[GHC] #9434: GHC.List.reverse does not fuse

[Dan Doel]

Make foldl's inline phased, and see what happens?

Presumably the reason it doesn't have a phase limit yet is that it never
participated in any fusion before, so there was never a reason to not just
inline.

Other than that it seems like:

    reverse xs
      => rewrite
    build (\c n -> foldl (noinlineFlip c) n xs)
      => inline
    foldl (noinlineFlip (:)) [] xs
      => rewrite
    reverse xs

where I assume you need a special flip which may or may not exist in these
modules already.


On Fri, Aug 15, 2014 at 12:46 PM, David Feuer <david.feuer at gmail.com> wrote:

> Yes, but I'm not sure how to do that, especially because foldl doesn't
> have the phased NOINLINE that foldr does.
> On Aug 15, 2014 12:45 PM, "Dan Doel" <dan.doel at gmail.com> wrote:
>
>> Isn't this kind of thing fixed for other functions by rewriting back into
>> the direct recursive definition if no fusion happens?
>>
>>
>> On Fri, Aug 15, 2014 at 11:41 AM, David Feuer <david.feuer at gmail.com>
>> wrote:
>>
>>> I'm having trouble when it doesn't fuse—it ends up with duplicate
>>> bindings at the top level, because build gets inlined n times, and the
>>> result lifted out. Nothing's *wrong* with the code, except that there are
>>> multiple copies of it.
>>> On Aug 15, 2014 10:58 AM, "GHC" <ghc-devs at haskell.org> wrote:
>>>
>>>> #9434: GHC.List.reverse does not fuse
>>>>
>>>> -------------------------------------+-------------------------------------
>>>>               Reporter:  dfeuer      |            Owner:
>>>>                   Type:  bug         |           Status:  new
>>>>               Priority:  normal      |        Milestone:
>>>>              Component:              |          Version:  7.9
>>>>   libraries/base                     |         Keywords:
>>>>             Resolution:              |     Architecture:
>>>> Unknown/Multiple
>>>>       Operating System:              |       Difficulty:  Easy (less
>>>> than 1
>>>>   Unknown/Multiple                   |  hour)
>>>>        Type of failure:  Runtime     |       Blocked By:
>>>>   performance bug                    |  Related Tickets:
>>>>              Test Case:              |
>>>>               Blocking:              |
>>>> Differential Revisions:              |
>>>>
>>>> -------------------------------------+-------------------------------------
>>>>
>>>> Comment (by simonpj):
>>>>
>>>>  Great.  Just check that when fusion ''doesn't'' take place, the result
>>>> is
>>>>  good. And do a `nofib` comparison for good luck. Then submit a patch.
>>>>
>>>>  Thanks for doing all this work on fusion, David.
>>>>
>>>>  Simon
>>>>
>>>> --
>>>> Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9434#comment:2>
>>>> GHC <http://www.haskell.org/ghc/>
>>>> The Glasgow Haskell Compiler
>>>>
>>>
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>>>
>>
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