% (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
-- XXX This define is a bit of a hack, and should be done more nicely
import CoreSyn
import CoreFVs
-import CoreUtils ( exprIsTrivial, isDefaultAlt )
+import CoreUtils ( exprIsTrivial, isDefaultAlt )
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, intersectUFM_C, foldUFM_Directly )
-import Util ( mapAndUnzip )
+import Maybes ( orElse )
+import Digraph ( stronglyConnCompR, SCC(..) )
+import PrelNames ( buildIdKey, foldrIdKey, runSTRepIdKey, augmentIdKey )
+import Unique ( Unique )
+import UniqFM ( keysUFM, intersectUFM_C, foldUFM_Directly )
+import Util ( mapAndUnzip )
import Outputable
import Data.List
%************************************************************************
-%* *
+%* *
\subsection[OccurAnal-main]{Counting occurrences: main function}
-%* *
+%* *
%************************************************************************
Here's the externally-callable interface:
where
go :: OccEnv -> [CoreBind] -> (UsageDetails, [CoreBind])
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
+ = (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
+ | 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 [Rules are extra RHSs]
+ | 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...
+ 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...
+ 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
+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
To that end, we build a Rec group for each cyclic strongly
connected component,
- *treating f's rules as extra RHSs for 'f'*.
+ *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.
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
+ 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
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
+ 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
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
+ 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
+ Rec { f = f_rhs
RULE f [] = g
-
- h = h_rhs
- g = h
- ...more...
+
+ h = h_rhs
+ g = h
+ ...more...
}
Remmber that we simplify the RULES before any RHS (see Note
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
+ 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. Why both?
+ The VarSet in a SpecInfo is used for dependency analysis in the
+ occurrence analyser. We must track free vars in *both* lhs and rhs. Why both?
Consider
- x = y
- RULE f x = 4
+ 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
So 'f' and 'fs' are in the same Rec group (since f refers to fs via its RULE).
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
+ - 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
This showed up when compiling Control.Concurrent.Chan.getChanContents.
\begin{code}
occAnalBind env (Rec pairs) body_usage
- | not (any (`usedIn` body_usage) bndrs) -- NB: look at body_usage, not total_usage
- = (body_usage, []) -- Dead code
+ | not (any (`usedIn` body_usage) bndrs) -- NB: look at body_usage, not total_usage
+ = (body_usage, []) -- Dead code
| otherwise
= (final_usage, map ({-# SCC "occAnalBind.dofinal" #-} do_final_bind) sccs)
where
bndrs = map fst pairs
bndr_set = mkVarSet bndrs
- ---------------------------------------
- -- See Note [Loop breaking]
- ---------------------------------------
+ ---------------------------------------
+ -- See Note [Loop breaking]
+ ---------------------------------------
-------------Dependency analysis ------------------------------
occ_anald :: [(Id, (UsageDetails, CoreExpr))]
- -- The UsageDetails here are strictly those arising from the RHS
- -- *not* from any rules in the Id
+ -- The UsageDetails here are strictly those arising from the RHS
+ -- *not* from any rules in the Id
occ_anald = [(bndr, occAnalRhs env bndr rhs) | (bndr,rhs) <- pairs]
total_usage = foldl add_usage body_usage occ_anald
add_usage body_usage (bndr, (rhs_usage, _))
- = body_usage +++ addRuleUsage rhs_usage bndr
+ = body_usage +++ addRuleUsage rhs_usage bndr
(final_usage, tagged_bndrs) = tagBinders total_usage bndrs
final_bndrs | isEmptyVarSet all_rule_fvs = tagged_bndrs
- | otherwise = map tag_rule_var tagged_bndrs
-
+ | otherwise = map tag_rule_var tagged_bndrs
+
tag_rule_var bndr | bndr `elemVarSet` all_rule_fvs = makeLoopBreaker True bndr
- | otherwise = bndr
+ | otherwise = bndr
all_rule_fvs = bndr_set `intersectVarSet` foldr (unionVarSet . idRuleVars) emptyVarSet bndrs
- -- Mark the binder with OccInfo saying "no preInlineUnconditionally" if
- -- it is used in any rule (lhs or rhs) of the recursive group
+ -- Mark the binder with OccInfo saying "no preInlineUnconditionally" if
+ -- it is used in any rule (lhs or rhs) of the recursive group
---- stuff for dependency analysis of binds -------------------------------
sccs :: [SCC (Node Details)]
sccs = {-# SCC "occAnalBind.scc" #-} stronglyConnCompR rec_edges
- rec_edges :: [Node Details] -- The binders are tagged with correct occ-info
+ rec_edges :: [Node Details] -- The binders are tagged with correct occ-info
rec_edges = {-# SCC "occAnalBind.assoc" #-} zipWith make_node final_bndrs occ_anald
make_node tagged_bndr (_bndr, (rhs_usage, rhs))
- = ((tagged_bndr, rhs, rhs_fvs), idUnique tagged_bndr, out_edges)
- where
- rhs_fvs = intersectUFM_C (\b _ -> b) bndr_set rhs_usage
- out_edges = keysUFM (rhs_fvs `unionVarSet` idRuleVars tagged_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!
+ = ((tagged_bndr, rhs, rhs_fvs), idUnique tagged_bndr, out_edges)
+ where
+ rhs_fvs = intersectUFM_C (\b _ -> b) bndr_set rhs_usage
+ out_edges = keysUFM (rhs_fvs `unionVarSet` idRuleVars tagged_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!
---- Stuff to "re-constitute" bindings from dependency-analysis info ------
do_final_bind (AcyclicSCC ((bndr, rhs, _), _, _)) = NonRec bndr rhs
do_final_bind (CyclicSCC cycle)
- | no_rules = Rec (reOrderCycle cycle)
- | otherwise = Rec (concatMap reOrderRec (stronglyConnCompR loop_breaker_edges))
- where -- See Note [Choosing loop breakers] for looop_breker_edges
- loop_breaker_edges = map mk_node cycle
- mk_node (details@(_bndr, _rhs, rhs_fvs), k, _) = (details, k, new_ks)
- where
- new_ks = keysUFM (extendFvs rule_fv_env rhs_fvs rhs_fvs)
-
-
+ | no_rules = Rec (reOrderCycle cycle)
+ | otherwise = Rec (concatMap reOrderRec (stronglyConnCompR loop_breaker_edges))
+ where -- See Note [Choosing loop breakers] for looop_breker_edges
+ loop_breaker_edges = map mk_node cycle
+ mk_node (details@(_bndr, _rhs, 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 :: 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
+ | 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
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
+ 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, -- Binder
- CoreExpr, -- RHS
- IdSet) -- RHS free vars (*not* include rules)
+type Node details = (details, Unique, [Unique]) -- The Ints are gotten from the Unique,
+ -- which is gotten from the Id.
+type Details = (Id, -- Binder
+ CoreExpr, -- RHS
+ IdSet) -- RHS free vars (*not* include rules)
reOrderRec :: SCC (Node Details)
- -> [(Id,CoreExpr)]
+ -> [(Id,CoreExpr)]
-- Sorted into a plausible order. Enough of the Ids have
--- IAmALoopBreaker pragmas that there are no loops left.
+-- IAmALoopBreaker pragmas that there are no loops left.
reOrderRec (AcyclicSCC ((bndr, rhs, _), _, _)) = [(bndr, rhs)]
-reOrderRec (CyclicSCC cycle) = reOrderCycle cycle
+reOrderRec (CyclicSCC cycle) = reOrderCycle cycle
reOrderCycle :: [Node Details] -> [(Id,CoreExpr)]
reOrderCycle []
= panic "reOrderCycle"
-reOrderCycle [bind] -- Common case of simple self-recursion
+reOrderCycle [bind] -- Common case of simple self-recursion
= [(makeLoopBreaker False bndr, rhs)]
where
((bndr, rhs, _), _, _) = bind
reOrderCycle (bind : binds)
- = -- Choose a loop breaker, mark it no-inline,
- -- do SCC analysis on the rest, and recursively sort them out
+ = -- Choose a loop breaker, mark it no-inline,
+ -- do SCC analysis on the rest, and recursively sort them out
concatMap reOrderRec (stronglyConnCompR unchosen) ++
[(makeLoopBreaker False bndr, rhs)]
(chosen_bind, unchosen) = choose_loop_breaker bind (score bind) [] binds
(bndr, rhs, _) = chosen_bind
- -- This loop looks for the bind with the lowest score
- -- to pick as the loop breaker. The rest accumulate in
+ -- 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
+ = (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
-
- | 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
+ | 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, _), _, _)
| workerExists (idWorkerInfo bndr) = 10
-- Note [Worker inline loop]
- | 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 = 2 -- Data types help with cases
+ | 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 = 2 -- Data types help with cases
-- Note [conapp]
- | inlineCandidate bndr rhs = 1 -- Likely to be inlined
- -- Note [Inline candidates]
+ | inlineCandidate bndr rhs = 1 -- Likely to be inlined
+ -- Note [Inline candidates]
- | otherwise = 0
+ | otherwise = 0
inlineCandidate :: Id -> CoreExpr -> Bool
inlineCandidate _ (Note InlineMe _) = True
-- 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".
