compiler/stgSyn/StgFVs.hs

-- | Free variable analysis on STG terms.
module StgFVs (
    annTopBindingsFreeVars,
    annBindingFreeVars
  ) where

import GhcPrelude

import StgSyn
import Id
import VarSet
import CoreSyn    ( 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 [Tacking 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 [Tacking 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 [Tacking 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 [Tacking 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 [Tacking 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 [Tacking local binders]
    (e', rhs_fvs) = expr (addLocals bndrs env) e
    fvs = delDVarSetList rhs_fvs bndrs