[GHC] #9719: Improve `mkInteger` interface

[GHC]

#9719: Improve `mkInteger` interface
-------------------------------------+-------------------------------------
       Reporter:  hvr                |                   Owner:
           Type:  task               |                  Status:  new
       Priority:  low                |               Milestone:  7.12.1
      Component:  Compiler           |                 Version:
       Keywords:  Integer            |        Operating System:
   Architecture:  Unknown/Multiple   |  Unknown/Multiple
     Difficulty:  Unknown            |         Type of failure:
     Blocked By:                     |  None/Unknown
Related Tickets:                     |               Test Case:
                                     |                Blocking:
                                     |  Differential Revisions:
-------------------------------------+-------------------------------------
 `mkInteger` is the current operation provided by the `Integer` libraries
 to construct (large) integer values. The current type-signature is

 {{{#!hs
 mkInteger :: Bool   -- sign-bit
           -> [Int]  -- absolute value in 31 bit chunks, least significant
 first
           -> Integer
 }}}

 A somewhat pathological example of why this representation is not so nice
 is the following simple CAF

 {{{#!hs
 c :: Integer
 c = 0xf000000000000000
 }}}

 that (this is for a 64bit arch!) gets compiled into the following STG:

 {{{
 ==================== STG syntax: ====================
 sat_sJQ :: GHC.Types.Int
 [LclId, Str=DmdType] =
     NO_CCS GHC.Types.I#! [3];
 sat_sJR :: [GHC.Types.Int]
 [LclId, Str=DmdType] =
     NO_CCS :! [sat_sJQ GHC.Types.[]];
 sat_sJP :: GHC.Types.Int
 [LclId, Str=DmdType] =
     NO_CCS GHC.Types.I#! [1610612736];
 sat_sJS :: [GHC.Types.Int]
 [LclId, Str=DmdType] =
     NO_CCS :! [sat_sJP sat_sJR];
 sat_sJO :: GHC.Types.Int
 [LclId, Str=DmdType] =
     NO_CCS GHC.Types.I#! [0];
 sat_sJT :: [GHC.Types.Int]
 [LclId, Str=DmdType] =
     NO_CCS :! [sat_sJO sat_sJS];
 Foo.c :: GHC.Integer.Type.Integer
 [GblId, Str=DmdType] =
     \u srt:SRT:[0Y :-> GHC.Integer.Type.mkInteger, sJT :-> sat_sJT] []
         GHC.Integer.Type.mkInteger GHC.Types.True sat_sJT;
 }}}



 Instead a more "natural" `mkInteger` would be desirable, possibly in the
 style of `unpackCString#`, in terms of a packed/unboxed vector of machine-
 word-sized chunks. A better `mkInteger` could then take a `ByteArray#` or
 a `Addr#` + length instead, e.g.

 {{{#!hs
 mkInteger :: Int#    -- signum(n) = sign of encoded Integer
                      -- abs(n)    = number of machine-word sized chunks
           -> Addr#   -- pointer to start of machine-word sized chunks,
                      -- least-significant chunk first
           -> Integer
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9719>
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