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- commit 5efbf0d243984444cf352ad6f0d147e226c64498
- Author: Sergei Trofimovich <slyfox@gentoo.org>
- Date: Thu Sep 1 17:34:58 2016 +0100
- restore -fmax-worker-args handling (Trac #11565)
-
- maxWorkerArgs handling was accidentally lost 3 years ago
- in a major update of demand analysis
- commit 0831a12ea2fc73c33652eeec1adc79fa19700578
-
- Old regression is noticeable as:
- - code bloat (requires stack reshuffling)
- - compilation slowdown (more code to optimise/generate)
- - and increased heap usage (DynFlags unboxing/reboxing?)
-
- On a simple compile benchmark this change causes heap
- allocation drop from 70G don to 67G (ghc perf build).
-
- Signed-off-by: Sergei Trofimovich <siarheit@google.com>
-
- Reviewers: simonpj, ezyang, goldfire, austin, bgamari
-
- Reviewed By: simonpj, ezyang
-
- Subscribers: thomie
-
- Differential Revision: https://phabricator.haskell.org/D2503
-
- GHC Trac Issues: #11565
- diff --git a/compiler/specialise/SpecConstr.hs b/compiler/specialise/SpecConstr.hs
- index 10d5614..7166f57 100644
- --- a/compiler/specialise/SpecConstr.hs
- +++ b/compiler/specialise/SpecConstr.hs
- @@ -29,7 +29,7 @@ import CoreFVs ( exprsFreeVarsList )
- import CoreMonad
- import Literal ( litIsLifted )
- import HscTypes ( ModGuts(..) )
- -import WwLib ( mkWorkerArgs )
- +import WwLib ( isWorkerSmallEnough, mkWorkerArgs )
- import DataCon
- import Coercion hiding( substCo )
- import Rules
- @@ -1533,10 +1533,14 @@ specialise env bind_calls (RI { ri_fn = fn, ri_lam_bndrs = arg_bndrs
-
- | Just all_calls <- lookupVarEnv bind_calls fn
- = -- pprTrace "specialise entry {" (ppr fn <+> ppr (length all_calls)) $
- - do { (boring_call, pats) <- callsToPats env specs arg_occs all_calls
- -
- + do { (boring_call, all_pats) <- callsToPats env specs arg_occs all_calls
- -- Bale out if too many specialisations
- - ; let n_pats = length pats
- + ; let pats = filter (is_small_enough . fst) all_pats
- + is_small_enough vars = isWorkerSmallEnough (sc_dflags env) vars
- + -- We are about to construct w/w pair in 'spec_one'.
- + -- Omit specialisation leading to high arity workers.
- + -- See Note [Limit w/w arity]
- + n_pats = length pats
- spec_count' = n_pats + spec_count
- ; case sc_count env of
- Just max | not (sc_force env) && spec_count' > max
- diff --git a/compiler/stranal/WwLib.hs b/compiler/stranal/WwLib.hs
- index 09bc204..d9460d9 100644
- --- a/compiler/stranal/WwLib.hs
- +++ b/compiler/stranal/WwLib.hs
- @@ -8,6 +8,7 @@
-
- module WwLib ( mkWwBodies, mkWWstr, mkWorkerArgs
- , deepSplitProductType_maybe, findTypeShape
- + , isWorkerSmallEnough
- ) where
-
- #include "HsVersions.h"
- @@ -144,7 +145,8 @@ mkWwBodies dflags fam_envs fun_ty demands res_info one_shots
- wrapper_body = wrap_fn_args . wrap_fn_cpr . wrap_fn_str . applyToVars work_call_args . Var
- worker_body = mkLams work_lam_args. work_fn_str . work_fn_cpr . work_fn_args
-
- - ; if useful1 && not (only_one_void_argument) || useful2
- + ; if isWorkerSmallEnough dflags work_args
- + && (useful1 && not only_one_void_argument || useful2)
- then return (Just (worker_args_dmds, wrapper_body, worker_body))
- else return Nothing
- }
- @@ -165,6 +167,12 @@ mkWwBodies dflags fam_envs fun_ty demands res_info one_shots
- | otherwise
- = False
-
- +-- See Note [Limit w/w arity]
- +isWorkerSmallEnough :: DynFlags -> [Var] -> Bool
- +isWorkerSmallEnough dflags vars = count isId vars <= maxWorkerArgs dflags
- + -- We count only Free variables (isId) to skip Type, Kind
- + -- variables which have no runtime representation.
- +
- {-
- Note [Always do CPR w/w]
- ~~~~~~~~~~~~~~~~~~~~~~~~
- @@ -178,6 +186,30 @@ a disaster, because then the enclosing function might say it has the CPR
- property, but now doesn't and there a cascade of disaster. A good example
- is Trac #5920.
-
- +Note [Limit w/w arity]
- +~~~~~~~~~~~~~~~~~~~~~~~~
- +Guard against high worker arity as it generates a lot of stack traffic.
- +A simplified example is Trac #11565#comment:6
- +
- +Current strategy is very simple: don't perform w/w transformation at all
- +if the result produces a wrapper with arity higher than -fmax-worker-args=.
- +
- +It is a bit all or nothing, consider
- +
- + f (x,y) (a,b,c,d,e ... , z) = rhs
- +
- +Currently we will remove all w/w ness entirely. But actually we could
- +w/w on the (x,y) pair... it's the huge product that is the problem.
- +
- +Could we instead refrain from w/w on an arg-by-arg basis? Yes, that'd
- +solve f. But we can get a lot of args from deeply-nested products:
- +
- + g (a, (b, (c, (d, ...)))) = rhs
- +
- +This is harder to spot on an arg-by-arg basis. Previously mkWwStr was
- +given some "fuel" saying how many arguments it could add; when we ran
- +out of fuel it would stop w/wing.
- +Still not very clever because it had a left-right bias.
-
- ************************************************************************
- * *
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