[GHC] #10069: CPR related performance issue

9 Feb 2015

      #10069: CPR related performance issue
-------------------------------------+-------------------------------------
              Reporter:  pacak       |             Owner:
                  Type:  bug         |            Status:  new
              Priority:  normal      |         Milestone:
             Component:  Compiler    |           Version:  7.10.1-rc2
              Keywords:              |  Operating System:  Unknown/Multiple
          Architecture:              |   Type of failure:  Runtime
  Unknown/Multiple                   |  performance bug
             Test Case:              |        Blocked By:
              Blocking:              |   Related Tickets:
Differential Revisions:              |
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 By default CRP analysis can be too aggressive in trying to pass as much as
 possible in unboxed tuples, in general it's not a problem but when one big
 datatype is passed to several consumers it might end up pushed to stack
 several times instead of once - to heap, things are getting worse when
 there are sufficient fields to cause stack overflow which otherwise is
 possible to avoid - in our codebase adding one field with
 ExistentialQuantification (unused, but that prevents ghc from doing CRP
 transformation) reduces number of stack overflow by a factor of 1000 and
 increases overall performance by 10%.

 In provided example performance for both A and B should be identical and
 yet B is consistently faster by 3-5%

 It's possible to increase this performance gap by adding more and more
 fields.

 I was able to replicate this issue in ghc 7.8.3 and 7.10,1rc2

 {{{#!hs

 {-# LANGUAGE ExistentialQuantification #-}

 module Blah where

 import Criterion
 import Criterion.Main
 import Data.Typeable

 data A = A ()
     !Int !Int !Int !Int !Int !Int !Int !Int !Int !Int !Int !Int !Int !Int
 !Int !Int
     !Int !Int !Int !Int !Int !Int !Int !Int !Int !Int !Int !Int !Int !Int
 !Int !Int

 data B = forall rep. (Typeable rep) => B rep
     !Int !Int !Int !Int !Int !Int !Int !Int !Int !Int !Int !Int !Int !Int
 !Int !Int
     !Int !Int !Int !Int !Int !Int !Int !Int !Int !Int !Int !Int !Int !Int
 !Int !Int

 a :: A
 a = A () 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8

 b :: B
 b = B () 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8

 {-# NOINLINE a1 #-}
 a1 :: A -> Int
 a1 (A _ f1 f2 f3 f4 f5 f6 f7 f8 g1 g2 g3 g4 g5 g6 g7 g8 _ _ _ _ _ _ _ _ _
 _ _ _ _ _ _ _) = f1

 {-# NOINLINE a2 #-}
 a2 :: A -> Int
 a2 (A _ f1 f2 f3 f4 f5 f6 f7 f8 g1 g2 g3 g4 g5 g6 g7 g8 _ _ _ _ _ _ _ _ _
 _ _ _ _ _ _ _) = f2

 {-# NOINLINE a3 #-}
 a3 :: A -> Int
 a3 (A _ f1 f2 f3 f4 f5 f6 f7 f8 g1 g2 g3 g4 g5 g6 g7 g8 _ _ _ _ _ _ _ _ _
 _ _ _ _ _ _ _) = f3

 {-# NOINLINE a4 #-}
 a4 :: A -> Int
 a4 (A _ f1 f2 f3 f4 f5 f6 f7 f8 g1 g2 g3 g4 g5 g6 g7 g8 _ _ _ _ _ _ _ _ _
 _ _ _ _ _ _ _) = f4

 {-# NOINLINE b1 #-}
 b1 :: B -> Int
 b1 (B _ f1 f2 f3 f4 f5 f6 f7 f8 g1 g2 g3 g4 g5 g6 g7 g8 _ _ _ _ _ _ _ _ _
 _ _ _ _ _ _ _) = f1

 {-# NOINLINE b2 #-}
 b2 :: B -> Int
 b2 (B _ f1 f2 f3 f4 f5 f6 f7 f8 g1 g2 g3 g4 g5 g6 g7 g8 _ _ _ _ _ _ _ _ _
 _ _ _ _ _ _ _) = f2

 {-# NOINLINE b3 #-}
 b3 :: B -> Int
 b3 (B _ f1 f2 f3 f4 f5 f6 f7 f8 g1 g2 g3 g4 g5 g6 g7 g8 _ _ _ _ _ _ _ _ _
 _ _ _ _ _ _ _) = f3

 {-# NOINLINE b4 #-}
 b4 :: B -> Int
 b4 (B _ f1 f2 f3 f4 f5 f6 f7 f8 g1 g2 g3 g4 g5 g6 g7 g8 _ _ _ _ _ _ _ _ _
 _ _ _ _ _ _ _) = f4

 {-# NOINLINE fa #-}
 fa :: A -> Int
 fa a = a1 a + a2 a + a3 a + a4 a

 {-# NOINLINE fb #-}
 fb :: B -> Int
 fb b = b1 b + b2 b + b3 b + b4 b

 main :: IO ()
 main = defaultMain [
    bgroup "single call" [
      bench "A" $ whnf fa a
    , bench "B" $ whnf fb b
    ]
    ]

 }}}

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Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10069
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