% (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
%
%************************************************************************
-%* *
+%* *
\section[OccurAnal]{Occurrence analysis pass}
-%* *
+%* *
%************************************************************************
The occurrence analyser re-typechecks a core expression, returning a new
\begin{code}
module OccurAnal (
- occurAnalysePgm, occurAnalyseExpr
+ occurAnalysePgm, occurAnalyseExpr
) where
#include "HsVersions.h"
import CoreSyn
-import CoreFVs ( idRuleVars )
-import CoreUtils ( exprIsTrivial, isDefaultAlt )
-import Id ( isDataConWorkId, isOneShotBndr, setOneShotLambda,
- idOccInfo, setIdOccInfo, isLocalId,
- isExportedId, idArity, idHasRules,
- idUnique, Id
- )
-import BasicTypes ( OccInfo(..), isOneOcc, InterestingCxt )
+import CoreFVs
+import CoreUtils ( exprIsTrivial, isDefaultAlt )
+import Coercion ( mkSymCoercion )
+import Id
+import IdInfo
+import BasicTypes
import VarSet
import VarEnv
-import Maybes ( orElse )
-import Digraph ( stronglyConnCompR, SCC(..) )
-import PrelNames ( buildIdKey, foldrIdKey, runSTRepIdKey, augmentIdKey )
-import Unique ( Unique )
-import UniqFM ( keysUFM, intersectsUFM )
-import Util ( mapAndUnzip, mapAccumL )
+import Maybes ( orElse )
+import Digraph ( SCC(..), stronglyConnCompFromEdgedVerticesR )
+import PrelNames ( buildIdKey, foldrIdKey, runSTRepIdKey, augmentIdKey )
+import Unique ( Unique )
+import UniqFM ( keysUFM, intersectUFM_C, foldUFM_Directly )
+import Util ( mapAndUnzip )
import Outputable
+
+import Data.List
\end{code}
%************************************************************************
-%* *
+%* *
\subsection[OccurAnal-main]{Counting occurrences: main function}
-%* *
+%* *
%************************************************************************
Here's the externally-callable interface:
= snd (go initOccEnv binds)
where
go :: OccEnv -> [CoreBind] -> (UsageDetails, [CoreBind])
- go env []
- = (emptyDetails, [])
- go env (bind:binds)
- = (final_usage, bind' ++ binds')
- where
- (bs_usage, binds') = go env binds
- (final_usage, bind') = occAnalBind env bind bs_usage
+ go _ []
+ = (emptyDetails, [])
+ go env (bind:binds)
+ = (final_usage, bind' ++ binds')
+ where
+ (bs_usage, binds') = go env binds
+ (final_usage, bind') = occAnalBind env bind bs_usage
occurAnalyseExpr :: CoreExpr -> CoreExpr
- -- Do occurrence analysis, and discard occurence info returned
+ -- Do occurrence analysis, and discard occurence info returned
occurAnalyseExpr expr = snd (occAnal initOccEnv expr)
\end{code}
%************************************************************************
-%* *
+%* *
\subsection[OccurAnal-main]{Counting occurrences: main function}
-%* *
+%* *
%************************************************************************
Bindings
\begin{code}
occAnalBind :: OccEnv
- -> CoreBind
- -> UsageDetails -- Usage details of scope
- -> (UsageDetails, -- Of the whole let(rec)
- [CoreBind])
+ -> CoreBind
+ -> UsageDetails -- Usage details of scope
+ -> (UsageDetails, -- Of the whole let(rec)
+ [CoreBind])
occAnalBind env (NonRec binder rhs) body_usage
- | not (binder `usedIn` body_usage) -- It's not mentioned
+ | isTyVar binder -- A type let; we don't gather usage info
+ = (body_usage, [NonRec binder rhs])
+
+ | not (binder `usedIn` body_usage) -- It's not mentioned
= (body_usage, [])
- | otherwise -- It's mentioned in the body
- = (body_usage' +++ addRuleUsage rhs_usage binder, -- Note [RulesOnly]
+ | otherwise -- It's mentioned in the body
+ = (body_usage' +++ addRuleUsage rhs_usage binder, -- Note [Rules are extra RHSs]
[NonRec tagged_binder rhs'])
where
(body_usage', tagged_binder) = tagBinder body_usage binder
- (rhs_usage, rhs') = occAnalRhs env tagged_binder rhs
+ (rhs_usage, rhs') = occAnalRhs env tagged_binder rhs
\end{code}
+Note [Dead code]
+~~~~~~~~~~~~~~~~
Dropping dead code for recursive bindings is done in a very simple way:
- the entire set of bindings is dropped if none of its binders are
- mentioned in its body; otherwise none are.
+ the entire set of bindings is dropped if none of its binders are
+ mentioned in its body; otherwise none are.
This seems to miss an obvious improvement.
-@
- letrec f = ...g...
- g = ...f...
- in
- ...g...
+ letrec f = ...g...
+ g = ...f...
+ in
+ ...g...
===>
+ letrec f = ...g...
+ g = ...(...g...)...
+ in
+ ...g...
+
+Now 'f' is unused! But it's OK! Dependency analysis will sort this
+out into a letrec for 'g' and a 'let' for 'f', and then 'f' will get
+dropped. It isn't easy to do a perfect job in one blow. Consider
+
+ letrec f = ...g...
+ g = ...h...
+ h = ...k...
+ k = ...m...
+ m = ...m...
+ in
+ ...m...
+
+
+Note [Loop breaking and RULES]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Loop breaking is surprisingly subtle. First read the section 4 of
+"Secrets of the GHC inliner". This describes our basic plan.
+
+However things are made quite a bit more complicated by RULES. Remember
+
+ * Note [Rules are extra RHSs]
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~
+ A RULE for 'f' is like an extra RHS for 'f'. That way the "parent"
+ keeps the specialised "children" alive. If the parent dies
+ (because it isn't referenced any more), then the children will die
+ too (unless they are already referenced directly).
+
+ To that end, we build a Rec group for each cyclic strongly
+ connected component,
+ *treating f's rules as extra RHSs for 'f'*.
+
+ When we make the Rec groups we include variables free in *either*
+ LHS *or* RHS of the rule. The former might seems silly, but see
+ Note [Rule dependency info].
+
+ So in Example [eftInt], eftInt and eftIntFB will be put in the
+ same Rec, even though their 'main' RHSs are both non-recursive.
+
+ * Note [Rules are visible in their own rec group]
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+ We want the rules for 'f' to be visible in f's right-hand side.
+ And we'd like them to be visible in other functions in f's Rec
+ group. E.g. in Example [Specialisation rules] we want f' rule
+ to be visible in both f's RHS, and fs's RHS.
+
+ This means that we must simplify the RULEs first, before looking
+ at any of the definitions. This is done by Simplify.simplRecBind,
+ when it calls addLetIdInfo.
+
+ * Note [Choosing loop breakers]
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+ We avoid infinite inlinings by choosing loop breakers, and
+ ensuring that a loop breaker cuts each loop. But what is a
+ "loop"? In particular, a RULE is like an equation for 'f' that
+ is *always* inlined if it is applicable. We do *not* disable
+ rules for loop-breakers. It's up to whoever makes the rules to
+ make sure that the rules themselves alwasys terminate. See Note
+ [Rules for recursive functions] in Simplify.lhs
+
+ Hence, if
+ f's RHS mentions g, and
+ g has a RULE that mentions h, and
+ h has a RULE that mentions f
+
+ then we *must* choose f to be a loop breaker. In general, take the
+ free variables of f's RHS, and augment it with all the variables
+ reachable by RULES from those starting points. That is the whole
+ reason for computing rule_fv_env in occAnalBind. (Of course we
+ only consider free vars that are also binders in this Rec group.)
+
+ Note that when we compute this rule_fv_env, we only consider variables
+ free in the *RHS* of the rule, in contrast to the way we build the
+ Rec group in the first place (Note [Rule dependency info])
+
+ Note that in Example [eftInt], *neither* eftInt *nor* eftIntFB is
+ chosen as a loop breaker, because their RHSs don't mention each other.
+ And indeed both can be inlined safely.
+
+ Note that the edges of the graph we use for computing loop breakers
+ are not the same as the edges we use for computing the Rec blocks.
+ That's why we compute
+ rec_edges for the Rec block analysis
+ loop_breaker_edges for the loop breaker analysis
+
+
+ * Note [Weak loop breakers]
+ ~~~~~~~~~~~~~~~~~~~~~~~~~
+ There is a last nasty wrinkle. Suppose we have
+
+ Rec { f = f_rhs
+ RULE f [] = g
+
+ h = h_rhs
+ g = h
+ ...more...
+ }
+
+ Remmber that we simplify the RULES before any RHS (see Note
+ [Rules are visible in their own rec group] above).
+
+ So we must *not* postInlineUnconditionally 'g', even though
+ its RHS turns out to be trivial. (I'm assuming that 'g' is
+ not choosen as a loop breaker.)
+
+ We "solve" this by making g a "weak" or "rules-only" loop breaker,
+ with OccInfo = IAmLoopBreaker True. A normal "strong" loop breaker
+ has IAmLoopBreaker False. So
+
+ Inline postInlineUnconditinoally
+ IAmLoopBreaker False no no
+ IAmLoopBreaker True yes no
+ other yes yes
+
+ The **sole** reason for this kind of loop breaker is so that
+ postInlineUnconditionally does not fire. Ugh.
+
+ * Note [Rule dependency info]
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~
+ The VarSet in a SpecInfo is used for dependency analysis in the
+ occurrence analyser. We must track free vars in *both* lhs and rhs.
+ Hence use of idRuleVars, rather than idRuleRhsVars in addRuleUsage.
+ Why both? Consider
+ x = y
+ RULE f x = 4
+ Then if we substitute y for x, we'd better do so in the
+ rule's LHS too, so we'd better ensure the dependency is respected
+
+
+Example [eftInt]
+~~~~~~~~~~~~~~~
+Example (from GHC.Enum):
+
+ eftInt :: Int# -> Int# -> [Int]
+ eftInt x y = ...(non-recursive)...
+
+ {-# INLINE [0] eftIntFB #-}
+ eftIntFB :: (Int -> r -> r) -> r -> Int# -> Int# -> r
+ eftIntFB c n x y = ...(non-recursive)...
+
+ {-# RULES
+ "eftInt" [~1] forall x y. eftInt x y = build (\ c n -> eftIntFB c n x y)
+ "eftIntList" [1] eftIntFB (:) [] = eftInt
+ #-}
+
+Example [Specialisation rules]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Consider this group, which is typical of what SpecConstr builds:
+
+ fs a = ....f (C a)....
+ f x = ....f (C a)....
+ {-# RULE f (C a) = fs a #-}
- letrec f = ...g...
- g = ...(...g...)...
- in
- ...g...
-@
+So 'f' and 'fs' are in the same Rec group (since f refers to fs via its RULE).
-Now @f@ is unused. But dependency analysis will sort this out into a
-@letrec@ for @g@ and a @let@ for @f@, and then @f@ will get dropped.
-It isn't easy to do a perfect job in one blow. Consider
+But watch out! If 'fs' is not chosen as a loop breaker, we may get an infinite loop:
+ - the RULE is applied in f's RHS (see Note [Self-recursive rules] in Simplify
+ - fs is inlined (say it's small)
+ - now there's another opportunity to apply the RULE
-@
- letrec f = ...g...
- g = ...h...
- h = ...k...
- k = ...m...
- m = ...m...
- in
- ...m...
-@
+This showed up when compiling Control.Concurrent.Chan.getChanContents.
\begin{code}
occAnalBind env (Rec pairs) body_usage
- = foldr (_scc_ "occAnalBind.dofinal" do_final_bind) (body_usage, []) sccs
+ = foldr occAnalRec (body_usage, []) sccs
+ -- For a recursive group, we
+ -- * occ-analyse all the RHSs
+ -- * compute strongly-connected components
+ -- * feed those components to occAnalRec
where
- analysed_pairs :: [Details]
- analysed_pairs = [ (bndr, rhs_usage, rhs')
- | (bndr, rhs) <- pairs,
- let (rhs_usage, rhs') = occAnalRhs env bndr rhs
- ]
+ -------------Dependency analysis ------------------------------
+ bndr_set = mkVarSet (map fst pairs)
sccs :: [SCC (Node Details)]
- sccs = _scc_ "occAnalBind.scc" stronglyConnCompR edges
-
-
- ---- stuff for dependency analysis of binds -------------------------------
- edges :: [Node Details]
- edges = _scc_ "occAnalBind.assoc"
- [ (details, idUnique id, edges_from id rhs_usage)
- | details@(id, rhs_usage, rhs) <- analysed_pairs
- ]
-
- -- (a -> b) means a mentions b
- -- Given the usage details (a UFM that gives occ info for each free var of
- -- the RHS) we can get the list of free vars -- or rather their Int keys --
- -- by just extracting the keys from the finite map. Grimy, but fast.
- -- Previously we had this:
- -- [ bndr | bndr <- bndrs,
- -- maybeToBool (lookupVarEnv rhs_usage bndr)]
- -- which has n**2 cost, and this meant that edges_from alone
- -- consumed 10% of total runtime!
- edges_from :: Id -> UsageDetails -> [Unique]
- edges_from bndr rhs_usage = _scc_ "occAnalBind.edges_from"
- keysUFM (addRuleUsage rhs_usage bndr)
-
- ---- Stuff to "re-constitute" bindings from dependency-analysis info ------
-
- -- Non-recursive SCC
- do_final_bind (AcyclicSCC ((bndr, rhs_usage, rhs'), _, _)) (body_usage, binds_so_far)
- | not (bndr `usedIn` body_usage)
- = (body_usage, binds_so_far) -- Dead code
- | otherwise
- = (body_usage' +++ addRuleUsage rhs_usage bndr, new_bind : binds_so_far)
- where
- (body_usage', tagged_bndr) = tagBinder body_usage bndr
- new_bind = NonRec tagged_bndr rhs'
-
- -- Recursive SCC
- do_final_bind (CyclicSCC cycle) (body_usage, binds_so_far)
- | not (any (`usedIn` body_usage) bndrs) -- NB: look at body_usage, not total_usage
- = (body_usage, binds_so_far) -- Dead code
- | otherwise -- If any is used, they all are
- = (final_usage, final_bind : binds_so_far)
- where
- details = [details | (details, _, _) <- cycle]
- bndrs = [bndr | (bndr, _, _) <- details]
- bndr_usages = [addRuleUsage rhs_usage bndr | (bndr, rhs_usage, _) <- details]
- total_usage = foldr (+++) body_usage bndr_usages
- (final_usage, tagged_cycle) = mapAccumL tag_bind total_usage cycle
- tag_bind usg ((bndr,rhs_usg,rhs),k,ks) = (usg', ((bndr',rhs_usg,rhs),k,ks))
- where
- (usg', bndr') = tagBinder usg bndr
- final_bind = Rec (reOrderCycle (mkVarSet bndrs) tagged_cycle)
-
-{- An alternative; rebuild the edges. No semantic difference, but perf might change
-
- -- Hopefully 'bndrs' is a relatively small group now
- -- Now get ready for the loop-breaking phase
- -- We've done dead-code elimination already, so no worries about un-referenced binders
- keys = map idUnique bndrs
- mk_node tagged_bndr (_, rhs_usage, rhs')
- = ((tagged_bndr, rhs'), idUnique tagged_bndr, used)
- where
- used = [key | key <- keys, used_outside_rule rhs_usage key ]
+ sccs = {-# SCC "occAnalBind.scc" #-} stronglyConnCompFromEdgedVerticesR rec_edges
+
+ rec_edges :: [Node Details]
+ rec_edges = {-# SCC "occAnalBind.assoc" #-} map make_node pairs
+
+ make_node (bndr, rhs)
+ = (ND bndr rhs' rhs_usage rhs_fvs, idUnique bndr, out_edges)
+ where
+ (rhs_usage, rhs') = occAnalRhs env bndr rhs
+ rhs_fvs = intersectUFM_C (\b _ -> b) bndr_set rhs_usage
+ out_edges = keysUFM (rhs_fvs `unionVarSet` idRuleVars bndr)
+ -- (a -> b) means a mentions b
+ -- Given the usage details (a UFM that gives occ info for each free var of
+ -- the RHS) we can get the list of free vars -- or rather their Int keys --
+ -- by just extracting the keys from the finite map. Grimy, but fast.
+ -- Previously we had this:
+ -- [ bndr | bndr <- bndrs,
+ -- maybeToBool (lookupVarEnv rhs_usage bndr)]
+ -- which has n**2 cost, and this meant that edges_from alone
+ -- consumed 10% of total runtime!
