[GHC] #10649: Performance issue with unnecessary reboxing

17 Jul 2015

      #10649: Performance issue with unnecessary reboxing
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              Reporter:  pacak       |             Owner:
                  Type:  bug         |            Status:  new
              Priority:  normal      |         Milestone:
             Component:  Compiler    |           Version:  7.10.1
              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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 Depending on number of fields in a structure (A and F), $fBA_$cfoo or
 $fBF_$cfoo will or will not be using $w$cfoo
 for 16 fields it uses it, for 12 - not

 With lazy fields behaviour, it starts using $w$cfoo around 100-200 fields.

 Adding -funfolding-use-threshold=90 helps in this case, but given enough
 fields (about 50 of them, of different types) value of 1000000000 is not
 helping and with very cheap operation (like addition or allocation of
 cons-like structure) overhead from sending those parameters via stack into
 worker becomes very significant - I have code that works ~3-5 times
 slower.

 This issue is not specific to generics, I can provide more examples if
 necessary

 {{{#!hs

 {-# LANGUAGE FlexibleContexts, FlexibleInstances, DeriveGeneric,
 DefaultSignatures #-}
 {-# LANGUAGE TypeOperators, BangPatterns  #-}
 {-# OPTIONS -funbox-strict-fields -ddump-to-file -ddump-simpl -ddump-stg
 -dsuppress-all -ddump-asm #-}

 import Data.Word
 import GHC.Generics

 data A = A !Word !Word !Word !Word   !Word !Word !Word !Word   !Word !Word
 !Word !Word  !Word
   deriving Generic

 data F = F !Word !Word !Word !Word   !Word !Word !Word !Word   !Word !Word
 !Word !Word  !Word !Word !Word !Word
   deriving Generic

 class B a where
     foo :: a -> Word

     {-# INLINE foo #-}
     default foo :: (Generic a, GB (Rep a)) => a -> Word
     foo !x = gfoo (from x)

 class GB f where
     gfoo :: (f a) -> Word

 instance GB x => GB (M1 D d (M1 C c x)) where
     {-# INLINE gfoo #-}
     gfoo (M1 (M1 x)) = gfoo x

 instance (GB a, GB b) => GB (a :*: b) where
     {-# INLINE gfoo #-}
     gfoo (a :*: b) = gfoo a + gfoo b

 instance GB (M1 S s (Rec0 Word)) where
     {-# INLINE gfoo #-}
     gfoo (M1 (K1 x)) = x

 instance B A
 instance B F

 main :: IO ()
 main = return ()
 }}}

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Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10649
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