#13825: Allow multiple constructor fields occupy the same word -------------------------------------+------------------------------------- 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: -------------------------------------+------------------------------------- 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 -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13825 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler