% (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
core expression with (hopefully) improved usage information.
\begin{code}
-{-# OPTIONS -w #-}
--- The above warning supression flag is a temporary kludge.
--- While working on this module you are encouraged to remove it and fix
--- any warnings in the module. See
--- http://hackage.haskell.org/trac/ghc/wiki/Commentary/CodingStyle#Warnings
--- for details
-
module OccurAnal (
- occurAnalysePgm, occurAnalyseExpr
+ occurAnalysePgm, occurAnalyseExpr
) where
#include "HsVersions.h"
import CoreSyn
import CoreFVs
-import CoreUtils ( exprIsTrivial, isDefaultAlt )
+import CoreUtils ( exprIsTrivial, isDefaultAlt, isExpandableApp, mkCoerce )
import Id
-import IdInfo
-import BasicTypes ( OccInfo(..), isOneOcc, InterestingCxt )
+import NameEnv
+import NameSet
+import Name ( Name, localiseName )
+import BasicTypes
+import Coercion
import VarSet
import VarEnv
-
-import Maybes ( orElse )
-import Digraph ( stronglyConnCompR, SCC(..) )
-import PrelNames ( buildIdKey, foldrIdKey, runSTRepIdKey, augmentIdKey )
-import Unique ( Unique )
-import UniqFM ( keysUFM, intersectsUFM, intersectUFM_C, foldUFM_Directly )
-import Util ( mapAndUnzip )
+import Var
+
+import Maybes ( orElse )
+import Digraph ( SCC(..), stronglyConnCompFromEdgedVerticesR )
+import PrelNames ( buildIdKey, foldrIdKey, runSTRepIdKey, augmentIdKey )
+import Unique
+import UniqFM
+import Util ( mapAndUnzip, filterOut )
+import Bag
import Outputable
-
+import FastString
import Data.List
\end{code}
%************************************************************************
-%* *
+%* *
\subsection[OccurAnal-main]{Counting occurrences: main function}
-%* *
+%* *
%************************************************************************
Here's the externally-callable interface:
\begin{code}
-occurAnalysePgm :: [CoreBind] -> [CoreBind]
-occurAnalysePgm binds
- = snd (go initOccEnv binds)
+occurAnalysePgm :: Maybe (Activation -> Bool) -> [CoreRule] -> [CoreVect]
+ -> [CoreBind] -> [CoreBind]
+occurAnalysePgm active_rule imp_rules vects binds
+ = snd (go (initOccEnv active_rule imp_rules) binds)
where
+ initial_uds = addIdOccs emptyDetails
+ (rulesFreeVars imp_rules `unionVarSet` vectsFreeVars vects)
+ -- The RULES and VECTORISE declarations keep things alive!
+
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 _ []
+ = (initial_uds, [])
+ go env (bind:binds)
+ = (final_usage, bind' ++ binds')
+ where
+ (bs_usage, binds') = go env binds
+ (final_usage, bind') = occAnalBind env env bind bs_usage
occurAnalyseExpr :: CoreExpr -> CoreExpr
- -- Do occurrence analysis, and discard occurence info returned
-occurAnalyseExpr expr = snd (occAnal initOccEnv expr)
+ -- Do occurrence analysis, and discard occurence info returned
+occurAnalyseExpr expr
+ = snd (occAnal (initOccEnv all_active_rules []) expr)
+ where
+ -- To be conservative, we say that all inlines and rules are active
+ all_active_rules = Just (\_ -> True)
\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])
-
-occAnalBind env (NonRec binder rhs) body_usage
- | not (binder `usedIn` body_usage) -- It's not mentioned
+occAnalBind :: OccEnv -- The incoming OccEnv
+ -> OccEnv -- Same, but trimmed by (binderOf bind)
+ -> CoreBind
+ -> UsageDetails -- Usage details of scope
+ -> (UsageDetails, -- Of the whole let(rec)
+ [CoreBind])
+
+occAnalBind env _ (NonRec binder rhs) body_usage
+ | 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 [Rules are extra RHSs]
- [NonRec tagged_binder rhs'])
+ | otherwise -- It's mentioned in the body
+ = (body_usage' +++ rhs_usage3, [NonRec tagged_binder rhs'])
where
(body_usage', tagged_binder) = tagBinder body_usage binder
- (rhs_usage, rhs') = occAnalRhs env tagged_binder rhs
+ (rhs_usage1, rhs') = occAnalRhs env (Just tagged_binder) rhs
+ rhs_usage2 = addIdOccs rhs_usage1 (idUnfoldingVars binder)
+ rhs_usage3 = addIdOccs rhs_usage2 (idRuleVars binder)
+ -- See Note [Rules are extra RHSs] and Note [Rule dependency info]
\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'*.
-
- 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.
+ *treating f's rules as extra RHSs for 'f'*.
+ More concretely, the SCC analysis runs on a graph with an edge
+ from f -> g iff g is mentioned in
+ (a) f's rhs
+ (b) f's RULES
+ These are rec_edges.
+
+ Under (b) 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 avoid infinite inlinings by choosing loop breakers, and
ensuring that a loop breaker cuts each loop. But what is a
- "loop"? In particular, a RULES is like an equation for 'f' that
- is *always* inlined if it are applicable. We do *not* disable
+ "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
+ make sure that the rules themselves always 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 (or its INLINE template if it has one) 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.)
+ See also Note [Finding rule RHS free vars]
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 if 'g' has RHS that mentions 'w', we should add w to
+ g's loop-breaker edges. More concretely there is an edge from f -> g
+ iff
+ (a) g is mentioned in f's RHS
+ (b) h is mentioned in f's RHS, and
+ g appears in the RHS of a RULE of h
+ or a transitive sequence of rules starting with h
+
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
-
-
+ That's why we compute
+ rec_edges for the Rec block analysis
+ loop_breaker_edges for the loop breaker analysis
+
+ * Note [Finding rule RHS free vars]
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+ Consider this real example from Data Parallel Haskell
+ tagZero :: Array Int -> Array Tag
+ {-# INLINE [1] tagZeroes #-}
+ tagZero xs = pmap (\x -> fromBool (x==0)) xs
+
+ {-# RULES "tagZero" [~1] forall xs n.
+ pmap fromBool <blah blah> = tagZero xs #-}
+ So tagZero's RHS mentions pmap, and pmap's RULE mentions tagZero.
+ However, tagZero can only be inlined in phase 1 and later, while
+ the RULE is only active *before* phase 1. So there's no problem.
+
+ To make this work, we look for the RHS free vars only for
+ *active* rules. That's the reason for the is_active argument
+ to idRhsRuleVars, and the occ_rule_act field of the OccEnv.
+
* 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
+ Remember 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.)
+ not choosen as a loop breaker.) Why not? Because then we
+ drop the binding for 'g', which leaves it out of scope in the
+ RULE!
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
+ Inline postInlineUnconditionally
+ 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?
- Consider
- x = y
- RULE f x = 4
+ 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 occAnalBind.
+ 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
+ * Note [Inline rules]
+ ~~~~~~~~~~~~~~~~~~~
+ None of the above stuff about RULES applies to Inline Rules,
+ stored in a CoreUnfolding. The unfolding, if any, is simplified
+ at the same time as the regular RHS of the function, so it should
+ be treated *exactly* like an extra RHS.
+
+ There is a danger that we'll be sub-optimal if we see this
+ f = ...f...
+ [INLINE f = ..no f...]
+ where f is recursive, but the INLINE is not. This can just about
+ happen with a sufficiently odd set of rules; eg
+
+ foo :: Int -> Int
+ {-# INLINE [1] foo #-}
+ foo x = x+1
+
+ bar :: Int -> Int
+ {-# INLINE [1] bar #-}
+ bar x = foo x + 1
+
+ {-# RULES "foo" [~1] forall x. foo x = bar x #-}
+
+ Here the RULE makes bar recursive; but it's INLINE pragma remains
+ non-recursive. It's tempting to then say that 'bar' should not be
+ a loop breaker, but an attempt to do so goes wrong in two ways:
+ a) We may get
+ $df = ...$cfoo...
+ $cfoo = ...$df....
+ [INLINE $cfoo = ...no-$df...]
+ But we want $cfoo to depend on $df explicitly so that we
+ put the bindings in the right order to inline $df in $cfoo
+ and perhaps break the loop altogether. (Maybe this
+ b)
+
+
+
Example [eftInt]
~~~~~~~~~~~~~~~
Example (from GHC.Enum):
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
- | otherwise
- = (final_usage, map ({-# SCC "occAnalBind.dofinal" #-} do_final_bind) sccs)
+occAnalBind _ env (Rec pairs) body_usage
+ = foldr (occAnalRec env) (body_usage, []) sccs
+ -- For a recursive group, we
+ -- * occ-analyse all the RHSs
+ -- * compute strongly-connected components
+ -- * feed those components to occAnalRec
where
- bndrs = map fst pairs
- bndr_set = mkVarSet bndrs
+ -------------Dependency analysis ------------------------------
+ bndr_set = mkVarSet (map fst pairs)
+
+ sccs :: [SCC (Node Details)]
+ sccs = {-# SCC "occAnalBind.scc" #-} stronglyConnCompFromEdgedVerticesR rec_edges
+
+ rec_edges :: [Node Details]
+ rec_edges = {-# SCC "occAnalBind.assoc" #-} map make_node pairs
+
+ make_node (bndr, rhs)
+ = (details, varUnique bndr, keysUFM out_edges)
+ where
+ details = ND { nd_bndr = bndr, nd_rhs = rhs'
+ , nd_uds = rhs_usage3, nd_inl = inl_fvs}
+
+ (rhs_usage1, rhs') = occAnalRhs env Nothing rhs
+ rhs_usage2 = addIdOccs rhs_usage1 rule_fvs -- Note [Rules are extra RHSs]
+ rhs_usage3 = addIdOccs rhs_usage2 unf_fvs
+ unf = realIdUnfolding bndr -- Ignore any current loop-breaker flag
+ unf_fvs = stableUnfoldingVars unf
+ rule_fvs = idRuleVars bndr -- See Note [Rule dependency info]
+
+ inl_fvs = rhs_fvs `unionVarSet` unf_fvs
+ rhs_fvs = intersectUFM_C (\b _ -> b) bndr_set rhs_usage1
+ out_edges = intersectUFM_C (\b _ -> b) bndr_set rhs_usage3
+ -- (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 :: OccEnv -> SCC (Node Details)
+ -> (UsageDetails, [CoreBind])
+ -> (UsageDetails, [CoreBind])
+
+ -- The NonRec case is just like a Let (NonRec ...) above
+occAnalRec _ (AcyclicSCC (ND { nd_bndr = bndr, nd_rhs = rhs, nd_uds = rhs_usage}, _, _))
+ (body_usage, binds)
+ | not (bndr `usedIn` body_usage)
+ = (body_usage, binds)
- ---------------------------------------
+ | otherwise -- It's mentioned in the body
+ = (body_usage' +++ rhs_usage,
+ 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 env (CyclicSCC nodes) (body_usage, binds)
+ | not (any (`usedIn` body_usage) bndrs) -- NB: look at body_usage, not total_usage
+ = (body_usage, binds) -- Dead code
- -------------Dependency analysis ------------------------------
- occ_anald :: [(Id, (UsageDetails, CoreExpr))]
- -- 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
-
- (final_usage, tagged_bndrs) = tagBinders total_usage bndrs
- final_bndrs | isEmptyVarSet all_rule_fvs = tagged_bndrs
- | otherwise = map tag_rule_var tagged_bndrs
-
- tag_rule_var bndr | bndr `elemVarSet` all_rule_fvs = makeLoopBreaker True 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
-
- ---- stuff for dependency analysis of binds -------------------------------
- sccs :: [SCC (Node Details)]
- sccs = {-# SCC "occAnalBind.scc" #-} stronglyConnCompR rec_edges
+ | otherwise -- At this point we always build a single Rec
+ = (final_usage, Rec pairs : binds)
- 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
+ bndrs = [b | (ND { nd_bndr = b }, _, _) <- nodes]
+ bndr_set = mkVarSet bndrs
+ non_boring bndr = isId bndr &&
+ (isStableUnfolding (realIdUnfolding bndr) || idHasRules bndr)
+
+ ----------------------------
+ -- Tag the binders with their occurrence info
+ total_usage = foldl add_usage body_usage nodes
+ add_usage usage_so_far (ND { nd_uds = rhs_usage }, _, _) = usage_so_far +++ rhs_usage
+ (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 (details@ND { nd_bndr = bndr }, k, ks)
+ = (usage `delVarEnv` bndr, (details { nd_bndr = bndr2 }, 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 | any non_boring bndrs
+ = foldr (reOrderRec 0) [] $
+ stronglyConnCompFromEdgedVerticesR loop_breaker_edges
+ | otherwise
+ = reOrderCycle 0 tagged_nodes []
+
+ -- See Note [Choosing loop breakers] for loop_breaker_edges
+ loop_breaker_edges = map mk_node tagged_nodes
+ mk_node (details@(ND { nd_inl = inl_fvs }), k, _) = (details, k, new_ks)
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)
+ new_ks = keysUFM (fst (extendFvs rule_fv_env inl_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
+ 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 = transClosureFV init_rule_fvs
+ init_rule_fvs
+ | Just is_active <- occ_rule_act env -- See Note [Finding rule RHS free vars]
+ = [ (b, rule_fvs)
+ | b <- bndrs
+ , isId b
+ , let rule_fvs = idRuleRhsVars is_active b
+ `intersectVarSet` bndr_set
+ , not (isEmptyVarSet rule_fvs)]
+ | otherwise
+ = []
\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)
+data Details
+ = ND { nd_bndr :: Id -- Binder
+ , nd_rhs :: CoreExpr -- RHS
-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 (AcyclicSCC ((bndr, rhs, _), _, _)) = [(bndr, rhs)]
-reOrderRec (CyclicSCC cycle) = reOrderCycle cycle
+ , nd_uds :: UsageDetails -- Usage from RHS,
+ -- including RULES and InlineRule unfolding
-reOrderCycle :: [Node Details] -> [(Id,CoreExpr)]
-reOrderCycle []
- = panic "reOrderCycle"
-reOrderCycle [bind] -- Common case of simple self-recursion
- = [(makeLoopBreaker False bndr, rhs)]
- where
- ((bndr, rhs, _), _, _) = bind
+ , nd_inl :: IdSet -- Other binders *from this Rec group* mentioned in
+ } -- its InlineRule unfolding (if present)
+ -- AND the RHS
+ -- but *excluding* any RULES
+ -- This is the IdSet that may be used if the Id is inlined
-reOrderCycle (bind : binds)
- = -- 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)]
+reOrderRec :: Int -> 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.
+reOrderRec _ (AcyclicSCC (ND { nd_bndr = bndr, nd_rhs = rhs }, _, _))
+ pairs = (bndr, rhs) : pairs
+reOrderRec depth (CyclicSCC cycle) pairs = reOrderCycle depth cycle pairs
+reOrderCycle :: Int -> [Node Details] -> [(Id,CoreExpr)] -> [(Id,CoreExpr)]
+reOrderCycle _ [] _
+ = panic "reOrderCycle"
+reOrderCycle _ [(ND { nd_bndr = bndr, nd_rhs = rhs }, _, _)] pairs
+ = -- Common case of simple self-recursion
+ (makeLoopBreaker False bndr, rhs) : pairs
+
+reOrderCycle depth (bind : binds) pairs
+ = -- Choose a loop breaker, mark it no-inline,
+ -- do SCC analysis on the rest, and recursively sort them out
+-- pprTrace "reOrderCycle" (ppr [b | (ND { nd_bndr = b }, _, _) <- bind:binds]) $
+ foldr (reOrderRec new_depth)
+ ([ (makeLoopBreaker False bndr, rhs)
+ | (ND { nd_bndr = bndr, nd_rhs = rhs }, _, _) <- chosen_binds] ++ pairs)
+ (stronglyConnCompFromEdgedVerticesR unchosen)
where
- (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
- 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
+ (chosen_binds, unchosen) = choose_loop_breaker [bind] (score bind) [] binds
+
+ approximate_loop_breaker = depth >= 2
+ new_depth | approximate_loop_breaker = 0
+ | otherwise = depth+1
+ -- After two iterations (d=0, d=1) give up
+ -- and approximate, returning to d=0
+
+ -- This loop looks for the bind with the lowest score
+ -- to pick as the loop breaker. The rest accumulate in
+ choose_loop_breaker loop_binds _loop_sc acc []
+ = (loop_binds, acc) -- Done
+
+ -- If approximate_loop_breaker is True, we pick *all*
+ -- nodes with lowest score, else just one
+ -- See Note [Complexity of loop breaking]
+ choose_loop_breaker loop_binds loop_sc acc (bind : binds)
+ | sc < loop_sc -- Lower score so pick this new one
+ = choose_loop_breaker [bind] sc (loop_binds ++ acc) binds
+
+ | approximate_loop_breaker && sc == loop_sc
+ = choose_loop_breaker (bind : loop_binds) loop_sc acc binds
+
+ | otherwise -- Higher score so don't pick it
+ = choose_loop_breaker loop_binds 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 { nd_bndr = bndr, nd_rhs = rhs }, _, _)
+ | not (isId bndr) = 100 -- A type or cercion variable is never a loop breaker
+
+ | isDFunId bndr = 9 -- Never choose a DFun as a loop breaker
+ -- Note [DFuns should not be loop breakers]
+
+ | Just inl_source <- isStableCoreUnfolding_maybe (idUnfolding bndr)
+ = case inl_source of
+ InlineWrapper {} -> 10 -- Note [INLINE pragmas]
+ _other -> 3 -- Data structures are more important than this
+ -- so that dictionary/method recursion unravels
+ -- Note that this case hits all InlineRule things, so we
+ -- never look at 'rhs for InlineRule stuff. That's right, because
+ -- 'rhs' is irrelevant for inlining things with an InlineRule
+
+ | is_con_app rhs = 5 -- Data types help with cases: Note [Constructor applications]
+
+ | exprIsTrivial rhs = 10 -- 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
- | 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
- -- Note [conapp]
-
- | inlineCandidate bndr rhs = 1 -- Likely to be inlined
- -- Note [Inline candidates]
-
- | otherwise = 0
-
- inlineCandidate :: Id -> CoreExpr -> Bool
- inlineCandidate id (Note InlineMe _) = True
- inlineCandidate id rhs = isOneOcc (idOccInfo id)
-
- -- Note [conapp]
- --
- -- It's really really important to inline dictionaries. Real
- -- example (the Enum Ordering instance from GHC.Base):
+
+-- 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
+
+ | isOneOcc (idOccInfo bndr) = 2 -- Likely to be inlined
+
+ | canUnfold (realIdUnfolding bndr) = 1
+ -- The Id has some kind of unfolding
+ -- Ignore loop-breaker-ness here because that is what we are setting!
+
+ | otherwise = 0
+
+ -- Checking for a constructor application
+ -- 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
--
- -- 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
+ -- However we *also* treat (\x. C p q) as a con-app-like thing,
+ -- Note [Closure conversion]
+ is_con_app (Var v) = isConLikeId v
is_con_app (App f _) = is_con_app f
- is_con_app (Lam b e) = is_con_app e
+ is_con_app (Lam _ e) = is_con_app e
is_con_app (Note _ e) = is_con_app e
- is_con_app other = False
+ 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)
+-- Set the loop-breaker flag: see Note [Weak loop breakers]
+makeLoopBreaker weak bndr
+ = ASSERT2( isId bndr, ppr bndr ) setIdOccInfo bndr (IAmALoopBreaker weak)
\end{code}
-Note [Worker inline loop]
-~~~~~~~~~~~~~~~~~~~~~~~~
-Never choose a wrapper as the loop breaker! Because
-wrappers get auto-generated inlinings when importing, and
-that can lead to an infinite inlining loop. For example:
- rec {
- $wfoo x = ....foo x....
+Note [Complexity of loop breaking]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+The loop-breaking algorithm knocks out one binder at a time, and
+performs a new SCC analysis on the remaining binders. That can
+behave very badly in tightly-coupled groups of bindings; in the
+worst case it can be (N**2)*log N, because it does a full SCC
+on N, then N-1, then N-2 and so on.
+
+To avoid this, we switch plans after 2 (or whatever) attempts:
+ Plan A: pick one binder with the lowest score, make it
+ a loop breaker, and try again
+ Plan B: pick *all* binders with the lowest score, make them
+ all loop breakers, and try again
+Since there are only a small finite number of scores, this will
+terminate in a constant number of iterations, rather than O(N)
+iterations.
+
+You might thing that it's very unlikely, but RULES make it much
+more likely. Here's a real example from Trac #1969:
+ Rec { $dm = \d.\x. op d
+ {-# RULES forall d. $dm Int d = $s$dm1
+ forall d. $dm Bool d = $s$dm2 #-}
- {-loop brk-} foo x = ...$wfoo x...
- }
+ dInt = MkD .... opInt ...
+ dInt = MkD .... opBool ...
+ opInt = $dm dInt
+ opBool = $dm dBool
+
+ $s$dm1 = \x. op dInt
+ $s$dm2 = \x. op dBool }
+The RULES stuff means that we can't choose $dm as a loop breaker
+(Note [Choosing loop breakers]), so we must choose at least (say)
+opInt *and* opBool, and so on. The number of loop breakders is
+linear in the number of instance declarations.
+
+Note [INLINE pragmas]
+~~~~~~~~~~~~~~~~~~~~~
+Avoid choosing 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.
+
+Usually this is just a question of optimisation. But a particularly
+bad 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 $wfoo, and sees that foo is
strict (and hence it gets an auto-generated wrapper). Result: an
infinite inlining in the importing scope. So be a bit careful if you
change this. A good example is Tree.repTree in
nofib/spectral/minimax. If the repTree wrapper is chosen as the loop
-breaker then compiling Game.hs goes into an infinite loop (this
-happened when we gave is_con_app a lower score than inline candidates).
+breaker then compiling Game.hs goes into an infinite loop. This
+happened when we gave is_con_app a lower score than inline candidates:
+
+ Tree.repTree
+ = __inline_me (/\a. \w w1 w2 ->
+ case Tree.$wrepTree @ a w w1 w2 of
+ { (# ww1, ww2 #) -> Branch @ a ww1 ww2 })
+ Tree.$wrepTree
+ = /\a w w1 w2 ->
+ (# w2_smP, map a (Tree a) (Tree.repTree a w1 w) (w w2) #)
+
+Here we do *not* want to choose 'repTree' as the loop breaker.
+
+Note [DFuns should not be loop breakers]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+It's particularly bad to make a DFun into a loop breaker. See
+Note [How instance declarations are translated] in TcInstDcls
+
+We give DFuns a higher score than ordinary CONLIKE things because
+if there's a choice we want the DFun to be the non-looop breker. Eg
+
+rec { sc = /\ a \$dC. $fBWrap (T a) ($fCT @ a $dC)
+
+ $fCT :: forall a_afE. (Roman.C a_afE) => Roman.C (Roman.T a_afE)
+ {-# DFUN #-}
+ $fCT = /\a \$dC. MkD (T a) ((sc @ a $dC) |> blah) ($ctoF @ a $dC)
+ }
+
+Here 'sc' (the superclass) looks CONLIKE, but we'll never get to it
+if we can't unravel the DFun first.
+
+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]
~~~~~~~~~~~~~~~~~~~~~~~~~
; 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)
-
-occAnalRhs env id rhs
+ -> Maybe Id -> CoreExpr -- Binder and rhs
+ -- Just b => non-rec, and alrady tagged with occurrence info
+ -- Nothing => Rec, no occ info
+ -> (UsageDetails, CoreExpr)
+ -- Returned usage details covers only the RHS,
+ -- and *not* the RULE or INLINE template for the Id
+occAnalRhs env mb_bndr 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...
-
- certainly_inline id = case idOccInfo id of
- OneOcc in_lam one_br _ -> not in_lam && one_br
- other -> False
-\end{code}
-
-
-
-\begin{code}
-addRuleUsage :: UsageDetails -> Id -> UsageDetails
--- Add the usage from RULES in Id to the usage
-addRuleUsage usage id
- = foldVarSet add usage (idRuleVars id)
+ -- See Note [Cascading inlines]
+ ctxt = case mb_bndr of
+ Just b | certainly_inline b -> env
+ _other -> rhsCtxt env
+
+ certainly_inline bndr -- See Note [Cascading inlines]
+ = case idOccInfo bndr of
+ OneOcc in_lam one_br _ -> not in_lam && one_br && active && not_stable
+ _ -> False
+ where
+ active = isAlwaysActive (idInlineActivation bndr)
+ not_stable = not (isStableUnfolding (idUnfolding bndr))
+
+addIdOccs :: UsageDetails -> VarSet -> UsageDetails
+addIdOccs usage id_set = foldVarSet add usage id_set
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 | isId v = addOneOcc u v NoOccInfo
+ | otherwise = u
+ -- Give a non-committal binder info (i.e NoOccInfo) 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}
+Note [Cascading inlines]
+~~~~~~~~~~~~~~~~~~~~~~~~
+By default we use an rhsCtxt for the RHS of a binding. This tells the
+occ anal n that it's looking at an RHS, which has an effect in
+occAnalApp. In particular, for constructor applications, it makes
+the arguments appear to have NoOccInfo, so that we don't inline into
+them. Thus x = f y
+ k = Just x
+we do not want to inline x.
+
+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.
+
+So, when analysing the RHS of x3 we notice that x3 will itself
+definitely inline the next time round, and so we analyse x3's rhs in
+an ordinary context, not rhsCtxt. Hence the "certainly_inline" stuff.
+
+Annoyingly, we have to approximiate SimplUtils.preInlineUnconditionally.
+If we say "yes" when preInlineUnconditionally says "no" the simplifier iterates
+indefinitely:
+ x = f y
+ k = Just x
+inline ==>
+ k = Just (f y)
+float ==>
+ x1 = f y
+ k = Just x1
+
+This is worse than the slow cascade, so we only want to say "certainly_inline"
+if it really is certain. Look at the note with preInlineUnconditionally
+for the various clauses.
+
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 env (Var v) = (mkOneOcc env v False, Var v)
+occAnal _ expr@(Type _) = (emptyDetails, expr)
+occAnal _ expr@(Lit _) = (emptyDetails, expr)
+occAnal env expr@(Var v) = (mkOneOcc env v False, expr)
-- 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.
-\end{code}
-
-We regard variables that occur as constructor arguments as "dangerousToDup":
-\begin{verbatim}
-module A where
-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}
-
-We feel free to duplicate the WHNF (True,y), but that means
-that y may be duplicated thereby.
+occAnal _ (Coercion co)
+ = (addIdOccs emptyDetails (coVarsOfCo co), Coercion co)
+ -- See Note [Gather occurrences of coercion veriables]
+\end{code}
-If we aren't careful we duplicate the (expensive x) call!
-Constructors are rather like lambdas in this way.
+Note [Gather occurrences of coercion veriables]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+We need to gather info about what coercion variables appear, so that
+we can sort them into the right place when doing dependency analysis.
\begin{code}
-occAnal env expr@(Lit lit) = (emptyDetails, expr)
\end{code}
\begin{code}
-occAnal env (Note InlineMe 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.
+ let usage1 = markManyIf (isRhsEnv env) usage
+ usage2 = addIdOccs usage1 (coVarsOfCo co)
+ -- See Note [Gather occurrences of coercion veriables]
+ in (usage2, 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.
}
\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.
occAnal env expr@(Lam _ _)
= 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
+ (final_usage, tagged_binders) = tagLamBinders body_usage binders'
+ -- Use binders' to put one-shot info on the lambdas
+
+ -- 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 (trimOccEnv env binders)
+ -- 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_usage1, tagged_bndr) = tag_case_bndr 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) }
- addCaseBndrUsage usage = case lookupVarEnv usage bndr of
- 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
+ -- 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) }
+ tag_case_bndr usage bndr
+ = case lookupVarEnv usage bndr of
+ Nothing -> (usage, setIdOccInfo bndr IAmDead)
+ Just _ -> (usage `delVarEnv` bndr, setIdOccInfo bndr NoOccInfo)
+
+ alt_env = mkAltEnv env scrut bndr
+ occ_anal_alt = occAnalAlt alt_env bndr
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 occAnalBind env bind body_usage of { (final_usage, new_binds) ->
+ = case occAnal env_body body of { (body_usage, body') ->
+ case occAnalBind env env_body bind body_usage of { (final_usage, new_binds) ->
(final_usage, mkLets new_binds body') }}
+ where
+ env_body = trimOccEnv env (bindersOf bind)
+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.
+Note [Arguments of let-bound constructors]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Consider
+ f x = let y = expensive x in
+ let z = (True,y) in
+ (case z of {(p,q)->q}, case z of {(p,q)->q})
+We feel free to duplicate the WHNF (True,y), but that means
+that y may be duplicated thereby.
+
+If we aren't careful we duplicate the (expensive x) call!
+Constructors are rather like lambdas in this way.
+
\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
+ final_args_uds = markManyIf (isRhsEnv env && is_exp) args_uds
+ -- 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
+ -- arguments are just variables, or trivial expressions.
+ --
+ -- This is the *whole point* of the isRhsEnv predicate
+ -- See Note [Arguments of let-bound constructors]
in
(fun_uds +++ final_args_uds, mkApps (Var fun) args') }
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_exp = isExpandableApp fun (valArgCount args)
+ -- See Note [CONLIKE pragma] in BasicTypes
+ -- The definition of is_exp should match that in
+ -- Simplify.prepareRhs
+
+ -- 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
--- 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
--- arguments are just variables, or trivial expressions.
---
--- 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
-
-
-appSpecial :: OccEnv
- -> Int -> CtxtTy -- Argument number, and context to use for it
- -> [CoreExpr]
- -> (UsageDetails, [CoreExpr])
+
+markManyIf :: Bool -- If this is true
+ -> UsageDetails -- Then do markMany on this
+ -> UsageDetails
+markManyIf True uds = mapVarEnv markMany uds
+markManyIf False uds = uds
+
+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 [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.) It really helps 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 :: OccEnv
+ -> CoreBndr
+ -> CoreAlt
+ -> (UsageDetails, Alt IdWithOccInfo)
occAnalAlt env case_bndr (con, bndrs, rhs)
- = case occAnal env rhs of { (rhs_usage, rhs') ->
+ = let
+ env' = trimOccEnv env bndrs
+ in
+ case occAnal env' rhs of { (rhs_usage1, rhs1) ->
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
--}
+ proxies = getProxies env' case_bndr
+ (rhs_usage2, rhs2) = foldrBag wrapProxy (rhs_usage1, rhs1) proxies
+ (alt_usg, tagged_bndrs) = tagLamBinders rhs_usage2 bndrs
+ bndrs' = tagged_bndrs -- See Note [Binders in case alternatives]
in
- (final_usage, (con, final_bndrs, rhs')) }
+ (alt_usg, (con, bndrs', rhs2)) }
+
+wrapProxy :: ProxyBind -> (UsageDetails, CoreExpr) -> (UsageDetails, CoreExpr)
+wrapProxy (bndr, rhs_var, co) (body_usg, body)
+ | not (bndr `usedIn` body_usg)
+ = (body_usg, body)
+ | otherwise
+ = (body_usg' +++ rhs_usg, Let (NonRec tagged_bndr rhs) body)
+ where
+ (body_usg', tagged_bndr) = tagBinder body_usg bndr
+ rhs_usg = unitVarEnv rhs_var NoOccInfo -- We don't need exact info
+ rhs = mkCoerce co (Var (zapIdOccInfo rhs_var)) -- See Note [Zap case binders in proxy bindings]
\end{code}
%************************************************************************
-%* *
-\subsection[OccurAnal-types]{OccEnv}
-%* *
+%* *
+ 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_proxy :: ProxyEnv
+ , occ_rule_fvs :: ImpRuleUsage
+ , occ_rule_act :: Maybe (Activation -> Bool) -- Nothing => Rules are inactive
+ -- See Note [Finding rule RHS free vars]
+ }
+
+-----------------------------
-- 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
+
+instance Outputable OccEncl where
+ ppr OccRhs = ptext (sLit "occRhs")
+ ppr OccVanilla = ptext (sLit "occVanilla")
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 []
+initOccEnv :: Maybe (Activation -> Bool) -> [CoreRule]
+ -> OccEnv
+initOccEnv active_rule imp_rules
+ = OccEnv { occ_encl = OccVanilla
+ , occ_ctxt = []
+ , occ_proxy = PE emptyVarEnv emptyVarSet
+ , occ_rule_fvs = findImpRuleUsage active_rule imp_rules
+ , occ_rule_act = active_rule }
-vanillaCtxt = OccEnv OccVanilla []
-rhsCtxt = OccEnv OccRhs []
+vanillaCtxt :: OccEnv -> OccEnv
+vanillaCtxt env = env { occ_encl = OccVanilla, occ_ctxt = [] }
-isRhsEnv (OccEnv OccRhs _) = True
-isRhsEnv (OccEnv OccVanilla _) = False
+rhsCtxt :: OccEnv -> OccEnv
+rhsCtxt env = env { occ_encl = OccRhs, occ_ctxt = [] }
-setVanillaCtxt :: OccEnv -> OccEnv
-setVanillaCtxt (OccEnv OccRhs ctxt_ty) = OccEnv OccVanilla ctxt_ty
-setVanillaCtxt other_env = other_env
+setCtxtTy :: OccEnv -> CtxtTy -> OccEnv
+setCtxtTy env ctxt = env { occ_ctxt = ctxt }
-setCtxt :: OccEnv -> CtxtTy -> OccEnv
-setCtxt (OccEnv encl _) ctxt = OccEnv encl 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}
%************************************************************************
-%* *
+%* *
+ ImpRuleUsage
+%* *
+%************************************************************************
+
+\begin{code}
+type ImpRuleUsage = NameEnv UsageDetails
+ -- Maps an *imported* Id f to the UsageDetails for *local* Ids
+ -- used on the RHS for a *local* rule for f.
+\end{code}
+
+Note [ImpRuleUsage]
+~~~~~~~~~~~~~~~~
+Consider this, where A.g is an imported Id
+
+ f x = A.g x
+ {-# RULE "foo" forall x. A.g x = f x #-}
+
+Obviously there's a loop, but the danger is that the occurrence analyser
+will say that 'f' is not a loop breaker. Then the simplifier will
+optimise 'f' to
+ f x = f x
+and then gaily inline 'f'. Result infinite loop. More realistically,
+these kind of rules are generated when specialising imported INLINABLE Ids.
+
+Solution: treat an occurrence of A.g as an occurrence of all the local Ids
+that occur on the RULE's RHS. This mapping from imported Id to local Ids
+is held in occ_rule_fvs.
+
+\begin{code}
+findImpRuleUsage :: Maybe (Activation -> Bool) -> [CoreRule] -> ImpRuleUsage
+-- Find the *local* Ids that can be reached transitively,
+-- via local rules, from each *imported* Id.
+-- Sigh: this function seems more complicated than it is really worth
+findImpRuleUsage Nothing _ = emptyNameEnv
+findImpRuleUsage (Just is_active) rules
+ = mkNameEnv [ (f, mapUFM (\_ -> NoOccInfo) ls)
+ | f <- rule_names
+ , let ls = find_lcl_deps f
+ , not (isEmptyVarSet ls) ]
+ where
+ rule_names = map ru_fn rules
+ rule_name_set = mkNameSet rule_names
+
+ imp_deps :: NameEnv VarSet
+ -- (f,g) means imported Id 'g' appears in RHS of
+ -- rule for imported Id 'f', *or* does so transitively
+ imp_deps = foldr add_imp emptyNameEnv rules
+ add_imp rule acc
+ | is_active (ruleActivation rule)
+ = extendNameEnv_C unionVarSet acc (ru_fn rule)
+ (exprSomeFreeVars keep_imp (ru_rhs rule))
+ | otherwise = acc
+ keep_imp v = isId v && (idName v `elemNameSet` rule_name_set)
+ full_imp_deps = transClosureFV (ufmToList imp_deps)
+
+ lcl_deps :: NameEnv VarSet
+ -- (f, l) means localId 'l' appears immediately
+ -- in the RHS of a rule for imported Id 'f'
+ -- Remember, many rules might have the same ru_fn
+ -- so we do need to fold
+ lcl_deps = foldr add_lcl emptyNameEnv rules
+ add_lcl rule acc = extendNameEnv_C unionVarSet acc (ru_fn rule)
+ (exprFreeIds (ru_rhs rule))
+
+ find_lcl_deps :: Name -> VarSet
+ find_lcl_deps f
+ = foldVarSet (unionVarSet . lookup_lcl . idName) (lookup_lcl f)
+ (lookupNameEnv full_imp_deps f `orElse` emptyVarSet)
+ lookup_lcl :: Name -> VarSet
+ lookup_lcl g = lookupNameEnv lcl_deps g `orElse` emptyVarSet
+
+-------------
+transClosureFV :: Uniquable a => [(a, VarSet)] -> UniqFM VarSet
+-- If (f,g), (g,h) are in the input, then (f,h) is in the output
+transClosureFV fv_list
+ | no_change = env
+ | otherwise = transClosureFV new_fv_list
+ where
+ env = listToUFM fv_list
+ (no_change, new_fv_list) = mapAccumL bump True fv_list
+ bump no_change (b,fvs)
+ | no_change_here = (no_change, (b,fvs))
+ | otherwise = (False, (b,new_fvs))
+ where
+ (new_fvs, no_change_here) = extendFvs env fvs
+
+-------------
+extendFvs :: UniqFM VarSet -> VarSet -> (VarSet, Bool)
+-- (extendFVs env s) returns
+-- (s `union` env(s), env(s) `subset` s)
+extendFvs env s
+ = foldVarSet add (s, True) s
+ where
+ add v (vs, no_change_so_far)
+ = case lookupUFM env v of
+ Just fvs | not (fvs `subVarSet` s)
+ -> (vs `unionVarSet` fvs, False)
+ _ -> (vs, no_change_so_far)
+\end{code}
+
+
+%************************************************************************
+%* *
+ ProxyEnv
+%* *
+%************************************************************************
+
+\begin{code}
+data ProxyEnv -- See Note [ProxyEnv]
+ = PE (IdEnv -- Domain = scrutinee variables
+ (Id, -- The scrutinee variable again
+ [(Id,Coercion)])) -- The case binders that it maps to
+ VarSet -- Free variables of both range and domain
+\end{code}
+
+Note [ProxyEnv]
+~~~~~~~~~~~~~~~
+The ProxyEnv keeps track of the connection between case binders and
+scrutinee. Specifically, if
+ sc |-> (sc, [...(cb, co)...])
+is a binding in the ProxyEnv, then
+ cb = sc |> coi
+Typically we add such a binding when encountering the case expression
+ case (sc |> coi) of cb { ... }
+
+Things to note:
+ * The domain of the ProxyEnv is the variable (or casted variable)
+ scrutinees of enclosing cases. This is additionally used
+ to ensure we gather occurrence info even for GlobalId scrutinees;
+ see Note [Binder swap for GlobalId scrutinee]
+
+ * The ProxyEnv is just an optimisation; you can throw away any
+ element without losing correctness. And we do so when pushing
+ it inside a binding (see trimProxyEnv).
+
+ * One scrutinee might map to many case binders: Eg
+ case sc of cb1 { DEFAULT -> ....case sc of cb2 { ... } .. }
+
+INVARIANTS
+ * If sc1 |-> (sc2, [...(cb, co)...]), then sc1==sc2
+ It's a UniqFM and we sometimes need the domain Id
+
+ * Any particular case binder 'cb' occurs only once in entire range
+
+ * No loops
+
+The Main Reason for having a ProxyEnv is so that when we encounter
+ case e of cb { pi -> ri }
+we can find all the in-scope variables derivable from 'cb',
+and effectively add let-bindings for them (or at least for the
+ones *mentioned* in ri) thus:
+ case e of cb { pi -> let { x = ..cb..; y = ...cb.. }
+ in ri }
+In this way we'll replace occurrences of 'x', 'y' with 'cb',
+which implements the Binder-swap idea (see Note [Binder swap])
+
+The function getProxies finds these bindings; then we
+add just the necessary ones, using wrapProxy.
+
+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 [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 [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 one use for the (small) occ_proxy env 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].
+
+Note [getProxies is subtle]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~
+The code for getProxies isn't all that obvious. Consider
+
+ case v |> cov of x { DEFAULT ->
+ case x |> cox1 of y { DEFAULT ->
+ case x |> cox2 of z { DEFAULT -> r
+
+These will give us a ProxyEnv looking like:
+ x |-> (x, [(y, cox1), (z, cox2)])
+ v |-> (v, [(x, cov)])
+
+From this we want to extract the bindings
+ x = z |> sym cox2
+ v = x |> sym cov
+ y = x |> cox1
+
+Notice that later bindings may mention earlier ones, and that
+we need to go "both ways".
+
+Note [Zap case binders in proxy bindings]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+From the original
+ case x of cb(dead) { p -> ...x... }
+we will get
+ case x of cb(live) { p -> let x = cb in ...x... }
+
+Core Lint never expects to find an *occurence* of an Id marked
+as Dead, so we must zap the OccInfo on cb before making the
+binding x = cb. See Trac #5028.
+
+Historical note [no-case-of-case]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+We *used* to suppress the binder-swap in case expressions 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.
+
+\begin{code}
+extendProxyEnv :: ProxyEnv -> Id -> Coercion -> Id -> ProxyEnv
+-- (extendPE x co y) typically arises from
+-- case (x |> co) of y { ... }
+-- It extends the proxy env with the binding
+-- y = x |> co
+extendProxyEnv pe scrut co case_bndr
+ | scrut == case_bndr = PE env1 fvs1 -- If case_bndr shadows scrut,
+ | otherwise = PE env2 fvs2 -- don't extend
+ where
+ PE env1 fvs1 = trimProxyEnv pe [case_bndr]
+ env2 = extendVarEnv_Acc add single env1 scrut1 (case_bndr,co)
+ single cb_co = (scrut1, [cb_co])
+ add cb_co (x, cb_cos) = (x, cb_co:cb_cos)
+ fvs2 = fvs1 `unionVarSet` tyCoVarsOfCo co
+ `extendVarSet` case_bndr
+ `extendVarSet` scrut1
+
+ scrut1 = mkLocalId (localiseName (idName scrut)) (idType scrut)
+ -- Localise the scrut_var before shadowing it; 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]
+ -- Also we don't want any INLINE or NOINLINE pragmas!
+
+-----------
+type ProxyBind = (Id, Id, Coercion)
+ -- (scrut variable, case-binder variable, coercion)
+
+getProxies :: OccEnv -> Id -> Bag ProxyBind
+-- Return a bunch of bindings [...(xi,ei)...]
+-- such that let { ...; xi=ei; ... } binds the xi using y alone
+-- See Note [getProxies is subtle]
+getProxies (OccEnv { occ_proxy = PE pe _ }) case_bndr
+ = -- pprTrace "wrapProxies" (ppr case_bndr) $
+ go_fwd case_bndr
+ where
+ fwd_pe :: IdEnv (Id, Coercion)
+ fwd_pe = foldVarEnv add1 emptyVarEnv pe
+ where
+ add1 (x,ycos) env = foldr (add2 x) env ycos
+ add2 x (y,co) env = extendVarEnv env y (x,co)
+
+ go_fwd :: Id -> Bag ProxyBind
+ -- Return bindings derivable from case_bndr
+ go_fwd case_bndr = -- pprTrace "go_fwd" (vcat [ppr case_bndr, text "fwd_pe =" <+> ppr fwd_pe,
+ -- text "pe =" <+> ppr pe]) $
+ go_fwd' case_bndr
+
+ go_fwd' case_bndr
+ | Just (scrut, co) <- lookupVarEnv fwd_pe case_bndr
+ = unitBag (scrut, case_bndr, mkSymCo co)
+ `unionBags` go_fwd scrut
+ `unionBags` go_bwd scrut [pr | pr@(cb,_) <- lookup_bwd scrut
+ , cb /= case_bndr]
+ | otherwise
+ = emptyBag
+
+ lookup_bwd :: Id -> [(Id, Coercion)]
+ -- Return case_bndrs that are connected to scrut
+ lookup_bwd scrut = case lookupVarEnv pe scrut of
+ Nothing -> []
+ Just (_, cb_cos) -> cb_cos
+
+ go_bwd :: Id -> [(Id, Coercion)] -> Bag ProxyBind
+ go_bwd scrut cb_cos = foldr (unionBags . go_bwd1 scrut) emptyBag cb_cos
+
+ go_bwd1 :: Id -> (Id, Coercion) -> Bag ProxyBind
+ go_bwd1 scrut (case_bndr, co)
+ = -- pprTrace "go_bwd1" (ppr case_bndr) $
+ unitBag (case_bndr, scrut, co)
+ `unionBags` go_bwd case_bndr (lookup_bwd case_bndr)
+
+-----------
+mkAltEnv :: OccEnv -> CoreExpr -> Id -> OccEnv
+-- Does two things: a) makes the occ_ctxt = OccVanilla
+-- b) extends the ProxyEnv if possible
+mkAltEnv env scrut cb
+ = env { occ_encl = OccVanilla, occ_proxy = pe' }
+ where
+ pe = occ_proxy env
+ pe' = case scrut of
+ Var v -> extendProxyEnv pe v (mkReflCo (idType v)) cb
+ Cast (Var v) co -> extendProxyEnv pe v co cb
+ _other -> trimProxyEnv pe [cb]
+
+-----------
+trimOccEnv :: OccEnv -> [CoreBndr] -> OccEnv
+trimOccEnv env bndrs = env { occ_proxy = trimProxyEnv (occ_proxy env) bndrs }
+
+-----------
+trimProxyEnv :: ProxyEnv -> [CoreBndr] -> ProxyEnv
+-- We are about to push this ProxyEnv inside a binding for 'bndrs'
+-- So dump any ProxyEnv bindings which mention any of the bndrs
+trimProxyEnv (PE pe fvs) bndrs
+ | not (bndr_set `intersectsVarSet` fvs)
+ = PE pe fvs
+ | otherwise
+ = PE pe' (fvs `minusVarSet` bndr_set)
+ where
+ pe' = mapVarEnv trim pe
+ bndr_set = mkVarSet bndrs
+ trim (scrut, cb_cos) | scrut `elemVarSet` bndr_set = (scrut, [])
+ | otherwise = (scrut, filterOut discard cb_cos)
+ discard (cb,co) = bndr_set `intersectsVarSet`
+ extendVarSet (tyCoVarsOfCo co) cb
+\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
+v `usedIn` details = isExportedId v || v `elemVarEnv` details
type IdWithOccInfo = Id
-tagBinders :: UsageDetails -- Of scope
- -> [Id] -- Binders
- -> (UsageDetails, -- Details with binders removed
- [IdWithOccInfo]) -- Tagged binders
-
-tagBinders usage binders
- = let
- usage' = usage `delVarEnvList` binders
- uss = map (setBinderOcc usage) binders
- in
- usage' `seq` (usage', uss)
-
-tagBinder :: UsageDetails -- Of scope
- -> Id -- Binders
- -> (UsageDetails, -- Details with binders removed
- IdWithOccInfo) -- Tagged binders
+tagLamBinders :: UsageDetails -- Of scope
+ -> [Id] -- Binders
+ -> (UsageDetails, -- Details with binders removed
+ [IdWithOccInfo]) -- Tagged binders
+-- Used for lambda and case binders
+-- It copes with the fact that lambda bindings can have InlineRule
+-- unfoldings, used for join points
+tagLamBinders usage binders = usage' `seq` (usage', bndrs')
+ where
+ (usage', bndrs') = mapAccumR tag_lam usage binders
+ tag_lam usage bndr = (usage2, setBinderOcc usage bndr)
+ where
+ usage1 = usage `delVarEnv` bndr
+ usage2 | isId bndr = addIdOccs usage1 (idUnfoldingVars bndr)
+ | otherwise = usage1
+
+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
+ | PE env _ <- occ_proxy env
+ , id `elemVarEnv` env = unitVarEnv id NoOccInfo
+ | Just uds <- lookupNameEnv (occ_rule_fvs env) (idName id)
+ = uds
+ | 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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