From fea9ecdbcdf68eb59449478075efd2df6aaea0a9 Mon Sep 17 00:00:00 2001
From: Sebastian Graf <sebastian.graf@kit.edu>
Date: Fri, 15 Sep 2023 20:18:36 +0200
Subject: [PATCH] CorePrep: Refactor FloatingBind (#23442)
A drastically improved architecture for local floating in CorePrep
that decouples the decision of whether a float is going to be let- or case-bound
from how far it can float (out of strict contexts, out of lazy contexts, to
top-level).
There are a couple of new Notes describing the effort:
* `Note [Floating in CorePrep]` for the overview
* `Note [BindInfo and FloatInfo]` for the new classification of floats
* `Note [Floats and FloatDecision]` for how FloatInfo is used to inform
floating decisions
This is necessary ground work for proper treatment of Strict fields and
unlifted values at top-level.
Fixes #23442.
NoFib results (omitted = 0.0%):
```
--------------------------------------------------------------------------------
Program Allocs Instrs
--------------------------------------------------------------------------------
pretty 0.0% -1.6%
scc 0.0% -1.7%
--------------------------------------------------------------------------------
Min 0.0% -1.7%
Max 0.0% -0.0%
Geometric Mean -0.0% -0.0%
```
---
compiler/GHC/CoreToStg/Prep.hs | 720 +++++++++++++++++++++------------
compiler/GHC/Data/OrdList.hs | 23 +-
2 files changed, 473 insertions(+), 270 deletions(-)
diff --git a/compiler/GHC/CoreToStg/Prep.hs b/compiler/GHC/CoreToStg/Prep.hs
index 9d9e17eda9af..02180b86c8b2 100644
--- a/compiler/GHC/CoreToStg/Prep.hs
+++ b/compiler/GHC/CoreToStg/Prep.hs
@@ -1,4 +1,5 @@
{-# LANGUAGE BangPatterns #-}
+{-# LANGUAGE ViewPatterns #-}
{-# OPTIONS_GHC -Wno-incomplete-uni-patterns #-}
@@ -97,7 +98,8 @@ The goal of this pass is to prepare for code generation.
(The code generator can't deal with anything else.)
Type lambdas are ok, however, because the code gen discards them.
-5. [Not any more; nuked Jun 2002] Do the seq/par munging.
+5. ANF-isation results in additional bindings that can obscure values.
+ We float these out; see Note [Floating in CorePrep].
6. Clone all local Ids.
This means that all such Ids are unique, rather than the
@@ -165,7 +167,7 @@ Here is the syntax of the Core produced by CorePrep:
Expressions
body ::= app
| let(rec) x = rhs in body -- Boxed only
- | case body of pat -> body
+ | case app of pat -> body
| /\a. body | /\c. body
| body |> co
@@ -217,7 +219,7 @@ corePrepPgm logger cp_cfg pgm_cfg
binds_out = initUs_ us $ do
floats1 <- corePrepTopBinds initialCorePrepEnv binds
floats2 <- corePrepTopBinds initialCorePrepEnv implicit_binds
- return (deFloatTop (floats1 `appendFloats` floats2))
+ return (deFloatTop (floats1 `zipFloats` floats2))
endPassIO logger (cpPgm_endPassConfig pgm_cfg)
binds_out []
@@ -244,7 +246,7 @@ corePrepTopBinds initialCorePrepEnv binds
-- Only join points get returned this way by
-- cpeBind, and no join point may float to top
floatss <- go env' binds
- return (floats `appendFloats` floatss)
+ return (floats `zipFloats` floatss)
mkDataConWorkers :: Bool -> ModLocation -> [TyCon] -> [CoreBind]
-- See Note [Data constructor workers]
@@ -268,7 +270,40 @@ mkDataConWorkers generate_debug_info mod_loc data_tycons
LexicalFastString $ mkFastString $ renderWithContext defaultSDocContext $ ppr name
span1 file = realSrcLocSpan $ mkRealSrcLoc (mkFastString file) 1 1
-{-
+{- Note [Floating in CorePrep]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+ANFisation risks producing a lot of nested lets that obscures values:
+ let v = (:) (f 14) [] in e
+ ==> { ANF in CorePrep }
+ let v = let sat = f 14 in (:) sat [] in e
+Here, `v` is not a value anymore, and we'd allocate a thunk closure for `v` that
+allocates a thunk for `sat` and then allocates the cons cell.
+Hence we carry around a bunch of floated bindings with us so that we again
+expose the values:
+ let v = let sat = f 14 in (:) sat [] in e
+ ==> { Float sat }
+ let sat = f 14 in
+ let v = (:) sat [] in e
+(We will not do this transformation if `v` does not become a value afterwards;
+see Note [wantFloatLocal].)
+If `v` is bound at the top-level, we might even float `sat` to top-level;
+see Note [Floating out of top level bindings].
+For nested let bindings, we have to keep in mind Note [Core letrec invariant]
+and may exploit strict contexts; see Note [wantFloatLocal].
+
+There are 3 main categories of floats, encoded in the `FloatingBind` type:
+
+ * `Float`: A floated binding, as `sat` above.
+ These come in different flavours as described by their `FloatInfo` and
+ `BindInfo`, which captures how far the binding can be floated and whether or
+ not we want to case-bind. See Note [BindInfo and FloatInfo].
+ * `UnsafeEqualityCase`: Used for floating around unsafeEqualityProof bindings;
+ see (U3) of Note [Implementing unsafeCoerce].
+ It's exactly a `Float` that is `CaseBound` and `LazyContextFloatable`
+ (see `mkNonRecFloat`), but one that has a non-DEFAULT Case alternative to
+ bind the unsafe coercion field of the Refl constructor.
+ * `FloatTick`: A floated `Tick`. See Note [Floating Ticks in CorePrep].
+
Note [Floating out of top level bindings]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
NB: we do need to float out of top-level bindings
@@ -557,9 +592,9 @@ cpeBind top_lvl env (NonRec bndr rhs)
floats1 | triv_rhs, isInternalName (idName bndr)
= floats
| otherwise
- = addFloat floats new_float
+ = snocFloat floats new_float
- new_float = mkFloat env dmd is_unlifted bndr1 rhs1
+ new_float = mkNonRecFloat env dmd is_unlifted bndr1 rhs1
; return (env2, floats1, Nothing) }
@@ -578,15 +613,21 @@ cpeBind top_lvl env (Rec pairs)
; stuff <- zipWithM (cpePair top_lvl Recursive topDmd False env')
bndrs1 rhss
- ; let (floats_s, rhss1) = unzip stuff
- -- Glom all floats into the Rec, *except* FloatStrings which can
- -- (and must, because unlifted!) float further.
- (string_floats, all_pairs) =
- foldrOL add_float (emptyFloats, bndrs1 `zip` rhss1)
- (concatFloats floats_s)
+ ; let (zipManyFloats -> floats, rhss1) = unzip stuff
+ -- Glom all floats into the Rec, *except* FloatStrings; see
+ -- see Note [ANF-ising literal string arguments], Wrinkle (FS1)
+ is_lit (Float (NonRec _ rhs) CaseBound TopLvlFloatable) = exprIsTickedString rhs
+ is_lit _ = False
+ (string_floats, top) = partitionOL is_lit (fs_binds floats)
+ -- Strings will *always* be in `top_floats` (we made sure of
+ -- that in `snocOL`), so that's the only field we need to
+ -- partition.
+ floats' = floats { fs_binds = top }
+ all_pairs = foldrOL add_float (bndrs1 `zip` rhss1) (getFloats floats')
-- use env below, so that we reset cpe_rec_ids
; return (extendCorePrepEnvList env (bndrs `zip` bndrs1),
- string_floats `addFloat` FloatLet (Rec all_pairs),
+ snocFloat (emptyFloats { fs_binds = string_floats })
+ (Float (Rec all_pairs) LetBound TopLvlFloatable),
Nothing) }
| otherwise -- See Note [Join points and floating]
@@ -604,10 +645,11 @@ cpeBind top_lvl env (Rec pairs)
-- Flatten all the floats, and the current
-- group into a single giant Rec
- add_float (FloatLet (NonRec b r)) (ss, prs2) = (ss, (b,r) : prs2)
- add_float (FloatLet (Rec prs1)) (ss, prs2) = (ss, prs1 ++ prs2)
- add_float s@FloatString{} (ss, prs2) = (addFloat ss s, prs2)
- add_float b _ = pprPanic "cpeBind" (ppr b)
+ add_float (Float bind bound _) prs2
+ | bound /= CaseBound = case bind of
+ NonRec x e -> (x,e) : prs2
+ Rec prs1 -> prs1 ++ prs2
+ add_float f _ = pprPanic "cpeBind" (ppr f)
---------------
cpePair :: TopLevelFlag -> RecFlag -> Demand -> Bool
@@ -620,7 +662,8 @@ cpePair top_lvl is_rec dmd is_unlifted env bndr rhs
do { (floats1, rhs1) <- cpeRhsE env rhs
-- See if we are allowed to float this stuff out of the RHS
- ; (floats2, rhs2) <- float_from_rhs floats1 rhs1
+ ; let dec = want_float_from_rhs floats1 rhs1
+ ; (floats2, rhs2) <- executeFloatDecision dec floats1 rhs1
-- Make the arity match up
; (floats3, rhs3)
@@ -629,8 +672,8 @@ cpePair top_lvl is_rec dmd is_unlifted env bndr rhs
else warnPprTrace True "CorePrep: silly extra arguments:" (ppr bndr) $
-- Note [Silly extra arguments]
(do { v <- newVar (idType bndr)
- ; let float = mkFloat env topDmd False v rhs2
- ; return ( addFloat floats2 float
+ ; let float = mkNonRecFloat env topDmd False v rhs2
+ ; return ( snocFloat floats2 float
, cpeEtaExpand arity (Var v)) })
-- Wrap floating ticks
@@ -640,35 +683,9 @@ cpePair top_lvl is_rec dmd is_unlifted env bndr rhs
where
arity = idArity bndr -- We must match this arity
- ---------------------
- float_from_rhs floats rhs
- | isEmptyFloats floats = return (emptyFloats, rhs)
- | isTopLevel top_lvl = float_top floats rhs
- | otherwise = float_nested floats rhs
-
- ---------------------
- float_nested floats rhs
- | wantFloatNested is_rec dmd is_unlifted floats rhs
- = return (floats, rhs)
- | otherwise = dontFloat floats rhs
-
- ---------------------
- float_top floats rhs
- | allLazyTop floats
- = return (floats, rhs)
-
- | otherwise
- = dontFloat floats rhs
-
-dontFloat :: Floats -> CpeRhs -> UniqSM (Floats, CpeBody)
--- Non-empty floats, but do not want to float from rhs
--- So wrap the rhs in the floats
--- But: rhs1 might have lambdas, and we can't
--- put them inside a wrapBinds
-dontFloat floats1 rhs
- = do { (floats2, body) <- rhsToBody rhs
- ; return (emptyFloats, wrapBinds floats1 $
- wrapBinds floats2 body) }
+ want_float_from_rhs floats rhs
+ | isTopLevel top_lvl = wantFloatTop floats
+ | otherwise = wantFloatLocal is_rec dmd is_unlifted floats rhs
{- Note [Silly extra arguments]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
@@ -754,14 +771,14 @@ cpeRhsE env (Let bind body)
; (body_floats, body') <- cpeRhsE env' body
; let expr' = case maybe_bind' of Just bind' -> Let bind' body'
Nothing -> body'
- ; return (bind_floats `appendFloats` body_floats, expr') }
+ ; return (bind_floats `appFloats` body_floats, expr') }
cpeRhsE env (Tick tickish expr)
-- Pull out ticks if they are allowed to be floated.
| tickishFloatable tickish
= do { (floats, body) <- cpeRhsE env expr
-- See [Floating Ticks in CorePrep]
- ; return (unitFloat (FloatTick tickish) `appendFloats` floats, body) }
+ ; return (FloatTick tickish `consFloat` floats, body) }
| otherwise
= do { body <- cpeBodyNF env expr
; return (emptyFloats, mkTick tickish' body) }
@@ -805,12 +822,12 @@ cpeRhsE env (Case scrut bndr _ alts@[Alt con bs _])
; (floats_rhs, rhs) <- cpeBody env rhs
-- ... but we want to float `floats_rhs` as in (U3) so that rhs' might
-- become a value
- ; let case_float = FloatCase scrut bndr con bs True
- -- NB: True <=> ok-for-spec; it is OK to "evaluate" the proof eagerly.
+ ; let case_float = UnsafeEqualityCase scrut bndr con bs
+ -- NB: It is OK to "evaluate" the proof eagerly.
-- Usually there's the danger that we float the unsafeCoerce out of
-- a branching Case alt. Not so here, because the regular code path
-- for `cpeRhsE Case{}` will not float out of alts.
- floats = addFloat floats_scrut case_float `appendFloats` floats_rhs
+ floats = snocFloat floats_scrut case_float `appFloats` floats_rhs
; return (floats, rhs) }
cpeRhsE env (Case scrut bndr ty alts)
@@ -859,7 +876,7 @@ cpeBody :: CorePrepEnv -> CoreExpr -> UniqSM (Floats, CpeBody)
cpeBody env expr
= do { (floats1, rhs) <- cpeRhsE env expr
; (floats2, body) <- rhsToBody rhs
- ; return (floats1 `appendFloats` floats2, body) }
+ ; return (floats1 `appFloats` floats2, body) }
--------
rhsToBody :: CpeRhs -> UniqSM (Floats, CpeBody)
@@ -882,7 +899,7 @@ rhsToBody expr@(Lam {}) -- See Note [No eta reduction needed in rhsToBody]
| otherwise -- Some value lambdas
= do { let rhs = cpeEtaExpand (exprArity expr) expr
; fn <- newVar (exprType rhs)
- ; let float = FloatLet (NonRec fn rhs)
+ ; let float = Float (NonRec fn rhs) LetBound TopLvlFloatable
; return (unitFloat float, Var fn) }
where
(bndrs,_) = collectBinders expr
@@ -1125,7 +1142,8 @@ cpeApp top_env expr
:: CorePrepEnv
-> [ArgInfo] -- The arguments (inner to outer)
-> CpeApp -- The function
- -> Floats
+ -> Floats -- INVARIANT: These floats don't bind anything that is in the CpeApp!
+ -- Just stuff floated out from the head of the application.
-> [Demand]
-> Maybe Arity
-> UniqSM (CpeApp
@@ -1170,7 +1188,7 @@ cpeApp top_env expr
(ss1 : ss_rest, False) -> (ss1, ss_rest)
([], _) -> (topDmd, [])
(fs, arg') <- cpeArg top_env ss1 arg
- rebuild_app' env as (App fun' arg') (fs `appendFloats` floats) ss_rest rt_ticks (req_depth-1)
+ rebuild_app' env as (App fun' arg') (fs `zipFloats` floats) ss_rest rt_ticks (req_depth-1)
CpeCast co
-> rebuild_app' env as (Cast fun' co) floats ss rt_ticks req_depth
@@ -1182,7 +1200,7 @@ cpeApp top_env expr
rebuild_app' env as fun' floats ss (tickish:rt_ticks) req_depth
| otherwise
-- See [Floating Ticks in CorePrep]
- -> rebuild_app' env as fun' (addFloat floats (FloatTick tickish)) ss rt_ticks req_depth
+ -> rebuild_app' env as fun' (snocFloat floats (FloatTick tickish)) ss rt_ticks req_depth
isLazyExpr :: CoreExpr -> Bool
-- See Note [lazyId magic] in GHC.Types.Id.Make
@@ -1261,8 +1279,7 @@ Other relevant Notes:
* Note [runRW arg] below, describing a non-obvious case where the
late-inlining could go wrong.
-
- Note [runRW arg]
+Note [runRW arg]
~~~~~~~~~~~~~~~~~~~
Consider the Core program (from #11291),
@@ -1294,7 +1311,6 @@ the function and the arguments) will forgo binding it to a variable. By
contrast, in the non-bottoming case of `hello` above the function will be
deemed non-trivial and consequently will be case-bound.
-
Note [Simplification of runRW#]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Consider the program,
@@ -1408,8 +1424,7 @@ But with -O0, there is no FloatOut, so CorePrep must do the ANFisation to
foo = Foo s
(String literals are the only kind of binding allowed at top-level and hence
-their floats are `OkToSpec` like lifted bindings, whereas all other unlifted
-floats are `IfUnboxedOk` so that they don't float to top-level.)
+their `FloatInfo` is `TopLvlFloatable`.)
This appears to lead to bad code if the arg is under a lambda, because CorePrep
doesn't float out of RHSs, e.g., (T23270)
@@ -1432,24 +1447,13 @@ But actually, it doesn't, because "turtle"# is already an HNF. Here is the Cmm:
Wrinkles:
-(FS1) It is crucial that we float out String literals out of RHSs that could
- become values, e.g.,
-
- let t = case "turtle"# of s { __DEFAULT -> MkT s }
- in f t
-
- where `MkT :: Addr# -> T`. We want
-
- let s = "turtle"#; t = MkT s
- in f t
-
- because the former allocates an extra thunk for `t`.
- Normally, the `case turtle# of s ...` becomes a `FloatCase` and
- we don't float `FloatCase` outside of (recursive) RHSs, so we get the
- former program (this is the 'allLazyNested' test in 'wantFloatNested').
- That is what we use `FloatString` for: It is essentially a `FloatCase`
- which is always ok-to-spec/can be regarded as a non-allocating value and
- thus be floated aggressively to expose more value bindings.
+(FS1) We detect string literals in `cpeBind Rec{}` and float them out anyway;
+ otherwise we'd try to bind a string literal in a letrec, violating
+ Note [Core letrec invariant]. Since we know that literals don't have
+ free variables, we float further.
+ Arguably, we could just as well relax the letrec invariant for
+ string literals, or anthing that is a value (lifted or not).
+ This is tracked in #24036.
-}
-- This is where we arrange that a non-trivial argument is let-bound
@@ -1459,10 +1463,9 @@ cpeArg env dmd arg
= do { (floats1, arg1) <- cpeRhsE env arg -- arg1 can be a lambda
; let arg_ty = exprType arg1
is_unlifted = isUnliftedType arg_ty
- want_float = wantFloatNested NonRecursive dmd is_unlifted
- ; (floats2, arg2) <- if want_float floats1 arg1
- then return (floats1, arg1)
- else dontFloat floats1 arg1
+ dec = wantFloatLocal NonRecursive dmd is_unlifted
+ floats1 arg1
+ ; (floats2, arg2) <- executeFloatDecision dec floats1 arg1
-- Else case: arg1 might have lambdas, and we can't
-- put them inside a wrapBinds
@@ -1474,8 +1477,8 @@ cpeArg env dmd arg
else do { v <- newVar arg_ty
-- See Note [Eta expansion of arguments in CorePrep]
; let arg3 = cpeEtaExpandArg env arg2
- arg_float = mkFloat env dmd is_unlifted v arg3
- ; return (addFloat floats2 arg_float, varToCoreExpr v) }
+ arg_float = mkNonRecFloat env dmd is_unlifted v arg3
+ ; return (snocFloat floats2 arg_float, varToCoreExpr v) }
}
cpeEtaExpandArg :: CorePrepEnv -> CoreArg -> CoreArg
@@ -1508,20 +1511,6 @@ See Note [Eta expansion for join points] in GHC.Core.Opt.Arity
Eta expanding the join point would introduce crap that we can't
generate code for
-Note [Floating unlifted arguments]
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-Consider C (let v* = expensive in v)
-
-where the "*" indicates "will be demanded". Usually v will have been
-inlined by now, but let's suppose it hasn't (see #2756). Then we
-do *not* want to get
-
- let v* = expensive in C v
-
-because that has different strictness. Hence the use of 'allLazy'.
-(NB: the let v* turns into a FloatCase, in mkLocalNonRec.)
-
-
------------------------------------------------------------------------------
-- Building the saturated syntax
-- ---------------------------------------------------------------------------
@@ -1714,7 +1703,9 @@ Since call-by-value is much cheaper than call-by-need, we case-bind arguments
that are either
1. Strictly evaluated anyway, according to the DmdSig of the callee, or
- 2. ok-for-spec, according to 'exprOkForSpeculation'
+ 2. ok-for-spec, according to 'exprOkForSpeculation'.
+ This includes DFuns `$fEqList a`, for example.
+ (Could identify more in the future; see reference to !1866 below.)
While (1) is a no-brainer and always beneficial, (2) is a bit
more subtle, as the careful haddock for 'exprOkForSpeculation'
@@ -1791,159 +1782,262 @@ of the very function whose termination properties we are exploiting.
It is also similar to Note [Do not strictify a DFun's parameter dictionaries],
where marking recursive DFuns (of undecidable *instances*) strict in dictionary
*parameters* leads to quite the same change in termination as above.
+
+Note [BindInfo and FloatInfo]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+The `BindInfo` of a `Float` describes whether it will be case-bound or
+let-bound:
+
+ * `LetBound`: A let binding `let x = rhs in ...`, can be Rec or NonRec.
+ * `CaseBound`: A case binding `case rhs of x -> { __DEFAULT -> .. }`.
+ (So always NonRec.)
+ Some case-bound things (string literals, lifted bindings)
+ can float to top-level (but not all), hence it is similar
+ to, but not the same as `StrictContextFloatable :: FloatInfo`
+ described below.
+
+This info is used in `wrapBinds` to pick the corresponding binding form.
+
+We want to case-bind iff the binding is (non-recursive, and) either
+
+ * ok-for-spec-eval (and perhaps lifted, see Note [Speculative evaluation]), or
+ * unlifted, or
+ * strictly used
+
+The `FloatInfo` of a `Float` describes how far it can float without
+(a) violating Core invariants and (b) changing semantics.
+
+ * Any binding is at least `StrictContextFloatable`, meaning we may float it
+ out of a strict context such as `f <>` where `f` is strict.
+
+ * A binding is `LazyContextFloatable` if we may float it out of a lazy context
+ such as `let x = <> in Just x`.
+ Counterexample: A strict or unlifted binding that isn't ok-for-spec-eval
+ such as `case divInt# x y of r -> { __DEFAULT -> I# r }`.
+ Here, we may not foat out the strict `r = divInt# x y`.
+
+ * A binding is `TopLvlFloatable` if it is `LazyContextFloatable` and also can
+ be bound at the top level.
+ Counterexample: A strict or unlifted binding (ok-for-spec-eval or not)
+ such as `case x +# y of r -> { __DEFAULT -> I# r }`.
+
+This meaning of "at least" is encoded in `floatsAtLeastAsFarAs`.
+Note that today, `LetBound` implies `TopLvlFloatable`, so we could make do with
+the the following enum (check `mkNonRecFloat` for whether this is up to date):
+
+ LetBoundTopLvlFloatable (lifted or boxed values)
+ CaseBoundTopLvlFloatable (strings, ok-for-spec-eval and lifted)
+ CaseBoundLazyContextFloatable (ok-for-spec-eval and unlifted)
+ CaseBoundStrictContextFloatable (not ok-for-spec-eval and unlifted)
+
+Although there is redundancy in the current encoding, SG thinks it is cleaner
+conceptually.
+
+See also Note [Floats and FloatDecision] for how we maintain whole groups of
+floats and how far they go.
+
+Note [Floats and FloatDecision]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+We have a special datatype `Floats` for modelling a telescope of `FloatingBind`
+and caching its "maximum" `FloatInfo`, according to `floatsAtLeastAsFarAs`
+(see Note [BindInfo and FloatInfo] for the ordering).
+There are several operations for creating and combining `Floats` that maintain
+scoping and the cached `FloatInfo`.
+
+When deciding whether we want to float out a `Floats` out of a binding context
+such as `let x = <> in e` (let), `f <>` (app), or `x = <>; ...` (top-level),
+we consult the cached `FloatInfo` of the `Floats`:
+
+ * If we want to float to the top-level (`x = <>; ...`), we check whether
+ we may float-at-least-as-far-as `TopLvlFloatable`, in which case we
+ respond with `FloatAll :: FloatDecision`; otherwise we say `FloatNone`.
+ * If we want to float locally (let or app), then the floating decision is
+ described in Note [wantFloatLocal].
+
+`executeFloatDecision` is then used to act on the particular `FloatDecision`.
-}
+-- See Note [BindInfo and FloatInfo]
+data BindInfo
+ = CaseBound -- ^ A strict binding
+ | LetBound -- ^ A lazy or value binding
+ deriving Eq
+
+-- See Note [BindInfo and FloatInfo]
+data FloatInfo
+ = TopLvlFloatable
+ -- ^ Anything that can be bound at top-level, such as arbitrary lifted
+ -- bindings or anything that responds True to `exprIsHNF`, such as literals or
+ -- saturated DataCon apps where unlifted or strict args are values.
+
+ | LazyContextFloatable
+ -- ^ Anything that can be floated out of a lazy context.
+ -- In addition to any 'TopLvlFloatable' things, this includes (unlifted)
+ -- bindings that are ok-for-spec that we intend to case-bind.
+
+ | StrictContextFloatable
+ -- ^ Anything that can be floated out of a strict evaluation context.
+ -- That is possible for all bindings; this is the Top element of 'FloatInfo'.
+
+ deriving Eq
+
+instance Outputable BindInfo where
+ ppr CaseBound = text "Case"
+ ppr LetBound = text "Let"
+
+instance Outputable FloatInfo where
+ ppr TopLvlFloatable = text "top-lvl"
+ ppr LazyContextFloatable = text "lzy-ctx"
+ ppr StrictContextFloatable = text "str-ctx"
+
+-- See Note [Floating in CorePrep]
+-- and Note [BindInfo and FloatInfo]
data FloatingBind
- -- | Rhs of bindings are CpeRhss
- -- They are always of lifted type;
- -- unlifted ones are done with FloatCase
- = FloatLet CoreBind
-
- -- | Float a literal string binding.
- -- INVARIANT: The `CoreExpr` matches `Lit (LitString bs)`.
- -- It's just more convenient to keep around the expr rather than
- -- the wrapped `bs` and reallocate the expr.
- -- This is a special case of `FloatCase` that is unconditionally ok-for-spec.
- -- We want to float out strings quite aggressively out of RHSs if doing so
- -- saves allocation of a thunk ('wantFloatNested'); see Wrinkle (FS1)
- -- in Note [ANF-ising literal string arguments].
- | FloatString !CoreExpr !Id
-
- | FloatCase
- CpeBody -- ^ Scrutinee
- Id -- ^ Case binder
- AltCon [Var] -- ^ Single alternative
- Bool -- ^ Ok-for-speculation; False of a strict,
- -- but lifted binding that is not OK for
- -- Note [Speculative evaluation].
-
- -- | See Note [Floating Ticks in CorePrep]
+ = Float !CoreBind !BindInfo !FloatInfo
+ | UnsafeEqualityCase !CoreExpr !CoreBndr !AltCon ![CoreBndr]
| FloatTick CoreTickish
-data Floats = Floats OkToSpec (OrdList FloatingBind)
+-- See Note [Floats and FloatDecision]
+data Floats
+ = Floats
+ { fs_info :: !FloatInfo
+ , fs_binds :: !(OrdList FloatingBind)
+ }
instance Outputable FloatingBind where
- ppr (FloatLet b) = ppr b
- ppr (FloatString e b) = text "string" <> braces (ppr b <> char '=' <> ppr e)
- ppr (FloatCase r b k bs ok) = text "case" <> braces (ppr ok) <+> ppr r
+ ppr (Float b bi fi) = ppr bi <+> ppr fi <+> ppr b
+ ppr (FloatTick t) = ppr t
+ ppr (UnsafeEqualityCase scrut b k bs) = text "case" <+> ppr scrut
<+> text "of"<+> ppr b <> text "@"
<> case bs of
[] -> ppr k
_ -> parens (ppr k <+> ppr bs)
- ppr (FloatTick t) = ppr t
instance Outputable Floats where
- ppr (Floats flag fs) = text "Floats" <> brackets (ppr flag) <+>
- braces (vcat (map ppr (fromOL fs)))
-
-instance Outputable OkToSpec where
- ppr OkToSpec = text "OkToSpec"
- ppr IfUnliftedOk = text "IfUnliftedOk"
- ppr NotOkToSpec = text "NotOkToSpec"
-
--- Can we float these binds out of the rhs of a let? We cache this decision
--- to avoid having to recompute it in a non-linear way when there are
--- deeply nested lets.
-data OkToSpec
- = OkToSpec -- ^ Lazy bindings of lifted type. Float as you please
- | IfUnliftedOk -- ^ A mixture of lazy lifted bindings and n
- -- ok-to-speculate unlifted bindings.
- -- Float out of lets, but not to top-level!
- | NotOkToSpec -- ^ Some not-ok-to-speculate unlifted bindings
-
-mkFloat :: CorePrepEnv -> Demand -> Bool -> Id -> CpeRhs -> FloatingBind
-mkFloat env dmd is_unlifted bndr rhs
- | Lit LitString{} <- rhs = FloatString rhs bndr
-
- | is_strict || ok_for_spec
- , not is_hnf = FloatCase rhs bndr DEFAULT [] ok_for_spec
- -- See Note [Speculative evaluation]
- -- Don't make a case for a HNF binding, even if it's strict
- -- Otherwise we get case (\x -> e) of ...!
-
- | is_unlifted = FloatCase rhs bndr DEFAULT [] True
- -- we used to assertPpr ok_for_spec (ppr rhs) here, but it is now disabled
- -- because exprOkForSpeculation isn't stable under ANF-ing. See for
- -- example #19489 where the following unlifted expression:
- --
- -- GHC.Prim.(#|_#) @LiftedRep @LiftedRep @[a_ax0] @[a_ax0]
- -- (GHC.Types.: @a_ax0 a2_agq a3_agl)
- --
- -- is ok-for-spec but is ANF-ised into:
- --
- -- let sat = GHC.Types.: @a_ax0 a2_agq a3_agl
- -- in GHC.Prim.(#|_#) @LiftedRep @LiftedRep @[a_ax0] @[a_ax0] sat
- --
- -- which isn't ok-for-spec because of the let-expression.
-
- | is_hnf = FloatLet (NonRec bndr rhs)
- | otherwise = FloatLet (NonRec (setIdDemandInfo bndr dmd) rhs)
- -- See Note [Pin demand info on floats]
- where
- is_hnf = exprIsHNF rhs
- is_strict = isStrUsedDmd dmd
- ok_for_spec = exprOkForSpecEval (not . is_rec_call) rhs
- is_rec_call = (`elemUnVarSet` cpe_rec_ids env)
+ ppr (Floats info binds) = text "Floats" <> brackets (ppr info) <> braces (ppr binds)
+
+lubFloatInfo :: FloatInfo -> FloatInfo -> FloatInfo
+lubFloatInfo StrictContextFloatable _ = StrictContextFloatable
+lubFloatInfo _ StrictContextFloatable = StrictContextFloatable
+lubFloatInfo LazyContextFloatable _ = LazyContextFloatable
+lubFloatInfo _ LazyContextFloatable = LazyContextFloatable
+lubFloatInfo TopLvlFloatable TopLvlFloatable = TopLvlFloatable
+
+floatsAtLeastAsFarAs :: FloatInfo -> FloatInfo -> Bool
+-- See Note [Floats and FloatDecision]
+floatsAtLeastAsFarAs l r = l `lubFloatInfo` r == r
emptyFloats :: Floats
-emptyFloats = Floats OkToSpec nilOL
+emptyFloats = Floats TopLvlFloatable nilOL
isEmptyFloats :: Floats -> Bool
-isEmptyFloats (Floats _ bs) = isNilOL bs
+isEmptyFloats (Floats _ b) = isNilOL b
-wrapBinds :: Floats -> CpeBody -> CpeBody
-wrapBinds (Floats _ binds) body
- = foldrOL mk_bind body binds
- where
- mk_bind (FloatCase rhs bndr con bs _) body = Case rhs bndr (exprType body) [Alt con bs body]
- mk_bind (FloatString rhs bndr) body = Case rhs bndr (exprType body) [Alt DEFAULT [] body]
- mk_bind (FloatLet bind) body = Let bind body
- mk_bind (FloatTick tickish) body = mkTick tickish body
-
-addFloat :: Floats -> FloatingBind -> Floats
-addFloat (Floats ok_to_spec floats) new_float
- = Floats (combine ok_to_spec (check new_float)) (floats `snocOL` new_float)
- where
- check FloatLet {} = OkToSpec
- check FloatTick{} = OkToSpec
- check FloatString{} = OkToSpec
- check (FloatCase _ _ _ _ ok_for_spec)
- | ok_for_spec = IfUnliftedOk
- | otherwise = NotOkToSpec
- -- The ok-for-speculation flag says that it's safe to
- -- float this Case out of a let, and thereby do it more eagerly
- -- We need the IfUnliftedOk flag because it's never ok to float
- -- an unlifted binding to the top level.
- -- There is one exception: String literals! But those will become
- -- FloatString and thus OkToSpec.
- -- See Note [ANF-ising literal string arguments]
+getFloats :: Floats -> OrdList FloatingBind
+getFloats = fs_binds
unitFloat :: FloatingBind -> Floats
-unitFloat = addFloat emptyFloats
-
-appendFloats :: Floats -> Floats -> Floats
-appendFloats (Floats spec1 floats1) (Floats spec2 floats2)
- = Floats (combine spec1 spec2) (floats1 `appOL` floats2)
-
-concatFloats :: [Floats] -> OrdList FloatingBind
-concatFloats = foldr (\ (Floats _ bs1) bs2 -> appOL bs1 bs2) nilOL
-
-combine :: OkToSpec -> OkToSpec -> OkToSpec
-combine NotOkToSpec _ = NotOkToSpec
-combine _ NotOkToSpec = NotOkToSpec
-combine IfUnliftedOk _ = IfUnliftedOk
-combine _ IfUnliftedOk = IfUnliftedOk
-combine _ _ = OkToSpec
+unitFloat = snocFloat emptyFloats
+
+floatInfo :: FloatingBind -> FloatInfo
+floatInfo (Float _ _ info) = info
+floatInfo UnsafeEqualityCase{} = LazyContextFloatable -- See Note [Floating in CorePrep]
+floatInfo FloatTick{} = TopLvlFloatable -- We filter these out in cpePair,
+ -- see Note [Floating Ticks in CorePrep]
+
+-- | Append a `FloatingBind` `b` to a `Floats` telescope `bs` that may reference any
+-- binding of the 'Floats'.
+snocFloat :: Floats -> FloatingBind -> Floats
+snocFloat floats fb =
+ Floats { fs_info = lubFloatInfo (fs_info floats) (floatInfo fb)
+ , fs_binds = fs_binds floats `snocOL` fb }
+
+-- | Cons a `FloatingBind` `b` to a `Floats` telescope `bs` which scopes over
+-- `b`.
+consFloat :: FloatingBind -> Floats -> Floats
+consFloat fb floats =
+ Floats { fs_info = lubFloatInfo (fs_info floats) (floatInfo fb)
+ , fs_binds = fb `consOL` fs_binds floats }
+
+-- | Append two telescopes, nesting the right inside the left.
+appFloats :: Floats -> Floats -> Floats
+appFloats outer inner =
+ Floats { fs_info = lubFloatInfo (fs_info outer) (fs_info inner)
+ , fs_binds = fs_binds outer `appOL` fs_binds inner }
+
+-- | Zip up two `Floats`, none of which scope over the other
+zipFloats :: Floats -> Floats -> Floats
+-- We may certainly just nest one telescope in the other, so appFloats is a
+-- valid implementation strategy.
+zipFloats = appFloats
+
+-- | `zipFloats` a bunch of independent telescopes.
+zipManyFloats :: [Floats] -> Floats
+zipManyFloats = foldr zipFloats emptyFloats
+
+mkNonRecFloat :: CorePrepEnv -> Demand -> Bool -> Id -> CpeRhs -> FloatingBind
+mkNonRecFloat env dmd is_unlifted bndr rhs = -- pprTraceWith "mkNonRecFloat" ppr $
+ Float (NonRec bndr' rhs) bound info
+ where
+ bndr' = setIdDemandInfo bndr dmd -- See Note [Pin demand info on floats]
+ (bound,info)
+ | is_lifted, is_hnf = (LetBound, TopLvlFloatable)
+ -- is_lifted: We currently don't allow unlifted values at the
+ -- top-level or inside letrecs
+ -- (but SG thinks that in principle, we should)
+ | is_data_con bndr = (LetBound, TopLvlFloatable)
+ -- We need this special case for unlifted DataCon workers/wrappers
+ -- until #17521 is fixed
+ | exprIsTickedString rhs = (CaseBound, TopLvlFloatable)
+ -- String literals are unboxed (so must be case-bound) and float to
+ -- the top-level
+ | is_unlifted, ok_for_spec = (CaseBound, LazyContextFloatable)
+ | is_lifted, ok_for_spec = (CaseBound, TopLvlFloatable)
+ -- See Note [Speculative evaluation]
+ -- Ok-for-spec-eval things will be case-bound, lifted or not.
+ -- But when it's lifted we are ok with floating it to top-level
+ -- (where it is actually bound lazily).
+ | is_unlifted || is_strict = (CaseBound, StrictContextFloatable)
+ -- These will never be floated out of a lazy RHS context
+ | otherwise = assertPpr is_lifted (ppr rhs) $
+ (LetBound, TopLvlFloatable)
+ -- And these float freely but can't be speculated, hence LetBound
+
+ is_lifted = not is_unlifted
+ is_hnf = exprIsHNF rhs
+ is_strict = isStrUsedDmd dmd
+ ok_for_spec = exprOkForSpecEval (not . is_rec_call) rhs
+ is_rec_call = (`elemUnVarSet` cpe_rec_ids env)
+ is_data_con = isJust . isDataConId_maybe
+-- | Wrap floats around an expression
+wrapBinds :: Floats -> CpeBody -> CpeBody
+wrapBinds floats body
+ = -- pprTraceWith "wrapBinds" (\res -> ppr floats $$ ppr body $$ ppr res) $
+ foldrOL mk_bind body (getFloats floats)
+ where
+ -- See Note [BindInfo and FloatInfo] on whether we pick Case or Let here
+ mk_bind f@(Float bind CaseBound _) body
+ | NonRec bndr rhs <- bind
+ = mkDefaultCase rhs bndr body
+ | otherwise
+ = pprPanic "wrapBinds" (ppr f)
+ mk_bind (Float bind _ _) body
+ = Let bind body
+ mk_bind (UnsafeEqualityCase scrut b con bs) body
+ = mkSingleAltCase scrut b con bs body
+ mk_bind (FloatTick tickish) body
+ = mkTick tickish body
+
+-- | Put floats at top-level
deFloatTop :: Floats -> [CoreBind]
--- For top level only; we don't expect any FloatCases
-deFloatTop (Floats _ floats)
- = foldrOL get [] floats
+-- Precondition: No Strict or LazyContextFloatable 'FloatInfo', no ticks!
+deFloatTop floats
+ = foldrOL get [] (getFloats floats)
where
- get (FloatLet b) bs = get_bind b : bs
- get (FloatString body var) bs = get_bind (NonRec var body) : bs
- get (FloatCase body var _ _ _) bs = get_bind (NonRec var body) : bs
- get b _ = pprPanic "corePrepPgm" (ppr b)
+ get (Float b _ TopLvlFloatable) bs
+ = get_bind b : bs
+ get b _ = pprPanic "corePrepPgm" (ppr b)
-- See Note [Dead code in CorePrep]
get_bind (NonRec x e) = NonRec x (occurAnalyseExpr e)
@@ -1951,25 +2045,113 @@ deFloatTop (Floats _ floats)
---------------------------------------------------------------------------
-wantFloatNested :: RecFlag -> Demand -> Bool -> Floats -> CpeRhs -> Bool
-wantFloatNested is_rec dmd rhs_is_unlifted floats rhs
- = isEmptyFloats floats
- || isStrUsedDmd dmd
- || rhs_is_unlifted
- || (allLazyNested is_rec floats && exprIsHNF rhs)
- -- Why the test for allLazyNested?
- -- v = f (x `divInt#` y)
- -- we don't want to float the case, even if f has arity 2,
- -- because floating the case would make it evaluated too early
-
-allLazyTop :: Floats -> Bool
-allLazyTop (Floats OkToSpec _) = True
-allLazyTop _ = False
-
-allLazyNested :: RecFlag -> Floats -> Bool
-allLazyNested _ (Floats OkToSpec _) = True
-allLazyNested _ (Floats NotOkToSpec _) = False
-allLazyNested is_rec (Floats IfUnliftedOk _) = isNonRec is_rec
+{- Note [wantFloatLocal]
+~~~~~~~~~~~~~~~~~~~~~~~~
+Consider
+ let x = let y = e1 in e2
+ in e
+Similarly for `(\x. e) (let y = e1 in e2)`.
+Do we want to float out `y` out of `x`?
+(This is discussed in detail in the paper
+"Let-floating: moving bindings to give faster programs".)
+
+`wantFloatLocal` is concerned with answering this question.
+It considers the Demand on `x`, whether or not `e2` is unlifted and the
+`FloatInfo` of the `y` binding (e.g., it might itself be unlifted, a value,
+strict, or ok-for-spec).
+
+We float out if ...
+ 1. ... the binding context is strict anyway, so either `x` is used strictly
+ or has unlifted type.
+ Doing so is trivially sound and won`t increase allocations, so we
+ return `FloatAll`.
+ This might happen while ANF-ising `f (g (h 13))` where `f`,`g` are strict:
+ f (g (h 13))
+ ==> { ANF }
+ case (case h 13 of r -> g r) of r2 -> f r2
+ ==> { Float }
+ case h 13 of r -> case g r of r2 -> f r2
+ The latter is easier to read and grows less stack.
+ 2. ... `e2` becomes a value in doing so, in which case we won't need to
+ allocate a thunk for `x`/the arg that closes over the FVs of `e1`.
+ In general, this is only sound if `y=e1` is `LazyContextFloatable`.
+ (See Note [BindInfo and FloatInfo].)
+ Nothing is won if `x` doesn't become a value
+ (i.e., `let x = let sat = f 14 in g sat in e`),
+ so we return `FloatNone` if there is any float that is
+ `StrictContextFloatable`, and return `FloatAll` otherwise.
+
+To elaborate on (2), consider the case when the floated binding is
+`e1 = divInt# a b`, e.g., not `LazyContextFloatable`:
+ let x = I# (a `divInt#` b)
+ in e
+this ANFises to
+ let x = case a `divInt#` b of r { __DEFAULT -> I# r }
+ in e
+If `x` is used lazily, we may not float `r` further out.
+A float binding `x +# y` is OK, though, and so every ok-for-spec-eval
+binding is `LazyContextFloatable`.
+
+Wrinkles:
+
+ (W1) When the outer binding is a letrec, i.e.,
+ letrec x = case a +# b of r { __DEFAULT -> f y r }
+ y = [x]
+ in e
+ we don't want to float `LazyContextFloatable` bindings such as `r` either
+ and require `TopLvlFloatable` instead.
+ The reason is that we don't track FV of FloatBindings, so we would need
+ to park them in the letrec,
+ letrec r = a +# b -- NB: r`s RHS might scope over x and y
+ x = f y r
+ y = [x]
+ in e
+ and now we have violated Note [Core letrec invariant].
+ So we preempt this case in `wantFloatLocal`, responding `FloatNone` unless
+ all floats are `TopLvlFloatable`.
+-}
+
+data FloatDecision
+ = FloatNone
+ | FloatAll
+
+executeFloatDecision :: FloatDecision -> Floats -> CpeRhs -> UniqSM (Floats, CpeRhs)
+executeFloatDecision dec floats rhs = do
+ let (float,stay) = case dec of
+ _ | isEmptyFloats floats -> (emptyFloats,emptyFloats)
+ FloatNone -> (emptyFloats, floats)
+ FloatAll -> (floats, emptyFloats)
+ -- Wrap `stay` around `rhs`.
+ -- NB: `rhs` might have lambdas, and we can't
+ -- put them inside a wrapBinds, which expects a `CpeBody`.
+ if isEmptyFloats stay -- Fast path where we don't need to call `rhsToBody`
+ then return (float, rhs)
+ else do
+ (floats', body) <- rhsToBody rhs
+ return (float, wrapBinds stay $ wrapBinds floats' body)
+
+wantFloatTop :: Floats -> FloatDecision
+wantFloatTop fs
+ | fs_info fs `floatsAtLeastAsFarAs` TopLvlFloatable = FloatAll
+ | otherwise = FloatNone
+
+wantFloatLocal :: RecFlag -> Demand -> Bool -> Floats -> CpeRhs -> FloatDecision
+-- See Note [wantFloatLocal]
+wantFloatLocal is_rec rhs_dmd rhs_is_unlifted floats rhs
+ | isEmptyFloats floats -- Well yeah...
+ || isStrUsedDmd rhs_dmd -- Case (1) of Note [wantFloatLocal]
+ || rhs_is_unlifted -- dito
+ || (fs_info floats `floatsAtLeastAsFarAs` max_float_info && exprIsHNF rhs)
+ -- Case (2) of Note [wantFloatLocal]
+ = FloatAll
+
+ | otherwise
+ = FloatNone
+ where
+ max_float_info | isRec is_rec = TopLvlFloatable
+ | otherwise = LazyContextFloatable
+ -- See Note [wantFloatLocal], Wrinkle (W1)
+ -- for 'is_rec'
{-
************************************************************************
@@ -2224,26 +2406,28 @@ newVar ty
-- | Like wrapFloats, but only wraps tick floats
wrapTicks :: Floats -> CoreExpr -> (Floats, CoreExpr)
-wrapTicks (Floats flag floats0) expr =
- (Floats flag (toOL $ reverse floats1), foldr mkTick expr (reverse ticks1))
- where (floats1, ticks1) = foldlOL go ([], []) $ floats0
+wrapTicks floats expr
+ | (floats1, ticks1) <- fold_fun go floats
+ = (floats1, foldrOL mkTick expr ticks1)
+ where fold_fun f floats =
+ let (binds, ticks) = foldlOL f (nilOL,nilOL) (fs_binds floats)
+ in (floats { fs_binds = binds }, ticks)
-- Deeply nested constructors will produce long lists of
-- redundant source note floats here. We need to eliminate
-- those early, as relying on mkTick to spot it after the fact
-- can yield O(n^3) complexity [#11095]
- go (floats, ticks) (FloatTick t)
+ go (flt_binds, ticks) (FloatTick t)
= assert (tickishPlace t == PlaceNonLam)
- (floats, if any (flip tickishContains t) ticks
- then ticks else t:ticks)
- go (floats, ticks) f@FloatString{}
- = (f:floats, ticks) -- don't need to wrap the tick around the string; nothing to execute.
- go (floats, ticks) f
- = (foldr wrap f (reverse ticks):floats, ticks)
-
- wrap t (FloatLet bind) = FloatLet (wrapBind t bind)
- wrap t (FloatCase r b con bs ok) = FloatCase (mkTick t r) b con bs ok
- wrap _ other = pprPanic "wrapTicks: unexpected float!"
- (ppr other)
+ (flt_binds, if any (flip tickishContains t) ticks
+ then ticks else ticks `snocOL` t)
+ go (flt_binds, ticks) f@UnsafeEqualityCase{}
+ -- unsafe equality case will be erased; don't wrap anything!
+ = (flt_binds `snocOL` f, ticks)
+ go (flt_binds, ticks) f@Float{}
+ = (flt_binds `snocOL` foldrOL wrap f ticks, ticks)
+
+ wrap t (Float bind bound info) = Float (wrapBind t bind) bound info
+ wrap _ f = pprPanic "Unexpected FloatingBind" (ppr f)
wrapBind t (NonRec binder rhs) = NonRec binder (mkTick t rhs)
wrapBind t (Rec pairs) = Rec (mapSnd (mkTick t) pairs)
diff --git a/compiler/GHC/Data/OrdList.hs b/compiler/GHC/Data/OrdList.hs
index 6b122b1833d9..ddf0c2088d5f 100644
--- a/compiler/GHC/Data/OrdList.hs
+++ b/compiler/GHC/Data/OrdList.hs
@@ -16,8 +16,8 @@ module GHC.Data.OrdList (
OrdList, pattern NilOL, pattern ConsOL, pattern SnocOL,
nilOL, isNilOL, unitOL, appOL, consOL, snocOL, concatOL, lastOL,
headOL,
- mapOL, mapOL', fromOL, toOL, foldrOL, foldlOL, reverseOL, fromOLReverse,
- strictlyEqOL, strictlyOrdOL
+ mapOL, mapOL', fromOL, toOL, foldrOL, foldlOL,
+ partitionOL, reverseOL, fromOLReverse, strictlyEqOL, strictlyOrdOL
) where
import GHC.Prelude
@@ -220,6 +220,25 @@ foldlOL k z (Snoc xs x) = let !z' = (foldlOL k z xs) in k z' x
foldlOL k z (Two b1 b2) = let !z' = (foldlOL k z b1) in foldlOL k z' b2
foldlOL k z (Many xs) = foldl' k z xs
+partitionOL :: (a -> Bool) -> OrdList a -> (OrdList a, OrdList a)
+partitionOL _ None = (None,None)
+partitionOL f (One x)
+ | f x = (One x, None)
+ | otherwise = (None, One x)
+partitionOL f (Two xs ys) = (Two ls1 ls2, Two rs1 rs2)
+ where !(!ls1,!rs1) = partitionOL f xs
+ !(!ls2,!rs2) = partitionOL f ys
+partitionOL f (Cons x xs)
+ | f x = (Cons x ls, rs)
+ | otherwise = (ls, Cons x rs)
+ where !(!ls,!rs) = partitionOL f xs
+partitionOL f (Snoc xs x)
+ | f x = (Snoc ls x, rs)
+ | otherwise = (ls, Snoc rs x)
+ where !(!ls,!rs) = partitionOL f xs
+partitionOL f (Many xs) = (toOL ls, toOL rs)
+ where !(!ls,!rs) = NE.partition f xs
+
toOL :: [a] -> OrdList a
toOL [] = None
toOL [x] = One x
--
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