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    1 
    2 
    3 module GHC.Stg.Subst where
    4 
    5 import GHC.Prelude
    6 
    7 import GHC.Types.Id
    8 import GHC.Types.Var.Env
    9 import GHC.Utils.Monad.State.Strict
   10 
   11 import GHC.Utils.Outputable
   12 import GHC.Utils.Misc
   13 import GHC.Utils.Panic
   14 import GHC.Utils.Trace
   15 
   16 -- | A renaming substitution from 'Id's to 'Id's. Like 'RnEnv2', but not
   17 -- maintaining pairs of substitutions. Like 'GHC.Core.Subst.Subst', but
   18 -- with the domain being 'Id's instead of entire 'CoreExpr'.
   19 data Subst = Subst InScopeSet IdSubstEnv
   20 
   21 type IdSubstEnv = IdEnv Id
   22 
   23 -- | @emptySubst = 'mkEmptySubst' 'emptyInScopeSet'@
   24 emptySubst :: Subst
   25 emptySubst = mkEmptySubst emptyInScopeSet
   26 
   27 -- | Constructs a new 'Subst' assuming the variables in the given 'InScopeSet'
   28 -- are in scope.
   29 mkEmptySubst :: InScopeSet -> Subst
   30 mkEmptySubst in_scope = Subst in_scope emptyVarEnv
   31 
   32 -- | Substitutes an 'Id' for another one according to the 'Subst' given in a way
   33 -- that avoids shadowing the 'InScopeSet', returning the result and an updated
   34 -- 'Subst' that should be used by subsequent substitutions.
   35 substBndr :: Id -> Subst -> (Id, Subst)
   36 substBndr id (Subst in_scope env)
   37   = (new_id, Subst new_in_scope new_env)
   38   where
   39     new_id = uniqAway in_scope id
   40     no_change = new_id == id -- in case nothing shadowed
   41     new_in_scope = in_scope `extendInScopeSet` new_id
   42     new_env
   43       | no_change = delVarEnv env id
   44       | otherwise = extendVarEnv env id new_id
   45 
   46 -- | @substBndrs = runState . traverse (state . substBndr)@
   47 substBndrs :: Traversable f => f Id -> Subst -> (f Id, Subst)
   48 substBndrs = runState . traverse (state . substBndr)
   49 
   50 -- | Substitutes an occurrence of an identifier for its counterpart recorded
   51 -- in the 'Subst'.
   52 lookupIdSubst :: HasCallStack => Id -> Subst -> Id
   53 lookupIdSubst id (Subst in_scope env)
   54   | not (isLocalId id) = id
   55   | Just id' <- lookupVarEnv env id = id'
   56   | Just id' <- lookupInScope in_scope id = id'
   57   | otherwise = warnPprTrace True (text "StgSubst.lookupIdSubst" <+> ppr id $$ ppr in_scope) id
   58 
   59 -- | Substitutes an occurrence of an identifier for its counterpart recorded
   60 -- in the 'Subst'. Does not generate a debug warning if the identifier to
   61 -- to substitute wasn't in scope.
   62 noWarnLookupIdSubst :: HasCallStack => Id -> Subst -> Id
   63 noWarnLookupIdSubst id (Subst in_scope env)
   64   | not (isLocalId id) = id
   65   | Just id' <- lookupVarEnv env id = id'
   66   | Just id' <- lookupInScope in_scope id = id'
   67   | otherwise = id
   68 
   69 -- | Add the 'Id' to the in-scope set and remove any existing substitutions for
   70 -- it.
   71 extendInScope :: Id -> Subst -> Subst
   72 extendInScope id (Subst in_scope env) = Subst (in_scope `extendInScopeSet` id) env
   73 
   74 -- | Add a substitution for an 'Id' to the 'Subst': you must ensure that the
   75 -- in-scope set is such that TyCoSubst Note [The substitution invariant]
   76 -- holds after extending the substitution like this.
   77 extendSubst :: Id -> Id -> Subst -> Subst
   78 extendSubst id new_id (Subst in_scope env)
   79   = assertPpr (new_id `elemInScopeSet` in_scope) (ppr id <+> ppr new_id $$ ppr in_scope) $
   80     Subst in_scope (extendVarEnv env id new_id)