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{- |
Non-global free variable analysis on STG terms. This pass annotates
non-top-level closure bindings with captured variables. Global variables are not
captured. For example, in a top-level binding like (pseudo-STG)
f = \[x,y] .
let g = \[p] . reverse (x ++ p)
in g y
In g, `reverse` and `(++)` are global variables so they're not considered free.
`p` is an argument, so `x` is the only actual free variable here. The annotated
version is thus:
f = \[x,y] .
let g = [x] \[p] . reverse (x ++ p)
in g y
Note that non-top-level recursive bindings are also considered free within the
group:
map = {} \r [f xs0]
let {
Rec {
go = {f, go} \r [xs1]
case xs1 of {
[] -> [] [];
: x xs2 ->
let { xs' = {go, xs2} \u [] go xs2; } in
let { x' = {f, x} \u [] f x; } in
: [x' xs'];
};
end Rec }
} in go xs0;
Here go is free in its RHS.
Top-level closure bindings never capture variables as all of their free
variables are global.
-}
module GHC.Stg.FVs (
annTopBindingsFreeVars,
annBindingFreeVars
) where
import GhcPrelude
import GHC.Stg.Syntax
import Id
import VarSet
import GHC.Core ( Tickish(Breakpoint) )
import Outputable
import Util
import Data.Maybe ( mapMaybe )
newtype Env
= Env
{ locals :: IdSet
}
emptyEnv :: Env
emptyEnv = Env emptyVarSet
addLocals :: [Id] -> Env -> Env
addLocals bndrs env
= env { locals = extendVarSetList (locals env) bndrs }
-- | Annotates a top-level STG binding group with its free variables.
annTopBindingsFreeVars :: [StgTopBinding] -> [CgStgTopBinding]
annTopBindingsFreeVars = map go
where
go (StgTopStringLit id bs) = StgTopStringLit id bs
go (StgTopLifted bind)
= StgTopLifted (annBindingFreeVars bind)
-- | Annotates an STG binding with its free variables.
annBindingFreeVars :: StgBinding -> CgStgBinding
annBindingFreeVars = fst . binding emptyEnv emptyDVarSet
boundIds :: StgBinding -> [Id]
boundIds (StgNonRec b _) = [b]
boundIds (StgRec pairs) = map fst pairs
-- Note [Tracking local binders]
-- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-- 'locals' contains non-toplevel, non-imported binders.
-- We maintain the set in 'expr', 'alt' and 'rhs', which are the only
-- places where new local binders are introduced.
-- Why do it there rather than in 'binding'? Two reasons:
--
-- 1. We call 'binding' from 'annTopBindingsFreeVars', which would
-- add top-level bindings to the 'locals' set.
-- 2. In the let(-no-escape) case, we need to extend the environment
-- prior to analysing the body, but we also need the fvs from the
-- body to analyse the RHSs. No way to do this without some
-- knot-tying.
-- | This makes sure that only local, non-global free vars make it into the set.
mkFreeVarSet :: Env -> [Id] -> DIdSet
mkFreeVarSet env = mkDVarSet . filter (`elemVarSet` locals env)
args :: Env -> [StgArg] -> DIdSet
args env = mkFreeVarSet env . mapMaybe f
where
f (StgVarArg occ) = Just occ
f _ = Nothing
binding :: Env -> DIdSet -> StgBinding -> (CgStgBinding, DIdSet)
binding env body_fv (StgNonRec bndr r) = (StgNonRec bndr r', fvs)
where
-- See Note [Tracking local binders]
(r', rhs_fvs) = rhs env r
fvs = delDVarSet body_fv bndr `unionDVarSet` rhs_fvs
binding env body_fv (StgRec pairs) = (StgRec pairs', fvs)
where
-- See Note [Tracking local binders]
bndrs = map fst pairs
(rhss, rhs_fvss) = mapAndUnzip (rhs env . snd) pairs
pairs' = zip bndrs rhss
fvs = delDVarSetList (unionDVarSets (body_fv:rhs_fvss)) bndrs
expr :: Env -> StgExpr -> (CgStgExpr, DIdSet)
expr env = go
where
go (StgApp occ as)
= (StgApp occ as, unionDVarSet (args env as) (mkFreeVarSet env [occ]))
go (StgLit lit) = (StgLit lit, emptyDVarSet)
go (StgConApp dc as tys) = (StgConApp dc as tys, args env as)
go (StgOpApp op as ty) = (StgOpApp op as ty, args env as)
go StgLam{} = pprPanic "StgFVs: StgLam" empty
go (StgCase scrut bndr ty alts) = (StgCase scrut' bndr ty alts', fvs)
where
(scrut', scrut_fvs) = go scrut
-- See Note [Tracking local binders]
(alts', alt_fvss) = mapAndUnzip (alt (addLocals [bndr] env)) alts
alt_fvs = unionDVarSets alt_fvss
fvs = delDVarSet (unionDVarSet scrut_fvs alt_fvs) bndr
go (StgLet ext bind body) = go_bind (StgLet ext) bind body
go (StgLetNoEscape ext bind body) = go_bind (StgLetNoEscape ext) bind body
go (StgTick tick e) = (StgTick tick e', fvs')
where
(e', fvs) = go e
fvs' = unionDVarSet (tickish tick) fvs
tickish (Breakpoint _ ids) = mkDVarSet ids
tickish _ = emptyDVarSet
go_bind dc bind body = (dc bind' body', fvs)
where
-- See Note [Tracking local binders]
env' = addLocals (boundIds bind) env
(body', body_fvs) = expr env' body
(bind', fvs) = binding env' body_fvs bind
rhs :: Env -> StgRhs -> (CgStgRhs, DIdSet)
rhs env (StgRhsClosure _ ccs uf bndrs body)
= (StgRhsClosure fvs ccs uf bndrs body', fvs)
where
-- See Note [Tracking local binders]
(body', body_fvs) = expr (addLocals bndrs env) body
fvs = delDVarSetList body_fvs bndrs
rhs env (StgRhsCon ccs dc as) = (StgRhsCon ccs dc as, args env as)
alt :: Env -> StgAlt -> (CgStgAlt, DIdSet)
alt env (con, bndrs, e) = ((con, bndrs, e'), fvs)
where
-- See Note [Tracking local binders]
(e', rhs_fvs) = expr (addLocals bndrs env) e
fvs = delDVarSetList rhs_fvs bndrs
|
