%
% (c) The GRASP/AQUA Project, Glasgow University, 19921998
%
%************************************************************************
%* *
\section[OccurAnal]{Occurrence analysis pass}
%* *
%************************************************************************
The occurrence analyser retypechecks a core expression, returning a new
core expression with (hopefully) improved usage information.
\begin{code}
module OccurAnal (
occurAnalysePgm, occurAnalyseExpr
) where
#include "HsVersions.h"
import CoreSyn
import CoreFVs
import CoreUtils ( exprIsTrivial, isDefaultAlt, isExpandableApp, mkCast )
import Id
import Name( localiseName )
import BasicTypes
import Module( Module )
import Coercion
import VarSet
import VarEnv
import Var
import Maybes ( orElse )
import Digraph ( SCC(..), stronglyConnCompFromEdgedVerticesR )
import PrelNames ( buildIdKey, foldrIdKey, runSTRepIdKey, augmentIdKey )
import Unique
import UniqFM
import Util ( mapAndUnzip, filterOut, fstOf3 )
import Bag
import Outputable
import FastString
import Data.List
\end{code}
%************************************************************************
%* *
\subsection[OccurAnalmain]{Counting occurrences: main function}
%* *
%************************************************************************
Here's the externallycallable interface:
\begin{code}
occurAnalysePgm :: Module
-> (Activation -> Bool)
-> [CoreRule] -> [CoreVect]
-> CoreProgram -> CoreProgram
occurAnalysePgm this_mod active_rule imp_rules vects binds
| isEmptyVarEnv final_usage
= binds'
| otherwise
= WARN( True, hang (text "Glomming in" <+> ppr this_mod <> colon)
2 (ppr final_usage ) )
[Rec (flattenBinds binds')]
where
(final_usage, binds') = go (initOccEnv active_rule) binds
initial_uds = addIdOccs emptyDetails
(rulesFreeVars imp_rules `unionVarSet` vectsFreeVars vects)
go :: OccEnv -> [CoreBind] -> (UsageDetails, [CoreBind])
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
occurAnalyseExpr expr
= snd (occAnal (initOccEnv all_active_rules) expr)
where
all_active_rules = \_ -> True
\end{code}
%************************************************************************
%* *
\subsection[OccurAnalmain]{Counting occurrences: main function}
%* *
%************************************************************************
Bindings
~~~~~~~~
\begin{code}
occAnalBind :: OccEnv
-> OccEnv
-> CoreBind
-> UsageDetails
-> (UsageDetails,
[CoreBind])
occAnalBind env _ (NonRec binder rhs) body_usage
| isTyVar binder
= (body_usage, [NonRec binder rhs])
| not (binder `usedIn` body_usage)
= (body_usage, [])
| otherwise
= (body_usage' +++ rhs_usage3, [NonRec tagged_binder rhs'])
where
(body_usage', tagged_binder) = tagBinder body_usage binder
(rhs_usage1, rhs') = occAnalRhs env (Just tagged_binder) rhs
rhs_usage2 = addIdOccs rhs_usage1 (idUnfoldingVars binder)
rhs_usage3 = addIdOccs rhs_usage2 (idRuleVars binder)
occAnalBind _ env (Rec pairs) body_usage
= foldr occAnalRec (body_usage, []) sccs
where
bndr_set = mkVarSet (map fst pairs)
sccs :: [SCC (Node Details)]
sccs = stronglyConnCompFromEdgedVerticesR nodes
nodes :: [Node Details]
nodes = map (makeNode env bndr_set) pairs
\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.
This seems to miss an obvious improvement.
letrec f = ...g...
g = ...f...
in
...g...
===>
letrec f = ...g...
g = ...(...g...)...
in
...g...
Now 'f' is unused! But it's OK! Dependency analysis will sort this
out into a letrec for 'g' and a 'let' for 'f', and then 'f' will get
dropped. It isn't easy to do a perfect job in one blow. Consider
letrec f = ...g...
g = ...h...
h = ...k...
k = ...m...
m = ...m...
in
...m...
Note [Forming Rec groups]
~~~~~~~~~~~~~~~~~~~~~~~~~
We put bindings {f = ef; g = eg } in a Rec group if "f uses g"
and "g uses f", no matter how indirectly. We do a SCC analysis
with an edge f -> g if "f uses g".
More precisely, "f uses g" iff g should be in scope whereever f is.
That is, g is free in:
a) the rhs 'ef'
b) or the RHS of a rule for f (Note [Rules are extra RHSs])
c) or the LHS or a rule for f (Note [Rule dependency info])
These conditions apply regardless of the activation of the RULE (eg it might be
inactive in this phase but become active later). Once a Rec is broken up
it can never be put back together, so we must be conservative.
The principle is that, regardless of rule firings, every variale is
always in scope.
* Note [Rules are extra RHSs]
~~~~~~~~~~~~~~~~~~~~~~~~~~~
A RULE for 'f' is like an extra RHS for 'f'. That way the "parent"
keeps the specialised "children" alive. If the parent dies
(because it isn't referenced any more), then the children will die
too (unless they are already referenced directly).
To that end, we build a Rec group for each cyclic strongly
connected component,
*treating f's rules as extra RHSs for 'f'*.
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 nonrecursive.
* Note [Rule dependency info]
~~~~~~~~~~~~~~~~~~~~~~~~~~~
The VarSet in a SpecInfo is used for dependency analysis in the
occurrence analyser. We must track free vars in *both* lhs and rhs.
Hence use of idRuleVars, rather than idRuleRhsVars in occAnalBind.
Why both? Consider
x = y
RULE f x = v+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 RULE appears to mention 'x'
as well as 'v'
* Note [Rules are visible in their own rec group]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
We want the rules for 'f' to be visible in f's righthand side.
And we'd like them to be visible in other functions in f's Rec
group. E.g. in Note [Specialisation rules] we want f' rule
to be visible in both f's RHS, and fs's RHS.
This means that we must simplify the RULEs first, before looking
at any of the definitions. This is done by Simplify.simplRecBind,
when it calls addLetIdInfo.
Note [Choosing loop breakers]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Loop breaking is surprisingly subtle. First read the section 4 of
"Secrets of the GHC inliner". This describes our basic plan.
We avoid infinite inlinings by choosing loop breakers, and
ensuring that a loop breaker cuts each loop.
Fundamentally, we do SCC analysis on a graph. For each recursive
group we choose a loop breaker, delete all edges to that node,
reanalyse the SCC, and iterate.
But what is the graph? NOT the same graph as was used for Note
[Forming Rec groups]! In particular, a RULE is like an equation for
'f' that is *always* inlined if it is applicable. We do *not* disable
rules for loopbreakers. It's up to whoever makes the rules to make
sure that the rules themselves always terminate. See Note [Rules for
recursive functions] in Simplify.lhs
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. Example: see Note
[Specialisation rules].
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
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 loopbreaker edges. More concretely there is an edge from f -> g
iff
(a) g is mentioned in f's RHS `xor` f's INLINE rhs
(see Note [Inline rules])
(b) or h is mentioned in f's RHS, and
g appears in the RHS of an active RULE of h
or a transitive sequence of active rules starting with h
Why "active rules"? See Note [Finding rule RHS free vars]
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 again that the edges of the graph we use for computing loop breakers
are not the same as the edges we use for computing the Rec blocks.
That's why we compute
rec_edges for the Rec block analysis
loop_breaker_edges for the loop breaker analysis
* Note [Finding rule RHS free vars]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Consider this real example from Data Parallel Haskell
tagZero :: Array Int -> Array Tag
tagZero xs = pmap (\x -> fromBool (x==0)) 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 occ_rule_act field
of the OccEnv.
* Note [Weak loop breakers]
~~~~~~~~~~~~~~~~~~~~~~~~~
There is a last nasty wrinkle. Suppose we have
Rec { f = f_rhs
RULE f [] = g
h = h_rhs
g = h
...more...
}
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.) Why not? Because then we
drop the binding for 'g', which leaves it out of scope in the
RULE!
Here's a somewhat different example of the same thing
Rec { g = h
; h = ...f...
; f = f_rhs
RULE f [] = g }
Here the RULE is "below" g, but we *still* can't postInlineUnconditionally
g, because the RULE for f is active throughout. So the RHS of h
might rewrite to h = ...g...
So g must remain in scope in the output program!
We "solve" this by:
Make g a "weak" loop breaker (OccInfo = IAmLoopBreaker True)
iff g is a "missing free variable" of the Rec group
A "missing free variable" x is one that is mentioned in an RHS or
INLINE or RULE of a binding in the Rec group, but where the
dependency on x may not show up in the loop_breaker_edges (see
note [Choosing loop breakers} above).
A normal "strong" loop breaker has IAmLoopBreaker False. So
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. (Typically it'll
inline via the usual callSiteInline stuff, so it'll be dead in the
next pass, so the main Ugh is the tiresome complication.)
Note [Rules for imported functions]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Consider this
f = /\a. B.g a
RULE B.g Int = 1 + f Int
Note that
* The RULE is for an imported function.
* f is nonrecursive
Now we
can get
f Int --> B.g Int Inlining f
--> 1 + f Int Firing RULE
and so the simplifier goes into an infinite loop. This
would not happen if the RULE was for a local function,
because we keep track of dependencies through rules. But
that is pretty much impossible to do for imported Ids. Suppose
f's definition had been
f = /\a. C.h a
where (by some long and devious process), C.h eventually inlines to
B.g. We could only spot such loops by exhaustively following
unfoldings of C.h etc, in case we reach B.g, and hence (via the RULE)
f.
Note that RULES for imported functions are important in practice; they
occur a lot in the libraries.
We regard this potential infinite loop as a *programmer* error.
It's up the programmer not to write silly rules like
RULE f x = f x
and the example above is just a more complicated version.
Note [Specialising imported functions]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
BUT for *automaticallygenerated* rules, the programmer can't be
responsible for the "programmer error" in Note [Rules for imported
functions]. In paricular, consider specialising a recursive function
defined in another module. If we specialise a recursive function B.g,
we get
g_spec = .....(B.g Int).....
RULE B.g Int = g_spec
Here, g_spec doesn't look recursive, but when the rule fires, it
becomes so. And if B.g was mutually recursive, the loop might
not be as obvious as it is here.
To avoid this,
* When specialising a function that is a loop breaker,
give a NOINLINE pragma to the specialised function
Note [Glomming]
~~~~~~~~~~~~~~~
RULES for imported Ids can make something at the top refer to something at the bottom:
f = \x -> B.g (q x)
h = \y -> 3
RULE: B.g (q x) = h x
Applying this rule makes f refer to h, although f doesn't appear to
depend on h. (And, as in Note [Rules for imported functions], the
dependency might be more indirect. For example, f might mention C.t
rather than B.g, where C.t eventually inlines to B.g.)
NOTICE that this cannot happen for rules whose head is a
locallydefined function, because we accurately track dependencies
through RULES. It only happens for rules whose head is an imported
function (B.g in the example above).
Solution:
When simplifying, bring all top level identifiers into
scope at the start, ignoring the Rec/NonRec structure, so
that when 'h' pops up in f's rhs, we find it in the inscope set
(as the simplifier generally expects). This happens in simplTopBinds.
In the occurrence analyser, if there are any outofscope
occurrences that pop out of the top, which will happen after
firing the rule: f = \x -> h x
h = \y -> 3
then just glom all the bindings into a single Rec, so that
the *next* iteration of the occurrence analyser will sort
them all out. This part happens in occurAnalysePgm.
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 (ie *not* like
Note [Rules are visible in their own rec group]), so it should be
treated *exactly* like an extra RHS.
Or, rather, when computing loopbreaker edges,
* If f has an INLINE pragma, and it is active, we treat the
INLINE rhs as f's rhs
* If it's inactive, we treat f as having no rhs
* If it has no INLINE pragma, we look at f's actual rhs
There is a danger that we'll be suboptimal 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
foo x = x+1
bar :: Int -> Int
bar x = foo x + 1
Here the RULE makes bar recursive; but it's INLINE pragma remains
nonrecursive. 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):
eftInt :: Int# -> Int# -> [Int]
eftInt x y = ...(nonrecursive)...
eftIntFB :: (Int -> r -> r) -> r -> Int# -> Int# -> r
eftIntFB c n x y = ...(nonrecursive)...
Note [Specialisation rules]
~~~~~~~~~~~~~~~~~~~~~~~~~~~
Consider this group, which is typical of what SpecConstr builds:
fs a = ....f (C a)....
f x = ....f (C a)....
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 [Selfrecursive 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}
type Node details = (details, Unique, [Unique])
data Details
= ND { nd_bndr :: Id
, nd_rhs :: CoreExpr
, nd_uds :: UsageDetails
, nd_inl :: IdSet
, nd_weak :: IdSet
, nd_active_rule_fvs :: IdSet
}
instance Outputable Details where
ppr nd = ptext (sLit "ND") <> braces
(sep [ ptext (sLit "bndr =") <+> ppr (nd_bndr nd)
, ptext (sLit "uds =") <+> ppr (nd_uds nd)
, ptext (sLit "inl =") <+> ppr (nd_inl nd)
, ptext (sLit "weak =") <+> ppr (nd_weak nd)
, ptext (sLit "rule =") <+> ppr (nd_active_rule_fvs nd)
])
makeNode :: OccEnv -> VarSet -> (Var, CoreExpr) -> Node Details
makeNode env bndr_set (bndr, rhs)
= (details, varUnique bndr, keysUFM node_fvs)
where
details = ND { nd_bndr = bndr
, nd_rhs = rhs'
, nd_uds = rhs_usage3
, nd_weak = node_fvs `minusVarSet` inl_fvs
, nd_inl = inl_fvs
, nd_active_rule_fvs = active_rule_fvs }
(rhs_usage1, rhs') = occAnalRhs env Nothing rhs
rhs_usage2 = addIdOccs rhs_usage1 all_rule_fvs
rhs_usage3 = case mb_unf_fvs of
Just unf_fvs -> addIdOccs rhs_usage2 unf_fvs
Nothing -> rhs_usage2
node_fvs = udFreeVars bndr_set rhs_usage3
is_active = occ_rule_act env :: Activation -> Bool
rules = filterOut isBuiltinRule (idCoreRules bndr)
rules_w_fvs :: [(Activation, VarSet)]
rules_w_fvs = [ (ru_act rule, fvs)
| rule <- rules
, let fvs = exprFreeVars (ru_rhs rule)
`delVarSetList` ru_bndrs rule
, not (isEmptyVarSet fvs) ]
all_rule_fvs = foldr (unionVarSet . snd) rule_lhs_fvs rules_w_fvs
rule_lhs_fvs = foldr (unionVarSet . (\ru -> exprsFreeVars (ru_args ru)
`delVarSetList` ru_bndrs ru))
emptyVarSet rules
active_rule_fvs = unionVarSets [fvs | (a,fvs) <- rules_w_fvs, is_active a]
unf = realIdUnfolding bndr
mb_unf_fvs = stableUnfoldingVars isLocalId unf
inl_fvs = case mb_unf_fvs of
Nothing -> udFreeVars bndr_set rhs_usage1
Just unf_fvs -> unf_fvs
occAnalRec :: SCC (Node Details)
-> (UsageDetails, [CoreBind])
-> (UsageDetails, [CoreBind])
occAnalRec (AcyclicSCC (ND { nd_bndr = bndr, nd_rhs = rhs, nd_uds = rhs_uds}, _, _))
(body_uds, binds)
| not (bndr `usedIn` body_uds)
= (body_uds, binds)
| otherwise
= (body_uds' +++ rhs_uds,
NonRec tagged_bndr rhs : binds)
where
(body_uds', tagged_bndr) = tagBinder body_uds bndr
occAnalRec (CyclicSCC nodes) (body_uds, binds)
| not (any (`usedIn` body_uds) bndrs)
= (body_uds, binds)
| otherwise
=
(final_uds, Rec pairs : binds)
where
bndrs = [b | (ND { nd_bndr = b }, _, _) <- nodes]
bndr_set = mkVarSet bndrs
tagged_nodes = map tag_node nodes
total_uds = foldl add_uds body_uds nodes
final_uds = total_uds `minusVarEnv` bndr_set
add_uds usage_so_far (nd, _, _) = usage_so_far +++ nd_uds nd
tag_node :: Node Details -> Node Details
tag_node (details@ND { nd_bndr = bndr }, k, ks)
= (details { nd_bndr = setBinderOcc total_uds bndr }, k, ks)
pairs :: [(Id,CoreExpr)]
pairs | isEmptyVarSet weak_fvs = reOrderNodes 0 bndr_set weak_fvs tagged_nodes []
| otherwise = loopBreakNodes 0 bndr_set weak_fvs loop_breaker_edges []
weak_fvs :: VarSet
weak_fvs = foldr (unionVarSet . nd_weak . fstOf3) emptyVarSet nodes
loop_breaker_edges = map mk_node tagged_nodes
mk_node (details@(ND { nd_inl = inl_fvs }), k, _)
= (details, k, keysUFM (extendFvs_ rule_fv_env inl_fvs))
rule_fv_env :: IdEnv IdSet
rule_fv_env = transClosureFV (mkVarEnv init_rule_fvs)
init_rule_fvs
= [ (b, trimmed_rule_fvs)
| (ND { nd_bndr = b, nd_active_rule_fvs = rule_fvs },_,_) <- nodes
, let trimmed_rule_fvs = rule_fvs `intersectVarSet` bndr_set
, not (isEmptyVarSet trimmed_rule_fvs)]
\end{code}
@loopBreakSCC@ 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
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
for these noinline guys.
Furthermore, the order of the binds is such that if we neglect dependencies
on the noinline Ids then the binds are topologically sorted. This means
that the simplifier will generally do a good job if it works from top bottom,
recording inlinings for any Ids which aren't marked as "no-inline" as it goes.
\begin{code}
type Binding = (Id,CoreExpr)
mk_loop_breaker :: Node Details -> Binding
mk_loop_breaker (ND { nd_bndr = bndr, nd_rhs = rhs}, _, _)
= (setIdOccInfo bndr strongLoopBreaker, rhs)
mk_non_loop_breaker :: VarSet -> Node Details -> Binding
mk_non_loop_breaker used_in_rules (ND { nd_bndr = bndr, nd_rhs = rhs}, _, _)
| bndr `elemVarSet` used_in_rules = (setIdOccInfo bndr weakLoopBreaker, rhs)
| otherwise = (bndr, rhs)
udFreeVars :: VarSet -> UsageDetails -> VarSet
udFreeVars bndrs uds = intersectUFM_C (\b _ -> b) bndrs uds
loopBreakNodes :: Int
-> VarSet
-> VarSet
-> [Node Details]
-> [Binding]
-> [Binding]
loopBreakNodes depth bndr_set weak_fvs nodes binds
= go (stronglyConnCompFromEdgedVerticesR nodes) binds
where
go [] binds = binds
go (scc:sccs) binds = loop_break_scc scc (go sccs binds)
loop_break_scc scc binds
= case scc of
AcyclicSCC node -> mk_non_loop_breaker weak_fvs node : binds
CyclicSCC [node] -> mk_loop_breaker node : binds
CyclicSCC nodes -> reOrderNodes depth bndr_set weak_fvs nodes binds
reOrderNodes :: Int -> VarSet -> VarSet -> [Node Details] -> [Binding] -> [Binding]
reOrderNodes _ _ _ [] _ = panic "reOrderNodes"
reOrderNodes depth bndr_set weak_fvs (node : nodes) binds
=
loopBreakNodes new_depth bndr_set weak_fvs unchosen $
(map mk_loop_breaker chosen_nodes ++ binds)
where
(chosen_nodes, unchosen) = choose_loop_breaker (score node) [node] [] nodes
approximate_loop_breaker = depth >= 2
new_depth | approximate_loop_breaker = 0
| otherwise = depth+1
choose_loop_breaker :: Int
-> [Node Details]
-> [Node Details]
-> [Node Details]
-> ([Node Details], [Node Details])
choose_loop_breaker _ loop_nodes acc []
= (loop_nodes, acc)
choose_loop_breaker loop_sc loop_nodes acc (node : nodes)
| sc < loop_sc
= choose_loop_breaker sc [node] (loop_nodes ++ acc) nodes
| approximate_loop_breaker && sc == loop_sc
= choose_loop_breaker loop_sc (node : loop_nodes) acc nodes
| otherwise
= choose_loop_breaker loop_sc loop_nodes (node : acc) nodes
where
sc = score node
score :: Node Details -> Int
score (ND { nd_bndr = bndr, nd_rhs = rhs }, _, _)
| not (isId bndr) = 100
| isDFunId bndr = 9
| Just inl_source <- isStableCoreUnfolding_maybe (idUnfolding bndr)
= case inl_source of
InlineWrapper {} -> 10
_other -> 3
| is_con_app rhs = 5
| exprIsTrivial rhs = 10
| isOneOcc (idOccInfo bndr) = 2
| canUnfold (realIdUnfolding bndr) = 1
| otherwise = 0
is_con_app (Var v) = isConLikeId v
is_con_app (App f _) = is_con_app f
is_con_app (Lam _ e) = is_con_app e
is_con_app (Tick _ e) = is_con_app e
is_con_app _ = False
\end{code}
Note [Complexity of loop breaking]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The loopbreaking algorithm knocks out one binder at a time, and
performs a new SCC analysis on the remaining binders. That can
behave very badly in tightlycoupled groups of bindings; in the
worst case it can be (N**2)*log N, because it does a full SCC
on N, then N1, then N2 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
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 mutuallyrecursive with a nonINLINE thing,
then the latter should be the loopbreaker.
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 {
$wfoo x = ....foo x....
foo x = ...$wfoo x...
}
The interface file sees the unfolding for $wfoo, and sees that foo is
strict (and hence it gets an autogenerated wrapper). Result: an
infinite inlining in the importing scope. So be a bit careful if you
change this. A good example is Tree.repTree in
nofib/spectral/minimax. If the repTree wrapper is chosen as the loop
breaker then compiling Game.hs goes into an infinite loop. This
happened when we gave is_con_app a lower score than inline candidates:
Tree.repTree
= __inline_me (/\a. \w w1 w2 ->
case Tree.$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 nonlooop 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)
$fCT = /\a \$dC. MkD (T a) ((sc @ a $dC) |> blah) ($ctoF @ a $dC)
}
Here 'sc' (the superclass) looks CONLIKE, but we'll never get to it
if we can't unravel the DFun first.
Note [Constructor applications]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
It's really really important to inline dictionaries. Real
example (the Enum Ordering instance from GHC.Base):
rec f = \ x -> case d of (p,q,r) -> p x
g = \ x -> case d of (p,q,r) -> q x
d = (v, f, g)
Here, f and g occur just once; but we can't inline them into d.
On the other hand we *could* simplify those case expressions if
we didn't stupidly choose d as the loop breaker.
But we won't because constructor args are marked "Many".
Inlining dictionaries is really essential to unravelling
the loops in static numeric dictionaries, see GHC.Float.
Note [Closure conversion]
~~~~~~~~~~~~~~~~~~~~~~~~~
We treat (\x. C p q) as a highscore candidate in the letrec scoring algorithm.
The immediate motivation came from the result of a closureconversion transformation
which generated code like this:
data Clo a b = forall c. Clo (c -> a -> b) c
($:) :: Clo a b -> a -> b
Clo f env $: x = f env x
rec { plus = Clo plus1 ()
; plus1 _ n = Clo plus2 n
; plus2 Zero n = n
; plus2 (Succ m) n = Succ (plus $: m $: n) }
If we inline 'plus' and 'plus1', everything unravels nicely. But if
we choose 'plus1' as the loop breaker (which is entirely possible
otherwise), the loop does not unravel nicely.
@occAnalRhs@ deals with the question of bindings where the Id is marked
by an INLINE pragma. For these we record that anything which occurs
in its RHS occurs many times. This pessimistically assumes that ths
inlined binder also occurs many times in its scope, but if it doesn't
we'll catch it next time round. At worst this costs an extra simplifier pass.
ToDo: try using the occurrence info for the inline'd binder.
[March 97] We do the same for atomic RHSs. Reason: see notes with loopBreakSCC.
[June 98, SLPJ] I've undone this change; I don't understand it. See notes with loopBreakSCC.
\begin{code}
occAnalRhs :: OccEnv
-> Maybe Id -> CoreExpr
-> (UsageDetails, CoreExpr)
occAnalRhs env mb_bndr rhs
= occAnal ctxt rhs
where
ctxt = case mb_bndr of
Just b | certainly_inline b -> env
_other -> rhsCtxt env
certainly_inline bndr
= 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 | isId v = addOneOcc u v NoOccInfo
| otherwise = u
\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
letbound constructor ==> give them a manyoccurrence.
