path: root/compiler/GHC/Core/Opt/Exitify.hs

module GHC.Core.Opt.Exitify ( exitifyProgram ) where

{-
Note [Exitification]
~~~~~~~~~~~~~~~~~~~~

This module implements Exitification. The goal is to pull as much code out of
recursive functions as possible, as the simplifier is better at inlining into
call-sites that are not in recursive functions.

Example:

  let t = foo bar
  joinrec go 0     x y = t (x*x)
          go (n-1) x y = jump go (n-1) (x+y)
  in …

We’d like to inline `t`, but that does not happen: Because t is a thunk and is
used in a recursive function, doing so might lose sharing in general. In
this case, however, `t` is on the _exit path_ of `go`, so called at most once.
How do we make this clearly visible to the simplifier?

A code path (i.e., an expression in a tail-recursive position) in a recursive
function is an exit path if it does not contain a recursive call. We can bind
this expression outside the recursive function, as a join-point.

Example result:

  let t = foo bar
  join exit x = t (x*x)
  joinrec go 0     x y = jump exit x
          go (n-1) x y = jump go (n-1) (x+y)
  in …

Now `t` is no longer in a recursive function, and good things happen!
-}

import GHC.Prelude
import GHC.Types.Var
import GHC.Types.Id
import GHC.Types.Id.Info
import GHC.Core
import GHC.Core.Utils
import GHC.Utils.Monad.State.Strict
import GHC.Builtin.Uniques
import GHC.Types.Var.Set
import GHC.Types.Var.Env
import GHC.Core.FVs
import GHC.Data.FastString
import GHC.Core.Type
import GHC.Utils.Misc( mapSnd )

import Data.Bifunctor
import Control.Monad

-- | Traverses the AST, simply to find all joinrecs and call 'exitify' on them.
-- The really interesting function is exitifyRec
exitifyProgram :: CoreProgram -> CoreProgram
exitifyProgram binds = map goTopLvl binds
  where
    goTopLvl (NonRec v e) = NonRec v (go in_scope_toplvl e)
    goTopLvl (Rec pairs) = Rec (map (second (go in_scope_toplvl)) pairs)
      -- Top-level bindings are never join points

    in_scope_toplvl = emptyInScopeSet `extendInScopeSetList` bindersOfBinds binds

    go :: InScopeSet -> CoreExpr -> CoreExpr
    go _    e@(Var{})       = e
    go _    e@(Lit {})      = e
    go _    e@(Type {})     = e
    go _    e@(Coercion {}) = e
    go in_scope (Cast e' c) = Cast (go in_scope e') c
    go in_scope (Tick t e') = Tick t (go in_scope e')
    go in_scope (App e1 e2) = App (go in_scope e1) (go in_scope e2)

    go in_scope (Lam v e')
      = Lam v (go in_scope' e')
      where in_scope' = in_scope `extendInScopeSet` v

    go in_scope (Case scrut bndr ty alts)
      = Case (go in_scope scrut) bndr ty (map go_alt alts)
      where
        in_scope1 = in_scope `extendInScopeSet` bndr
        go_alt (Alt dc pats rhs) = Alt dc pats (go in_scope' rhs)
           where in_scope' = in_scope1 `extendInScopeSetList` pats

    go in_scope (Let (NonRec bndr rhs) body)
      = Let (NonRec bndr (go in_scope rhs)) (go in_scope' body)
      where
        in_scope' = in_scope `extendInScopeSet` bndr

    go in_scope (Let (Rec pairs) body)
      | is_join_rec = mkLets (exitifyRec in_scope' pairs') body'
      | otherwise   = Let (Rec pairs') body'
      where
        is_join_rec = any (isJoinId . fst) pairs
        in_scope'   = in_scope `extendInScopeSetList` bindersOf (Rec pairs)
        pairs'      = mapSnd (go in_scope') pairs
        body'       = go in_scope' body


-- | State Monad used inside `exitify`
type ExitifyM =  State [(JoinId, CoreExpr)]

-- | Given a recursive group of a joinrec, identifies “exit paths” and binds them as
--   join-points outside the joinrec.
exitifyRec :: InScopeSet -> [(Var,CoreExpr)] -> [CoreBind]
exitifyRec in_scope pairs
  = [ NonRec xid rhs | (xid,rhs) <- exits ] ++ [Rec pairs']
  where
    -- We need the set of free variables of many subexpressions here, so
    -- annotate the AST with them
    -- see Note [Calculating free variables]
    ann_pairs = map (second freeVars) pairs

    -- Which are the recursive calls?
    recursive_calls = mkVarSet $ map fst pairs

    (pairs',exits) = (`runState` []) $
        forM ann_pairs $ \(x,rhs) -> do
            -- go past the lambdas of the join point
            let (args, body) = collectNAnnBndrs (idJoinArity x) rhs
            body' <- go args body
            let rhs' = mkLams args body'
            return (x, rhs')

    ---------------------
    -- 'go' is the main working function.
    -- It goes through the RHS (tail-call positions only),
    -- checks if there are no more recursive calls, if so, abstracts over
    -- variables bound on the way and lifts it out as a join point.