+
+-----------------------------
+occAnalRec :: SCC (Node Details) -> (UsageDetails, [CoreBind])
+ -> (UsageDetails, [CoreBind])
+
+ -- The NonRec case is just like a Let (NonRec ...) above
+occAnalRec (AcyclicSCC (ND bndr rhs rhs_usage _, _, _)) (body_usage, binds)
+ | not (bndr `usedIn` body_usage)
+ = (body_usage, binds)
- used_outside_rule usage uniq = case lookupUFM_Directly usage uniq of
- Nothing -> False
- Just RulesOnly -> False -- Ignore rules
- other -> True
--}
+ | otherwise -- It's mentioned in the body
+ = (body_usage' +++ addRuleUsage rhs_usage bndr, -- Note [Rules are extra RHSs]
+ NonRec tagged_bndr rhs : binds)
+ where
+ (body_usage', tagged_bndr) = tagBinder body_usage bndr
+
+
+ -- The Rec case is the interesting one
+ -- See Note [Loop breaking]
+occAnalRec (CyclicSCC nodes) (body_usage, binds)
+ | not (any (`usedIn` body_usage) bndrs) -- NB: look at body_usage, not total_usage
+ = (body_usage, binds) -- Dead code
+
+ | otherwise -- At this point we always build a single Rec
+ = (final_usage, Rec pairs : binds)
+
+ where
+ bndrs = [b | (ND b _ _ _, _, _) <- nodes]
+ bndr_set = mkVarSet bndrs
+
+ ----------------------------
+ -- Tag the binders with their occurrence info
+ total_usage = foldl add_usage body_usage nodes
+ add_usage body_usage (ND bndr _ rhs_usage _, _, _)
+ = body_usage +++ addRuleUsage rhs_usage bndr
+ (final_usage, tagged_nodes) = mapAccumL tag_node total_usage nodes
+
+ tag_node :: UsageDetails -> Node Details -> (UsageDetails, Node Details)
+ -- (a) Tag the binders in the details with occ info
+ -- (b) Mark the binder with "weak loop-breaker" OccInfo
+ -- saying "no preInlineUnconditionally" if it is used
+ -- in any rule (lhs or rhs) of the recursive group
+ -- See Note [Weak loop breakers]
+ tag_node usage (ND bndr rhs rhs_usage rhs_fvs, k, ks)
+ = (usage `delVarEnv` bndr, (ND bndr2 rhs rhs_usage rhs_fvs, k, ks))
+ where
+ bndr2 | bndr `elemVarSet` all_rule_fvs = makeLoopBreaker True bndr1
+ | otherwise = bndr1
+ bndr1 = setBinderOcc usage bndr
+ all_rule_fvs = bndr_set `intersectVarSet` foldr (unionVarSet . idRuleVars)
+ emptyVarSet bndrs
+
+ ----------------------------
+ -- Now reconstruct the cycle
+ pairs | no_rules = reOrderCycle tagged_nodes
+ | otherwise = concatMap reOrderRec (stronglyConnCompFromEdgedVerticesR loop_breaker_edges)
+
+ -- See Note [Choosing loop breakers] for looop_breaker_edges
+ loop_breaker_edges = map mk_node tagged_nodes
+ mk_node (details@(ND _ _ _ rhs_fvs), k, _) = (details, k, new_ks)
+ where
+ new_ks = keysUFM (extendFvs rule_fv_env rhs_fvs rhs_fvs)
+
+ ------------------------------------
+ rule_fv_env :: IdEnv IdSet -- Variables from this group mentioned in RHS of rules
+ -- Domain is *subset* of bound vars (others have no rule fvs)
+ rule_fv_env = rule_loop init_rule_fvs
+
+ no_rules = null init_rule_fvs
+ init_rule_fvs = [(b, rule_fvs)
+ | b <- bndrs
+ , let rule_fvs = idRuleRhsVars b `intersectVarSet` bndr_set
+ , not (isEmptyVarSet rule_fvs)]
+
+ rule_loop :: [(Id,IdSet)] -> IdEnv IdSet -- Finds fixpoint
+ rule_loop fv_list
+ | no_change = env
+ | otherwise = rule_loop new_fv_list
+ where
+ env = mkVarEnv init_rule_fvs
+ (no_change, new_fv_list) = mapAccumL bump True fv_list
+ bump no_change (b,fvs)
+ | new_fvs `subVarSet` fvs = (no_change, (b,fvs))
+ | otherwise = (False, (b,new_fvs `unionVarSet` fvs))
+ where
+ new_fvs = extendFvs env emptyVarSet fvs
+
+idRuleRhsVars :: Id -> VarSet
+-- Just the variables free on the *rhs* of a rule
+-- See Note [Choosing loop breakers]
+idRuleRhsVars id = foldr (unionVarSet . ruleRhsFreeVars) emptyVarSet (idCoreRules id)
+
+extendFvs :: IdEnv IdSet -> IdSet -> IdSet -> IdSet
+-- (extendFVs env fvs s) returns (fvs `union` env(s))
+extendFvs env fvs id_set
+ = foldUFM_Directly add fvs id_set
+ where
+ add uniq _ fvs
+ = case lookupVarEnv_Directly env uniq of
+ Just fvs' -> fvs' `unionVarSet` fvs
+ Nothing -> fvs
\end{code}
@reOrderRec@ is applied to the list of (binder,rhs) pairs for a cyclic
strongly connected component (there's guaranteed to be a cycle). It returns the
-same pairs, but
- a) in a better order,
- b) with some of the Ids having a IAmALoopBreaker pragma
+same pairs, but
+ a) in a better order,
+ b) with some of the Ids having a IAmALoopBreaker pragma
The "loop-breaker" Ids are sufficient to break all cycles in the SCC. This means
that the simplifier can guarantee not to loop provided it never records an inlining
recording inlinings for any Ids which aren't marked as "no-inline" as it goes.
==============
-[June 98: I don't understand the following paragraphs, and I've
- changed the a=b case again so that it isn't a special case any more.]
+[June 98: I don't understand the following paragraphs, and I've
+ changed the a=b case again so that it isn't a special case any more.]
Here's a case that bit me:
- letrec
- a = b
- b = \x. BIG
- in
- ...a...a...a....
+ letrec
+ a = b
+ b = \x. BIG
+ in
+ ...a...a...a....
Re-ordering doesn't change the order of bindings, but there was no loop-breaker.
\begin{code}
type Node details = (details, Unique, [Unique]) -- The Ints are gotten from the Unique,
-- which is gotten from the Id.
-type Details = (Id, UsageDetails, CoreExpr)
-
-reOrderRec :: IdSet -- Binders of this group
- -> SCC (Node Details)
- -> [(Id,CoreExpr)]
+data Details = ND Id -- Binder
+ CoreExpr -- RHS
+ UsageDetails -- Full usage from RHS (*not* including rules)
+ IdSet -- Other binders from this Rec group mentioned on RHS
+ -- (derivable from UsageDetails but cached here)
+
+reOrderRec :: SCC (Node Details)
+ -> [(Id,CoreExpr)]
-- Sorted into a plausible order. Enough of the Ids have
--- IAmALoopBreaker pragmas that there are no loops left.
-reOrderRec bndrs (AcyclicSCC ((bndr, _, rhs), _, _)) = [(bndr, rhs)]
-reOrderRec bndrs (CyclicSCC cycle) = reOrderCycle bndrs cycle
+-- IAmALoopBreaker pragmas that there are no loops left.
+reOrderRec (AcyclicSCC (ND bndr rhs _ _, _, _)) = [(bndr, rhs)]
+reOrderRec (CyclicSCC cycle) = reOrderCycle cycle
-reOrderCycle :: IdSet -> [Node Details] -> [(Id,CoreExpr)]
-reOrderCycle bndrs []
+reOrderCycle :: [Node Details] -> [(Id,CoreExpr)]
+reOrderCycle []
= panic "reOrderCycle"
-reOrderCycle bndrs [bind] -- Common case of simple self-recursion
- = [(makeLoopBreaker bndrs rhs_usg bndr, rhs)]
+reOrderCycle [bind] -- Common case of simple self-recursion
+ = [(makeLoopBreaker False bndr, rhs)]
where
- ((bndr, rhs_usg, rhs), _, _) = bind
+ (ND bndr rhs _ _, _, _) = bind
-reOrderCycle bndrs (bind : binds)
- = -- Choose a loop breaker, mark it no-inline,
- -- do SCC analysis on the rest, and recursively sort them out
- concatMap (reOrderRec bndrs) (stronglyConnCompR unchosen) ++
- [(makeLoopBreaker bndrs rhs_usg bndr, rhs)]
+reOrderCycle (bind : binds)
+ = -- Choose a loop breaker, mark it no-inline,
+ -- do SCC analysis on the rest, and recursively sort them out
+ concatMap reOrderRec (stronglyConnCompFromEdgedVerticesR unchosen) ++
+ [(makeLoopBreaker False bndr, rhs)]
where
(chosen_bind, unchosen) = choose_loop_breaker bind (score bind) [] binds
- (bndr, rhs_usg, rhs) = chosen_bind
+ ND bndr rhs _ _ = chosen_bind
- -- This loop looks for the bind with the lowest score
- -- to pick as the loop breaker. The rest accumulate in
- choose_loop_breaker (details,_,_) loop_sc acc []
- = (details, acc) -- Done
+ -- This loop looks for the bind with the lowest score
+ -- to pick as the loop breaker. The rest accumulate in
+ choose_loop_breaker (details,_,_) _loop_sc acc []
+ = (details, acc) -- Done
choose_loop_breaker loop_bind loop_sc acc (bind : binds)
- | sc < loop_sc -- Lower score so pick this new one
- = choose_loop_breaker bind sc (loop_bind : acc) binds
+ | sc < loop_sc -- Lower score so pick this new one
+ = choose_loop_breaker bind sc (loop_bind : acc) binds
- | otherwise -- No lower so don't pick it
- = choose_loop_breaker loop_bind loop_sc (bind : acc) binds
- where
- sc = score bind
-
- score :: Node Details -> Int -- Higher score => less likely to be picked as loop breaker
- score ((bndr, _, rhs), _, _)
- | exprIsTrivial rhs = 4 -- Practically certain to be inlined
- -- Used to have also: && not (isExportedId bndr)
- -- But I found this sometimes cost an extra iteration when we have
- -- rec { d = (a,b); a = ...df...; b = ...df...; df = d }
- -- where df is the exported dictionary. Then df makes a really
- -- bad choice for loop breaker
-
- | is_con_app rhs = 3 -- Data types help with cases
- -- This used to have a lower score than inlineCandidate, but
- -- it's *really* helpful if dictionaries get inlined fast,
- -- so I'm experimenting with giving higher priority to data-typed things
-
- | inlineCandidate bndr rhs = 2 -- Likely to be inlined
-
- | idHasRules bndr = 1
- -- Avoid things with specialisations; we'd like
- -- to take advantage of them in the subsequent bindings
-
- | otherwise = 0
+ | otherwise -- No lower so don't pick it
+ = choose_loop_breaker loop_bind loop_sc (bind : acc) binds
+ where
+ sc = score bind
+
+ score :: Node Details -> Int -- Higher score => less likely to be picked as loop breaker
+ score (ND bndr rhs _ _, _, _)
+ | workerExists (idWorkerInfo bndr) = 10
+ -- Note [Worker inline loop]
+
+ | exprIsTrivial rhs = 5 -- Practically certain to be inlined
+ -- Used to have also: && not (isExportedId bndr)
+ -- But I found this sometimes cost an extra iteration when we have
+ -- rec { d = (a,b); a = ...df...; b = ...df...; df = d }
+ -- where df is the exported dictionary. Then df makes a really
+ -- bad choice for loop breaker
+
+ | is_con_app rhs = 3 -- Data types help with cases
+ -- Note [Constructor applictions]
+
+-- If an Id is marked "never inline" then it makes a great loop breaker
+-- The only reason for not checking that here is that it is rare
+-- and I've never seen a situation where it makes a difference,
+-- so it probably isn't worth the time to test on every binder
+-- | isNeverActive (idInlinePragma bndr) = -10
+
+ | inlineCandidate bndr rhs = 2 -- Likely to be inlined
+ -- Note [Inline candidates]
+
+ | not (neverUnfold (idUnfolding bndr)) = 1
+ -- the Id has some kind of unfolding
+
+ | otherwise = 0
inlineCandidate :: Id -> CoreExpr -> Bool
- inlineCandidate id (Note InlineMe _) = True
- inlineCandidate id rhs = isOneOcc (idOccInfo id)
-
- -- Real example (the Enum Ordering instance from PrelBase):
- -- rec f = \ x -> case d of (p,q,r) -> p x
- -- g = \ x -> case d of (p,q,r) -> q x
- -- d = (v, f, g)
- --
- -- Here, f and g occur just once; but we can't inline them into d.
- -- On the other hand we *could* simplify those case expressions if
- -- we didn't stupidly choose d as the loop breaker.
- -- But we won't because constructor args are marked "Many".
-
- -- Cheap and cheerful; the simplifer moves casts out of the way
+ inlineCandidate _ (Note InlineMe _) = True
+ inlineCandidate id _ = isOneOcc (idOccInfo id)
+
+ -- Note [conapp]
+ --
+ -- It's really really important to inline dictionaries. Real
+ -- example (the Enum Ordering instance from GHC.Base):
+ --
+ -- rec f = \ x -> case d of (p,q,r) -> p x
+ -- g = \ x -> case d of (p,q,r) -> q x
+ -- d = (v, f, g)
+ --
+ -- Here, f and g occur just once; but we can't inline them into d.
+ -- On the other hand we *could* simplify those case expressions if
+ -- we didn't stupidly choose d as the loop breaker.
+ -- But we won't because constructor args are marked "Many".
+ -- Inlining dictionaries is really essential to unravelling
+ -- the loops in static numeric dictionaries, see GHC.Float.
+
+ -- Cheap and cheerful; the simplifer moves casts out of the way
+ -- The lambda case is important to spot x = /\a. C (f a)
+ -- which comes up when C is a dictionary constructor and
+ -- f is a default method.
+ -- Example: the instance for Show (ST s a) in GHC.ST
+ --
+ -- However we *also* treat (\x. C p q) as a con-app-like thing,
+ -- Note [Closure conversion]
is_con_app (Var v) = isDataConWorkId v
is_con_app (App f _) = is_con_app f
+ is_con_app (Lam _ e) = is_con_app e
is_con_app (Note _ e) = is_con_app e
- is_con_app other = False
-
-makeLoopBreaker :: VarSet -- Binders of this group
- -> UsageDetails -- Usage of this rhs (neglecting rules)
- -> Id -> Id
--- Set the loop-breaker flag, recording whether the thing occurs only in
--- the RHS of a RULE (in this recursive group)
-makeLoopBreaker bndrs rhs_usg bndr
- = setIdOccInfo bndr (IAmALoopBreaker rules_only)
- where
- rules_only = bndrs `intersectsUFM` rhs_usg
+ is_con_app _ = False
+
+makeLoopBreaker :: Bool -> Id -> Id
+-- Set the loop-breaker flag: see Note [Weak loop breakers]
+makeLoopBreaker weak bndr = setIdOccInfo bndr (IAmALoopBreaker weak)
\end{code}
+Note [INLINE pragmas]
+~~~~~~~~~~~~~~~~~~~~~
+Never choose a function with an INLINE pramga as the loop breaker!
+If such a function is mutually-recursive with a non-INLINE thing,
+then the latter should be the loop-breaker.
+
+A particular case is wrappers generated by the demand analyser.
+If you make then into a loop breaker you may get an infinite
+inlining loop. For example:
+ rec {
+ $wfoo x = ....foo x....
+
+ {-loop brk-} foo x = ...$wfoo x...
+ }
+The interface file sees the unfolding for $wfoo, and sees that foo is
+strict (and hence it gets an auto-generated wrapper). Result: an
+infinite inlining in the importing scope. So be a bit careful if you
+change this. A good example is Tree.repTree in
+nofib/spectral/minimax. If the repTree wrapper is chosen as the loop
+breaker then compiling Game.hs goes into an infinite loop (this
+happened when we gave is_con_app a lower score than inline candidates).
+
+Note [Constructor applications]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+It's really really important to inline dictionaries. Real
+example (the Enum Ordering instance from GHC.Base):
+
+ rec f = \ x -> case d of (p,q,r) -> p x
+ g = \ x -> case d of (p,q,r) -> q x
+ d = (v, f, g)
+
+Here, f and g occur just once; but we can't inline them into d.
+On the other hand we *could* simplify those case expressions if
+we didn't stupidly choose d as the loop breaker.
+But we won't because constructor args are marked "Many".
+Inlining dictionaries is really essential to unravelling
+the loops in static numeric dictionaries, see GHC.Float.
+
+Note [Closure conversion]
+~~~~~~~~~~~~~~~~~~~~~~~~~
+We treat (\x. C p q) as a high-score candidate in the letrec scoring algorithm.
+The immediate motivation came from the result of a closure-conversion transformation
+which generated code like this:
+
+ data Clo a b = forall c. Clo (c -> a -> b) c
+
+ ($:) :: Clo a b -> a -> b
+ Clo f env $: x = f env x
+
+ rec { plus = Clo plus1 ()
+
+ ; plus1 _ n = Clo plus2 n
+
+ ; plus2 Zero n = n
+ ; plus2 (Succ m) n = Succ (plus $: m $: n) }
+
+If we inline 'plus' and 'plus1', everything unravels nicely. But if
+we choose 'plus1' as the loop breaker (which is entirely possible
+otherwise), the loop does not unravel nicely.
+
+
@occAnalRhs@ deals with the question of bindings where the Id is marked
by an INLINE pragma. For these we record that anything which occurs
in its RHS occurs many times. This pessimistically assumes that ths
\begin{code}
occAnalRhs :: OccEnv
- -> Id -> CoreExpr -- Binder and rhs
- -- For non-recs the binder is alrady tagged
- -- with occurrence info
- -> (UsageDetails, CoreExpr)
+ -> Id -> CoreExpr -- Binder and rhs
+ -- For non-recs the binder is alrady tagged
+ -- with occurrence info
+ -> (UsageDetails, CoreExpr)
occAnalRhs env id rhs
= occAnal ctxt rhs
where
ctxt | certainly_inline id = env
- | otherwise = rhsCtxt
- -- Note that we generally use an rhsCtxt. This tells the occ anal n
- -- that it's looking at an RHS, which has an effect in occAnalApp
- --
- -- But there's a problem. Consider
- -- x1 = a0 : []
- -- x2 = a1 : x1
- -- x3 = a2 : x2
- -- g = f x3
- -- First time round, it looks as if x1 and x2 occur as an arg of a
- -- let-bound constructor ==> give them a many-occurrence.
- -- But then x3 is inlined (unconditionally as it happens) and
- -- next time round, x2 will be, and the next time round x1 will be
- -- Result: multiple simplifier iterations. Sigh.
- -- Crude solution: use rhsCtxt for things that occur just once...
+ | otherwise = rhsCtxt env
+ -- Note that we generally use an rhsCtxt. This tells the occ anal n
+ -- that it's looking at an RHS, which has an effect in occAnalApp
+ --
+ -- But there's a problem. Consider
+ -- x1 = a0 : []
+ -- x2 = a1 : x1
+ -- x3 = a2 : x2
+ -- g = f x3
+ -- First time round, it looks as if x1 and x2 occur as an arg of a
+ -- let-bound constructor ==> give them a many-occurrence.
+ -- But then x3 is inlined (unconditionally as it happens) and
+ -- next time round, x2 will be, and the next time round x1 will be
+ -- Result: multiple simplifier iterations. Sigh.
+ -- Crude solution: use rhsCtxt for things that occur just once...
certainly_inline id = case idOccInfo id of
- OneOcc in_lam one_br _ -> not in_lam && one_br
- other -> False
+ OneOcc in_lam one_br _ -> not in_lam && one_br
+ _ -> False
\end{code}
-Note [RulesOnly]
-~~~~~~~~~~~~~~~~~~
-If the binder has RULES inside it then we count the specialised Ids as
-"extra rhs's". That way the "parent" keeps the specialised "children"
-alive. If the parent dies (because it isn't referenced any more),
-then the children will die too unless they are already referenced
-directly.
-
-That's the basic idea. However in a recursive situation we want to be a bit
-cleverer. Example (from GHC.Enum):
-
- eftInt :: Int# -> Int# -> [Int]
- eftInt x y = ...(non-recursive)...
-
- {-# INLINE [0] eftIntFB #-}
- eftIntFB :: (Int -> r -> r) -> r -> Int# -> Int# -> r
- eftIntFB c n x y = ...(non-recursive)...
-
- {-# RULES
- "eftInt" [~1] forall x y. eftInt x y = build (\ c n -> eftIntFB c n x y)
- "eftIntList" [1] eftIntFB (:) [] = eftInt
- #-}
-
-The two look mutually recursive only because of their RULES; we don't want
-that to inhibit inlining!
-
-So when we identify a LoopBreaker, we mark it to say whether it only mentions
-the other binders in its recursive group in a RULE. If so, we can inline it,
-because doing so will not expose new occurrences of binders in its group.
\begin{code}
-
addRuleUsage :: UsageDetails -> Id -> UsageDetails
-- Add the usage from RULES in Id to the usage
addRuleUsage usage id
= foldVarSet add usage (idRuleVars id)
+ -- idRuleVars here: see Note [Rule dependency info]
where
- add v u = addOneOcc u v NoOccInfo -- Give a non-committal binder info
- -- (i.e manyOcc) because many copies
- -- of the specialised thing can appear
+ add v u = addOneOcc u v NoOccInfo
+ -- Give a non-committal binder info (i.e manyOcc) because
+ -- a) Many copies of the specialised thing can appear
+ -- b) We don't want to substitute a BIG expression inside a RULE
+ -- even if that's the only occurrence of the thing
+ -- (Same goes for INLINE.)
\end{code}
Expressions
~~~~~~~~~~~
\begin{code}
occAnal :: OccEnv
- -> CoreExpr
- -> (UsageDetails, -- Gives info only about the "interesting" Ids
- CoreExpr)
+ -> CoreExpr
+ -> (UsageDetails, -- Gives info only about the "interesting" Ids
+ CoreExpr)
-occAnal env (Type t) = (emptyDetails, Type t)
+occAnal _ (Type t) = (emptyDetails, Type t)
occAnal env (Var v) = (mkOneOcc env v False, Var v)
-- At one stage, I gathered the idRuleVars for v here too,
-- which in a way is the right thing to do.
- -- Btu that went wrong right after specialisation, when
+ -- But that went wrong right after specialisation, when
-- the *occurrences* of the overloaded function didn't have any
-- rules in them, so the *specialised* versions looked as if they
-- weren't used at all.
\begin{verbatim}
module A where
-f x = let y = expensive x in
- let z = (True,y) in
+f x = let y = expensive x in
+ let z = (True,y) in
(case z of {(p,q)->q}, case z of {(p,q)->q})
\end{verbatim}
Constructors are rather like lambdas in this way.
\begin{code}
-occAnal env expr@(Lit lit) = (emptyDetails, expr)
+occAnal _ expr@(Lit _) = (emptyDetails, expr)
\end{code}
\begin{code}
occAnal env (Note InlineMe body)
- = case occAnal env body of { (usage, body') ->
+ = case occAnal env body of { (usage, body') ->
(mapVarEnv markMany usage, Note InlineMe body')
}
-occAnal env (Note note@(SCC cc) body)
+occAnal env (Note note@(SCC _) body)
= case occAnal env body of { (usage, body') ->
(mapVarEnv markInsideSCC usage, Note note body')
}
occAnal env (Cast expr co)
= case occAnal env expr of { (usage, expr') ->
(markRhsUds env True usage, Cast expr' co)
- -- If we see let x = y `cast` co
- -- then mark y as 'Many' so that we don't
- -- immediately inline y again.
+ -- If we see let x = y `cast` co
+ -- then mark y as 'Many' so that we don't
+ -- immediately inline y again.
}
\end{code}
\begin{code}
-occAnal env app@(App fun arg)
- = occAnalApp env (collectArgs app) False
+occAnal env app@(App _ _)
+ = occAnalApp env (collectArgs app)
-- Ignore type variables altogether
-- (a) occurrences inside type lambdas only not marked as InsideLam
-- (b) type variables not in environment
-occAnal env expr@(Lam x body) | isTyVar x
+occAnal env (Lam x body) | isTyVar x
= case occAnal env body of { (body_usage, body') ->
(body_usage, Lam x body')
}
-- For value lambdas we do a special hack. Consider
--- (\x. \y. ...x...)
+-- (\x. \y. ...x...)
-- If we did nothing, x is used inside the \y, so would be marked
-- as dangerous to dup. But in the common case where the abstraction
-- is applied to two arguments this is over-pessimistic.
= case occAnal env_body body of { (body_usage, body') ->
let
(final_usage, tagged_binders) = tagBinders body_usage binders
- -- URGH! Sept 99: we don't seem to be able to use binders' here, because
- -- we get linear-typed things in the resulting program that we can't handle yet.
- -- (e.g. PrelShow) TODO
-
- really_final_usage = if linear then
- final_usage
- else
- mapVarEnv markInsideLam final_usage
+ -- URGH! Sept 99: we don't seem to be able to use binders' here, because
+ -- we get linear-typed things in the resulting program that we can't handle yet.
+ -- (e.g. PrelShow) TODO
+
+ really_final_usage = if linear then
+ final_usage
+ else
+ mapVarEnv markInsideLam final_usage
in
(really_final_usage,
mkLams tagged_binders body') }
where
- env_body = vanillaCtxt -- Body is (no longer) an RhsContext
+ env_body = vanillaCtxt env -- Body is (no longer) an RhsContext
(binders, body) = collectBinders expr
- binders' = oneShotGroup env binders
- linear = all is_one_shot binders'
+ binders' = oneShotGroup env binders
+ linear = all is_one_shot binders'
is_one_shot b = isId b && isOneShotBndr b
occAnal env (Case scrut bndr ty alts)
- = case occ_anal_scrut scrut alts of { (scrut_usage, scrut') ->
- case mapAndUnzip (occAnalAlt alt_env bndr) alts of { (alts_usage_s, alts') ->
+ = case occ_anal_scrut scrut alts of { (scrut_usage, scrut') ->
+ case mapAndUnzip occ_anal_alt alts of { (alts_usage_s, alts') ->
let
- alts_usage = foldr1 combineAltsUsageDetails alts_usage_s
- alts_usage' = addCaseBndrUsage alts_usage
- (alts_usage1, tagged_bndr) = tagBinder alts_usage' bndr
+ alts_usage = foldr1 combineAltsUsageDetails alts_usage_s
+ alts_usage' = addCaseBndrUsage alts_usage
+ (alts_usage1, tagged_bndr) = tagBinder alts_usage' bndr
total_usage = scrut_usage +++ alts_usage1
in
total_usage `seq` (total_usage, Case scrut' tagged_bndr ty alts') }}
where
- -- The case binder gets a usage of either "many" or "dead", never "one".
- -- Reason: we like to inline single occurrences, to eliminate a binding,
- -- but inlining a case binder *doesn't* eliminate a binding.
- -- We *don't* want to transform
- -- case x of w { (p,q) -> f w }
- -- into
- -- case x of w { (p,q) -> f (p,q) }
+ -- Note [Case binder usage]
+ -- ~~~~~~~~~~~~~~~~~~~~~~~~
+ -- The case binder gets a usage of either "many" or "dead", never "one".
+ -- Reason: we like to inline single occurrences, to eliminate a binding,
+ -- but inlining a case binder *doesn't* eliminate a binding.
+ -- We *don't* want to transform
+ -- case x of w { (p,q) -> f w }
+ -- into
+ -- case x of w { (p,q) -> f (p,q) }
addCaseBndrUsage usage = case lookupVarEnv usage bndr of
- Nothing -> usage
- Just occ -> extendVarEnv usage bndr (markMany occ)
+ Nothing -> usage
+ Just _ -> extendVarEnv usage bndr NoOccInfo
- alt_env = setVanillaCtxt env
- -- Consider x = case v of { True -> (p,q); ... }
- -- Then it's fine to inline p and q
+ alt_env = mkAltEnv env bndr_swap
+ -- Consider x = case v of { True -> (p,q); ... }
+ -- Then it's fine to inline p and q
+
+ bndr_swap = case scrut of
+ Var v -> Just (v, Var bndr)
+ Cast (Var v) co -> Just (v, Cast (Var bndr) (mkSymCoercion co))
+ _other -> Nothing
+
+ occ_anal_alt = occAnalAlt alt_env bndr bndr_swap
occ_anal_scrut (Var v) (alt1 : other_alts)
- | not (null other_alts) || not (isDefaultAlt alt1)
- = (mkOneOcc env v True, Var v)
- occ_anal_scrut scrut alts = occAnal vanillaCtxt scrut
- -- No need for rhsCtxt
+ | not (null other_alts) || not (isDefaultAlt alt1)
+ = (mkOneOcc env v True, Var v) -- The 'True' says that the variable occurs
+ -- in an interesting context; the case has
+ -- at least one non-default alternative
+ occ_anal_scrut scrut _alts
+ = occAnal (vanillaCtxt env) scrut -- No need for rhsCtxt
occAnal env (Let bind body)
- = case occAnal env body of { (body_usage, body') ->
+ = case occAnal env body of { (body_usage, body') ->
case occAnalBind env bind body_usage of { (final_usage, new_binds) ->
(final_usage, mkLets new_binds body') }}
+occAnalArgs :: OccEnv -> [CoreExpr] -> (UsageDetails, [CoreExpr])
occAnalArgs env args
- = case mapAndUnzip (occAnal arg_env) args of { (arg_uds_s, args') ->
+ = case mapAndUnzip (occAnal arg_env) args of { (arg_uds_s, args') ->
(foldr (+++) emptyDetails arg_uds_s, args')}
where
- arg_env = vanillaCtxt
+ arg_env = vanillaCtxt env
\end{code}
Applications are dealt with specially because we want
the "build hack" to work.
\begin{code}
-occAnalApp env (Var fun, args) is_rhs
+occAnalApp :: OccEnv
+ -> (Expr CoreBndr, [Arg CoreBndr])
+ -> (UsageDetails, Expr CoreBndr)
+occAnalApp env (Var fun, args)
= case args_stuff of { (args_uds, args') ->
let
final_args_uds = markRhsUds env is_pap args_uds
where
fun_uniq = idUnique fun
fun_uds = mkOneOcc env fun (valArgCount args > 0)
- is_pap = isDataConWorkId fun || valArgCount args < idArity fun
-
- -- Hack for build, fold, runST
- args_stuff | fun_uniq == buildIdKey = appSpecial env 2 [True,True] args
- | fun_uniq == augmentIdKey = appSpecial env 2 [True,True] args
- | fun_uniq == foldrIdKey = appSpecial env 3 [False,True] args
- | fun_uniq == runSTRepIdKey = appSpecial env 2 [True] args
- -- (foldr k z xs) may call k many times, but it never
- -- shares a partial application of k; hence [False,True]
- -- This means we can optimise
- -- foldr (\x -> let v = ...x... in \y -> ...v...) z xs
- -- by floating in the v
-
- | otherwise = occAnalArgs env args
-
-
-occAnalApp env (fun, args) is_rhs
- = case occAnal (addAppCtxt env args) fun of { (fun_uds, fun') ->
- -- The addAppCtxt is a bit cunning. One iteration of the simplifier
- -- often leaves behind beta redexs like
- -- (\x y -> e) a1 a2
- -- Here we would like to mark x,y as one-shot, and treat the whole
- -- thing much like a let. We do this by pushing some True items
- -- onto the context stack.
-
- case occAnalArgs env args of { (args_uds, args') ->
+ is_pap = isConLikeId fun || valArgCount args < idArity fun
+
+ -- Hack for build, fold, runST
+ args_stuff | fun_uniq == buildIdKey = appSpecial env 2 [True,True] args
+ | fun_uniq == augmentIdKey = appSpecial env 2 [True,True] args
+ | fun_uniq == foldrIdKey = appSpecial env 3 [False,True] args
+ | fun_uniq == runSTRepIdKey = appSpecial env 2 [True] args
+ -- (foldr k z xs) may call k many times, but it never
+ -- shares a partial application of k; hence [False,True]
+ -- This means we can optimise
+ -- foldr (\x -> let v = ...x... in \y -> ...v...) z xs
+ -- by floating in the v
+
+ | otherwise = occAnalArgs env args
+
+
+occAnalApp env (fun, args)
+ = case occAnal (addAppCtxt env args) fun of { (fun_uds, fun') ->
+ -- The addAppCtxt is a bit cunning. One iteration of the simplifier
+ -- often leaves behind beta redexs like
+ -- (\x y -> e) a1 a2
+ -- Here we would like to mark x,y as one-shot, and treat the whole
+ -- thing much like a let. We do this by pushing some True items
+ -- onto the context stack.
+
+ case occAnalArgs env args of { (args_uds, args') ->
let
- final_uds = fun_uds +++ args_uds
+ final_uds = fun_uds +++ args_uds
in
(final_uds, mkApps fun' args') }}
-
-markRhsUds :: OccEnv -- Check if this is a RhsEnv
- -> Bool -- and this is true
- -> UsageDetails -- The do markMany on this
- -> UsageDetails
--- We mark the free vars of the argument of a constructor or PAP
+
+markRhsUds :: OccEnv -- Check if this is a RhsEnv
+ -> Bool -- and this is true
+ -> UsageDetails -- The do markMany on this
+ -> UsageDetails
+-- We mark the free vars of the argument of a constructor or PAP
-- as "many", if it is the RHS of a let(rec).
-- This means that nothing gets inlined into a constructor argument
-- position, which is what we want. Typically those constructor
-- This is the *whole point* of the isRhsEnv predicate
markRhsUds env is_pap arg_uds
| isRhsEnv env && is_pap = mapVarEnv markMany arg_uds
- | otherwise = arg_uds
+ | otherwise = arg_uds
-appSpecial :: OccEnv
- -> Int -> CtxtTy -- Argument number, and context to use for it
- -> [CoreExpr]
- -> (UsageDetails, [CoreExpr])
+appSpecial :: OccEnv
+ -> Int -> CtxtTy -- Argument number, and context to use for it
+ -> [CoreExpr]
+ -> (UsageDetails, [CoreExpr])
appSpecial env n ctxt args
= go n args
where
- arg_env = vanillaCtxt
+ arg_env = vanillaCtxt env
- go n [] = (emptyDetails, []) -- Too few args
+ go _ [] = (emptyDetails, []) -- Too few args
+
+ go 1 (arg:args) -- The magic arg
+ = case occAnal (setCtxtTy arg_env ctxt) arg of { (arg_uds, arg') ->
+ case occAnalArgs env args of { (args_uds, args') ->
+ (arg_uds +++ args_uds, arg':args') }}
- go 1 (arg:args) -- The magic arg
- = case occAnal (setCtxt arg_env ctxt) arg of { (arg_uds, arg') ->
- case occAnalArgs env args of { (args_uds, args') ->
- (arg_uds +++ args_uds, arg':args') }}
-
go n (arg:args)
- = case occAnal arg_env arg of { (arg_uds, arg') ->
- case go (n-1) args of { (args_uds, args') ->
- (arg_uds +++ args_uds, arg':args') }}
+ = case occAnal arg_env arg of { (arg_uds, arg') ->
+ case go (n-1) args of { (args_uds, args') ->
+ (arg_uds +++ args_uds, arg':args') }}
\end{code}
-
-Case alternatives
-~~~~~~~~~~~~~~~~~
-If the case binder occurs at all, the other binders effectively do too.
-For example
- case e of x { (a,b) -> rhs }
-is rather like
- let x = (a,b) in rhs
-If e turns out to be (e1,e2) we indeed get something like
- let a = e1; b = e2; x = (a,b) in rhs
-
-Note [Aug 06]: I don't think this is necessary any more, and it helpe
- to know when binders are unused. See esp the call to
- isDeadBinder in Simplify.mkDupableAlt
+
+Note [Binder swap]
+~~~~~~~~~~~~~~~~~~
+We do these two transformations right here:
+
+ (1) case x of b { pi -> ri }
+ ==>
+ case x of b { pi -> let x=b in ri }
+
+ (2) case (x |> co) of b { pi -> ri }
+ ==>
+ case (x |> co) of b { pi -> let x = b |> sym co in ri }
+
+ Why (2)? See Note [Case of cast]
+
+In both cases, in a particular alternative (pi -> ri), we only
+add the binding if
+ (a) x occurs free in (pi -> ri)
+ (ie it occurs in ri, but is not bound in pi)
+ (b) the pi does not bind b (or the free vars of co)
+We need (a) and (b) for the inserted binding to be correct.
+
+For the alternatives where we inject the binding, we can transfer
+all x's OccInfo to b. And that is the point.
+
+Notice that
+ * The deliberate shadowing of 'x'.
+ * That (a) rapidly becomes false, so no bindings are injected.
+
+The reason for doing these transformations here is because it allows
+us to adjust the OccInfo for 'x' and 'b' as we go.
+
+ * Suppose the only occurrences of 'x' are the scrutinee and in the
+ ri; then this transformation makes it occur just once, and hence
+ get inlined right away.
+
+ * If we do this in the Simplifier, we don't know whether 'x' is used
+ in ri, so we are forced to pessimistically zap b's OccInfo even
+ though it is typically dead (ie neither it nor x appear in the
+ ri). There's nothing actually wrong with zapping it, except that
+ it's kind of nice to know which variables are dead. My nose
+ tells me to keep this information as robustly as possible.
+
+The Maybe (Id,CoreExpr) passed to occAnalAlt is the extra let-binding
+{x=b}; it's Nothing if the binder-swap doesn't happen.
+
+There is a danger though. Consider
+ let v = x +# y
+ in case (f v) of w -> ...v...v...
+And suppose that (f v) expands to just v. Then we'd like to
+use 'w' instead of 'v' in the alternative. But it may be too
+late; we may have substituted the (cheap) x+#y for v in the
+same simplifier pass that reduced (f v) to v.
+
+I think this is just too bad. CSE will recover some of it.
+
+Note [Binder swap on GlobalId scrutinees]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+When the scrutinee is a GlobalId we must take care in two ways
+
+ i) In order to *know* whether 'x' occurs free in the RHS, we need its
+ occurrence info. BUT, we don't gather occurrence info for
+ GlobalIds. That's what the (small) occ_scrut_ids set in OccEnv is
+ for: it says "gather occurrence info for these.
+
+ ii) We must call localiseId on 'x' first, in case it's a GlobalId, or
+ has an External Name. See, for example, SimplEnv Note [Global Ids in
+ the substitution].
+
+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]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+This old note describes a problem that is also fixed by doing the
+binder-swap in OccAnal:
+
+ 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.
+
+It's fixed by doing the binder-swap in OccAnal because we can do the
+binder-swap unconditionally and still get occurrence analysis
+information right.
+
+Note [Case of cast]
+~~~~~~~~~~~~~~~~~~~
+Consider case (x `cast` co) of b { I# ->
+ ... (case (x `cast` co) of {...}) ...
+We'd like to eliminate the inner case. That is the motivation for
+equation (2) in Note [Binder swap]. When we get to the inner case, we
+inline x, cancel the casts, and away we go.
+
+Note [Binders in case alternatives]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Consider
+ case x of y { (a,b) -> f y }
+We treat 'a', 'b' as dead, because they don't physically occur in the
+case alternative. (Indeed, a variable is dead iff it doesn't occur in
+its scope in the output of OccAnal.) This invariant is It really
+helpe to know when binders are unused. See esp the call to
+isDeadBinder in Simplify.mkDupableAlt
+
+In this example, though, the Simplifier will bring 'a' and 'b' back to
+life, beause it binds 'y' to (a,b) (imagine got inlined and
+scrutinised y).
\begin{code}
-occAnalAlt env case_bndr (con, bndrs, rhs)
+occAnalAlt :: OccEnv
+ -> CoreBndr
+ -> Maybe (Id, CoreExpr) -- Note [Binder swap]
+ -> CoreAlt
+ -> (UsageDetails, Alt IdWithOccInfo)
+occAnalAlt env case_bndr mb_scrut_var (con, bndrs, rhs)
= case occAnal env rhs of { (rhs_usage, rhs') ->
let
- (final_usage, tagged_bndrs) = tagBinders rhs_usage bndrs
- final_bndrs = tagged_bndrs -- See Note [Aug06] above
-{-
- final_bndrs | case_bndr `elemVarEnv` final_usage = bndrs
- | otherwise = tagged_bndrs
- -- Leave the binders untagged if the case
- -- binder occurs at all; see note above
--}
+ (alt_usg, tagged_bndrs) = tagBinders rhs_usage bndrs
+ bndrs' = tagged_bndrs -- See Note [Binders in case alternatives]
in
- (final_usage, (con, final_bndrs, rhs')) }
+ case mb_scrut_var of
+ Just (scrut_var, scrut_rhs) -- See Note [Binder swap]
+ | scrut_var `localUsedIn` alt_usg -- (a) Fast path, usually false
+ , not (any shadowing bndrs) -- (b)
+ -> (addOneOcc usg_wo_scrut case_bndr NoOccInfo,
+ -- See Note [Case binder usage] for the NoOccInfo
+ (con, bndrs', Let (NonRec scrut_var' scrut_rhs) rhs'))
+ where
+ (usg_wo_scrut, scrut_var') = tagBinder alt_usg (localiseId scrut_var)
+ -- Note the localiseId; we're making a new binding
+ -- for it, and it might have an External Name, or
+ -- even be a GlobalId; Note [Binder swap on GlobalId scrutinees]
+ shadowing bndr = bndr `elemVarSet` rhs_fvs
+ rhs_fvs = exprFreeVars scrut_rhs
+
+ _other -> (alt_usg, (con, bndrs', rhs')) }
\end{code}
%************************************************************************
-%* *
+%* *
\subsection[OccurAnal-types]{OccEnv}
-%* *
+%* *
%************************************************************************
\begin{code}
data OccEnv
- = OccEnv OccEncl -- Enclosing context information
- CtxtTy -- Tells about linearity
+ = OccEnv { occ_encl :: !OccEncl -- Enclosing context information
+ , occ_ctxt :: !CtxtTy -- Tells about linearity
+ , occ_scrut_ids :: !GblScrutIds }
+
+type GblScrutIds = IdSet -- GlobalIds that are scrutinised, and for which
+ -- we want to gather occurence info; see
+ -- Note [Binder swap for GlobalId scrutinee]
+ -- No need to prune this if there's a shadowing binding
+ -- because it's OK for it to be too big
-- OccEncl is used to control whether to inline into constructor arguments
-- For example:
--- x = (p,q) -- Don't inline p or q
--- y = /\a -> (p a, q a) -- Still don't inline p or q
--- z = f (p,q) -- Do inline p,q; it may make a rule fire
+-- x = (p,q) -- Don't inline p or q
+-- y = /\a -> (p a, q a) -- Still don't inline p or q
+-- z = f (p,q) -- Do inline p,q; it may make a rule fire
-- So OccEncl tells enought about the context to know what to do when
-- we encounter a contructor application or PAP.
data OccEncl
- = OccRhs -- RHS of let(rec), albeit perhaps inside a type lambda
- -- Don't inline into constructor args here
- | OccVanilla -- Argument of function, body of lambda, scruintee of case etc.
- -- Do inline into constructor args here
+ = OccRhs -- RHS of let(rec), albeit perhaps inside a type lambda
+ -- Don't inline into constructor args here
+ | OccVanilla -- Argument of function, body of lambda, scruintee of case etc.