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,
CoreExpr)
occAnal _ expr@(Type _) = (emptyDetails, expr)
occAnal _ expr@(Lit _) = (emptyDetails, expr)
occAnal env expr@(Var v) = (mkOneOcc env v False, expr)
occAnal _ (Coercion co)
= (addIdOccs emptyDetails (coVarsOfCo co), Coercion co)
\end{code}
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 (Tick tickish body)
| Breakpoint _ ids <- tickish
= (mapVarEnv markInsideSCC usage
+++ mkVarEnv (zip ids (repeat NoOccInfo)), Tick tickish body')
| tickishScoped tickish
= (mapVarEnv markInsideSCC usage, Tick tickish body')
| otherwise
= (usage, Tick tickish body')
where
!(usage,body') = occAnal env body
occAnal env (Cast expr co)
= case occAnal env expr of { (usage, expr') ->
let usage1 = markManyIf (isRhsEnv env) usage
usage2 = addIdOccs usage1 (coVarsOfCo co)
in (usage2, Cast expr' co)
}
\end{code}
\begin{code}
occAnal env app@(App _ _)
= occAnalApp env (collectArgs app)
occAnal env (Lam x body) | isTyVar x
= case occAnal env body of { (body_usage, body') ->
(body_usage, Lam x body')
}
occAnal env expr@(Lam _ _)
= case occAnal env_body body of { (body_usage, body') ->
let
(final_usage, tagged_binders) = tagLamBinders body_usage binders'
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 (trimOccEnv env binders)
(binders, body) = collectBinders expr
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 occ_anal_alt alts of { (alts_usage_s, alts') ->
let
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
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 env) scrut
occAnal env (Let bind body)
= 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') ->
(foldr (+++) emptyDetails arg_uds_s, args')}
where
arg_env = vanillaCtxt env
\end{code}
Applications are dealt with specially because we want
the "build hack" to work.
Note [Arguments of letbound 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 :: OccEnv
-> (Expr CoreBndr, [Arg CoreBndr])
-> (UsageDetails, Expr CoreBndr)
occAnalApp env (Var fun, args)
= case args_stuff of { (args_uds, args') ->
let
final_args_uds = markManyIf (isRhsEnv env && is_exp) args_uds
in
(fun_uds +++ final_args_uds, mkApps (Var fun) args') }
where
fun_uniq = idUnique fun
fun_uds = mkOneOcc env fun (valArgCount args > 0)
is_exp = isExpandableApp fun (valArgCount args)
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
| otherwise = occAnalArgs env args
occAnalApp env (fun, args)
= case occAnal (addAppCtxt env args) fun of { (fun_uds, fun') ->
case occAnalArgs env args of { (args_uds, args') ->
let
final_uds = fun_uds +++ args_uds
in
(final_uds, mkApps fun' args') }}
markManyIf :: Bool
-> UsageDetails
-> UsageDetails
markManyIf True uds = mapVarEnv markMany uds
markManyIf False uds = uds
appSpecial :: OccEnv
-> Int -> CtxtTy
-> [CoreExpr]
-> (UsageDetails, [CoreExpr])
appSpecial env n ctxt args
= go n args
where
arg_env = vanillaCtxt env
go _ [] = (emptyDetails, [])
go 1 (arg:args)
= 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 n (arg:args)
= case occAnal arg_env arg of { (arg_uds, arg') ->
case go (n1) args of { (args_uds, args') ->
(arg_uds +++ args_uds, arg':args') }}
\end{code}
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)
= let
env' = trimOccEnv env bndrs
in
case occAnal env' rhs of { (rhs_usage1, rhs1) ->
let
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
in
(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
rhs = mkCast (Var (zapIdOccInfo rhs_var)) co
\end{code}
%************************************************************************
%* *
OccEnv
%* *
%************************************************************************
\begin{code}
data OccEnv
= OccEnv { occ_encl :: !OccEncl
, occ_ctxt :: !CtxtTy
, occ_proxy :: ProxyEnv
, occ_rule_act :: Activation -> Bool
}
data OccEncl
= OccRhs
| OccVanilla
instance Outputable OccEncl where
ppr OccRhs = ptext (sLit "occRhs")
ppr OccVanilla = ptext (sLit "occVanilla")
type CtxtTy = [Bool]
initOccEnv :: (Activation -> Bool) -> OccEnv
initOccEnv active_rule
= OccEnv { occ_encl = OccVanilla
, occ_ctxt = []
, occ_proxy = PE emptyVarEnv emptyVarSet
, occ_rule_act = active_rule }
vanillaCtxt :: OccEnv -> OccEnv
vanillaCtxt env = env { occ_encl = OccVanilla, occ_ctxt = [] }
rhsCtxt :: OccEnv -> OccEnv
rhsCtxt env = env { occ_encl = OccRhs, occ_ctxt = [] }
setCtxtTy :: OccEnv -> CtxtTy -> OccEnv
setCtxtTy env ctxt = env { occ_ctxt = ctxt }
isRhsEnv :: OccEnv -> Bool
isRhsEnv (OccEnv { occ_encl = OccRhs }) = True
isRhsEnv (OccEnv { occ_encl = OccVanilla }) = False
oneShotGroup :: OccEnv -> [CoreBndr] -> [CoreBndr]
oneShotGroup (OccEnv { occ_ctxt = ctxt }) bndrs
= go ctxt bndrs []
where
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
go ctxt (bndr:bndrs) rev_bndrs = go ctxt bndrs (bndr:rev_bndrs)
addAppCtxt :: OccEnv -> [Arg CoreBndr] -> OccEnv
addAppCtxt env@(OccEnv { occ_ctxt = ctxt }) args
= env { occ_ctxt = replicate (valArgCount args) True ++ ctxt }
\end{code}
\begin{code}
transClosureFV :: UniqFM VarSet -> UniqFM VarSet
transClosureFV env
| no_change = env
| otherwise = transClosureFV (listToUFM new_fv_list)
where
(no_change, new_fv_list) = mapAccumL bump True (ufmToList env)
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
extendFvs_ env s = fst (extendFvs env s)
extendFvs :: UniqFM VarSet -> VarSet -> (VarSet, Bool)
extendFvs env s
| isNullUFM env
= (s, True)
| otherwise
= (s `unionVarSet` extras, extras `subVarSet` s)
where
extras :: VarSet
extras = foldUFM unionVarSet emptyVarSet $
intersectUFM_C (\x _ -> x) env s
\end{code}
%************************************************************************
%* *
ProxyEnv
%* *
%************************************************************************
\begin{code}
data ProxyEnv
= PE (IdEnv
(Id,
[(Id,Coercion)]))
VarSet
\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 inscope variables derivable from 'cb',
and effectively add letbindings 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 Binderswap 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 letbinding
{x=b}; it's Nothing if the binderswap 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
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
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[OccurAnaltypes]{OccEnv}
%* *
%************************************************************************
\begin{code}
type UsageDetails = IdEnv OccInfo
(+++), combineAltsUsageDetails
:: UsageDetails -> UsageDetails -> UsageDetails
(+++) usage1 usage2
= plusVarEnv_C addOccInfo usage1 usage2
combineAltsUsageDetails usage1 usage2
= plusVarEnv_C orOccInfo usage1 usage2
addOneOcc :: UsageDetails -> Id -> OccInfo -> UsageDetails
addOneOcc usage id info
= plusVarEnv_C addOccInfo usage (unitVarEnv id info)
emptyDetails :: UsageDetails
emptyDetails = (emptyVarEnv :: UsageDetails)
usedIn :: Id -> UsageDetails -> Bool
v `usedIn` details = isExportedId v || v `elemVarEnv` details
type IdWithOccInfo = Id
tagLamBinders :: UsageDetails
-> [Id]
-> (UsageDetails,
[IdWithOccInfo])
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
-> Id
-> (UsageDetails,
IdWithOccInfo)
tagBinder usage binder
= let
usage' = usage `delVarEnv` binder
binder' = setBinderOcc usage binder
in
usage' `seq` (usage', binder')
setBinderOcc :: UsageDetails -> CoreBndr -> CoreBndr
setBinderOcc usage bndr
| isTyVar bndr = bndr
| isExportedId bndr = case idOccInfo bndr of
NoOccInfo -> bndr
_ -> setIdOccInfo bndr NoOccInfo
| otherwise = setIdOccInfo bndr occ_info
where
occ_info = lookupVarEnv usage bndr `orElse` IAmDead
\end{code}
%************************************************************************
%* *
\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)
| PE env _ <- occ_proxy env
, id `elemVarEnv` env
= unitVarEnv id NoOccInfo
| otherwise
= emptyDetails
markMany, markInsideLam, markInsideSCC :: OccInfo -> OccInfo
markMany _ = NoOccInfo
markInsideSCC occ = markInsideLam occ
markInsideLam (OneOcc _ one_br int_cxt) = OneOcc True one_br int_cxt
markInsideLam occ = occ
addOccInfo, orOccInfo :: OccInfo -> OccInfo -> OccInfo
addOccInfo a1 a2 = ASSERT( not (isDeadOcc a1 || isDeadOcc a2) )
NoOccInfo
orOccInfo (OneOcc in_lam1 _ int_cxt1)
(OneOcc in_lam2 _ int_cxt2)
= OneOcc (in_lam1 || in_lam2)
False
(int_cxt1 && int_cxt2)
orOccInfo a1 a2 = ASSERT( not (isDeadOcc a1 || isDeadOcc a2) )
NoOccInfo
\end{code}