+ -- 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]
+ -- 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
wrappers get auto-generated inlinings when importing, and
that can lead to an infinite inlining loop. For example:
rec {
- $wfoo x = ....foo x....
-
- {-loop brk-} foo x = ...$wfoo x...
+ $wfoo x = ....foo x....
+
+ {-loop brk-} foo x = ...$wfoo x...
}
The interface file sees the unfolding for $wfoo, and sees that foo is
; plus1 _ n = Clo plus2 n
- ; plus2 Zero n = n
- ; plus2 (Succ m) n = Succ (plus $: m $: 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
\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
+ -- 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
- _ -> False
+ OneOcc in_lam one_br _ -> not in_lam && one_br
+ _ -> False
\end{code}
addRuleUsage usage id
= foldVarSet add usage (idRuleVars id)
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 many copies
+ -- of the specialised thing can appear
\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 _ (Type t) = (emptyDetails, Type t)
occAnal env (Var v) = (mkOneOcc env v False, Var v)
\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}
\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 (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}
}
-- 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 -- 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 (occAnalAlt alt_env bndr) 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) }
+ -- 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 occ -> extendVarEnv usage bndr (markMany occ)
alt_env = setVanillaCtxt env
- -- Consider x = case v of { True -> (p,q); ... }
- -- Then it's fine to inline p and q
+ -- Consider x = case v of { True -> (p,q); ... }
+ -- Then it's fine to inline p and q
occ_anal_scrut (Var v) (alt1 : other_alts)
- | not (null other_alts) || not (isDefaultAlt alt1)
- = (mkOneOcc env v True, Var v)
+ | 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
+ -- 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
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
+ -- 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
+ | 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') ->
+ = 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
- go _ [] = (emptyDetails, []) -- Too few args
+ go _ [] = (emptyDetails, []) -- Too few 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 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.
+If the case binder occurs at all, the other binders effectively do too.
For example
- case e of x { (a,b) -> rhs }
+ case e of x { (a,b) -> rhs }
is rather like
- let x = (a,b) in rhs
+ 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
+ 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
+ to know when binders are unused. See esp the call to
+ isDeadBinder in Simplify.mkDupableAlt
\begin{code}
occAnalAlt :: OccEnv
= 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 = 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
+ 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
-}
in
(final_usage, (con, final_bndrs, rhs')) }
%************************************************************************
-%* *
+%* *
\subsection[OccurAnal-types]{OccEnv}
-%* *
+%* *
%************************************************************************
\begin{code}
data OccEnv
- = OccEnv OccEncl -- Enclosing context information
- CtxtTy -- Tells about linearity
+ = OccEnv OccEncl -- Enclosing context information
+ CtxtTy -- Tells about linearity
-- 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 []
setVanillaCtxt :: OccEnv -> OccEnv
setVanillaCtxt (OccEnv OccRhs ctxt_ty) = OccEnv OccVanilla ctxt_ty
-setVanillaCtxt other_env = other_env
+setVanillaCtxt other_env = other_env
setCtxt :: OccEnv -> CtxtTy -> OccEnv
setCtxt (OccEnv encl _) ctxt = OccEnv encl ctxt
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
= go ctxt 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 -> [Arg CoreBndr] -> OccEnv
-addAppCtxt (OccEnv encl ctxt) args
+addAppCtxt (OccEnv encl ctxt) args
= OccEnv encl (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
(+++), 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)
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
- _ -> 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}
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
orOccInfo IAmDead info2 = info2
orOccInfo info1 IAmDead = info1
orOccInfo (OneOcc in_lam1 _ int_cxt1)
- (OneOcc in_lam2 _ int_cxt2)
+ (OneOcc in_lam2 _ int_cxt2)
= OneOcc (in_lam1 || in_lam2)
- False -- False, because it occurs in both branches
- (int_cxt1 && int_cxt2)
+ False -- False, because it occurs in both branches
+ (int_cxt1 && int_cxt2)
orOccInfo _ _ = NoOccInfo
\end{code}
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