[GHC] #5218: Add unpackCStringLen# to create Strings from string literals

[GHC]

#5218: Add unpackCStringLen# to create Strings from string literals
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        Reporter:  tibbe             |                Owner:  thoughtpolice
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  7.0.3
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Runtime           |  Unknown/Multiple
  performance bug                    |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #5877 #10064      |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by jscholl):

 After switching to {{{(# Int#, Addr# #)}}}, I have a sort-of working
 implementation. The builtin rules seem to work, rewriting string literals
 from user-code works and has access to the length, but ghci wouldn't even
 start up. Maybe I can take a look this evening into it, but if I
 understand unboxed tuples correctly, it should be possible to just flatten
 them if they appear in an argument position, right? E.g. turn {{{ foo (#
 a, b, c #) }}} into {{{ foo a b c }}}.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/5218#comment:47>
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