import CoreUtils ( coreExprType, nonErrorRHSs, maybeErrorApp,
unTagBinders, squashableDictishCcExpr
)
-import Id ( idType, idWantsToBeINLINEd, idMustNotBeINLINEd, addIdArity, getIdArity,
+import Id ( idType, idMustBeINLINEd, idWantsToBeINLINEd, idMustNotBeINLINEd,
+ addIdArity, getIdArity,
getIdDemandInfo, addIdDemandInfo,
GenId{-instance NamedThing-}
)
import SimplVar ( completeVar )
import Unique ( Unique )
import SimplUtils
-import Type ( mkTyVarTy, mkTyVarTys, mkAppTy, applyTy, mkFunTys,
+import Type ( mkTyVarTy, mkTyVarTys, mkAppTy, applyTy, mkFunTys, maybeAppDataTyCon,
splitFunTy, splitFunTyExpandingDicts, getFunTy_maybe, eqTy
)
import TysWiredIn ( realWorldStateTy )
-> InExpr
-> OutId -- The new binder (used only for its type)
-> SmplM (OutExpr, ArityInfo)
+\end{code}
--- First a special case for variable right-hand sides
--- v = w
--- It's OK to simplify the RHS, but it's often a waste of time. Often
--- these v = w things persist because v is exported, and w is used
--- elsewhere. So if we're not careful we'll eta expand the rhs, only
--- to eta reduce it in competeNonRec.
---
--- If we leave the binding unchanged, we will certainly replace v by w at
--- every occurrence of v, which is good enough.
---
--- In fact, it's better to replace v by w than to inline w in v's rhs,
--- even if this is the only occurrence of w. Why? Because w might have
--- IdInfo (like strictness) that v doesn't.
+First a special case for variable right-hand sides
+ v = w
+It's OK to simplify the RHS, but it's often a waste of time. Often
+these v = w things persist because v is exported, and w is used
+elsewhere. So if we're not careful we'll eta expand the rhs, only
+to eta reduce it in competeNonRec.
+
+If we leave the binding unchanged, we will certainly replace v by w at
+every occurrence of v, which is good enough.
+
+In fact, it's *better* to replace v by w than to inline w in v's rhs,
+even if this is the only occurrence of w. Why? Because w might have
+IdInfo (like strictness) that v doesn't.
+Furthermore, there might be other uses of w; if so, inlining w in
+v's rhs will duplicate w's rhs, whereas replacing v by w doesn't.
+
+HOWEVER, we have to be careful if w is something that *must* be
+inlined. In particular, its binding may have been dropped. Here's
+an example that actually happened:
+ let x = let y = e in y
+ in f x
+The "let y" was floated out, and then (since y occurs once in a
+definitely inlinable position) the binding was dropped, leaving
+ {y=e} let x = y in f x
+But now using the reasoning of this little section,
+y wasn't inlined, because it was a let x=y form.
+\begin{code}
simplRhsExpr env binder@(id,occ_info) (Var v) new_id
- = case (runEager $ lookupId env v) of
- LitArg lit -> returnSmpl (Lit lit, ArityExactly 0)
- VarArg v' -> returnSmpl (Var v', getIdArity v')
+ | maybeToBool maybe_stop_at_var
+ = returnSmpl (Var the_var, getIdArity the_var)
+ where
+ maybe_stop_at_var
+ = case (runEager $ lookupId env v) of
+ VarArg v' | not (must_unfold v') -> Just v'
+ other -> Nothing
+
+ Just the_var = maybe_stop_at_var
+
+ must_unfold v' = idMustBeINLINEd v'
+ || case lookupOutIdEnv env v' of
+ Just (_, _, InUnfolding _ _) -> True
+ other -> False
+\end{code}
+\begin{code}
simplRhsExpr env binder@(id,occ_info) rhs new_id
+ | maybeToBool (maybeAppDataTyCon rhs_ty)
+ -- Deal with the data type case, in which case the elaborate
+ -- eta-expansion nonsense is really quite a waste of time.
+ = simplExpr rhs_env rhs [] rhs_ty `thenSmpl` \ rhs' ->
+ returnSmpl (rhs', ArityExactly 0)
+
+ | otherwise -- OK, use the big hammer
= -- Deal with the big lambda part
ASSERT( null uvars ) -- For now
mapSmpl cloneTyVarSmpl tyvars `thenSmpl` \ tyvars' ->
let
- rhs_ty = idType new_id
new_tys = mkTyVarTys tyvars'
body_ty = foldl applyTy rhs_ty new_tys
lam_env = extendTyEnvList rhs_env (zipEqual "simplRhsExpr" tyvars new_tys)
returnSmpl (rhs', arity)
where
+ rhs_ty = idType new_id
rhs_env | idWantsToBeINLINEd id -- Don't ever inline in a INLINE thing's rhs
= switchOffInlining env1 -- See comments with switchOffInlining
| otherwise
%************************************************************************
%* *
-\subsection[Simplify-let]{Let-expressions}
+\subsection[Simplify-bind]{Binding groups}
%* *
%************************************************************************
-> (SimplEnv -> SmplM OutExpr)
-> OutType
-> SmplM OutExpr
+
+simplBind env (NonRec binder rhs) body_c body_ty = simplNonRec env binder rhs body_c body_ty
+simplBind env (Rec pairs) body_c body_ty = simplRec env pairs body_c body_ty
\end{code}
+
+%************************************************************************
+%* *
+\subsection[Simplify-let]{Let-expressions}
+%* *
+%************************************************************************
+
+Float switches
+~~~~~~~~~~~~~~
+The booleans controlling floating have to be set with a little care.
+Here's one performance bug I found:
+
+ let x = let y = let z = case a# +# 1 of {b# -> E1}
+ in E2
+ in E3
+ in E4
+
+Now, if E2, E3 aren't HNFs we won't float the y-binding or the z-binding.
+Before case_floating_ok included float_exposes_hnf, the case expression was floated
+*one level per simplifier iteration* outwards. So it made th s
+
+
+Floating case from let
+~~~~~~~~~~~~~~~~~~~~~~
When floating cases out of lets, remember this:
let x* = case e of alts
ToDo: check this is OK with andy
+Let to case: two points
+~~~~~~~~~~~
+
+Point 1. We defer let-to-case for all data types except single-constructor
+ones. Suppose we change
+
+ let x* = e in b
+to
+ case e of x -> b
+
+It can be the case that we find that b ultimately contains ...(case x of ..)....
+and this is the only occurrence of x. Then if we've done let-to-case
+we can't inline x, which is a real pain. On the other hand, we lose no
+transformations by not doing this transformation, because the relevant
+case-of-X transformations are also implemented by simpl_bind.
+
+If x is a single-constructor type, then we go ahead anyway, giving
+
+ case e of (y,z) -> let x = (y,z) in b
+
+because now we can squash case-on-x wherever they occur in b.
+
+We do let-to-case on multi-constructor types in the tidy-up phase
+(tidyCoreExpr) mainly so that the code generator doesn't need to
+spot the demand-flag.
+
+
+Point 2. It's important to try let-to-case before doing the
+strict-let-of-case transformation, which happens in the next equation
+for simpl_bind.
+
+ let a*::Int = case v of {p1->e1; p2->e2}
+ in b
+
+(The * means that a is sure to be demanded.)
+If we do case-floating first we get this:
+
+ let k = \a* -> b
+ in case v of
+ p1-> let a*=e1 in k a
+ p2-> let a*=e2 in k a
+
+Now watch what happens if we do let-to-case first:
+
+ case (case v of {p1->e1; p2->e2}) of
+ Int a# -> let a*=I# a# in b
+===>
+ let k = \a# -> let a*=I# a# in b
+ in case v of
+ p1 -> case e1 of I# a# -> k a#
+ p1 -> case e2 of I# a# -> k a#
+
+The latter is clearly better. (Remember the reboxing let-decl for a
+is likely to go away, because after all b is strict in a.)
