out-types
- -- with the InExpr in the argument, so we simply substitute
- -- to make it all consistent. It's a bit messy.
- -- But it isn't a common case.
- --
- -- Example of use: Trac #995
+ , isFunTy s1s2 -- t1t2 must be a function type, becuase it's applied
+ -- (e |> (g :: s1s2 ~ t1->t2)) f
+ -- ===>
+ -- (e (f |> (arg g :: t1~s1))
+ -- |> (res g :: s2->t2)
+ --
+ -- t1t2 must be a function type, t1->t2, because it's applied
+ -- to something but s1s2 might conceivably not be
+ --
+ -- When we build the ApplyTo we can't mix the out-types
+ -- with the InExpr in the argument, so we simply substitute
+ -- to make it all consistent. It's a bit messy.
+ -- But it isn't a common case.
+ --
+ -- Example of use: Trac #995
= ApplyTo dup new_arg (zapSubstEnv env) (addCoerce co2 cont)
where
- -- we split coercion t1->t2 :=: s1->s2 into t1 :=: s1 and
- -- t2 :=: s2 with left and right on the curried form:
- -- (->) t1 t2 :=: (->) s1 s2
+ -- we split coercion t1->t2 ~ s1->s2 into t1 ~ s1 and
+ -- t2 ~ s2 with left and right on the curried form:
+ -- (->) t1 t2 ~ (->) s1 s2
[co1, co2] = decomposeCo 2 co
new_arg = mkCoerce (mkSymCoercion co1) arg'
- arg' = substExpr arg_se arg
+ arg' = substExpr (arg_se `setInScope` env) arg
add_coerce co _ cont = CoerceIt co cont
\end{code}
%************************************************************************
-%* *
+%* *
\subsection{Lambdas}
-%* *
+%* *
%************************************************************************
\begin{code}
simplLam :: SimplEnv -> [InId] -> InExpr -> SimplCont
- -> SimplM (SimplEnv, OutExpr)
+ -> SimplM (SimplEnv, OutExpr)
simplLam env [] body cont = simplExprF env body cont
- -- Type-beta reduction
-simplLam env (bndr:bndrs) body (ApplyTo _ (Type ty_arg) arg_se cont)
- = ASSERT( isTyVar bndr )
- do { tick (BetaReduction bndr)
- ; ty_arg' <- simplType (arg_se `setInScope` env) ty_arg
- ; simplLam (extendTvSubst env bndr ty_arg') bndrs body cont }
-
- -- Ordinary beta reduction
+ -- Beta reduction
simplLam env (bndr:bndrs) body (ApplyTo _ arg arg_se cont)
- = do { tick (BetaReduction bndr)
- ; simplNonRecE env bndr (arg, arg_se) (bndrs, body) cont }
+ = do { tick (BetaReduction bndr)
+ ; simplNonRecE env bndr (arg, arg_se) (bndrs, body) cont }
- -- Not enough args, so there are real lambdas left to put in the result
+ -- Not enough args, so there are real lambdas left to put in the result
simplLam env bndrs body cont
- = do { (env, bndrs') <- simplLamBndrs env bndrs
- ; body' <- simplExpr env body
- ; new_lam <- mkLam bndrs' body'
- ; rebuild env new_lam cont }
+ = do { (env', bndrs') <- simplLamBndrs env bndrs
+ ; body' <- simplExpr env' body
+ ; new_lam <- mkLam bndrs' body'
+ ; rebuild env' new_lam cont }
------------------
-simplNonRecE :: SimplEnv
- -> InId -- The binder
- -> (InExpr, SimplEnv) -- Rhs of binding (or arg of lambda)
- -> ([InId], InExpr) -- Body of the let/lambda
- -- \xs.e
- -> SimplCont
- -> SimplM (SimplEnv, OutExpr)
+simplNonRecE :: SimplEnv
+ -> InBndr -- The binder
+ -> (InExpr, SimplEnv) -- Rhs of binding (or arg of lambda)
+ -> ([InBndr], InExpr) -- Body of the let/lambda
+ -- \xs.e
+ -> SimplCont
+ -> SimplM (SimplEnv, OutExpr)
-- simplNonRecE is used for
-- * non-top-level non-recursive lets in expressions
--
-- The "body" of the binding comes as a pair of ([InId],InExpr)
-- representing a lambda; so we recurse back to simplLam
--- Why? Because of the binder-occ-info-zapping done before
--- the call to simplLam in simplExprF (Lam ...)
+-- Why? Because of the binder-occ-info-zapping done before
+-- the call to simplLam in simplExprF (Lam ...)
+
+ -- First deal with type applications and type lets
+ -- (/\a. e) (Type ty) and (let a = Type ty in e)
+simplNonRecE env bndr (Type ty_arg, rhs_se) (bndrs, body) cont
+ = ASSERT( isTyVar bndr )
+ do { ty_arg' <- simplType (rhs_se `setInScope` env) ty_arg
+ ; simplLam (extendTvSubst env bndr ty_arg') bndrs body cont }
simplNonRecE env bndr (rhs, rhs_se) (bndrs, body) cont
| preInlineUnconditionally env NotTopLevel bndr rhs
- = do { tick (PreInlineUnconditionally bndr)
- ; simplLam (extendIdSubst env bndr (mkContEx rhs_se rhs)) bndrs body cont }
+ = do { tick (PreInlineUnconditionally bndr)
+ ; simplLam (extendIdSubst env bndr (mkContEx rhs_se rhs)) bndrs body cont }
| isStrictId bndr
- = do { simplExprF (rhs_se `setFloats` env) rhs
- (StrictBind bndr bndrs body env cont) }
+ = do { simplExprF (rhs_se `setFloats` env) rhs
+ (StrictBind bndr bndrs body env cont) }
| otherwise
- = do { (env, bndr') <- simplNonRecBndr env bndr
- ; env <- simplLazyBind env NotTopLevel NonRecursive bndr bndr' rhs rhs_se
- ; simplLam env bndrs body cont }
+ = ASSERT( not (isTyVar bndr) )
+ do { (env1, bndr1) <- simplNonRecBndr env bndr
+ ; let (env2, bndr2) = addBndrRules env1 bndr bndr1
+ ; env3 <- simplLazyBind env2 NotTopLevel NonRecursive bndr bndr2 rhs rhs_se
+ ; simplLam env3 bndrs body cont }
\end{code}
%************************************************************************
-%* *
+%* *
\subsection{Notes}
-%* *
+%* *
%************************************************************************
\begin{code}
--- Hack alert: we only distinguish subsumed cost centre stacks for the
+-- Hack alert: we only distinguish subsumed cost centre stacks for the
-- purposes of inlining. All other CCCSs are mapped to currentCCS.
+simplNote :: SimplEnv -> Note -> CoreExpr -> SimplCont
+ -> SimplM (SimplEnv, OutExpr)
simplNote env (SCC cc) e cont
- = do { e' <- simplExpr (setEnclosingCC env currentCCS) e
- ; rebuild env (mkSCC cc e') cont }
-
--- See notes with SimplMonad.inlineMode
-simplNote env InlineMe e cont
- | contIsRhsOrArg cont -- Totally boring continuation; see notes above
- = do { -- Don't inline inside an INLINE expression
- e' <- simplExpr (setMode inlineMode env) e
- ; rebuild env (mkInlineMe e') cont }
-
- | otherwise -- Dissolve the InlineMe note if there's
- -- an interesting context of any kind to combine with
- -- (even a type application -- anything except Stop)
- = simplExprF env e cont
+ = do { e' <- simplExpr (setEnclosingCC env currentCCS) e
+ ; rebuild env (mkSCC cc e') cont }
simplNote env (CoreNote s) e cont
- = simplExpr env e `thenSmpl` \ e' ->
- rebuild env (Note (CoreNote s) e') cont
+ = do { e' <- simplExpr env e
+ ; rebuild env (Note (CoreNote s) e') cont }
\end{code}
%************************************************************************
-%* *
+%* *
\subsection{Dealing with calls}
-%* *
+%* *
%************************************************************************
\begin{code}
+simplVar :: SimplEnv -> Id -> SimplCont -> SimplM (SimplEnv, OutExpr)
simplVar env var cont
= case substId env var of
- DoneEx e -> simplExprF (zapSubstEnv env) e cont
- ContEx tvs ids e -> simplExprF (setSubstEnv env tvs ids) e cont
- DoneId var1 -> completeCall (zapSubstEnv env) var1 cont
- -- Note [zapSubstEnv]
- -- The template is already simplified, so don't re-substitute.
- -- This is VITAL. Consider
- -- let x = e in
- -- let y = \z -> ...x... in
- -- \ x -> ...y...
- -- We'll clone the inner \x, adding x->x' in the id_subst
- -- Then when we inline y, we must *not* replace x by x' in
- -- the inlined copy!!
+ DoneEx e -> simplExprF (zapSubstEnv env) e cont
+ ContEx tvs ids e -> simplExprF (setSubstEnv env tvs ids) e cont
+ DoneId var1 -> completeCall (zapSubstEnv env) var1 cont
+ -- Note [zapSubstEnv]
+ -- The template is already simplified, so don't re-substitute.
+ -- This is VITAL. Consider
+ -- let x = e in
+ -- let y = \z -> ...x... in
+ -- \ x -> ...y...
+ -- We'll clone the inner \x, adding x->x' in the id_subst
+ -- Then when we inline y, we must *not* replace x by x' in
+ -- the inlined copy!!
---------------------------------------------------------
--- Dealing with a call site
+-- Dealing with a call site
+completeCall :: SimplEnv -> Id -> SimplCont -> SimplM (SimplEnv, OutExpr)
completeCall env var cont
- = do { dflags <- getDOptsSmpl
- ; let (args,call_cont) = contArgs cont
- -- The args are OutExprs, obtained by *lazily* substituting
- -- in the args found in cont. These args are only examined
- -- to limited depth (unless a rule fires). But we must do
- -- the substitution; rule matching on un-simplified args would
- -- be bogus
-
- ------------- First try rules ----------------
- -- Do this before trying inlining. Some functions have
- -- rules *and* are strict; in this case, we don't want to
- -- inline the wrapper of the non-specialised thing; better
- -- to call the specialised thing instead.
- --
- -- We used to use the black-listing mechanism to ensure that inlining of
- -- the wrapper didn't occur for things that have specialisations till a
- -- later phase, so but now we just try RULES first
- --
- -- You might think that we shouldn't apply rules for a loop breaker:
- -- doing so might give rise to an infinite loop, because a RULE is
- -- rather like an extra equation for the function:
- -- RULE: f (g x) y = x+y
- -- Eqn: f a y = a-y
- --
- -- But it's too drastic to disable rules for loop breakers.
- -- Even the foldr/build rule would be disabled, because foldr
- -- is recursive, and hence a loop breaker:
- -- foldr k z (build g) = g k z
- -- So it's up to the programmer: rules can cause divergence
- ; rules <- getRules
- ; let in_scope = getInScope env
- maybe_rule = case activeRule dflags env of
- Nothing -> Nothing -- No rules apply
- Just act_fn -> lookupRule act_fn in_scope
- rules var args
- ; case maybe_rule of {
- Just (rule, rule_rhs) ->
- tick (RuleFired (ru_name rule)) `thenSmpl_`
- (if dopt Opt_D_dump_rule_firings dflags then
- pprTrace "Rule fired" (vcat [
- text "Rule:" <+> ftext (ru_name rule),
- text "Before:" <+> ppr var <+> sep (map pprParendExpr args),
- text "After: " <+> pprCoreExpr rule_rhs,
- text "Cont: " <+> ppr call_cont])
- else
- id) $
- simplExprF env rule_rhs (dropArgs (ruleArity rule) cont)
- -- The ruleArity says how many args the rule consumed
-
- ; Nothing -> do -- No rules
-
- ------------- Next try inlining ----------------
- { let arg_infos = [interestingArg arg | arg <- args, isValArg arg]
- n_val_args = length arg_infos
- interesting_cont = interestingCallContext (notNull args)
- (notNull arg_infos)
- call_cont
- active_inline = activeInline env var
- maybe_inline = callSiteInline dflags active_inline
- var arg_infos interesting_cont
- ; case maybe_inline of {
- Just unfolding -- There is an inlining!
- -> do { tick (UnfoldingDone var)
- ; (if dopt Opt_D_dump_inlinings dflags then
- pprTrace "Inlining done" (vcat [
- text "Before:" <+> ppr var <+> sep (map pprParendExpr args),
- text "Inlined fn: " <+> nest 2 (ppr unfolding),
- text "Cont: " <+> ppr call_cont])
- else
- id)
- simplExprF env unfolding cont }
-
- ; Nothing -> -- No inlining!
-
- ------------- No inlining! ----------------
- -- Next, look for rules or specialisations that match
- --
- rebuildCall env (Var var) (idType var)
- (mkArgInfo var n_val_args call_cont) cont
+ = do { dflags <- getDOptsSmpl
+ ; let (args,call_cont) = contArgs cont
+ -- The args are OutExprs, obtained by *lazily* substituting
+ -- in the args found in cont. These args are only examined
+ -- to limited depth (unless a rule fires). But we must do
+ -- the substitution; rule matching on un-simplified args would
+ -- be bogus
+
+ ------------- First try rules ----------------
+ -- Do this before trying inlining. Some functions have
+ -- rules *and* are strict; in this case, we don't want to
+ -- inline the wrapper of the non-specialised thing; better
+ -- to call the specialised thing instead.
+ --
+ -- We used to use the black-listing mechanism to ensure that inlining of
+ -- the wrapper didn't occur for things that have specialisations till a
+ -- later phase, so but now we just try RULES first
+ --
+ -- Note [Rules for recursive functions]
+ -- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+ -- You might think that we shouldn't apply rules for a loop breaker:
+ -- doing so might give rise to an infinite loop, because a RULE is
+ -- rather like an extra equation for the function:
+ -- RULE: f (g x) y = x+y
+ -- Eqn: f a y = a-y
+ --
+ -- But it's too drastic to disable rules for loop breakers.
+ -- Even the foldr/build rule would be disabled, because foldr
+ -- is recursive, and hence a loop breaker:
+ -- foldr k z (build g) = g k z
+ -- So it's up to the programmer: rules can cause divergence
+ ; rule_base <- getSimplRules
+ ; let in_scope = getInScope env
+ rules = getRules rule_base var
+ maybe_rule = case activeRule dflags env of
+ Nothing -> Nothing -- No rules apply
+ Just act_fn -> lookupRule act_fn in_scope
+ var args rules
+ ; case maybe_rule of {
+ Just (rule, rule_rhs) -> do
+ tick (RuleFired (ru_name rule))
+ (if dopt Opt_D_dump_rule_firings dflags then
+ pprTrace "Rule fired" (vcat [
+ text "Rule:" <+> ftext (ru_name rule),
+ text "Before:" <+> ppr var <+> sep (map pprParendExpr args),
+ text "After: " <+> pprCoreExpr rule_rhs,
+ text "Cont: " <+> ppr call_cont])
+ else
+ id) $
+ simplExprF env rule_rhs (dropArgs (ruleArity rule) cont)
+ -- The ruleArity says how many args the rule consumed
+
+ ; Nothing -> do -- No rules
+
+ ------------- Next try inlining ----------------
+ { let arg_infos = [interestingArg arg | arg <- args, isValArg arg]
+ n_val_args = length arg_infos
+ interesting_cont = interestingCallContext call_cont
+ active_inline = activeInline env var
+ maybe_inline = callSiteInline dflags active_inline var
+ (null args) arg_infos interesting_cont
+ ; case maybe_inline of {
+ Just unfolding -- There is an inlining!
+ -> do { tick (UnfoldingDone var)
+ ; (if dopt Opt_D_dump_inlinings dflags then
+ pprTrace ("Inlining done: " ++ showSDoc (ppr var)) (vcat [
+ text "Before:" <+> ppr var <+> sep (map pprParendExpr args),
+ text "Inlined fn: " <+> nest 2 (ppr unfolding),
+ text "Cont: " <+> ppr call_cont])
+ else
+ id)
+ simplExprF env unfolding cont }
+
+ ; Nothing -> -- No inlining!
+
+ ------------- No inlining! ----------------
+ -- Next, look for rules or specialisations that match
+ --
+ rebuildCall env (Var var)
+ (mkArgInfo var n_val_args call_cont) cont
}}}}
rebuildCall :: SimplEnv
- -> OutExpr -> OutType -- Function and its type
- -> (Bool, [Bool]) -- See SimplUtils.mkArgInfo
- -> SimplCont
- -> SimplM (SimplEnv, OutExpr)
-rebuildCall env fun fun_ty (has_rules, []) cont
+ -> OutExpr -- Function
+ -> ArgInfo
+ -> SimplCont
+ -> SimplM (SimplEnv, OutExpr)
+rebuildCall env fun (ArgInfo { ai_strs = [] }) cont
-- When we run out of strictness args, it means
-- that the call is definitely bottom; see SimplUtils.mkArgInfo
-- Then we want to discard the entire strict continuation. E.g.
- -- * case (error "hello") of { ... }
- -- * (error "Hello") arg
- -- * f (error "Hello") where f is strict
- -- etc
+ -- * 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.
- | not (contIsTrivial cont) -- Only do this if there is a non-trivial
+ | not (contIsTrivial cont) -- Only do this if there is a non-trivial
= return (env, mk_coerce fun) -- contination to discard, else we do it
- where -- again and again!
- cont_ty = contResultType cont
+ where -- again and again!
+ fun_ty = exprType fun
+ cont_ty = contResultType env fun_ty cont
co = mkUnsafeCoercion fun_ty cont_ty
- mk_coerce expr | cont_ty `coreEqType` fun_ty = fun
- | otherwise = mkCoerce co fun
+ mk_coerce expr | cont_ty `coreEqType` fun_ty = expr
+ | otherwise = mkCoerce co expr
-rebuildCall env fun fun_ty info (ApplyTo _ (Type arg_ty) se cont)
- = do { ty' <- simplType (se `setInScope` env) arg_ty
- ; rebuildCall env (fun `App` Type ty') (applyTy fun_ty ty') info cont }
+rebuildCall env fun info (ApplyTo _ (Type arg_ty) se cont)
+ = do { ty' <- simplType (se `setInScope` env) arg_ty
+ ; rebuildCall env (fun `App` Type ty') info cont }
-rebuildCall env fun fun_ty (has_rules, str:strs) (ApplyTo _ arg arg_se cont)
- | str || isStrictType arg_ty -- Strict argument
+rebuildCall env fun
+ (ArgInfo { ai_rules = has_rules, ai_strs = str:strs, ai_discs = disc:discs })
+ (ApplyTo _ arg arg_se cont)
+ | str -- Strict argument
= -- pprTrace "Strict Arg" (ppr arg $$ ppr (seIdSubst env) $$ ppr (seInScope env)) $
simplExprF (arg_se `setFloats` env) arg
- (StrictArg fun fun_ty (has_rules, strs) cont)
- -- Note [Shadowing]
-
- | otherwise -- Lazy argument
- -- DO NOT float anything outside, hence simplExprC
- -- There is no benefit (unlike in a let-binding), and we'd
- -- have to be very careful about bogus strictness through
- -- floating a demanded let.
- = do { arg' <- simplExprC (arg_se `setInScope` env) arg
- (mkLazyArgStop arg_ty has_rules)
- ; rebuildCall env (fun `App` arg') res_ty (has_rules, strs) cont }
+ (StrictArg fun cci arg_info' cont)
+ -- Note [Shadowing]
+
+ | otherwise -- Lazy argument
+ -- DO NOT float anything outside, hence simplExprC
+ -- There is no benefit (unlike in a let-binding), and we'd
+ -- have to be very careful about bogus strictness through
+ -- floating a demanded let.
+ = do { arg' <- simplExprC (arg_se `setInScope` env) arg
+ (mkLazyArgStop cci)
+ ; rebuildCall env (fun `App` arg') arg_info' cont }
where
- (arg_ty, res_ty) = splitFunTy fun_ty
+ arg_info' = ArgInfo { ai_rules = has_rules, ai_strs = strs, ai_discs = discs }
+ cci | has_rules || disc > 0 = ArgCtxt has_rules disc -- Be keener here
+ | otherwise = BoringCtxt -- Nothing interesting
-rebuildCall env fun fun_ty info cont
+rebuildCall env fun _ cont
= rebuild env fun cont
\end{code}
~~~~~~~~~~~~~~~~
This part of the simplifier may break the no-shadowing invariant
Consider
- f (...(\a -> e)...) (case y of (a,b) -> e')
+ f (...(\a -> e)...) (case y of (a,b) -> e')
where f is strict in its second arg
If we simplify the innermost one first we get (...(\a -> e)...)
Simplifying the second arg makes us float the case out, so we end up with
- case y of (a,b) -> f (...(\a -> e)...) e'
+ case y of (a,b) -> f (...(\a -> e)...) e'
So the output does not have the no-shadowing invariant. However, there is
no danger of getting name-capture, because when the first arg was simplified
we used an in-scope set that at least mentioned all the variables free in its
static environment, and that is enough.
We can't just do innermost first, or we'd end up with a dual problem:
- case x of (a,b) -> f e (...(\a -> e')...)
+ case x of (a,b) -> f e (...(\a -> e')...)
I spent hours trying to recover the no-shadowing invariant, but I just could
not think of an elegant way to do it. The simplifier is already knee-deep in
all this at once is TOO HARD!
%************************************************************************
-%* *
- Rebuilding a cse expression
-%* *
+%* *
+ Rebuilding a cse expression
+%* *
%************************************************************************
-Blob of helper functions for the "case-of-something-else" situation.
+Note [Case elimination]
+~~~~~~~~~~~~~~~~~~~~~~~
+The case-elimination transformation discards redundant case expressions.
+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!
+
+The code in SimplUtils.prepareAlts has the effect of 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:
+
+ case x of
+ 0# -> ...
+ DEFAULT -> ...(case x of
+ 0# -> ...
+ DEFAULT -> ...) ...
+
+Here the inner case is first trimmed to have only one alternative, the
+DEFAULT, after which it's an instance of the previous case. 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
+ - e is already evaluated (it may so if e is a variable)
+ - x is used strictly, or
+
+Lastly, the code in SimplUtils.mkCase combines identical RHSs. So
+
+ case e of ===> case e of DEFAULT -> r
+ True -> r
+ False -> r
+
+Now again the case may be elminated by the CaseElim transformation.
+
+
+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}
---------------------------------------------------------
--- Eliminate the case if possible
+-- Eliminate the case if possible
rebuildCase :: SimplEnv
- -> OutExpr -- Scrutinee
- -> InId -- Case binder
- -> [InAlt] -- Alternatives (inceasing order)
- -> SimplCont
- -> SimplM (SimplEnv, OutExpr)
+ -> OutExpr -- Scrutinee
+ -> InId -- Case binder
+ -> [InAlt] -- Alternatives (inceasing order)
+ -> SimplCont
+ -> SimplM (SimplEnv, OutExpr)
--------------------------------------------------
--- 1. Eliminate the case if there's a known constructor
+-- 1. Eliminate the case if there's a known constructor
--------------------------------------------------
rebuildCase env scrut case_bndr alts cont
- | Just (con,args) <- exprIsConApp_maybe scrut
- -- Works when the scrutinee is a variable with a known unfolding
- -- as well as when it's an explicit constructor application
+ | Just (con,args) <- exprIsConApp_maybe scrut
+ -- Works when the scrutinee is a variable with a known unfolding
+ -- as well as when it's an explicit constructor application
= knownCon env scrut (DataAlt con) args case_bndr alts cont
- | Lit lit <- scrut -- No need for same treatment as constructors
- -- because literals are inlined more vigorously
+ | Lit lit <- scrut -- No need for same treatment as constructors
+ -- because literals are inlined more vigorously
= knownCon env scrut (LitAlt lit) [] case_bndr alts cont
--------------------------------------------------
--- 2. Eliminate the case if scrutinee is evaluated
+-- 2. Eliminate the case if scrutinee is evaluated
--------------------------------------------------
-rebuildCase env scrut case_bndr [(con,bndrs,rhs)] cont
+rebuildCase env scrut case_bndr [(_, bndrs, rhs)] cont
-- 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,
+ -- See Note [Case eliminiation]
+ -- mkCase made sure that if all the alternatives are equal,
-- then there is now only one (DEFAULT) rhs
- | all isDeadBinder bndrs -- bndrs are [InId]
+ | all isDeadBinder bndrs -- bndrs are [InId]
- -- Check that the scrutinee can be let-bound instead of case-bound
+ -- 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
- = do { tick (CaseElim case_bndr)
- ; env <- simplNonRecX env case_bndr scrut
- ; simplExprF env rhs cont }
+ -- 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
+ = do { tick (CaseElim case_bndr)
+ ; env' <- simplNonRecX env case_bndr scrut
+ ; simplExprF env' rhs cont }
where
- -- The case binder is going to be evaluated later,
- -- and the scrutinee is a simple variable
+ -- 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)
- && not (isTickBoxOp v)
- -- ugly hack; covering this case is what
- -- exprOkForSpeculation was intended for.
- var_demanded_later other = False
+ && not (isTickBoxOp v)
+ -- ugly hack; covering this case is what
+ -- exprOkForSpeculation was intended for.
+ var_demanded_later _ = False
--------------------------------------------------
--- 3. Catch-all case
+-- 3. Catch-all case
--------------------------------------------------
rebuildCase env scrut case_bndr alts cont
- = do { -- Prepare the continuation;
- -- The new subst_env is in place
- (env, dup_cont, nodup_cont) <- prepareCaseCont env alts cont
-
- -- Simplify the alternatives
- ; (scrut', case_bndr', alts') <- simplAlts env scrut case_bndr alts dup_cont
- ; let res_ty' = contResultType dup_cont
- ; case_expr <- mkCase scrut' case_bndr' res_ty' alts'
-
- -- Notice that rebuildDone returns the in-scope set from env, not alt_env
+ = do { -- Prepare the continuation;
+ -- The new subst_env is in place
+ (env', dup_cont, nodup_cont) <- prepareCaseCont env alts cont
+
+ -- Simplify the alternatives
+ ; (scrut', case_bndr', alts') <- simplAlts env' scrut case_bndr alts dup_cont
+
+ -- Check for empty alternatives
+ ; if null alts' then
+ -- This isn't strictly an error, although it is unusual.
+ -- 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 instead.
+ pprTrace "mkCase: null alts" (ppr case_bndr <+> ppr scrut) $
+ let res_ty' = contResultType env' (substTy env' (coreAltsType alts)) dup_cont
+ lit = mkStringLit "Impossible alternative"
+ in return (env', mkApps (Var rUNTIME_ERROR_ID) [Type res_ty', lit])
+
+ else do
+ { case_expr <- mkCase scrut' case_bndr' alts'
+
+ -- Notice that rebuild gets the in-scope set from env, not alt_env
-- The case binder *not* scope over the whole returned case-expression
- ; rebuild env case_expr nodup_cont }
+ ; rebuild env' case_expr nodup_cont } }
\end{code}
simplCaseBinder checks whether the scrutinee is a variable, v. If so,
way, there's a chance that v will now only be used once, and hence
inlined.
-Note [no-case-of-case]
-~~~~~~~~~~~~~~~~~~~~~~
-There is a time we *don't* want to do that, namely when
--fno-case-of-case is on. This happens in the first simplifier pass,
-and enhances full laziness. Here's the bad case:
- f = \ y -> ...(case x of I# v -> ...(case x of ...) ... )
-If we eliminate the inner case, we trap it inside the I# v -> arm,
-which might prevent some full laziness happening. I've seen this
-in action in spectral/cichelli/Prog.hs:
- [(m,n) | m <- [1..max], n <- [1..max]]
-Hence the check for NoCaseOfCase.
-
-Note [Suppressing the case binder-swap]
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-There is another situation when it might make sense to suppress the
-case-expression binde-swap. If we have
-
- case x of w1 { DEFAULT -> case x of w2 { A -> e1; B -> e2 }
- ...other cases .... }
-
-We'll perform the binder-swap for the outer case, giving
-
- case x of w1 { DEFAULT -> case w1 of w2 { A -> e1; B -> e2 }
- ...other cases .... }
-
-But there is no point in doing it for the inner case, because w1 can't
-be inlined anyway. Furthermore, doing the case-swapping involves
-zapping w2's occurrence info (see paragraphs that follow), and that
-forces us to bind w2 when doing case merging. So we get
-
- case x of w1 { A -> let w2 = w1 in e1
- B -> let w2 = w1 in e2
- ...other cases .... }
-
-This is plain silly in the common case where w2 is dead.
-
-Even so, I can't see a good way to implement this idea. I tried
-not doing the binder-swap if the scrutinee was already evaluated
-but that failed big-time:
-
- data T = MkT !Int
-
- case v of w { MkT x ->
- case x of x1 { I# y1 ->
- case x of x2 { I# y2 -> ...
-
-Notice that because MkT is strict, x is marked "evaluated". But to
-eliminate the last case, we must either make sure that x (as well as
-x1) has unfolding MkT y1. THe straightforward thing to do is to do
-the binder-swap. So this whole note is a no-op.
+Historical note: we use to do the "case binder swap" in the Simplifier
+so there were additional complications if the scrutinee was a variable.
+Now the binder-swap stuff is done in the occurrence analyer; see
+OccurAnal Note [Binder swap].
