[GHC] #10064: Add support for "foo"## literals to MagicHash

[GHC]

#10064: Add support for "foo"## literals to MagicHash
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              Reporter:  chadaustin  |             Owner:
                  Type:  feature     |            Status:  new
  request                            |         Milestone:
              Priority:  normal      |           Version:  7.8.4
             Component:  Compiler    |  Operating System:  Unknown/Multiple
              Keywords:              |   Type of failure:  None/Unknown
          Architecture:              |        Blocked By:
  Unknown/Multiple                   |   Related Tickets:
             Test Case:              |
              Blocking:              |
Differential Revisions:              |
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 I'm working on a high-performance library for building up buffers of data.
 Appending ByteStrings works pretty well, but, for bytestring literals,
 it's faster to avoid the CAF (and it's associated tag check + indirect
 jump) entirely and append a zero-terminate MagicHash string literal via
 Addr#.

 Then I discovered that it's 30% faster if you can avoid the strlen with an
 explicit length.  See this benchmark: https://github.com/chadaustin
 /buffer-
 builder/blob/ac662962d6d3c21f206ab8e8121323b9f18677fa/bench/Bench.hs#L42

 That API is not very pleasant, of course.  :)

 Thus, here is a feature request:

 It would be great if "foo"## produced (# Int#, Addr# #), where the Int# is
 the length of the buffer in bytes.

 The compiler could enforce that the literal only contains 8-bit values.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10064>
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