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

15 Aug 2014

      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  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"  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 
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"  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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