[GHC] #8763: forM_ [1..N] does not get fused (allocates 50% more)

[GHC]

#8763: forM_ [1..N] does not get fused (allocates 50% more)
-------------------------------------+-------------------------------------
        Reporter:  nh2               |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.8.1
       Component:  Compiler          |              Version:  7.6.3
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Runtime           |  Unknown/Multiple
  performance bug                    |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #7206             |  Differential Rev(s):  Phab:D5131
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by Krzysztof Gogolewski <krz.gogolewski@…>):

 In [changeset:"e655aac18c5b240f27fcaf26317ad87be5ce8b96/ghc"
 e655aac1/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="e655aac18c5b240f27fcaf26317ad87be5ce8b96"
 Make sure forM_ and related functions fuse cleanly

 Summary:
 It was revealed in #8763 that it's hard to come up with a list fusion
 helper for `efdtIntFB` that doesn't duplicated occurrences of `c`,
 which is crucial in guaranteeing that it is inlined.

 Not inlining `c` led to spoiled join points, in turn leading to
 unnecessary
 heap allocation. This patch tackles the problem from a different angle:
 Fixing all consumers instead of the less often used producer
 `efdtIntFB` by inserting an INLINE pragma in the appropriate places.
 See https://ghc.haskell.org/trac/ghc/ticket/8763#comment:76 and the new
 Note [List fusion and continuations in 'c'].

 A quick run of NoFib revealed no regression or improvements whatsoever.

 Reviewers: hvr, bgamari, simonpj

 Reviewed By: simonpj

 Subscribers: simonpj, rwbarton, carter

 GHC Trac Issues: #8763

 Differential Revision: https://phabricator.haskell.org/D5131
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8763#comment:85>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler