[GHC] #11564: Possible overzealous unfolding

[GHC]

#11564: Possible overzealous unfolding
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        Reporter:  simonmar          |                Owner:
            Type:  bug               |               Status:  new
        Priority:  high              |            Milestone:
       Component:  Compiler          |              Version:  8.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):  Phab:D1900
       Wiki Page:                    |
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Comment (by Simon Marlow <marlowsd@…>):

 In [changeset:"a47b62cb36853d03c77ef63b3208b3d869fb687e/ghc"
 a47b62cb/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="a47b62cb36853d03c77ef63b3208b3d869fb687e"
 Second attempt to fix sizeExpr

 Summary:
 Background:
 * sizeExpr was calculating expressions like ((e `cast` T) x) wrongly
 * Fixing it caused regressions in compile performance, and one nofib
   program (k-nucleotide)

 I managed to fix the source of the compiler regressions.  I think it was
 due to traceTc not being inlined, which I fixed in a more robust way by
 putting an export list on TcRnMonad.

 The k-nucleotide regression is more difficult.  I don't think anything
 is actually going wrong, but this program has been highly tuned and is
 quite sensitive to changing in inlining behaviour.  I managed to recover
 most of the performance by manual lambda-lifting which makes it a bit
 less fragile, but the end result was a bit slower.  I don't think this
 is disastrous, the program is pretty horrible to begin with and we could
 probably make a faster one by starting from scratch.

 Test Plan: validate, nofib

 Reviewers: simonpj, bgamari, niteria, austin, erikd

 Subscribers: thomie

 Differential Revision: https://phabricator.haskell.org/D2338

 GHC Trac Issues: #11564
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11564#comment:10>
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