Note [zapOccInfo]
~~~~~~~~~~~~~~~~~
-If we replace the scrutinee, v, by tbe case binder, then we have to nuke
-any occurrence info (eg IAmDead) in the case binder, because the
-case-binder now effectively occurs whenever v does. AND we have to do
-the same for the pattern-bound variables! Example:
-
- (case x of { (a,b) -> a }) (case x of { (p,q) -> q })
-
-Here, b and p are dead. But when we move the argment inside the first
-case RHS, and eliminate the second case, we get
-
- case x of { (a,b) -> a b }
-
-Urk! b is alive! Reason: the scrutinee was a variable, and case elimination
-happened.
-
-Indeed, this can happen anytime the case binder isn't dead:
- case <any> of x { (a,b) ->
+If the case binder is not dead, then neither are the pattern bound
+variables:
+ case <any> of x { (a,b) ->
case x of { (p,q) -> p } }
Here (a,b) both look dead, but come alive after the inner case is eliminated.
The point is that we bring into the envt a binding
- let x = (a,b)
+ let x = (a,b)
after the outer case, and that makes (a,b) alive. At least we do unless
the case binder is guaranteed dead.
+In practice, the scrutinee is almost always a variable, so we pretty
+much always zap the OccInfo of the binders. It doesn't matter much though.
+
+
Note [Case of cast]
~~~~~~~~~~~~~~~~~~~
-Consider case (v `cast` co) of x { I# ->
- ... (case (v `cast` co) of {...}) ...
-We'd like to eliminate the inner case. We can get this neatly by
+Consider case (v `cast` co) of x { I# ->
+ ... (case (v `cast` co) of {...}) ...
+We'd like to eliminate the inner case. We can get this neatly by
arranging that inside the outer case we add the unfolding
- v |-> x `cast` (sym co)
+ v |-> x `cast` (sym co)
to v. Then we should inline v at the inner case, cancel the casts, and away we go
-
+
Note [Improving seq]
~~~~~~~~~~~~~~~~~~~
Consider
- type family F :: * -> *
- type instance F Int = Int
+ type family F :: * -> *
+ type instance F Int = Int
- ... case e of x { DEFAULT -> rhs } ...
+ ... case e of x { DEFAULT -> rhs } ...
where x::F Int. Then we'd like to rewrite (F Int) to Int, getting
- case e `cast` co of x'::Int
- I# x# -> let x = x' `cast` sym co
- in rhs
+ case e `cast` co of x'::Int
+ I# x# -> let x = x' `cast` sym co
+ in rhs
-so that 'rhs' can take advantage of hte form of x'. Notice that Note
+so that 'rhs' can take advantage of the form of x'. Notice that Note
[Case of cast] may then apply to the result.
This showed up in Roman's experiments. Example:
where
bar 0 = 0
bar n = bar (n - case t of TI i -> i)
-Here we'd like to avoid repeated evaluating t inside the loop, by
+Here we'd like to avoid repeated evaluating t inside the loop, by
taking advantage of the `seq`.
At one point I did transformation in LiberateCase, but it's more robust here.
(Otherwise, there's a danger that we'll simply drop the 'seq' altogether, before
LiberateCase gets to see it.)
-Note [Case elimination]
-~~~~~~~~~~~~~~~~~~~~~~~
-The case-elimination transformation discards redundant case expressions.
-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!
-
-The code in SimplUtils.prepareAlts has the effect of 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:
-
- case x of
- 0# -> ...
- DEFAULT -> ...(case x of
- 0# -> ...
- DEFAULT -> ...) ...
-
-Here the inner case is first trimmed to have only one alternative, the
-DEFAULT, after which it's an instance of the previous case. 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
- - e is already evaluated (it may so if e is a variable)
- - x is used strictly, or
-
-Lastly, the code in SimplUtils.mkCase combines identical RHSs. So
-
- case e of ===> case e of DEFAULT -> r
- True -> r
- False -> r
-
-Now again the case may be elminated by the CaseElim transformation.
+Historical note [no-case-of-case]
+~~~~~~~~~~~~~~~~~~~~~~
+We *used* to suppress the binder-swap in case expressoins when
+-fno-case-of-case is on. Old remarks:
+ "This happens in the first simplifier pass,
+ and enhances full laziness. Here's the bad case:
+ f = \ y -> ...(case x of I# v -> ...(case x of ...) ... )
+ If we eliminate the inner case, we trap it inside the I# v -> arm,
+ which might prevent some full laziness happening. I've seen this
+ in action in spectral/cichelli/Prog.hs:
+ [(m,n) | m <- [1..max], n <- [1..max]]
+ Hence the check for NoCaseOfCase."
+However, now the full-laziness pass itself reverses the binder-swap, so this
+check is no longer necessary.
+
+Historical note [Suppressing the case binder-swap]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+There is another situation when it might make sense to suppress the
+case-expression binde-swap. If we have
-Further notes about case elimination
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-Consider: test :: Integer -> IO ()
- test = print
+ case x of w1 { DEFAULT -> case x of w2 { A -> e1; B -> e2 }
+ ...other cases .... }
-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:
+We'll perform the binder-swap for the outer case, giving
-f x y = if x < 0 then jtos x
- else if y==0 then "" else jtos x
+ case x of w1 { DEFAULT -> case w1 of w2 { A -> e1; B -> e2 }
+ ...other cases .... }
-At a particular call site we have (f v 1). So we inline to get
+But there is no point in doing it for the inner case, because w1 can't
+be inlined anyway. Furthermore, doing the case-swapping involves
+zapping w2's occurrence info (see paragraphs that follow), and that
+forces us to bind w2 when doing case merging. So we get
- if v < 0 then jtos x
- else if 1==0 then "" else jtos x
+ case x of w1 { A -> let w2 = w1 in e1
+ B -> let w2 = w1 in e2
+ ...other cases .... }
-Now simplify the 1==0 conditional:
+This is plain silly in the common case where w2 is dead.
- if v<0 then jtos v else jtos v
+Even so, I can't see a good way to implement this idea. I tried
+not doing the binder-swap if the scrutinee was already evaluated
+but that failed big-time:
-Now common-up the two branches of the case:
+ data T = MkT !Int
- case (v<0) of DEFAULT -> jtos v
+ case v of w { MkT x ->
+ case x of x1 { I# y1 ->
+ case x of x2 { I# y2 -> ...
-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.
+Notice that because MkT is strict, x is marked "evaluated". But to
+eliminate the last case, we must either make sure that x (as well as
+x1) has unfolding MkT y1. THe straightforward thing to do is to do
+the binder-swap. So this whole note is a no-op.
\begin{code}
-simplCaseBinder :: SimplEnv -> OutExpr -> OutId -> [InAlt]
- -> SimplM (SimplEnv, OutExpr, OutId)
-simplCaseBinder env scrut case_bndr alts
- = do { (env1, case_bndr1) <- simplBinder env case_bndr
-
- ; fam_envs <- getFamEnvs
- ; (env2, scrut2, case_bndr2) <- improve_seq fam_envs env1 scrut
- case_bndr case_bndr1 alts
- -- Note [Improving seq]
-
- ; let (env3, case_bndr3) = improve_case_bndr env2 scrut2 case_bndr2
- -- Note [Case of cast]
-
- ; return (env3, scrut2, case_bndr3) }
- where
-
- improve_seq fam_envs env1 scrut case_bndr case_bndr1 [(DEFAULT,_,_)]
- | Just (co, ty2) <- topNormaliseType fam_envs (idType case_bndr1)
- = do { case_bndr2 <- newId FSLIT("nt") ty2
- ; let rhs = DoneEx (Var case_bndr2 `Cast` mkSymCoercion co)
- env2 = extendIdSubst env1 case_bndr rhs
- ; return (env2, scrut `Cast` co, case_bndr2) }
-
- improve_seq fam_envs env1 scrut case_bndr case_bndr1 alts
- = return (env1, scrut, case_bndr1)
-
-
+improveSeq :: (FamInstEnv, FamInstEnv) -> SimplEnv
+ -> OutExpr -> InId -> OutId -> [InAlt]
+ -> SimplM (SimplEnv, OutExpr, OutId)
+-- Note [Improving seq]
+improveSeq fam_envs env scrut case_bndr case_bndr1 [(DEFAULT,_,_)]
+ | Just (co, ty2) <- topNormaliseType fam_envs (idType case_bndr1)
+ = do { case_bndr2 <- newId (fsLit "nt") ty2
+ ; let rhs = DoneEx (Var case_bndr2 `Cast` mkSymCoercion co)
+ env2 = extendIdSubst env case_bndr rhs
+ ; return (env2, scrut `Cast` co, case_bndr2) }
+
+improveSeq _ env scrut _ case_bndr1 _
+ = return (env, scrut, case_bndr1)
+
+{-
improve_case_bndr env scrut case_bndr
- | switchIsOn (getSwitchChecker env) NoCaseOfCase
- -- See Note [no-case-of-case]
- = (env, case_bndr)
-
- | otherwise -- Failed try [see Note 2 above]
- -- not (isEvaldUnfolding (idUnfolding v))
- = case scrut of
- Var v -> (modifyInScope env1 v case_bndr', case_bndr')
- -- Note about using modifyInScope for v here
- -- We could extend the substitution instead, but it would be
- -- a hack because then the substitution wouldn't be idempotent
- -- any more (v is an OutId). And this does just as well.
-
- Cast (Var v) co -> (addBinderUnfolding env1 v rhs, case_bndr')
- where
- rhs = Cast (Var case_bndr') (mkSymCoercion co)
-
- other -> (env, case_bndr)
- where
- case_bndr' = zapOccInfo case_bndr
- env1 = modifyInScope env case_bndr case_bndr'
-
-
-zapOccInfo :: InId -> InId -- See Note [zapOccInfo]
-zapOccInfo b = b `setIdOccInfo` NoOccInfo
+ -- See Note [no-case-of-case]
+ -- | switchIsOn (getSwitchChecker env) NoCaseOfCase
+ -- = (env, case_bndr)
+
+ | otherwise -- Failed try; see Note [Suppressing the case binder-swap]
+ -- not (isEvaldUnfolding (idUnfolding v))
+ = case scrut of
+ Var v -> (modifyInScope env1 v case_bndr', case_bndr')
+ -- Note about using modifyInScope for v here
+ -- We could extend the substitution instead, but it would be
+ -- a hack because then the substitution wouldn't be idempotent
+ -- any more (v is an OutId). And this does just as well.
+
+ Cast (Var v) co -> (addBinderUnfolding env1 v rhs, case_bndr')
+ where
+ rhs = Cast (Var case_bndr') (mkSymCoercion co)
+
+ _ -> (env, case_bndr)
+ where
+ case_bndr' = zapIdOccInfo case_bndr
+ env1 = modifyInScope env case_bndr case_bndr'
+-}
\end{code}
2. If the DEFAULT alternative can match only one possible constructor,
then make that constructor explicit.
e.g.
- case e of x { DEFAULT -> rhs }
+ case e of x { DEFAULT -> rhs }
===>
- case e of x { (a,b) -> rhs }
+ case e of x { (a,b) -> rhs }
where the type is a single constructor type. This gives better code
when rhs also scrutinises x or e.
data Colour = Red | Green | Blue
f x = case x of
- Red -> ..
- Green -> ..
- DEFAULT -> h x
+ Red -> ..
+ Green -> ..
+ DEFAULT -> h x
h y = case y of
- Blue -> ..
- DEFAULT -> [ case y of ... ]
+ Blue -> ..
+ DEFAULT -> [ case y of ... ]
If we inline h into f, the default case of the inlined h can't happen.
If we don't notice this, we may end up filtering out *all* the cases
\begin{code}
-simplAlts :: SimplEnv
- -> OutExpr
- -> InId -- Case binder
- -> [InAlt] -> SimplCont
- -> SimplM (OutExpr, OutId, [OutAlt]) -- Includes the continuation
+simplAlts :: SimplEnv
+ -> OutExpr
+ -> InId -- Case binder
+ -> [InAlt] -- Non-empty
+ -> SimplCont
+ -> SimplM (OutExpr, OutId, [OutAlt]) -- Includes the continuation
-- Like simplExpr, this just returns the simplified alternatives;
-- it not return an environment
simplAlts env scrut case_bndr alts cont'
= -- pprTrace "simplAlts" (ppr alts $$ ppr (seIdSubst env)) $
- do { let alt_env = zapFloats env
- ; (alt_env, scrut', case_bndr') <- simplCaseBinder alt_env scrut case_bndr alts
+ do { let env0 = zapFloats env
+
+ ; (env1, case_bndr1) <- simplBinder env0 case_bndr
+
+ ; fam_envs <- getFamEnvs
+ ; (alt_env', scrut', case_bndr') <- improveSeq fam_envs env1 scrut
+ case_bndr case_bndr1 alts
- ; (imposs_deflt_cons, in_alts) <- prepareAlts scrut case_bndr' alts
+ ; (imposs_deflt_cons, in_alts) <- prepareAlts alt_env' scrut' case_bndr' alts
- ; alts' <- mapM (simplAlt alt_env imposs_deflt_cons case_bndr' cont') in_alts
- ; return (scrut', case_bndr', alts') }
+ ; alts' <- mapM (simplAlt alt_env' imposs_deflt_cons case_bndr' cont') in_alts
+ ; return (scrut', case_bndr', alts') }
------------------------------------
simplAlt :: SimplEnv
- -> [AltCon] -- These constructors can't be present when
- -- matching the DEFAULT alternative
- -> OutId -- The case binder
- -> SimplCont
- -> InAlt
- -> SimplM OutAlt
+ -> [AltCon] -- These constructors can't be present when
+ -- matching the DEFAULT alternative
+ -> OutId -- The case binder
+ -> SimplCont
+ -> InAlt
+ -> SimplM OutAlt
simplAlt env imposs_deflt_cons case_bndr' cont' (DEFAULT, bndrs, rhs)
= ASSERT( null bndrs )
- do { let env' = addBinderOtherCon env case_bndr' imposs_deflt_cons
- -- Record the constructors that the case-binder *can't* be.
- ; rhs' <- simplExprC env' rhs cont'
- ; return (DEFAULT, [], rhs') }
+ do { let env' = addBinderOtherCon env case_bndr' imposs_deflt_cons
+ -- Record the constructors that the case-binder *can't* be.
+ ; rhs' <- simplExprC env' rhs cont'
+ ; return (DEFAULT, [], rhs') }
-simplAlt env imposs_deflt_cons case_bndr' cont' (LitAlt lit, bndrs, rhs)
+simplAlt env _ case_bndr' cont' (LitAlt lit, bndrs, rhs)
= ASSERT( null bndrs )
- do { let env' = addBinderUnfolding env case_bndr' (Lit lit)
- ; rhs' <- simplExprC env' rhs cont'
- ; return (LitAlt lit, [], rhs') }
-
-simplAlt env imposs_deflt_cons case_bndr' cont' (DataAlt con, vs, rhs)
- = do { -- Deal with the pattern-bound variables
- (env, vs') <- simplBinders env (add_evals con vs)
-
- -- Mark the ones that are in ! positions in the
- -- data constructor as certainly-evaluated.
- ; let vs'' = add_evals con vs'
-
- -- Bind the case-binder to (con args)
- ; let inst_tys' = tyConAppArgs (idType case_bndr')
- con_args = map Type inst_tys' ++ varsToCoreExprs vs''
- env' = addBinderUnfolding env case_bndr' (mkConApp con con_args)
-
- ; rhs' <- simplExprC env' rhs cont'
- ; return (DataAlt con, vs'', rhs') }
+ do { let env' = addBinderUnfolding env case_bndr' (Lit lit)
+ ; rhs' <- simplExprC env' rhs cont'
+ ; return (LitAlt lit, [], rhs') }
+
+simplAlt env _ case_bndr' cont' (DataAlt con, vs, rhs)
+ = do { -- Deal with the pattern-bound variables
+ -- Mark the ones that are in ! positions in the
+ -- data constructor as certainly-evaluated.
+ -- NB: simplLamBinders preserves this eval info
+ let vs_with_evals = add_evals (dataConRepStrictness con)
+ ; (env', vs') <- simplLamBndrs env vs_with_evals
+
+ -- Bind the case-binder to (con args)
+ ; let inst_tys' = tyConAppArgs (idType case_bndr')
+ con_args = map Type inst_tys' ++ varsToCoreExprs vs'
+ env'' = addBinderUnfolding env' case_bndr'
+ (mkConApp con con_args)
+
+ ; rhs' <- simplExprC env'' rhs cont'
+ ; return (DataAlt con, vs', rhs') }
where
- -- add_evals records the evaluated-ness of the bound variables of
- -- a case pattern. This is *important*. Consider
- -- data T = T !Int !Int
- --
- -- case x of { T a b -> T (a+1) b }
- --
- -- We really must record that b is already evaluated so that we don't
- -- go and re-evaluate it when constructing the result.
- -- See Note [Data-con worker strictness] in MkId.lhs
- add_evals dc vs = cat_evals dc vs (dataConRepStrictness dc)
-
- cat_evals dc vs strs
- = go vs strs
- where
- go [] [] = []
- go (v:vs) strs | isTyVar v = v : go vs strs
- go (v:vs) (str:strs)
- | isMarkedStrict str = evald_v : go vs strs
- | otherwise = zapped_v : go vs strs
- where
- zapped_v = zap_occ_info v
- evald_v = zapped_v `setIdUnfolding` evaldUnfolding
- go _ _ = pprPanic "cat_evals" (ppr dc $$ ppr vs $$ ppr strs)
-
- -- If the case binder is alive, then we add the unfolding
- -- case_bndr = C vs
- -- to the envt; so vs are now very much alive
- -- Note [Aug06] I can't see why this actually matters
- zap_occ_info | isDeadBinder case_bndr' = \id -> id
- | otherwise = zapOccInfo
+ -- add_evals records the evaluated-ness of the bound variables of
+ -- a case pattern. This is *important*. Consider
+ -- data T = T !Int !Int
+ --
+ -- case x of { T a b -> T (a+1) b }
+ --
+ -- We really must record that b is already evaluated so that we don't
+ -- go and re-evaluate it when constructing the result.
+ -- See Note [Data-con worker strictness] in MkId.lhs
+ add_evals the_strs
+ = go vs the_strs
+ where
+ go [] [] = []
+ go (v:vs') strs | isTyVar v = v : go vs' strs
+ go (v:vs') (str:strs)
+ | isMarkedStrict str = evald_v : go vs' strs
+ | otherwise = zapped_v : go vs' strs
+ where
+ zapped_v = zap_occ_info v
+ evald_v = zapped_v `setIdUnfolding` evaldUnfolding
+ go _ _ = pprPanic "cat_evals" (ppr con $$ ppr vs $$ ppr the_strs)
+
+ -- See Note [zapOccInfo]
+ -- zap_occ_info: if the case binder is alive, then we add the unfolding
+ -- case_bndr = C vs
+ -- to the envt; so vs are now very much alive
+ -- Note [Aug06] I can't see why this actually matters, but it's neater
+ -- case e of t { (a,b) -> ...(case t of (p,q) -> p)... }
+ -- ==> case e of t { (a,b) -> ...(a)... }
+ -- Look, Ma, a is alive now.
