[GHC] #13344: Core string literal patch regresses compiler performance considerably

[GHC]

#13344: Core string literal patch regresses compiler performance considerably
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        Reporter:  bgamari           |                Owner:  bgamari
            Type:  bug               |               Status:  new
        Priority:  high              |            Milestone:  8.2.1
       Component:  Compiler          |              Version:  8.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Compile-time      |  Unknown/Multiple
  performance bug                    |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by simonpj):

 I'm all for making interface-file deserialisation cheaper.

 But I don't feel that we have a well-characterised problem.  ''What
 exactly is the problem we are trying to solve here?''

 Is it ONLY that
 {{{
 x :: Addr#
 x = "foo"#
 y :: T
 y = K x
 }}}
 is more expensive than
 {{{
 y :: T
 y = K "foo"#
 }}}
 And if it is more expensive, how much more expensive? And does that cost
 come from simplifying, spitting out an interface file, code generation,
 reading in an interface file?

 I don't think there is much to do with strings here.  The same will happen
 for
 {{{
 x = [True, False, True, ..., True]
 }}}
 Perhaps we just don't want to expose the unfolding for a top level
 constructor.  That would be a bit less good on optimisation, but perhaps
 not a lot.  We could measure it.

 I feel as if we are at risk of taking actions without data to guide them.
 I'd love to have data.

 Simon

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13344#comment:16>
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