+ -- Do inline into constructor args here
type CtxtTy = [Bool]
- -- [] No info
- --
- -- True:ctxt Analysing a function-valued expression that will be
- -- applied just once
- --
- -- False:ctxt Analysing a function-valued expression that may
- -- be applied many times; but when it is,
- -- the CtxtTy inside applies
+ -- [] No info
+ --
+ -- True:ctxt Analysing a function-valued expression that will be
+ -- applied just once
+ --
+ -- False:ctxt Analysing a function-valued expression that may
+ -- be applied many times; but when it is,
+ -- the CtxtTy inside applies
initOccEnv :: OccEnv
-initOccEnv = OccEnv OccRhs []
-
-vanillaCtxt = OccEnv OccVanilla []
-rhsCtxt = OccEnv OccRhs []
-
-isRhsEnv (OccEnv OccRhs _) = True
-isRhsEnv (OccEnv OccVanilla _) = False
-
-setVanillaCtxt :: OccEnv -> OccEnv
-setVanillaCtxt (OccEnv OccRhs ctxt_ty) = OccEnv OccVanilla ctxt_ty
-setVanillaCtxt other_env = other_env
-
-setCtxt :: OccEnv -> CtxtTy -> OccEnv
-setCtxt (OccEnv encl _) ctxt = OccEnv encl ctxt
+initOccEnv = OccEnv { occ_encl = OccRhs
+ , occ_ctxt = []
+ , occ_scrut_ids = emptyVarSet }
+
+vanillaCtxt :: OccEnv -> OccEnv
+vanillaCtxt env = OccEnv { occ_encl = OccVanilla, occ_ctxt = []
+ , occ_scrut_ids = occ_scrut_ids env }
+
+rhsCtxt :: OccEnv -> OccEnv
+rhsCtxt env = OccEnv { occ_encl = OccRhs, occ_ctxt = []
+ , occ_scrut_ids = occ_scrut_ids env }
+
+mkAltEnv :: OccEnv -> Maybe (Id, CoreExpr) -> OccEnv
+-- Does two things: a) makes the occ_ctxt = OccVanilla
+-- b) extends the scrut_ids if necessary
+mkAltEnv env (Just (scrut_id, _))
+ | not (isLocalId scrut_id)
+ = OccEnv { occ_encl = OccVanilla
+ , occ_scrut_ids = extendVarSet (occ_scrut_ids env) scrut_id
+ , occ_ctxt = occ_ctxt env }
+mkAltEnv env _
+ | isRhsEnv env = env { occ_encl = OccVanilla }
+ | otherwise = env
+
+setCtxtTy :: OccEnv -> CtxtTy -> OccEnv
+setCtxtTy env ctxt = env { occ_ctxt = ctxt }
+
+isRhsEnv :: OccEnv -> Bool
+isRhsEnv (OccEnv { occ_encl = OccRhs }) = True
+isRhsEnv (OccEnv { occ_encl = OccVanilla }) = False
oneShotGroup :: OccEnv -> [CoreBndr] -> [CoreBndr]
- -- The result binders have one-shot-ness set that they might not have had originally.
- -- This happens in (build (\cn -> e)). Here the occurrence analyser
- -- linearity context knows that c,n are one-shot, and it records that fact in
- -- the binder. This is useful to guide subsequent float-in/float-out tranformations
+ -- The result binders have one-shot-ness set that they might not have had originally.
+ -- This happens in (build (\cn -> e)). Here the occurrence analyser
+ -- linearity context knows that c,n are one-shot, and it records that fact in
+ -- the binder. This is useful to guide subsequent float-in/float-out tranformations
-oneShotGroup (OccEnv encl ctxt) bndrs
+oneShotGroup (OccEnv { occ_ctxt = ctxt }) bndrs
= go ctxt bndrs []
where
- go ctxt [] rev_bndrs = reverse rev_bndrs
+ go _ [] rev_bndrs = reverse rev_bndrs
go (lin_ctxt:ctxt) (bndr:bndrs) rev_bndrs
- | isId bndr = go ctxt bndrs (bndr':rev_bndrs)
- where
- bndr' | lin_ctxt = setOneShotLambda bndr
- | otherwise = bndr
+ | isId bndr = go ctxt bndrs (bndr':rev_bndrs)
+ where
+ bndr' | lin_ctxt = setOneShotLambda bndr
+ | otherwise = bndr
go ctxt (bndr:bndrs) rev_bndrs = go ctxt bndrs (bndr:rev_bndrs)
-addAppCtxt (OccEnv encl ctxt) args
- = OccEnv encl (replicate (valArgCount args) True ++ ctxt)
+addAppCtxt :: OccEnv -> [Arg CoreBndr] -> OccEnv
+addAppCtxt env@(OccEnv { occ_ctxt = ctxt }) args
+ = env { occ_ctxt = replicate (valArgCount args) True ++ ctxt }
\end{code}
%************************************************************************
-%* *
+%* *
\subsection[OccurAnal-types]{OccEnv}
-%* *
+%* *
%************************************************************************
\begin{code}
-type UsageDetails = IdEnv OccInfo -- A finite map from ids to their usage
+type UsageDetails = IdEnv OccInfo -- A finite map from ids to their usage
+ -- INVARIANT: never IAmDead
+ -- (Deadness is signalled by not being in the map at all)
(+++), combineAltsUsageDetails
- :: UsageDetails -> UsageDetails -> UsageDetails
+ :: UsageDetails -> UsageDetails -> UsageDetails
(+++) usage1 usage2
= plusVarEnv_C addOccInfo usage1 usage2
addOneOcc :: UsageDetails -> Id -> OccInfo -> UsageDetails
addOneOcc usage id info
= plusVarEnv_C addOccInfo usage (unitVarEnv id info)
- -- ToDo: make this more efficient
+ -- ToDo: make this more efficient
+emptyDetails :: UsageDetails
emptyDetails = (emptyVarEnv :: UsageDetails)
-usedIn :: Id -> UsageDetails -> Bool
-v `usedIn` details = isExportedId v || v `elemVarEnv` details
+localUsedIn, usedIn :: Id -> UsageDetails -> Bool
+v `localUsedIn` details = v `elemVarEnv` details
+v `usedIn` details = isExportedId v || v `localUsedIn` details
type IdWithOccInfo = Id
-tagBinders :: UsageDetails -- Of scope
- -> [Id] -- Binders
- -> (UsageDetails, -- Details with binders removed
- [IdWithOccInfo]) -- Tagged binders
+tagBinders :: UsageDetails -- Of scope
+ -> [Id] -- Binders
+ -> (UsageDetails, -- Details with binders removed
+ [IdWithOccInfo]) -- Tagged binders
tagBinders usage binders
= let
in
usage' `seq` (usage', uss)
-tagBinder :: UsageDetails -- Of scope
- -> Id -- Binders
- -> (UsageDetails, -- Details with binders removed
- IdWithOccInfo) -- Tagged binders
+tagBinder :: UsageDetails -- Of scope
+ -> Id -- Binders
+ -> (UsageDetails, -- Details with binders removed
+ IdWithOccInfo) -- Tagged binders
tagBinder usage binder
= let
setBinderOcc usage bndr
| isTyVar bndr = bndr
| isExportedId bndr = case idOccInfo bndr of
- NoOccInfo -> bndr
- other -> setIdOccInfo bndr NoOccInfo
- -- Don't use local usage info for visible-elsewhere things
- -- BUT *do* erase any IAmALoopBreaker annotation, because we're
- -- about to re-generate it and it shouldn't be "sticky"
-
+ NoOccInfo -> bndr
+ _ -> setIdOccInfo bndr NoOccInfo
+ -- Don't use local usage info for visible-elsewhere things
+ -- BUT *do* erase any IAmALoopBreaker annotation, because we're
+ -- about to re-generate it and it shouldn't be "sticky"
+
| otherwise = setIdOccInfo bndr occ_info
where
occ_info = lookupVarEnv usage bndr `orElse` IAmDead
%************************************************************************
-%* *
+%* *
\subsection{Operations over OccInfo}
-%* *
+%* *
%************************************************************************
\begin{code}
mkOneOcc :: OccEnv -> Id -> InterestingCxt -> UsageDetails
mkOneOcc env id int_cxt
| isLocalId id = unitVarEnv id (OneOcc False True int_cxt)
- | otherwise = emptyDetails
+ | id `elemVarSet` occ_scrut_ids env = unitVarEnv id NoOccInfo
+ | otherwise = emptyDetails
markMany, markInsideLam, markInsideSCC :: OccInfo -> OccInfo
-markMany IAmDead = IAmDead
-markMany other = NoOccInfo
+markMany _ = NoOccInfo
markInsideSCC occ = markMany occ
markInsideLam (OneOcc _ one_br int_cxt) = OneOcc True one_br int_cxt
-markInsideLam occ = occ
+markInsideLam occ = occ
addOccInfo, orOccInfo :: OccInfo -> OccInfo -> OccInfo
-addOccInfo IAmDead info2 = info2
-addOccInfo info1 IAmDead = info1
-addOccInfo info1 info2 = NoOccInfo
+addOccInfo a1 a2 = ASSERT( not (isDeadOcc a1 || isDeadOcc a2) )
+ NoOccInfo -- Both branches are at least One
+ -- (Argument is never IAmDead)
-- (orOccInfo orig new) is used
-- when combining occurrence info from branches of a case
-orOccInfo IAmDead info2 = info2
-orOccInfo info1 IAmDead = info1
-orOccInfo (OneOcc in_lam1 one_branch1 int_cxt1)
- (OneOcc in_lam2 one_branch2 int_cxt2)
+orOccInfo (OneOcc in_lam1 _ int_cxt1)
+ (OneOcc in_lam2 _ int_cxt2)
= OneOcc (in_lam1 || in_lam2)
- False -- False, because it occurs in both branches
- (int_cxt1 && int_cxt2)
-orOccInfo info1 info2 = NoOccInfo
+ False -- False, because it occurs in both branches
+ (int_cxt1 && int_cxt2)
+orOccInfo a1 a2 = ASSERT( not (isDeadOcc a1 || isDeadOcc a2) )
+ NoOccInfo
\end{code}
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