[GHC] #10012: Cheap-to-compute values aren't pushed into case branches inducing unnecessary register pressure

[GHC]

#10012: Cheap-to-compute values aren't pushed into case branches inducing
unnecessary register pressure
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        Reporter:  bgamari           |                Owner:  bgamari
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  7.8.4
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Runtime           |  Unknown/Multiple
  performance bug                    |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by nomeata):

 In comment:4 simon (or ben?) says “duplicating work”, but it seems that
 the question at hand is not about loss of sharing, but rather code size,
 right? Floating a let binding into the branches of a case does not
 duplicate any work, only code.

 A `{-# INLINE_THROUGH #-}` pragma seems to be a bit unspecific. Do we
 really want the compiler to inline through everything, no matter how
 large? Or should this just shift the heuristics a bit? But what is so
 special about this `case` that makes this a worthwhile thing here, and not
 in other cases (sic)? Maybe the heuristics can be improved in general?

 It seems to be more a general question of whether the user wants more
 speed or smaller code.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10012#comment:7>
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