[GHC] #14186: CSE fails to CSE two identical large top-level functions

[GHC]

#14186: CSE fails to CSE two identical large top-level functions
-------------------------------------+-------------------------------------
        Reporter:  nomeata           |                Owner:  (none)
            Type:  bug               |               Status:  patch
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.3
      Resolution:                    |             Keywords:  CSE
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #13589            |  Differential Rev(s):  Phab:D3939
       Wiki Page:                    |
-------------------------------------+-------------------------------------
Changes (by bgamari):

 * keywords:   => CSE
 * related:   => #13589


Old description:

> Consider this code:
>
> {{{
> module CSEBug where
>
> data Succs a = Succs a [a]
>
> instance Functor Succs where
>     fmap f (Succs o s) = Succs (f o) (map f s)
>
> foo, bar :: (a -> b) -> (b -> c) -> Succs a -> Succs c
> foo f g x = fmap (g . f) x
> bar f g x = fmap (g . f) x
> }}}
>
> If I compile this with `-O`, `foo` and `bar` are not CSEd, which can be
> seen with `-ddump-cse`.
>
> Removing either the first or the second argument of `Succs` makes CSE
> work.
>
> Is there a size limit on CSE?

New description:

 Consider this code:

 {{{#!hs
 module CSEBug where

 data Succs a = Succs a [a]

 instance Functor Succs where
     fmap f (Succs o s) = Succs (f o) (map f s)

 foo, bar :: (a -> b) -> (b -> c) -> Succs a -> Succs c
 foo f g x = fmap (g . f) x
 bar f g x = fmap (g . f) x
 }}}

 If I compile this with `-O`, `foo` and `bar` are not CSEd, which can be
 seen with `-ddump-cse`.

 Removing either the first or the second argument of `Succs` makes CSE
 work.

 Is there a size limit on CSE?

--

Comment:

 #131589 is also somewhat relevant here.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14186#comment:6>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler