Exitify.hs 18.8 KB
 Joachim Breitner committed Oct 29, 2017 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 ``````module 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 GhcPrelude import Var import Id import IdInfo import CoreSyn import CoreUtils import State import Unique import VarSet import VarEnv import CoreFVs import FastString import Type `````` Simon Peyton Jones committed Apr 27, 2018 51 ``````import Util( mapSnd ) `````` Joachim Breitner committed Oct 29, 2017 52 53 54 55 56 `````` import Data.Bifunctor import Control.Monad -- | Traverses the AST, simply to find all joinrecs and call 'exitify' on them. `````` Simon Peyton Jones committed Apr 27, 2018 57 ``````-- The really interesting function is exitifyRec `````` Joachim Breitner committed Oct 29, 2017 58 59 60 61 62 ``````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) `````` Joachim Breitner committed Apr 09, 2018 63 `````` -- Top-level bindings are never join points `````` Joachim Breitner committed Oct 29, 2017 64 65 66 67 `````` in_scope_toplvl = emptyInScopeSet `extendInScopeSetList` bindersOfBinds binds go :: InScopeSet -> CoreExpr -> CoreExpr `````` Simon Peyton Jones committed Apr 27, 2018 68 69 70 71 72 73 74 `````` 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) `````` Joachim Breitner committed Oct 29, 2017 75 `````` `````` Simon Peyton Jones committed Apr 27, 2018 76 77 `````` go in_scope (Lam v e') = Lam v (go in_scope' e') `````` Joachim Breitner committed Oct 29, 2017 78 `````` where in_scope' = in_scope `extendInScopeSet` v `````` Simon Peyton Jones committed Apr 27, 2018 79 `````` `````` Joachim Breitner committed Oct 29, 2017 80 `````` go in_scope (Case scrut bndr ty alts) `````` Simon Peyton Jones committed Apr 27, 2018 81 82 83 84 85 `````` = Case (go in_scope scrut) bndr ty (map go_alt alts) where in_scope1 = in_scope `extendInScopeSet` bndr go_alt (dc, pats, rhs) = (dc, pats, go in_scope' rhs) where in_scope' = in_scope1 `extendInScopeSetList` pats `````` Joachim Breitner committed Oct 29, 2017 86 `````` `````` Simon Peyton Jones committed Apr 27, 2018 87 88 89 90 `````` 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 `````` Joachim Breitner committed Oct 29, 2017 91 `````` `````` Simon Peyton Jones committed Apr 27, 2018 92 93 94 95 96 97 98 99 `````` 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 `````` Joachim Breitner committed Oct 29, 2017 100 `````` `````` Joachim Breitner committed Apr 09, 2018 101 102 103 104 `````` -- | State Monad used inside `exitify` type ExitifyM = State [(JoinId, CoreExpr)] `````` Joachim Breitner committed Oct 29, 2017 105 106 ``````-- | Given a recursive group of a joinrec, identifies “exit paths” and binds them as -- join-points outside the joinrec. `````` Simon Peyton Jones committed Apr 27, 2018 107 108 109 ``````exitifyRec :: InScopeSet -> [(Var,CoreExpr)] -> [CoreBind] exitifyRec in_scope pairs = [ NonRec xid rhs | (xid,rhs) <- exits ] ++ [Rec pairs'] `````` Joachim Breitner committed Oct 29, 2017 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 `````` 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` []) \$ do 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') `````` Simon Peyton Jones committed May 08, 2018 127 128 129 `````` --------------------- -- 'go' is the main working function. -- It goes through the RHS (tail-call positions only), `````` Joachim Breitner committed Oct 29, 2017 130 131 132 `````` -- 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. -- `````` Joachim Breitner committed Apr 09, 2018 133 `````` -- ExitifyM is a state monad to keep track of floated binds `````` Simon Peyton Jones committed Apr 27, 2018 134 135 136 137 138 `````` 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 `````` Joachim Breitner committed Apr 09, 2018 139 `````` -> ExitifyM CoreExpr `````` Joachim Breitner committed Oct 29, 2017 140 `````` `````` Joachim Breitner committed Apr 09, 2018 141 142 `````` -- We first look at the expression (no matter what it shape is) -- and determine if we can turn it into a exit join point `````` Joachim Breitner committed Oct 29, 2017 143 `````` go captured ann_e `````` Simon Peyton Jones committed May 08, 2018 144 145 146 147 `````` | -- An exit expression has no recursive calls let fvs = dVarSetToVarSet (freeVarsOf ann_e) , disjointVarSet fvs recursive_calls = go_exit captured (deAnnotate ann_e) fvs `````` Joachim Breitner committed Oct 29, 2017 148 `````` `````` Joachim Breitner committed Apr 09, 2018 149 150 151 `````` -- We could not turn it into a exit joint point. So now recurse -- into all expression where eligible exit join points might sit, -- i.e. into all tail-call positions: `````` Joachim Breitner committed Oct 29, 2017 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 `````` -- Case right hand sides are in tail-call position go captured (_, AnnCase scrut bndr ty alts) = do alts' <- forM alts \$ \(dc, pats, rhs) -> do rhs' <- go (captured ++ [bndr] ++ pats) rhs return (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 `````` Joachim Breitner committed Apr 09, 2018 189 190 `````` -- Cannot be turned into an exit join point, but also has no -- tail-call subexpression. Nothing to do here. `````` Joachim Breitner committed Oct 29, 2017 191 192 `````` go _ ann_e = return (deAnnotate ann_e) `````` Simon Peyton Jones committed May 08, 2018 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239 240 241 242 243 244 245 246 247 `````` --------------------- 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 expressoins 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 `````` Ryan Scott committed Mar 15, 2019 248 `````` -- zap any IdInfo they have; see #15005 `````` Simon Peyton Jones committed May 08, 2018 249 250 251 252 253 254 255 `````` -- cf. SetLevels.abstractVars zap v | isId v = setIdInfo v vanillaIdInfo | otherwise = v -- We cannot abstract over join points captures_join_points = any isJoinId abs_vars `````` Joachim Breitner committed Oct 29, 2017 256 257 258 259 260 261 262 263 264 265 266 267 268 269 270 `````` -- 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 ty `asJoinId` join_arity `````` Simon Peyton Jones committed May 08, 2018 271 272 ``````addExit :: InScopeSet -> JoinArity -> CoreExpr -> ExitifyM JoinId addExit in_scope join_arity rhs = do `````` Joachim Breitner committed Oct 29, 2017 273 `````` -- Pick a suitable name `````` Simon Peyton Jones committed May 08, 2018 274 `````` let ty = exprType rhs `````` Joachim Breitner committed Oct 29, 2017 275 276 277 278 279 280 281 282 283 284 285 286 287 288 289 290 291 292 293 294 295 296 297 298 299 300 301 302 303 304 305 306 307 308 309 310 311 312 313 314 315 316 317 318 319 320 321 322 323 324 325 326 327 328 329 330 331 332 333 334 `````` 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) `````` Simon Peyton Jones committed Apr 06, 2018 335 336 337 338 339 340 341 342 343 344 345 346 347 348 349 350 351 352 353 354 355 ``````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): `````` Joachim Breitner committed Oct 29, 2017 356 357 358 359 360 361 `````` 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 … `````` Simon Peyton Jones committed Apr 06, 2018 362 ``````Again, `j` would not be inlined because we do not inline something that looks `````` Joachim Breitner committed Oct 29, 2017 363 364 365 366 367 368 369 370 371 372 373 374 375 376 377 378 379 380 381 382 383 384 385 386 387 388 389 390 391 392 393 394 395 396 397 398 399 400 401 402 403 404 405 406 407 408 409 410 411 412 413 ``````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. `````` Joachim Breitner committed Apr 09, 2018 414 415 ``````We therefore choose B, and calculate the free variables in `exitify`. `````` Joachim Breitner committed Oct 29, 2017 416 417 418 419 420 421 422 423 424 425 426 427 428 429 430 431 432 433 `````` 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 `````` Brian Wignall committed Nov 28, 2019 434 ``````occurrence in a recursive group, and can be recognized (after the occurrence `````` Joachim Breitner committed Apr 09, 2018 435 ``````analyzer ran!) using `isExitJoinId`. `````` Joachim Breitner committed Oct 29, 2017 436 437 438 439 440 441 442 443 ``````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) `````` Simon Peyton Jones committed Apr 06, 2018 444 445 446 447 ``````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. `````` Joachim Breitner committed Oct 29, 2017 448 449 450 451 452 453 454 455 456 457 458 459 460 461 462 463 464 465 466 467 468 469 470 471 472 473 474 475 476 477 478 479 480 481 482 483 484 485 `````` 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). `````` Joachim Breitner committed May 01, 2018 486 487 488 489 490 491 492 493 494 495 ``````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 `````` Brian Wignall committed Nov 28, 2019 496 ``````these cases we want to abstract only over the last occurrence, hence the `foldr` `````` Joachim Breitner committed May 01, 2018 497 498 ``````(with emphasis on the `r`). This is #15110. `````` Joachim Breitner committed Oct 29, 2017 499 ``-}``