+ zap_occ_info = zapCasePatIdOcc case_bndr'
addBinderUnfolding :: SimplEnv -> Id -> CoreExpr -> SimplEnv
addBinderUnfolding env bndr rhs
- = modifyInScope env bndr (bndr `setIdUnfolding` mkUnfolding False rhs)
+ = modifyInScope env (bndr `setIdUnfolding` mkUnfolding False rhs)
addBinderOtherCon :: SimplEnv -> Id -> [AltCon] -> SimplEnv
addBinderOtherCon env bndr cons
- = modifyInScope env bndr (bndr `setIdUnfolding` mkOtherCon cons)
+ = modifyInScope env (bndr `setIdUnfolding` mkOtherCon cons)
+
+zapCasePatIdOcc :: Id -> Id -> Id
+-- Consider case e of b { (a,b) -> ... }
+-- Then if we bind b to (a,b) in "...", and b is not dead,
+-- then we must zap the deadness info on a,b
+zapCasePatIdOcc case_bndr
+ | isDeadBinder case_bndr = \ pat_id -> pat_id
+ | otherwise = \ pat_id -> zapIdOccInfo pat_id
\end{code}
%************************************************************************
-%* *
+%* *
\subsection{Known constructor}
-%* *
+%* *
%************************************************************************
We are a bit careful with occurrence info. Here's an example
- (\x* -> case x of (a*, b) -> f a) (h v, e)
+ (\x* -> case x of (a*, b) -> f a) (h v, e)
where the * means "occurs once". This effectively becomes
- case (h v, e) of (a*, b) -> f a)
+ case (h v, e) of (a*, b) -> f a)
and then
- let a* = h v; b = e in f a
+ let a* = h v; b = e in f a
and then
- f (h v)
+ f (h v)
All this should happen in one sweep.
\begin{code}
-knownCon :: SimplEnv -> OutExpr -> AltCon -> [OutExpr]
- -> InId -> [InAlt] -> SimplCont
- -> SimplM (SimplEnv, OutExpr)
+knownCon :: SimplEnv -> OutExpr -> AltCon
+ -> [OutExpr] -- Args *including* the universal args
+ -> InId -> [InAlt] -> SimplCont
+ -> SimplM (SimplEnv, OutExpr)
knownCon env scrut con args bndr alts cont
- = do { tick (KnownBranch bndr)
- ; knownAlt env scrut args bndr (findAlt con alts) cont }
+ = do { tick (KnownBranch bndr)
+ ; knownAlt env scrut args bndr (findAlt con alts) cont }
-knownAlt env scrut args bndr (DEFAULT, bs, rhs) cont
+knownAlt :: SimplEnv -> OutExpr -> [OutExpr]
+ -> InId -> (AltCon, [CoreBndr], InExpr) -> SimplCont
+ -> SimplM (SimplEnv, OutExpr)
+knownAlt env scrut _ bndr (DEFAULT, bs, rhs) cont
= ASSERT( null bs )
- do { env <- simplNonRecX env bndr scrut
- -- This might give rise to a binding with non-atomic args
- -- like x = Node (f x) (g x)
- -- but simplNonRecX will atomic-ify it
- ; simplExprF env rhs cont }
+ do { env' <- simplNonRecX env bndr scrut
+ -- This might give rise to a binding with non-atomic args
+ -- like x = Node (f x) (g x)
+ -- but simplNonRecX will atomic-ify it
+ ; simplExprF env' rhs cont }
-knownAlt env scrut args bndr (LitAlt lit, bs, rhs) cont
+knownAlt env scrut _ bndr (LitAlt _, bs, rhs) cont
= ASSERT( null bs )
- do { env <- simplNonRecX env bndr scrut
- ; simplExprF env rhs cont }
-
-knownAlt env scrut args bndr (DataAlt dc, bs, rhs) cont
- = do { let dead_bndr = isDeadBinder bndr -- bndr is an InId
- n_drop_tys = length (dataConUnivTyVars dc)
- ; env <- bind_args env dead_bndr bs (drop n_drop_tys args)
- ; let
- -- It's useful to bind bndr to scrut, rather than to a fresh
- -- binding x = Con arg1 .. argn
- -- because very often the scrut is a variable, so we avoid
- -- creating, and then subsequently eliminating, a let-binding
- -- BUT, if scrut is a not a variable, we must be careful
- -- about duplicating the arg redexes; in that case, make
- -- a new con-app from the args
- bndr_rhs = case scrut of
- Var v -> scrut
- other -> con_app
- con_app = mkConApp dc (take n_drop_tys args ++ con_args)
- con_args = [substExpr env (varToCoreExpr b) | b <- bs]
- -- args are aready OutExprs, but bs are InIds
-
- ; env <- simplNonRecX env bndr bndr_rhs
- ; -- pprTrace "knownCon2" (ppr bs $$ ppr rhs $$ ppr (seIdSubst env)) $
- simplExprF env rhs cont }
-
--- Ugh!
-bind_args env dead_bndr [] _ = return env
-
-bind_args env dead_bndr (b:bs) (Type ty : args)
- = ASSERT( isTyVar b )
- bind_args (extendTvSubst env b ty) dead_bndr bs args
-
-bind_args env dead_bndr (b:bs) (arg : args)
- = ASSERT( isId b )
- do { let b' = if dead_bndr then b else zapOccInfo b
- -- Note that the binder might be "dead", because it doesn't occur
- -- in the RHS; and simplNonRecX may therefore discard it via postInlineUnconditionally
- -- Nevertheless we must keep it if the case-binder is alive, because it may
- -- be used in the con_app. See Note [zapOccInfo]
- ; env <- simplNonRecX env b' arg
- ; bind_args env dead_bndr bs args }
-
-bind_args _ _ _ _ = panic "bind_args"
+ do { env' <- simplNonRecX env bndr scrut
+ ; simplExprF env' rhs cont }
+
+knownAlt env scrut the_args bndr (DataAlt dc, bs, rhs) cont
+ = do { let n_drop_tys = length (dataConUnivTyVars dc)
+ ; env' <- bind_args env bs (drop n_drop_tys the_args)
+ ; let
+ -- It's useful to bind bndr to scrut, rather than to a fresh
+ -- binding x = Con arg1 .. argn
+ -- because very often the scrut is a variable, so we avoid
+ -- creating, and then subsequently eliminating, a let-binding
+ -- BUT, if scrut is a not a variable, we must be careful
+ -- about duplicating the arg redexes; in that case, make
+ -- a new con-app from the args
+ bndr_rhs = case scrut of
+ Var _ -> scrut
+ _ -> con_app
+ con_app = mkConApp dc (take n_drop_tys the_args ++ con_args)
+ con_args = [substExpr env' (varToCoreExpr b) | b <- bs]
+ -- args are aready OutExprs, but bs are InIds
+
+ ; env'' <- simplNonRecX env' bndr bndr_rhs
+ ; simplExprF env'' rhs cont }
+ where
+ zap_occ = zapCasePatIdOcc bndr -- bndr is an InId
+
+ -- Ugh!
+ bind_args env' [] _ = return env'
+
+ bind_args env' (b:bs') (Type ty : args)
+ = ASSERT( isTyVar b )
+ bind_args (extendTvSubst env' b ty) bs' args
+
+ bind_args env' (b:bs') (arg : args)
+ = ASSERT( isId b )
+ do { let b' = zap_occ b
+ -- Note that the binder might be "dead", because it doesn't
+ -- occur in the RHS; and simplNonRecX may therefore discard
+ -- it via postInlineUnconditionally.
+ -- Nevertheless we must keep it if the case-binder is alive,
+ -- because it may be used in the con_app. See Note [zapOccInfo]
+ ; env'' <- simplNonRecX env' b' arg
+ ; bind_args env'' bs' args }
+
+ bind_args _ _ _ =
+ pprPanic "bind_args" $ ppr dc $$ ppr bs $$ ppr the_args $$
+ text "scrut:" <+> ppr scrut
\end{code}
%************************************************************************
-%* *
+%* *
\subsection{Duplicating continuations}
-%* *
+%* *
%************************************************************************
\begin{code}
prepareCaseCont :: SimplEnv
- -> [InAlt] -> SimplCont
- -> SimplM (SimplEnv, SimplCont,SimplCont)
- -- Return a duplicatable continuation, a non-duplicable part
- -- plus some extra bindings (that scope over the entire
- -- continunation)
-
- -- No need to make it duplicatable if there's only one alternative
-prepareCaseCont env [alt] cont = return (env, cont, mkBoringStop (contResultType cont))
-prepareCaseCont env alts cont = mkDupableCont env cont
+ -> [InAlt] -> SimplCont
+ -> SimplM (SimplEnv, SimplCont,SimplCont)
+ -- Return a duplicatable continuation, a non-duplicable part
+ -- plus some extra bindings (that scope over the entire
+ -- continunation)
+
+ -- No need to make it duplicatable if there's only one alternative
+prepareCaseCont env [_] cont = return (env, cont, mkBoringStop)
+prepareCaseCont env _ cont = mkDupableCont env cont
\end{code}
\begin{code}
-mkDupableCont :: SimplEnv -> SimplCont
- -> SimplM (SimplEnv, SimplCont, SimplCont)
+mkDupableCont :: SimplEnv -> SimplCont
+ -> SimplM (SimplEnv, SimplCont, SimplCont)
mkDupableCont env cont
| contIsDupable cont
- = returnSmpl (env, cont, mkBoringStop (contResultType cont))
+ = return (env, cont, mkBoringStop)
-mkDupableCont env (Stop {}) = panic "mkDupableCont" -- Handled by previous eqn
+mkDupableCont _ (Stop {}) = panic "mkDupableCont" -- Handled by previous eqn
mkDupableCont env (CoerceIt ty cont)
- = do { (env, dup, nodup) <- mkDupableCont env cont
- ; return (env, CoerceIt ty dup, nodup) }
+ = do { (env', dup, nodup) <- mkDupableCont env cont
+ ; return (env', CoerceIt ty dup, nodup) }
-mkDupableCont env cont@(StrictBind bndr _ _ se _)
- = return (env, mkBoringStop (substTy se (idType bndr)), cont)
- -- See Note [Duplicating strict continuations]
+mkDupableCont env cont@(StrictBind {})
+ = return (env, mkBoringStop, cont)
+ -- See Note [Duplicating strict continuations]
-mkDupableCont env cont@(StrictArg _ fun_ty _ _)
- = return (env, mkBoringStop (funArgTy fun_ty), cont)
- -- See Note [Duplicating strict continuations]
+mkDupableCont env cont@(StrictArg {})
+ = return (env, mkBoringStop, cont)
+ -- See Note [Duplicating strict continuations]
mkDupableCont env (ApplyTo _ arg se cont)
- = -- e.g. [...hole...] (...arg...)
- -- ==>
- -- let a = ...arg...
