Performance issue with unnecessary reboxing

Depending on number of fields in a structure (A and F), fBA_cfoo or fBF_cfoo will or will not be using wcfoo for 16 fields it uses it, for 12 - not

With lazy fields behaviour, it starts using wcfoo 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

{-# 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 ()

Trac metadata

Trac field	Value
Version	7.10.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture