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|
%
% (c) The GRASP/AQUA Project, Glasgow University, 1993-1998
%
\section[Specialise]{Stamping out overloading, and (optionally) polymorphism}
\begin{code}
module Specialise ( specProgram ) where
#include "HsVersions.h"
import CmdLineOpts ( opt_D_verbose_core2core, opt_D_dump_spec, opt_D_dump_rules )
import Id ( Id, idName, idType, mkTemplateLocals, mkUserLocal,
idSpecialisation, setIdNoDiscard, isExportedId,
modifyIdInfo, idUnfolding
)
import IdInfo ( zapSpecPragInfo )
import VarSet
import VarEnv
import Type ( Type, mkTyVarTy, splitSigmaTy, splitFunTysN,
tyVarsOfType, tyVarsOfTypes, tyVarsOfTheta, applyTys,
mkForAllTys, boxedTypeKind
)
import PprType ( {- instance Outputable Type -} )
import Subst ( Subst, mkSubst, substTy, mkSubst, substBndrs, extendSubstList,
substId, substAndCloneId, substAndCloneIds, lookupIdSubst
)
import Var ( TyVar, mkSysTyVar, setVarUnique )
import VarSet
import VarEnv
import CoreSyn
import CoreUtils ( applyTypeToArgs )
import CoreUnfold ( certainlyWillInline )
import CoreFVs ( exprFreeVars, exprsFreeVars )
import CoreLint ( beginPass, endPass )
import PprCore ( pprCoreRules )
import Rules ( addIdSpecialisations )
import UniqSupply ( UniqSupply,
UniqSM, initUs_, thenUs, thenUs_, returnUs, getUniqueUs,
getUs, setUs, uniqFromSupply, splitUniqSupply, mapUs
)
import Name ( nameOccName, mkSpecOcc, getSrcLoc )
import FiniteMap
import Maybes ( MaybeErr(..), catMaybes )
import ErrUtils ( dumpIfSet )
import Bag
import List ( partition )
import Util ( zipEqual, zipWithEqual, mapAccumL )
import Outputable
infixr 9 `thenSM`
\end{code}
%************************************************************************
%* *
\subsection[notes-Specialise]{Implementation notes [SLPJ, Aug 18 1993]}
%* *
%************************************************************************
These notes describe how we implement specialisation to eliminate
overloading.
The specialisation pass works on Core
syntax, complete with all the explicit dictionary application,
abstraction and construction as added by the type checker. The
existing type checker remains largely as it is.
One important thought: the {\em types} passed to an overloaded
function, and the {\em dictionaries} passed are mutually redundant.
If the same function is applied to the same type(s) then it is sure to
be applied to the same dictionary(s)---or rather to the same {\em
values}. (The arguments might look different but they will evaluate
to the same value.)
Second important thought: we know that we can make progress by
treating dictionary arguments as static and worth specialising on. So
we can do without binding-time analysis, and instead specialise on
dictionary arguments and no others.
The basic idea
~~~~~~~~~~~~~~
Suppose we have
let f = <f_rhs>
in <body>
and suppose f is overloaded.
STEP 1: CALL-INSTANCE COLLECTION
We traverse <body>, accumulating all applications of f to types and
dictionaries.
(Might there be partial applications, to just some of its types and
dictionaries? In principle yes, but in practice the type checker only
builds applications of f to all its types and dictionaries, so partial
applications could only arise as a result of transformation, and even
then I think it's unlikely. In any case, we simply don't accumulate such
partial applications.)
STEP 2: EQUIVALENCES
So now we have a collection of calls to f:
f t1 t2 d1 d2
f t3 t4 d3 d4
...
Notice that f may take several type arguments. To avoid ambiguity, we
say that f is called at type t1/t2 and t3/t4.
We take equivalence classes using equality of the *types* (ignoring
the dictionary args, which as mentioned previously are redundant).
STEP 3: SPECIALISATION
For each equivalence class, choose a representative (f t1 t2 d1 d2),
and create a local instance of f, defined thus:
f@t1/t2 = <f_rhs> t1 t2 d1 d2
f_rhs presumably has some big lambdas and dictionary lambdas, so lots
of simplification will now result. However we don't actually *do* that
simplification. Rather, we leave it for the simplifier to do. If we
*did* do it, though, we'd get more call instances from the specialised
RHS. We can work out what they are by instantiating the call-instance
set from f's RHS with the types t1, t2.
Add this new id to f's IdInfo, to record that f has a specialised version.
Before doing any of this, check that f's IdInfo doesn't already
tell us about an existing instance of f at the required type/s.
(This might happen if specialisation was applied more than once, or
it might arise from user SPECIALIZE pragmas.)
Recursion
~~~~~~~~~
Wait a minute! What if f is recursive? Then we can't just plug in
its right-hand side, can we?
But it's ok. The type checker *always* creates non-recursive definitions
for overloaded recursive functions. For example:
f x = f (x+x) -- Yes I know its silly
becomes
f a (d::Num a) = let p = +.sel a d
in
letrec fl (y::a) = fl (p y y)
in
fl
We still have recusion for non-overloaded functions which we
speciailise, but the recursive call should get specialised to the
same recursive version.
Polymorphism 1
~~~~~~~~~~~~~~
All this is crystal clear when the function is applied to *constant
types*; that is, types which have no type variables inside. But what if
it is applied to non-constant types? Suppose we find a call of f at type
t1/t2. There are two possibilities:
(a) The free type variables of t1, t2 are in scope at the definition point
of f. In this case there's no problem, we proceed just as before. A common
example is as follows. Here's the Haskell:
g y = let f x = x+x
in f y + f y
After typechecking we have
g a (d::Num a) (y::a) = let f b (d'::Num b) (x::b) = +.sel b d' x x
in +.sel a d (f a d y) (f a d y)
Notice that the call to f is at type type "a"; a non-constant type.
Both calls to f are at the same type, so we can specialise to give:
g a (d::Num a) (y::a) = let f@a (x::a) = +.sel a d x x
in +.sel a d (f@a y) (f@a y)
(b) The other case is when the type variables in the instance types
are *not* in scope at the definition point of f. The example we are
working with above is a good case. There are two instances of (+.sel a d),
but "a" is not in scope at the definition of +.sel. Can we do anything?
Yes, we can "common them up", a sort of limited common sub-expression deal.
This would give:
g a (d::Num a) (y::a) = let +.sel@a = +.sel a d
f@a (x::a) = +.sel@a x x
in +.sel@a (f@a y) (f@a y)
This can save work, and can't be spotted by the type checker, because
the two instances of +.sel weren't originally at the same type.
Further notes on (b)
* There are quite a few variations here. For example, the defn of
+.sel could be floated ouside the \y, to attempt to gain laziness.
It certainly mustn't be floated outside the \d because the d has to
be in scope too.
* We don't want to inline f_rhs in this case, because
that will duplicate code. Just commoning up the call is the point.
* Nothing gets added to +.sel's IdInfo.
* Don't bother unless the equivalence class has more than one item!
Not clear whether this is all worth it. It is of course OK to
simply discard call-instances when passing a big lambda.
Polymorphism 2 -- Overloading
~~~~~~~~~~~~~~
Consider a function whose most general type is
f :: forall a b. Ord a => [a] -> b -> b
There is really no point in making a version of g at Int/Int and another
at Int/Bool, because it's only instancing the type variable "a" which
buys us any efficiency. Since g is completely polymorphic in b there
ain't much point in making separate versions of g for the different
b types.
That suggests that we should identify which of g's type variables
are constrained (like "a") and which are unconstrained (like "b").
Then when taking equivalence classes in STEP 2, we ignore the type args
corresponding to unconstrained type variable. In STEP 3 we make
polymorphic versions. Thus:
f@t1/ = /\b -> <f_rhs> t1 b d1 d2
We do this.
Dictionary floating
~~~~~~~~~~~~~~~~~~~
Consider this
f a (d::Num a) = let g = ...
in
...(let d1::Ord a = Num.Ord.sel a d in g a d1)...
Here, g is only called at one type, but the dictionary isn't in scope at the
definition point for g. Usually the type checker would build a
definition for d1 which enclosed g, but the transformation system
might have moved d1's defn inward. Solution: float dictionary bindings
outwards along with call instances.
Consider
f x = let g p q = p==q
h r s = (r+s, g r s)
in
h x x
Before specialisation, leaving out type abstractions we have
f df x = let g :: Eq a => a -> a -> Bool
g dg p q = == dg p q
h :: Num a => a -> a -> (a, Bool)
h dh r s = let deq = eqFromNum dh
in (+ dh r s, g deq r s)
in
h df x x
After specialising h we get a specialised version of h, like this:
h' r s = let deq = eqFromNum df
in (+ df r s, g deq r s)
But we can't naively make an instance for g from this, because deq is not in scope
at the defn of g. Instead, we have to float out the (new) defn of deq
to widen its scope. Notice that this floating can't be done in advance -- it only
shows up when specialisation is done.
User SPECIALIZE pragmas
~~~~~~~~~~~~~~~~~~~~~~~
Specialisation pragmas can be digested by the type checker, and implemented
by adding extra definitions along with that of f, in the same way as before
f@t1/t2 = <f_rhs> t1 t2 d1 d2
Indeed the pragmas *have* to be dealt with by the type checker, because
only it knows how to build the dictionaries d1 and d2! For example
g :: Ord a => [a] -> [a]
{-# SPECIALIZE f :: [Tree Int] -> [Tree Int] #-}
Here, the specialised version of g is an application of g's rhs to the
Ord dictionary for (Tree Int), which only the type checker can conjure
up. There might not even *be* one, if (Tree Int) is not an instance of
Ord! (All the other specialision has suitable dictionaries to hand
from actual calls.)
Problem. The type checker doesn't have to hand a convenient <f_rhs>, because
it is buried in a complex (as-yet-un-desugared) binding group.
Maybe we should say
f@t1/t2 = f* t1 t2 d1 d2
where f* is the Id f with an IdInfo which says "inline me regardless!".
Indeed all the specialisation could be done in this way.
That in turn means that the simplifier has to be prepared to inline absolutely
any in-scope let-bound thing.
Again, the pragma should permit polymorphism in unconstrained variables:
h :: Ord a => [a] -> b -> b
{-# SPECIALIZE h :: [Int] -> b -> b #-}
We *insist* that all overloaded type variables are specialised to ground types,
(and hence there can be no context inside a SPECIALIZE pragma).
We *permit* unconstrained type variables to be specialised to
- a ground type
- or left as a polymorphic type variable
but nothing in between. So
{-# SPECIALIZE h :: [Int] -> [c] -> [c] #-}
is *illegal*. (It can be handled, but it adds complication, and gains the
programmer nothing.)
SPECIALISING INSTANCE DECLARATIONS
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Consider
instance Foo a => Foo [a] where
...
{-# SPECIALIZE instance Foo [Int] #-}
The original instance decl creates a dictionary-function
definition:
dfun.Foo.List :: forall a. Foo a -> Foo [a]
The SPECIALIZE pragma just makes a specialised copy, just as for
ordinary function definitions:
dfun.Foo.List@Int :: Foo [Int]
dfun.Foo.List@Int = dfun.Foo.List Int dFooInt
The information about what instance of the dfun exist gets added to
the dfun's IdInfo in the same way as a user-defined function too.
Automatic instance decl specialisation?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Can instance decls be specialised automatically? It's tricky.
We could collect call-instance information for each dfun, but
then when we specialised their bodies we'd get new call-instances
for ordinary functions; and when we specialised their bodies, we might get
new call-instances of the dfuns, and so on. This all arises because of
the unrestricted mutual recursion between instance decls and value decls.
Still, there's no actual problem; it just means that we may not do all
the specialisation we could theoretically do.
Furthermore, instance decls are usually exported and used non-locally,
so we'll want to compile enough to get those specialisations done.
Lastly, there's no such thing as a local instance decl, so we can
survive solely by spitting out *usage* information, and then reading that
back in as a pragma when next compiling the file. So for now,
we only specialise instance decls in response to pragmas.
SPITTING OUT USAGE INFORMATION
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
To spit out usage information we need to traverse the code collecting
call-instance information for all imported (non-prelude?) functions
and data types. Then we equivalence-class it and spit it out.
This is done at the top-level when all the call instances which escape
must be for imported functions and data types.
*** Not currently done ***
Partial specialisation by pragmas
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
What about partial specialisation:
k :: (Ord a, Eq b) => [a] -> b -> b -> [a]
{-# SPECIALIZE k :: Eq b => [Int] -> b -> b -> [a] #-}
or even
{-# SPECIALIZE k :: Eq b => [Int] -> [b] -> [b] -> [a] #-}
Seems quite reasonable. Similar things could be done with instance decls:
instance (Foo a, Foo b) => Foo (a,b) where
...
{-# SPECIALIZE instance Foo a => Foo (a,Int) #-}
{-# SPECIALIZE instance Foo b => Foo (Int,b) #-}
Ho hum. Things are complex enough without this. I pass.
Requirements for the simplifer
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The simplifier has to be able to take advantage of the specialisation.
* When the simplifier finds an application of a polymorphic f, it looks in
f's IdInfo in case there is a suitable instance to call instead. This converts
f t1 t2 d1 d2 ===> f_t1_t2
Note that the dictionaries get eaten up too!
* Dictionary selection operations on constant dictionaries must be
short-circuited:
+.sel Int d ===> +Int
The obvious way to do this is in the same way as other specialised
calls: +.sel has inside it some IdInfo which tells that if it's applied
to the type Int then it should eat a dictionary and transform to +Int.
In short, dictionary selectors need IdInfo inside them for constant
methods.
* Exactly the same applies if a superclass dictionary is being
extracted:
Eq.sel Int d ===> dEqInt
* Something similar applies to dictionary construction too. Suppose
dfun.Eq.List is the function taking a dictionary for (Eq a) to
one for (Eq [a]). Then we want
dfun.Eq.List Int d ===> dEq.List_Int
Where does the Eq [Int] dictionary come from? It is built in
response to a SPECIALIZE pragma on the Eq [a] instance decl.
In short, dfun Ids need IdInfo with a specialisation for each
constant instance of their instance declaration.
All this uses a single mechanism: the SpecEnv inside an Id
What does the specialisation IdInfo look like?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The SpecEnv of an Id maps a list of types (the template) to an expression
[Type] |-> Expr
For example, if f has this SpecInfo:
[Int, a] -> \d:Ord Int. f' a
it means that we can replace the call
f Int t ===> (\d. f' t)
This chucks one dictionary away and proceeds with the
specialised version of f, namely f'.
What can't be done this way?
~~~~~~~~~~~~~~~~~~~~~~~~~~~~
There is no way, post-typechecker, to get a dictionary for (say)
Eq a from a dictionary for Eq [a]. So if we find
==.sel [t] d
we can't transform to
eqList (==.sel t d')
where
eqList :: (a->a->Bool) -> [a] -> [a] -> Bool
Of course, we currently have no way to automatically derive
eqList, nor to connect it to the Eq [a] instance decl, but you
can imagine that it might somehow be possible. Taking advantage
of this is permanently ruled out.
Still, this is no great hardship, because we intend to eliminate
overloading altogether anyway!
A note about non-tyvar dictionaries
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Some Ids have types like
forall a,b,c. Eq a -> Ord [a] -> tau
This seems curious at first, because we usually only have dictionary
args whose types are of the form (C a) where a is a type variable.
But this doesn't hold for the functions arising from instance decls,
which sometimes get arguements with types of form (C (T a)) for some
type constructor T.
Should we specialise wrt this compound-type dictionary? We used to say
"no", saying:
"This is a heuristic judgement, as indeed is the fact that we
specialise wrt only dictionaries. We choose *not* to specialise
wrt compound dictionaries because at the moment the only place
they show up is in instance decls, where they are simply plugged
into a returned dictionary. So nothing is gained by specialising
wrt them."
But it is simpler and more uniform to specialise wrt these dicts too;
and in future GHC is likely to support full fledged type signatures
like
f ;: Eq [(a,b)] => ...
%************************************************************************
%* *
\subsubsection{The new specialiser}
%* *
%************************************************************************
Our basic game plan is this. For let(rec) bound function
f :: (C a, D c) => (a,b,c,d) -> Bool
* Find any specialised calls of f, (f ts ds), where
ts are the type arguments t1 .. t4, and
ds are the dictionary arguments d1 .. d2.
* Add a new definition for f1 (say):
f1 = /\ b d -> (..body of f..) t1 b t3 d d1 d2
Note that we abstract over the unconstrained type arguments.
* Add the mapping
[t1,b,t3,d] |-> \d1 d2 -> f1 b d
to the specialisations of f. This will be used by the
simplifier to replace calls
(f t1 t2 t3 t4) da db
by
(\d1 d1 -> f1 t2 t4) da db
All the stuff about how many dictionaries to discard, and what types
to apply the specialised function to, are handled by the fact that the
SpecEnv contains a template for the result of the specialisation.
We don't build *partial* specialisations for f. For example:
f :: Eq a => a -> a -> Bool
{-# SPECIALISE f :: (Eq b, Eq c) => (b,c) -> (b,c) -> Bool #-}
Here, little is gained by making a specialised copy of f.
There's a distinct danger that the specialised version would
first build a dictionary for (Eq b, Eq c), and then select the (==)
method from it! Even if it didn't, not a great deal is saved.
We do, however, generate polymorphic, but not overloaded, specialisations:
f :: Eq a => [a] -> b -> b -> b
{#- SPECIALISE f :: [Int] -> b -> b -> b #-}
Hence, the invariant is this:
*** no specialised version is overloaded ***
%************************************************************************
%* *
\subsubsection{The exported function}
%* *
%************************************************************************
\begin{code}
specProgram :: UniqSupply -> [CoreBind] -> IO [CoreBind]
specProgram us binds
= do
beginPass "Specialise"
let binds' = initSM us (go binds `thenSM` \ (binds', uds') ->
returnSM (dumpAllDictBinds uds' binds'))
endPass "Specialise" (opt_D_dump_spec || opt_D_verbose_core2core) binds'
dumpIfSet opt_D_dump_rules "Top-level specialisations"
(vcat (map dump_specs (concat (map bindersOf binds'))))
return binds'
where
-- We need to start with a Subst that knows all the things
-- that are in scope, so that the substitution engine doesn't
-- accidentally re-use a unique that's already in use
-- Easiest thing is to do it all at once, as if all the top-level
-- decls were mutually recursive
top_subst = mkSubst (mkVarSet (bindersOfBinds binds)) emptySubstEnv
go [] = returnSM ([], emptyUDs)
go (bind:binds) = go binds `thenSM` \ (binds', uds) ->
specBind top_subst bind uds `thenSM` \ (bind', uds') ->
returnSM (bind' ++ binds', uds')
dump_specs var = pprCoreRules var (idSpecialisation var)
\end{code}
%************************************************************************
%* *
\subsubsection{@specExpr@: the main function}
%* *
%************************************************************************
\begin{code}
specVar :: Subst -> Id -> CoreExpr
specVar subst v = case lookupIdSubst subst v of
DoneEx e -> e
DoneId v _ -> Var v
specExpr :: Subst -> CoreExpr -> SpecM (CoreExpr, UsageDetails)
-- We carry a substitution down:
-- a) we must clone any binding that might flaot outwards,
-- to avoid name clashes
-- b) we carry a type substitution to use when analysing
-- the RHS of specialised bindings (no type-let!)
---------------- First the easy cases --------------------
specExpr subst (Type ty) = returnSM (Type (substTy subst ty), emptyUDs)
specExpr subst (Var v) = returnSM (specVar subst v, emptyUDs)
specExpr subst (Lit lit) = returnSM (Lit lit, emptyUDs)
specExpr subst (Note note body)
= specExpr subst body `thenSM` \ (body', uds) ->
returnSM (Note (specNote subst note) body', uds)
---------------- Applications might generate a call instance --------------------
specExpr subst expr@(App fun arg)
= go expr []
where
go (App fun arg) args = specExpr subst arg `thenSM` \ (arg', uds_arg) ->
go fun (arg':args) `thenSM` \ (fun', uds_app) ->
returnSM (App fun' arg', uds_arg `plusUDs` uds_app)
go (Var f) args = case specVar subst f of
Var f' -> returnSM (Var f', mkCallUDs f' args)
e' -> returnSM (e', emptyUDs) -- I don't expect this!
go other args = specExpr subst other
---------------- Lambda/case require dumping of usage details --------------------
specExpr subst e@(Lam _ _)
= specExpr subst' body `thenSM` \ (body', uds) ->
let
(filtered_uds, body'') = dumpUDs bndrs' uds body'
in
returnSM (mkLams bndrs' body'', filtered_uds)
where
(bndrs, body) = collectBinders e
(subst', bndrs') = substBndrs subst bndrs
-- More efficient to collect a group of binders together all at once
-- and we don't want to split a lambda group with dumped bindings
specExpr subst (Case scrut case_bndr alts)
= specExpr subst scrut `thenSM` \ (scrut', uds_scrut) ->
mapAndCombineSM spec_alt alts `thenSM` \ (alts', uds_alts) ->
returnSM (Case scrut' case_bndr' alts', uds_scrut `plusUDs` uds_alts)
where
(subst_alt, case_bndr') = substId subst case_bndr
-- No need to clone case binder; it can't float like a let(rec)
spec_alt (con, args, rhs)
= specExpr subst_rhs rhs `thenSM` \ (rhs', uds) ->
let
(uds', rhs'') = dumpUDs args uds rhs'
in
returnSM ((con, args', rhs''), uds')
where
(subst_rhs, args') = substBndrs subst_alt args
---------------- Finally, let is the interesting case --------------------
specExpr subst (Let bind body)
= -- Clone binders
cloneBindSM subst bind `thenSM` \ (rhs_subst, body_subst, bind') ->
-- Deal with the body
specExpr body_subst body `thenSM` \ (body', body_uds) ->
-- Deal with the bindings
specBind rhs_subst bind' body_uds `thenSM` \ (binds', uds) ->
-- All done
returnSM (foldr Let body' binds', uds)
-- Must apply the type substitution to coerceions
specNote subst (Coerce t1 t2) = Coerce (substTy subst t1) (substTy subst t2)
specNote subst note = note
\end{code}
%************************************************************************
%* *
\subsubsection{Dealing with a binding}
%* *
%************************************************************************
\begin{code}
specBind :: Subst -- Use this for RHSs
-> CoreBind
-> UsageDetails -- Info on how the scope of the binding
-> SpecM ([CoreBind], -- New bindings
UsageDetails) -- And info to pass upstream
specBind rhs_subst bind body_uds
= specBindItself rhs_subst bind (calls body_uds) `thenSM` \ (bind', bind_uds) ->
let
bndrs = bindersOf bind
all_uds = zapCalls bndrs (body_uds `plusUDs` bind_uds)
-- It's important that the `plusUDs` is this way round,
-- because body_uds may bind dictionaries that are
-- used in the calls passed to specDefn. So the
-- dictionary bindings in bind_uds may mention
-- dictionaries bound in body_uds.
in
case splitUDs bndrs all_uds of
(_, ([],[])) -- This binding doesn't bind anything needed
-- in the UDs, so put the binding here
-- This is the case for most non-dict bindings, except
-- for the few that are mentioned in a dict binding
-- that is floating upwards in body_uds
-> returnSM ([bind'], all_uds)
(float_uds, (dict_binds, calls)) -- This binding is needed in the UDs, so float it out
-> returnSM ([], float_uds `plusUDs` mkBigUD bind' dict_binds calls)
-- A truly gruesome function
mkBigUD bind@(NonRec _ _) dbs calls
= -- Common case: non-recursive and no specialisations
-- (if there were any specialistions it would have been made recursive)
MkUD { dict_binds = listToBag (mkDB bind : dbs),
calls = listToCallDetails calls }
mkBigUD bind dbs calls
= -- General case
MkUD { dict_binds = unitBag (mkDB (Rec (bind_prs bind ++ dbsToPairs dbs))),
-- Make a huge Rec
calls = listToCallDetails calls }
where
bind_prs (NonRec b r) = [(b,r)]
bind_prs (Rec prs) = prs
dbsToPairs [] = []
dbsToPairs ((bind,_):dbs) = bind_prs bind ++ dbsToPairs dbs
-- specBindItself deals with the RHS, specialising it according
-- to the calls found in the body (if any)
specBindItself rhs_subst (NonRec bndr rhs) call_info
= specDefn rhs_subst call_info (bndr,rhs) `thenSM` \ ((bndr',rhs'), spec_defns, spec_uds) ->
let
new_bind | null spec_defns = NonRec bndr' rhs'
| otherwise = Rec ((bndr',rhs'):spec_defns)
-- bndr' mentions the spec_defns in its SpecEnv
-- Not sure why we couln't just put the spec_defns first
in
returnSM (new_bind, spec_uds)
specBindItself rhs_subst (Rec pairs) call_info
= mapSM (specDefn rhs_subst call_info) pairs `thenSM` \ stuff ->
let
(pairs', spec_defns_s, spec_uds_s) = unzip3 stuff
spec_defns = concat spec_defns_s
spec_uds = plusUDList spec_uds_s
new_bind = Rec (spec_defns ++ pairs')
in
returnSM (new_bind, spec_uds)
specDefn :: Subst -- Subst to use for RHS
-> CallDetails -- Info on how it is used in its scope
-> (Id, CoreExpr) -- The thing being bound and its un-processed RHS
-> SpecM ((Id, CoreExpr), -- The thing and its processed RHS
-- the Id may now have specialisations attached
[(Id,CoreExpr)], -- Extra, specialised bindings
UsageDetails -- Stuff to fling upwards from the RHS and its
) -- specialised versions
specDefn subst calls (fn, rhs)
-- The first case is the interesting one
| n_tyvars == length rhs_tyvars -- Rhs of fn's defn has right number of big lambdas
&& n_dicts <= length rhs_bndrs -- and enough dict args
&& not (null calls_for_me) -- And there are some calls to specialise
&& not (certainlyWillInline fn) -- And it's not small
-- If it's small, it's better just to inline
-- it than to construct lots of specialisations
= -- Specialise the body of the function
specExpr subst rhs `thenSM` \ (rhs', rhs_uds) ->
-- Make a specialised version for each call in calls_for_me
mapSM spec_call calls_for_me `thenSM` \ stuff ->
let
(spec_defns, spec_uds, spec_env_stuff) = unzip3 stuff
fn' = addIdSpecialisations zapped_fn spec_env_stuff
in
returnSM ((fn',rhs'),
spec_defns,
rhs_uds `plusUDs` plusUDList spec_uds)
| otherwise -- No calls or RHS doesn't fit our preconceptions
= specExpr subst rhs `thenSM` \ (rhs', rhs_uds) ->
returnSM ((zapped_fn, rhs'), [], rhs_uds)
where
zapped_fn = modifyIdInfo zapSpecPragInfo fn
-- If the fn is a SpecPragmaId, make it discardable
-- It's role as a holder for a call instance is o'er
-- But it might be alive for some other reason by now.
fn_type = idType fn
(tyvars, theta, tau) = splitSigmaTy fn_type
n_tyvars = length tyvars
n_dicts = length theta
(rhs_tyvars, rhs_ids, rhs_body) = collectTyAndValBinders rhs
rhs_dicts = take n_dicts rhs_ids
rhs_bndrs = rhs_tyvars ++ rhs_dicts
body = mkLams (drop n_dicts rhs_ids) rhs_body
-- Glue back on the non-dict lambdas
calls_for_me = case lookupFM calls fn of
Nothing -> []
Just cs -> fmToList cs
----------------------------------------------------------
-- Specialise to one particular call pattern
spec_call :: ([Maybe Type], ([DictExpr], VarSet)) -- Call instance
-> SpecM ((Id,CoreExpr), -- Specialised definition
UsageDetails, -- Usage details from specialised body
([CoreBndr], [CoreExpr], CoreExpr)) -- Info for the Id's SpecEnv
spec_call (call_ts, (call_ds, call_fvs))
= ASSERT( length call_ts == n_tyvars && length call_ds == n_dicts )
-- Calls are only recorded for properly-saturated applications
-- Suppose f's defn is f = /\ a b c d -> \ d1 d2 -> rhs
-- Supppose the call is for f [Just t1, Nothing, Just t3, Nothing] [dx1, dx2]
-- Construct the new binding
-- f1 = SUBST[a->t1,c->t3, d1->d1', d2->d2'] (/\ b d -> rhs)
-- PLUS the usage-details
-- { d1' = dx1; d2' = dx2 }
-- where d1', d2' are cloned versions of d1,d2, with the type substitution applied.
--
-- Note that the substitution is applied to the whole thing.
-- This is convenient, but just slightly fragile. Notably:
-- * There had better be no name clashes in a/b/c/d
--
let
-- poly_tyvars = [b,d] in the example above
-- spec_tyvars = [a,c]
-- ty_args = [t1,b,t3,d]
poly_tyvars = [tv | (tv, Nothing) <- rhs_tyvars `zip` call_ts]
spec_tyvars = [tv | (tv, Just _) <- rhs_tyvars `zip` call_ts]
ty_args = zipWithEqual "spec_call" mk_ty_arg rhs_tyvars call_ts
where
mk_ty_arg rhs_tyvar Nothing = Type (mkTyVarTy rhs_tyvar)
mk_ty_arg rhs_tyvar (Just ty) = Type ty
rhs_subst = extendSubstList subst spec_tyvars [DoneTy ty | Just ty <- call_ts]
in
cloneBinders rhs_subst rhs_dicts `thenSM` \ (rhs_subst', rhs_dicts') ->
let
inst_args = ty_args ++ map Var rhs_dicts'
-- Figure out the type of the specialised function
spec_id_ty = mkForAllTys poly_tyvars (applyTypeToArgs rhs fn_type inst_args)
in
newIdSM fn spec_id_ty `thenSM` \ spec_f ->
specExpr rhs_subst' (mkLams poly_tyvars body) `thenSM` \ (spec_rhs, rhs_uds) ->
let
-- The rule to put in the function's specialisation is:
-- forall b,d, d1',d2'. f t1 b t3 d d1' d2' = f1 b d
spec_env_rule = (poly_tyvars ++ rhs_dicts',
inst_args,
mkTyApps (Var spec_f) (map mkTyVarTy poly_tyvars))
-- Add the { d1' = dx1; d2' = dx2 } usage stuff
final_uds = foldr addDictBind rhs_uds (my_zipEqual "spec_call" rhs_dicts' call_ds)
in
returnSM ((spec_f, spec_rhs),
final_uds,
spec_env_rule)
where
my_zipEqual doc xs ys
| length xs /= length ys = pprPanic "my_zipEqual" (ppr xs $$ ppr ys $$ (ppr fn <+> ppr call_ts) $$ ppr rhs)
| otherwise = zipEqual doc xs ys
\end{code}
%************************************************************************
%* *
\subsubsection{UsageDetails and suchlike}
%* *
%************************************************************************
\begin{code}
data UsageDetails
= MkUD {
dict_binds :: !(Bag DictBind),
-- Floated dictionary bindings
-- The order is important;
-- in ds1 `union` ds2, bindings in ds2 can depend on those in ds1
-- (Remember, Bags preserve order in GHC.)
calls :: !CallDetails
}
type DictBind = (CoreBind, VarSet)
-- The set is the free vars of the binding
-- both tyvars and dicts
type DictExpr = CoreExpr
emptyUDs = MkUD { dict_binds = emptyBag, calls = emptyFM }
type ProtoUsageDetails = ([DictBind],
[(Id, [Maybe Type], ([DictExpr], VarSet))]
)
------------------------------------------------------------
type CallDetails = FiniteMap Id CallInfo
type CallInfo = FiniteMap [Maybe Type] -- Nothing => unconstrained type argument
([DictExpr], VarSet) -- Dict args and the vars of the whole
-- call (including tyvars)
-- [*not* include the main id itself, of course]
-- The finite maps eliminate duplicates
-- The list of types and dictionaries is guaranteed to
-- match the type of f
unionCalls :: CallDetails -> CallDetails -> CallDetails
unionCalls c1 c2 = plusFM_C plusFM c1 c2
singleCall :: (Id, [Maybe Type], [DictExpr]) -> CallDetails
singleCall (id, tys, dicts)
= unitFM id (unitFM tys (dicts, call_fvs))
where
call_fvs = exprsFreeVars dicts `unionVarSet` tys_fvs
tys_fvs = tyVarsOfTypes (catMaybes tys)
-- The type args (tys) are guaranteed to be part of the dictionary
-- types, because they are just the constrained types,
-- and the dictionary is therefore sure to be bound
-- inside the binding for any type variables free in the type;
-- hence it's safe to neglect tyvars free in tys when making
-- the free-var set for this call
-- BUT I don't trust this reasoning; play safe and include tys_fvs
--
-- We don't include the 'id' itself.
listToCallDetails calls
= foldr (unionCalls . mk_call) emptyFM calls
where
mk_call (id, tys, dicts_w_fvs) = unitFM id (unitFM tys dicts_w_fvs)
-- NB: the free vars of the call are provided
callDetailsToList calls = [ (id,tys,dicts)
| (id,fm) <- fmToList calls,
(tys,dicts) <- fmToList fm
]
mkCallUDs f args
| null theta
|| length spec_tys /= n_tyvars
|| length dicts /= n_dicts
= emptyUDs -- Not overloaded
| otherwise
= MkUD {dict_binds = emptyBag,
calls = singleCall (f, spec_tys, dicts)
}
where
(tyvars, theta, tau) = splitSigmaTy (idType f)
constrained_tyvars = tyVarsOfTheta theta
n_tyvars = length tyvars
n_dicts = length theta
spec_tys = [mk_spec_ty tv ty | (tv, Type ty) <- tyvars `zip` args]
dicts = [dict_expr | (_, dict_expr) <- theta `zip` (drop n_tyvars args)]
mk_spec_ty tyvar ty | tyvar `elemVarSet` constrained_tyvars
= Just ty
| otherwise
= Nothing
------------------------------------------------------------
plusUDs :: UsageDetails -> UsageDetails -> UsageDetails
plusUDs (MkUD {dict_binds = db1, calls = calls1})
(MkUD {dict_binds = db2, calls = calls2})
= MkUD {dict_binds = d, calls = c}
where
d = db1 `unionBags` db2
c = calls1 `unionCalls` calls2
plusUDList = foldr plusUDs emptyUDs
-- zapCalls deletes calls to ids from uds
zapCalls ids uds = uds {calls = delListFromFM (calls uds) ids}
mkDB bind = (bind, bind_fvs bind)
bind_fvs (NonRec bndr rhs) = exprFreeVars rhs
bind_fvs (Rec prs) = foldl delVarSet rhs_fvs bndrs
where
bndrs = map fst prs
rhs_fvs = unionVarSets [exprFreeVars rhs | (bndr,rhs) <- prs]
addDictBind (dict,rhs) uds = uds { dict_binds = mkDB (NonRec dict rhs) `consBag` dict_binds uds }
dumpAllDictBinds (MkUD {dict_binds = dbs}) binds
= foldrBag add binds dbs
where
add (bind,_) binds = bind : binds
dumpUDs :: [CoreBndr]
-> UsageDetails -> CoreExpr
-> (UsageDetails, CoreExpr)
dumpUDs bndrs uds body
= (free_uds, foldr add_let body dict_binds)
where
(free_uds, (dict_binds, _)) = splitUDs bndrs uds
add_let (bind,_) body = Let bind body
splitUDs :: [CoreBndr]
-> UsageDetails
-> (UsageDetails, -- These don't mention the binders
ProtoUsageDetails) -- These do
splitUDs bndrs uds@(MkUD {dict_binds = orig_dbs,
calls = orig_calls})
= if isEmptyBag dump_dbs && null dump_calls then
-- Common case: binder doesn't affect floats
(uds, ([],[]))
else
-- Binders bind some of the fvs of the floats
(MkUD {dict_binds = free_dbs,
calls = listToCallDetails free_calls},
(bagToList dump_dbs, dump_calls)
)
where
bndr_set = mkVarSet bndrs
(free_dbs, dump_dbs, dump_idset)
= foldlBag dump_db (emptyBag, emptyBag, bndr_set) orig_dbs
-- Important that it's foldl not foldr;
-- we're accumulating the set of dumped ids in dump_set
-- Filter out any calls that mention things that are being dumped
orig_call_list = callDetailsToList orig_calls
(dump_calls, free_calls) = partition captured orig_call_list
captured (id,tys,(dicts, fvs)) = fvs `intersectsVarSet` dump_idset
|| id `elemVarSet` dump_idset
dump_db (free_dbs, dump_dbs, dump_idset) db@(bind, fvs)
| dump_idset `intersectsVarSet` fvs -- Dump it
= (free_dbs, dump_dbs `snocBag` db,
dump_idset `unionVarSet` mkVarSet (bindersOf bind))
| otherwise -- Don't dump it
= (free_dbs `snocBag` db, dump_dbs, dump_idset)
\end{code}
%************************************************************************
%* *
\subsubsection{Boring helper functions}
%* *
%************************************************************************
\begin{code}
lookupId:: IdEnv Id -> Id -> Id
lookupId env id = case lookupVarEnv env id of
Nothing -> id
Just id' -> id'
----------------------------------------
type SpecM a = UniqSM a
thenSM = thenUs
thenSM_ = thenUs_
returnSM = returnUs
getUniqSM = getUniqueUs
getUniqSupplySM = getUs
setUniqSupplySM = setUs
mapSM = mapUs
initSM = initUs_
mapAndCombineSM f [] = returnSM ([], emptyUDs)
mapAndCombineSM f (x:xs) = f x `thenSM` \ (y, uds1) ->
mapAndCombineSM f xs `thenSM` \ (ys, uds2) ->
returnSM (y:ys, uds1 `plusUDs` uds2)
cloneBindSM :: Subst -> CoreBind -> SpecM (Subst, Subst, CoreBind)
-- Clone the binders of the bind; return new bind with the cloned binders
-- Return the substitution to use for RHSs, and the one to use for the body
cloneBindSM subst (NonRec bndr rhs)
= getUs `thenUs` \ us ->
let
(subst', us', bndr') = substAndCloneId subst us bndr
in
setUs us' `thenUs_`
returnUs (subst, subst', NonRec bndr' rhs)
cloneBindSM subst (Rec pairs)
= getUs `thenUs` \ us ->
let
(subst', us', bndrs') = substAndCloneIds subst us (map fst pairs)
in
setUs us' `thenUs_`
returnUs (subst', subst', Rec (bndrs' `zip` map snd pairs))
cloneBinders subst bndrs
= getUs `thenUs` \ us ->
let
(subst', us', bndrs') = substAndCloneIds subst us bndrs
in
setUs us' `thenUs_`
returnUs (subst', bndrs')
newIdSM old_id new_ty
= getUniqSM `thenSM` \ uniq ->
let
-- Give the new Id a similar occurrence name to the old one
name = idName old_id
new_id = mkUserLocal (mkSpecOcc (nameOccName name)) uniq new_ty (getSrcLoc name)
-- If the old Id was exported, make the new one non-discardable,
-- else we will discard it since it doesn't seem to be called.
new_id' | isExportedId old_id = setIdNoDiscard new_id
| otherwise = new_id
in
returnSM new_id'
newTyVarSM
= getUniqSM `thenSM` \ uniq ->
returnSM (mkSysTyVar uniq boxedTypeKind)
\end{code}
Old (but interesting) stuff about unboxed bindings
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
What should we do when a value is specialised to a *strict* unboxed value?
map_*_* f (x:xs) = let h = f x
t = map f xs
in h:t
Could convert let to case:
map_*_Int# f (x:xs) = case f x of h# ->
let t = map f xs
in h#:t
This may be undesirable since it forces evaluation here, but the value
may not be used in all branches of the body. In the general case this
transformation is impossible since the mutual recursion in a letrec
cannot be expressed as a case.
There is also a problem with top-level unboxed values, since our
implementation cannot handle unboxed values at the top level.
Solution: Lift the binding of the unboxed value and extract it when it
is used:
map_*_Int# f (x:xs) = let h = case (f x) of h# -> _Lift h#
t = map f xs
in case h of
_Lift h# -> h#:t
Now give it to the simplifier and the _Lifting will be optimised away.
The benfit is that we have given the specialised "unboxed" values a
very simplep lifted semantics and then leave it up to the simplifier to
optimise it --- knowing that the overheads will be removed in nearly
all cases.
In particular, the value will only be evaluted in the branches of the
program which use it, rather than being forced at the point where the
value is bound. For example:
filtermap_*_* p f (x:xs)
= let h = f x
t = ...
in case p x of
True -> h:t
False -> t
==>
filtermap_*_Int# p f (x:xs)
= let h = case (f x) of h# -> _Lift h#
t = ...
in case p x of
True -> case h of _Lift h#
-> h#:t
False -> t
The binding for h can still be inlined in the one branch and the
_Lifting eliminated.
Question: When won't the _Lifting be eliminated?
Answer: When they at the top-level (where it is necessary) or when
inlining would duplicate work (or possibly code depending on
options). However, the _Lifting will still be eliminated if the
strictness analyser deems the lifted binding strict.
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