- -- in [...hole...] a
- do { (env, dup_cont, nodup_cont) <- mkDupableCont env cont
- ; arg <- simplExpr (se `setInScope` env) arg
- ; (env, arg) <- makeTrivial env arg
- ; let app_cont = ApplyTo OkToDup arg (zapSubstEnv env) dup_cont
- ; return (env, app_cont, nodup_cont) }
-
-mkDupableCont env cont@(Select _ case_bndr [(_,bs,rhs)] se case_cont)
+ = -- e.g. [...hole...] (...arg...)
+ -- ==>
+ -- let a = ...arg...
+ -- in [...hole...] a
+ do { (env', dup_cont, nodup_cont) <- mkDupableCont env cont
+ ; arg' <- simplExpr (se `setInScope` env') arg
+ ; (env'', arg'') <- makeTrivial env' arg'
+ ; let app_cont = ApplyTo OkToDup arg'' (zapSubstEnv env'') dup_cont
+ ; return (env'', app_cont, nodup_cont) }
+
+mkDupableCont env cont@(Select _ case_bndr [(_, bs, _rhs)] _ _)
-- See Note [Single-alternative case]
-- | not (exprIsDupable rhs && contIsDupable case_cont)
-- | not (isDeadBinder case_bndr)
- | all isDeadBinder bs -- InIds
- = return (env, mkBoringStop scrut_ty, cont)
- where
- scrut_ty = substTy se (idType case_bndr)
+ | all isDeadBinder bs -- InIds
+ && not (isUnLiftedType (idType case_bndr))
+ -- Note [Single-alternative-unlifted]
+ = return (env, mkBoringStop, cont)
mkDupableCont env (Select _ case_bndr alts se cont)
- = -- e.g. (case [...hole...] of { pi -> ei })
- -- ===>
- -- let ji = \xij -> ei
- -- in case [...hole...] of { pi -> ji xij }
- do { tick (CaseOfCase case_bndr)
- ; (env, dup_cont, nodup_cont) <- mkDupableCont env cont
- -- NB: call mkDupableCont here, *not* prepareCaseCont
- -- We must make a duplicable continuation, whereas prepareCaseCont
- -- doesn't when there is a single case branch
-
- ; let alt_env = se `setInScope` env
- ; (alt_env, case_bndr') <- simplBinder alt_env case_bndr
- ; alts' <- mapM (simplAlt alt_env [] case_bndr' dup_cont) alts
- -- Safe to say that there are no handled-cons for the DEFAULT case
- -- NB: simplBinder does not zap deadness occ-info, so
- -- a dead case_bndr' will still advertise its deadness
- -- This is really important because in
- -- case e of b { (# p,q #) -> ... }
- -- b is always dead, and indeed we are not allowed to bind b to (# p,q #),
- -- which might happen if e was an explicit unboxed pair and b wasn't marked dead.
- -- In the new alts we build, we have the new case binder, so it must retain
- -- its deadness.
- -- NB: we don't use alt_env further; it has the substEnv for
- -- the alternatives, and we don't want that
-
- ; (env, alts') <- mkDupableAlts env case_bndr' alts'
- ; return (env, -- Note [Duplicated env]
- Select OkToDup case_bndr' alts' (zapSubstEnv env)
- (mkBoringStop (contResultType dup_cont)),
- nodup_cont) }
+ = -- e.g. (case [...hole...] of { pi -> ei })
+ -- ===>
+ -- let ji = \xij -> ei
+ -- in case [...hole...] of { pi -> ji xij }
+ do { tick (CaseOfCase case_bndr)
+ ; (env', dup_cont, nodup_cont) <- mkDupableCont env cont
+ -- NB: call mkDupableCont here, *not* prepareCaseCont
+ -- We must make a duplicable continuation, whereas prepareCaseCont
+ -- doesn't when there is a single case branch
+
+ ; let alt_env = se `setInScope` env'
+ ; (alt_env', case_bndr') <- simplBinder alt_env case_bndr
+ ; alts' <- mapM (simplAlt alt_env' [] case_bndr' dup_cont) alts
+ -- Safe to say that there are no handled-cons for the DEFAULT case
+ -- NB: simplBinder does not zap deadness occ-info, so
+ -- a dead case_bndr' will still advertise its deadness
+ -- This is really important because in
+ -- case e of b { (# p,q #) -> ... }
+ -- b is always dead, and indeed we are not allowed to bind b to (# p,q #),
+ -- which might happen if e was an explicit unboxed pair and b wasn't marked dead.
+ -- In the new alts we build, we have the new case binder, so it must retain
+ -- its deadness.
+ -- NB: we don't use alt_env further; it has the substEnv for
+ -- the alternatives, and we don't want that
+
+ ; (env'', alts'') <- mkDupableAlts env' case_bndr' alts'
+ ; return (env'', -- Note [Duplicated env]
+ Select OkToDup case_bndr' alts'' (zapSubstEnv env'') mkBoringStop,
+ nodup_cont) }
mkDupableAlts :: SimplEnv -> OutId -> [InAlt]
- -> SimplM (SimplEnv, [InAlt])
+ -> SimplM (SimplEnv, [InAlt])
-- Absorbs the continuation into the new alternatives
-mkDupableAlts env case_bndr' alts
- = go env alts
+mkDupableAlts env case_bndr' the_alts
+ = go env the_alts
where
- go env [] = return (env, [])
- go env (alt:alts)
- = do { (env, alt') <- mkDupableAlt env case_bndr' alt
- ; (env, alts') <- go env alts
- ; return (env, alt' : alts' ) }
-
-mkDupableAlt env case_bndr' (con, bndrs', rhs')
- | exprIsDupable rhs' -- Note [Small alternative rhs]
- = return (env, (con, bndrs', rhs'))
+ go env0 [] = return (env0, [])
+ go env0 (alt:alts)
+ = do { (env1, alt') <- mkDupableAlt env0 case_bndr' alt
+ ; (env2, alts') <- go env1 alts
+ ; return (env2, alt' : alts' ) }
+
+mkDupableAlt :: SimplEnv -> OutId -> (AltCon, [CoreBndr], CoreExpr)
+ -> SimplM (SimplEnv, (AltCon, [CoreBndr], CoreExpr))
+mkDupableAlt env case_bndr1 (con, bndrs1, rhs1)
+ | exprIsDupable rhs1 -- Note [Small alternative rhs]
+ = return (env, (con, bndrs1, rhs1))
| otherwise
- = do { let rhs_ty' = exprType rhs'
- used_bndrs' = filter abstract_over (case_bndr' : bndrs')
- abstract_over bndr
- | isTyVar bndr = True -- Abstract over all type variables just in case
- | otherwise = not (isDeadBinder bndr)
- -- The deadness info on the new Ids is preserved by simplBinders
-
- ; (final_bndrs', final_args) -- Note [Join point abstraction]
- <- if (any isId used_bndrs')
- then return (used_bndrs', varsToCoreExprs used_bndrs')
- else do { rw_id <- newId FSLIT("w") realWorldStatePrimTy
- ; return ([rw_id], [Var realWorldPrimId]) }
-
- ; join_bndr <- newId FSLIT("$j") (mkPiTypes final_bndrs' rhs_ty')
- -- Note [Funky mkPiTypes]
-
- ; let -- We make the lambdas into one-shot-lambdas. The
- -- join point is sure to be applied at most once, and doing so
- -- prevents the body of the join point being floated out by
- -- the full laziness pass
- really_final_bndrs = map one_shot final_bndrs'
- one_shot v | isId v = setOneShotLambda v
- | otherwise = v
- join_rhs = mkLams really_final_bndrs rhs'
- join_call = mkApps (Var join_bndr) final_args
-
- ; return (addNonRec env join_bndr join_rhs, (con, bndrs', join_call)) }
- -- See Note [Duplicated env]
+ = do { let abstract_over bndr
+ | isTyVar bndr = True -- Abstract over all type variables just in case
+ | otherwise = not (isDeadBinder bndr)
+ -- The deadness info on the new Ids is preserved by simplBinders
+
+ inst_tys1 = tyConAppArgs (idType case_bndr1)
+ con_app dc = mkConApp dc (map Type inst_tys1 ++ varsToCoreExprs bndrs1)
+
+ (rhs2, final_bndrs) -- See Note [Passing the case binder to join points]
+ | isDeadBinder case_bndr1
+ = (rhs1, filter abstract_over bndrs1)
+ | opt_PassCaseBndrToJoinPoints, not (null bndrs1)
+ = (rhs1, (case_bndr1 : filter abstract_over bndrs1))
+ | otherwise
+ = case con of
+ DataAlt dc -> (Let (NonRec case_bndr1 (con_app dc)) rhs1, bndrs1)
+ LitAlt lit -> ASSERT( null bndrs1 ) (Let (NonRec case_bndr1 (Lit lit)) rhs1, [])
+ DEFAULT -> ASSERT( null bndrs1 ) (rhs1, [case_bndr1])
+
+ ; (final_bndrs1, final_args) -- Note [Join point abstraction]
+ <- if (any isId final_bndrs)
+ then return (final_bndrs, varsToCoreExprs final_bndrs)
+ else do { rw_id <- newId (fsLit "w") realWorldStatePrimTy
+ ; return (rw_id : final_bndrs,
+ Var realWorldPrimId : varsToCoreExprs final_bndrs) }
+
+ ; let rhs_ty1 = exprType rhs1
+ ; join_bndr <- newId (fsLit "$j") (mkPiTypes final_bndrs1 rhs_ty1)
+ -- Note [Funky mkPiTypes]
+
+ ; let -- We make the lambdas into one-shot-lambdas. The
+ -- join point is sure to be applied at most once, and doing so
+ -- prevents the body of the join point being floated out by
+ -- the full laziness pass
+ really_final_bndrs = map one_shot final_bndrs1
+ one_shot v | isId v = setOneShotLambda v
+ | otherwise = v
+ join_rhs = mkLams really_final_bndrs rhs2
+ join_call = mkApps (Var join_bndr) final_args
+
+ ; env1 <- addPolyBind NotTopLevel env (NonRec join_bndr join_rhs)
+ ; return (env1, (con, bndrs1, join_call)) }
+ -- See Note [Duplicated env]
\end{code}
+Note [Passing the case binder to join points]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Suppose we have
+ case e of cb { C1 -> r1[cb]; C2 x y z -> r2[cb,x] }
+and we want to make join points for the two alternatives,
+which mention the case binder 'cb'. Should we pass 'cb' to
+the join point, or reconstruct it? Here are the two alternatives
+for the C2 alternative:
+
+ Plan A(pass cb): j2 cb x = r2[cb,x]
+
+ Plan B(reconstruct cb): j2 x y z = let cb = C2 x y z in r2[cb,x]
+
+The advantge of Plan B is that we can "see" the definition of cb
+in r2, and that may be important when we inline stuff in r2. The
+disadvantage is that if this optimisation doesn't happen, we end up
+re-allocating C2, when it already exists. This does happen occasionally;
+an example is the function nofib/spectral/cichelli/Auxil.$whinsert.
+
+Plan B is always better if the constructor is nullary.
+
+In both cases we don't have liveness info for cb on a branch-by-branch
+basis, and it's possible that 'cb' is used in some branches but not
+others. Well, the absence analyser will find that out later, so it's
+not too bad.
+
+Sadly, at the time of writing, neither choice seems an unequivocal
+win. Here are nofib results, for adding -fpass-case-bndr-to-join-points
+(all others are zero effect):
+
+ Program Size Allocs Runtime Elapsed
+--------------------------------------------------------------------------------
+ cichelli +0.0% -4.4% 0.13 0.13
+ pic +0.0% -0.7% 0.01 0.04
+ transform -0.0% +2.8% -0.4% -9.1%
+ wave4main +0.0% +10.5% +3.1% +3.4%
+--------------------------------------------------------------------------------
+ Min -0.0% -4.4% -7.0% -31.9%
+ Max +0.1% +10.5% +3.1% +15.0%
+ Geometric Mean +0.0% +0.1% -1.7% -6.1%
+
+
Note [Duplicated env]
~~~~~~~~~~~~~~~~~~~~~
Some of the alternatives are simplified, but have not been turned into a join point
inlined, but after the join points had been inlined it looked smaller, and so
was inlined.
-NB: we have to check the size of rhs', not rhs.
+NB: we have to check the size of rhs', not rhs.
Duplicating a small InAlt might invalidate occurrence information
However, if it *is* dupable, we return the *un* simplified alternative,
because otherwise we'd need to pair it up with an empty subst-env....
it's possible that the join point will be abstracted over
type varaibles as well as term variables.
Example: Suppose we have
- data T = forall t. C [t]
+ data T = forall t. C [t]
Then faced with
- case (case e of ...) of
- C t xs::[t] -> rhs
+ case (case e of ...) of
+ C t xs::[t] -> rhs
We get the join point
- let j :: forall t. [t] -> ...
- j = /\t \xs::[t] -> rhs
- in
- case (case e of ...) of
- C t xs::[t] -> j t xs
+ let j :: forall t. [t] -> ...
+ j = /\t \xs::[t] -> rhs
+ in
+ case (case e of ...) of
+ C t xs::[t] -> j t xs
Note [Join point abstaction]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~
where v::State# RealWorld#. The value passed to this function
is realworld#, which generates (almost) no code.
-There's a slight infelicity here: we pass the overall
+There's a slight infelicity here: we pass the overall
case_bndr to all the join points if it's used in *any* RHS,
because we don't know its usage in each RHS separately
We used to say "&& isUnLiftedType rhs_ty'" here, but now
we make the join point into a function whenever used_bndrs'
-is empty. This makes the join-point more CPR friendly.
-Consider: let j = if .. then I# 3 else I# 4
- in case .. of { A -> j; B -> j; C -> ... }
+is empty. This makes the join-point more CPR friendly.
+Consider: let j = if .. then I# 3 else I# 4
+ in case .. of { A -> j; B -> j; C -> ... }
Now CPR doesn't w/w j because it's a thunk, so
that means that the enclosing function can't w/w either,
which is a lose. Here's the example that happened in practice:
- kgmod :: Int -> Int -> Int
- kgmod x y = if x > 0 && y < 0 || x < 0 && y > 0
- then 78
- else 5
+ kgmod :: Int -> Int -> Int
+ kgmod x y = if x > 0 && y < 0 || x < 0 && y > 0
+ then 78
+ else 5
I have seen a case alternative like this:
- True -> \v -> ...
+ True -> \v -> ...
It's a bit silly to add the realWorld dummy arg in this case, making
- $j = \s v -> ...
- True -> $j s
+ $j = \s v -> ...
+ True -> $j s
(the \v alone is enough to make CPR happy) but I think it's rare
Note [Duplicating strict continuations]
Do *not* duplicate StrictBind and StritArg continuations. We gain
nothing by propagating them into the expressions, and we do lose a
lot. Here's an example:
- && (case x of { T -> F; F -> T }) E
+ && (case x of { T -> F; F -> T }) E
Now, && is strict so we end up simplifying the case with
an ArgOf continuation. If we let-bind it, we get
- let $j = \v -> && v E
- in simplExpr (case x of { T -> F; F -> T })
- (ArgOf (\r -> $j r)
+ let $j = \v -> && v E
+ in simplExpr (case x of { T -> F; F -> T })
+ (ArgOf (\r -> $j r)
And after simplifying more we get
- let $j = \v -> && v E
- in case x of { T -> $j F; F -> $j T }
+ let $j = \v -> && v E
+ in case x of { T -> $j F; F -> $j T }
Which is a Very Bad Thing
The desire not to duplicate is the entire reason that
mkDupableCont returns a pair of continuations.
The original plan had:
-e.g. (...strict-fn...) [...hole...]
- ==>
- let $j = \a -> ...strict-fn...
- in $j [...hole...]
+e.g. (...strict-fn...) [...hole...]
+ ==>
+ let $j = \a -> ...strict-fn...
+ in $j [...hole...]
Note [Single-alternative cases]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
This case is just like the ArgOf case. Here's an example:
- data T a = MkT !a
- ...(MkT (abs x))...
+ data T a = MkT !a
+ ...(MkT (abs x))...
Then we get
- case (case x of I# x' ->
- case x' <# 0# of
- True -> I# (negate# x')
- False -> I# x') of y {
- DEFAULT -> MkT y
+ case (case x of I# x' ->
+ case x' <# 0# of
+ True -> I# (negate# x')
+ False -> I# x') of y {
+ DEFAULT -> MkT y
Because the (case x) has only one alternative, we'll transform to
- case x of I# x' ->
- case (case x' <# 0# of
- True -> I# (negate# x')
- False -> I# x') of y {
- DEFAULT -> MkT y
-But now we do *NOT* want to make a join point etc, giving
- case x of I# x' ->
- let $j = \y -> MkT y
- in case x' <# 0# of
- True -> $j (I# (negate# x'))
- False -> $j (I# x')
+ case x of I# x' ->
+ case (case x' <# 0# of
+ True -> I# (negate# x')
+ False -> I# x') of y {
+ DEFAULT -> MkT y
+But now we do *NOT* want to make a join point etc, giving
+ case x of I# x' ->
+ let $j = \y -> MkT y
+ in case x' <# 0# of
+ True -> $j (I# (negate# x'))
+ False -> $j (I# x')
In this case the $j will inline again, but suppose there was a big
strict computation enclosing the orginal call to MkT. Then, it won't
"see" the MkT any more, because it's big and won't get duplicated.
And, what is worse, nothing was gained by the case-of-case transform.
-When should use this case of mkDupableCont?
+When should use this case of mkDupableCont?
However, matching on *any* single-alternative case is a *disaster*;
- e.g. case (case ....) of (a,b) -> (# a,b #)
+ e.g. case (case ....) of (a,b) -> (# a,b #)
We must push the outer case into the inner one!
Other choices:
- * Match [(DEFAULT,_,_)], but in the common case of Int,
+ * Match [(DEFAULT,_,_)], but in the common case of Int,
the alternative-filling-in code turned the outer case into
- case (...) of y { I# _ -> MkT y }
+ case (...) of y { I# _ -> MkT y }
* Match on single alternative plus (not (isDeadBinder case_bndr))
Rationale: pushing the case inwards won't eliminate the construction.
But there's a risk of
- case (...) of y { (a,b) -> let z=(a,b) in ... }
+ case (...) of y { (a,b) -> let z=(a,b) in ... }
Now y looks dead, but it'll come alive again. Still, this
seems like the best option at the moment.
case_cont *btoo, because case_cont might be big!
HOWEVER: I found that this version doesn't work well, because
- we can get let x = case (...) of { small } in ...case x...
+ we can get let x = case (...) of { small } in ...case x...
When x is inlined into its full context, we find that it was a bad
idea to have pushed the outer case inside the (...) case.
+Note [Single-alternative-unlifted]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Here's another single-alternative where we really want to do case-of-case:
+
+data Mk1 = Mk1 Int#
+data Mk1 = Mk2 Int#
+
+M1.f =
+ \r [x_s74 y_s6X]
+ case
+ case y_s6X of tpl_s7m {
+ M1.Mk1 ipv_s70 -> ipv_s70;
+ M1.Mk2 ipv_s72 -> ipv_s72;
+ }
+ of
+ wild_s7c
+ { __DEFAULT ->
+ case
+ case x_s74 of tpl_s7n {
+ M1.Mk1 ipv_s77 -> ipv_s77;
+ M1.Mk2 ipv_s79 -> ipv_s79;
+ }
+ of
+ wild1_s7b
+ { __DEFAULT -> ==# [wild1_s7b wild_s7c];
+ };
+ };
+
+So the outer case is doing *nothing at all*, other than serving as a
+join-point. In this case we really want to do case-of-case and decide
+whether to use a real join point or just duplicate the continuation.
+
+Hence: check whether the case binder's type is unlifted, because then
+the outer case is *not* a seq.
projects / ghc-hetmet.git / blobdiff