[GHC] #13825: Allow multiple constructor fields occupy the same word

[GHC]

#13825: Allow multiple constructor fields occupy the same word
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           Reporter:  michalt        |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.1
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 The main goal is to reduce the overhead of things like:
 {{{#!hs
 data Bloated =
   Bloated {-# UNPACK #-} !Word8
           {-# UNPACK #-} !Int8
           {-# UNPACK #-} !Bool
 }}}
 Assuming 64-bit architecture, currently those fields will take 8 bytes
 each! So for this example we'd need: 8 bytes for header + 3 * 8 bytes for
 fields = 32 bytes. But we should be able to pack the fields into a single
 word (a word is 8 bytes and each field really only needs 1 byte) for a
 total of 16 bytes (8 bytes header + 8 bytes for fields, with the 5 bytes
 being "overhead" due to heap alignment).

 My understanding is that we need a few things to make this happen:
 - Ability to refer to fields that are packed into a single word (currently
 everything in GHC assumes that each field occupies a single word). Simon
 Marlow started working on this in https://phabricator.haskell.org/D38
 - Introduce primitives like `Word8#`, `Int8#`, ... (currently `WordX` and
 `IntX` are defined as wrappers of `Word#` and `Int#` respectively) and
 change `WordX`/`IntX` definitions to use those primitives.
 - Figure out what to do with `Bool` (should it be just `Word8#`? should we
 have `Bool#`?) and change its definition (using pattern synonyms for
 `True`/`False`)

 Some additional info:
 - Thread on ghc-devs: https://mail.haskell.org/pipermail/ghc-
 devs/2017-June/014304.html

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13825>
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