    --
    -- ExitifyM is a state monad to keep track of floated binds
    go :: [Var]           -- ^ Variables that are in-scope here, but
                          -- not in scope at the joinrec; that is,
                          -- we must potentially abstract over them.
                          -- Invariant: they are kept in dependency order
       -> CoreExprWithFVs -- ^ Current expression in tail position
       -> ExitifyM CoreExpr

    -- We first look at the expression (no matter what it shape is)
    -- and determine if we can turn it into a exit join point
    go captured ann_e
        | -- An exit expression has no recursive calls
          let fvs = dVarSetToVarSet (freeVarsOf ann_e)
        , disjointVarSet fvs recursive_calls
        = go_exit captured (deAnnotate ann_e) fvs

    -- We could not turn it into a exit join point. So now recurse
    -- into all expression where eligible exit join points might sit,
    -- i.e. into all tail-call positions:

    -- Case right hand sides are in tail-call position
    go captured (_, AnnCase scrut bndr ty alts) = do
        alts' <- forM alts $ \(AnnAlt dc pats rhs) -> do
            rhs' <- go (captured ++ [bndr] ++ pats) rhs
            return (Alt dc pats rhs')
        return $ Case (deAnnotate scrut) bndr ty alts'

    go captured (_, AnnLet ann_bind body)
        -- join point, RHS and body are in tail-call position
        | AnnNonRec j rhs <- ann_bind
        , Just join_arity <- isJoinId_maybe j
        = do let (params, join_body) = collectNAnnBndrs join_arity rhs
             join_body' <- go (captured ++ params) join_body
             let rhs' = mkLams params join_body'
             body' <- go (captured ++ [j]) body
             return $ Let (NonRec j rhs') body'

        -- rec join point, RHSs and body are in tail-call position
        | AnnRec pairs <- ann_bind
        , isJoinId (fst (head pairs))
        = do let js = map fst pairs
             pairs' <- forM pairs $ \(j,rhs) -> do
                 let join_arity = idJoinArity j
                     (params, join_body) = collectNAnnBndrs join_arity rhs
                 join_body' <- go (captured ++ js ++ params) join_body
                 let rhs' = mkLams params join_body'
                 return (j, rhs')
             body' <- go (captured ++ js) body
             return $ Let (Rec pairs') body'

        -- normal Let, only the body is in tail-call position
        | otherwise
        = do body' <- go (captured ++ bindersOf bind ) body
             return $ Let bind body'
      where bind = deAnnBind ann_bind

    -- Cannot be turned into an exit join point, but also has no
    -- tail-call subexpression. Nothing to do here.
    go _ ann_e = return (deAnnotate ann_e)

    ---------------------
    go_exit :: [Var]      -- Variables captured locally
            -> CoreExpr   -- An exit expression
            -> VarSet     -- Free vars of the expression
            -> ExitifyM CoreExpr
    -- go_exit deals with a tail expression that is floatable
    -- out as an exit point; that is, it mentions no recursive calls
    go_exit captured e fvs
      -- Do not touch an expression that is already a join jump where all arguments
      -- are captured variables. See Note [Idempotency]
      -- But _do_ float join jumps with interesting arguments.
      -- See Note [Jumps can be interesting]
      | (Var f, args) <- collectArgs e
      , isJoinId f
      , all isCapturedVarArg args
      = return e

      -- Do not touch a boring expression (see Note [Interesting expression])
      | not is_interesting
      = return e

      -- Cannot float out if local join points are used, as
      -- we cannot abstract over them
      | captures_join_points
      = return e

      -- We have something to float out!
      | otherwise
      = do { -- Assemble the RHS of the exit join point
             let rhs   = mkLams abs_vars e
                 avoid = in_scope `extendInScopeSetList` captured
             -- Remember this binding under a suitable name
           ; v <- addExit avoid (length abs_vars) rhs
             -- And jump to it from here
           ; return $ mkVarApps (Var v) abs_vars }

      where
        -- Used to detect exit expressions that are already proper exit jumps
        isCapturedVarArg (Var v) = v `elem` captured
        isCapturedVarArg _ = False

        -- An interesting exit expression has free, non-imported
        -- variables from outside the recursive group
        -- See Note [Interesting expression]
        is_interesting = anyVarSet isLocalId $
                         fvs `minusVarSet` mkVarSet captured

        -- The arguments of this exit join point
        -- See Note [Picking arguments to abstract over]
        abs_vars = snd $ foldr pick (fvs, []) captured
          where
            pick v (fvs', acc) | v `elemVarSet` fvs' = (fvs' `delVarSet` v, zap v : acc)
                               | otherwise           = (fvs',               acc)

        -- We are going to abstract over these variables, so we must
        -- zap any IdInfo they have; see #15005
        -- cf. GHC.Core.Opt.SetLevels.abstractVars
        zap v | isId v = setIdInfo v vanillaIdInfo
              | otherwise = v

        -- We cannot abstract over join points
        captures_join_points = any isJoinId abs_vars


-- Picks a new unique, which is disjoint from
--  * the free variables of the whole joinrec
--  * any bound variables (captured)
--  * any exit join points created so far.
mkExitJoinId :: InScopeSet -> Type -> JoinArity -> ExitifyM JoinId
mkExitJoinId in_scope ty join_arity = do
    fs <- get
    let avoid = in_scope `extendInScopeSetList` (map fst fs)
                         `extendInScopeSet` exit_id_tmpl -- just cosmetics
    return (uniqAway avoid exit_id_tmpl)
  where
    exit_id_tmpl = mkSysLocal (fsLit "exit") initExitJoinUnique Many ty
                    `asJoinId` join_arity

addExit :: InScopeSet -> JoinArity -> CoreExpr -> ExitifyM JoinId
addExit in_scope join_arity rhs = do
    -- Pick a suitable name
    let ty = exprType rhs
    v <- mkExitJoinId in_scope ty join_arity
    fs <- get
    put ((v,rhs):fs)
    return v

{-
Note [Interesting expression]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
We do not want this to happen:

  joinrec go 0     x y = x
          go (n-1) x y = jump go (n-1) (x+y)
  in …
==>
  join exit x = x
  joinrec go 0     x y = jump exit x
          go (n-1) x y = jump go (n-1) (x+y)
  in …

because the floated exit path (`x`) is simply a parameter of `go`; there are
not useful interactions exposed this way.

Neither do we want this to happen

  joinrec go 0     x y = x+x
          go (n-1) x y = jump go (n-1) (x+y)
  in …
==>
  join exit x = x+x
  joinrec go 0     x y = jump exit x
          go (n-1) x y = jump go (n-1) (x+y)
  in …

where the floated expression `x+x` is a bit more complicated, but still not
intersting.

Expressions are interesting when they move an occurrence of a variable outside
the recursive `go` that can benefit from being obviously called once, for example:
 * a local thunk that can then be inlined (see example in note [Exitification])
 * the parameter of a function, where the demand analyzer then can then
   see that it is called at most once, and hence improve the function’s
   strictness signature

So we only hoist an exit expression out if it mentiones at least one free,
non-imported variable.

Note [Jumps can be interesting]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
A jump to a join point can be interesting, if its arguments contain free
non-exported variables (z in the following example):

  joinrec go 0     x y = jump j (x+z)
          go (n-1) x y = jump go (n-1) (x+y)
  in …
==>
  join exit x y = jump j (x+z)
  joinrec go 0     x y = jump exit x
          go (n-1) x y = jump go (n-1) (x+y)


The join point itself can be interesting, even if none if its
arguments have free variables free in the joinrec.  For example

  join j p = case p of (x,y) -> x+y
  joinrec go 0     x y = jump j (x,y)
          go (n-1) x y = jump go (n-1) (x+y) y
  in …

Here, `j` would not be inlined because we do not inline something that looks
like an exit join point (see Note [Do not inline exit join points]). But
if we exitify the 'jump j (x,y)' we get

  join j p = case p of (x,y) -> x+y
  join exit x y = jump j (x,y)
  joinrec go 0     x y = jump exit x y
          go (n-1) x y = jump go (n-1) (x+y) y
  in …

and now 'j' can inline, and we get rid of the pair. Here's another
example (assume `g` to be an imported function that, on its own,
does not make this interesting):

  join j y = map f y
  joinrec go 0     x y = jump j (map g x)
          go (n-1) x y = jump go (n-1) (x+y)
  in …

Again, `j` would not be inlined because we do not inline something that looks
like an exit join point (see Note [Do not inline exit join points]).

But after exitification we have

  join j y = map f y
  join exit x = jump j (map g x)
  joinrec go 0     x y = jump j (map g x)
              go (n-1) x y = jump go (n-1) (x+y)
  in …

and now we can inline `j` and this will allow `map/map` to fire.


Note [Idempotency]
~~~~~~~~~~~~~~~~~~

We do not want this to happen, where we replace the floated expression with
essentially the same expression:

  join exit x = t (x*x)
  joinrec go 0     x y = jump exit x
          go (n-1) x y = jump go (n-1) (x+y)
  in …
==>
  join exit x = t (x*x)
  join exit' x = jump exit x
  joinrec go 0     x y = jump exit' x
          go (n-1) x y = jump go (n-1) (x+y)
  in …

So when the RHS is a join jump, and all of its arguments are captured variables,
then we leave it in place.

Note that `jump exit x` in this example looks interesting, as `exit` is a free
variable. Therefore, idempotency does not simply follow from floating only
interesting expressions.

Note [Calculating free variables]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
We have two options where to annotate the tree with free variables:

 A) The whole tree.
 B) Each individual joinrec as we come across it.

Downside of A: We pay the price on the whole module, even outside any joinrecs.
Downside of B: We pay the price per joinrec, possibly multiple times when
joinrecs are nested.

Further downside of A: If the exitify function returns annotated expressions,
it would have to ensure that the annotations are correct.

We therefore choose B, and calculate the free variables in `exitify`.


Note [Do not inline exit join points]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
When we have

  let t = foo bar
  join exit x = t (x*x)
  joinrec go 0     x y = jump exit x
          go (n-1) x y = jump go (n-1) (x+y)
  in …

we do not want the simplifier to simply inline `exit` back in (which it happily
would).

To prevent this, we need to recognize exit join points, and then disable
inlining.

Exit join points, recognizeable using `isExitJoinId` are join points with an
occurrence in a recursive group, and can be recognized (after the occurrence
analyzer ran!) using `isExitJoinId`.
This function detects joinpoints with `occ_in_lam (idOccinfo id) == True`,
because the lambdas of a non-recursive join point are not considered for
`occ_in_lam`.  For example, in the following code, `j1` is /not/ marked
occ_in_lam, because `j2` is called only once.

  join j1 x = x+1
  join j2 y = join j1 (y+2)

To prevent inlining, we check for isExitJoinId
* In `preInlineUnconditionally` directly.
* In `simplLetUnfolding` we simply give exit join points no unfolding, which
  prevents inlining in `postInlineUnconditionally` and call sites.

Note [Placement of the exitification pass]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
I (Joachim) experimented with multiple positions for the Exitification pass in
the Core2Core pipeline:

 A) Before the `simpl_phases`
 B) Between the `simpl_phases` and the "main" simplifier pass
 C) After demand_analyser
 D) Before the final simplification phase

Here is the table (this is without inlining join exit points in the final
simplifier run):

        Program |                       Allocs                      |                      Instrs
                | ABCD.log     A.log     B.log     C.log     D.log  | ABCD.log     A.log     B.log     C.log     D.log
----------------|---------------------------------------------------|-------------------------------------------------
 fannkuch-redux |   -99.9%     +0.0%    -99.9%    -99.9%    -99.9%  |    -3.9%     +0.5%     -3.0%     -3.9%     -3.9%
          fasta |    -0.0%     +0.0%     +0.0%     -0.0%     -0.0%  |    -8.5%     +0.0%     +0.0%     -0.0%     -8.5%
            fem |     0.0%      0.0%      0.0%      0.0%     +0.0%  |    -2.2%     -0.1%     -0.1%     -2.1%     -2.1%
           fish |     0.0%      0.0%      0.0%      0.0%     +0.0%  |    -3.1%     +0.0%     -1.1%     -1.1%     -0.0%
   k-nucleotide |   -91.3%    -91.0%    -91.0%    -91.3%    -91.3%  |    -6.3%    +11.4%    +11.4%     -6.3%     -6.2%
            scs |    -0.0%     -0.0%     -0.0%     -0.0%     -0.0%  |    -3.4%     -3.0%     -3.1%     -3.3%     -3.3%
         simple |    -6.0%      0.0%     -6.0%     -6.0%     +0.0%  |    -3.4%     +0.0%     -5.2%     -3.4%     -0.1%
  spectral-norm |    -0.0%      0.0%      0.0%     -0.0%     +0.0%  |    -2.7%     +0.0%     -2.7%     -5.4%     -5.4%
----------------|---------------------------------------------------|-------------------------------------------------
            Min |   -95.0%    -91.0%    -95.0%    -95.0%    -95.0%  |    -8.5%     -3.0%     -5.2%     -6.3%     -8.5%
            Max |    +0.2%     +0.2%     +0.2%     +0.2%     +1.5%  |    +0.4%    +11.4%    +11.4%     +0.4%     +1.5%
 Geometric Mean |    -4.7%     -2.1%     -4.7%     -4.7%     -4.6%  |    -0.4%     +0.1%     -0.1%     -0.3%     -0.2%

Position A is disqualified, as it does not get rid of the allocations in
fannkuch-redux.
Position A and B are disqualified because it increases instructions in k-nucleotide.
Positions C and D have their advantages: C decreases allocations in simpl, but D instructions in fasta.

Assuming we have a budget of _one_ run of Exitification, then C wins (but we
could get more from running it multiple times, as seen in fish).

Note [Picking arguments to abstract over]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

When we create an exit join point, so we need to abstract over those of its
free variables that are be out-of-scope at the destination of the exit join
point. So we go through the list `captured` and pick those that are actually
free variables of the join point.

We do not just `filter (`elemVarSet` fvs) captured`, as there might be
shadowing, and `captured` may contain multiple variables with the same Unique. I
these cases we want to abstract only over the last occurrence, hence the `foldr`
(with emphasis on the `r`). This is #15110.

-}