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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