+
+We do not do let to case for WHNFs, e.g.
+
+ let x = a:b in ...
+ =/=>
+ case a:b of x in ...
+
+as this is less efficient. but we don't mind doing let-to-case for
+"bottom", as that will allow us to remove more dead code, if anything:
+
+ let x = error in ...
+ ===>
+ case error of x -> ...
+ ===>
+ error
+
+Notice that let to case occurs only if x is used strictly in its body
+(obviously).
+
\begin{code}
-- Dead code is now discarded by the occurrence analyser,
-simplBind env (NonRec binder@(id,occ_info) rhs) body_c body_ty
+simplNonRec env binder@(id,occ_info) rhs body_c body_ty
+ | inlineUnconditionally ok_to_dup occ_info
+ = -- The binder is used in definitely-inline way in the body
+ -- So add it to the environment, drop the binding, and continue
+ body_c (extendEnvGivenInlining env id occ_info rhs)
+
| idWantsToBeINLINEd id
= complete_bind env rhs -- Don't mess about with floating or let-to-case on
-- INLINE things
bindLargeRhs env [binder] body_ty body_c `thenSmpl` \ (extra_binding, new_body) ->
let
body_c' = \env -> simplExpr env new_body [] body_ty
- case_c = \env rhs -> simplBind env (NonRec binder rhs) body_c' body_ty
+ case_c = \env rhs -> simplNonRec env binder rhs body_c' body_ty
in
simplCase env scrut alts case_c body_ty `thenSmpl` \ case_expr ->
returnSmpl (Let extra_binding case_expr)
-- See note below
\end{code}
-Float switches
-~~~~~~~~~~~~~~
-The booleans controlling floating have to be set with a little care.
-Here's one performance bug I found:
-
- let x = let y = let z = case a# +# 1 of {b# -> E1}
- in E2
- in E3
- in E4
-
-Now, if E2, E3 aren't HNFs we won't float the y-binding or the z-binding.
-Before case_floating_ok included float_exposes_hnf, the case expression was floated
-*one level per simplifier iteration* outwards. So it made th s
-
-Let to case: two points
-~~~~~~~~~~~
-
-Point 1. We defer let-to-case for all data types except single-constructor
-ones. Suppose we change
-
- let x* = e in b
-to
- case e of x -> b
-
-It can be the case that we find that b ultimately contains ...(case x of ..)....
-and this is the only occurrence of x. Then if we've done let-to-case
-we can't inline x, which is a real pain. On the other hand, we lose no
-transformations by not doing this transformation, because the relevant
-case-of-X transformations are also implemented by simpl_bind.
-
-If x is a single-constructor type, then we go ahead anyway, giving
-
- case e of (y,z) -> let x = (y,z) in b
-
-because now we can squash case-on-x wherever they occur in b.
-
-We do let-to-case on multi-constructor types in the tidy-up phase
-(tidyCoreExpr) mainly so that the code generator doesn't need to
-spot the demand-flag.
-
-
-Point 2. It's important to try let-to-case before doing the
-strict-let-of-case transformation, which happens in the next equation
-for simpl_bind.
-
- let a*::Int = case v of {p1->e1; p2->e2}
- in b
-
-(The * means that a is sure to be demanded.)
-If we do case-floating first we get this:
-
- let k = \a* -> b
- in case v of
- p1-> let a*=e1 in k a
- p2-> let a*=e2 in k a
-
-Now watch what happens if we do let-to-case first:
-
- case (case v of {p1->e1; p2->e2}) of
- Int a# -> let a*=I# a# in b
-===>
- let k = \a# -> let a*=I# a# in b
- in case v of
- p1 -> case e1 of I# a# -> k a#
- p1 -> case e2 of I# a# -> k a#
-
-The latter is clearly better. (Remember the reboxing let-decl for a
-is likely to go away, because after all b is strict in a.)
-
-We do not do let to case for WHNFs, e.g.
-
- let x = a:b in ...
- =/=>
- case a:b of x in ...
-
-as this is less efficient. but we don't mind doing let-to-case for
-"bottom", as that will allow us to remove more dead code, if anything:
-
- let x = error in ...
- ===>
- case error of x -> ...
- ===>
- error
-
-Notice that let to case occurs only if x is used strictly in its body
-(obviously).
-
-
-Letrec expressions
-~~~~~~~~~~~~~~~~~~
-
-Simplify each RHS, float any let(recs) from the RHSs (if let-floating is
-on and it'll expose a HNF), and bang the whole resulting mess together
-into a huge letrec.
-
-1. Any "macros" should be expanded. The main application of this
-macro-expansion is:
-
- letrec
- f = ....g...
- g = ....f...
- in
- ....f...
-
-Here we would like the single call to g to be inlined.
-
-We can spot this easily, because g will be tagged as having just one
-occurrence. The "inlineUnconditionally" predicate is just what we want.
-
-A worry: could this lead to non-termination? For example:
-
- letrec
- f = ...g...
- g = ...f...
- h = ...h...
- in
- ..h..
-
-Here, f and g call each other (just once) and neither is used elsewhere.
-But it's OK:
-
-* the occurrence analyser will drop any (sub)-group that isn't used at
- all.
-
-* If the group is used outside itself (ie in the "in" part), then there
- can't be a cyle.
-
-** IMPORTANT: check that NewOccAnal has the property that a group of
- bindings like the above has f&g dropped.! ***
-
-
-2. We'd also like to pull out any top-level let(rec)s from the
-rhs of the defns:
-
- letrec
- f = let h = ... in \x -> ....h...f...h...
- in
- ...f...
-====>
- letrec
- h = ...
- f = \x -> ....h...f...h...
- in
- ...f...
-
-But floating cases is less easy? (Don't for now; ToDo?)
-
-
-3. We'd like to arrange that the RHSs "know" about members of the
-group that are bound to constructors. For example:
-
- let rec
- d.Eq = (==,/=)
- f a b c d = case d.Eq of (h,_) -> let x = (a,b); y = (c,d) in not (h x y)
- /= a b = unpack tuple a, unpack tuple b, call f
- in d.Eq
-
-here, by knowing about d.Eq in f's rhs, one could get rid of
-the case (and break out the recursion completely).
-[This occurred with more aggressive inlining threshold (4),
-nofib/spectral/knights]
-
-How to do it?
- 1: we simplify constructor rhss first.
- 2: we record the "known constructors" in the environment
- 3: we simplify the other rhss, with the knowledge about the constructors
-
-
-
-\begin{code}
-simplBind env (Rec pairs) body_c body_ty
- = -- Do floating, if necessary
- floatBind env False (Rec pairs) `thenSmpl` \ [Rec pairs'] ->
- let
- binders = map fst pairs'
- in
- cloneIds env binders `thenSmpl` \ ids' ->
- let
- env_w_clones = extendIdEnvWithClones env binders ids'
- in
- simplRecursiveGroup env_w_clones ids' pairs' `thenSmpl` \ (pairs', new_env) ->
-
- body_c new_env `thenSmpl` \ body' ->
-
- returnSmpl (Let (Rec pairs') body')
-\end{code}
-
-\begin{code}
--- The env passed to simplRecursiveGroup already has
--- bindings that clone the variables of the group.
-simplRecursiveGroup env new_ids []
- = returnSmpl ([], env)
-
-simplRecursiveGroup env (new_id : new_ids) ((binder@(_, occ_info), rhs) : pairs)
- = simplRhsExpr env binder rhs new_id `thenSmpl` \ (new_rhs, arity) ->
- let
- new_id' = new_id `withArity` arity
-
- -- ToDo: this next bit could usefully share code with completeNonRec
-
- new_env
- | idMustNotBeINLINEd new_id -- Occurrence analyser says "don't inline"
- = env
-
- | is_atomic eta'd_rhs -- If rhs (after eta reduction) is atomic
- = extendIdEnvWithAtom env binder the_arg
-
- | otherwise -- Non-atomic
- = extendEnvGivenBinding env occ_info new_id new_rhs
- -- Don't eta if it doesn't eliminate the binding
-
- eta'd_rhs = etaCoreExpr new_rhs
- the_arg = case eta'd_rhs of
- Var v -> VarArg v
- Lit l -> LitArg l
- in
- simplRecursiveGroup new_env new_ids pairs `thenSmpl` \ (new_pairs, final_env) ->
- returnSmpl ((new_id', new_rhs) : new_pairs, final_env)
-\end{code}
-
-@completeLet@ looks at the simplified post-floating RHS of the
+@completeNonRec@ looks at the simplified post-floating RHS of the
let-expression, and decides what to do. There's one interesting
aspect to this, namely constructor reuse. Consider
@
Lit l -> LitArg l
\end{code}
+%************************************************************************
+%* *
+\subsection[Simplify-letrec]{Letrec-expressions}
+%* *
+%************************************************************************
+
+Letrec expressions
+~~~~~~~~~~~~~~~~~~
+Here's the game plan
+
+1. Float any let(rec)s out of the RHSs
+2. Clone all the Ids and extend the envt with these clones
+3. Simplify one binding at a time, adding each binding to the
+ environment once it's done.
+
+This relies on the occurrence analyser to
+ a) break all cycles with an Id marked MustNotBeInlined
+ b) sort the decls into topological order
+The former prevents infinite inlinings, and the latter means
+that we get maximum benefit from working top to bottom.
+
+
+\begin{code}
+simplRec env pairs body_c body_ty
+ = -- Do floating, if necessary
+ floatBind env False (Rec pairs) `thenSmpl` \ [Rec pairs'] ->
+ let
+ binders = map fst pairs'
+ in
+ cloneIds env binders `thenSmpl` \ ids' ->
+ let
+ env_w_clones = extendIdEnvWithClones env binders ids'
+ in
+ simplRecursiveGroup env_w_clones ids' pairs' `thenSmpl` \ (pairs', new_env) ->
+
+ body_c new_env `thenSmpl` \ body' ->
+
+ returnSmpl (Let (Rec pairs') body')
+\end{code}
+
+\begin{code}
+-- The env passed to simplRecursiveGroup already has
+-- bindings that clone the variables of the group.
+simplRecursiveGroup env new_ids []
+ = returnSmpl ([], env)
+
+simplRecursiveGroup env (new_id : new_ids) ((binder@(_, occ_info), rhs) : pairs)
+ | inlineUnconditionally ok_to_dup occ_info
+ = -- Single occurrence, so drop binding and extend env with the inlining
+ let
+ new_env = extendEnvGivenInlining env new_id occ_info rhs
+ in
+ simplRecursiveGroup new_env new_ids pairs
+
+ | otherwise
+ = simplRhsExpr env binder rhs new_id `thenSmpl` \ (new_rhs, arity) ->
+ let
+ new_id' = new_id `withArity` arity
+
+ -- ToDo: this next bit could usefully share code with completeNonRec
+
+ new_env
+ | idMustNotBeINLINEd new_id -- Occurrence analyser says "don't inline"
+ = env
+
+ | is_atomic eta'd_rhs -- If rhs (after eta reduction) is atomic
+ = extendIdEnvWithAtom env binder the_arg
+
+ | otherwise -- Non-atomic
+ = extendEnvGivenBinding env occ_info new_id new_rhs
+ -- Don't eta if it doesn't eliminate the binding
+
+ eta'd_rhs = etaCoreExpr new_rhs
+ the_arg = case eta'd_rhs of
+ Var v -> VarArg v
+ Lit l -> LitArg l
+ in
+ simplRecursiveGroup new_env new_ids pairs `thenSmpl` \ (new_pairs, final_env) ->
+ returnSmpl ((new_id', new_rhs) : new_pairs, final_env)
+ where
+ ok_to_dup = switchIsSet env SimplOkToDupCode
+\end{code}
+
+
\begin{code}
floatBind :: SimplEnv
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