[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 osa1):

 I didn't follow the whole thread but yeah, if you want to compile string
 literals to unboxed tuples (`(# len, ptr #)`) it'd be a problem for GHCi
 as the bytecode interpreter/compiler can't handle unboxed tuple returns.
 (it has some special cases for handling primops that return state tokens
 in an unboxed tuple)

 We had briefly discussed doing bytecode generation after unarise in the
 mailing list (https://mail.haskell.org/pipermail/ghc-
 devs/2016-July/012502.html see Simon's response). I don't understand
 native calling conventions good enough to comment, but maybe we can add
 one more special case in the bytecode compiler to handle `(# len, ptr #)`
 returns, like the special cases for returning `(# RealWorld#, a #)` etc.
 (search for `isUnboxedTupleCon` in `ByteCodeGen.hs`)

 I'm willing to work on extending unboxed tuple support in the bytecode
 interpreter, but I don't have any concrete proposals right now -- I need
 to study native calling conventions for returning tuples first.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/5218#comment:46>
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